\ Canadian Galactic Plane Survey Publications
[Photograph of the CGPS field in the Milky Way]

Canadian Galactic Plane Survey Publications

This page lists refereed journal publications related to the Canadian Galactic Plane Survey and authored by CGPS consortium members. It is maintained as a service to the consortium, and feedback on its use is appreciated. Please tell me if you find this page useful. There are many unrefereed works -- conference talks, posters, proceedings, colloquia, etc. --that are too numerous to list here. For a small sample of these, see the Program and Abstracts from the ISM Workshop, NEW PERSPECTIVES ON THE INTERSTELLAR MEDIUM, held 1998 August in Naramata, B.C.

CGPS members: Please tell me about any refereed CGPS-related papers not listed here.

An ASCII histogram of the number of papers published each year can be found here. For a "pretty" version to use in presentations, please click here.


pre-CGPS | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008


Chandra Confirmation of a Pusar Wind Nebula in DA 495

Arzoumanian, Z., Safi-Harb, S., Landekcer, T.L., Kothes, R.,& Camilo, F.
2007 ApJ, submitted

As part of a multiwavelength study of the unusual radio supernova remnant DA 495, we present observations made with the Chandra X-ray Observatory. Imaging and spectroscopic analysis confirms the previously detected X-ray source at the heart of the annular radio nebula, establishing the radiative properties of two key emission components: a soft unresolved source with a blackbody temperature of 1 MK consistent with a neutron star, surrounded by a nonthermal nebula 40 arcseconds in diameter exhibiting a power-law spectrum with photon index Γ= 1.6+/−0.3, typical of a pulsar wind nebula. The implied spin-down luminosity of the neutron star, assuming a conversion efficiency to nebular flux appropriate to Vela-like pulsars, is ~1035 ergs s−1, again typical of objects a few tens of kyr old. Morphologically, the nebular flux is slightly enhanced along a direction, in projection on the sky, previously demonstrated to be of significance in radio and ASCA observations; we argue that this represents the orientation of the pulsar spin axis. At smaller scales, a narrow X-ray feature is seen extending out 5 arcseconds from the point source, a distance consistent with the sizes of resolved wind termination shocks around many Vela-like pulsars. Finally, we argue based on synchrotron lifetimes in the nebular magnetic field that DA 495 represents the first example of a pulsar wind nebula in which electromagnetic flux makes up a significant part, together with particle flux, of the neutron star's wind, and that this high magnetization factor may account for the nebula's low luminosity.

Comments on the Radio Spectrum of HB 3

Green, D.A.
2007 Bulletin of the Astronomical Society of India, 35, 77

It has recently been suggested that the radio aspectrum of the Galactic supernova remnant HB 3 shows flattening at high frequencies (above about 1 GHz). Here I review the radio spectrum of HB 3, noting the difficulties in deriving accurate flux densities for this remnant, particularly at high frequencies, dues to the proximity of bright, thermal emission from W3 and its surroundings. A flux density for HB 3 at 2696 MHz is derived from Effelsberg survey data. The spectrum of HB 3 is well represented by a single power-law spectrum from 22 to 2695 MHz, with a spectral index of 0.56+/−0.03. It is concluded that contamination with thermal emission from adjacent regions is the cause for the reported spectral flattening for HB 3.

An Analysis of the Broadband (22-3900 MHz) Radio Spectrum of HB 3 (G132.7+1.3): The Detection of Thermal Radio Emission from an Evolved Supernova Remnant?

Urosevic, D., Pannuti, T.G., & Leahy, D.
2007 ApJ, 655, L41

We present an analysis of the broadband radio spectrum (from 22 to 3900 MHz) of the Galactic supernova remnant (SNR) HB 3 (G132.7+1.3). Published observations have revealed that a curvature is present in the radio spectrum of this SNR, indicating that a single synchrotron component appears insufficient to adequately fit the spectrum. We present here a fit to this spectrum using a combination of a synchrotron component and a thermal bremsstrahlung component. We discuss properties of the latter component and estimate the ambient density implied by the presence of this component to be n~10 cm−3. We have also analyzed X-ray spectra extracted from archived ASCA GIS observations of different regions of HB 3 to obtain independent estimates of the density of the surrounding interstellar medium (ISM). From this analysis we have derived electron densities of 0.1 - 0.4f−0.5 cm−3 for the ISM for the three different regions of the SNR, where f is the filling factor. By comparing these density estimates with the estimate derived from the thermal bremsstrahlung component, we argue that the radio thermal bremsstrahlung emission is emitted from a thin shell enclosing HB 3. The presence of this thermal bremsstrahlung component in the radio spectrum of HB 3 suggests that this SNR is in fact interacting with an adjacent molecular cloud associated with the H II region W3. By extension, we argue that the the presence of thermal emission at radio wavelengths may be a useful tool for identifying interactions between SNRs and molecular clouds, and for estimating the ambient density near SNRs using radio continuum data.

Correcting the Polarization Leakage Amplitudes and Phases Throughout the Primary Beam of an Interferometer

Reid, R.I., Gray, A.D., Landecker, T.L. & Willis, A.G.
2007 Radio Science, submitted (June 2007)

Polarimetric observations are affected by leakage of unpolarized light into the polarization channels, in a way that varies with the angular position of the source relative to the optical axis. The off-axis part of the leakage is often corrected by subtracting from each polarization image the product of the unpolarized map and a leakage map, but it is seldom realized that heterogeneities in the array shift the loci of the leaked radiation in a baseline-dependent fashion. We present here a method to measure and remove the wide-field polarization leakage of a heterogeneous array. The process also maps the complex voltage patterns of each antenna, which can be used to correct in all Stokes parameters imaging errors due to the primary beam.

A New Model For The Loop-I (North Polar Spur) Region

Maik Wolleben
2007 The Astrophysical Journal, 664, 349

The North Polar Spur (NPS) is the brightest filament of Loop I, a large circular feature in the radio continuum sky. In this paper, a model consisting of two synchrotron emitting shells is presented that reproduces large-scale structures revealed by recent polarization surveys. The polarized emission of the NPS is reproduced by one of these shells. The other shell, which passes close to the Sun, gives rise to polarized emission towards the Galactic poles. It is proposed that X-ray emission seen towards the NPS is produced by interaction of the two shells. Two OB-associations coincide with the centers of the shells. A formation scenario of the Loop I region is suggested.

Tracking the Outer Spiral Arms of the Galaxy in H I Absorption

Strasser, S.T., Dickey, J.M, Boothroyd, A.I., Kavars, D.W., Lockman, F.J., Martin, P.G., McClure-Griffiths, N.M., Rothwell, T.A., Stil, J.M. & Taylor, A.R.
2007 Astronomical Journal, 134, 2252

We have analyzed over 800 atomic hydrogen emission and absorption spectrum pairs toward continuum background sources in the northern and southern Galactic plane. In this paper we focus on absorption features from cold gas in the outermost arms of the Galaxy. We find HI absorption associated on a global scale with the Outer Spiral arm (first and second Galactic quadrants). To a lesser extent we also see absorption associated with the most distant spiral arm in the third and forth Galactic quadrants. Cool-phase gas therefore exists throughout these outermost spiral arms. Mean distances between absorbing clouds are on the order of 90 pc to 220 pc. We identify a number of striking HI structures with masses on the order of 104 Msun to 106 Msun containing cool-phase gas with temperatures below 100 K. These clouds are only marginally stable against gravitational collapse.

XMM-Newton and CHANDRA Observations of the Supernova Remnants G85.4+0.7 and G85.9−0.6

Jackson, M. S., Safi-Harb, S., Kothes, R. & Foster, T.
2008 Astrophysical Journal, in press

The study of low surface brightness supernova remnants (SNRs) is vital to address the explosions of massive stars in a low-density ambient medium, and XMM-Newton is the best satellite to date to detect such SNRs in the 0.5-10 keV band because of its large effective area. We present an XMM-Newton detection of two low radio surface brightness SNRs, G85.4+0.7 and G85.9−0.6, discovered with the Canadian Galactic Plane Survey (CGPS), and found to be weak X-ray emitters in the ROSAT All Sky Survey. High-resolution XMM-Newton X-ray images revealing the morphology of the diffuse X-ray emission, as well as a number of point sources for each object, are presented and correlated with the radio images and with recent Chandra images, in which the point sources appearing in the field are better resolved. In addition the new data allow for the first time a spectroscopic and timing analysis of the diffuse emission regions as well as the discrete X-ray point sources in the fields. Distances have been determined from H I and CO data to be 3.5+/−1.0 kpc for SNR G85.4+0.7 and 4.8+/−1.6 kpc for SNR G85.9−0.6. The SNR G85.4+0.7 is found to have a temperature of ~12-13 MK and a 0.5-2.5 keV luminosity of ~1-4×1033D23.5 erg/s (where D3.5 is the distance in units of 3.5 kpc), with an electron density ne of ~0.09-0.14(fD3.5)0.5 cm−3 (where f is the volume filling factor, expected to be around 0.2 for this SNR), and a shock age of ~18-23(fD3.5)−1/2 kyr. The SNR G85.9−0.6 is found to have a temperature of ~15-19 MK and a 0.5-2.5 keV luminosity of ~1-4×1034D24.8 erg/s (where D4.8 is the distance in units of 4.8 kpc), with an electron density ne of ~0.06-0.08(fD4.8)0.5 cm−3 and a shock age of ~22-30(fD4.8)−1/2 kyr, where f~0.1 for this SNR. Based on the combination of optical to X-ray flux ratio (assuming the optical sources are the true counterparts), distances from SNR centres, spectral fits, and comparison of column densities, none of the point sources appears to be the neutron star associated with either SNR.

A sharper view of the outer Galaxy at 1420 and 408 MHz from the Canadian Galactic Plane Survey II: The catalogue of extended radio sources

Kerton, C. R., Murphy, J. & Patterson, J.
2007 MNRAS, 379, 289

A new catalogue of extended radio sources has been prepared based on arcminute-resolution 1420 MHz images from the Canadian Galactic Plane Survey (CGPS). The new catalogue provides both 1420 MHz and 408 MHz flux density measurements on sources found near the Galactic plane in the second quadrant of our Galaxy. In addition cross-identifications are made with other major radio catalogues and information is provided to facilitate the recovery of CGPS image data associated with each catalogued source. Numerous new radio sources are identified and the catalogue provides a comprehensive summary of both newly discovered and previously known H II regions and supernova remnants in the outer Galaxy. The catalogue should be of use both for synoptic studies of Galactic structure and for placing higher resolution observations, at radio and other wavelengths, in context.

Revealing the Galactic H II region G84.9+0.5 through 5 GHz continuum and polarization emission, and a Voigt Profile analysis of RRL Observations

Foster, T. J., Kothes, R., Kerton, C. & Arvidsson, K.
2007 Astrophysical Journal, 667, 248

We present new λ6 cm radio observations (Stokes I, Q, U and hydrogen recombination line) of the Galactic object G84.9+0.5, previously classified as a supernova remnant. Radio recombination line (RRL) emission near λ6 cm is detected in deep GBT observations, and we are able to separate the 7.6 mK line detected from this object (appearing at vLSR=−40 km s−1) from the line emitted by ionized gas of W 80 in the foreground (Tl=5.4 mK; vLSR=0 km s−1) along the same line-of-sight (LOS). Detection of RRL emission from G84.9+0.5 and the absence of polarized emission at λ6 cm imply this object is an H II region. Rather than a Gaussian a Voigt function better describes the extended line profile of G84.9+0.5, which has a low-level "wing" extending into its negative velocity end. A Monte Carlo analysis of noisy synthetic spectra is presented, and it is concluded the wing is not spurious. Two physical explanations for the wing (pressure broadening and an outflow of gas) are considered. We favor that of a champagne-type outflow in the gas flowing along the inside wall of a known molecular cloud in the vicinity of the nebula (at −40 km s−1), making G84.9+0.5 "blister" type H II region viewed face-on. We find Te=9900 K and ne=20 cm−3 from a non-LTE analysis of the peak towards the RRL, and a total H II mass of 440 Msolar. A distance of 4.9 kpc is determined for this object. An IR analysis using MSX and 2MASS data is presented, showing H II region colours for G84.9+0.5 and identifying a possible exciting star for this H II region.

Infrared Excess Sources in the Canadian Galactic Plane Survey

Douglas, K. & Taylor, A. R.
2007 Astrophysical Journal, 659, 426

Using datasets from the Canadian Galactic Plane Survey, we have conducted a multiwavelength study of interstellar components, covering the region 102.o5 < l < 141.o5 and −3.o03 < b < 5.o41. By comparing column density tracers of dust, atomic hydrogen and molecular gas (traced by CO emission), we have found regions where the dust optical depth shows evidence of more gas than is predicted by the H I and CO observations. Within this population of infrared excess sources, it is possible to discriminate between sources associated with low and high dust temperatures. We interpret the colder-temperature sources as molecular clouds/clumps not traced by the CO J = 1-0 transition. Possible reasons include the depletion of CO onto dust grains in the coldest, densest regions of molecular clouds, or photodissociation of CO on the outskirts of molecular clouds.

Discovery of a new faint radio SNR G108.2-0.6

Tian, W. W., Leahy, D. A., & Foster, T. J.
2007 Astronomy & Astrophysics, 465, 907 .

A new faint and large shell-type radio Supernova Remnant (SNR) G108.2-0.6 has been discovered in the Canadian Galactic Plane Survey (CGPS). The SNR shows an elliptical shell-type structure at 1420 MHz, and has a 408-1420 MHz TT-plot spectral index of α=−0.5+/−0.1 (Sν∝ν−α), typical of a shell-type SNR. The remnant's flux density at 1420 MHz is 6.6+/−0.7 Jy, and at 408 MHz is 11.5+/−1.2 Jy. Both of these are corrected for compact sources. An integrated spectral index of −0.45+/−0.13 is determined. This new SNR has among the lowest surface brightness of any known remnant (Σ1 GHz=2.4×10−22} W m−2 Hz−1 sr−1). 21 cm Stokes Q and U CGPS data (plus preliminary Effelsberg Q and U maps) show some suggestive features that correlate with total power. H I observations show structures associated with G108.2−0.6 in the radial velocity range −53 to −58 km s−1, and indicate it is located in the Perseus arm shock at a distance of 3.2+/−0.6 kpc. At this distance the diameter of G108.2−0.6 is 58 pc. IRAS maps (12, 25, 60 and 100µm) of the new SNR show rich infrared emission surrounding G108.2−0.6.

A sharper view of the outer Galaxy at 1420 and 408 MHz from the Canadian Galactic Plane Survey I: Revisiting the KR catalogue and new Gigahertz Peaked Spectrum sources

Kerton, C.
2006 MNRAS, 373, 1203.

Arcminute-resolution radio continuum images at 408 and 1420 MHz from the Canadian Galactic Plane Survey (CGPS) have been used to reexamine radio sources listed in the Kallas & Reich (1980) catalogue. This catalogue is of particular interest to Galactic studies as it lists both extended and compact radio sources found in the second Galactic quadrant. We have determined the nature (extended vs. compact, Galactic vs. extragalactic) of all of these bright radio sources. A number of large regions with no optical counterparts are highlighted along with a sample of large radio galaxies. Many sources previously thought to be extended Galactic objects are shown to be point sources. A sample of point sources with flat or rising spectra between 408 and 1420 MHz has been compiled, and within this sample likely Gigahertz Peaked Spectrum sources have been identified.

Automatic Detection of Expanding H I Shells in the CGPS Data

Daigle, A., Joncas, G. & Parizeau, M.
2006 Astrophysical Journal, 661, 285.

The identification of expanding H I shells is difficult because of their variable morphology. In this paper, we present an automatic detector for H I shells, based on the more stable dynamical characteristics of expanding bubbles with radii<40 pc. The detection is performed in two stages. First, artificial neural networks are trained to recognize the dynamical signature of an expanding bubble in the velocity spectra of 21-cm data. The second stage consists in subsequent validations based on the potential bubble's morphology. The technique is tested on 11 known bubbles, and 10 are successfully detected. Conducting a systematic detection on a 48×9 region in the Perseus Arm, we obtain 7100 detections with spatial distribution following the stellar distribution of the Galactic disk. The estimated radius and expansion velocity distributions for objects with R<=10pc agree with the distributions predicted by models of adiabatically expanding bubble populations. The fraction of the Perseus Arm volume occupied by the detected objects, which can be interpreted as the small bubbles' contribution to the Galactic porosity Q, is calculated to QR<40pc= 0.007+0.025−0.003. Four new bubble cases and eight serious candidates, related to known progenitors, are proposed.

Radio Spectrum and Distance of the SNR HB9

Leahy, D. A. & Tian, W. W.
2007 Astronomy & Astrophysics, 461, 1013.

New images are presented of the supernova remnant (SNR) HB9 based on 408 MHz and 1420 MHz continuum emission and HI-line emission data of the Canadian Galactic Plane Survey (CGPS) by the Dominion Radio Astrophysical Observatory (DRAO). Two methods of spectral index analysis for HB9 are presented and compared: one removes compact sources at both frequencies but is limited to the resolution of the 408 MHz image; the other removes compact sources only in the 1420 MHz image so is effective at higher spatial resolution. The second allows more detailed spectral index variation studies than the first. The two T-T plot methods and new integrated flux densities give spectral index (Sν∝ν−α) for the whole of HB9 of 0.48+/−0.03; and 0.47+/−0.06, respectively. These are lower than previous spectral index for HB9 (α=0.61). Spatial variations of spectral index are derived using the second method and yield a steeper spectral index for interior regions than for the rim. This can be explained by a standard curved interstellar electron energy spectrum combined with lower interior magnetic field compared to that near the outer shock, which results in a larger proportion of steep spectrum emission for lines-of-sight through the central body of the SNR. HI observations show structures probably associated with the SNR in the radial velocity range −3 to −9 km s−1 and suggest a distance of 0.8+/−0.4 kpc for the SNR. This is consistent with the distance to the radio pulsar 0458+46, offset from the center of HB9 by 23'. However the pulsar spindown and kinematic ages are significantly greater than estimates of the SNR age: the Sedov age for HB9 is 6600 yr and the evaporative cloud model yields ages of 4000-7,000 yr.

DA 495 - an Aging Pulsar Wind Nebula

Kothes, R., Landecker, T. L., Reich, W., Safi-Harb, S. & Arzoumanian, Z.
2006 The Astrophysical Journal, submitted.

We present a radio continuum study of the filled--center supernova remnant (SNR) DA 495 (G65.7+1.2), including images of total intensity and linear polarization from 408 to 10550 MHz based on the Canadian Galactic Plane Survey and observations with the Effelsberg 100-m Radio Telescope. Removal of flux density contributions from a superimposed H II region and from compact extragalactic sources reveals a break in the spectrum of DA 495 at 1.3 GHz, with a spectral index α=−0.45+/−0.2 below the break and α=−0.87+/−0.1 above it (Sν∝ν−α). The spectral break is more than three times lower in frequency than the lowest break detected in any other filled--center SNR. The break in the spectrum is likely the result of synchrotron cooling, and DA 495, at an age of ~17000 yr, may have evolved from an object similar to the Vela X nebula, with a similarly energetic pulsar. The nebula's magnetic field is ~1.5μG. After correcting for the resulting high internal rotation measure, the magnetic field structure is quite simple, resembling a dipole field projected onto the plane of the sky, although a toroidal component is probably also present. The dipole field axis, which should be parallel to the spin axis of the putative pulsar, lies at an angle of ~50o east of the North Celestial Pole and is pointing away from us towards the south-west. The upper limit to any shell-type SNR emission around DA 495 is ~5.4× 10−23 Watt m−2 Hz−1 sr−1 (assuming α = −0.5), lower than the faintest shell-type remnant known to date.

The Fragmenting Superbubble Associated with the H II Region W4

West, J. L., English, J., Normandeau, M., & Landecker, T. L.
2006 The Astrophysical Journal, 656, 914.

New observations at high latitudes above the H II region W4 show that the structure formerly identified as a chimney candidate, an opening to the Galactic halo, is instead a superbubble in the process of fragmenting and possibly evolving into a chimney. Data at high Galactic latitudes (b > 5o) above the W3/W4 star forming region at 1420 and 408 MHz Stokes I (total power) and 1420 MHz Stokes Q and U (linear polarization) reveal an egg-shaped structure with morphological correlations between our data and the Hα data of Dennison, Topasna, & Simonetti. Polarized intensity images show depolarization extending from W4 up the walls of the superbubble, providing strong evidence that the radio continuum is generated by thermal emission coincident with the Hα emission regions. We conclude that the parts of the HII region hitherto known as W4 and the newly revealed thermal emission are all ionized by the open cluster OCl 352. Assuming a distance of 2.35 kpc, the ovoid structure is ~164 pc wide and extends ~246 pc above the mid-plane of the Galaxy. The shell's emission decreases in total-intensity and polarized intensity in various locations, appearing to have a break at its top and another on one side. Using a geometric analysis of the depolarization in the shell's walls, we estimate that a magnetic field line-of-sight component of 3 to 5 μG exists in the shell. We explore the connection between W4 and the Galactic halo, considering whether sufficient radiation can escape from the fragmenting superbubble to ionize the kpc-scale Hα loop discovered by Reynolds, Sterling & Haffner.

Radio Spectral Index Variations in HB 21

Leahy, Denis
2006, Astrophysical Journal, 647, 1125

New observations are presented of the supernova remnant HB 21 at 408 and 1420 MHz made with the Synthesis Telescope (ST) of the Dominion Radio Astrophysical Observatory. A new method of spectral index analysis is presented that allows for removal of point-source contamination at the spatial resolution of the higher spatial resolution map. This is a great improvement in point-source contamination removal (by a factor >~10), and allows more detailed spectral index variation studies than previously possible. Significant spatial variations in spectral index are found in HB 21. Different physical mechanisms for spectral index changes are considered, including a detailed consideration of ionization losses in the dense molecular gas interacting with HB 21. The lack of detailed spatial correlation of spectral flattening with positions of shocked clumps indicates that absorption by thermal plasma is the preferred mechanism.

The radio SNR G65.1+0.6 and its associated pulsar J1957+2831

Tian, W. & Leahy, D.
2006, Astronomy & Astrophysics,455, 1053

New images of the radio Supernova Remnant (SNR) G65.1+0.6 are presented, based on the 408 MHz and 1420 MHz continuum emission and the H I-line emission data of the Canadian Galactic Plane Survey. H I observations show structures associated with the SNR G65.1+0.6 in the radial velocity range of −20 to −26 km/s and suggest a distance of 9.2 kpc for the SNR. The estimated Sedov age for G65.1+0.6 is 4 to 14×104yr. The pulsar J1957+2831 is associated with the SNR G65.1+0.6 due to their consistent distances and ages. The EGRET source 3EG J1958+2909 and Gamma-ray source 2CG 065+00 are also near the eastern edge of the SNR but do not agree in position with the pulsar and are likely not associated with the SNR. The SNR's flux densities at 408 MHz (8.6 +/−0.8 Jy), 1420 MHz (4.9 +/− 0.5 Jy) and 2695 MHz (3.3 +/− 0.5 Jy) have been corrected for flux densities from compact sources within the SNR. The integrated flux density based spectral index (Sν∝ν−α) between 1420 MHz and 408 MHz is 0.45 +/− 0.11 and agrees with the T-T plot spectral index of 0.34 +/− 0.20. The nearby SNR DA495 has a T-T plot spectral index of 0.50 +/− 0.01.

A Catalogue of Galactic Supernova Remnants from the Canadian Galactic Plane Survey I. Flux Densities, Spectra, and Polarization Characteristics

Kothes, R., Fedotov, K., Foster, T., & Uyaniker, B.
2006, Astronomy & Astrophysics, 457, 1081.

We present a new catalogue containing all known Galactic supernova remnants observed in phase I and II of the Canadian Galactic Plane Survey (65 degrees < l < 175 degrees, −5 degrees < b < +7 degrees). Stokes I images at 1420 MHz of 34 SNRs and at 408 MHz of two SNRs (not observed at 1420 MHz) are displayed. From these 1' and 3'-resolution data (respectively) we determine accurate centre positions and angular dimensions. The flux densities at 1420 and 408 MHz are integrated and combined with values from the literature to define more accurate radio spectra. The CGPS polarization data at 1420 MHz were examined and significant linear polarization from 18 SNRs is found, with a modest signal from 6 others.

1051 Ergs Less: The Galactic H II Region OA 184

Foster, T., Kothes, R., Sun, X., Reich, W., & Han, J. L.
2006, Astronomy & Astrophysics, 454, 517-526.

The identification of the object OA 184 as a Galactic Supernova Remnant (SNR) is re-examined, and evidence to the contrary is presented. An integrated spectral index of α=−0.14 to −0.2 (S(Jy)∝να ) is determined from four radio frequencies, two from new 408 & 1420 MHz Canadian Galactic Plane Survey data, one from existing Effelsberg 2695 MHz data, and one from new Urumqi 5 GHz radio observations in Stokes I, Q, & U. Additionally, eight recombination lines (H103-110α) were observed for this study with the Green Bank Telescope. The line appears at vLSR=−25.8 km s−1 and at the intensity predicted for free-free thermal emission. Q & U maps show OA 184 as a depolarizing source, affecting a background filament of polarized non-thermal Galactic emission. An H II shell of thickness less than 0.2 degrees is indicated by this depolarization. CGPS 1-arcminute resolution H I line maps are presented, and the systemic velocity of the ISM immediately surrounding OA 184 is −26.8 km s−1. The distance of OA 184 is 2.2+/−0.4 kpc, obtained from a Galactic H I modelling procedure that determines the most probable velocity field along the line of sight. A simple model as a shell of ionized hydrogen 4 pc thick is considered, and from the observations the line of sight magnetic field in the ionized shell is found to be ⟨B||⟩≅−2.5μG (directed away from the Sun). We conclude that OA 184 is a Galactic H II region energized by the lone O7.5V star BD+41o 1144.

Evidence for Shocked Molecular Gas in the Galactic SNR CTB 109

Sasaki, M., Kothes, R., Plucinsky, P. P., Gaetz, T. J., & Brunt C. M.
2006, Astrophysical Journal Letters, 642, 149.

We report the detection of molecular clouds around the X-ray bright interior feature in the Galactic supernova remnant CTB109 (G109.1-1.0). This feature, called the Lobe, has been previously suggested to be the result of an interaction of the SNR shock wave with a molecular cloud complex. We present new high resolution CO data from the Five College Radio Observatory which show the interaction region with the cloud complex in greater detail. The CO data reveal three clouds around the Lobe in the velocity interval −57 < v < −52 km s−1. The velocity profiles of 12CO at various parts of the east cloud are well fit with a Gaussian; however, at the position where the CO cloud and the Lobe overlap, the velocity profile has an additional component towards higher negative velocities. The molecular hydrogen density in this part of the cloud is relatively high (NH∼ 4.5×1021 cm−2 ), obtained from Chandra data, is lower than outer parts of the cloud and in the north and south cloud. These results indicate that this cloud has been hit by the SNR blast wave on the western side, corroborating that the shock-cloud interaction caused the bright X-ray lobe.

Modelling the Neutral Hydrogen ISM: A Better Kinematic Distance Tool

Foster, T. & MacWilliams, J.
2006, Astrophysical Journal, 644, no. 1, 214, .

An advanced approach to the kinematic distance method is developed. The method is applicable to 2nd and 3rd quadrant Galactic objects with known velocities. It is based on fitting a model of the density and velocity features in an isothermal H I disk to observed H I data. The velocity field of the gas is modelled with a power-law for basic circular rotation, underlying non-circular motions from a two-armed density wave pattern. With a reasonable number of adjustable parameters and constraints the model reproduces observations towards many Galactic objects, and accurate distances are found from the modelled velocity field. High resolution H I spectral line data from the Canadian Galactic Plane Survey (CGPS, Taylor et al. 2003) are used to discriminate clouds from the intercloud medium (the ''stratum'') for which the model is intended. The ability of the model to reproduce these data is demonstrated in one (Tb(v)) and two (Tb(l,v)) dimensional fits. Distances to 22 H II regions and SNRs calculated by the fitted velocity field compare extremely well with other kinematically-independent distances.

Morphological Analysis of H I Features. II. Wavelet-Based Multifractal Formalism

Khalil, A., Joncas, G., Nekka, F., Kestener, P., & Arneodo, A.
2006, Astrophysical Journal Supplement Series, 165, 512.

For the first time in the study of the Interstellar Medium (ISM), the 2-D Wavelet Transform Modulus Maxima (WTMM) Method is used to obtain quantitative information on the anisotropic structure of Galactic atomic hydrogen (H I) from the Canadian Galactic Plane Survey (CGPS). Three mosaics were analyzed: the Perseus spiral arm, the Local spiral arm, and the ``Inter-arm'' region located between these two spiral arms. Evidence of a longitudinal anisotropic structure is detected on all physical length scales studied, from ~ 1 pc in the Local arm to ~ 65 pc in the Perseus arm. The anisotropic signature is stronger in the dense media (Local and Perseus arms) than in the diffuse media (Inter-arm). Also, the strength of the anisotropy decreases with the scale and size of the mosaics.

Radio Spectra of three Supernova Remnants

Tian, W., & Leahy, D. A.
2006, Chinese Journal of Astronomy & Astrophysics, 6, 543.

New images of the Supernova Remnants (SNRs) G114.3+0.3, G116.5+1.1 and G116.9+0.2 (CTB 1) are presented at 408 MHz from the Canadian Galactic Plane Survey (CGPS). We also use the 1420 MHz images from the CGPS to study their 408-1420 MHz spectral indices. The flux densities at 408 MHz and 1420 MHz, corrected for flux densities from compact sources within the SNRs, are 12 +/- 6 Jy and 9.8 +/- 0.8 Jy for G114.3+0.3, 15.0 +/- 1.5 Jy and 10.6 +/- 0.6 Jy for G116.5+1.1, 15.0 +/- 1.5 Jy and 8.1 +/- 0.4 Jy for G116.9+0.2.The integrated flux density-based spectral indices (Snu ~ nu-alpha) are alpha = 0.16 +/- 0.41, 0.28 +/- 0.10 and 0.49 +/- 0.09 for G114.3+0.3, G116.5+1.1 and G116.9+0.2, respectively. Their T-T plot-based spectral indexes are 0.68 +/- 0.48, 0.28 +/- 0.15, and 0.48 +/- 0.04, in agreement with the integrated flux density-based spectral indices. New flux densities are derived at 2695 MHz which are significantly larger than previous values. The new 408, 1420 and 2695 MHz flux densities and published values at other frequencies, where images are not available, are fitted after correcting for contributions from compact sources, to derive their multi-frequency spectral indices.

Radio Observations and Spectrum of the SNR G127.1+0.5 and its Central Source 0125+628

Leahy, D. A., & Tian, W.
2006, Astronomy & Astrophysics, 451, 251.

We present new images of the Supernova Remnant (SNR) G127.1+0.5 (R5), based on the 408 MHz and 1420 MHz continuum emission and the HI-line emission data of the Canadian Galactic Plane Survey (CGPS). The radio spectrum of the central compact source (0125+628) is analyzed in the range 178 MHz - 8.7 GHz, indicating a flat spectrum with synchrotron self-absorption below 800 MHz. The SNR's flux density at 408 MHz is 17.1 +/- 1.7 Jy and at 1420 MHz is 10.0 +/- 0.8 Jy, corrected for flux densities from compact sources within the SNR. The SNR's integrated flux density based spectral index (Snu ~ nu-alpha) is 0.43 +/- 0.10. The respective T-T plot spectral index (derived from the relative size of brightness temperature variations between two frequencies, see text for details) is 0.46 +/- 0.01. There is no evidence at 1-sigma for spatial variations in spectral index within G127.1+0.5. In particular, we compared the northern shell, southern shell and central diffuse region. HI observations show structures associated with the SNR in the radial velocity range of -12 to -16 km/s, suggesting G127.1+0.5's distance is 1.15 kpc. The estimated Sedov age is 2 - 3 x 10^4 yr.

An absolutely calibrated survey of polarized emission from the northern sky at 1.4 GHz

Wolleben, M., Landecker, T. L., Reich, W., & Wielebinski, R.
2006, Astronomy & Astrophysics, 448, 411.

A new polarization survey of the northern sky at 1.41 GHz is presented. The observations were carried out using the 25.6 m telescope at the Dominion Radio Astrophysical Observatory in Canada, with an angular resolution of 36 arcmin. The data are corrected for ground radiation to obtain absolutely calibrated Stokes U and Q maps, containing emission structures of large angular extent, with an rms noise of 12 mK. Survey observations were carried out by drift scanning the sky between -29 and +90 degrees declination. The fully sampled drift scans, observed in steps of 0.25 to 2.5 degrees in declination, result in a northern sky coverage of 41.7% of full Nyquist sampling. The survey surpasses by a factor of 200 the coverage, and by a factor of 5 the sensitivity, of the Leiden/Dwingeloo polarization survey (1972A&AS....5..205S) that was until now the most complete large-scale survey. The temperature scale is tied to the Effelsberg scale. Absolute zero-temperature levels are taken from the Leiden/Dwingeloo survey after rescaling those data by the factor of 0.94. The paper describes the observations, data processing, and calibration steps. The data are publicly available at the links below.

Intermediate Velocity Gas in the Canadian Galactic Plane Survey

Kerton, C. R., Knee, L. B. G., & Schaeffer, A. J.
2006, Astronomical Journal, 131, 1501.

We present observations of intermediate velocity clouds (IVCs) found at positive velocities in the Canadian Galactic Plane Survey Phase I region (74 deg < l < 148 deg, -3.5 deg < b < +5.6 deg). A catalog of 138 distinct IVCs and 13 IVC complexes has been compiled. From the global characteristics of our sample we demonstrate that the majority of these IVCs are directly associated with energetic phenomena, such as supernova explosions and massive star-forming regions, within the disk of the Galaxy and thus form a different population than the high velocity clouds and high-latitude IVCs. We report what we believe to be the first detection of H I 21~cm absorption of a background source by an IVC and attempt to determine its physical properties and distance. We discuss in some detail a few of the IVCs which illustrate the connection between the clouds and energetic phenomena in the Galaxy. We suggest that the velocity shift of these IVCs away from the bulk of the Galactic H I emission provides an opportunity to study the structure of the low latitude H I emission in a relatively confusion-free environment and present two examples of large (degree-scale) complexes of H I emission particularly suitable for such studies.

Radio observations and spectral index study of SNR G126.2+1.6

Tian, W., & Leahy, D. A.
2006, Astronomy & Astrophysics, 447, 205 .

We present new images of the low radio surface brightness Supernova Remnant (SNR) G126.2+1.6, based on the 408 MHz and 1420 MHz continuum emission and the H I line emission data of the Canadian Galactic Plane Survey (CGPS). The SNR's flux densities at 408 MHz and 1420 MHz are found and corrected for flux densities from compact sources within the SNR. The integrated flux density based spectral index (Snu ~ nu-alpha) is 0.30 +/- 0.22. This agrees with the respective T-T plot spectral index of 0.30 +/- 0.08. We also find spatial variations of spectral index within the SNR. The east-to-south arc emission regions have a flatter spectrum of 0.2 - 0.3, the west-to-north and center emission regions have a steeper spectrum of 0.4 - 0.6. H I observations show structures associated with the SNR, i.e, features associated with the SNR's southeastern filaments in the radial velocity range of -33 to -42 km/s, and with its northwestern filaments in -47 to -52 km/s. This association suggests a distance of 5.6 kpc for SNR G126.2+1.6. The estimated Sedov age for G126.2+1.6 is 2 x 10^5 years.

The Boomerang and the Magnetic Field Structure in Pulsar Wind Nebulae

Kothes, R., Reich, W., & Uyaniker, B.
2006, Astrophysical Journal, 638, 225.

We present new observations at high radio frequencies made with the Effelsberg 100 m radiotelescope of the Boomerang pulsar wind nebula G106.65+2.96, which is part of the supernova remnant G106.3+2.7. Including low frequency data from the Canadian Galactic Plane Survey allows the development of a complete picture of the PWN and its evolution in a very complex environment. The radio continuum spectrum reveals a break between 4 GHz and 5 GHz which is the result of synchrotron cooling. We find evidence that the reverse shock of the initial supernova shock wave has driven away the original PWN, which explains why the current nebula around the pulsar has such a low radio luminosity. An analysis of the energy budget of the pulsar, the magnetic field in the nebula, and the lifetime of relativistic electrons gives a magnetic field of 2.6 mG in the PWN with an age of 3900 yr. This is not the age of the SNR but the time that has passed since the reverse shock crushed the original PWN. A comparison of our polarization measurements with those of other pulsar wind nebulae leads to a simple model of the magnetic field structure in such an object. With this model we derive an angle of about 60 degrees between the Galactic plane and the spin axis of the pulsar projected onto the plane of the sky. The magnetic field axis is pointing away from us towards the north west. The nebula contains a toroidal magnetic field structure, yet the rotation measure distribution indicates a radial magnetic field as its cause. Apparently the observed radio emission originates from the far side of the object and passes through the inner region which has a radial magnetic field.

The ionizing star of the North American and Pelican Nebulae

Comeron, F., & Pasquali, A.
2005, Astronomy & Astrophysics, 430, 541.

We present the results of a search for the ionizing star of the North America (NGC 7000) and the Pelican (IC 5070) nebulae complex. The application of adequate selection criteria to the 2MASS JH KS broad-band photometry allows us to narrow the search down to 19 preliminary candidates in a circle of 0o 5 radius containing most of the L935 dark cloud that separates both nebulae. Follow-up near-infrared spectroscopy shows that most of these objects are carbon stars and mid-to-late-type giants, including some AGB stars. Two of the three remaining objects turn out to be later than spectral type B and thus cannot account for the ionization of the nebula, but a third object, 2MASS J205551.25+435224.6, has infrared properties consistent with it being a mid O-type star at the distance of the nebulae complex and reddened by AV . 9.6. We confirm its O5V spectral type by means of visible spectroscopy in the blue. This star has the spectral type required by the ionization conditions of the nebulae and photometric properties consistent with the most recent estimates of their distance. Moreover, it lies close to the geometric center of the complex that other studies have proposed as the most likely location for the ionizing star, and is also very close to the position inferred from the morphology of cloud rims detected in radio continuum. Given the fulfillment of all the conditions and the existence of only one star in the whole search area that satisfies them, we thus propose 2MASS J205551.25+435224.6 as the ionizing star of the North America/Pelican complex.

Two New Perseus Arm Supernova Remnants Discovered in the Canadian Galactic Plane Survey

Kothes, R., Uyaniker, B., & Reid, R. I.
2005, Astronomy & Astrophysics, 444, 871.

We report the discovery of two new second-quadrant supernova remnants, G96.0+2.0 and G113.0+0.2, in the data of the Canadian Galactic Plane Survey. The two SNRs are residents of the Perseus spiral arm at distances of 4.0 kpc and 3.1 kpc, respectively. The distances were determined kinematically by associating the objects with neutral hydrogen and molecular material. G96.0+2.0 is located at the edge of a large stellar wind bubble with a systemic velocity of about -44 km/s. It has a diameter of about 30 pc and a radio spectral index of alpha ~ -0.66 (S ~ nualpha), indicating that it is a shell-type remnant in an early stage of development. It consists of a relatively bright shell where the shock is encountering the wall of H I and slowly fades away towards the interior of the stellar wind bubble. The SNR is most likely the remnant of a type Ib/c supernova explosion. G113.0+0.2 is located in an area of confusing thermal emission not far from the radio-bright supernova remnant Cassiopeia A. It consists of a long polarized filament and a more complex head structure that is interacting with a small molecular cloud; it resides in a butterfly-shaped H I cavity, probably a stellar wind bubble. It is about 36 pc long and 15 pc wide at a position angle of 70 degrees with the Galactic Plane. A pulsar with a relatively low period derivative, giving it a characteristic age of 10 million years, is located close to the centre of the radio continuum emission at a Perseus arm distance. Whether the pulsar is the result of the same supernova explosion that created G113.0+0.2 or if it was left behind by an earlier supernova that also shaped the stellar wind bubble remains uncertain.

Polarization properties of reflector antennas used as radio telescopes

Ng, T., Landecker, T. L., Cazzolato, F., Routledge, D., Gray, A. D., Reid, R. I., & Veidt, B. G.
2005, Radio Science, 40(#5), paper RS5014.

The distribution of cross-polarization across the main beam and near sidelobes of a reflector antenna is calculated. Results are expressed in terms relevant to imaging in radio astronomy, using Stokes Parameters, as plots of instrumental polarization Q/I, U/I, and V/I, showing conversion of total intensity of a signal which is unpolarized into apparent linear and circular polarization. The calculations use GRASP8, software based on Physical Optics and the Physical Theory of Diffraction. For purposes of calculation, the symmetrical paraboloidal reflector (diameter ~ 40 wavelengths) is fed at the prime focus with a linearly polarized signal. Computed radiation patterns at a number of feed orientations are averaged to establish the antenna response to an unpolarized radio astronomy signal. The results of the computations are consistent with measurements of instrumental polarization of the DRAO Synthesis Telescope at 1420 MHz made using unpolarized radio sources. For this telescope the dominant source of instrumental polarization across the field is the cross-polarization of the feed. The next most significant effect is scattering by the feed struts; both three-strut and four-strut configurations are examined. Struts affect performance in linear polarization, but also introduce some instrumental circular polarization. The contribution to instrumental polarization from the reflector itself is comparatively small. Roughness of the reflector surface has relatively little effect in the main beam in Q and U, but introduces V, and also randomizes the polarization of the sidelobes. In all cases considered, the computations show that the first and subsequent sidelobes are highly polarized, with levels of instrumental polarization up to 50%.

The Galactic Plane Region near l=93o. III. Multi-Wavelength Emission from SNR 3C434.1

Foster, T.
2005, Astronomy & Astrophysics, 441, 1043.

New Canadian Galactic Plane Survey radio continuum, ROSAT X-ray, and optical line observations of supernova remnant (SNR) 3C434.1 (G94.0+1.0) are presented. A radio spectrum of index alpha = 0.4 (where S ~ nu-alpha ) confirms this SNR's emission signature as predominantly synchrotron, and suggests the SNR is in the Sedov expansion phase. The morphology of the remnant is compared in X-ray, optical, and radio continuum, and the brightest emission in all three wavelength regimes is from the eastern hemisphere of 3C434.1, which marks where the SNR shock is interacting with the inside wall of its stellar wind bubble (SWB) home. The system is determined to be 4.5 kpc distant, residing in the Perseus Arm Spiral shock. From a deep H-alpha mosaic of the region, 656 nm H-alpha line emission is observed that correlates well with radio synchrotron emission and anticorrelates with X-ray emission from the SNR. The origin of this optical emission is likely dense (ne = 40 cm-3) cooling H II from the wall of the SWB, where the SNR shock has penetrated and become radiative (vs ~ 100 km/s). The X-ray spectrum of this SNR between 0.5 and 2.4 keV is well modelled by a single-temperature thermal plasma (Te = 4.5 x 106 K, ne = 0.2 cm-3). The magnetic field of the bright radio synchrotron emission region is found (under the assumption of near equipartition) to be B ~ 16 uG, a factor of 8 compression of the ambient ISM field (2 uG). The westward extension of 3C434.1 is the result of ongoing free expansion of the shock into the lower density interior of the SWB. I use multiwavelength observations to arrive at a unique solution for an interaction model of 3C434.1 with the SWB, from which the age (t = 25000 yr) and mass ejected in the explosion (Mej = 15.5 Msun) are determined. I also find an initial blast-wave velocity of 1350 km/s, typical of type Ib SNe.

CO in H I Self-Absorbed Clouds in Perseus

Klaassen, P. D., Plume, R., Gibson, S. J., Taylor, A. R., & Brunt, C. M.
2005, Astrophysical Journal, 631, 1001.

We have observed 12CO J = 2 - 1 and J = 1 - 0, and 13CO J = 1 - 0 emission in two regions of H I Self-Absorption (HISA) in Perseus: a small, isolated HISA feature called the globule and a more extended HISA cloud called the complex. Using both Large Velocity Gradient and Monte Carlo radiative transfer codes we found that, in the globule, N(12CO) < 6.0 x 1015 cm-2 which, using PDR models, implies that N(H2) < 9.9 x 1020 cm-2. In the complex we found that the H2 column densities ranged from 1.2 - 2.2 x 1021 cm-2. By comparing the HISA and CO observations we are able to constrain the physical conditions and atomic gas fraction (f). In the globule, 8 K < Tspin < 22 K and 0.02 < f < 0.2 depending on whether the (unknown) gas density is 102, 103, or 104 cm-3. In the complex, 12 K < Tspin < 24 K, 0.02 < f < 0.05, and we were also able to constrain the gas density (100 < n < 1200 cm-3). These results imply that the gas in the HISA clouds is colder and denser than that usually associated with the atomic ISM and, indeed, is similar to that seen in molecular clouds. The small atomic gas fractions also imply that there is a significant molecular component in these HISA clouds, even when little or no 12CO is detected. The level of 12CO detected and the visual extinction due to dust is consistent with the idea that these HISA clouds are undergoing a transition from the atomic to molecular phase.

Radio Spectral Index Study of the SNRs OA184 and VRO42.05.01

Leahy, D., & Tian, W.
2005, Astronomy & Astrophysics, 440, 929.

New images of the Supernova Remnants (SNRs) OA184 and VRO42.05.01 are presented at 408 MHz and 1420 MHz, from the Canadian Galactic Plane Survey (CGPS) data. The SNRs' flux densities at both 408 MHz and 1420 MHz are found and corrected for flux from compact sources within the SNRs. The integrated flux-density based spectral indices (Snu ~ nu-alpha) are 0.25 +/- 0.03 for OA184 and 0.36 +/- 0.06 for VRO42.05.01. These agree with the respective T-T plot spectral indexes of 0.23 +/- 0.06 and 0.36 +/- 0.03. OA184's spectral index is smaller than previously published values. The older flux density values of OA184 from lower resolution data include a contribution from a non-SNR ridge emission region and from compact sources within OA184. Subtracting these contributions results in a spectral index of 0.32 +/- 0.06 (38 MHz to 2695 MHz) or 0.28 +/- 0.06 (408 MHz to 2695 MHz). Correction of published fluxes for point sources for VRO.42.05.01 results in a spectral index of 0.32 +/- 0.05 for 38 MHz to 2695 MHz. We also find spatial variations of spectral index in OA184: ~0.1 - ~0.3. For VRO42.05.01, the shell region has alpha = 0.3 and the wing region has alpha = 0.47.

An Unusual System of H I Filaments near WR5 and HD 17603

Knee, L. B. G., Wallace, B. J., & Normandeau, M.
2005, Astrophysical Journal, 628, 758.

We report the discovery of a system of unusual H I filaments that appear to be associated with molecular clouds in the Perseus spiral arm of our Galaxy. We investigate the hypothesis that this system is the result of a directed flow of dissociated gas from clouds trapped within an extended wind flow from massive stars. The Wolf-Rayet star WR 5 and the O Ib(f) star HD 17603 are identified as candidate driving sources. However, an examination of this hypothesis within the context of the theory of mass-loaded winds shows that these two stars alone cannot account for the energetics and kinematics of the required spherically symmetric wind flow. Unless the apparent association between H I, molecular gas, and stars is an accidental one, we suggest that other as yet unidentified stars must have contributed to driving the filaments.

A Search for Narrow Vertical Structures in the Canadian Galactic Plane Survey

Asgekar A., English J., Safi-Harb S., Kothes R.
2005, Astronomical Journal, 130, 674.

Worms are defined to be dusty, atomic hydrogen (H I) structures which are observed in low resolution data to rise perpendicular to the Galactic plane. Data from the 1' resolution Canadian Galactic Plane Survey (CGPS) were systematically searched for narrow vertical H I structures which could be resolved worms. Another motivation for the search was to explore the scenario that mushroom-shaped worms like GW123.4-1.5, studied by English and collaborators, could be generated by a single supernova. However no other vertical structures of mushroom-shape morphology were found. We also examined objects previously classified as worm candidates by Koo and collaborators; only 7 have a significant portion of their structure falling in the CGPS range of l = 74 deg to 147 deg, -3.5 deg < b < +5.5 deg. Apart from GW123.4-1.5 we could not confirm that any of these are coherent structures that extend towards the Milky Way's halo. However a list of 10 narrow, vertical structures found in our search is furnished; one structure is >= 500 pc tall, thus extending from the Galactic plane into the halo. We provide details about these narrow vertical structures, including comparisons between H I, radio continuum, IR, and CO observations. Our search was conducted by visual inspection and we describe the limitations of this approach since it indicates that only 6 disk-halo features may exist throughout the Milky Way. We also discuss possible origins of structures at high latitudes and the relationship between mushroom-shaped clouds and old supernova remnants.

Radio Spectral Index Variations of the SNR HB3

Tian, W.-W., & Leahy, D.
2005, Astronomy & Astrophysics, 436, 187.

We present images of the SNR HB3 at both 408 MHz and 1420 MHz from the Canadian Galactic Plane Survey mainly based on data from the Synthesis Telescope of the Dominion Radio Astrophysical Observatory. We reproduce the 100m-Effelsberg HB3 image at 2695 MHz at large scale, and find that there exists a background emission gradient across the HB3 area. Based on our analysis of background emission and the boundary between W3 and HB3, we give HB3's flux density as 68.6 +/- 11.5 Jy at 408 MHz and 44.8 +/- 12.0 Jy at 1420 MHz, after subtracting flux from compact sources within HB3. The integrated flux-density-based spectral index between 408 MHz and 1420 MHz is 0.34 +/- 0.15. The averaged T-T plot spectral index using all subareas is 0.36. Our measurement values are less than a previously published value of 0.6. The 408-1420 MHz spectral index varies spatially in HB3 in the range 0.1 to 0.7. We investigate the data used by previous authors, and consider more data at 232 MHz, 3650 MHz and 3900 MHz which are not included in previous calculations. There is evidence for two spectral indices for HB3 in the radio band, i.e., 0.63 (38 - 610 MHz) and 0.32 (408 - 3900 MHz). This is consistent with the spatial variations: the low frequency data mainly reflects the steeper indices and the high frequency data mainly reflects the flatter indices.

An Automated Method for the Detection and Extraction of H I Self-Absorption in High-Resolution 21cm Line Surveys

Gibson, S. J., Taylor, A. R., Higgs, L. A., Brunt, C. M., & Dewdney, P. E.
2005, Astrophysical Journal, 626, 214.

We describe algorithms that detect 21cm line H I self-absorption (HISA) in large data sets and extract it for analysis. Our search method identifies HISA as spatially and spectrally confined dark H I features that appear as negative residuals after removing larger-scale emission components with a modified CLEAN algorithm. Adjacent HISA volume-pixels (voxels) are grouped into features in (l,b,v) space, and the H I brightness of voxels outside the 3-D feature boundaries is smoothly interpolated to estimate the absorption amplitude and the unabsorbed H I emission brightness. The reliability and completeness of our HISA detection scheme have been tested extensively with model data. We detect most features over a wide range of sizes, linewidths, amplitudes, and background levels, with poor detection only where the absorption brightness temperature amplitude is weak, the absorption scale approaches that of the correlated noise, or the background level is too faint for HISA to be distinguished reliably from emission gaps. False detection rates are very low in all parts of the parameter space except at sizes and amplitudes approaching those of noise fluctuations. Absorption measurement biases introduced by the method are generally small and appear to arise from cases of incomplete HISA detection. This paper is the third in a series examining HISA at high angular resolution. A companion paper (Paper II) uses our HISA search and extraction method to investigate the cold atomic gas distribution in the Canadian Galactic Plane Survey.

A Self-Absorption Census of Cold H I Clouds in the Canadian Galactic Plane Survey

Gibson, S. J., Taylor, A. R., Higgs, L. A., Brunt, C. M., & Dewdney, P. E.
2005, Astrophysical Journal, 626, 195.

We present a 21cm line H I self-absorption (HISA) survey of cold atomic gas within Galactic longitudes l = 75 degrees to 146 degrees and latitudes b = -3 degrees to +5 degrees. We identify HISA as spatially and spectrally confined dark H I features and extract it from the surrounding H I emission in the arcminute-resolution Canadian Galactic Plane Survey (CGPS). We compile a catalog of the most significant features in our survey and compare our detections against those in the literature. Within the parameters of our search, we find nearly all previously detected features and identify many new ones. The CGPS shows HISA in much greater detail than any prior survey and allows both new and previously-discovered features to be placed into the larger context of Galactic structure. In space and radial velocity, faint HISA is detected virtually everywhere that the H I emission background is sufficiently bright. This ambient HISA population may arise from small turbulent fluctuations of temperature and velocity in the neutral interstellar medium. By contrast, stronger HISA is organized into discrete complexes, many of which follow a longitude-velocity distribution that suggests they have been made visible by the velocity reversal of the Perseus arm's spiral density wave. The cold H I revealed in this way may have recently passed through the spiral shock and be on its way to forming molecules and, eventually, new stars. This paper is the second in a series examining HISA at high angular resolution. A companion paper (Paper III) describes our HISA search and extraction algorithms in detail.

The Low-Resolution DRAO Survey of H I Emission from the Galactic Plane: Part II

Higgs, L. A., Landecker, T. L., Asgekar, A., Davison, O. S., Rothwell, T. A., & Yar-Uyaniker, A.
2005, Astronomical Journal, 129, 2750.

In 2000, a survey of H I line emission in the Galactic plane between Galactic longitudes l = 72.2 deg and 149.0 deg, covering latitudes from b = -5.4 deg to +7.2 deg, made using the 26-m Telescope at DRAO was published (Paper I), as part of the Canadian Galactic Plane Survey project. The survey with the 26-m Telescope has now been extended to cover longitudes from l = 50.0 deg to 195.0 deg, between the same latitude limits. In addition, a region at higher Galactic latitudes (b = +7.4 deg to +24.0 deg) has been surveyed between longitudes l = 96.4 deg and 120.2 deg. As in the initial survey, spectra were observed at 0.2 deg intervals of longitude and latitude (0.33 half-power beamwidths). (In an appendix, it is demonstrated that, when single-antenna data and interferometer data are to be combined, the single-antenna data must be sampled at an interval no greater than 0.5 times the half-power beamwidth of the single antenna. Coarser sampling will result in the introduction of spurious structure into the resulting images). The spectral resolution is 1.32 km/s and the LSR velocity range is -260 to +161 km/s. The spectra have been corrected for stray radiation, and have an estimated Tb scaling accuracy of 1%, and an estimated RMS noise in one channel (0.8245 km/s) of 0.25 K. Regions of suspected strong H I self-absorption have been identified using a statistic characterizing the ``complexity'' of the observed spectra. Comparison with spectra from the Leiden/Dwingeloo Survey supports the conclusion from Paper I that a small percentage (~ 1% -- 2%) of the spectra in the latter survey suffer from calibration errors exceeding 10%. A table of positions of the more discordant spectra is given.

Theoretical and Observational Aspects of Expanding H I Shells

Cazzolato F., & Pineault S.
2005, Astronomical Journal, 129, 2731.

We have modeled H I shells expanding into a homogeneous medium in order to explain some of their observational peculiarities. Such peculiarities include difficult-to-observe caps, the presence of stationary rings, expansion velocity determination problems, inaccurate mass measurements, and a systematic discrepancy between H I missing masses and shell masses. Velocity dispersion within the shell, in the form of either thermal or turbulent motions, has been found to be the likely major cause for the absence of observable caps and the presence of stationary rings, hence explaining the apparent lack of ring transition. We discuss different methods generally used to calculate H I shell masses and show that, if one does not take into account the varying shape of the H I background local to the shell, shell masses are likely to be underestimated by a significant factor whose value depends on the relative shell thickness and the ratio of the dispersion to the expansion velocity.

The Physical Properties of a Cold H I Feature Observed Transitioning from Absorption to Emission

Kerton, C. R.
2005, Astrophysical Journal, 623, 235.

Large scale surveys of neutral hydrogen (H I) 21-cm line emission from the Galactic plane at one arcminute resolution have revealed an extensive population of H I self-absorption features or H ISA, i.e., presumably cold H I seen in absorption against warmer background H I. These observations provide previously unknown information regarding the morphology and distribution of cold H I gas in our Galaxy, but determining the physical properties of such features primarily depends upon some sort of radiative transfer modeling that only loosely constrains various model parameters. We present here observations and analysis of one H ISA feature that is seen to move from absorption into emission and show how observations made at the transition point allow the physical properties of the H ISA feature to be tightly constrained. The derived physical properties are consistent with a diffuse, cold H I cloud with minimal molecular gas content. Identification of objects like this is currently uncommon in the interferometer-based Galactic plane surveys which, necessarily, have focused on lower Galactic latitudes. As high-resolution Galactic surveys are extended to higher latitudes we expect that many additional H I features will be seen transitioning from absorption to emission and thus be amenable to the analysis techniques presented here.

A Multiwavelength Study of IC 63 and IC 59

Karr, J. L., Noriega-Crespo, A., & Martin, P. G.
2005, Astronomical Journal, 129, 954.

IC 63 and IC 59 are two nearby arc-shaped nebulae with relatively simple geometries and minimal obscuring material. The two regions, in spite of a similar projected distance from their ionizing star, have very different observational properties, both in continuum emission and in the presence and strength of line emission from molecular species. This paper conducts a multiwavelength study of the two regions using archived data from a variety of sources, including the Canadian Galactic Plane Survey and the Infrared Space Observatory. The multiwavelength morphology and structure of the two nebulae are studied in detail, particularly the ionization fronts in IC 63. The possibility of triggered star formation in IC 63 is investigated and determined to be spurious. H2 and polycyclic aromatic hydrocarbon emission is detected in both IC 63 and IC 59, confirming the presence of molecular hydrogen in IC 59. The averaged line ratios are similar in the two regions, but variations are seen within each region. Temperatures and densities were calculated from the S(3) and S(5) pure rotational lines of molecular hydrogen. We derived a temperature of 630 K in IC 63, comparable to previous results, and a column density of 5.81017 cm-2, somewhat lower than previous values. New results for IC 59 show values of 590 K and 3.41017 cm-2, slightly cooler and with lower column density than IC 63. The contrast in appearance between IC 63 and IC 59 is consistent with a difference in actual (rather than projected) distances and a small variation in temperature and column density.

Numerical simulations of expanding supershells in dwarf irregular galaxies II. Formation of giant H I rings

Vorobyov, E. I., & Basu, Shantanu
2005, Astronomy & Astrophysics, 431, 451.

We perform numerical hydrodynamic modeling of various physical processes that can form an H I ring as is observed in Holmberg I (Ho I). Three energetic mechanisms are considered: multiple supernova explosions (SNe), a hypernova explosion associated with a gamma ray burst (GRB), and the vertical impact of a high velocity cloud (HVC). The total released energy has an upper limit of ~ 1054 ergs. We find that multiple SNe are in general more effective in producing shells that break out of the disk than a hypernova explosion of the same total energy. As a consequence, multiple SNe form rings with a high ring-to-center contrast K <~ 100 in the H I column density, whereas single hypernova explosions form rings with K <~ 10. Only multiple SNe can reproduce both the size (diameter ~ 1.7 kpc) and the ring-to-center contrast (K ~ 15-20) of the H I ring in Ho I. High velocity clouds create H I rings that are much smaller in size (<~ 0.8 kpc) and contrast (K <~ 4.5) than seen in Ho I. We construct model position-velocity (pV) diagrams and find that they can be used to distinguish among different H I ring formation mechanisms. The observed pV-diagrams of Ho I (Ott et al. 2001) are best reproduced by multiple SNe. We conclude that the giant H I ring in Ho I is most probably formed by multiple SNe. We also find that the appearance of the SNe-driven shell in the integrated H I image depends on the inclination angle of the galaxy. In nearly face-on galaxies, the integrated H I image shows a ring of roughly constant H I column density surrounding a deep central depression, whereas in considerably inclined galaxies (i > 45 degrees) the H I image is characterized by two kidney-shaped density enhancements and a mild central depression.

A Global Model of MHD Wave Support of a Stratified Molecular Cloud

Kudoh, T. & Basu, S.
2004, Astrophysics & Space Science, 292, 161.

We perform numerical simulations of nonlinear MHD waves in a gravitationally stratified molecular cloud that is bounded by a hot and tenuous external medium, within a 1.5-dimensional approximation. Under the influence of a driving source of Alfvénic disturbances, the cloud is lifted up by the pressure of MHD waves and reaches a steady state characterized by oscillations about a new time-averaged equilibrium state. The nonlinear effect results in the generation of longitudinal motions and many shock waves. Models of an ensemble of clouds show that, for various strengths of the input energy, the velocity dispersion in the cloud sigma ~ Z^0.5, where Z is a characteristic size of the cloud. Furthermore, sigma is always comparable to the mean Alfvén velocity of the cloud, consistent with observational results.

Distance of Three Supernova Remnants from H I Line Observations in a Complex Region: G114.3+0.3, G116.5+1.1 and CTB 1 (G116.9+0.2)

Yar-Uyaniker, A., Uyaniker, B., & Kothes, R.
2004, Astrophysical Journal, 616, 247.

We present new radio continuum and H I images toward the supernova remnants (SNRs) G114.3+0.3, G116.5+1.1, and G116.9+0.2 (CTB 1) taken from the Canadian Galactic Plane Survey (CGPS). We discuss the dynamics of their H I environment and a possible relationship of these SNRs with each other. We discovered patches of H I emission surrounding G114.3+0.3 indicating a location in the Local arm at a distance of about 700 pc, in contrast to previous publications, which proposed a Perseus arm location. The other two SNRs have radial velocities of -17 km/s (G116.5+1.1) and -27 km/s (CTB 1) according to related H I. However, the structure of the H I and its dynamics in velocity space suggest a possible relation between them, placing both remnants at a distance of about 1.6 kpc. CTB 1 appears to be embedded in an H I feature that is moving as a whole toward us with a velocity of about 10 km s-1. Furthermore, the off-centered location of CTB 1 in a large H I bubble indicates that the so-called breakout region of the remnant is in fact due to its expansion toward the low-density interior of this bubble. We believe that the progenitor star of CTB 1 was an early B- or O-type star shaping its environment with a strong stellar wind, in which case it exploded in a Ib or Ic event.

The Universality of Turbulence in Galactic Molecular Clouds

M. H. Heyer & C. M. Brunt
2004, Astrophysical Journal Letters, 615, L45.

The universality of interstellar turbulence is examined from observed structure functions of 27 giant molecular clouds and Monte Carlo modeling. We show that the structure functions, dv=v0 l^gamma, derived from wide field imaging of CO J=1-0 emission from individual clouds are described by a narrow range in the scaling exponent, gamma, and the scaling coefficient, v0. The similarity of turbulent structure functions emphasizes the universality of turbulence in the molecular interstellar medium and accounts for the cloud-to-cloud size-line width relationship initially identified by Larson (1981). The degree of turbulence universality is quantified by Monte Carlo simulations that reproduce the mean squared velocity residuals of the observed cloud-to-cloud relationship. Upper limits to the variation of the scaling amplitudes and exponents for molecular clouds are ~10-20%. The measured invariance of turbulence for molecular clouds with vastly different sizes, environments, and star formation activity suggests a common formation mechanism such as converging turbulent flows within the diffuse ISM and a limited contribution of energy from sources within the cloud with respect to large scale driving mechanisms.

Sharpless 170 and the surrounding interstellar medium

Roger, R. S., McCutcheon, W. H., Dewdney, P. E., & Purton, C. R.
2004, Astronomy & Astrophysics, 425, 553.

Sharpless 170 is a diffuse H II region ionized by a single main sequence O-star located near the periphery of a small dense molecular cloud at a distance of ~ 2 kpc. We describe wide-field observations of the region in the radio continuum, in H I and CO-lines, and in the far-infrared which delineate the major ionized, atomic, molecular and dust components of the gas affected by the exciting star. From the thermal continuum emission we estimate the mass of ionized gas at ~ 350 Msun within a radius of ~ 7 pc. The H I (21 cm) and far-infrared observations show an extended low-density atomic component, of ~ 1000 Msun, within an irregular boundary surrounding the ionized gas of mean radius ~ 10 pc. Mean densities in the H I and H II are similar, in the range 9-16 nucleons cm-3. CO emission shows a molecular cloud of ~ 1150 Msun within an area 6 pc x 4 pc with densities ~ 2000 nucleons cm-3. A compact infrared component coincides with the cloud. The exciting star is located on the near side of the cloud, just inside the southern periphery.

Sh 170 is an example of a young blister H II region with the ionized gas, seen in H-alpha emission, streaming outward in the manner of a ``champagne flow''. Although the observed velocities of the H I are close to the mean velocity of the CO cloud, the morphology of the associated atomic hydrogen closely resembles that seen in the surrounds of other young H II regions which show clear evidence of expansion of their H I. We propose that much of the H I is part of a diffuse dissociation zone beyond the ionization front, in directions from the star within a wide annulus, approximately transverse to the line-of-sight, which constitutes the boundary between the dense, photon-bounded region on the far side of the star and the density-bounded ionized flow region on the near side. In this view, much of the associated atomic gas, like the ionized gas, has been eroded from the molecular cloud in a small fraction (<= 10%) of the star's main sequence lifetime.

A mushroom-shaped structure from the impact of a cloud with the Galactic disk

Kudoh T., & Basu S.
2004, Astronomy & Astrophysics, 432, 183.

We propose that the mushroom-shaped structure of the Galactic worm GW 123.4-1.5 is created by a cloud collision with the Galactic gas disk. A hydrodynamic simulation shows that a mushroom-shaped structure is created after the cloud crosses the Galactic midplane. The lifetime of the mushroom-shaped structure is of order the dynamical time scale of the disk, ~ 10^7 years. We find that the velocities across the cap of the mushroom-shaped structure in the simulation are consistent with the observed values. The simulation also predicts a structure on the opposite side of the Galactic plane which is created by the Kelvin-Helmholtz instability after the cloud passes through the disk.

A Central X-Ray Source in the Nonthermal Radio Nebula DA 495

Arzoumanian Z., Safi-Harb S., Landecker T. L., & Kothes R.
2004, Astrophysical Journal Letters, 610, L101.

We report the detection, in archival ROSAT and ASCA observations, of X-ray emission from the direction of DA 495 (G65.7+1.2), a likely supernova remnant of uncertain classification but with similarities to the Crab Nebula. An unusual feature of the radio nebula is its annular morphology, with a flux minimum at the geometrical center. In the soft X-ray band, the ROSAT data reveal a compact source near the edge of the central radio ``hole'' the hard X-ray morphology, at the limit of ASCA's angular resolution, is suggestive of extended emission coincident with the ROSAT source. The X-ray flux is roughly constant with time, and its spectrum is well described by a power law with photon index ~1.7. Taken together, this evidence suggests identification of the X-ray source with a magnetospherically active neutron star and its associated wind nebula. Timing analysis of the ASCA data yields only a weak upper bound on pulsations with periods >~30 ms. These results reveal for the first time the high-energy engine that powers the DA 495 radio nebula and strengthen its classification as a plerionic supernova remnant, one that may represent the poorly explored late evolutionary stages of Crab-like nebulae.

I Absorption of Polarized Emission: A New Technique for Determining Kinematic Distances to Supernova Remnants

Kothes, R., Landecker, T. L., & Wolleben, M.
2004, Astrophysical Journal, 607, 855.

We present a new method of determining the systemic velocity of Galactic supernova remnants based on H I absorption of the ir linearly polarized radio continuum emission. Conventional H I observations of total power emission are limited by H I emission and self-absorption along the line of sight, but, since H I emission is unpolarized, the only limits on measurements of absorption of the polarized emission are noise and velocity resolution. This leads to lower uncertainties, and makes it possible to obtain absorption profiles for virtually all Galactic supernova remnants, with very precise H I column densities. To demonstrate the new technique we have obtained H I absorption profiles of both the total power and the polarized emission from Tycho's supernova remnant (G120.1+1.4), the Boomerang pulsar wind nebula (part of G106.3+2.7), and the plerionic SNR DA 495 (G65.7+1.2).

The Galactic Plane Region near l=93o. II. A Stellar Wind Bubble Surrounding SNR 3C434.1

Foster, T., Routledge, D., & Kothes, R.
2004, Astronomy & Astrophysics, 417, 79.

New Canadian Galactic Plane Survey 21 cm H I line observations towards supernova remnant (SNR) 3C434.1 (G94.0+1.0) are presented. We find a fragmented and thin-walled atomic hydrogen shell inside which the SNR is seen to be contained at v ~ -80 km/s, which we report to be a highly evolved stellar wind bubble (SWB) associated with the remnant. A dark area in the midst of otherwise bright line emission is also seen near -71 km/s. An absorption profile to the extragalactic continuum source 4C51.45 (superimposed on the shell's north face) allows us to probe the shell's optical depth, kinetic temperature and expansion velocity. The material in the dark area has the same properties as material in the fragmented shell, suggesting that the dark area is actually the far-side "cap" of the shell seen absorbing emission from warm background gas, the first instance of H I Self Absorption (HISA) seen in such a structure. We show that the kinematic distance of 10 kpc derived from a flat Galactic rotation model is highly improbable, and that this bubble/SNR system is most likely resident in the Perseus Spiral Arm, lying 5.2 kpc distant. We model the SWB shell in three dimensions as a homologously expanding ellipsoid. Physical and dynamical characteristics of the bubble are determined, showing its advanced evolutionary state. Finally, from a photometric search for one or more stars associated with the SWB, we determine that three B0V stars and one O4V star currently inhabit this bubble, and that the progenitor of 3C434.1 was at latest also an O4 type star.

Modification of Projected Velocity Power Spectra by Density Inhomogeneities in Compressible Supersonic Turbulence

Brunt, C. M., & Mac Low, M.-M.
2004, Astrophysical Journal, 604, 196.

The scaling of velocity fluctuation δv as a function of spatial scale L in molecular clouds can be measured from size-line width relations, principal component analysis, or line centroid variation. Differing values of the power-law index of the scaling relation <dv2>1/2 ~ L gamma3D in three dimensions are given by these different methods: the first two give gamma3D ~= 0.5, while line centroid analysis gives gamma3D ~= 0. This discrepancy has previously not been fully appreciated, as the variation of projected velocity line centroid fluctuations (<dvlc2>1/2 ~ L gamma2D) is indeed described, in two dimensions, by gamma2D ~= 0.5. However, if projection smoothing is accounted for, this implies, in three dimensions, that gamma3D ~ 0. We suggest that a resolution of this discrepancy can be achieved by accounting for the effect of density inhomogeneity on the observed gamma2D obtained from velocity line centroid analysis. Numerical simulations of compressible turbulence are used to show that the effect of density inhomogeneity statistically, but not identically, reverses the effect of projection smoothing in the case of driven turbulence, so that velocity line centroid analysis does indeed predict that gamma2D ~ gamma3D ~ 0.5. For decaying turbulence, the effect of density inhomogeneity on the velocity line centroids diminishes with time so that at late times, gamma2D ~ gamma3D + 0.5 as a result of projection smoothing alone. Deprojecting the observed line centroid statistics thus requires some knowledge of the state of the flow. This information can be inferred from the spectral slope of the column density power spectrum and a measure of the standard deviation in column density relative to the mean. Using our numerical results, we can restore consistency between line centroid analysis, principal component analysis, and size-line width relations, and we derive gamma3D ~ 0.5, corresponding to shock-dominated (Burgers) turbulence, which also describes the simulations of driven turbulence at scales on which numerical dissipation is negligible. We find that this consistency requires that molecular clouds are continually driven on large scales or are only recently formed.

1420 MHz Continuum Absorption toward Extragalactic Sources in the Galactic Plane

Strasser, S., & Taylor, A. R.
2004, Astrophysical Journal, 603, 560.

We present a 21-cm emission-absorption study towards extragalactic sources in the Canadian Galactic Plane Survey (CGPS). We have analyzed H I spectra towards 437 sources with S > 150 mJy, giving us a source density of 0.6 sources per square degree at arcminute resolution. We present the results of a first analysis of the H I temperatures, densities, and feature statistics. Particular emphasis is placed on 5 features with observed spin temperatures below 40 K.

We find most spin temperatures in the range from 40 K to 300 K. A simple H I two-component model constrains the bulk of the cold component to temperatures (T_c) between 40 K and 100 K. T_c peaks in the Perseus arm region and clearly drops off with Galactocentric radius, R, beyond that. The H I density follows this trend, ranging from a local value of 0.4 cm^(-3) to less than 0.1 cm^(-3) at R = 20 kpc. We find that H I emission alone on average traces about 75% of the total H I column density, as compared to the total inferred by the emission and absorption. Comparing the neutral hydrogen absorption to CO emission no correlation is found in general, but all strong CO emission is accompanied by a visible H I spectral feature. Finally, the number of spectral H I absorption features per kpc drop off exponentially with increasing R.

The Overlooked H II Region DA 568

Kerton, C. R., Brunt, C. M., & Kothes, R.
2004, Astronomical Journal, 127, 1059.

During the inspection of Canadian Galactic Plane Survey 1420 MHz continuum images of the outer Galaxy we have rediscovered the large, bright, H II region DA 568. Intriguingly, this region does not appear in any of the commonly used catalogs of H II regions and radio sources in the outer Galaxy. After demonstrating how DA 568 was essentially overlooked, we present new radio, infrared, and molecular line observations of the region. We show that, far from being a minor overlooked H II region, DA 568 is actually the most prominent part of a much larger star-forming complex in the Perseus arm of our Galaxy. The complex spans ~45 x 20 pc, contains over 104 Msolar of ionized and molecular gas, and includes the BFS 10 H II region and numerous associated IRAS sources. The star-forming complex is apparently quite evolved with most of the original giant molecular cloud having been severely disrupted by the action of DA 568.

The Cygnus X region XXII: A Herbig Ae/Be star with a giant bipolar outflow in DR 16

Behre, O. P., Wendker, H. J., Higgs, L. A., & Landecker, T. L.
2004, Astronomy & Astrophysics, 415, 217.

From medium-resolution radio images, DR 16 was suspected to be a large cometary nebula. To test this suggestion we obtained a higher resolution (15'') VLA continuum map. We also analyzed data from the Canadian Galactic Plane Survey in continuum, H I line, and IR. These data were supplemented by published near-infrared (J, H, K) stellar photometric results and MSX 8.28 µm data. We suggest that DR 16 is the diffuse H II region of an ongoing star formation site at a distance of about 3 kpc. The complicated radio picture arises from the superposition of diffuse H II with the remains of a giant bipolar outflow. The outflow was generated by a probable Herbig AeBe star, and the lobes could be the equivalent of HH working surfaces. Additional ring-like features are discussed. DR 16 is part of a larger volume of space in the local spiral arm where star formation is an ongoing process.

Morphological Analysis of H I Features I : Metric Space Technique

Khalil, A., Joncas, G., & Nekka, F.
2004, Astrophysical Journal, 601, 352.

This is the first of two papers on the morphological analysis of H I features. In this first paper, we use the so-called Metric Space Technique, developed by F. C. Adams and J. Wiseman. The Metric Space Technique is an image analysis, mathematical formalism used to quantitatively-compare astrophysical maps according to complexity. Instead of comparing maps on a pixel-by-pixel basis, we compare the maps' one-dimensional ``output functions'' which characterize specific morphological/physical aspects of the maps. The tool is used to analyze 28 H I features of known origin taken from the Canadian Galactic Plane Survey (CGPS) where the maps are scaled at 18 arc seconds per pixel (resolution of 1' x cos(DEC)). Technical and mathematical improvements to the formalism are presented. After classifying the 28 maps according to complexity, we searched for correlations between this complexity ranking and other quantifiable aspects of the H I features such as age, area, H I area, distance, flux from the ionizing star(s), fractal dimension, H I mass, and |z| (the absolute value of the height of the objects, above or below the Galactic plane). The most interesting correlations are (1) the higher the flux of UV photons the more complex is the photodissociated H I feature, and (2) the older the supernova remnant the more complex the H I associated with it. There is no correlation between the fractal dimension of the maps and their complexity or their physical characteristics, thus showing that the Metric Space Technique could be used as solution to the degeneracy of the fractal dimension.

On the Intrinsic Shapes of Molecular Clouds

Kerton, C. R., Brunt, C. M., Jones, C. E., & Basu, S.
2003, Astronomy & Astrophysics, 411, 149.

Knowledge of the intrinsic shape of molecular clouds and molecular cloud cores provides useful information on both the formation and collapse mechanisms associated with the objects and on the initial conditions for star formation. We compare the shapes of molecular clouds as determined by the Heyer et al. (2001, HSC01) and Bunt et al. (2003, BKP03) catalogues of 12CO (J=1-0) emission in the outer Galaxy. The catalogues are based upon different versions of the FCRAO Outer Galaxy Survey and also utilize different techniques for defining both the extent and shape of the clouds, which allows us to examine the effects of using different cloud definition and shape-fitting algorithms. In order to compare the two catalogues we use a subset of the cloud population where the clouds are well-defined in both data sets. We model the clouds in terms of triaxial ellipsoids and use a Monte Carlo technique to determine the best-fit intrinsic shape distribution which matches the observed axis ratio distributions. Our analysis shows that the observed shapes of molecular clouds can be best described in terms of an intrinsic distribution of triaxial ellipsoids that are intermediate between near-oblate and near-prolate ellipsoids. The lack of high axis ratio clouds seen in the HCS01 catalogue is shown to be an artifact of the cloud definition algorithm and not an intrinsic property of the molecular clouds.

A New Distance Technique for Galactic Plane Objects

Foster, T., & Routledge, D.
2003, Astrophysical Journal, 598, 1005.

We present a new method based on absorption for determination of distances within the disk of the Galaxy. The technique is useful for all Galactic plane objects, including H II regions and supernova remnants (SNR's). Our method uses radio spectral line data to find the hydrogen column density to an object, and beyond it to the Galactic edge. Models of the Galactic distribution of absorbing material (principally traced by H I, plus H2) are constructed, entailing exponential density variation of absorbing material with Galactocentric radius and with height above the midplane. Our models account for scale-height variation with increasing Galactocentric distance, and include the Galactic warp, which is prominent in the first and second quadrants of the Galaxy. The model's ability to trace the observed distribution of H I is tested and confirmed. We then demonstrate the new technique on several Sharpless H II regions with known photometric distances, and show the exemplary results. Our method yields consistent distances to these objects, and also shows them to be markedly closer than they appear using kinematic distances from a flat Galactic rotation curve.

Thermal and Non-thermal Plasmas in the Galaxy Cluster 3C 129

Krawczynski, H., Harris, D. E., Grossman, R., Lane, W., Kassim, N., & Willis, A. G.
2003, Monthly Notices of the Royal Astronomical Society, 345, 1255.

We describe new Chandra spectroscopy data of the cluster which harbors the prototypical ``head tail'' radio galaxy 3C 129 and the weaker radio galaxy 3C 129.1. We combined the Chandra data with Very Large Array (VLA) radio data taken at 0.33, 5, and 8 GHz (archival data) and 1.4 GHz (new data). We also obtained new H I observations at the Dominion Radio Astrophysical Observatory (DRAO) to measure the neutral Hydrogen column density in the direction of the cluster with arcminute angular resolution. The Chandra observation reveals extended X-ray emission from the radio galaxy 3C 129.1 with a total luminosity of 1.5E+41 erg/s. The X-ray excess is resolved into an extended central source of ~2 arcsec (1 kpc) diameter and several point sources with an individual luminosity up to 2.1E+40 erg/s. In the case of the radio galaxy 3C 129, the Chandra observation shows, in addition to core and jet X-ray emission reported in an earlier paper, some evidence for extended, diffuse X-ray emission from a region east of the radio core. The 12 arcsec x 36 arcsec (6 kpc x 17 kpc) region lies ``in front'' of the radio core, in the same direction into which the radio galaxy is moving. We use the radio and X-ray data to study in detail the pressure balance between the non-thermal radio plasma and the thermal Intra Cluster Medium (ICM) along the tail of 3C 129 which extends over 15 arcmin (427 kpc). Depending on the assumed lower energy cutoff of the electron energy spectrum, the minimum pressure of the radio plasma lies a factor of between 10 and 40 below the ICM pressure for a large part of the tail. We discuss several possibilities to explain the apparent pressure mismatch.

Triggered Star Formation in the W5 H II Region

Karr, J. L., & Martin, P. G.
2003, Astrophysical Journal, 595, 900.

Young, massive stars can have a profound effect on the surrounding interstellar medium. The triggering of star formation by an H II region expanding into nearby molecular material is an example of this type of interaction. This paper presents a multiwavelength study of the W5 star-forming region investigating the possibility of triggered star formation. The ionizing sources and the morphologies of the ionized and molecular gas and the dust are examined. A population of YSOs is identified. Statistical evidence for triggering of these YSOs is discussed in terms of the spatial dependence of the star formation efficiency and the clustering and correlation of the YSO sources with respect to the molecular gas and the H II region. Evidence for direct interactions between the components is discussed, on both large scales and the scale of individual objects. Timescales for the expansion of the H II region, triggering processes, and the ages of the YSOs are compared to establish a plausible sequence of events.

Three Lynds Bright Nebulae

Karr, J. L., & Martin, P. G.
2003, Astrophysical Journal, 595, 880.

The region l = 139.5:141.0, b = -2.5:+3.0 contains three faint Lynds Bright Nebulae. LBN 140.77-1.42 and the lower portion of LBN 140.07+1.64 are associated with an intriguing arc of neutral hydrogen at Perseus Arm velocities and morphologically associated CO and dust emission. Young stellar objects (YSOs) are found along the length of the H I feature, several with velocities indicating a connection with the features in the molecular and neutral material. The arc-shaped ionized emission from LBN 139.57+2.70 is associated with a hole in H I also at Perseus Arm velocities and contains a YSO at its edge. The hole is consistent with a wind-driven bubble blown by an early B star. This paper uses data from the Canadian Galactic Plane Survey to characterize these regions, studying the gas, dust, and YSO components of the nebulae and the sources of illumination for the ionized gas.

Nonlinear Hydromagnetic Wave Support of a Stratified Molecular Cloud

Kudoh, T., & Basu, S.
2003, Astrophysical Journal, 595, 842.

We perform numerical simulations of nonlinear MHD waves in a gravitationally stratified molecular cloud that is bounded by a hot and tenuous external medium. We study the relation between the strength of the turbulence and various global properties of a molecular cloud, within a 1.5-dimensional approximation. Under the influence of a driving source of Alfvénic disturbances, the cloud is lifted up by the pressure of MHD waves and reaches a steady state characterized by oscillations about a new time-averaged equilibrium state. The nonlinear effect results in the generation of longitudinal motions and many shock waves; however, the wave kinetic energy remains predominantly in transverse, rather than longitudinal, motions. There is an approximate equipartition of energy between the transverse velocity and fluctuating magnetic field (as predicted by small-amplitude theory) in the region of the stratified cloud that contains most of the mass; however, this relation breaks down in the outer regions, particularly near the cloud surface, where the motions have a standing-wave character. This means that the Chandrasekhar-Fermi formula applied to molecular clouds must be significantly modified in such regions. Models of an ensemble of clouds show that for various strengths of the input energy, the velocity dispersion in the cloud sigma ~ Z0.5, where Z is a characteristic size of the cloud. Furthermore, sigma is always comparable to the mean Alfvén velocity of the cloud, consistent with observational results.

Intrinsic, Observed, and Retrieved Properties of Interstellar Turbulence

Brunt, C. M., Heyer, M. H., Vazquez-Semadeni, E., & Pichardo, B.
2003, Astrophysical Journal, 595, 824.

We generate synthetic observations of 3D, self-gravitating, magnetohydrodynamic (MHD) simulations of the ISM to evaluate the ability of principal component analysis (PCA) to measure the scale dependence of turbulent velocity fluctuations in molecular clouds. Scaling exponents, alpha, observationally obtained from the coupled characteristic scales for line profile variability in velocity, dv, and in space, L, where dv ~ L^alpha, are compared to the intrinsic scaling exponents of the MHD velocity fields. We determine the approximate structure function order at which PCA operates, in order to then verify a previously established calibration of the PCA method. We also analyze the statistical properties of projected velocity line centroid fields, including effects of intermittent velocity fluctuations, density inhomogeneity and opacity, and examine the relationship of the projected 2D statistics to the intrinsic 3D statistics. Using PCA, we infer steep three-dimensional energy spectra in the molecular ISM, generally steeper than can be accounted for by Kolmogorov turbulence or possibly even shock-dominated turbulence.

G107.5-1.5: A New SNR Discovered through its Highly Polarized Radio Emission

Kothes, R.
2003, Astronomy & Astrophysics, 408, 187.

A new highly polarized shell-type supernova remnant (SNR) has been discovered in the Canadian Galactic Plane Survey (CGPS). A comparison of 408 MHz and 1420 MHz continuum emission reveals a spectral index of alpha = -0.6, typical for a mature shell-type SNR. The polarized intensity averaged over the object is 50% of the total intensity, and the peak fractional polarization is close to the theoretical maximum, making this remnant the most highly polarized SNR known. At the projected centre of the radio shell is the unidentified X-ray point source 1RXS J225203.8+574249, which could be the neutron star left behind by the supernova explosion or its pulsar wind nebula. From the low rotation measure and possibly related H I features a distance of about 1.1 kpc is proposed. At this distance the shell is about 4 pc long and the radius of G107.5-1.5 is about 6 pc. The morphology and the structure of the ambient neutral hydrogen around the SNR suggests that this supernova remnant is in a late stage of evolution.

The interstellar medium local to HD 10125

Cichowolski, S., Arnal, E. M., Cappa, C. E., Pineault, S., & St-Louis, N.
2003, Monthly Notices of the Royal Astronomical Society, 343, 47.

Based on an H I line and 408- and 1420-MHz radio continuum survey carried out at the Dominion Radio Astrophysical Observatory (DRAO), the environment of the O star HD 10125 has been studied. In addition, complementary radio continuum images, as well as infrared data of the same region have been analysed. An arc-like structure is found in all the radio continuum data. From the 21-cm line data, an H I minimum is found in the velocity range -27 to -32 km/s. Although HD 10125 is not at the centre of the H I cavity, its eccentric position is consistent with the observed stellar proper motion. The H I cavity and the continuum arc-like structure show an excellent morphological correlation. The radio continuum emission has a spectral index (Snu ~ nualpha) alpha = 0.0 +/- 0.1, which establishes the thermal nature of the arc-like feature. The dust temperature obtained from the infrared data is higher in the area where the continuum emission is present. A distance of 3 kpc is derived for the star, the H I cavity and the radio continuum structure. We conclude that all the features we have found are physically related to each other. The O star has enough energetic photons to both ionize the surrounding gas and heat up the dust and, through its powerful wind, also sweep up the H I and H II gas.

On Large-Scale Magnetic Field Reversals in the Outer Galaxy

Brown, J. C., Taylor, A. R., Wielebinski, R., & Mueller, P.
2003, Astrophysical Journal Letters, 592, L29.

We combine the observations of rotation measures of extragalactic radio sources from the Canadian Galactic Plane Survey and pulsars to investigate the question of magnetic field reversals in the outer Galaxy. Our results are consistent with there being no reversal in the Galactic magnetic field beyond the solar radius. We reconcile our conclusions with the results of previous studies that have been used to argue the presence of such reversals.

Automatic Detection of Expanding H I Shells Using Artificial Neural Networks

Daigle, A., Joncas, G., Parizeau, M., & Miville-Deschenes, M.-A.
2003, Publications of the Astronomical Society of the Pacific, 115, 662.

The identification of expanding H I shells is difficult because of their variable morphological characteristics. The detection of H I bubbles on a global scale has therefore never been attempted. In this paper, an automatic detector for expanding H I shells is presented. The detection is based on the more stable dynamical characteristics of expanding shells and is performed in two stages. The first one is the recognition of the dynamical signature of an expanding bubble in the velocity spectra, based on the classification of an artificial neural network. The pixels associated with these recognized spectra are identified on each velocity channel. The second stage consists of looking for concentrations of those pixels that were first pointed out and deciding if they are potential detections by morphological and 21 cm emission variation considerations. Two test bubbles are correctly detected, and a potentially new case of a shell that is visually very convincing is discovered. About 0.6% of the surveyed pixels are identified as part of a bubble. These may be false detections but still constitute regions of space with high probability of finding an expanding shell. The subsequent search field is thus significantly reduced. In the near future, we intend to conduct a large-scale H I shell detection over the Perseus arm using our detector.

The Impact of Ionizing Stars on the Diffuse Interstellar Medium: A Swept-up Shell and Ionized Halo around the W4 Chimney/Superbubble

Terebey, S., Fich, M., Taylor, R., Cao, Y., & Hancock, T.
2003, Astrophysical Journal, 590, 906.

Normandeau et al. have suggested that W4 is a Galactic chimney powered by the OCl 352 cluster, and subsequent work adds to its status as the prime candidate chimney in our Galaxy. The model proposes that the massive star cluster OCl 352 powers a superbubble that achieved ``blowout'' to form a Galactic chimney. In this picture blowout occurs above the Galactic plane because of a density gradient in the interstellar medium. Using the ~1' resolution far-infrared IRAS IGA and radio DRAO CGPS Galactic plane surveys, we analyze the neutral and ionized components of the W4 chimney/superbubble. We show that W4 has a swept-up, partially ionized shell of gas and dust that is powered by the OCl 352 star cluster. Dust optical depth maps show dense interstellar material below the shell and provide direct evidence of the dense gas that caused the lower shell expansion to stall. Although parts of the shell are found to be ionization-bounded, the shell is very inhomogenous, and an ionized halo provides evidence of significant Lyman continuum leakage. A large fraction (40%) of the OCl 352 cluster photons escape to large distances and are available to ionize the Galactic halo and the diffuse ionized gas component of the interstellar medium.

The Canadian Galactic Plane Survey

Taylor, A. R., Gibson, S. J., Peracaula, M., Martin, P. G., Landecker, T. L., Brunt, C. M., Dewdney, P. E., Dougherty, S. M., Gray, A. D., Higgs, L. A., Kerton, C. R., Knee, L. B. G., Kothes, R., Purton, C. R., Uyaniker, B., Wallace, B. J., Willis, A. G., & Durand, D.
2003, Astronomical Journal, 125, 3145.

The Canadian Galactic Plane Survey (CGPS) is a project to combine radio, millimetre and infrared surveys of the Galactic Plane to provide arc-minute scale images of all major components of the interstellar medium over a large portion of the Galactic disk. We describe in detail the observations for the low-frequency component of the CGPS, the radio surveys carried out at the Dominion Radio Astrophysical Observatory (DRAO), and summarize the properties of the merged database of surveys that comprises the CGPS.

The DRAO Synthesis Telescope surveys have imaged a 73 degree section of the Galactic Plane, using ~85% of the telescope time between April 1995 and June 2000. The observations provide simultaneous radio continuum images at two frequencies, 408 MHz and 1420 MHz, and spectral-line images of the 21cm transition of neutral atomic hydrogen. In the radio continuum at 1420 MHz dual-polarization receivers provide images in all four Stokes parameters. The surveys cover the region 74.2 < l < 147.3 degrees, with latitude extent of -3.6 < b < +5.6 degrees at 1420 MHz and -6.7 < b < +8.7 degrees at 408 MHz. By integration of data from single-antenna observations, the survey images provide complete information on all scales of emission structures down to the resolution limit, which is just below 1' x 1' cosec(DEC) at 1420 MHz, and 3.4' x 3.4' cosec(DEC) at 408 MHz. The continuum images have dynamic range of several thousand, yielding essentially noise-limited images with rms of ~0.3 mJy/beam at 1420 MHz and ~3 mJy/beam at 408 MHz. The spectral-line data are noise limited with rms brightness temperature dTB ~ 3 K in a 0.82 km/s channel.

The complete CGPS data set, including the DRAO surveys and data at similar resolution in 12CO (1--0) and in infrared emission from dust, all imaged to an identical Galactic co-ordinate grid and map projection, are being made publicly available through the Canadian Astronomy Data Centre.

The Distance to SNR CTB 87 and the Radial Velocity of the Perseus Arm towards l = 75 Degrees Derived from H I and CO Observations

Kothes, R., Reich, W., Foster, T., & Byun, D.-Y.
2003, Astrophysical Journal, 588, 852.

We present the results of an analysis of H I data from the Canadian Galactic Plane Survey and new CO observations in the direction of the radio bright filled-centre supernova remnant CTB 87. Our CO(1-0) and CO(3-2) data reveal evidence for associated molecular material surrounding parts of CTB 87 at a systemic velocity of v ~ -58 km/s. There is no evidence of interacting H I or the existence of a stellar wind bubble around the SNR. This indicates a supernova explosion of type II and a B2 or later type progenitor star. We established a new distance of 6.1 +/- 0.9 kpc based on the extinction-distance relation introduced by Foster & Routledge (2003), which agrees with a location within the Perseus spiral arm. Our result contradicts previous distance estimates based on H I absorption measurements, which indicated a distance of about 12 kpc using a flat rotation curve. This discrepancy arises from a significant shift of the radial velocities in the direction of CTB 87 due to a spiral shock. The new distance reduces the derived physical size and the luminosity of CTB 87 by a factor of two and four, respectively. The properties of this plerionic supernova remnant are then more similar to the other SNRs of its kind except the Crab Nebula, which is an exceptionally luminous object.

Large-Scale Structure and Dynamics of Cassiopeia OB7

Cazzolato. F. & Pineault, S.
2003, Astronomical Journal, 125, 2050.

A large region (9 deg x 9 deg) surrounding the OB association Cassiopeia OB7 has been studied for signs of interaction between the stars and the ambient gas. A large half-shell has been detected in H I around Cas OB7, as well as in the infrared and partially in CO. This half-shell covers a range of 20 km/s but could reach 40 km/s when extrapolating to the whole shell. The structure is 120 pc across if located at 2 kpc, the distance to Cas OB7. At this distance, we estimate the mass of the shell to be approximately 30,000 M_sun. Interestingly, all Cas OB7 members are located well within the edge of this shell. We believe that there is a strong hint of interaction between the gas and the stars based on morphological evidence. The velocity of both objects (association and shell) and their respective distance are also in agreement. We explain the missing half by an expansion into the interarm region.

Rotation Measures of Compact Sources in the Canadian Galactic Plane Survey

Brown, J. C., Taylor, A. R., & Jackel, B. J.
2003, Astrophysical Journal Supplement Series, 145, 213.

The Canadian Galactic Plane Survey is providing new rotation measures (RMs) for compact extragalactic sources in the Galactic plane at a solid-angle density of roughly 1 source per square degree. To date, we have derived reliable RM values for 380 sources along lines of sight through the disk of the Galaxy in the first and second quadrants. The purpose is to provide a data set useful for studies of the magneto-ionic component of the Galactic interstellar medium (ISM). We present the method used to obtain the measurements and the resulting RMs.

The Association of IRAS Sources and 12CO Emission in the Outer Galaxy

Kerton, C. R., & Brunt, C. M.
2003, Astronomy & Astrophysics, 399, 1083.

We have revisited the question of the association of CO emission with IRAS sources in the outer Galaxy using data from the FCRAO Outer Galaxy Survey (OGS). The availability of a large-scale high-resolution CO survey allows us to approach the question of IRAS-CO associations from a new direction - namely we examined all of the IRAS sources within the OGS region for associated molecular material. By investigating the association of molecular material with random lines of sight in the OGS region we were able to construct a quantitative means to judge the likelihood that any given IRAS-CO association is valid and to disentangle multiple emission components along the line of sight. The paper presents a list of all of the IRAS-CO associations in the OGS region. We show that, within the OGS region, there is a significant increase (~25%) in the number of probable star forming regions over previous targeted CO surveys towards IRAS sources. As a demonstration of the utility of the IRAS-CO association table we present the results of three brief studies on candidate zone-of-avoidance galaxies with IRAS counterparts, far outer Galaxy CO clouds, and very bright CO clouds with no associated IRAS sources. We find that ~25% of such candidate ZOAGs are Galactic objects. We have discovered two new far outer Galaxy star-forming regions, and have discovered six bright molecular clouds that we believe are ideal targets for the investigation of the earliest stages of sequential star formation around H II regions. Finally, this paper provides readers with the necessary data to compare other catalogued data sets with the OGS data.

Radio Polarization from the Galactic Plane in Cygnus

Uyaniker, B., Landecker, T. L., Gray, A. D., & Kothes, R.
2003, Astrophysical Journal, 585, 785.

We present 1420 MHz (21 cm) observations of polarized emission from an area of 117 deg2 in the Galactic plane in Cygnus, covering 82 < l < 95 deg, -3.5 < b < +5.5 deg, a complex region where the line of sight is directed nearly along the Local spiral arm. The angular resolution is ~ 1', and structures as large as 45' are fully represented in the images. Polarization features bear little resemblance to features detected in total power: while the polarized signal arises in diffuse Galactic synchrotron emission regions, the appearance of the polarized sky is dominated by Faraday rotation occurring in small-scale structure in the intervening Warm Ionized Medium. There is no concentration of polarization structure towards the Galactic plane, indicating that both the emission and Faraday rotation occur nearby. We develop a conceptual framework for interpretation of the observations. We can detect only that polarized emission which has its origin closer than the polarization horizon, at a distance d_ph; more distant polarized emission is undetectable because of depth depolarization (differential Faraday rotation) and/or beam depolarization (due to internal and external Faraday dispersion). d_ph depends on the instrument used (frequency and beamwidth) as well as the direction being studied. In our data we find that d_ph ~= 2 kpc, consistent with the polarization features originating in the Local arm. The filling factor of the Warm Ionized Medium is constrained by our data to be considerably less than unity: polarized signals that pass through multiple regions of Faraday rotation experience severe depolarization, but polarized fractions up to ~ 10% are seen, implying that there are lines of sight that intersect only one Faraday rotation region within the polarization horizon. The Rotation Measure (RM) of the extended polarized emission has a distribution which peaks at -30 rad m-2 and has a width to half-maximum of 300 rad m-2. The peak and half-width of the distribution of RMs of extragalactic sources in the region are -125 rad m-2 and 600 rad m-2 respectively. This suggests that RM increases monotonically with length of propagation path through the interstellar medium in this direction.

Among localized polarization features that we discuss, G83.2+1.8 and G91.8-2.5, stand out for their circular or quasi-circular form and extent of more than 1 degree; both are probably related to the impact of stellar activity on the surrounding medium, although the stars responsible cannot be identified. Another polarization feature, G91.5+4.5, extends 2.5 deg x 1 deg, and coincides with a molecular cloud; it plausibly arises in an ionized skin on the outside of the cloud. Polarized emission seen across the face of the large, dense H II region, W80, must be generated in the 500 pc between the Sun and W80, since W80 must depolarize all extended non-thermal emission generated behind it. Of the supernova remnants G84.2-0.8, G89.0+4.7 (HB21), G93.7-0.2 (CTB104A), and G94.0+1.0 (3C434.1), only HB21 and CTB104A show polarized emission; the other two lie beyond the polarization horizon and their emission suffers beam depolarization. Emission from the surrounding medium is depolarized on passage through HB21.

The Universality of Turbulence in the Molecular Interstellar Medium and its Exploitation as a Distance Estimator

C. M. Brunt
2003, Astrophysical Journal, 584, 293.

The turbulent energy spectrum of molecular clouds in a variety of environments is measured via principal component analysis (PCA) of spectral line imaging observations at millimeter wavelengths. Molecular clouds with known distances have been previously shown to accurately obey a universal scale dependence of turbulent velocity dispersion over spatial scales of 1-50 pc, via both standard object-based analysis and, more recently, PCA. The PCA-based spectrum is accurately obeyed such that it may be used as a distance estimator for molecular clouds with ~30% accuracy, where the error budget is contributed to strongly by input H II region distances used for the calibration. The use of 13CO spectral line data for distance estimation is examined and compared to the distance calibration established for 12CO observations. We show that distances estimated using 13CO are in good agreement with those obtained using 12CO, with a possible ~10% distance overestimation for 13CO relative to the 12CO calibration. Several molecular clouds with known distances are subjected to PCA, and we demonstrate that the universal spectrum is closely respected by all clouds; PCA-based distances estimated under the assumption of exact adherence to the universal spectrum are derived and are shown to be in excellent agreement with optically estimated distances. We examine the possibility that the PCA distance estimation method may be used to solve the kinematic distance ambiguity in the inner Galaxy. We establish how PCA may be used to diagnose severe blending of near/far emission and, in cases of little or no blending, to accurately establish the near or far distance. The inner Galaxy results provide initial support for the global validity of the universal PCA spectrum previously demonstrated for the outer Galaxy only. In conjunction with the accurate velocity information provided by millimeter wavelength spectral line data, PCA can provide useful information for studies of Galactic structure and kinematics.

Large Scale Turbulence in Molecular Clouds

C. M. Brunt
2003, Astrophysical Journal, 583, 280.

Principal component analysis of 12CO (J=1-0) emission is used to diagnose the presence of large-scale, global velocity fluctuations in molecular clouds. We search for, and find, large-scale flows of atomic material in which the globally turbulent molecular clouds are embedded. This is consistent with the picture of molecular clouds existing as short-lived, turbulent density fluctuations within larger scale atomic flows. Large-scale driving of turbulence in molecular clouds, by converging flows of atomic material, can reconcile recent numerical determinations of the dissipation rate of turbulent energy with the observations, that otherwise, for small-scale driving, leads to unacceptably high cloud luminosities. On the other hand, lack of large-scale shock signatures in the molecular gas, as expected for large-scale driving, may, if not due to an observational bias, require that the large-scale flows are themselves driven by energy injection occurring on smaller scales within molecular clouds.

Magnetic Reconnection Triggered by the Parker Instability in the Galaxy: Two-dimensional Numerical Magnetohydrodynamic Simulations and Application to the Origin of X-Ray Gas in the Galactic Halo

Tanuma, S., Yokoyama, T., Kudoh, T., & Shibata, K.
2003, Astrophysical Journal, 582, 215.

We propose the Galactic flare model for the origin of the X-ray gas in the Galactic halo. For this purpose, we examine the magnetic reconnection triggered by Parker instability (magnetic buoyancy instability), by performing the two-dimensional resistive numerical magnetohydrodynamic simulations. As a result of numerical simulations, the system evolves through the following phases. Parker instability occurs in the Galactic disk. In the nonlinear phase of Parker instability, the magnetic loop inflates from the Galactic disk into the Galactic halo and collides with the antiparallel magnetic field, so that the current sheets are created in the Galactic halo. The tearing instability occurs and creates the plasmoids (magnetic islands). Just after the plasmoid ejection, further current sheet thinning occurs in the sheet, and the anomalous resistivity sets in. Petschek reconnection starts and heats the gas quickly in the Galactic halo. It also creates the slow and fast shock regions in the Galactic halo. The magnetic field (B ~ 3 µG), for example, can heat the gas (n ~ 10-3 cm-3) to a temperature of 106 K via the reconnection in the Galactic halo. The gas is accelerated to Alfvén velocity (~ 300 km s-1). Such high-velocity jets are the evidence of the Galactic flare model we present in this paper, if the Doppler shift of the bipolar jet is detected in the Galactic halo.

An Outer Galaxy Molecular Cloud Catalog

C. M. Brunt, C. R. Kerton, & C. Pomerleau
2003, Astrophysical Journal Supplement Series, 144, 47.

We have generated a molecular cloud catalog from the Five College Radio Astronomy Observatory Outer Galaxy Survey of 12CO (J=1-0) emission using a two phase object identification procedure. The first phase consists of grouping pixels into contiguous structures above a radiation temperature threshold of 0.8 K. The second phase decomposes the first-phase objects by an enhanced version of the CLUMPFIND algorithm, using dynamic thresholding, and again with a threshold of 0.8 K used for discrimination. A detailed comparison of our method with the CLUMPFIND algorithm is given, highlighting the advantages of the use of dynamic (rather than quantized) thresholding. Basic attributes of the clouds: coordinates, bounding boxes, integrated intensities, peak observed temperatures are tabulated in the catalog. A two dimensional elliptical Gaussian is fitted to the velocity-integrated map of each cloud; the major and minor axis sizes and major axis position angles thus derived are included in the catalog. To the spatially-integrated emission line of each cloud, a Gaussian profile is fitted to measure the global linewidth. Model Gaussian clouds, truncated at 0.8 K, are examined to determine the effects of biases on measured quantities, induced by truncation. Coupled with detailed analysis of the catalogued clouds, statistical corrections for the effects of truncation on measured sizes, linewidths and integrated intensities are derived and applied, along with corrections for the effects of finite resolution on the measured attributes. The catalogued emission accounts for 76.4 % of the total emission in the Outer Galaxy Survey. The deficit is shown to arise mainly from low intensity emission on the periphery of larger objects, rather than from a large number of small and/or low intensity features. The deficit may be globally accounted for by the derived result that the emission is more `compact' (lacking extended wings) than a Gaussian distribution, a property that may be attributed to the combined effects of photo-ionization and diminished excitation in low column density regions.

From the measured parameters, Gaussian reconstructions of the emission are carried out, and these compare favorably to the raw data. A detailed analysis of the decomposition in complex regions is performed, showing that severe truncation at levels much in excess of 0.8 K is countered by the operation of a `thermostat', resulting in concatenation of emission into a larger object if severe blending is present, rather than the identification of a number of smaller, more heavily truncated objects. Two other tests are carried out : (1) an association test, that examines the utility of using the decomposed 12CO (J=1-0) emission, in comparison to CS emission, to identify possible sites of star formation as traced by IRAS point sources; (2) a test comparison of 12CO and 13CO decompositions to gauge the effects of emission saturation and blending. Overall, the results of these two tests show that emission enhancements in 12CO (J=1-0) emission, induced by internal heating by embedded star formation, are in general usefully recorded in the decomposition, but that emission blending and saturation on scales of ~ few arcminutes in complex regions can limit the precision to which associations with other tracers can be made. A statistical approach to source association is developed and described, that makes good use of the information contained in the catalog.

The catalog covers the Galactic longitude range 102.5° to 141.5°, Galactic latitude range -3° to +5.4° and lsr velocity range +20.8 to -120.2 km s-1 and contains 14592 objects. Due to its size, and for ease of access, the catalog is made available in electronic form.

Detecting Embedded Intermediate-mass Stars Using Mid-infrared and H I 21-cm Emission

C. R. Kerton
2002, Astronomical Journal, 124, 3449.

A technique is presented to detect distant, young embedded B stars (and possible clusters) from their imprint on the surrounding interstellar medium. H I 21-cm and 12CO (J=1-0) line data from the Canadian Galactic Plane Survey are combined with mid-infrared images from the Midcourse Space Experiment (MSX) Galactic Plane Survey to detect the photodissociation regions (PDRs) and molecular gas associated with the embedded intermediate-mass stars. The technique has been applied to a section of the Galaxy covered by the 12CO FCRAO Outer Galaxy Survey (102.5° < l < 141.5° -3.0° < b < +5.4°) and has led to the detection of 15 embedded intermediate-mass stars. The basic physical properties of the PDRs are presented and compared with PDR models. The PDR models, combined with previous observations of early-type Herbig Ae/Be stars and the non-detection of many of the sources in OH, H2O and CH3OH surveys, imply that the objects are ~ 106 years old. The IRAS colors of the objects form a well-defined group and have been combined with 12CO and MSX observations to identify approximately 100 additional IRAS sources within the survey area as possible embedded intermediate-mass stars.

The Distance to the SNR CTB 109 Deduced from Its Environment

R. Kothes, B. Uyaniker & A. Yar
2002, Astrophysical Journal, 576, 169.

We conducted a study of the environment around the supernova remnant (SNR) CTB 109. We found that the SNR is part of a large complex of H II regions extending over an area of 400 pc along the Galactic plane at a distance of about 3 kpc at the closer edge of the Perseus spiral arm. At this distance, CTB 109 has a diameter of about 24 pc. We demonstrated that including spiral shocks in the distance estimation is an ultimate requirement to determine reliable distances to objects located in the Perseus arm. The most likely explanation for the high concentration of H II regions and SNRs is that the star formation in this part of the Perseus arm is triggered by the spiral shock.

A Highly Ordered Faraday-Rotation Structure in the Interstellar Medium

B. Uyaniker & T. Landecker
2002, Astrophysical Journal, 575, 225.

We describe a Faraday rotation structure in the interstellar medium detected through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane Survey (CGPS). The structure, at l = 91.8,b = -2.5, has an extent of ~2 deg, within which polarization angle varies smoothly over a range of ~100 deg. Polarized intensity also varies smoothly, showing a central peak within an outer shell. This region is in sharp contrast to its surroundings, where low-level chaotic polarization structure occurs on arcminute scales. The Faraday rotation structure has no counterpart in radio total intensity and is unrelated to known objects along the line of sight, which include a Lynds Bright Nebula, LBN 416, and the star cluster M39 (NGC 7092). It is interpreted as a smooth enhancement of electron density. The absence of a counterpart, in either optical emission or total intensity, establishes a lower limit to its distance. An upper limit is determined by the strong beam depolarization in this direction. At a probable distance of 350 ± 50 pc, the size of the object is 10 pc, the enhancement of electron density is 1.7 cm-3, and the mass of ionized gas is 23 Msun. It has a very smooth internal magnetic field of strength 3 uG, slightly enhanced above the ambient field. G91.8-2.5 is the second such object to be discovered in the CGPS, and it seems likely that such structures are common in the magneto-ionic medium.

A Large Atomic Hydrogen Shell in the Outer Galaxy: SNR or Stellar Wind Bubble?

B. Uyaniker & R. Kothes
2002, Astrophysical Journal, 574, 805.

We report the detection of a ringlike H I structure toward l=90.0d, b=2.8d, with a velocity of vLSR=-99 km s-1. This velocity implies a distance of d=13 kpc, corresponding to a Galactocentric radius of Rgal=15kpc. The l-vLSR diagram implies an expansion velocity of vexp~=15 km s-1 for the shell. The structure has an oblate, irregular shell-like appearance, which surrounds weak infrared emission as seen in the 60 µm Infrared Astronomical Satellite data. At a distance of 13 kpc the size of the object is about 110x220 pc and placed 500 pc above the Galactic plane, with a mass of 105 Msolar. An expanding shell with such a high mass and diameter cannot be explained by a single supernova explosion or by a single stellar wind bubble. We interpret the structure as a relic of a distant stellar activity region powered by the joint action of strong stellar winds from early-type stars and supernova explosions.

Expanding Shells of Shocked Neutral Hydrogen around Compact H II Regions

R. Kothes & C. R. Kerton
2002, Astronomy & Astrophysics, 390, 337.

By comparing radial velocities of radio bright compact H II regions with their H I absorption profiles, we discovered expanding shells of neutral hydrogen around them. These shells are revealed by absorption of the radio continuum emission from the H II regions at velocities indicating greater distances than the observed radial velocity. We believe that these shells are shock zones at the outer edge of the expanding ionized region. Additionally we found evidence for a velocity inversion inside the Perseus arm caused by a spiral shock, which results in a deep absorption line in the spectra of compact H II regions behind it.

The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales

Jones, C. E., & Basu, S.
2002, Astrophysical Journal, 569, 280.

We decipher intrinsic three-dimensional shape distributions of molecular clouds, cloud cores, Bok globules, and condensations using recently compiled catalogues of observed axis ratios for these objects mapped in carbon monoxide, ammonia, through optical selection, or in continuum dust emission. We apply statistical techniques to compare assumed intrinsic axis ratio distributions with observed projected axis ratio distributions. Intrinsically triaxial shapes produce projected distributions which agree with observations. Molecular clouds mapped in 12CO are intrinsically triaxial but more nearly prolate than oblate, while the smaller cloud cores, Bok globules, and condensations are also intrinsically triaxial but more nearly oblate than prolate.

Molecular Cloud Distances via Principal Component Analysis

C. M. Brunt & C. R. Kerton
2002, Astrophysical Journal Letters, 567, L41.

We demonstrate that principal component analysis of 12CO J = 10 emission can be used to estimate distances to molecular clouds. The method rests on the assumption of a universal relationship between velocity dispersion and spatial scale within the clouds. Our data do not in fact support an exact universal relationship, but the influence of intercloud variability results in a 30% uncertainty in derived distances only. We establish a calibration of the method for giant molecular clouds associated with H II regions of known distance.

Interstellar Turbulence. II. Energy Spectra of Molecular Regions in the Outer Galaxy

C. M. Brunt & M. H. Heyer
2002, Astrophysical Journal, 566, 289.

The multivariate tool of principal component analysis (PCA) is applied to 23 fields in the FCRAO CO Survey of the Outer Galaxy. PCA enables the identification of line profile differences, which are assumed to be generated from fluctuations within a turbulent velocity field. The variation of these velocity differences with spatial scale within a molecular region is described by a singular power law, delta-v = c * L^alpha, which can be used as a powerful diagnostic to turbulent motions. For the ensemble of 23 fields, we find a mean value = 0.62 +/- 0.11. From a recent calibration of this method using fractal Brownian motion simulations (Brunt & Heyer), the measured velocity difference-size relationship corresponds to an energy spectrum, E(k), which varies as k - beta;, where beta = 2.17 +/- 0.31. We compare our results to both decaying and forced hydrodynamic simulations of turbulence. We conclude that energy must be continually injected into the regions to replenish that lost by dissipative processes such as shocks. The absence of large, widely distributed shocks within the targeted fields suggests that the energy is injected at spatial scales less than several parsecs.

Interstellar Turbulence. I. Retrieval of Velocity Field Statistics

C. Brunt & M. H. Heyer
2002, Astrophysical Journal, 566, 276.

We demonstrate the capability of Principal Component Analysis (PCA) as applied by Heyer & Schloerb (1997) to extract the statistics of turbulent interstellar velocity fields as measured by the energy spectrum, E(k)= k^-beta. Turbulent velocity and density fields with known statistics are generated from fractional Brownian motion simulations. These fields are translated to the observational domain, T(x,y,v), considering the excitation of molecular rotational energy levels and radiative transfer. Using PCA and the characterization of velocity and spatial scales from the eigenvectors and eigenimages respectively, a relationship is identified which describes the magnitude of line profile differences, delta-v, and the scale, L, over which these differences occur, delta-v ~ L^alpha. From a series of models with varying values of beta, we find alpha = 0.33 * beta - 0.05 for 1 < beta < 3. This provides the basic calibration between the intrinsic velocity field statistics to observational measures and a diagnostic for turbulent flows in the interstellar medium. We also investigate the effects of noise, line opacity, and finite resolution on these results.

The Supernova Remnant CTB104A : Magnetic Field Structure and Interaction with the Environment

B. Uyaniker, R. Kothes, & C. M. Brunt
2002, Astrophysical Journal, 565, 1022.

We present new, high-resolution 1420 and 408 MHz continuum images and H I and 12CO (J = 1-0) spectral line maps of the diffuse supernova remnant (SNR) CTB 104A (G93.7-0.3). Analysis of the complex continuum emission reveals no significant spectral index variations across the remnant. Three prominences around CTB 104A are found to be related to the SNR, while one extension to the east is identified as an H II region associated with a background molecular shell. Small-scale polarization and rotation measure (RM) structures are turbulent in nature, but we find a well-ordered RM gradient across the remnant, extending from southeast to northwest. This gradient does not agree with the direction of the global Galactic magnetic field but does agree with a large-scale RM anomaly inferred from RM data by Clegg et al. We show that the observed morphology of CTB 104A is consistent with expansion in a uniform magnetic field, and this is supported by the observed RM distribution. By modeling the RM gradient with a simple compression model we have determined the magnetic field strength within the remnant as B0 ~ 2.3 uG. We have identified signatures of the interaction of CTB 104A with the surrounding neutral material and determined its distance, from the kinematics of the H I structure encompassing the radio emission, as 1.5 kpc. We also observed clear breaks in the H I shell that correspond well to the positions of two of the prominences, indicating regions where hot gas is escaping from the interior of the SNR.

GSH 138-01-94: An Old Supernova Remnant in the Far Outer Galaxy

J. M. Stil & J. A. Irwin
2001, Astrophysical Journal, 563, 816.

The properties of the Galactic H I shell GSH 138-01-94 are derived from data of the Canadian Galactic Plane Survey. The basic parameters of GSH 138-01-94 were determined by fitting the expansion of a thin shell to the expansion velocity field on the sky. The kinematic distance is 16.6 kpc for vLSR=-94.2+/-0.5kms-1. The radius is 180+/-10pc, the expansion velocity vexp=11.8+/-0.9kms-1, and the mass 2×105Msolar. No radio-continuum counterpart of the shell was detected at 21 cm or at 74 cm. Absorption of a background continuum source constrains the spin temperature of H I in the shell to Ts=230367173K. The expansion age of GSH 138-01-94 is 4.3 Myr. These observables are in excellent agreement with predictions from hydrodynamic models for a supernova remnant in a low-density, low-metallicity environment such as the outer Galaxy. GSH 138-01-94 is then the largest and the oldest supernova remnant known. It provides direct evidence for the release of mechanical energy in the interstellar medium by stars in the outer Galaxy. We argue that such old supernova remnants are to be found in low-density, low-metallicity environments such as the outer Galaxy, dwarf galaxies, and low surface brightness galaxies.

A Peculiar Intermediate-Velocity Hydrogen Cloud near the Galactic Plane

L. A. Higgs, C. R. Kerton, & L. B. G. Knee
2001, Astronomical Journal, 122, 3155.

An H I cloud with a double-lobed structure and a radial velocity of +50 km/s has been found near l = 92o, b = -4o in a low-resolution H I survey carried out at the Dominion Radio Astrophysical Observatory (DRAO). This velocity is forbidden for circular Galactic rotation. Because of its unusual morphology and small variation in radial velocity over the cloud, further observations of the area were made with the DRAO Synthesis Telescope, both in the H I line and in the radio continuum. The cloud has dimensions of about 3.3 × 0.5 degrees, and has a morphology somewhat similar to high-velocity H I clouds (HVCs). Several possible associations with other objects are examined in this paper. A variable inverted-spectrum radio continuum source exists near the morphological center of the H I cloud. It is probably a rare example of a gigahertz peaked spectrum (GPS) extragalactic source having a peak above 5 GHz. A luminous B star is found close to the peculiar H I cloud, and the cloud is also located near the center of an H I supershell, supposedly located at a distance of about 9 kpc. Energetic and other considerations lead to the conclusion that the cloud is probably not related to the HVC phenomenon nor to any of the above objects but is a relic of a nearby (D ~ 1 kpc) supernova event, all other evidence of which has now dissipated. Maps of continuum radio emission (at 1420 and 408 MHz) in the area around the cloud are presented, as well as lists of continuum radio sources. A few isolated high-velocity knots of H I emission have also been detected in the vicinity of the cloud.

The Structure of the Magnetic Field in the Outer Galaxy from Rotation Measure Observations Through the Disk

J. C. Brown & A. R. Taylor
2001, Astrophysical Journal Letters, 563, L31.

High-resolution 21 cm polarization data from the Canadian Galactic Plane Survey are yielding radio sources with well-defined rotation measures (RMs) at a solid angle density of roughly 1 source deg-2, approximately 10 times greater than previous RM surveys in the plane of the Galaxy. The predominance of negative RMs in the second quadrant suggests, in contradiction to previous work, that there is no global magnetic field reversal between the solar circle and the Perseus arm. In addition, these data reveal details in the magnetoionic medium not previously observed and suggest that the random component of the magnetic field may be preferentially aligned to the uniform component, in contrast to the common assumption of an isotropic distribution.

The SNR G106.3+2.7 and its Pulsar Wind Nebula: relics of triggered star formation in a complex environment

R. Kothes, B. Uyaniker, & S. Pineault
2001, Astrophysical Journal, 560, 236.

We propose that the pulsar nebula associated with the pulsar J2229+6114 and the supernova remnant (SNR) G106.3+2.7 are the result of the same supernova explosion. The whole structure is located at the edge of an H I bubble with extended regions of molecular gas inside. The radial velocities of both the atomic hydrogen and the molecular material suggest a distance of 800 pc. At this distance the SNR is 14 pc long and 6 pc wide. Apparently the bubble was created by the stellar wind and supernova explosions of a group of stars in its center which also triggered the formation of the progenitor star of G106.3+2.7. The progenitor star exploded at or close to the current position of the pulsar, which is at one end of the SNR rather than at its center. The expanding shock wave of the supernova explosion created a comet shaped supernova remnant by running into dense material and then breaking out into the inner part of the H I bubble. A synchrotron nebula with a shell-like structure (the ``Boomerang'') of length 0.8 pc was created by the pulsar wind interacting with the dense ambient medium. The expanding shock wave created an H I shell of mass 0.4 M_solar around this nebula by ionizing the atomic hydrogen in its vicinity.

Two supernova remnants of low radio surface brightness discovered in the Canadian Galactic Plane Survey

R. Kothes, T. L. Landecker, T. Foster, & D. A. Leahy
2001, Astronomy & Astrophysics, 376, 641.

Two new supernova remnants (SNRs), G85.4+0.7 and G85.9-0.6, superimposed on the radio source W 80, have been discovered in the radio continuum data from the Canadian Galactic Plane Survey (CGPS). Both SNRs consist of a thin incomplete radio shell surrounding a weak, extended X-ray source. G85.4+0.7 has a thin non-thermal shell of diameter ~0.4o lying within a thermal shell whose diameter is ~0.6o. Its radio surface brightness at 1 GHz is Sigma1 GHz <= 1x 10-22 Watt m-2 Hz-1 sr-1. It is located within a large H I bubble, whose systemic velocity, vLSR = -12 km s-1, implies a distance of 3.8 kpc. Two B1 stars detected within this bubble are most likely part of the OB association which formed it. The diameter of the H I bubble is about 100 pc. The SNR has a diameter of about 30 pc and probably is the result of a type II explosion of an early B star ~6300 years ago. G85.9-0.6 has a radio surface brightness of Sigma1 GHz <= 2x 10-22 Watt m-2 Hz-1 sr-1. No H I features corresponding to the SNR have been detected. This, and the low radio and X-ray brightnesses, suggest expansion in a low-density medium. The SNR may lie in the low-density region between the local and Perseus spiral arms, at a distance of about 5 kpc. Its diameter would then be ~ 35 pc.

A Massive Cold Atomic Hydrogen Supershell in the Outer Galaxy

L. B. G. Knee & C. M. Brunt
2001, Nature, 412, 308.

A large fraction of the mass of the interstellar medium in our Galaxy is in the form of warm (103 -104 K) and cool (50-100 K) atomic hydrogen (H I) gas. Cold (10-30 K) regions are thought to be dominated by dense clouds of molecular hydrogen. Cold H I is difficult to observe, and therefore our knowledge of its abundance and distribution in the interstellar medium is poor. The few known clouds of cold H I are much smaller in size and mass than typical molecular clouds. Here we report the discovery that the H I supershell GSH139-03-69 is very cold (10 K). It is about 2 kiloparsecs in size and as massive as the largest molecular complexes. The existence of such an immense structure composed of cold atomic hydrogen in the interstellar medium runs counter to the prevailing view that cold gas resides almost exclusively in clouds dominated by molecular hydrogen.

A Fast Technique for the Creation of Large-Scale High-Resolution IRAS (HIRES) Beam-matched Images

C. R. Kerton & P. G. Martin
2001, Publications of the Astronomical Society of the Pacific, 113, 872.

HIRES processing provides a significant improvement in both resolution and image quality over previous IRAS image products, but the characteristics of the HIRES beam make accurate comparisons between the various IRAS bandpasses and between HIRES data and data at other wavelengths non-trivial. We present a new, fast technique for the construction of HIRES beam-matched images that is especially well suited for the creation of large-scale (several square degrees) ratio maps. Other techniques for the construction of ratio maps are discussed and compared with the new algorithm. Examples of the large-scale ratio maps that can be constructed using this new technique are presented. The algorithm's application to the construction of multiwavelength difference images and multi-colour images is also demonstrated.

A Submillimeter View of Star Formation Near the H II Region KR 140

C. R. Kerton, P. G. Martin, D. Johnstone, & D. R. Ballantyne
2001, Astrophysical Journal, 552, 601.

We present the results of 450 and 850 µm continuum mapping of the H II region KR 140 using the SCUBA instrument on the JCMT. KR 140 is a small (5.7 pc diameter) H II region at a distance of 2.3 +/- 0.3 kpc. Five of the six IRAS point sources near KR 140 were mapped in this study. Our analysis shows that two of these IRAS sources are embedded late B type stars lying well outside the H II region, two are a part of the dust shell surrounding the H II region, and one is the combined emission from an ensemble of smaller sources unresolved by IRAS. We have discovered a number of relatively cold submillimeter sources not visible in the IRAS data, ranging in size from 0.2 to 0.7 pc and in mass from 0.5 to 130 solar masses. The distribution of masses for all sources is well characterized by a power law N(>M) ~ M-alpha with alpha = 0.5 +/- 0.04, in agreement with the typical mass function for clumped structures of this scale in molecular clouds. Several of the submillimeter sources are found at the H II -- molecular gas interface and have probably been formed as the result of the expansion of the H II region. Many of the submillimeter sources we detect are gravitationally bound and most of these follow a mass--size relationship expected for objects in virial equilibrium with non-thermal pressure support. Upon the loss of non-thermal support they could be sites of star formation. Along with the two B stars that we have identified as possible cluster members along with VES 735, we argue that five nearby highly-reddened stars are in a pre-main-sequence stage of evolution.

The Equilibrium State of Molecular Regions in the Outer Galaxy

M. H. Heyer, J. M. Carpenter, & R. J. Snell
2001, Astrophysical Journal, 551, 852.

A summary of global properties and an evaluation of the equilibrium state of molecular regions in the outer Galaxy are presented from the decomposition of the FCRAO Outer Galaxy Survey and targeted 12CO and 13CO observations of four giant molecular cloud complexes. The ensemble of identified objects includes both small, isolated clouds and clumps within larger cloud complexes. The 12CO luminosity function and size distribution of a subsample of objects with well-defined distances are determined such that ΔN/ΔLCO=(3×104)L-1.80+/- 0.03CO and ΔN/Δre=(1.7×104)r-3.2+/- 0.1e. 12CO velocity dispersions show little variation with cloud sizes for radii less than 10 pc. It is demonstrated that the internal motions of regions with MCO=XCOLCO>104 Msolar are bound by self-gravity, yet the constituent clumps of cloud complexes and isolated molecular clouds with MCO<103 Msolar are not in self-gravitational equilibrium. The required external pressures to maintain the equilibrium of this population are (1-2)×104 cm-3 K.

The Structure of Four Molecular Cloud Complexes in the BU-FCRAO Milky Way Galactic Ring Survey

R. Simon, J. M. Jackson, D. P. Clemens, T. M. Bania, & M. H. Heyer
2001, Astrophysical Journal, 551, 747.

We present a study of the structure of four molecular clouds from the Milky Way Galactic Ring Survey (GRS), a Boston University and Five College Radio Astronomy Observatory collaboration. The GRS is a new high-resolution survey in the 13CO J=1-0 spectral line of the inner Galaxy and the 5 kpc ring, the Milky Way's dominant star-forming structure. Because of the smaller line widths of 13CO compared to 12CO, we can avoid velocity crowding and establish accurate kinematic distances to the clouds. The kinematic distance ambiguity in the first Galactic quadrant is resolved using self-absorption in complementary high-resolution atomic hydrogen data. The four clouds are selected to span a large range of star formation activity, from the quiescent cloud GRSMC 45.60+0.30, which shows no signs of high-mass star formation, to W49, the most luminous star-forming region in the Galaxy. We use a three-dimensional Gaussian clump decomposition to identify clumps in the clouds and to investigate their properties. Each cloud has the same clump mass spectrum, dN/dM~M-1.8, independent of star formation activity. We do not find significant differences in the slopes of the relations of density, line width, and clump mass as a function of clump size among the clouds. The size-density and size-line width relations show considerable scatter. Compared to the conventional Larson scaling laws, we find systematically flatter slopes for the size-density and size-line width relations and a higher power-law index for the size-mass relation. In particular, the clump line widths for the most quiescent cloud GRSMC 45.60+0.30 are independent of clump size. While the clouds as a whole are gravitationally bound, most of the clumps are not; only a small fraction of the total number of clumps is self-gravitating. The active star-forming clouds have a higher fraction of gravitationally bound clumps and a higher mean cloud volume density than the more quiescent clouds. The gravitationally unbound clumps are possibly confined by the weight of the self-gravitating complex. The pressures needed to bind these clumps are largest for the active star-forming clouds, which have a much higher weight than the quiescent clouds. Alternatively, a high number of the gravitationally unbound clumps may be transient.

The galactic plane region near l=93o. I. H II region NRAO 655

T. Foster & D. Routledge
2001, Astronomy & Astrophysics, 367, 635.

We present new Canadian Galactic Plane Survey radio continuum and 21 cm H I line observations of NRAO 655 (G93.4+1.8), plus radio recombination line observations, and optical H-alpha line observations. The radio spectrum of NRAO 655 confirms its emission as thermal. From the 21 cm H I data we find an atomic hydrogen cavity associated with this object at v ~= -71.5 km/s. The cavity corresponds in position and size to the brightest radio continuum emission from NRAO 655. The corresponding kinematic distance is 8.8 kpc, placing NRAO 655 in the Perseus Arm. NRAO 655's linear size is therefore 70 pc x 130 pc. To confirm the 21 cm H I velocity we present the first recombination line detection of NRAO 655 (H-158-alpha line, v ~= -71 km/s, and the first observations of a molecular cloud coinciding with NRAO 655 (at v ~= -72 km/s. The first optical detection of 656 nm H-alpha emission line features in NRAO 655 is also presented, and the H-alpha emission line brightness is determined. We suggest that the eastward extension of this strongly asymmetric object originates in a champagne outflow, and we estimate its age. We show that a single early-type star cannot be responsible for the outflow, whereas a group of later-type stars would suffice. A partial H I shell is seen adjacent to the brightest part of NRAO 655; we suggest that it has been formed by dissociation of H2 in the molecular cloud.

G106.3+2.7: A Supernova Remnant in a Late Stage of Evolution

S. Pineault & G. Joncas
2000, Astronomical Journal, 120, 3218.

We report on observations of the candidate supernova remnant (SNR) G106.3+2.7 with the Dominion Radio Astrophysical Observatory's Synthesis Telescope in the continuum at both 408 and 1420 MHz and in the 21 cm line of neutral hydrogen. The general morphology of the object and its spectral index (alpha ~ 0.57 +/- 0.04, where S(nu) ~ nu^-alpha) confirm it as an SNR. The object consists of two distinct components, the head and the tail. The tail component is of lower surface brightness and has a marginally steeper spectral index than the head component. A deficiency of neutral hydrogen at an LSR velocity of about -105 km/s is most likely due to the effect of the SNR, suggesting that the SNR is expanding at a velocity of about 15 km/s, it is at a kinematic distance of 12 kpc, and its largest angular extent is of the order of 200 pc. These parameters are shown to be consistent with a dynamical model in which the SNR is in a very late stage of its isothermal evolution, where the pressure inside the SNR is approaching the pressure of the ambient interstellar medium. We also describe the H II region Sh 141, which is about 20' north of G106.3+2.7.

The Interstellar Environment of the Wolf-Rayet Star WR 143

F. Cazzolato & S. Pineault
2000, Astronomical Journal, 120, 3192.

As part of a systematic study of the environment of Galactic WR stars, a region along the line of sight to Cygnus (l=77.5°,b=0°) has been studied. The neutral hydrogen 21 cm line distribution shows the existence of a cavity expanding from the Wolf-Rayet star WR 143. This cavity, created by the stellar wind of the star, has a mean radius of 7 pc (assuming a distance of about 1 kpc), an expansion velocity of more than 8 km/s, and a missing mass of some 150 Msolar and seems to be surrounded by an H I shell, likely made up of the gas pushed by the star. Although this hole (also called bubble) is quite conspicuous in H I, there seems to be no trace of it at the other wavelengths studied (radio continuum and infrared). All radio observations were obtained at the Dominion Radio Astrophysical Observatory as part of the Canadian Galactic Plane Survey.

The Low-Resolution DRAO Survey of H I Emission from the Galactic Plane

L. A. Higgs & K. F. Tapping
2000, Astronomical Journal, 120, 2471.

A survey of H I line emission in the Galactic plane from Galactic longitude l = 72.2 deg to 149.0 deg, and latitude b = -5.4 deg to 7.2 deg, has been made using the 26-m Telescope at DRAO. Spectra were observed at 0.2 deg intervals of longitude and latitude, providing a survey which is fully sampled in the spatial domain. A detailed study of the antenna characteristics of the telescope allowed the correction of the spectra for stray radiation. The spectral resolution is 1.32 km/s and the LSR velocity range is -260 to 161 km/s. The final spectra have an estimated Tb scaling accuracy of 1%, and an estimated RMS noise in one channel (0.8245 km/s) of 0.25 K. A region of suspected strong H I self-absorption at longitude about 92 deg, b about 3 deg, has been identified. Comparison with spectra from the Leiden/Dwingeloo Survey is used to estimate the overall accuracy of the stray-radiation corrections. This comparison also shows that a small percentage (about 1%) of the spectra in the latter survey suffer from calibration errors exceeding 10%. A table of positions of the more discordant spectra is given.

Embedded Stellar Clusters in the W3/W4/W5 Molecular Cloud Complex

J. M. Carpenter, M. H. Heyer, & R. L. Snell
2000, Astrophysical Journal Supplement Series, 130, 381.

We analyze the embedded stellar content in the vicinity of the W3/W4/W5 H II regions using the FCRAO Outer Galaxy 12CO (1-0) Survey, the IRAS Point Source Catalog, published radio continuum surveys, and new near-infrared and molecular-line observations. Thirty-four IRAS point sources are identified that have far-infrared colors characteristic of embedded star forming regions, and we have obtained K′ mosaics and 13CO (1-0) maps for 32 of them. Ten of the IRAS sources are associated with an OB star and 19 with a stellar cluster, although three OB stars are not identified with a cluster. Half of the embedded stellar population identified in the K′ images is found in just the five richest clusters, and 61% is contained in IRAS sources associated with an embedded OB star. Thus, rich clusters around OB stars contribute substantially to the stellar population currently forming in the W3/W4/W5 region. Approximately 39% of the cluster population is embedded in small clouds with an average mass of ~130 Msolar that are located as far as 100 pc from the W3/W4/W5 cloud complex. We speculate that these small clouds are fragments of a cloud complex dispersed by previous episodes of massive star formation. Finally, we find that four of the five known embedded massive star forming sites in the W3 molecular cloud are found along the interface with the W4 H II region despite the fact that most of the molecular mass is contained in the interior regions of the cloud. These observations are consistent with the classical notion that the W4 H II region has triggered massive star formation along the eastern edge of the W3 molecular cloud.

The Synthesis Telescope at the Dominion Radio Astrophysical Observatory

T. L. Landecker, P. E. Dewdney, T. A. Burgess, A. D. Gray, L. A. Higgs, A. P. Hoffmann, G. J. Hovey, D. R. Karpa, J. D. Lacey, N. Prowse, C. R. Purton, R. S. Roger, A. G. Willis, W. Wyslouzil, D. Routledge, & J. F. Vaneldik
2000, Astronomy & Astrophysics Supplement Series, 145, 509.

We describe an aperture synthesis radio telescope optimized for studies of the Galactic interstellar medium (ISM), providing the ability to image extended structures with high angular resolution over wide fields. The telescope produces images of atomic hydrogen emission using the 21-cm H I spectral line, and, simultaneously, continuum emission in two bands centred at 1420 MHz and 408 MHz, including linearly polarized emission at 1420 MHz, with synthesized beams of 1' and 3.4' at the respective frequencies. A full synthesis can achieve a continuum sensitivity (rms) of 0.28 mJy/beam at 1420 MHz and 3.8 mJy/beam at 408 MHz, and the 256-channel H I spectrometer has an rms sensitivity of 3.5 B-0.5sin(delta) K per channel, for total spectrometer bandwidth B MHz and declination delta. The tuning range of the telescope permits studies of Galactic and nearby extragalactic objects. The array uses 9 m antennas, which provide very wide fields of view of 3.1° and 9.6° (at the 10% level) at the two frequencies, and also allow data to be gathered on short baselines, yielding extremely good sensitivity to extended structure. Single-antenna data are also routinely incorporated into images to ensure complete coverage of emission on all angular scales down to the resolution limit. In this paper we describe the telescope and its receiver and correlator systems in detail, together with calibration and observing strategies that make this instrument an efficient survey machine.

A New View of Cold H I Clouds in the Milky Way

S. J. Gibson, A. R. Taylor, L. A. Higgs, & P. E. Dewdney
2000, Astrophysical Journal, 540, 851.

We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail. Our 21cm synthesis maps, taken from the Canadian Galactic Plane Survey, show a numerous and diverse population of H I self-absorption (HISA) features in gas outside the Solar circle. These objects vary in size, shape, and contrast against the background H I. All display a high level of angular and velocity structure, and most would appear significantly diluted, if not invisible, in lower-resolution H I surveys. A number of Perseus arm features remain unresolved by the 1' beam of our survey, with apparent diameters < 0.6 pc at 2 kpc distance. The majority of HISA features we detect have no obvious 12CO emission counterparts. This suggests either HISA is not found predominantly in molecular clouds, as has often been presumed in the past, or CO is not a good tracer of H2. Some HISA lacking CO shows far-infrared dust emission, though whether this arises from shielded molecular gas or diffuse atomic clouds is not clear. Constraining the gas properties of HISA remains a difficult problem, but we introduce a new method which aids this process. Our approach relates a number of physical parameters via gas law and line integral relationships, and should prove powerful if the input variables are sufficiently well known. We explore the current allowed parameter ranges for three sample features of very different appearance. We find spin temperatures <~50 K and densities >~102 cm-3.

The H II Region KR 140: Spontaneous Formation of a High Mass Star

D. R. Ballantyne, C. R. Kerton, & P. G. Martin
2000, Astrophysical Journal, 539, 283.

We have used a multiwavelength data set from the Canadian Galactic Plane Survey (CGPS) to study the Galactic H II region KR 140, both on the scale of the nebula itself and in the context of the star forming activity in the nearby W3/W4/W5 complex of molecular clouds and H II regions. From both radio and infrared data we have found a covering factor of about 0.5 for KR 140 and we interpret the nebula as a bowl-shaped region viewed close to face on. Extinction measurements place the region on the near side of its parent molecular cloud. The nebula is kept ionized by one O8.5 V(e) star, VES 735, which is less than a few million years old. CO data show that VES 735 has disrupted much of the original molecular cloud for which the estimated mass and density are about 5000 MSun and 100 cm-3, respectively. KR 140 is isolated from the nearest star forming activity, in W3. Our data suggest that KR 140 is an example of spontaneous (i.e., non-triggered) formation of, unusually, a high mass star.

The Galactic Worm GW 123.4-1.5: A Mushroom-Shaped H I Cloud

J. English, A. R. Taylor, S. Y. Mashchenko, J. A. Irwin, S. Basu, & D. Johnstone
2000, Astrophysical Journal Letters, 533, L25.

The Dominion Radio Astrophysical Observatory's Synthesis Telescope provides the highest resolution data (1 arcmin and 0.82 km/s) to date of an H I worm candidate. Observed as part of the Canadian Galactic Plane Survey, mushroom-shaped GW 123.4-1.5 extends only a few hundred parsecs, contains ~105 M_solar of neutral hydrogen, and appears unrelated to a conventional shell or chimney structure. Our preliminary Zeus-2d models use a single off-plane explosion with a modest (~1051 ergs) energy input. These generic simulations generate, interior to an expanding outer blast wave, a buoyant cloud whose structure resembles the morphology of the observed feature. Unlike typical model superbubbles, the stem can be narrow because its width is not governed by the pressure behind the blast wave nor the disk scale height. Using this type of approach it should be possible to more accurately model the thin stem and other details of GW 123.4-1.5 in the future.

The H I shell G132.6-0.7-25.3: a supernova remnant or an old wind-blown bubble?

M. Normandeau, A.R. Taylor, P.E. Dewdney, & S. Basu
2000, Astronomical Journal, 119, 2982.

Data from the Canadian Galactic Plane SUrvey reveal an abundance of H I shells and arcs in the disk of our galaxy. While their shape is suggestive of stellar winds or supernovae influence, very few of these structures have been examined in detail thus far. A fune example is an H I shell in the outer Galaxy with no continuum counterpart discovered in the survey's pilot project. Its size and kinematics suggest that it was created by the winds of a single late-type ) star which has since evolved off the main sequence or by a supernova explosion. A B1 Ia star at the centre of the shell, in projection, is a possible candidate for energy source if the shell is assumed to be wind-blown. The shell's shape implies a surprisingly small scale height of less than about 30 pc for the surrounding gas if the elongation is due to evolution in a density gradient.

A Mid-Infrared Galaxy Atlas (MIGA)

C. R. Kerton and P. G. Martin
2000, Astrophysical Journal Supplement Series, 126, 85.

A mid-infrared atlas of part of the Galactic plane (75° < l < 148°, |b| < 6°) has been constructed using HIRES processed infrared data to provide a mid-infrared data set for the Canadian Galactic Plane Survey (CGPS). The addition of this data set to the CGPS will enable the study of the emission from the smallest components of interstellar dust at an angular resolution comparable to that of the radio, millimetre, and far-infrared data in the CGPS. The Mid-Infrared Galaxy Atlas (MIGA) is a mid-infrared (12 µm and 25 µm) counterpart to the far-infrared IRAS Galaxy Atlas (IGA), and consists of resolution enhanced (~0.5' resolution) HIRES images along with ancillary maps. This paper describes the processing and characteristics of the atlas, the cross-band simulation technique used to obtain high-resolution ratio maps, and future plans to extend both the IGA and MIGA.

Probing the Interstellar Medium Using H I Emission and Absorption toward the W3 H II Region

M. Normandeau
1999, Astronomical Journal, 117, 2440.

I spectra towards the W3 H II complex are presented and used to probe the Galactic structure and interstellar medium conditions between us and this region. The overall shape of the spectra is consistent with the predictions of the Two-Arm Spiral Shock model wherein the gas found in the -40 km/s to -50 km/s range has been accelerated by some 20 km/s from its rotation curve velocity. Spin temperatures of ~100 K are derived for the Local Arm gas, lower than found in a previous, similar study towards DR 7. For the interarm region, values on the order of 300 K are found, implying a negligible filling factor for the Cold Neutral Medium (<< 1%). Some of the absorbing gas at velocities near -40 km/s is confirmed to be associated with the H II regions.

Some Synthesis Telescope Imaging Algorithms to Remove Nonisoplanatic and other Nasty Artifacts

A. G. Willis
1999, Astronomy & Astrophysics Supplement Series, 136, 603.

The small diameter antennas of the DRAO Synthesis Telescope give this instrument a very wide field of view (107 arcmin FWHM at 21 cm wavelength and 330 arcmin at 74 cm wavelength). Consequently the telescope has recently begun a systematic survey of a large region of the galactic plane. However, due to the wide field of view, the varying quality of the components used to build the telescope, and the long time required to synthesize a given field, artifacts may appear in ST images which are not easily removable by standard self-calibration techniques. This paper describes the specialized algorithms which have been developed at DRAO to remove such image defects.

The Dynamical Evolution and Ionization Structure of an Expanding Superbubble: Application to W4

S. Basu, D. Johnstone, & P. G. Martin
1999, Astrophysical Journal, 516, 843.

Recent observations have revealed a superbubble associated with the young stellar cluster OCl 352 near the W4 H II region: a void in H I emission (Normandeau, Taylor, & Dewdney), and a bright shell in H-alpha emission (Dennison, Topasna, & Simonetti). We investigate the hypothesis that the bubble is blown by stellar winds from the O-type stars in the association. The Kompaneets approximation is adapted to model a wind-blown bubble in a stratified interstellar medium. We describe some general principles necessary for understanding the dynamics of an expanding bubble and the associated ionization structure in a stratified atmosphere. The Kompaneets model can be used to determine the mean scale height of the ambient medium as well as the age of the bubble. The ionization structure also places constraints on the ambient density near the cluster. We also estimate the surface brightness of the shell and the fraction of ionizing photons which escape the bubble. The prescription we use can be applied to any observed bubble that is blown by the effectively continuous energy output of stellar winds or multiple supernovae. Application to the W4 superbubble shows that the mean scale height of the ambient gas around the cluster is remarkably small, 25 pc for a cluster distance of 2.35 kpc. The age of the bubble is estimated to be about 2.5 Myr, consistent with the notion that the bubble is blown by stellar winds from a very young cluster in which no supernovae have yet occurred.

Radio Polarimetric Imaging of the Interstellar Medium: Magnetic Field and Diffuse Ionized Gas Structure near the W3/W4/W5/HB3 Complex

A. D. Gray, T. L. Landecker, P. E. Dewdney, A. R. Taylor, A. G. Willis, & M. Normandeau
1999, Astrophysical Journal, 514, 221

We have used polarimetric imaging to study the magneto-ionic medium of the Galaxy, obtaining 1420 MHz images with an angular resolution of 1 over more than 40 deg of sky around the W3/W4/W5/HB3 H II region/SNR complex in the Perseus Arm. Features detected in polarization angle are imposed on the linearly polarized Galactic synchrotron background emission by Faraday rotation arising in foreground ionized gas having an emission measure as low as 1 cm^-6 pc. Several new remarkable phenomena have been identified, including: mottled polarization arising from random fluctuations in a magneto-ionic screen that we identify with a medium in the Perseus Arm, probably in the vicinity of the H II regions themselves; depolarization arising from very high rotation measures (several times 10^3 rad m^-2) and rotation measure gradients due to the dense, turbulent environs of the H II regions; highly ordered features spanning up to several degrees; and an extended influence of the H II regions beyond the boundaries defined by earlier observations. In particular, the effects of an extended, low-density ionized halo around the H II region W4 are evident, probably an example of the extended H II envelopes postulated as the origin of weak recombination-line emission detected from the Galactic ridge. Our polarization observations can be understood if the uniform magnetic field component in this envelope scales with the square-root of electron density and is 20 uG at the edge of the depolarized region around W4, although this is probably an overestimate since the random field component will have a significant effect.

JCMT Observations of Molecular Clouds in the Galactic Chimney Near W4

A. R. Taylor, J. A. Irwin, H. E. Matthews, & M. H. Heyer
1999, Astrophysical Journal, 513, 339.

Recently, Normandeau and coworkers reported the discovery of a Galactic chimney emerging out of the Galactic plane above a cluster of O stars associated with the W4 H II region. Two compact molecular clouds have survived the evacuation of the chimney and show evidence of dynamical effects from the energetic radiation of the nearby O stars. The age of the star cluster indicates that the clouds have been subject to intense UV radiation for several Myr. We have obtained high-resolution images of the compact ``heads'' of these clouds in the CO(2-1) line using the James Clerk Maxwell Telescope (JCMT). We have also obtained CO(3-2) over a more limited area for one of the clouds. Our observations reveal a striking arrow-shaped CO globule, with the symmetry axis of the ``arrow'' pointing toward the most luminous cluster star. A large velocity gradient analysis, carried out using both the JCMT data and CO(1-0) observations from the Five College Radio Astronomy Observatory, yields a temperature of 30 K and a density of 10^4 cm^-3 for the molecular gas at the apex. The total mass of molecular hydrogen in the region surveyed in C18O (5.3 pc) is ~800 M_solar. The velocity gradients about the symmetry axis of the arrow are well represented by a simple model of symmetric advance about the cloud center of an ionization front driven by the UV radiation of the nearby O star. The overall morphology and kinematics of the globule are in good agreement with the hydrodynamical simulations of photoevaporation of cometary globules, although for our more massive cloud the survival timescale is much longer.

Statistical Characterization of Complex Structures in H I Maps

N. Ghazzali, G. Joncas, & S. Jean
1999, Astrophysical Journal, 511, 242.

Our study aims at the characterization of H I gas cloud images for classification purposes using their brightness temperature distribution. The morphology and velocity structure of the gas cloud is not taken into account in this first study. The analysis is based on spatial contiguity at a chosen radial velocity value. It is assumed that the form brightness temperature distribution of each cloud is dependent on the physical process that created it. We wish to identify the existing relations between different clouds as a first step toward understanding the evolutionary and physical processes that form them. Our study starts with an exploratory analysis based on principal components analysis and on cluster analysis. Because the inherent signal in each image is noise, the characterization is then studied by using a method of density estimation by deconvolution. The clustering of the images is done by considering single linkage and complete linkage criteria of cluster aggregation.

A Kinematical Study of Old H II Regions and Optical Counterparts to the DRAO Canadian Galactic Plane Survey

S. Godbout, G. Joncas, & L. Drissen
1998, Publications of the Astronomical Society of Australia, 15, 60.

This project aims to tackle a few unresolved problems related to the interstellar medium (ISM) by acting as an optical counterpart to the Canadian Galactic Plane Survey (CGPS). We have three main objectives:

  1. The study of large/old H II regions.
  2. Observations of targets-of-opportunity that may be found by the CGPS.
  3. Comparison of the kinematics of extragalactic (M33) and galactic H II regions.

(1) Old H II regions having large spatial extents will be observed to establish their kinematical structure when almost no molecular material is left to produce photodissociated flows. Does turbulence play a role in these objects as for younger nebulae? (2) Ionized nebulae observed by the CGPS that are peculiar, either morphologically or by special association with neutral hydrogen, will also be observed. (3) The kinematical behavior of old H II regions in M33 will be compared with that of galactic H II regions. With both data sets in hand we will check if H II regions, like supernovae, dump energy into the neutral ISM. The instrument used as well as some very preliminary data are presented.

G74.5+0.9: A New Bipolar Source in Cygnus

S. Pineault
1998, Astronomical Journal, 115, 2483.

Viewed at 1' resolution at 1420 MHz, the source G74.5+0.9 appears as a double lobe structure of total length 20' with a moderately strong compact source located slightly off-center. Although its morphology is reminiscent of a typical extragalactic radio source with a central jet-core source, its infrared-to-radio ratio is more typical of an H II region. Furthermore, neutral hydrogen observations show the presence of an H I cavity at an LSR velocity of about -70 km/s. Combining continuum radio, infrared, and optical observations, as well as H I data, we show that the source is compatible with the model of a bipolar H II region. If at its kinematic distance (about 12 kpc), the bipolar source has an overall length of about 60 pc and is powered by an O6 star. The possibility that the source is at a much closer distance of about 2 kpc, associated with material expanding toward us and away from the Cyg OB1/OB3 association, is also considered, in which case its overall dimension is only 12 pc and a B0 star is sufficient to power it. The nature of the central source is also discussed.

The Canadian Galactic Plane Survey

J. English, A. R. Taylor, J. A. Irwin, S. M. Dougherty, S. Basu, C. Beichman, J. Brown, Y. Cao, C. Carignan, D. Crabtree, P. Dewdney, N. Duric, M. Fich, E. Gagnon, J. Galt, S. Germain, N. Ghazzali, S. J. Gibson, S. Godbout, A. Gray, D.A. Green, C. Heiles, M. Heyer, L. Higgs, S. Jean, D. Johnstone, G. Joncas, L. Knee, T. Landecker, W. Langer, D. Leahy, P. Martin, H. Matthews, W. McCutcheon, G. Moriarity-Scheiven, S. Pineault, C. Purton, R. Roger, D. Routledge, N. St-Louis, K. Tapping, S. Terebey, F. Vaneldik, B. Wallace, D. Watson, T. Willis, H. Wendker, & X. Zhang
1998, Publications of the Astronomical Society of Australia, 15, 56.

The Dominion Radio Astrophysical Observatory (DRAO) is carrying out a survey as part of an international collaboration to image the northern Milky Way, at a common resolution, in emission from all major constituents of the interstellar medium; the neutral atomic gas, the molecular gas, the ionized gas, dust and relativistic plasma. For many of these constituents the angular resolution of the images (1 arcmin) will be more than a factor of 10 better than any previous studies. The aim is to produce a publicly-available database of high resolution, high-dynamic range images of the Galaxy for multi-phase studies of the physical states and processes in the interstellar medium. We will sketch the main scientific motivations as well as describe some preliminary results from the Canadian Galactic Plane Survey/Releve Canadien du Plan Galactique (CGPS/RCPG).

The Anatomy of the Perseus Spiral Arm: 12CO and IRAS Imaging Observations of the W3/4/5 Cloud Complex

M. H. Heyer & S. Terebey
1998, Astrophysical Journal, 502, 265.

Panoramic images of 12CO J=1-0 and thermal dust emissions from the W3/W4/W5 region of the outer Galaxy are presented. These data and recently published H I 21cm line emission images provide a ~1' resolution perspective to the dynamics and thermal energy content of the interstellar gas and dust components contained within a 9° arc of the Perseus spiral arm. We tabulate the molecular properties of 1560 clouds identified as closed surfaces within the l-b-v CO data cube at a threshold of 0.9 K T*R. Relative surface densities of the molecular (28:1) and atomic (2.5:1) gas components determined within the arm and interarm velocity intervals demonstrate that the gas component which enters the spiral arm is predominantly atomic. Molecular clouds must necessarily condense from the compressed atomic material which enters the spiral arm and are likely short lived within the interarm regions. From the distribution of centroid velocities of clouds, we determine a random cloud-to-cloud velocity dispersion of 4 km/s over the width of the spiral arm but find no clear evidence within the molecular gas for streaming motions induced by the spiral potential.

The far infrared images are analyzed with the CO J=1-0 and H I 21cm line emission. The enhanced UV radiation field from members of the CasOB6 association and embedded newborn stars provide a significant source of heating to the extended dust component within the Perseus arm relative to the quiescent cirrus regions. Much of the measured far infrared flux (69% at 60µm and 45% at 100µm) originates from regions associated with star formation rather than the extended, infrared cirrus component.

The Five College Radio Astronomy Observatory CO Survey of the Outer Galaxy

M. H. Heyer, C. Brunt, R. L. Snell, J. E. Howe, F. P. Schloerb, & J. M. Carpenter
1998, Astrophysical Journal Supplement Series, 115, 241.

Images from the FCRAO CO Survey of the Outer Galaxy and a description of the calibration and processing of the data are presented. The survey is comprised of 1,696,800 12CO J=1-0 spectra sampled every 50'' between Galactic longitudes 102.49 and 141.54 and latitudes -3.03 and 5.41. The VLSR ranges from -153 to +40 km/s sampled every 0.81 km/s with a velocity resolution of 0.98 km/s. The survey provides the highest spatial dynamic range imaging of the molecular interstellar medium ever obtained. We describe the observing procedure and characterize the random and systematic noise of the survey.

The images reveal a wide diversity of spatial structure within the molecular interstellar medium. We identify several voids with angular extents of 2-5o in diameter that may result from the cumulative interactions of ultraviolet radiation fields and stellar winds from newborn massive stars with ambient interstellar gas. It is demonstrated that a significant fraction of the integrated intensity summed over the survey field originates from regions with low peak antenna temperature and low column density assuming a conversion between integrated CO emission and molecular hydrogen column density. Assuming kinematic distances to the observed emission, we derive the radial profile of mass surface density and the variations of the z midplane and scale height of the molecular gas distribution with galactocentric radius.

A large-scale, interstellar Faraday rotation feature of unknown origin seen towards the W5 H II region

A. D. Gray, T. L. Landecker, P. E. Dewdney, & A. R. Taylor
1998, Nature, 393, 660.

The disc of the Milky Way contains free electrons and magnetic fields (a magneto-ionic medium), which contribute significantly to the energetics of the interstellar medium (ISM) [1]. Faraday rotation, the wavelength-dependent shift in linear polarization angle induced by a magneto-ionic medium [2], provides a means of investigating regions where concentrations of electrons and magnetic fields are too low to detect via direct methods. Structures in polarization angle arising from Faraday rotation have been detected recently at long radio wavelengths [3]. Those structures are thought to arise from ISM gas in the Milky Way near the Sun, and are unorganized and filamentary. Here we report the discovery of a more distant, highly ordered Faraday rotation structure: a large, smooth, elliptically-shaped feature, whose long axis is parallel to the plane of the Galaxy. Presently of mysterious origin, this magneto-ionic structure appears to be located in the inter-arm region of the Milky Way between the Sun and the Perseus spiral arm, the next spiral arm outside the radius of the Sun's orbit. The feature must arise from an organization of the magnetic field and electron density distributions on a scale of order 50 pc (165 light-years). The small scale structure characteristic of the turbulence seen in adjacent regions of the ISM is absent within this volume.

[1] Jura, M., in Interstellar Processes (eds Hollenbach, D.J. & Thronson, H.A., Jr.) 3-17 (D. Reidel, Dordrecht, 1987).

[2] Lang, K.R. Astrophysical Formulae (Springer-Verlag, Berlin, 1974).

[3] Wieringa, M.H., de Bruyn, A.G., Jansen, D., Brouw, W.N. & Katgert, P. Small scale polarization structure in the diffuse galactic emission at 325 MHz. A&A, 268, 215-229 (1993).

The 7C(G) survey of radio sources at 151 MHz -- the Galactic plane at 80° < l < 104° and 126° < l < 180°, for |b| < 5.5°

S. J. Vessey & D. A. Green
1998, Monthly Notices of the Royal Astronomical Society, 294, 607.

Results from a survey of the northern Galactic plane (at declination > 30°) at 151 MHz made with the Cambridge Low Frequency Synthesis Telescope are presented. This survey is designated the 7C(G) -- i.e. the Galactic portion of the ongoing 7C surveys. This covers the region 80° < l < 104° and 126° < l < 180°, for |b| < 5.5°, and has some coverage to |b| approximately 9°, with a resolution of approximately 70×70 cosec(delta) arcsec2 (RA×Dec). The observations, data reduction and calibration of this survey are described, and a catalogue of 6262 compact sources, with a completeness limit of approximately 0.25 Jy over most of the survey region, is presented. The catalogue has an rms position accuracy of better than 10 arcsec, and the flux densities are tied to the scale of Roger, Bridle & Costain (1973) with an accuracy of better than 10 per cent.

The High-Resolution IRAS Galaxy Atlas

Y. Cao, S. Terebey, T. A. Prince, & C. A. Beichman
1997, Astrophysical Journal Supplement Series, 111, 387.

An atlas of the Galactic plane (-4.7° < b < 4.7°), along with the molecular clouds in Orion, rho Oph, and Taurus-Auriga, has been produced at 60 and 100 µm from IRAS data. The atlas consists of resolution-enhanced co-added images with 1'--2' resolution and co-added images at the native IRAS resolution. The IRAS Galaxy Atlas, together with the Dominion Radio Astrophysical Observatory H I line/21 cm continuum and FCRAO CO (1--0) Galactic plane surveys, which both have similar (~1') resolution to the IRAS atlas, provides a powerful tool for studying the interstellar medium, star formation, and large-scale structure in our Galaxy. This paper documents the production and characteristics of the atlas.

Dynamical Effects of the Parker Instability in the Interstellar Medium

S. Basu, T. Ch. Mouschovias, & E. V. Paleologou
1997, Astrophysical Journal Letters, 480, L55.

We follow numerically the nonlinear evolution of the Parker instability in the interstellar medium in two spatial dimensions. The response of an initially unstable equilibrium state to small, random velocity perturbations is investigated. The horizontal wavelength with maximum growth rate in the linear theory grows preferentially and dominates the nonlinear evolution of the system during timescales of interest in the interstellar medium. The nonlinear evolution of the system also favors modes whose symmetry allows motions that cross the galactic plane. In a separate calculation, we investigate the dominant mode in more detail and discuss the effect of the nonlinear evolution on the structure of the interstellar medium.

Aperture Synthesis Polarimetry: Application to the DRAO Synthesis Telescope

R. J. Smegal, T. L. Landecker, J. F. Vaneldik, D. Routledge, & P. E. Dewdney
1997, Radio Science, 32, 643.

Aperture synthesis is a powerful technique for imaging the radio sky and can be used to make images in all four Stokes parameters, providing a complete measurement of the polarization state of the received radiation. In centimetre-wavelength continuum astronomy the received signals are generally partially linearly polarized, with a negligibly small fraction of circular polarization. For this application the preferred antenna configuration receives both left- and right-hand circular polarization. In this paper the effects of non-ideal antenna performance are analyzed, and calibration and data correction procedures are described which allow precise measurement of the four Stokes parameters. Three levels of data correction are identified. Level 1, complex channel gain correction, is the standard calibration of amplitude and gain required in every synthesis telescope. Level 2, orthogonality correction, makes the instrument appear as a set of interferometers, identical to the level of approximation involved, which have two orthogonal, but not precisely circular, polarizations. Level 3 correction converts the telescope into an array of identical antennas with exactly circular polarization. Level 3 correction gives the most accurate polarimetry; for this level of correction the polarization characteristics of one antenna, which is used as a reference, must be determined. In the absence of a precise determination of the polarization characteristics of the reference antenna, a measurement which may be very difficult, there is no value in proceeding to Level 3 correction. If the cross-hand contamination of the antennas is less than about 15%, then only Level 1 and Level 2 corrections are needed to achieve a sensitivity to polarized emission of 1% of the total intensity, an accuracy of measurement of polarized intensity of better than 10%, and a measurement of the position angle of linearly polarized emission better than 5 degrees. The implementation of polarimetry at 1420 MHz on the DRAO Synthesis Telescope is described and instrument performance to the above specifications is demonstrated. Observations of the supernova remnant DA530 are presented, demonstrating a usable field of view for polarimetry of at least 1.5 degrees.

Application of Principal Component Analysis to Large Scale Spectral Line Imaging Studies of the Interstellar Medium

M. H. Heyer & F. P. Schloerb
1997, Astrophysical Journal, 475, 173.

The multivariate statistical technique of principal component analysis (PCA) is described and demonstrated to be a valuable tool to consolidate the large amount of information obtained with spectroscopic imaging observations of the interstellar medium. Simple interstellar cloud models with varying degrees of complexity and Gaussian noise are constructed and analyzed to demonstrate the ability of PCA to statistically extract physical features and phenomena from the data and to gauge the effects of random noise upon the analysis. Principal components are calculated for high spatial dynamic range 12CO and 13CO data cubes of the Sh 155 (Cep OB3) cloud complex. These identify the three major emission components within the cloud and the spatial differences between 12CO and 13CO emissions. Higher order eigenimages identify small velocity fluctuations and therefore provide spatial information to the turbulent velocity field within the cloud. A size line width relationship delta-v ~ Ralpha is derived from spatial and kinematic characterizations of the principal components of 12CO emission from the Sh 155, Sh 235, Sh 140, and Gem OB1 cloud complexes. The power-law indices for these clouds range from 0.42 to 0.55 and are similar to those derived from an ensemble of clouds within the Galaxy found by Larson (1981) and Solomon et al. (1987). The size-line width relationship within a given cloud provides an important diagnostic to the variation of kinetic energy with size scale within turbulent flows of the interstellar medium.

The Infrared Environment of the Wolf-Rayet Star WR 134

S. Pineault & S. Terebey
1997, Astronomical Journal, 113, 433.

Infrared data obtained with the IRAS satellite are used to study the immediate environment of the Wolf-Rayet star WR 134. In addition to an incomplete hemispherical dust shell of diameter 18', a small-scale feature is seen projected inside the stellar wind bubble, at an angular separation of about 3' to the north and east of WR 134. Among the possible explanations for this feature, a model is proposed in which it is interpreted as the result of a bow shock interaction between the WR stellar wind and material blowing past the star and originating from the Cyg OB1/OB3 supershell, also quite conspicuous in the infrared.

The DRAO Galactic Plane Survey Pilot Project: the W3/4/5/HB3 Region

M. Normandeau, A. R. Taylor, & P. E. Dewdney
1997, Astrophysical Journal Supplement Series, 108, 279.

The Dominion Radio Astrophysical Observatory, in collaboration with other site, recently began a Galactic plane survey. The data from the pilot project for this survey are presented here. They cover the W3/W4/W5/HB3 Galactic complex in the Perseus arm. Ten fields were observed to produce mosaic images of this region at two continuum frequencies, 408 MHz and 1420 MHz, as well as in the 21 cm spectral line of atomic hydrogen at 127 velocities covering +55.5 km/s to -153.9 km/s. At 1420 MHz (continuum and spectral line), an area of approximately 8° x 6° is imaged with a resolution of 1.00' x 1.14' (EW x NS), whereas at 408 MHz, the coverage was 14° x 10° (l x b) with a resolution of 3.5' x 4.0' (EW x NS). The spectral line data cube constitutues the highest resolution atomic hydrogen study of the entire complex to date. A wealth of large scale filaments, arcs, bubbles and shells is revealed.

A Massive Cometary Cloud Associated with IC 1805

M. H. Heyer, C. Brunt, R. L. Snell, J. Howe, F. P. Schloerb, J. M. Carpenter, M. Normandeau, A. R. Taylor, P. E. Dewdney, Y. Cao, S. Terebey, & C. A. Beichman
1996, Astrophysical Journal Letters, 464, L175.

High resolution, wide field imaging of 12CO, 21 cm line and continuum and IRAS far infrared emissions from the outer Galaxy has identified a massive cometary molecular cloud associated with IC 1805. The dense cloud is remnant molecular material within a large 102 pc sized cavity evacuated by the stellar winds and UV radiation field of the cluster O stars. A 37 pc long molecular tail points directly away from the sources of ionizing radiation and is likely due to the effective shielding of radiation by the dense gas and associated dust within the cometary head region. Maps of C18O J=1-0 and CS J=2-1 emissions are presented which constrain the column and mean volume densities within the cometary head region tp 1022 cm-2 and 104 cm-3 respectively. A 1155 solar luminosity point source embedde4d within the dense gas of the cometary head region provides evidence for ongoing star formation which may have been triggered by shocks driven by the ionization front.

(see also the erratum in Astrophys. J. Lett. 469, L139. Mismatched grid in Figure 2 of the original paper)

I "Tails" from Cometary Globules in IC 1396

G. H. Moriarty-Schieven, T. Xie, & N. A. Patel
1996, Astrophysical Journal Letters, 463, L105.

IC 1396 is a relatively nearby (750 pc), large (>2°), HII region ionized by a single O6.5V star and containing bright-rimmed cometary globules. We have made the first arcmin resolution images of atomic hydrogen towards IC 1396, and have found remarkable "tail"-like structures associated with some of the globules and extending up to 6.5 pc radially away from the central ionizing star. These H I "tails" may be material which has been ablated from the globule through ionization and/or photodissociation and then accelerated away from the globule by the stellar wind, but which has since drifted into the "shadow" of the globules.

This report presents the first results of the Galactic Plane Survey Project recently begun by the Dominion Radio Astrophysical Observatory.

A galactic chimney in the Perseus arm of the Milky Way

M. Normandeau, A.R. Taylor, & P.E. Dewdney
1996, Nature, 380, 687.

Galaxies are surrounded by large haloes of hot gas which must be replenished as the gas cools. This has led to the concept of galactic 'chimneys' -cavities in the interstellar medium, created by multiple supernova explosions, that can act as conduits for the efficent transport of hot gas from a galaxy's disk to its halo. Here we present a high-resolution map of atomic hydrogen in the Perseus arm of our Galaxy, which shows clear evidence for the existence of such a chimney. This chimney appears to have been formed by the energetic winds from a cluster of young massive stars, and may currently have reached the stage of blowing out into the halo.

Optical and Radio Emission from the Galactic Supernova Remnant HB 3 (G132.6+1.5)

R. Fesen, R. Downes, D. Wallace, & M. Normandeau
1995, Astronomical Journal, 110, 2876.

Wide-field Halpha images of the galactic supernova remnant HB 3 are presented and compared to new DRAO radio continuum maps at 408 and 1420 MHz with angular resolutions of 3.5'x4.0' and 1.0'x1.13' respectively. HB 3's optical emission appears as broken shell of filamentary and diffuse emission, most incomplete along the northern rim. New optical filaments are detected along the remnant's southern radio boundary, in an area of suspected greater extinction due to a foreground molecular emission ridge. An optical spectrum of bright western limb filaments indicates a shock velocity =< 100 km s-1, a [S II] lambda6716/lambda6731 derived electron density of =< 150 cm-3, and an E(B-V) = 0.71 ± 0.04 consistent with X-ray derived column density estimates. The remnant's radio spectral index is -0.64 ± 0.01, evaluated in regions away from confusing emission from W3. HB 3's scalloped western structure suggests multiple shock fronts, possibly related to the remnant's large size and late evolutionary stage, where the remnant's shock is expanding into density irregularities in the local interstellar medium. Overall we find strong correlation between individual radio and optical emission features at both large and small spatial scales. We suggest that a strong radio-optical correlation plus a multi-shell structure may be better indicators of advanced remnant age than the often mentioned diffuse optical morphology.

Maintained by Tyler Foster, DRAO.