Abstracts for


A Workshop hosted by the
Dominion Radio Astrophysical Observatory
Herzberg Institute of Astrophysics
National Research Council of Canada
22-28 August 1998, Naramata, BC

Sponsored by the Herzberg Institute of Astrophysics,
the Canadian Galactic Plane Survey Consortium,
and the Canadian Institute for Theoretical Astrophysics

See PROGRAM for overview.



Continuum Results from the Canadian Galactic Plane Survey

A.R. Taylor (Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4)

Since April 1995 the Dominion Radio Astrophysical Observatory has been engaged in a project to image the radio emission from a 70° section of the Galactic Plane as part of the Canadian Galactic Plane Survey (CGPS). The DRAO observations provide simultaneous radio continuum images at two wavelengths, 74cm and 21cm, and spectral line images of the 21cm line of neutral atomic hydrogen. In the radio continuum at 21cm dual polarisation receivers provide images in all four Stokes parameters.

The survey covers the region 146° < l < 75°, with latitude extent of -3.5° < b < +5.5° at 21cm and -6.7° < b < +8.7° at 74cm. Complete coverage of this region will be achieved by a mosaic of 190 synthesis fields, each consisting of 12 separate 12-hour observations. Approximately 60% of the observations have been carried out, and the survey is scheduled for completion in April 2000. By integration of data from single dish observations, the survey images provide complete spatial frequency coverage down to the resolution limit, which is 1' × 1'csc(DEC) at 21cm, and 3.5' × 3.5'csc(DEC) at 74cm. The first mosaic image of a section of the survey was completed in December 1997. The continuum images have dynamic range of several thousand, yielding essentially noise-limited images with rms of ~0.2 mJy/beam at 21cm and ~3 mJy/beam at 74cm. The images are sensitive to synchrotron radiation from the relativistic plasma associated with supernovae remnants and the diffuse non-thermal radiation field, and to both diffuse and compact regions of thermal Bremsstrahlung radiation. Because of the different spectral characteristics of these two emission mechanisms, the simultaneous observations at two well-separated radiation frequencies will allow a first-order discrimination of these two components in the ISM. The images of polarised emission provide an additional constraint on the non-thermal component. The initial CGPS polarisation images show widespread structures arising from differential Faraday rotation of the diffuse polarised Galactic radiation field. These structures probe the magneto-ionic medium of the galaxy, a component that is not widely visible in the diffuse ISM by any other means.

The Canadian Galactic Plane Survey: Atomic Hydrogen Observations

L.A. Higgs (Herzberg Institute of Astrophysics, DRAO, Box 248, Penticton BC, Canada, V2A 6K3)

The imaging of atomic hydrogen resulting from the Canadian Galactic Plane Survey (CGPS) represents a major step in high-resolution mapping of the atomic interstellar medium. Previous large-scale surveys of HI have had resolutions generally poorer than 4', so the CGPS with a resolution approaching 1' yields images improved in clarity of detail of bright features by one or two orders of magnitude. The survey will cover 666 square degrees: Galactic longitude 74.2° to 147.3° and Galactic latitude -3.6° to 5.6°. The HI images will cover the LSR velocity range of -164.7 km/s to 58.7 km/s, in 272 channels spaced at intervals of 0.82446 km/s. The spectral resolution is 1.32 km/s. The spatial resolution is image dependent but is about 1.2'x1.0'.

The CGPS data will be presented in 36 ``mosaic" images, each 5.12° square, that overlap each other by 1.1°. For each mosaic, observations from about ten separate observations with the DRAO Synthesis Telescope are combined. The resulting r.m.s. sensitivity in one spectral line channel varies between 2.9 K and 4.6 K across the mosaic, in a honeycomb-like pattern. Since HI features generally have peak brightness temperatures less than 100 K, the dynamic range is noise-limited to 30:1 or less.

Short-spacing HI data (for spacings less than about 13 m) are being derived from a complementary survey of HI emission made in 1997 with the 26-m Telescope at DRAO. The 24,640 spectra observed in this survey are being corrected for stray radiation, based on theoretical analysis and observations of the 26-m Telescope antenna pattern. The resulting corrected spectra will have an r.m.s. sensitivity of about 0.17 K, mainly determined by baseline stability, and absolute brightness temperature accuracy of about 2% (based on an assumed brightness temperature of 100 K for the S7 standard region).

The methodology used to calibrate the CGPS HI data, to add short-spacing information and to produce the mosaics will be outlined. The survey is now about 50% completed, and examples of the resulting imagery for the MW1 mosaic will be presented. A wide range of morphological emission structures are seen as well as many examples of HI self-absorption features.

A survey of the northern Galactic plane at 151 MHz---the 7C(G) survey

D.A. Green (MRAO, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K.)

Results from the recently published survey of the northern Galactic plane made with the Cambridge Low Frequency Synthesis Telescope (CLFST) at 151 MHz are presented. This survey---which is designated the 7C(G), i.e. the Galactic portion of the ongoing 7C surveys---covers the Galactic plane at declinations above ~ 30°, i.e. the region 80° < l < 104°, and 126° < l < 180°, for |b| <= 5.5°, and has some coverage to |b| ~= 9°. (The region 104° < l < 126° is missed because of its proximity of Cassiopeia A.) The survey a resolution of ~= 70 × 70 csc(DEC) arcsec2 (RA × Dec). The data reduction is complicated by various factors: 1) at 151 MHz the ionosphere introduces phase changes across the telescope on short timescales, which requires positions to be tied by self-calibration to bright compact sources in each field; 2) the flux density sensitivity of the CLFST varies from field-to-field due to the varying large-scale background emission from the Galactic plane; and 3) the CLFST is not exactly east--west, which means that the synthesised beam varies across images, and this has to be carefully taken into account when determining the parameters of observed sources. A catalogue of 6262 compact sources, with a completeness limit of ~ 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 Molonglo Galactic Plane Surveys

A. J. Green (Astrophysics Dept., School of Physics, University of Sydney, NSW 2006, Australia)

The Molonglo Observatory Synthesis Telescope (MOST) is a radio interferometer operating in the continuum at 843 MHz with sub-arcmin resolution. It is a highly redundant synthesis array which is very sensitive to weak, extended emission. The MOST operates in a frequency regime where nonthermal (synchrotron) emission dominates, while at the same time, thermal sources resolvable by the telescope are still generally optically thin. Hence, this telescope is an excellent instrument for morphological studies of Galactic supernova remnants (SNRs) and HII regions and their relationship with the interstellar medium (ISM).

The first epoch Molonglo Galactic Plane Survey (MGPS--1) is now accepted for publication (Green et al. 1998, Ap.J. Suppl.). For this survey the region covered is 245° <= l <= 355°, |b| <= 1.5°, an area of 330 square-degrees. Complete coverage was achieved from more than 450 overlapping fields, each representing a 12-hr synthesis observation of an area 70'x70'csc(|DEC|). The MOST is an east-west array with a resolution of 43''x43''csc(|DEC|). The rms noise (1 sigma) of a 12-hr synthesis is ~1 mJy and the MOST detects all spatial scales from the resolution limit to ~30'. The MGPS--1 complements two other selected area surveys made with the MOST: the Galactic Centre (Gray 1994, MNRAS, 270, 822) and the Vela SNR (Bock et al. 1998, A.J. in press).

Recently, the MOST has been upgraded to widen the field of view to 160'x160'csc(|DEC|). Replacement of the pre-amplifiers and an innovative method of multi-plexing and phase switching has allowed a five-fold increase in observed field area without degradation of the sensitivity or resolution. A second epoch Galactic plane survey has commenced (MGPS--2); it is planned to cover the region 240° <= l <= 365°, |b| <= 10°, which totals 2500 square-degrees. This survey is the Galactic extension of the Sydney University Molonglo Sky Survey (SUMSS).

Emission is detected in MOST images over a wide range of angular scales, including much filamentary and diffuse structure not previously seen at radio frequencies. The expected outcomes from the two surveys include an inventory of the discrete extended sources (SNRs and HII regions), a catalogue of the unresolved sources, perhaps including an unknown Galactic population, and detailed tracking of the large-scale filamentary structures out of the Plane. The distribution of external galaxies with line-of-sight positions close to the Plane are also being studied.

An important outcome will be correlation with complementary surveys, either in progress or planned. In particular, collaborations exist for comparing MOST data with an HI/20 cm survey using the Australia Telescope Compact Array, with two H-alpha surveys being carried out at Siding Spring Observatory, and with new far-infrared observations with the MSX satellite. These multi-wavelength studies will provide insights into the physical processes producing radiation from the different constituents of the ISM and will give us a better overview of the global interactions taking place.

The FCRAO CO Survey of the Outer Galaxy

M.H. Heyer (FCRAO & Dept. of Physics and Astronomy, University of Massachusetts, Amherst, MA, USA, 01003)

Images and recent results from the FCRAO CO Survey of the Outer Galaxy are presented. The Survey provides the most detailed perspective of the molecular interstellar medium over wide areas on the sky. The high spatial dynamic range of the data allow one to investigate the relationship of the CO emission to spiral arm features and to determine the global, structural, and kinematic properties of the gas for a large number of molecular regions.

The Survey is decomposed into emission features which form a closed, topological surface within the observed VLSR-l-b volume. The ensemble of objects are comprised of isolated molecular clouds and clumps within larger cloud complexes. Over 10000 objects are identified -- 4000 of which are located beyond the local spiral arm and therefore, have reasonable kinematic distances. From this restricted list of objects, we find the following:

The star forming properties of the molecular gas component are investigated with a flux limited, color selected sample of IRAS point sources associated with CO emission in the Perseus arm and far outer Galaxy. From 13CO and 2µm imaging of limited fields centered on the far infrared point source, we find regions with large LFIR/MH2 ratios (>10 Lsun/Msun) and rich clusters of recently formed stars.

Polarimetric investigations of the Galactic Plane at GHz frequencies

A.R. Duncan (Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany)

Early polarimetric radio surveys of the Galaxy focused on the large-scale properties of the polarised emission (i.e. scale-sizes of many degrees), because of the large beam areas used. These low-resolution surveys revealed much of the global magnetic structure of the Galaxy, helping to show the importance of magnetic fields in the large-scale dynamics and evolution of the Milky Way.

In contrast, polarimetric surveys of the Galaxy incorporating high sensitivity, good resolution, and reasonable angular coverage, have been performed only in the last few years. It is becoming apparent that such sensitive, high resolution observations of the diffuse polarisation show considerable promise as a probe of the magnetoionic structure of the ISM. Indeed, observations in the GHz range may be particularly effective at probing Galactic structure in this way. This is because, first, there is a considerable amount of polarised emission to be detected; second, receiver sensitivity and (phase) stability are excellent in this frequency range; third, vector rotation from the ionosphere is much less of a problem than at longer wavelengths; fourth, GHz frequencies are appropriately sensitive to low thermal electron densities. This latter point arises because even low thermal densities can produce significant vector rotation at these wavelengths. Indeed, emission measure sensitivities of the order of several cm-6pc are readily attainable (assuming optically thin thermal emission at approximately 104K, and an uncertainty in vector angles of several degrees). However, at longer wavelengths, vector rotations are large enough to depolarise almost all of the distant emission, leaving the polarimetric data dominated by a local ``Faraday screen''. At GHz frequencies, uncertainties in the vector angles (arising from noise in the Stokes-Q and U signals) preclude the detection of this local screen.

Research groups from the Max-Planck-Institut für Radioastronomie and the Australia Telescope National Facility have recently completed the first detailed look at this diffuse emission, in the frequency range 2.4--2.7 GHz. These polarimetric data have revealed many unexpected and remarkable features, over a range of angular scales. Both local and distant emission is clearly detected, as are the effects of thermal particles (Faraday rotation and depolarisation) along the line-of-sight.

In the present work, polarimetric data south of l = 5° are taken from the Parkes 2.4 GHz survey, and data north of l = 5° are from the Effelsberg 2.7 GHz survey. The angular resolutions of these surveys are 10 and 5 arcmin, respectively. Together, these data cover the Galactic Plane within the region 74° >= l >= -122°, with a latitude range of |b| <= 5° (this has been extended to b = +7° and b = -8° over some southern longitudes).

The images reveal large amounts of polarised structure, over a wide range of intensities and angular sizes. A quasi-uniform ``background'' component of patchy, low surface brightness polarisation is detected over the length of the survey, extending to the latitude limit of the data. The intensity of this emission appears generally constant over large ranges of l and b, despite the line-of-sight traversing quite different distances through the Galactic disk. Although remarkably uniform, this faint emission is not entirely local in origin. The polarisation contains components from a range of distances, out to greater than 5 kpc, and shows considerable structure in the vector orientations, some of which is attributable to distant spiral arms.

Several bright HII complexes on the Carina arm exhibit bipolar, depolarising ``plumes'', several degrees in length, which are interpreted as outflows of low-density thermal material (with densities in the range 1--10 cm-3).

A number of bright, extended regions of polarised emission (~= 5° across) are visible along the Plane, including the Vela SNR and a large ``cap'' structure appearing to the north of Sgr-A. This latter object is associated with a large cloud of Faraday-rotating, thermal particles sitting above the Galactic Centre. Several other areas of bright emission, exhibiting remarkably uniform vector orientations, are detected over the surveyed area; the physical origins of such regions remain unclear. A general symmetry in the distribution of polarised intensities about the Galactic Centre is noted, suggesting that such enhancements may be related to large-scale Galactic structure.

Examination of a number of small patches show correlating enhancements in the total-power emission. From this we conclude that a significant proportion of the polarised structure seen over the Plane is due to changes in the synchrotron emissivity, rather than to any depolarisation or Faraday rotation of a uniform, synchrotron background. Hence, these variations in the polarised intensity reflect real variations in the characteristics of the source regions of the ISM.

High Rydberg State Carbon Recombination Lines from the Galactic Plane

K.R. Anantharamaiah (NRAO, Socorro, NM 87801, USA)

Nimisha G. Kantharia (Raman Research Institute, Bangalore 560 080, India)

We present an observational study of recombination lines of carbon in the frequency range 34.5 and 327 MHz in the direction of Cas A, the Galactic Centre and about 10 other directions in the Galactic Plane. The radio telescopes used for these observations are: the 34.5 MHz T-shaped array at Gauribidanur, near Bangalore, India, the Ooty Radio Telescope, the 140-ft telescope at Green Bank and the VLA. The principal quantum numbers which generate recombination lines at these frequencies is in the range 270 to 600. The transitions at higher quantum levels are very sensitive to the kinetic pressure (nT) and the radiation background in the line forming regions. These low-frequency lines are thus very useful diagnostics of the physical conditions such as density, temperature and ionization levels in these regions. A high signal to noise profile of the C578-alpha line near 34.5 MHz obtained towards Cas A shows, clearly, wide Lorentzian wings which is the signature of radiation and/or pressure broadening. The C578-alpha line was detected in absorption towards the Galactic Centre and 8 other directions in the Galactic plane. Towards the same directions, the C272-alpha line (near 325 MHz) was dected in emission. The turnover of the lines from absorption at high quantum levels to to emission at lower quantum levels is expected on simple physical grounds. The turnover frequency (i.e. the quantum number) also depends of the physcial conditions in the line forming region. We combine the data from all observations, including those published by Erickson et al. (1996) at 75 MHz, to derive the properties of the line forming region. To derive the properties, we make use of calculation of departure coefficients (from LTE) which include the effects of dielectonic-like recombination in carbon. The derived properties are compared with those of known components of the ISM to identify the possible sites of line formation. Our results show that that the high Rydberg state lines of carbon arise most likely in the neutral cold HI phase of the ISM.

A Study of the Extended Low-density Ionized Gas in the Galactic Disk using Observations of the Diffuse Hydrogen Recombination Lines near 327 MHz with the Ooty Radio Telescope

D. Anish Roshi (National Centre for Radio Astrophysics, TIFR, Pune, India)

K.R. Anantharamaiah (Raman Research Institute, Bangalore, India (Visiting Astronomer, National Radio Astronomy Observatory, Socorro, New Mexico, USA))

With the objective of understanding the distribution of the extended low-density ionized gas in the Galactic disk and its association with other components of the Interstellar Medium (ISM) a survey of Radio Recombination Lines (RRLs) near 327 MHz in the Galactic plane is carried out using the Ooty Radio Telescope (ORT). The angular resolution of the observations is 2° × 2°. Although the angular resolution is coarse, these observations represent the first contiguous survey of RRL emission in the longitude range -30° < l < 89° (inner Galaxy). Hydrogen RRLs are detected in almost all directions in the inner Galaxy and carbon lines in several of the positions. In the outer Galaxy (l = 170° to 240°) an unbiased sample of 14 positions were observed and lines are detected towards 4 of them. This confirms the earlier conclusion by Anantharamaiah (1985a, 1986) that the ionized gas responsible for RRL emission is not as widely distributed in the outer Galaxy as in the inner Galaxy. To study the latitude extent we observed RRLs along galactic latitude at two specific longitudes (l = 0° & 13.9°). RRLs have been detected up to b = ±3°.

The radial distribution of RRL emission near 327 MHz as a function of Galactocentric distance shows a sharp peak near 4 kpc with more than 70% of the emission originating between 2.5 and 6 kpc. The lv-diagram and the radial distribution obtained from RRL emission near 327 MHz do show good similarity with that of RRL emission near 1.4 GHz, ``intense'' 12CO emission, and to some extent with the RRLs observed near 3 cm from normal HII regions. Their distribution is distinctly different from that of the H-alpha emission and HI emission from the Galactic disk. We therefore conclude that the diffuse RRL emission in the Galactic disk is associated with the star forming region.

Combining RRL observations near 1.4 GHz with our data we tried to derive the physical properties of the gas producing line emission. The density of the ionized gas could be well constrained and is in the range 0.5 -- 10 cm-3. Using the measured continuum near 10 and 2.7 GHz, the upper limit on the RRL intensity near 75 MHz and the DM obtained from the Taylor & Cordes (1993) electron density model, we found that the temperature and thus the physical size of the ionized region should be in the range 2000 -- 10000 K and 20 -- 500 pc respectively. These values are consistent with that derived earlier by Anantharamaiah (1985a). Interestingly, in few of the positions the upper limit on the electron temperature, as obtained from the width of the line, are less than 4500 K.

Using the derived physical properties of the clouds producing RRL emission near 327 MHz we estimated the absorption of galactic non-thermal emission near 35 MHz due to the presence of these clouds. Considerable fraction, if not all, of the absorption of the background radiation at low frequencies (< 100 MHz) are due to these low density ionized gas. We also computed the C II 158 µm, N II 205 µm and H-alpha emission form the low-density ionized gas. We found that a considerable fraction of the far infrared fine structure line emission originate from the ionized gas producing RRL emission, as pointed out earlier by Heiles (1994).

The morphology of the ionized gas is not clear from the existing observations. On the basis of the association of the RRL emitting clouds with the star forming region, the physical properties and the low filling factor we speculate that these objects are envelopes of normal HII region. In our opinion they do not originate from a pervasive medium, like the extended low-density warm ionized medium, in the inner Galaxy.



High-resolution Mid-Infrared Images of the ISM from the MSX satellite

Martin Cohen (Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, U.S.A.)

The Midcourse Space Experiment (MSX) surveyed the Galactic plane in 1996-7, for all longitudes, and latitudes -5 to +5°, at a resolution of 18'', simultaneously in 6 bands from 4.2 to 26 µm. MSX enables study of the ISM at an unprecedented angular resolution, offering an astrophysical opportunity through multiwavelength combination of data from the Canadian Galactic Plane Survey (CGPS), Australia's Molonglo Observatory Synthesis Telescope survey of the southern plane, and new H-alpha sky survey capabilities for faint emission. The emphasis of a new Long Term Space Astrophysics proposal to NASA is on the interpretation of the MSX space-based data via the intercomparison of all these unprecedentedly high spatial resolution data sets, focusing on the morphology, dynamics, and physics of the various types of interface found in the ISM that can be elicited from this multiwavelength perspective.

Specifically, the proposal seeks to: (1) directly overlay radio and MSX images of the GP, seeking similarities and differences in morphology and relative intensity for a host of astronomical objects, as a function of HI radial velocity (CGPS) and Galactocentric distance; (2) take full advantage of MSX's complete coverage of the plane to seek large-scale structures; (3) interpret the unprecedented detail of the diffuse IR emission in the GP in terms of known phenomena, if possible, using MSX's capability to image thermal emission from dust grains and fluorescent emission from polycyclic aromatic hydrocarbons (PAHs); (4) determine whether MSX's ``hydrocarbon perspective'' is a unique and radically new view of the ISM; (5) catalog HII regions and examine global variations of their properties with Galactocentric distance, and create a census of ultracompact HII regions; (6) assess the global rate of star formation with Galactocentric distance, particularly beyond the solar circle, seeking evidence for a radial cut-off in star formation; (7) select, identify and study known radio objects in the MSX images, namely supernova remnants; wind blown bubbles around Wolf-Rayet stars; HII regions and complexes; planetary and reflection nebulae; thermal and nonthermal fila- ments; (8) construct 3-dimensional models of representative examples of these types of object to delineate and quantify the dominant physical emission processes, and to evaluate their influence on the ISM by sampling large enough areas of the sky to assure the robustness and relevance of our conclusions.

High Resolution Infrared Observations of the Galaxy

Charles A. Beichman (IPAC (Jet Propulsion Lab/California Institute of Technology, CA, USA))

The Two Micron All Sky Survey (2MASS) will be an important dataset for many aspects of galactic research. In addition to probing galactic structure using stars as probes, 2MASS will reveal areas of star formation, and allow large scale mapping of extinction. I will describe the status of the 2MASS project, plans for the release of 2MASS data, and highlight areas of on-going research with 2MASS.

Interstellar Dust in the WIRE to PLANCK Era

P.G. Martin (CITA, University of Toronto, Toronto, ON, Canada M5S 3H8)

Interstellar dust appears in a number of roles in the interstellar medium. Historically the most familiar is as a source of extinction in the optical. Absorption of optical and ultraviolet light of course heats the dust, whence infrared (including near-infrared to submillimetre) emission, the particular theme of this talk. In some distant galaxies, most of the luminosity of the galaxy is thus converted into the infrared, though in the Milky Way the situation is not generally as extreme, except in localized regions of star formation. I shall briefly review the range of physical conditions in which the dust emits in the interstellar medium and the various sizes of dust probed. Of interest astrophysically are observations of both the spectrum and the spatial distribution of the emission, preferably in combination.

In the past two decades, probes of dust emission have advanced significantly, for example through IRAS and more recently KAO, COBE experiments, ISO, and MSX. Satellite observations of dust emission are complemented by ground-based studies, for example extending to imaging and polarimetric studies in the submillimetre with JCMT. Looking ahead, the next decade promises to be equally exciting. I shall give an overview of some of the distinctive features of facilities anticipated in the near term (WIRE), at the turn of the millennium (SOFIA, SIRTF), and somewhat further out (FIRST, PLANCK).

The contribution of SCUBA to studies of the ISM

Henry Matthews (NRC of Canada, DAO, 5071 W. Saanich Road, Victoria, B.C. V8X 4M6, Canada & Joint Astronomy Centre, 660 N. A'ohoku Place, Hilo, HI 96720, U.S.A.)

SCUBA is a bolometer camera optimised for operation at submillimetre wavelengths. It has been in use at the JCMT since April 1997. The principal modes of operation applicable to studies of dust in the ISM are first described briefly in this paper. I then discuss the situations under which use of SCUBA is most useful, and contrast these with physical conditions where spectral line observations are more revealing. Specific recent observations, e.g. of the NGC6334 and Serpens star-forming clouds, are used to illustrate the complementary nature of continuum and heterodyne approaches.

The AAO/UKST H-alpha survey

Q.A. Parker (Anglo-Australian Observatory, Coonabarabran, NSW, Australia)

The UK Schmidt Telescope (UKST) of the Anglo-Australian Observatory (AAO) has recently commenced a H-alpha survey of the Southern Galactic Plane, Magellanic Clouds and selected areas. It uses a specially designed, high specification, single-element interference filter of exceptional quality. With a clear aperture of 305mm on a square 360x360mm substrate, it is probably the largest of its kind for astronomy. Used in combination with fine grained Kodak Tech Pan film-based emulsion, a survey with an unprecedented combination of coverage, resolution and sensitivity should result, superior to any other survey of optical line emission at high resolution. The survey initially includes 233 Galactic Plane and 40 Magellanic Cloud fields on 4-degree centres and will take 3 years to complete. Film copies of the survey fields may be produced but the release of a fully digitised, calibrated survey based on SuperCOSMOS scans of the original films is anticipated as a CDrom atlas. This is the first time that a UKST survey will be released to the general astronomical community in this way. Examples of the first survey discoveries; new resolved galactic planetary nebulae, Herbig Haro objects in Orion and probable SuperNova remnants are presented.



The Gaseous ISM: Observations with the Wisconsin H-alpha Mapper (WHAM)

R. J. Reynolds (Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706, USA)

WHAM is a new facility dedicated to the study of faint optical emission lines from diffuse interstellar gas. The instrument consists of a large (15 cm) aperture, dual etalon Fabry-Perot spectrometer coupled to a 0.6 m ``telescope'', which can provide either spectra or images of faint (0.05 R) sources at high (12 km/s) spectral resolution. WHAM is located at Kitt Peak and operated remotely from Madison, Wisconsin.

During its first 18 months of operation, WHAM carried out a survey of the interstellar H-alpha emission associated with the warm, ionized component of the interstellar medium. The observations consisted of 37,000 spectra obtained with a one degree diameter beam on a 0.98° × 0.85° grid (l × b), covering the sky above declination --30°. This survey provides for the first time a detailed picture of the distribution and kinematics of the diffuse ionized hydrogen through the H-alpha line comparable to surveys of the neutral hydrogen obtained through the 21 cm line. Initial results from selected portions of the sky reveal that the interstellar H II has a complex distribution, with long filaments and loop-like structures extending to high Galactic latitudes and superposed on a more diffuse background. Some of the H-alpha filaments are clearly associated with emission features seen at other wavelengths ---such as 21 cm and soft x-ray enhancements--- implying a close association with other phases of the medium. Other filaments have no clear correspondence to any of the other phases.

In addition to H-alpha, WHAM has detected for the first time faint diagnostic emission lines in selected directions, [O I] 6300Å, [O III] 5007Å, He I 5876Å, which provide information about the physical state of the gas and perhaps clues about the source of the ionization. Observations of the [O I] line, for example, suggest that most of the diffuse H II is located within regions of nearly fully ionized hydrogen and not in partially ionized H I clouds. In addition, maps of [S II] 6716Å and [N II] 6584Å over limited regions of the sky are providing information about variations in the temperature and ionization conditions, including changes in the properties of the gas as a function of distance from the Galactic midplane.

This work has been carried out in collaboration with S. L. Tufte, L. M. Haffner, N. R. Hausen, and M. Quigley. WHAM is funded by the National Science Foundation through grants AST 96-19424.

The Cosmic Ray and Magnetic Field Components of the ISM

N. Duric (Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131)

The ISM appears to be composed of a number of distinct components in pressure equilibrium with each other. In contrast to the stars in the galaxy, the gaseous atomic phases of the ISM are non-gravitating. The molecular clouds represent a transition between the gravitating and the non-gravitating regimes.

An often overlooked component of the ISM is the relativistic phase. Although cosmic rays are collisionless particles they form a gas whose energy density is comparable to that of the other gaseous phases. The magnetic field of the Galaxy, though not a gas (but rather a byproduct of it), falls into a similar category. The cosmic rays and the Galactic magnetic field are in approximate pressure equilibrium with the other phases of the ISM. It is therefore appropriate to treat them as legitimate non-gravitating phases of the ISM.

This review will be carried out in the context of the above remarks. The latest observational and theoretical work in these areas will be covered and connections with the overall ISM emphasized.

Dust Nature and Evolution among the various ISM Components

F. Boulanger (Institut d'Astrophysique Spatiale, Universite Paris XI, Batiment 121, Orsay 91405, France)

The wealth of photometric and spectroscopic data provided by the Infrared Space Observatory (ISO) is bringing new insights into the nature of interstellar dust. ISO observations of interstellar matter have allowed to detect the emission features attributed to Polycylic Aromatic Hydrocarbons (PAHs) over a wide range of environments, including cirrus clouds and the high Galactic latitude diffuse interstellar matter. From cirrus clouds to bright reflexion nebulae such as NGC 7023, the mid-infrared emission from the interstellar medium is observed to globally scale with the intensity of the radiation field with only small changes in the spectrum shape. This is a remarkable result confirming what is expected for the emission of particles that are small enough (< 1000 C atoms) to be heated to a temperature of a few 100 K by the absorption of individual photons. Close to hot ionizing stars the mid-IR spectra show different spectral features, which can be interpreted as spectral signatures from larger grains, including possibly coal-like particles and silicates. In a second part of my talk, I will present two recent results of the analysis of the high latitude far-infrared emission measured by the DIRBE and FIRAS experiments. First, the dust in nearby molecular clouds is observed to be significantly colder than that associated with the HI gas component, a result suggesting an evolution of dust properties in between the two components. Second, I will show that the residuals of the correlation between the far-infrared and HI emission, and the spatial variations in the emission ratio can be interpreted by the presence of dust in the Warm Ionized Medium with an abundance comparable to that in the neutral gas but with a different temperature and emissivity law.

Foundations for an Empirical and Theoretical Model of the Distribution and Heating of Galactic Dust

R. Weinberger, M. Gajdosik, S. Temporin (Institute of Astronomy, University of Innsbruck, Austria)

A break-through has been achieved in recent years in the exploration of the extragalactic space ``beyond'' the plane of the Milky Way; the basis for this research was laid by the searches for, discoveries and studies of galaxies at low galactic latitudes. Our institute was and is active in this field since its onset---for example, about 7000 galaxies have been detected there by us up to now. An important, promising, but hitherto almost entirely neglected aspect of the research on extragalactic objects along the galactic plane is their potential for studies of the Milky Way. We have made a first, fruitful attempt to open this field by investigating the effects of dust which strongly influences galaxies beyond the Galaxy's plane. The empirical basis of such investigations of the Milky Way will be presented and its empirical extension will be discussed. Furthermore, quite recently we were able to obtain first results of our theoretical efforts that might help to considerably improve the understanding of the large-scale distribution and heating of galactic dust: Our model is based on a specific spiral arm model and predicts the far-infrared emission from the Galaxy. Comparison with observational data enables tests of the various published spiral arm models.

Spectrum of Interstellar Dust from 30--3000 GHz

Douglas P. Finkbeiner and Marc Davis (UC Berkeley, Berkeley, CA 94720, USA)

We have recently published a map of Galactic dust based on the IRAS 100µm and DIRBE 100µm and 240µm skymaps (Schlegel, Finkbeiner, & Davis 1998). One purpose of this map is to estimate the effects of foreground dust emission on CMB measurements. Using the COBE/FIRAS data as well as our temperature map derived from the DIRBE 100/240µm ratio we can extrapolate the dust emission from 100µm (3000 GHz) to 1 mm (300 GHz) with confidence. The spectrum is consistent with nualpha Bnu(18 K) where alpha = 1.55± 0.1 and Bnu(18 K) is the Planck function for a temperature of 18 K.

At frequencies below 300 GHz, systematic uncertainties in FIRAS become great enough that we do not use it to fit the dust spectrum. However, we may extrapolate our fit to the DMR channels (31.5, 53, and 90 GHz), cross correlate the dust with these three microwave bands, and compare to our prediction. The 90 GHz channel exhibits an obvious correlation with our dust map, and agrees with our prediction. At 53 and 31.5 GHz an excess of dust-correlated flux is seen. This result is in agreement with Kogut et al. (1996) but is more significant because of the temperature correction, and our use of a greater portion of the sky.

Microwave emission by dust: mechanisms, properties and prospects for ISM studies

Alex Lazarian (Princeton University Observatory, Princeton, NJ 08544, USA)

Ongoing efforts to measure microwave background and its polarization may bring very useful information about the interstellar medium. For instance, recent observations revealed an anomalous foreground emissivity at 10-100 GHz. The initial claims that this emission is free-free were proven to be untenable. Our work with Bruce Draine explains how emission from dust can account for the anomalous emission. In particular, we show that two possibilities exist:

  1. rotational dipole emission from ultrasmall (<10-7 cm) grains

  2. magneto-dipole emission from from large (~10-6 -10-5 cm) grains.

We calculate expected emissivities from different phases of interstellar medium, including diffuse HI clouds, warm HI, low-density photoionized gas, and cold molecular gas. We also calculate the emission spectra for various grain candidates and show that emission from ferromagnetic and ferrimagnetic materials is the most important. As the relative role of the two mechanisms is still unclear and we discuss how it can be established via measurements of microwave emission from dark clouds. We also show that polarization from ferromagnetic single magnetic dipole grains will strongly depend on frequency, while no similar dependence is expected for rotational electric dipole emission from ultrasmall grains. New microwave window is a window of opportunity for interstellar studies. Magnetic fields well inside dark clouds, where other methods fail, may be studied successfully studied via microwave polarization. Microwave emissivity constrains the abundance of strongly magnetic materials. For instance, the available data at 90 GHz indicate that not more that ~ 5% of interstellar Fe is in the form of metallic iron grains or inclusions (e.g., in ``GEMS''). Future missions, e.g. MAP and PLANCK will bring a lot of microwave data that can be successfully used to study ISM. Such a study would also be appreciated by cosmologists who franticly try to remove all foregrounds from their data.



The Eridanus Superbubble in its Multiwavelength Glory

Carl Heiles (Astronomy Department, University of California, Berkeley, CA 94720-3411, USA)

Eridanus is the Rosetta Stone of superbubbles. It is in a middle evolutionary stage, having originated long ago but still being energized by massive stellar winds and supernovae; this means it exhibits the full range of astrophysical processes that occur whenever all of the interstellar gas phases lie in close proximity. It is nearby, so only modest angular resolution is required and, more importantly, it is the only object along the line of sight so its maps can be interpreted unambiguously. Its importance lies not in itself, but rather in serving as a laboratory where we can learn in detail about important astrophysical processes that occur everywhere. We bring together the full range of available data to show that the multiwavelength whole is far more than the sum of its parts.

The Cygnus Loop: a prototype for radio studies of SNRs

D.A. Leahy (Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4)

The Cygnus Loop is a large nearby supernova remnant (SNR) well suited to multiwavelength radio studies. It also serves to illustrate the types of analysis and physical results we might expect for other large SNRs, such as those observed in the Canadian Galactic Plane Survey (CGPS). The Cygnus Loop has been observed with the Dominion Radio Astrophysical Observatory Synthesis Telescope at 1420 MHz, in all four Stokes parameters, at 408 MHz, and in the 21 cm neutral hydrogen line, similar to other CGPS SNRs. Additionally, previous observations exist at 22 MHz, 34 MHz and 2695 MHz, and in infrared (IRAS 12, 25, 60 and 100 micron maps) and x-ray (e.g. ROSAT All-Sky-Survey and Pointed maps). A sample of observational results and interpretations are reviewed from: a 1420 MHz polarization study; a multi-frequency spectral index study; and an HI 21 cm study.

The Cygnus Loop shows up to 39% polarization in southern regions. However the bright northeastern rim has a mean of only 2.4% polarization. Comparison with X-ray images shows a correlation between regions bright in x-rays (i.e. high electron density) and depolarization. 5 GHz polarization data was used to derive intrinsic polarization vectors, showing the bright filaments have magnetic field aligned along their long dimension.

Analysis of 408 MHz and 1420 MHz data with existing 2.695 GHz data yields radio spectral indices in the Cygnus Loop. Several regions have significant spectral curvature between 408 and 2695 MHz, confirmed by comparison with lower resolution 22 and 34 MHz maps. The bright radio filaments show negative curvature (steeper at higher frequency); regions dominated by diffuse emission show positive curvature (flatter at higher frequency). The origin of the curvature is discussed.

HI spectral line maps show a great deal of structure in the region encompassing the Cygnus Loop. Several features are found which have interior edges closely matching the outer boundary of the Cygnus Loop. These are evidence for much of the wall of the cavity into which the Cygnus Loop is hypothesized to be expanding.

Compression, Self-Gravitation, and the Roots of Star Formation

Ralph E. Pudritz (Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1 )

Star formation occurs within the cores of molecular clouds. These clouds are highly filamentary structures whose origins may possibly be traced to the cooling of massive, swept-up magnetized, super-shells created by coherent supernova activity in the galaxy. Recent submm polarimetric observations reveal that molecular clouds are threaded by surprisingly ordered magnetic fields. They are supported against gravitational collapse by the pressure from a combination of ordered magnetic fields, as well as that from MHD waves or turbulence. Molecular cloud cores are also dominated by non-thermal motions and are rather elongated structures. The infrared camera observations of cores over the last few years reveal that the typical star forms as member of a star cluster within the most massive cores within a molecular cloud. The gravitational collapse of magnetized and turbulent gas within these cores proceeds in a rather different manner than expected for standard models of singular isothermal spheres. This review will address these central problems in the structure and physics of self-gravitating, magnetized clouds and the star formation that they engender.. It will also address the problem of how multiple star formation in star clusters might proceed; a problem for which there is, as yet, no existing comprehensive theory.



The Effelsberg continuum and polarization surveys of the Galactic plane

W. Reich (Max-Planck-Institut für Radioastronomie, Bonn, Germany)

We give a short review of the all-sky survey work at Bonn in the past and describe the Galactic plane surveys at 1.4 and 2.7 GHz. An internet access to all published surveys is provided. Recently reduction of polarized emission at 2.7 GHz from the first Galactic quadrant was completed, showing rather complex structures just outside the inner degree of Galactic latitude. Present observational activities concentrate on a medium latitude survey at 1.4 GHz including linear polarization, which is calibrated by absolute measurements. Strong and spatially variing structures in polarization are seen almost everywhere, which are at most not related to the total intensity features. Depolarized loops and filaments on degree scales are seen most pronaunced towards the anticentre direction. For some of them follow up high frequency observations have been made.

Polarized emission structures in the local ISM

Bülent Uyaniker (Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany)

The interaction between the Galactic magnetic field and interstellar clouds leads to a variety of radio emitting structures, shells, filaments, and loops. They are very faint, an order of magnitude fainter than the large scale diffuse Galactic emission. Many of these features are linearly polarized. They extend up to large Galactic latitudes (±20°). However, the diffuse faint component of the Galactic emission at these latitudes has never been studied in a systematic way, especially in polarization.

I report several polarization structures detected within the currently running continuum survey at 1.4 GHz with the Effelsberg 100-m telescope. These structures are of the order of several degrees in size and have no corresponding structures in the total-power emission. The reason of the unobserved total-power emission is not clear. Moreover, within the polarized emission there are numerous nearly straight, loop or arc shaped structures, which seem to be depolarized. They are most pronounced towards the Galactic anticentre, where the line of sight across the Galaxy is comparably short. In these regions the total intensity of diffuse Galactic emission as revealed from the Stockert 1.4 GHz survey is less than 0.5 K. In view of the observed maximal polarized intensities of up to 0.25 K, there are also areas with well organized magnetic fields and quite small Faraday rotation effects. They are most likely due to the spatial variations of the Faraday rotation by the interstellar medium in the line of sight, which modulate a significantly polarized smooth Galactic diffuse emission. It is not clear wheather fluctuations of the Galactic magnetic field in strength or direction are the reason for the Faraday rotation or these variations are due to changes in the electron density.

It seems that probing the properties of the diffuse Galactic emission via a polarization survey is a promising tool towards the improvement of our current understanding of the Galaxy and the local ISM.

Polarisation Structures in the Milky Way

M. Peracaula, A.R. Taylor, T.L. Bellchamber (Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4)

A.D. Gray, T.L. Landecker (DRAO, Box 248, Penticton BC, Canada, V2A 6K3)

As part of the Canadian Galactic Plane Survey (CGPS), the linearly polarised emission of the Milky Way is being mapped in the radio continuum at 21 cm. In this talk we present the results obtained from the recently processed Stokes Q and U interferometric images covering the region from l=74° to l=83° (Cygnus × region) and those covering the region from l=137° to l=147°. In them we see medium- and large-scale polarisation structures with generally no related counterparts in the continuum total power (Stokes I emission). The possibility of a correlation between these structures and those seen in CGPS HI images is being studied, but no conclusive results have yet been obtained. We discuss possible origins of these polarised structures, such as Faraday screens and depolarization phenomena imposed on the Galactic synchrotron radiation background by the magneto-ionic interstellar medium. We also look at the information we can potentially obtain about the Galactic ISM from the study of these kinds of phenomena.

Faraday Rotation and the Diffuse Ionized Medium

J.C. Brown, A.R. Taylor and M. Peracaula (Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4)

As part of the Canadian Galactic Plane Survey (CGPS), we have been studying the interstellar medium using polarisation maps. Non-thermal radiation from the Galactic disk produces a diffuse linearly polarised radiation field. This radiation field is acted upon and modified by propagation through the interstellar medium. By studying the properties of the polarised radiation in the CGPS images (Stokes Q and U in particular), we are developing techniques to deduce information about the ISM, such as the magnetic field structure. Part of this investigation involves studying how the Faraday rotation properties correlate with radiation from components of the ISM, such as the ionized gas (as seen in the Stokes I images) and the neutral gas (as seen in HI and CO images). As an example, we are currently investigating the region around the ionized filament CXR-11 in Cygnus-X. In the Q and U maps of this region, a distinct shift in the position angle of the linear polarisation of the background field corresponds to the location of the filament in the I map, which suggests Faraday rotation of this background field by the magnetoionic medium within the filament. By inferring the electron density of the filament from the Stokes I map, the magnitude of the line of sight component of the magnetic field within the filament may be estimated. Modelling techniques and observations at additional wavelengths provide constraints on the strength and geometry of the magnetic field. The measurement and modelling techniques developed by studying this field will allow us to extract information about the magnetoionic component of the ISM in other regions of the galaxy.

The Structure of the ISM on pc Scales from Small-scale Structure in the Polarized Background

P. Katgert (Leiden Observatory, The Netherlands)

No abstract available.



Disk-halo interactions in external spiral galaxies---an overview

Michael Dahlem (ESA/ESTEC, The Netherlands)

In this overview I present observational results on disk-halo interactions (DHIs) in external galaxies. Parameters determining the structure of the ISM and initiating DHIs are investigated. It appears that the level of energy input into the ISM by supernova remnants and high-mass stars, the magnetic field structure and the gravitational potential (which in many cases is influenced by interactions with companion galaxies) play a crucial role in the onset and maintenance of DHIs. After exclusion of galaxies with AGNs, the most reliable tool to identify galaxies with DHIs is a high IRAS flux ratio of f60/f100 > 0.4, indicating the presence of warm dust, which was originally defined by Heckman et al. (1990) to identify starburst galaxies with superwinds.

Observations and Modeling of the Disk-Halo Interaction in our Galaxy

Magdalen Normandeau (Astronomy Department, 601 Campbell Hall, University of California, Berkeley, CA, 94720-3411, United States)

Shantanu Basu (Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON, M5S 3H8, Canada)

Galaxies are surrounded by large halos of hot gas which must be replenished as the gas cools. This led Norman & Ikeuchi (1989 ApJ 345, 372) to propose the chimney model of the interstellar medium which predicts that there should be on the order of a thousand such conduits connecting the disk and the halo of a galaxy.

Where then are these structures and other possible disk-halo connections in our galaxy? What do they look like, how can we detect them, and what do they tell us about the interstellar medium and about the Galaxy?

We present a review of the observational evidence for Galactic disk-halo connections, beginning with large scale searches and then concentrating on the characteristics of selected candidates. We summarize how modeling these structures can provide information on the structure of the interstellar medium in which they evolved, focusing on the W4 superbubble and the Anchor as illustrations.

The Interaction of the Disk with the Halo

Mordecai-Mark Mac Low (Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany)

The high temperatures and ionization fractions of the gaseous haloes of star-forming galaxies must be explained by energetic processes occurring in the disks of the galaxies. How the energy is transferred from the disk to the halo will largely determine the structure and extent of the halo.

Most explanations of the properties of the halo rely on massive stars in one way or another. Stellar winds and supernovae drive shock waves into the ISM in the disk that can create superbubbles large enough to blow out of the disk into the halo. Numerical models of these blowouts reveal that rather little mass is lifted out of the disk by superbubbles (typically only a few percent of the total mass swept up by the shell), although that may still be enough mass to dominate the low-density halo.

However, radiation from massive stars that would normally be absorbed by dust in the galactic disk can escape out into the halo through the holes carved by superbubbles. This radiation will ionize the halo and maybe even drive gas motions, perhaps through radiative pressure on dust grains at high altitudes. The superbubble chimneys might also provide an avenue out of the disk for metals produced by the SNe of the massive stars in the plane, allowing metal redistribution, or, particularly for starburst galaxies, even metal ejection.

Understanding the overall structure of a halo controlled by massive stars requires understanding not just the structure of isolated superbubbles in a smoothly stratified background medium, but also the results of their interactions. Numerical models have started to appear that address this, though the challenge of including all the relevant physics (MHD, radiation, heating and cooling, cosmic rays) remains a daunting one. However, such models are required to understand the actual structure of the chimneys that allow the existence of the halo. They will also tell us a great deal about the structure of the ISM in the disk, naturally.

I will also briefly consider models of the galactic halo that rely primarily on magnetic field generation by gas motions in the disk followed by reconnection heating in the halo or even ejection of mass along open field lines in a galactic wind. In these models, the energy would primarily be carried by magnetic field advection or MHD waves, although the source of energy for both field generation in the disk and for transport of field into the halo may remain the massive stars in the disk. Alternatively, the shear from galactic rotation could play an important role at least in field generation.

OH 1720 MHz masers---the best indicators of C-shocks and maser saturation

Moshe Elitzur (Code 661, NASA/GSFC, Greenbelt, MD 20771 and Department of Physics & Astronomy, University of Kentucky, Lexington, KY 40506)

The OH ground state masers have long been important sources of information about the regions in which they are formed. The main lines at 1665 and 1667 MHz and the satellite line at 1612 MHz have been widely observed in star forming regions and the envelopes of late-type stars. Until recently, the satellite line at 1720 MHz has been the least commonly observed and has received the least attention. However, recent observations show that this maser is a powerful probe of the shocked region where a supernova remnant strikes a molecular cloud. Tight pumping constraints imply that this maser requires moderate temperatures (50 -- 125 K) and densities (about 105 cm-3), and OH column densities of order 1016 cm-2. These conditions can exist only if the shocks are of C-type. J-shocks fail by such a wide margin that this maser could become the most powerful indicator of C-shocks.

VLA observations of the these masers reveal significant circular polarization (upward of 20%), and the right- and left-hand components coincide on the sky, as expected from the Zeeman effect. Furthermore, the spectral shape of the Stokes parameter V follows an antisymmetric S-curve with sharp reversal at line center, the typical profile for Zeeman shift much smaller than the Doppler linewidth. The high-quality of this data allows detailed profile comparison of the Stokes parameters V and I, which proves to be a powerful new tool for maser data analysis. This analysis provides the first direct methods for unambiguous determination of (1) the maser saturation stage, (2) the amplification optical depth and intrinsic Doppler width of unsaturated masers, and (3) the comparative magnitudes of Zeeman splitting and Doppler linewidth. The circular polarization detected in OH 1720 MHz emission appears to provide the first direct evidence for maser saturation.

Outflows from the Orion Nebula

C. Robert O'Dell (MS-108, Rice University, P.O. Box 1892, Houston, TX 77251, USA)

The Orion Nebula is the closest region of star formation that includes massive stars. The presence of these hot stars fundamentally alters the nature of the interaction of recently formed stars with the placental interstellar material and may affect the presence of the circumstellar disks necessary for star and planet formation. The jet type outflow from the young low mass stars interacts with ambient ionized material, producing very different physics than observed in ordinary Herbig Haro objects.



Large-scale model of the interstellar medium, with new constraints on the hot gas component

K.M. Ferrière (Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France)

We review the currently available observational data on the properties and spatial distribution of the various constituents of the interstellar medium, which include the five components of the interstellar gas (molecular, cold atomic, warm atomic, warm ionized, and hot ionized), cosmic rays, magnetic fields, gravitating matter, and supernovae (both isolated supernovae which produce individual supernova remnants, and clustered supernovae which, together with the wind from their progenitor star, are responsible for the formation of superbubbles).

Based on these data, we construct a large-scale axi-symmetric model of the interstellar medium, providing analytical expressions for the mass density and thermal pressure ot the five gas components, for the cosmic-ray and magnetic pressures, for the gravitational acceleration, for the supernova and superbubble rates per unit volume, and for the superbubble luminosity function, all as a function of Galactic radius and height.

The hot gas parameters, which are poorly constrained observationally, are adjusted by requiring that the interstellar medium be in hydrostatic balance at midplane and by placing reasonable bounds on the radial dependence of the hot gas column density and scale height.

More specifically, the hot gas is assumed to be exponentially distributed along the vertical, so that the two parameters characterizing its vertical distribution are the midplane space-averaged density, n0(R), and the exponential scale height, H(R).

At the solar circle, the midplane space-averaged density is taken to be the product of the true density deduced from local measurements by the local filling factor computed numerically in the second part of this study, while the exponential scale height is estimated on the basis of UV observations of highly ionized species such as O VI and N V. With the values thus obtained, n(Rsun) = 5 × 10-4 cm-3 and H(Rsun) = 1.5 kpc, the interstellar medium at midplane is automatically in hydrostatic equilibrium to within less than 5 %.

Away from the solar circle, neither the midplane space-averaged density nor the exponential scale height can be inferred from observations, but two constraining relations between these parameters can be obtained on theoretical grounds. The first relation is provided by the condition of hydrostatic balance at midplane. The second relation expresses the fact that the hot gas column density through the Galactic disk falls off with radius at the same rate as the total supernova rate per unit area, sigmaSN(R): a steeper rate would be inconsistent with our physical expectation that supernovae are more efficient at producing hot gas at larger radii, whereas a gentler rate would lead to an exponential scale height increasing too rapidly outward (faster than R1.6).

Hence, the above considerations bring us to adopt

H(R) = (1.5 kpc) [ R / Rsun ]1.6


n0(R) = (5 × 10-4 cm-3) [sigmaSN(R) / sigmaSN(Rsun)] [R / Rsun ]-1.6

In a second step, we use our model interstellar medium to calculate the hot gas filling factor in the Galaxy. The calculation proceeds from the notion that the hot gas is generated by supernova explosions and by the associated stellar winds, and it is performed with the help of a numerical code designed to follow the expansion and contraction phases of individual supernova remnants and superbubbles evolving in the model interstellar medium.

Globally, the hot gas is found to occupy a rather small fraction of the interstellar volume. At the solar circle, this fraction is <= 20% at low altitude and it drops off gradually above the dense gas layer. The outer Galaxy remains largely devoid of hot gas, whereas the inner Galaxy may contain a large-scale region (around R ~ 6.5 kpc, |Z| ~ 250 pc) entirely filled with hot gas.

Interstellar turbulence and cloud formation

Enrique Vázquez-Semadeni (Instituto de Astronomía, UNAM, Apdo. Postal 70-264, México, D.F. 04510, Mexico)

I discuss HD and MHD compressible turbulence as a cloud-forming and cloud-structuring mechanism in the ISM. Results from a numerical model of the turbulent ISM at large scales suggest that the phase-like appearance of the medium, the typical values of the densities and magnetic field strengths in the intercloud medium, as well as the velocity dispersion-size scaling relation in clouds may be interpreted as consequences of the interstellar turbulence. However, the density-size relation appears to only hold for the densest clouds, suggesting that low-column density clouds, which are hardest to observe, may be turbulent transients.

I then explore the production of density fluctuations in highly compressible polytropic turbulence, in one and several dimensions. At low values of the polytropic exponent gamma, turbulence may induce the gravitational collapse of otherwise linearly stable clouds. Additionally, the nature and statistics of the density fluctuations in the high Mach-number limit depend on gamma.

Finally, I discuss some implications of interpreting clouds as turbulent density fluctuations, such as the role of pressure confinement, the identification of velocity features with actual clumps, and the possibility of forming quasi-hydrostatic structures out of turbulent fluctuations.

A simple approach to ISM turbulence

Itzhak Goldman (School of Physics and Astronomy, Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel)

A model for turbulence based on simple physical assumptions regarding the non-linear eddy interactions is presented. For a given rate of energy input from the source that generates the turbulence, It yields the full turbulent energy spectrum. The model is computationally much simpler than numerical simulations, and can provide a qualitative understanding of the dependence of the turbulence on the physical parameters of the problem. Moreover, it is capable of handling the very high Reynolds and very low Prandtl numbers that characterize turbulent astrophysical flows, and can be quite naturally extended to study turbulence in the presence of fast rotation. Some applications regarding the ISM and in particular molecular clouds are mentioned.

Supernova driven interstellar turbulence

M.J. Korpi, I. Tuominen (Astronomy Division, University of Oulu, P.O. BOX 333, 90571 Oulu, Finland)

A. Brandenburg, A. Shukurov (Mathematics Department, University of Newcastle, NE1 7RU, Newcastle Upon Tyne, UK)

We investigate supernova driven interstellar turbulence using local three-dimensional MHD simulations and adopting conditions corresponding to the Galaxy. Our model includes the effects of large-scale shear due to galactic differential rotation, density stratification, compressibility, magnetic fields, heating via supernova explosions and parameterized radiative cooling of the interstellar medium. We allow for multiple supernova explosions distributed randomly in the galactic disc and exponentially in the vertical direction. We discuss the general properties and dynamics of supernova driven interstellar turbulence as well as the long-term evolution of the galactic disk in the presence of supernovae.

MHD Turbulence and Scintillation

J.L. Maron, P.M. Goldreich and M. Stone (California Institute of Technology, USA)

Interstellar electron density fluctuations constitute a refractively distorting medium which is responsible for the scintillation and angular broadening of compact radio sources. The structure of the density fluctuations originates from a turbulent MHD cascade, where they play the role of a passive scalar. Any connection between scintillation observations and the turbulent cascade must therefore include the electron density.

Small scale MHD turbulence, down to the diffusion scale at ~ 107 cm, makes the dominant contribution to scintillation effects. This regime is characterized as an Alfven wave cascade superimposed on a strong uniform magnetic field. Goldreich and Sridhar theorized that this turbulence is anisotropic with the wavevectors clustered around the plane transverse to the magnetic field, having a transverse power spectrum with the Kolmogorov index. Our simulations confirm this and additionally show dramatic coherent structure which does not manifest itself in the power spectrum.

With a simulational understanding of the MHD cascade in hand, we make the connection to observation by generating artificial scintillation data from our simulated electron densities. With this we address a range of effects such as time variability, caustics, and anisotropy. For reference, we also show results based on electron density data generated from random-phase power spectra. Comparing the results from these two analyses brings out the role of the coherent turbulent structure.

Turbulent stellar winds feeding the ISM

Anthony F.J. Moffat (Département de physique, Univ. de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, QC, H3C 3J7)

Recent spectroscopic evidence shows that the strong winds of Wolf-Rayet stars are highly inhomogeneous on small scales. In fact, it is quite likely that all hot-star winds are clumped. Scaling laws among the clumps prevail, possibly like those seen in the ISM. The implication is that hot-star winds exhibit the signatures of supersonic, compressible turbulence, the driver being ultimately radiation pressure, as opposed to gravity/magnetic fields in the ISM. However, unlike in the ISM, we see clumps forming and dissipating in real time. The consequences of clumping in hot-star winds are manifold, including their impact on the ISM itself.

HST/WFPC2 H-alpha imagery of the nebula M1-67: interface between clumpy stellar winds and the clumpy ISM

Yves Grosdidier, Anthony F.J. Moffat (Université de Montréal, Département de Physique, C.P. 6128, Succursale Centre-Ville, Montréal (Québec), Canada, H3C 3J7)

Gilles Joncas (Université Laval, Département de Physique, Pavillon Alexandre-Vachon, Sainte-Foy (Québec), Canada, G1K 7P4)

Agnès Acker (Observatoire Astronomique de Strasbourg, UMR 7550, 11 rue de l'Université, F-67000 Strasbourg, France)

We present Hubble Space Telescope, WFPC2/H-alpha deep images of the ejected nebula M1-67. Our data suggest that M1-67 is the imprint of a previously slow Luminous Blue Variable (LBV) wind ejected from the central star WR124, which now exhibits a WR (WN8) spectrum. We find evidence for a highly variable and anisotropic LBV wind. Some dense, persisting clumps have been possibly ejected directly from the stellar surface. Finally, we have used wavelets to isolate stochastic structures of different characteristic size and look for scaling laws. We draw attention to a short inertial range at the smallest scales, which can be attributed to turbulence in the nebula, and a strong scale break at larger scales. The study of the structure functions of different orders shows that the turbulent regime is intermittent.

Compact, isolated high-velocity clouds: CHVCs

W.B. Burton (Leiden Observatory, P.O. Box 9513, Leiden, 2300 RA, The Netherlands)

R. Braun (Netherlands Foundation for Research in Astronomy, P.O. Box 2, Dwingeloo, 7990 AA, The Netherlands)

We have identified a class of high-velocity clouds which are compact and apparently isolated. The clouds are compact in that they are either point sources or unresolved in any detail in an antenna beam of angular resolution 0.5°; they are isolated in that they are separated from neighboring emission by expanses where no emission is seen to the detection limit of the available data. Candidates for inclusion in this class were extracted from the Leiden/Dwingeloo HI survey of Hartmann & Burton (1997) and from the Wakker & van Woerden (1991) catalogue of high--velocity clouds identified in the surveys of Hulsbosch & Wakker (1988) and of Bajaja et al. (1985). The candidates were subject to independent confirmation using either the 25-meter telescope in Dwingeloo or the 140-foot telescope in Green Bank. We argue that the resulting list, even if incomplete, is sufficiently representative of the ensemble of compact, isolated HVCs---CHVCs---that the characteristics of their disposition on the sky, and of their kinematics, are revealing of some physical aspects of the class. The sample may represent a more singular situation than would a sample which included any of the major HVC complexes. There are several indications that these objects are not confined to the Milky Way gaseous disk or lower halo. We consider the deployment of the ensemble of CHVCs in terms used by others to ascertain membership in the Local Group, and show that the positional and kinematic characteristics of the compact HVCs are similar in many regards to those of the Local Group galaxies. The velocity dispersion of the ensemble is minimized when the kinematics are considered in terms of the Local Group reference frame. If the CHVCs are deployed throughout the Local Group, then their average net velocity with respect to the mean of the Local Group galaxies implies a more rapid infall towards the barycenter than is the case for the galaxies.



The Structure of the Atomic Gas

Gilles Joncas (Dépt. de physique, Université Laval and Observatoire du mont Mégantic)

The great majority of HI feature studies are targeted observations of supernova remnants, stars with strong stellar winds or star forming regions. There have been very few attempts at searching HI surveys for new types of objects. The work of Heiles (1979) is an example of such an attempt. His investigation brought the listing of Galactic HI shells and the study of "worms", walls of broken through superbubbles (Koo, Heiles and Reach 1992). Often features of a given morphology (e.g. filaments, Tamanaka 1994) are looked for using spatial filtering. Such approaches are biased and may filter out important information. This presentation will describe first steps toward establishing a procedure for the taxonomy of HI features in the Galactic plane without any type of filtering. The aim is the classification within families characterized by the physical origin of the feature (e.g. dissociation of molecular gas). The HI gas features are characterized 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 of the brightness temperature distribution of each cloud is dependant on the physical process that created it. Our study starts with an exploratory analysis based on Principal Components Analysis and on Cluster Analysis. As 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. We rely first on known types as listed above but we might end up with new classifications. Ultimately it is expected that such study will bring quantitative information about the energetics of our Galaxy by giving the proportion of features related to different energy input phenomena (e.g. stellar winds). As a side effect new types of HI features may be found which may be links between the diffuse HI (recombined HII gas?) and molecular clouds. The long term goal is to establish an evolutionary scenario where the diffuse HI gas is transformed into star forming molecular clouds and where the energy transfer and dissipation is integrated quantitatively.

The large-scale structure of the outer-Galaxy HI-layer

Tom Voskes (Leiden University, The Netherlands)

Using the new Leiden/Dwingeloo all sky survey as a main part in a complete datacube, the structure of the HI-layer of the outer-Galaxy was displayed. As a controlled input medium we made a simulation of the same complete l,b,v-datacube taking some 12 parameters to model a fixed Galaxy of known dimensions. Varying this artificial input we were able to check our method of converting the datacube as well as putting constraints on the parameters needed. We wrote a parameter-checking program to use as a quantitative control mechanism on both the real galaxy-data and the simulated one. This approach enabled us to get the most objective look so far possible on elements as the line of nodes, central density of the warp and so on. Furthermore we used it to mask out HVC and IVC clouds disturbing the converting technique and to implement any possible lobsidedness of the Galaxy. We raised some questions about the importance of velocity dispersion as a fallacy in determining the outward extent of the HI-gas.

VLBA Imaging of Small Scale Structure in Galactic HI

M.D. Faison, W.M. Goss, P.J. Diamond, and G.B. Taylor (NRAO, PO Box 0, Socorro, NM 87801, USA)

We have used the Very Long Baseline Array (VLBA) with the phased Very Large Array (VLA) to image the Galactic HI in absorption in the direction of three bright extragalactic objects: 3C 138, QSO 2255+416, and CJ1 0404+768. The angular resolution of these images is 20 milliarcseconds (mas) in the case of 3C 138 and 10 mas for 2255+416 and 0404+768. HI optical depth variations up to a factor of two are observed in the directions of 3C 138 and 2255+416 on angular scales larger than 10 to 20 mas. The source 0404+768 shows no significant optical depth variations from 10 mas up to the angular size of the source, about 150 mas. This is the first source towards which significant small scale HI opacity variations are not observed, possibly indicating an inner scale of a few AU for the cold HI small scale structure. The small scale opacity structures seen towards 3C 138 and 2255+416 could be due to density variations, spin temperature variations, velocity turbulence in the atomic gas, or a combination of these effects. If the variations are due to density fluctuations, the data suggest high density structures (~ 106 cm-3) in the cold neutral medium on physical scales <10 AU. However, if the large column density variations are due to elongated structures such as filaments or sheets observed edge on, the densities (and pressures) can be reduced by invoking an elongation factor along the line of sight. Measurements of Stokes V in the HI absorbing gas show no indication of circular polarization resulting in 2 sigma upper limits for the line of sight magnetic fields of 40 µGauss for 3C 138, 100 µGauss for 2255+416, and 800 µGauss for 0404+768. These results as well as future work will be discussed.

Neutral Hydrogen Self-Absorption Features in the Canadian Galactic Plane Survey

S.J. Gibson, A.R. Taylor (Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada)

P.E. Dewdney, L.A. Higgs (Dominion Radio Astrophysical Observatory, Box 248, Penticton, BC, V2A 6K3, Canada)

W.H. McCutcheon (Department of Physics and Astronomy, University of British Columbia, 5224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada)

H.J. Wendker (Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany)

Neutral Hydrogen 21 cm line emission is widely used to chart the warm and cool diffuse components of the interstellar medium. Less studied is the colder constituent revealed by HI self-absorption (HISA) against background HI emission, though where detected, it has often shown a significant correlation with dense clouds of molecular gas and dust. While some evidence suggests HISA may be a more sensitive tracer of cold gas than CO emission, a detailed examination of the relations between these and other ISM constituents is only possible if several conditions are met. First, a multiwavelength dataset showing all relevant ISM phases at a common angular resolution is essential. Second, this resolution must allow HI spectra to be taken close to HISA feature edges, since measuring the properties of the absorbing material depends upon accurate knowledge of the background emission. Finally, a large area should be used to ensure an unbiased sample, rather than, e.g., a preselected set of known molecular clouds. All of these criteria are met by the Canadian Galactic Plane Survey, which is mapping the major phases of the ISM at arcminute resolution over hundreds of square degrees.

Although our investigation of HISA in the CGPS dataset has just begun, we have already found a wealth of remarkable features in the Perseus Arm and in local gas which will require careful study. Some of these have clear counterparts in 12CO and far-IR dust emission, while others are only visible in 21 cm absorption, often at extremely low contrast. All display a complex, multiscale structure of interconnected filaments, knots, and sheets. We present examples of various features along with the fruits of our current efforts to analyze their physical nature.

The Kinematic and Spatial Structure of the Molecular Gas Component

M.H. Heyer and C. Brunt (FCRAO & Dept. of Physics and Astronomy, Lederle Research Building, University of Massachusetts, Amherst, MA, USA, 01003)

Panoramic images of the molecular ISM provide valuable information to cloud structure and interstellar turbulence. The outer Galaxy is particularly well suited for such studies due to the absence of velocity crowding and the ability to isolate molecular features at low column densities. With suitable descriptions to the observed kinematic and spatial emission variability, one can constrain both phenomenological models and increasingly more sophisticated hydrodynamic simulations of turbulent flows.

I will briefly describe the multivariate tool of Principle Component Analysis (PCA) to decompose a data cube onto a basis set of orthogonal axes. From PCA, a size line-width relationship is derived which describes the magnitude of velocity differences between line profiles and the spatial scales over which these differences occur. If such profile differences are due to fluctuations of the turbulent velocity field, then the derived size line-width law provides a diagnostic to turbulent flows within a given cloud. A recent calibration of this technique with simulated velocity fields demonstrates that the size line-width relationship derived from PCA can be directly related to the energy spectrum, E(k), of the 3 dimensional velocity field. Several hydrodynamic simulations have shown that the energy spectrum varies as kinetic energy is dissipated over time. Thus, the decomposition of the data cube by PCA and the resultant size line-width description offers an observational means to classify the molecular interstellar medium according to cloud dynamics.

PCA is applied to targeted regions of the FCRAO CO Survey of the Outer Galaxy. In general, the inferred energy spectra for giant molecular cloud regions (W3, NGC 7538, Sh 152) are steeper than those derived for more diffuse fields and those expected from Kolmogorov turbulent flow.

To investigate the spatial variability of the CO emission and its relationship to turbulence, we have applied multifractal descriptions to velocity integrated images. This formalism which includes both generalized structure functions and singular measures provides a more complete accounting of the spatial structure than the more conventional power spectrum and autocorrelation function. Using phenomenological density fields, we calibrate these measures to constrain the 3 dimensional statistical properties of the density field. Recent hydrodynamic simulations suggest that there is a link between the variance of the log density histogram and the Mach number of the cloud. These measures provide a secondary means, in addition to PCA, to classify molecular regions according to the internal dynamics.

The Tokyo-Nobeyama CO J=2-1 Galactic Plane Survey

Seiichi Sakamoto (Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305, Japan)

Tetsuo Hasegawa, Toshihiro Handa (Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181-8588, Japan)

Jun-Ichi Morino (Department of Astronomy, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan)

Tomoharu Oka (The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan)

Kumiko S. Usuda, and Masahiko Hayashi (National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA)

We conducted an extensive out-of-plane CO J=2-1 survey of the Milky Way with the same angular resolution as the Columbia CO J=1-0 survey to extract physical conditions of molecular gas. The CO J=2-1/J=1-0 intensity ratio (= R2-1/1-0) shows a large-scale radial gradient from ~ 0.75 at 4 kpc to ~ 0.6 at 8 kpc in Galactocentric distance.

To interpret this gradient by population synthesis of molecular gas, we classify molecular gas into three categories in terms of R2-1/1-0 on the basis of our supplemental CO J=2-1 mapping of nearby molecular clouds and an LVG calculation. Very high ratio gas (VHRG; R2-1/1-0 > 1.0) is either dense, warm, and optically thin gas or externally heated, dense gas. High ratio gas (HRG; R2-1/1-0 = 0.7--1.0) is warm and dense gas, and is often observed in central regions of GMCs. Low ratio gas (LRG; R2-1/1-0 < 0.7) is less dense or cold or both, and is often observed in dark clouds and peripheries of GMCs. The CO J=2-1 emission is better characterized as a probe of density rather than that of temperature for molecular gas warmer than 10 K.

The observed large-scale radial decrease in R2-1/1-0 is ascribed to the fractional decrease of HRG and VHRG from ~ 40% near 5 kpc to ~ 20% near the solar circle. The HRG and VHRG are found predominantly along the Sagittarius and Scutum arms, probably in their downstream. This fact indicates that physical conditions of interstellar gas, density in particular, are affected by grand-design, nonlinear processes, such as gravitational collapse induced by spiral density waves.

A CO(2-1) Survey of the Galactic Plane with the 60-cm telescope to explore density and temperature of molecular gas

T. Handa et al. (Institute of Astronomy, University of Tokyo)

The density and temperature of molecular clouds are basic parameters to know their physical states. The line intensity ratio of CO(2-1) to CO(1-0) is an indicator of density and temperature of molecular gas. To explore these parameters through the Galaxy we are making a large CO(2-1) survey using the specially designed twin 60-cm telescope, VST1 and VST2, which were installed at Nobeyama, Japan, and La Silla, Chile, respectively. The beamsize of the survey is 9-arcmin which is the same as that of the Columbia survey in CO(1-0) and its southern counterpart. Due to the same beamsizes we can directly derive the line intensity ratio with minimum ambiguity. Until the last season we have observed the region of 300° < l < 60° (through l=0°), -0.5° < b < +0.5°, with 0.25° spacing, and a few other regions including the Galactic center. The distributions of the intensity ratio were derived using a kinematic model of the Galaxy. The ratio gives the temperature and density of the molecular gas averaged over the beam area using a single-zone radiative transfer model and our own results of several well-known nearby molecular clouds.

The northern hemisphere survey with VST1 is a collaboration of University of Tokyo and Nobeyama Radio Observatory; the southern hemisphere survey with VST2 is an international collaboration of University of Tokyo, Onsala Space Observatory, European Southern Observatory, and University of Chile. The CO(1-0) data from the north hemisphere are provided T.M. Dame and P. Thaddeus (Harvard-Smithsonian Center for Astrophysics).

Atomic gas streaming from a molecular cloud in the Perseus Arm?

L.B.G. Knee and B. Wallace (National Research Council Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, P.O. Box 248, Penticton BC, Canada, V2A 6K3)

We have begun a project within the Canadian Galactic Plane Survey to study anisotropic flows of HI associated with molecular clouds (see the poster contribution by Wallace & Knee in this workshop). The first object to be studied in detail by us is a peculiar anisotropic and elongated HI structure which appears to emanate from an anonymous molecular cloud in the Perseus Arm. Associated with it are two narrow and long HI streamers. The molecular cloud (which we designate G142.15+3.35) has a size of ~= 10 × 20 pc, a mass of ~= 1200 Msun, and lies some 150 pc above the Galactic Plane. This molecular cloud is located at the apex of the streamers seen in HI, is elongated along the axis defined by the HI streamers, and displays a radial velocity gradient along the same axis. The HI structure, when the streamers are included, is nearly 2° in length (a projected length of 80 pc at the assumed distance of the molecular cloud of ~= 2.5 kpc). The HI also shows radial velocity gradients along the axis. We propose that the morphological and kinematic propeties of the molecular and neutral atomic gas can best be explained in terms of the photo-dissociation and dispersal of the molecular cloud by the winds of young stars which formed in its vicinity. The streamers may either represent the limb-brightened edges of a hollow cone-like structure streaming away from the molecular cloud, or as local enhancements of a less highly structured HI flow created, for example, by ablation from compact molecular gas clumps caught up in the general flow away from G142.15+3.35. The HI is flowing away from G142.15+3.35 at several km s-1 and is at least comparable in mass to the molecular gas, suggesting that the cloud is in an advanced stage of its destruction and dispersal into the atomic medium. We see no evidence of radio continuum or optical emission from HII, which suggests that the exciting stars (as yet unidentified) are probably less massive than early B-type stars. Very young (protostellar) objects can drive large collimated flows, but the largest examples known at present are an order of magnitude shorter in length than the HI filaments.



What are some of the possible mechanisms of energy dissipation and redistribution among scales in the turbulent Interstellar Medium?

A. Pouquet, S. Galtier and H. Politano (Observatoire de la Côte d'Azur, Laboratoire Cassini, C.N.R.S. U.M.R. 6529, B.P. 4229, Nice Cedex 04, France, F06304)

The interstellar medium, at large, is observed to be both supersonic and sub-Alfvénic. In the presence of magnetic shocks, sources of energy are necessary to regenerate the flow on long time scales; such sources can be the galactic shear at the kiloparsec scale, as well as ionization winds, cosmic ray input and supernovae blast waves that originate at small scales but permeate the whole medium by their expansion. A discussion of these effects, and of their cohabitation, is presented in the framework of turbulence modeling, both phenomenological and numerical (see e.g. E. Vázquez-Semadeni, T. Passot & A. Pouquet 1996; ApJ, 473, 881). The possible competition between numerous dissipative mechanisms, both physical and numerical, is analyzed in the framework of simple one-dimensional models of compressible flows such as the Burgers equations and several of its variants for conducting fluids.

An explicit example of a sizable dissipation of magnetic energy for high magnetic Reynolds numbers will be given in the possibly somewhat simpler context of the solar corona, and the problem of its being heated to several millions degrees Kelvin (S. Galtier & A. Pouquet 1998; Solar Phys., 179, 141). A similar---though less extreme---example can be found in the presence of the Hall term in the generalized Ohm's law, leading to the formation of emission knots in proto-stellar jets (F. Bacciotti, C. Chiuderi & A. Pouquet 1997; ApJ, 478, 594).

In a magnetized compressible flow, there are many energetic exchanges between acoustic modes and shocks, vortices and magnetic structures, both flux tubes and current sheets. The temporal dynamics of such flows in the absence of driving has been studied by several authors who conclude to obtaining a decay which is slightly slower than for neutral fluids, with a power-law dependence EW(t) ~ (t-t*)-alpha; alpha ~ 0.9, and t* is a characteristic time of the onset of dissipation, i.e. of the formation of small scales (EW is the wave energy). An analysis is presented which recovers this behavior (S. Galtier, H. Politano & A. Pouquet 1997, Phys. Rev. Lett., 79, 2807) in the context of the so-called Iroshnikov-Kraichnan phenomenology. The relative importance of the nonlinear terms in the non-conducting case---stemming from an analysis of numerical simulations at low resolution---will be given as well (S. Kida & S. Orszag 1990, J. Scient. Comput., 5, 85).

Dissipative events are both sporadic in time and sparse in space. The role played by such intermittent behavior is important: chemistry can be activated locally when on average the temperature is insufficient (E. Falgarone 1995, Lecture Notes in Physics, 462, 377, Springer-Verlag). Intermittency can be quantified through the anomalous scaling exponents of the velocity and magnetic field structure functions. Measurement of high-order moments of the physical variables is difficult, but information is coming in from the experimental, numerical and observational sides (in the latter case, both in the solar wind and in the interstellar medium) that may be of interest as a measure of the granularity of these flows. Intermittency, measured in such a way, appears insensitive to the amount of compressibility for a neutral gas driven by shear waves (D. Porter, A. Pouquet & P. Woodward 1998, Advances in Turbulence VII, 255, U. Frisch Ed., Kluwer) but the MHD case is still open (S. Galtier, T. Gomez, H. Politano & A. Pouquet 1998, Advances in Turbulence VII, 453, op. cit.).

Finally, the inverse cascade of magnetic helicity HM = < a · b > (with b = nabla × a the magnetic induction) with excitation---here HM---being transferred to scales larger than that of the input, will be mentioned, both for incompressible and for compressible flows. This phenomenon is likely to be essential to the understanding of the nonlinear phase of the galactic dynamo, resulting in the formation of large-scale magnetic helical structures that could be observed.

In the presence of a strong quasi-uniform magnetic field that can arise from such an inverse cascade, the flow simplifies and becomes a quasi-bidimensional fluid that can be studied in the framework of the ``kinetic'' equations for weak MHD turbulence, providing us with a workable possibly useful tool for studying the complex dynamics of such flows, at least already in the simpler case of incompressible fluids.

Some of the analyses presented here have not yet reached completion. Collaborations over the years with F. Bacciotti (Firenze), D. Balsara (Urbana), S. Galtier & T. Gomez (Nice), S. Nazarenko & A. Newell (Warwick), T. Passot & H. Politano (Nice), D. Porter (Minneapolis), E. Vázquez-Semadeni (Mexico), P. Woodward (Minneapolis) & E. Zienicke (Dresden) are gratefully acknowledged. This work received partial support from a CNRS-PNPCMI grant.

Supershells formed by Supernovae and Stellar Winds of OB Associations: on the Nature of the Two-Component Kinematics

T.A. Lozinskaya (Sternberg Astronomical Institute, Moscow, Russia)

An analysis of the observations of the multishell complex in the Cyg OB1, Cyg OB3 region in the H-alpha line (Lozinskaya et al. 1998) and their comparison with the IRAS and IUE archival data have revealed patterns of behavior that are common to several well-studied supershells around OB associations. An important common property is their ``two-component kinematics'', i.e., the coexistence of bright emission at low velocities and weak high-velocity emission at the same location in the shell. We propose a possible scenario for the formation of this two-component kinematics: two sets of lines are shown to be formed at different stages and under the effect of different populations of a rich OB association. We briefly discuss the observational manifestations of this scenario in the supershells around Cyg OB1 and Cyg OB3; Ori OB1/Lambda Ori; Car OB1 and Car OB2.



Topology and Energy Injection: Review

J.M. Dickey (University of Minnesota, 116 Church St. SE Minneapolis, MN 55455, USA)

There are two approaches to describing small scale structure in the ISM, deterministic and statistical. In the first approach, we use maps and spectra to determine the physical parameters of a discrete structure, and to predict its dynamical evolution. Examples of such structures are clouds, shells and supershells, chimneys and galactic winds. From this approach we learn things like the size-linewidth relation, the shell size vs. expansion velocity relation, and the Sigma - D relation for supernova remnants.

The second approach is no less physical, though it uses statistics to describe the observed structure of the ISM and so it is non-deterministic in the way thermodynamics is. Examples of this approach include the spatial power spectrum of the density or column density, the fractal dimensionality of the medium, or various kinds of structure functions. Observations which are best understood or explained with this approach include pulsar scintillations, cosmic ray propagation, small scale magnetic structure, and the far IR - radio continuum correlation.

Merging the two approaches, specifically understanding how the physical processes we see and understand in the discrete objects give rise, in the ensemble, to the statistical quantities we measure for larger samples, is illuminated by computer simulations of the ISM. There has been much progress in recent years in explaining the structure of the ISM theoretically. But the large surveys now underway are begining to provide more precise and complete statistical measures of the ISM structure as seen in a variety of tracers, as well as providing many specific examples of discrete objects whose dynamics we can understand. This new data presents a challenge to theorists to explain the forest by the trees, i.e. to explain how the discrete processes lead in the aggregate to the overall structure which we see in the ISM.



Multiwavelength Studies of Interstellar Dust Using WIRE and MIGA

C.R. Kerton and P.G. Martin (Department of Astronomy and CITA, University of Toronto, 60 St. George Street, Toronto, ON, M5S 1A1, Canada)

In order to understand the evolution of dust in the interstellar medium it is important to use data spanning a wide range of wavelengths. Infrared data can provide information on dust temperature and composition, while radio data can provide information about gas temperature and structure, i.e. the physical condition of the medium surrounding the dust. This poster summarizes our ongoing studies of interstellar dust using the recently completed Mid-Infrared Galaxy Atlas (MIGA) and the soon to be launched Wide Field Infrared Explorer (WIRE) satellite along with complementary radio and millimetre data from the Canadian Galactic Plane Survey (CGPS). The MIGA is a mid-infrared (12 and 25 µm) atlas of the Galactic Plane between 74°WIRE is a small (30 cm aperture) IR astronomy satellite with detectors at 12 and 25 µm, providing ~20'' resolution over a 32' × 32' field of view. We are planning on using WIRE to study the structure and evolution of dust in two very different environments of the ISM. One project will look at PAHs and very small dust grains in translucent clouds (Av < 2) that have shown strong variations in their mid-infrared colours, and in high lattitude cirrus clouds. Another project will look at the interfaces between HII regions and molecular clouds. The 25 µm data should be especially interesting as this band was not available on ISO for imaging.

Resonance Paramagnetic Relaxation and Alignment of Ultrasmall Grains

A. Lazarian (Princeton University Observatory, Princeton, NJ 08544, USA)

While paramagnetic alignment was one of the first mechanisms of grain alignment to be discussed in the literature, it has been believed that ultrasmall grains rotate too fast to be subjected to paramagnetic relaxation. Indeed, it is easy to show that paramagnetic response of candidate materials is suppressed at 1010-1011 Hz, which are typical frequencies of ultrasmall grain rotation. As from the very beginning of the paramagnetic research it has been taken for granted that paramagnetic relaxation does not depend on whether a grain is rotating in magnetic field or magnetic field is rotating about the grain the conclusion about marginal alignment of rapidly rotating grains seemed self-evident. We show that the assumption above is incorrect as it disregards an important effect, namely, the Barnett magnetization. If this magnetization is accounted for, we prove that paramagnetic relaxation is being dramatically enhanced at high frequencies. In fact, we prove that the magnetization that arises from grain rotation ensures that paramagnetic absorption happens at its maximum efficiency, i.e. the conditions for paramagnetic resonance are being fulfilled. We call this effect ``resonance paramagnetic relaxation'' and discuss implications of this mechanism for the alignment of small carbonaceous and silicate grains. We also discuss the possibility of detecting this alignment at various parts of electromagnetic spectra. Our calculations prove that just the presence of free radicals in ultrasmall carbonaceous grains is sufficient to enable resonance relaxation. We study the effect of grain size on the relaxation efficiency and show that the resonance paramagnetic relaxation enables ultrasmall grains to be efficiently aligned. As these grains are likely to be responsible for the recently discovered anomalous emission, we predict that this emission should be partially polarized. We calculate the degree of its polarization as a function of frequency for various phases of interstellar medium. Our results indicate that microwave polarimetry can be a unique tool for studying magnetic fields in molecular clouds.



Constraints on the Formation and Evolution of Circumstellar Disks in Rotating Magnetized Cloud Cores

Shantanu Basu (Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON, M5S 3H8, Canada)

We use magnetic collapse models to place some constraints on the formation and angular momentum evolution of circumstellar disks which are embedded in magnetized cloud cores. Previous models have shown that the early evolution of a magnetized cloud core is governed by ambipolar diffusion and magnetic braking, and that the core takes the form of a nonequilibrium flattened envelope which ultimately collapses dynamically to form a protostar. In this paper, we focus on the inner centrifugally-supported disk, which is formed only after a central protostar exists, and grows by dynamical accretion from the flattened envelope. We estimate a centrifugal radius for the collapse of mass shells within a rotating, magnetized cloud core. The centrifugal radius of the inner disk is related to its mass through the two important parameters characterizing the background medium: the background rotation rate Omegab and the background magnetic field strength Bref. Our model predicts that a significant centrifugal disk (much larger than a stellar radius) will be present in the very early (Class 0) stage of protostellar evolution. Additionally, we derive an upper limit for the disk radius as it evolves due to internal torques, under the assumption that the star-disk system conserves its mass and angular momentum even while most of the mass is transferred to a central star.

Is the Extended Red Emission due to CO?

P.D. Bennett, G.M. Harper, A. Brown (Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA)

G.C. Clayton (Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA)

The Extended Red Emission (ERE) is a broad emission feature observed between 5000 and 8000 Å in a large number of galactic sources, including reflection nebulae, post-AGB stars, planetary nebulae, high-latitude dark clouds, HII regions, and novae, as well as in the halo of M82, and recently, in the diffuse ISM. The carrier of the ERE remains uncertain, although various large carbon molecules have been proposed: hydrogenated amorphous carbon (HAC), polycyclic aromatic hydrocarbons (PAHs), and C60, in both neutral and ionized forms. None of these identifications have been convincingly demonstrated to be correct. The strongest known source of the ERE is the bipolar nebula (the ``Red Rectangle'') associated with the peculiar post-AGB binary star HD 44179. HD 44179 is notable for spectral anomalies in addition to the strong ERE:

The formation of the a3pi--X1Sigma+ (Cameron) lines is puzzling because the observed excitation in the circumstellar environment is low. Direct radiative excitation can be ruled out because of the very weak oscillator strengths of these intercombination bands.

We propose that the upper levels of the Cameron bands are populated by a radiative cycle that allows selected rotational levels of the d3Delta, a'3Sigma+, e3Sigma-, I1Sigma- and D1Delta states to be radiatively pumped from the ground X1Sigma+ state. These excited states overlap the energy levels of the A1Pi state, which form the upper levels of the strong 4th Positive band system in the UV. Rotational levels in the overlapping d, a', e, I, and D states are populated when the energies of these levels are close to those of levels in the A1Pi state. In this case, transitions ``gain oscillator strength'' from mixing with the strongly allowed A--X transitions. These excited states decay preferentially to the metastable a3pi state of CO, which forms the upper level of the observed Cameron (a3pi--X1Sigma+) emission. Decays of these excited CO states will emit radiation, predominantly in the 5000--7500 Å region, consisting of a superposition of several fragmentary band systems. We propose that this emission from excited levels of CO is the source of the ERE.

Helical Fields and Filamentary Molecular Clouds

Jason D. Fiege and Ralph E. Pudritz (McMaster University, 1280 Main St. W., Hamilton ON, Canada, L8S 4M1)

We study the equilibrium of pressure truncated, filamentary molecular clouds with a rather general, helical magnetic field topology. We first derive a new virial equation appropriate for pressure truncated, filamentary clouds that are threaded by helical fields and supported by non-thermal motions of some sort. When compared with the data, our models predict that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds.

We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. Our models are constrained by using observational properties of filaments compiled from the literature. We examine the effects of the equation of state by comparing isothermal filaments with equilibria constructed using the pure logatrope of McLaughlin and Pudritz (1996). Both equations of state result in equilibria that agree with the observational results.

Our theoretical models involve 3 parameters; two to describe the mass loading of the toroidal and poloidal fields, and a third that specifies the external pressure. We show that by using 3 observed quantities of filamentary clouds, namely, their observed line-widths, masses per unit length, and radii, our theoretical models can be strongly constrained. We find that the best-fitting models have density profiles that vary as r-1.5 to r-1.8 in cylindrical radius. This behaviour arises entirely because of the toroidal, rather than the poloidal character, of the underlying helical magnetic field. We find that purely poloidal fields produce filaments with steep density gradients that not allowed by the observations.

We show that magnetized filaments that are initially in equilibrium are always stable in the sense of Bonnor (1956) and Ebert (1955); radial collapse is possible only if the mass per unit unit length exceeds a critical value that depends on the magnetic field. We also present our first results on the stability of our models against fragmentation into cores along the filament axis.

Protostars in the Molecular ISM: Probing their Structure with SCUBA at the JCMT

B.C. Matthews and C.D. Wilson (McMaster University, Hamilton, ON Canada L8S 4M1)

Although dense, cold molecular clouds do not dominate the volume of the ISM, it is now understood that ~ 90% of the mass of the ISM is in the form of molecular gas. These clouds are the subjects of intense study as it is within them that still denser cores form and then collapse, creating new stars. In the early 1980's, the IRAS satellite yielded striking images of the high densities of young stars hidden by dark dust inside these clouds. However, it is the submillimeter regime which is regarded as ideal for the study of protostellar candidates: sources which are actively accreting material through infall. Because these sources remain deeply embedded in their parent molecular cloud cores, they are virtually undetectable in the infra-red regime. Thus, single dish techniques favour the submillimeter, where their intensity is high and resolution is favorable to their detection. Nevertheless, until the commissioning of the SCUBA detector on the JCMT in 1997, only the brightest protostellar sources (> 5 Jy) were detectable due to limited receiver sensitivity. SCUBA now permits us to observe sources 100 times fainter than previously possible. Thus, we can now image the regions surrounding young stars rather than just detect the brightest central peaks. We will present maps of sources in several star-forming regions obtained with SCUBA at 850 and 450 µm. An understanding of the physical structure of these objects may be within observational grasp, and this will finally provide rigorous tests of theoretical models, which differ primarily in their predictions about the density structure of uncollapsed objects. Models of the singular isothermal sphere predict that uncollapsed material will obey rho ~ r-2 (Shu 1977), while the more recent logotropic model of McLaughlin & Pudritz (1996) predicts rho ~ r-1. Both predict rho ~ r-1.5 once material has collapsed.

Detailed Structure of Low-Density Molecular Gas in Giant Molecular Clouds

Seiichi Sakamoto (Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305, Japan)

Tetsuo Hasegawa (Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181-8588, Japan)

Jun-Ichi Morino (Department of Astronomy, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan)

Masahiko Hayashi, Kumiko S. Usuda (National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA)

We mapped a 18.6 pc × 3.0 pc region along the minor axis of the L1641 cloud in Orion in 12CO J=1-0 emission with a 15'' (0.03 pc) beam spaced by 34''. This is the first high-resolution imaging of peripheral regions of a GMC. We detected CO emission almost everywhere within the outermost boundary of the cloud, besides bright ridge-like emission in the middle of the cloud. At least two distinct populations of molecular gas exist: well-defined ``clumps'' (Tb ~ 25 K, Delta v ~ 1.5 km s-1), and ``extended components'' (Tb ~ 2.5 K, Delta v ~ 2.5 km s-1). The clumps dominate the CO luminosity from the central ridge region and are severely overlapped, while they are isolated in the periphery of the cloud. The extended component is widely distributed, especially in the eastern side of the cloud, and seems to be connected to the atomic gas layer in the Galactic plane. It lacks notable substructures in our spatial resolution and exhibits a surface filling factor very close to unity. The extended component has a line-of-sight velocity different from those of the clumps, and appears to be separating from the clumps in the central ridge. Since winds from the Orion OB association do not have sufficient energy to blow this component, it may be formed by interaction of the cloud with the Galactic plane.

Molecular gas density estimated from excitation analysis of 12CO J=2-1 and J=1-0 emissions is > 3x103 cm-3 in the clumps and ~ 2x102 cm-3 in the extended component. The peripheral region of the cloud is fainter in CO brightness not because the surface filling factor of emitting gas is smaller, but because the emission is dominated by the tenuous extended component with low brightness temperature. Our follow-up large-scale 12CO J=2-1 survey of nearby clouds confirms that it is the case for other molecular clouds.

Detailed Structure of High Latitude Clouds

Seiichi Sakamoto (Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305, Japan)

High-latitude clouds (HLCs) are nearby small molecular clouds of relatively low density. High-resolution observations of HLCs will provide the key to the detailed spatial and kinematical structure of molecular clouds in their early stages of evolution. It is not only because HLCs are located nearby and have relatively simple structure, but because they may maintain their initial structure and composition since dynamical and chemical evolution are slow in low-density regions. They are also expected to remain their initial velocity structure without an onset of perturbation due to internal star formation. With the above merit in mind, we made extensive (~ 1° in R.A.) and sensitive [~ 70 mK (km s-1)-1] strip-scan observations of MBM 32, MBM 54, and MBM 55 in 12CO J=1-0 emission with the 15'' (0.007 pc at 100 pc) beam of the Nobeyama 45 m telescope.

Although our observations are one-dimensional, our position-velocity diagrams kinematically resolve individual clumps in these clouds. The size of the smallest structure of low-density molecular gas traced by 12CO J=1-0 emission is ~ 0.03 pc and probably less. Clumps in these clouds tend to have larger line width compared to that expected from the line width-size relation for low-mass cores.

In addition, we discovered a number of ``pseudo-outflows'' which are subparsec-scale outflow-like velocity structures probably not powered by young stellar objects (YSOs). They are well-separated in the position-velocity diagrams from other components, and are easily identified. They have a typical size of ~ 0.05 pc and a typical velocity gradient of ~ 40 km s-1 pc-1. The small sizes and large velocity gradient suggest that these features are transient with their typical dynamical timescale of 2 × 104 yr. None of them accompanies YSO candidates (IRAS non-detection gives the luminosity of hidden YSO, if any, of < 0.06 Lsun) in their vicinity. Our near infrared follow-up observations failed to detect H2 v=1-0 S(1) emission near the corresponding positions. Considering these evidences and their narrow line widths (< 1 km s-1), it appears unlikely that these ``pseudo-outflows'' originate from strong shocks associated with YSO jets or supernova remnants. They may be weak shocks excited by collisions of clumps and the sites of turbulence decay. Further imaging of these features are expected.

Vertical Motion of Molecular and Ionized Gas in Nearby Face-on Galaxies

Seiichi Sakamoto (Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305, Japan)

Akihiko Tomita (Department of Earth and Astronomical Sciences, Wakayama University, Wakayama 640-8510, Japan)

Kinematical evidence for vertical motion including disk-halo connection of inter stellar matter (ISM) may be observed through high-dispersion spectroscopy of face-on galaxies. We carried out a high-resolution 12CO J=1-0 imaging of NGC 628 (i = 6°, D = 10 Mpc) for the vertical motion of molecular gas, and a high-dispersion scanning-slit spectroscopy of H-alpha emission in NGC 628 and NGC 3938 (i = 10°, D = 10 Mpc) for the vertical motion of ionized gas. Supplemental images in narrow-band H-alpha and J- and K' bands were also obtained.

Molecular gas.--- We observed the central ~ 4' × 4' region of NGC 628 in 12CO J=1-0 emission with a 11'' grid spacing. The 15''-resolution (680 pc at 10 Mpc) was sufficiently high to resolve spiral arms of NGC 628. We find that the velocity dispersion of molecular gas is 6.5±1.5 km s-1 throughout the inner (0.5--6 kpc) disk without any systematic radial variation. The velocity dispersion of molecular gas at 5 kpc from the center is smaller than that of atomic gas at the same distance by a factor of ~ 1.5. This factor ~ 1.5 difference naturally explains the fact that molecular disks in spiral galaxies are generally thinner than atomic gas disks by a factor of ~ 1.5. Although there is a significant point-by-point variation of the velocity dispersion, no simple correlations with CO brightness (gas surface density), K'-band brightness (stellar gravitational potential), nor H-alpha brightness (star-formation activity) are observed. There is no high-velocity (|Delta vperp| > 30 km s-1) molecular gas detected with our 5 sigma sensitivity of ~ 5 × 105 Msun per the 680 pc beam. It suggests that disk-halo connection of ISM in the form of molecular gas is a rare phenomenon.

Ionized gas.--- In contrast, high-dispersion H-alpha spectroscopy of NGC 628 and NGC 3938 revealed that there are evidences for peculiar vertical motion (|Delta vperp| > 30 km s-1) of ionized gas probably related with disk-halo connection of the ISM. The regions with the peculiar motion are associated with bright HII regions and may originate from expanding motions associated with massive star formation activities.



The DDO IVC Distance Project: First Results

T. E. Clarke, Michael D. Gladders, Christopher R. Burns, A. Attard, M. P. Casey, Devon Hamilton, Gabriela Mallén-Ornelas, J. L. Karr, Sara M. Poirier, Marcin Sawicki, L. Felipe Barrientos, and Stefan W. Mochnacki (University of Toronto, Toronto, ON, Canada)

We will present the first round of distance measurements from the David Dunlap Observatory's Intermediate Velocity Cloud (DDO IVC) survey. This ongoing project aims to measure the distances to all accessible IVCs using the interstellar absorption line method. We will discuss the survey strategy, and highlight differences between our techniques and those used previously in IVC distance studies. We will present distance estimates for approximately 6 northern IVCs (the exact number being dependent on ongoing observations). To illustrate our techniques, we will include a detailed discussion of our distance estimate for the Draco Cloud. The future of this survey work at the DDO will also be discussed.

An ATCA+Parkes 21-cm Survey of the Southern Galactic Plane

J.M. Dickey(1), A. Green(2), R. Haynes(3), and M. Wieringa(4) (1. University of Minnesota, Minneapolis, MN, USA); (2. University of Sydney, Sydney, NSW, Australia); (3. ATNF Marsfield, NSW, Australia); (4. ATNF Narrabri, NSW, Australia)

No abstract available.

A Large HI Shell Surrounding the Wolf-Rayet Star HD191765

Simon Gervais and Nicole St-Louis (Département de Physique et Observatoire du Mont Mégantic, Université de Montréal, C.P. 6128, Succ. Centre Ville, Montréal, Québec, Canada, H3C 3J7)

We surveyed a 2.6° × 2.6° field centered on the Wolf-Rayet (WR) star HD 191765 (=WR 134) in the 21 cm HI-line and in the 1420 and 408 MHz radio continuum, using the Synthesis Telescope of the DRAO in order to search for direct evidence of the interaction between the WR star wind and the interstellar medium.

An arc shaped structure is present in the 1420 MHz continuum image, which perfectly matches the optical nebula Anon (MR 100). This gas is not visible on our 408 MHz map indicating the thermal nature of the source. This ionized gas could be associated with LBV or RSG ejectae swept-up by the WR wind and photoionized by the strong UV flux from the star. However, this suggestion needs to be confirmed by clear CNO enrichment measurements in the north-west region of the nebula.

Our HI maps reveal the existance of a nearly complete shell at a velocity of vsys = -11.44 km s-1 (Rs ~= 20.9 pc, vexp ~= 9.9 km s-1, MHI ~= 1830 Msun) which, mainly for morphological reasons, we associate with HD 191765. The dynamical age of this bubble (~ 1.3 × 106 yr) suggests that it was mainly blown during the main sequence progenitor O-star phase. The kinematic distance corresponding to a LSR velocity of -11 km s-1 is approximately 5 kpc which is incompatible with the cluster distance of HD191765 of 2.1 kpc. Therefore, we conclude that the HI bubble we have detected was blown in gas already in movement with respect to normal Galactic rotation for this region. There is independant evidence that gas at this velocity is present in this line of sight.

Recent Hipparcos data analyzed by Moffat et al. (1997) give a supersonic proper motion for HD 191765 (~ 52 km s-1 at -38° in the sky). This velocity vector points back to a position very close to the center of the HI bubble which strengthens the hypothesis of a physical link with the WR star. The supersonic velocity of the star with respect to the interstellar medium might have generated a bow shock in the south-west region rendering the H-alpha gas brighter in that direction with respect to the surroundings.

Neutral Hydrogen at Intermediate Velocities

V. de Heij (Leiden Observatory, POBox 9513, 2300 RA, Leiden, Netherlands)

Looking up to the Galactic North pole (b~=90°), one expects to observe simple HI 21-cm spectra, produced by cold, neutral hydrogen in the galactic halo. The thermal and turbulent motions of the gas, which oppose the gravitational contraction of the halo, give rise to Gaussian like spectra, centered around 0 km s-1 and having a width of 5 to 15 km s-1. After the detection of the high velocity gas in 1963, a new component had to be added to the globally stationary halo. This component consists of the high velocity clouds, which impact into the galactic disc with velocities of some hundreds km s-1. But even in the low and intermediate velocity regime (|Vlsr| <= 90 km s-1), there are deviations from the stationary halo-model: the peak in the spectrum can be located at a negative velocity and in some directions there are additional bumps around -40 km s-1 and around 15 km s-1.

Using the Leiden-Dwingeloo HI-survey, we map these deviations. The spectra of a small part of the northern hemisphere (latitude >= 75°), are fitted by Gaussian components. According to the peak velocity these Gaussian components can be divided into five groups : high-negative-velocity gas, intermediate-negative-velocity gas (bump around -40 km s-1), low-velocity gas (``normal gas'') and intermediate-positive-velocity gas (bump around 15 km s-1). Using these Gaussian components one can unravel HI-spectra and produce maps of the different groups, which are shown by the poster.

Using Numerical Techniques to Gain Insight into the Structure of Superbubbles

Philip T. Komljenovic(1,2), Shantanu Basu(2) & Doug Johnstone(2) (1. Department of Physics and Astronomy, York University, 4700 Keele St., Toronto, ON, Canada M3J 1P3) (2. Canadian Institute for Theoretical Astrophysics, 60 Saint George St., Toronto, ON, Canada, M5S 3H8)

Results from the Pilot project of the arcminute resolution Canadian Galactic Plane Survey (CGPS) show evidence of a large cavity in HI above the W4 HII region. Further investigations of this region in H-alpha reveal a shell of HII, which is interpreted as the ionized inner shell of a Superbubble blown by stellar winds. These results have motivated us to study the structure and evolution of Superbubbles. We have been using various stellar wind models ( Kompaneets Model, Thinshell Approximation, and Numerical Hydrodynamics ) to help us understand the structure of Superbubbles and, in particular, gain insight about the W4 Superbubble.

The various models are used to track the evolution of Superbubbles in different density distributions. The Kompaneets Model describes the propagation of a strong adiabatic shock wave, and the Thinshell Approximation takes into account the forces acting on the shell of mass. Numerical hydrodynamic (and magnetohydrodynamic) calculations were computed using ZEUS-2D. By comparing these models, we gain useful information about the structure of Superbubbles.

The HI Cloud at (l,b) = (89.8,3.8)---the Cylon

D. Routledge (Dept. of Electrical Engineering, University of Alberta, Edmonton, AB, Canada, T6G 2G7)

B.J. Wallace (NRC Canada, HIA, DRAO, Box 248, Penticton BC, Canada, V2A 6K3)

A ``remarkable HI cloud'' was discovered serendipitously in 1989 in the field of HB21 by Roger, Landecker, and Tatematsu (JRASC 83, 318, 1989). It was described by Roger et al. as a linear HI cloud, nearly perpendicular to the Galactic Plane, with a progressive shift in velocity of about 15 km s-1 along its length (roughly 30 arcmin). The mean velocity was -106 km s-1. These authors suggested that it resembled an outflow source, but a search for 12CO with the Nagoya telescope in the range -150 to -50 km s-1 detected no emission. The kinematic distance corresponding to -106 km s-1 would be about 14 kpc.

We report here the discovery in the CGPS data of a possible positive-velocity counterpart to the negative-velocity HI. The positive-velocity HI, near +19 km s-1, has parallel linear structure, but is much fainter. The systemic velocity of the negative-velocity HI and positive-velocity HI taken together may therefore be about -44 km s-1. Faint linear structure is in fact visible near -45 km s-1, and it may be related to this phenomenon. Adopting -45 km s-1 as the systemic velocity gives a kinematic distance of about 6.5 kpc.

IRAS 20512+5042 is seen very close to the north end of the negative-velocity linear HI feature and IRAS 20527+5026 is seen very close to the junction of the negative-velocity and positive-velocity linear HI features. A model of the phenomenon is suggested.

HI streamers and cones: Signatures of molecular cloud destruction?

B.J. Wallace and L.B.G. Knee (National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, Box 248, Penticton BC, Canada, V2A 6K3)

Inspection of HI channel maps having ~ 1' resolution reveal numerous linear and cone-like structures which are abruptly truncated at one end. These structures are typically a degree or more in length, by 30--45' in width. Preliminary investigations of some of these structures suggest that they may form a new class of phenomena: anisotropic flows of atomic material streaming from molecular clouds. An in-depth study of one of these objects (see the talk by Knee & Wallace, these proceedings) suggests that the exciting objects are probably newly-formed stars. If confirmed, this hypothesis suggests that these anisotropic flows may play an important, but previously unrecognized, role in the evolution of star-forming molecular clouds.

We are investigating the ubiquity of this phenomenon by compiling a catalog of suspected anisotropic HI flows from the available HI line data of the Canadian Galactic Plane Survey (CGPS). Where CO line data is available, we have also searched for possibly associated molecular structures. We present preliminary results from this project, derive physical parameters, and discuss the physical requirements needed to power the flows.

J2029+3744---a pulsar with an HI envelope?

B.J. Wallace (National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, Box 248, Penticton BC, Canada, V2A 6K3)

An HI emission feature, ~ 1' in size, has been found in the direction towards the Galactic pulsar J2029+3744. The feature is elongated in roughly the N--S direction, and the center of the feature moves away from the pulsar as velocity becomes less negative. At the most negative velocity the peak of the feature lies 9.9 ± 4.5" from the position of the pulsar. The kinematic distance of the HI feature is consistent with the (highly uncertain) dispersion measure distance to the pulsar.

We speculate that the HI emission feature and the pulsar are physically associated, and discuss physical models which might give rise to the HI feature. We conclude that the most likely explanation is that the combination of the pulsars wind and supersonic motion cause it to sweep the ISM into a dense sheath around a less-dense tube in the ISM; the material in this dense sheath then recombines, forming a trail of neutral material behind the pulsar.

HI Emission and Zeeman Splitting in One of the Spider's Legs

L. Young (Astronomy Department, New Mexico State University, P.O. Box 30001, Las Cruces, NM 88003, USA)

M. Normandeau, C. Heiles (Astronomy Department, UC Berkeley, Berkeley, CA 94720-3411, USA)

Near the coordinates (l,b) = (135°, 41°) there is a remarkable spider-like structure in the Galactic interstellar medium. The spider's body connects numerous filamentary legs, which are bright in 100µm and HI emission. The features are kinematically coherent as well, suggesting that this is a long-lived structure despite the presence of expanding bubbles. It is possible that the spider legs (filaments) are held together by a magnetic field. We test this possibility by using the VLA to make high resolution HI images of the region and by searching for Zeeman splitting in HI emission. These observations allow us to assess the importance of the magnetic field in the evolution of this region.



Separation of Thermal and Non-thermal Emission in the Nucleus of M82

M.L. Allen and P.P. Kronberg (University of Toronto, Toronto, ON, Canada)

We report the preliminary results of an investigation of the ISM in the starburst nucleus of M82. Using VLA maps at five radio frequencies, we have used a least-squares approach to separate the emission into thermal and non-thermal components, at a resolution of 1.3-arcsec, or 23 pc. We identify several massive HII complexes. We discuss the morphology of both components in comparison with features at other wavelengths.

Spectral index variations in the Supernova Remnant HB9

D.A. Leahy (Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4)

Zhang, X. (Beijing Astronomical Observatory, Beijing, China)

Wu, X., Lin, J. (Department of Geophysics, Peking University, Beijing, China)

A new radio map of HB9 at 232 MHz, made at the Beijing Astronomical Observatory, and previously unpublished maps at 151 MHz, from the new Cambridge Low-Frequency Synthesis Telescope Survey, and at 4750 MHz, from the Effelsberg Telescope are presented. Spectral index maps are constructed using the running T-T plot method. Several properties of spectral index behavior are determined. The mechanisms which can most naturally account for the observed behavior are: variable thermal electron density near the rim causing variable low frequency absorption and spectral flattening; and variable magnetic field near the rim and a curved electron spectrum causing variable high frequency steepening near the rim.

New Images From The UKST H-alpha Survey

J.V. Precious, S. Phillipps (University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, U.K.)

Q.A. Parker (UK Schmidt Telescope, Anglo-Australian Observatory, Siding Spring, New South Wales, Australia)

Despite the obviously vital importance of star formation for the evolution of the structure of our galaxy (and other spirals), general surveys of star forming regions within the Galaxy are remarkable for their paucity. In particular, no complete large scale, yet detailed survey in H-alpha emission---the prime optical indicator of star formation and general ionised gas---has yet been carried out.

The Anglo-Australian Observatory has recently undertaken to carry out a complete survey of the Southern Galactic Plane using the U.K. Schmidt Telescope with a narrow band H-alpha filter. The survey will consist of some 200 fields, seperated by 40 rather than the usual 50 due to the physical size of the filter. Prior to the commencement of the full survey, a number of H-alpha films were obtained on behalf of the original ``H-alpha Consortium''. It is from these films that the images presented are taken from. The giant star forming regions NGC 6334 and NGC 6357 are shown, along with various HII regions and other objects of interest.

New detections of extraplanar diffuse ionized gas in a small sample of edge-on spiral galaxies

J. Rossa and R.-J. Dettmar (Astronomisches Institut der Ruhr-Universität Bochum, D-44780 Bochum, Germany)

We present results of an H-alpha-survey of edge-on spiral galaxies. This survey has been carried out aiming at the detection of extended diffuse ionized gas (eDIG) in the halos of these galaxies. Since star formation activity in the disk of spiral galaxies is most likely correlated with the presence of the DIG in the halo, the disk-halo connection can be studied. The DIG traced by H-alpha emission reaches typical scale-heights of usually 1-2 kpc, although extraplanar diffuse ionized gas has been detected even at scale-heights of z ~ 5 kpc.

Our sample of edge-on galaxies comprises of 10 late-type galaxies (Sb-Sc), which have been observed at La Silla, Chile with EFOSC II in imaging mode at the ESO 2.2m telescope and with EMMI attached to the NTT. The selection criteria for our sample have been the following. The fact that they are almost edge--on (i >= 75°), where the halo separates from the disk, make a detailed investigation possible. Additionally many of the galaxies were studied at radio wavelengths and some of them show `thick disks' as encountered by radio continuum observations, which implies that there is interaction between the disk and the halo of these galaxies.

We show new evidence for the disk-halo connection from our H-alpha imaging observations. A few of the galaxies in our sample show eDIG, such as in the case of NGC 4634, NGC 3044, where eDIG can be traced up to 1 kpc above the galactic plane. Some show only individual filaments, i.e. IC 2531, and other show no eDIG at all. NGC 4634 shows a bright eDIG with filaments more frequent but fainter in comparison to NGC 5775. We discuss the observations in detail and compare them to theoretical models.

A Sample Of New Galactic Emission Nebulae

R. Weinberger, H. Hartl, S. Temporin, and C. Zanin (Institute of Astronomy, University of Innsbruck, Austria)

As a byproduct of systematic optical searches for galaxies behind the plane of the Milky Way and as a result of quick-look examinations of POSS II film copies, a number of hitherto unregistered galactic emission nebulae has been discovered by us in recent years. A few of them have already been investigated in detail and were published very recently; we provide better images and other new information on them. Several others unpublished ones will be shown here too---all of them are of extremely low optical surface brightness and represent mainly supernova remnants, planetary nebulae and HII regions.

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