Star Formation

The formation of stars, particularly those of low mass like the Sun, is fast becoming one of the cornerstones of modern astrophysics. This is due in part to the advance of observational techniques, which have brought us to the point where studies of the formation of individual stars and their planetary systems are now feasible. One of the links in the chain of cosmic events leading from the birth of the Universe to the emergence of intelligent life, the problem of low mass star formation and planet formation is poised to make enormous advances in the early 21st Century. This effort also commands wide interest in society as a whole.

Since stars form in dense molecular clouds, proposed millimetre and submillimetre wave arrays will take a leading role in star formation research in the next century. However, it is clear that the SKA working at high angular resolution at centimetre wavelengths will make critical and unique contributions to star formation. There are many molecular line transitions available at low frequencies, some of which are uniquely capable of probing certain aspects of the physics of molecular clouds and their collapse to form stars. At radio continuum wavelengths emission from protostars is comprised of a dusty moleclar disk and collimated ionized jets (see Fig 2.11). Widespread study of the physical regimes and processes underlying these phenomena at AU scales will require the SKA.

**Figure 2.11:** VLA images of HL Tau at $\lambda$ 7 mm a showing structures on scales of 10's to 100's of AU. At low" resolution (0.2 arc sec) the extended emission from the disk dominates ( *left*), while at higher resolution (0.05 arc sec) the brighter jet dominates ( *right*). From Wilner, Ho and Rodriguez (1999).
$\begin{figure}\centering \centerline{\epsfxsize=11.5truecm \epsfbox{stars/formation/disk_jet.eps} \message{Figure disk_jet.eps}} \end{figure}$

Protostellar Cores

Dust around protostars

Observing the formation and evolution of circumstellar disks is crucial for understanding the star formation and planet-building processes. These disks, $\sim$ 100 AU in radius, tens to a few AU thick, and with masses $\sim$ 0.1 solar masses, are usually studied in the dust thermal continuum at (sub)millimetre wavelengths in the case of young deeply embedded objects, although for optically visible pre-Main sequence stars, such disks can be seen with advanced optical telescopes as the HST or the new generation of ground-based optical telescopes.

Little is yet known about the physical conditions and processes within protostellar disks. Planned (sub)mm interferometers will observe dusty disks at an angular resolution of $\sim$ 10 milli-arcseconds, corresponding to a size scale $\sim$ 5 AU at a distance of 500 pc. This angular/linear resolution will permit the study of the mass and thermal structure of disks, disk vertical stratification, the spatial distribution of dust properties, the distribution, kinematics, and chemistry of molecular components, and the development of circumstellar structures in binary systems. Given these capabilities in the millimetre and submillimetre range, the SKA will nevertheless have a unique advantage when observing protostellar disks at centimetre wavelengths: dust emission is optically thin at centimetre wavelengths, whereas it may be optically thick in the (sub)millimetre in very dense gas condensations. The consequence of optically thick dust emission is that it complicates the determination of dust masses and dust optical properties through measurement of the continuum spectral index, and attenuates the spectral line emission from molecules within the condensation. It is thus possible that only at centimetre wavelengths with the SKA we will be able to probe the bulk of the material in the inner tens of AU where surface mass densities may be greater than 10³ g cm^-2.

Recent observations of the embedded protostellar object L1641N (IRAS 05338-0624) by Chen et al. (1995) provide evidence that dust continuum emission in low mass protostars can be optically thick even at millimetre wavelengths. These researchers found that the continuum spectrum of L1641N between 5 GHz and 200 GHz can be understood in terms of two components: optically-thin free-free emission from an ionized protostellar wind (spectrum $\sim \nu^{-0.1}$ ) dominating at low frequencies, and dust thermal emission (spectrum $\sim \nu^{+2.1}$ ) dominating at high frequencies. Assuming the standard dust emissivity relationship ( $\beta \sim 2$ ), the spectral index of approximately +2.1 found between $\lambda$ 7mm and $\lambda$ 1.3cm wavelength suggests that the dust emission is optically thick in that wavelength range (in the optically-thick regime we expect the spectrum to go as $\nu^{+2}$ , whereas for optically-thin emission we expect $\nu^{(+2+\beta)}$ ). The alternative interpretation, that the emission is optically thin, would imply $\beta\sim0.1$ , meaning that the dust grains were really large fluffy ``snowballs" and implying an unreasonably massive disk. Note that the determination of the dust emissivity parameter $\beta$ is of great importance for understanding dust properties and determining dust masses, and can only be determined from observations in which the emission is optically thin. In the case of L1641N, observations longward of $\sim\lambda$ 1cm are required.

**Figure 2.12:** The wavelength for optical depth unity for a spherical cloud of total mass 1 M $_{\odot}$ (dust mass of 0.01 M $_{\odot}$ ), as a function of linear diameter. Typical ISM dust properties have been assumed ( $\kappa_o$ = 0.1 cm² gm^-1, $\lambda_o$ = 250 $\mu$ m and $\beta$ = 2). High resolution imaging in the cm regime is required to probe the cloud on scales below 10-20 AU. With linear resolution of $\sim$ 1 AU at distances of a few hundred pc, the SKA will uniquely image AU-scale structures within such dense protostellar cores.
$\begin{figure}\centering \centerline{\epsfxsize=11.0truecm \epsfbox{stars/formation/opacity.eps} \message{Figure opacity.eps}} \end{figure}$

The wavelength at which dust emission becomes optically thick depends critically on the dust properties. In the simplest model, the optical depth is given by

Dust emission from very young Class 0 protostellar objects (< 10⁵ years old?) such as NGC1333/IRAS-4 accounts for 20% to 100% of the emission at $\lambda$ 1.3cm (Mundy et al. 1993). Other somewhat older objects, Class I and T Tauri stars also have very steep centimetre wave spectral indices $\nu^{(2.3-3.2)}$ which cannot be explained by ionized gas emission, and thus must be dominated by the long wavelength tail of thermal dust emission. Most such observations of centimetre wave dust emission have been made with the current VLA. The SKA will have no difficulty in detecting dust emission from even the most evolved pre-Main sequence Class III objects, in which disk evolution has gone on the longest and in which the planet formation process may be in full swing.

Dynamics and Chemistry

SKA observations of the NH₃ molecule will allow dynamic imaging of the dense molecular gas associated with star formation cores in our Galaxy on sub-AU scales. Many regions of active star formation exist within a few hundred parsecs of the Sun. At these distances milli-arcsecond angular scales correspond to dimensions $\sim 0.1$ AU. As demonstrated above, it could well be that the optical depths due to dust in very dense regions may be high at (sub)millimetre wavelengths, meaning that molecular line emission from very high column density regions may be severely attenuated. If so, observations of molecular species such as NH₃, H₂CO, CH₃OH, and carbon chain molecules at centimetre wavelengths (where the dust is optically thin) will be required in order to probe the gas chemistry and dynamics in the densest molecular condensations and on (sub)AU scales. What the characteristics of these regions might be are suggested by recent new models of dust emission towards L1551/IRS5, which suggests the existence of a region at the core of size $\sim$ 100 AU having a gas density $\sim10^9$ cm^-3. (Men'shchikov & Henning 1997). There are no observations of molecular lines towards this source which probe densities > 10⁷ cm^-3, so we presently have no solid understanding of the structure and dynamics in the highest density regions. Opacities in the millimetre and submillimetre are predicted to be large, $\tau \sim 1$ at $\lambda$ 1mm.

Protostellar Jets

Strong mass loss occurs in star formation, in which processes occuring very near the protostellar object (accretion, rotation, magnetic fields) drive large scale bipolar outflows of mass. Driven probably by strong ionized winds which are produced and collimated very near the star/accretion disk (tens of AU scale). Recent studies of heavily obscured YSO's at centimeter wavelengths have revealed very weak thermal radio continuum jets on AU scales in a large fraction of objects. Approximately 80% of what are currently thought to be the youngest objects (extreme class``I'' sources or class ``O'' sources) have been detected in the centimeter range (Anglada 1996). Only a fraction of the centimeter sources have been resolved. In those cases the emission is in the form of thermal radio jets from collimated, partially ionized flows with dimensions of 10-100 AU and dynamical time scales of order 1 year. We need to understand how they are driven and collimated, how fast they are, how massive, and how mass outflows change as the star evolves. Of particular interest is the question of how the mass loss varies on the shorter timescales years to tens of years and its connection with short term variations in the protostellar object and/or episodic disk/accretion events. Some jets appear to be time variable up to the extent that the jets may be pulsed with monopolar phases. What role do magnetic fields play in driving and collimating the flows? Which objects precess, and why?

**Figure 2.13:** Approximate range of lengths of collimated outflows from Young Stellar Objects as observed at different wavelengths (after Anglada 1996).
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To answer such questions we need high angular resolution in order to resolve the jets and to be able to do variability and proper motion studies on them. The combined high resolution and high surface brightness sensitivity provided by the SKA will make it one of the most powerful tools for studying the nature of these deeply embedded objects, and thus understanding the final stages of the star formation process. The thermal jets are aligned with the larger scale molecular and optical outflows. Figure 2.13 shows a schematic illustration of the range of scales of the outflows that are seen around YSO's and the physical scales that will be probed by the SKA. Ionized jets such as those associated with L1551 (see Fig. 2.14), will be probed on sub-AU scales! The ionized jets die out as protostellar objects approach the Main sequence, so we need the sensitivity of the SKA in order to trace the long term evolution of the mass outflow phase. Multifrequency observations are required in order to determine spectral indices and thus probe jet fine structures (such as opening angles and temperatures).

The number of resolved protostellar jets, which emit strongest at centimetre wavelengths, is only about 20, and of those only about 10 are reasonably well studied, but there are at least 200 molecular outflow sources known, not to mention many more older pre-Main sequence objects without detectable molecular outflows but which have weak ionized winds. A typical jet may have a flux of < 1 mJy, and the VLA has the sensitivity to detect winds in only about 10% of nearby young stellar objects. The SKA will be able to detect and resolve these winds in essentially all nearby low mass protostellar and pre-Main sequence objects.

**Figure:** A $\lambda$ 3.6 cm VLA image of double? jets in L1551. The yellow rectangles indicate the position and size of two protoplanetary disks found a $\lambda$ 7 mm (Rodriguez et al. 1998). With the sensitivity and resolution limits of the VLA the nature and origin of this ionized gas cannot be uniquely defined. The SKA will allow such structures to be probed at sub-AU scales.
$\begin{figure}\centering \centerline{\epsfxsize=7.5truecm \epsfbox{stars/formation/L1551.eps} \message{Figure proper_motion.eps}} \end{figure}$

**Figure 2.15:** Multi-epoch images of the ionized jet of HH80-81 at $\lambda$ 3.6cm. The three images are difference maps from an image taken earlier at epoch 1990.2. High resolution images at low flux levels allow detection of faint components of the jet and measurements of proper motion. From Marti, Rodriguez, and Reipurth (1998).
$\begin{figure}\centering \centerline{\epsfxsize=7.0truecm \epsfbox{stars/formation/proper_motion.eps} \message{Figure proper_motion.eps}} \end{figure}$

Proper motions have been detected in a few objects, most notably in HH80-81 (1000 km/s) (see Fig 2.15), the Serpens jet source (300 km/s), and HH1-2 (400 km/s), with evidence that some jets are unipolar (HH111) or bipolar but asymmetric (Serpens, Re 50). These must be intrinsic effects, since variable dust extinction cannot affect these radio continuum observations. Current instruments which have some capability to resolve proper motions in ionized protostellar jets (VLA, VLBI) can observe only a handful of the fastest, nearest, and brightest jets. Confirming proper motions of $\sim 0.1''$ year^-1 with the VLA requires observations spaced over a $\sim$ 10 year period, while the brightness sensitivity of VLBI observations is rather poor. The SKA will excel in observing in essentially ``real time'' the ejection, development, and interactions of the jets which act as a dynamical and energetic interface between young stars and their circumstellar environments over many decades of size scale.

SKA will also be able to detect and map for the first time radio recombination lines from hydrogen in these jets. The understanding of how these jets are accelerated and collimated, most probably by magnetohydrodynamical mechanisms, requires of a knowledge of the kinematics of the ionized gas. Only the SKA will have the sensitivity to provide this information.

Uncovering the Evolutionary Sequence

One of the key stages in stellar evolution is the period just prior to the formation of a protostar when a cloud core achieves the critical state that transforms it into a collapsing object. The conditions which lead to this transformation, and the processes by which it occurs, determine how solar systems form and how galaxies evolve.

In the commonly presented scenario, a proto-stellar nebula forms out of one of the fragments of a collapsing cloud core. Observations of dense regions have in come cases shown evidence for a ``layered chemistry'', or chemistry that varies greatly through a section of a dense cloud. This could be an excitation effect due to increasing density towards the centers of knots, or an ``age'' effect in which a time dependent chemistry both creates and destroys various species. Or it could be time-dependent or density-dependent depletion on to grains. The structure of these regions also indicates fragmentation, which could lead to the coagulation of fragments to form proto-stellar systems.

**Figure 2.16:** L1498 is an example of a quiescent core, possibly in a pre-protostellar phase. Ammonia, a molecule which forms slowly, is concentrated near the center, indicating the presence of relatively old gas. CCS, a molecule with a short (few $\times 10^{5}$ yr) lifetime, is concentrated in an outer layer. This suggests that the cloud is growing by accretion.
$\begin{figure}\centering \centerline{\epsfxsize=10.0truecm \epsfbox{stars/formation/L1498.eps} \message{Figure L1498.eps}} \end{figure}$

The layered separation of two important species is illustrated in Fig. 2.16, where thorough observations of $\rm NH_3$ and CCS have been carried out for the dense cloud L1498. In this case Kuiper et al. (1996) argue that this pre-protostellar cloud shows evidence for growth by accretion. Ammonia, which takes about 10⁶ year to form (Herbst et al. 1989), is concentrated near the center. CCS, on the other hand, forms early in a chemically evolving cloud, and is destroyed within a few $\times 10^{5}$ yr (Millar and Herbst 1990). Although only the ends of L1498 could be mapped with high angular resolution, the pattern is strongly suggestive of layering.

**Figure 2.17:** Taurus Molecular Cloud 1 shows a linear progression from young CCS-rich gas in the SE to older gas with prominent NH₃ in the NW. (Adapted from original Hirahara et al. 1992.)
$\begin{figure}\centering \centerline{\epsfxsize=14.0truecm \epsfbox{stars/formation/TMC1-clr.ps} \message{Figure TMC1-clr.ps}} \end{figure}$

The dense ridge in Taurus MC1 illustrates the value of the SKA in studying star forming clouds (Fig. 2.17). Over a region about 20 arcmin in size, an evolutionary sequence appears to be laid out. The ridge in TMC1, however, appears to be quite unusual (at least in terms of its extraordinarily rich chemistry), and may not be indicative of typical cloud-collapse circumstances. At the NW end there is a concentration of $\rm NH_3$ , far-infrard, and outflow sources (Chandler et al. 1996). At the SE end, CCS predominates. There are five condensations, labeled A through E, along the cloud, and star formation is evident only near core A. If this is an age effect, it seems likely that, in time, star formation will occur, probably sequentially, in at least some of the other condensations. However, with the best present instrumentation, it took several years and several hundred hours of telescope time to observe Core D alone.

Although CCS illustrates the layering effect particularly well in L1498 and TMC1, other species have also been found to vary along the TMC1 ridge (e.g. $\rm C_4H$ and $\rm HC_7N$ (Olano etal. 1988)).

A quantitative explanation must be based on the statistics of many clouds and cloud-clumps that constitute the low mass end of the structure of the interstellar molecular medium. Sensitive ( $T_{A}^{*} \sim 0.1$ K) spectral line maps of molecular clouds, with high spectral resolution ( $\sim 0.05$ km s^-1) and good spatial resolution, are required to provide data on the density, mass, and temperature of the fragments, the space density of the fragments and their relative velocities. Comparison of the abundances of key molecular species in many objects could elucidate which of the possible processes is producing the layered effects. A large set of cases is needed to provide information on the ages of fragments, and to yield evolutionary sequences as the abundances respond to the changing conditions during the pre-star-formation assembly process.

Observations of this type are best done at centimeter wavelengths. These weak lines may be one of the best hopes of tracing star formation. At these low temperatures ammonia only emits significantly in a few transitions near 24 GHz. CCS and similar carbon chain molecules, because of their large moments of inertia, radiate predominantly at centimeter wavelengths. In dark cloud cores, CCS and NH₃ abundances are anti-correlated, with NH₃ abundant in cores with signs of star formation, and CCS is abundant in cores without star formation (Suzuki et al. 1992). Thus it currently appears that these are two key molecules which probe the beginning and end of the star formation evolutionary sequence.

Existing telescopes and arrays are not well suited to measuring on the relevant size scales. The largest single apertures do not have enough resolution, and the arrays cannot achieve the required aerial coverage and brightness sensitivity. As a phased array the SKA would take about 10 minutes to integrate to an r.m.s noise level of 20 mK with a spectral resolution of 0.05 km/s in the 22 GHz band and an angular resolution of about 10''. This would, for example, enable a map of the $3 \times 4$ arcmin area of L1498 to be carried out in the NH₃ and CCS lines near 24 and 22 GHz, respectively, in less than an hour.

Magnetic Fields in Protostellar Objects

Zeeman Splitting in Molecular Gas

Polarisation observations of molecular spectral lines with the SKA will yield measurements of magnetic field strengths in dense molecular regions via the Zeeman effect. At centimetre wavelengths the Zeeman effect has been observed in the interstellar medium for both the HIline and for the OH radical. The HIline does not, however, serve as a good magnetic field probe of primarily molecular regions. Neither does OH, since its abundance is much higher in the less dense envelopes around molecular gas than in the interior regions, and OH maser observations sample only a particular type of environment inside a cloud whereas a general probe is needed for mapping fields in dense molecular clouds. In the millimetre and submillimetre range, possible Zeeman probes include the SO, CN, CCS, and CCH molecules. However, there are strong advantages to be gained by making Zeeman observations in the centimetre wave lines of appropriate molecules: the splitting of the line into Zeeman components is approximately independent of the line frequency whereas the Doppler width of lines is proportional to frequency. Therefore, the ratio of Zeeman splitting to Doppler width (and hence the ability to detect the Zeeman effect) is greater for the lower frequency lines. Potential probes of the Zeeman effect in low frequency lines include SO (13 GHz) and CCS (11 GHz, 22 GHz). The transition which has the greatest sensitivity to this effect is the CCS line near 11 GHz. The measurements require high sensitivity and high spectral resolution to make very precise determinations of line profiles, and high angular resolution to resolve the magnetic structure in protostellar cores. Present efforts to observe this effect with single aperture telescopes involve many tens of hours of integration time for one position.

As an aside, attempts to detect the Zeeman effect in hydrogen and carbon radio recombination lines have not yet been successful, and the SKA will be the most powerful instrument for renewed attempts.

Non-Thermal Emission Processes

Classical T Tauri stars (CTTSs) have weak radio emission detected in about 10% of objects, $\sim$ 0.3 mJy at 5 GHz. All these seem to be associated with jets/collimated outflows. Herbig Ae/Be (HAEBE) stars, which are intermediate mass CTTSs, are detected for about 20% of all nearby objects. All of the above are thought to be dominated by thermal emission from ionized winds. In contrast, some weak lined Tauri stars (WTTSs) and related Class III sources are detected in the radio continuum much more often (up to 50% of objects) at $\sim$ 1 mJy at 5 GHz. Such objects have only remnant disks and weak ionized winds at best, and emit with nonthermal characteristics: they are variable on timescales of hours to days, have a moderate degree of circular polarization (several percent), and high brightness temperatures $\sim 10^7$ K. The emission is thought to be due to the gyrosynchrotron mechanism (electrons moving around large scale, dipolar-like stellar ``magnetospheres" up to 30 solar radii in diameter). The radio spectra are quite flat, with indices $\sim$ 0 during quiescence and $\sim$ 1 during outbursts at 1-5 GHz. At higher frequencies, 5-15 GHz, the opposite behaviour is seen, suggestive of a turnover in the spectrum around 5-10 GHz during flares. Little is known about the frequency/time dependence of the circular polarization of the nonthermal emission from these objects.

The current lack of detections of nonthermal emission from CTTS and younger Class I and Class 0 objects (in contrast to Class III/WTTS objects) brings up the question of what are the magnetic field strengths and structures around these very young objects and how do they evolve? Many theoretical models of protostellar outflows require strong, $\sim$ 1 kGauss, fields near the stellar surface. It has also been suggested that strong fields in CTTSs may couple the star to its accretion disk and thus provide a way for the star to regulate its angular momentum. This regulation is needed in order to keep CTTSs rotating well below breakup speed even though they are accreting high angular momentum material from their disks, and to explain why CTTSs (generally younger) rotate only half as fast as WTTSs (generally older). The SKA sensitivity would be essential for the concerted searches for nonthermal emission from CTTSs.

The above ``theoretical" requirements of CTTS magnetic fields can probably be satisfied with field strengths roughly similar to those of the nonthermally emitting WTTSs. There is, however, no radio evidence of large scale magnetic structures in very young objects. These objects have ionized winds/jets which, in a few cases at least, may have appreciable optical depths in their thermal free-free emission. It is thus possible that nonthermal emission from very young objects may be partially or completely absorbed in the ionized gas. What fraction of the nonthermal emission that would be masked will depend upon the size of the magnetosphere with respect to the scale length of the ionized jet gas. The SKA will have the sensitivity to go two orders of magnitude deeper than current centimetre wave telescopes in the search for spectral signatures of partially attenuated nonthermal emission. A key method to find evidence for the existance of magnetospheric-type structures around jet sources may be to look for short period variability and/or circular polarization at high angular resolution and sensitivity. A demonstration of this possibility is the polarized radio emission from around T Tauri: evidence for magnetic fields in young jet-driving sources can be detected when they extend outside of the inner obscuring free-free ``blanket", as seen from the MERLIN $\lambda$ 6cm map of this system. The near infrared companion to T Tauri is observed to have distinct lobes of right and left circularly polarized emission around the infrared source, separated by $\sim$ 20 AU, suggestive of magnetic structures extending on tens of AU scales. The most likely explanation of this phenomena is that the magnetic fields (a few Gauss) are part of a collimated flow from the star. The sensitivity of SKA will be needed to study similar phenomena in other young stellar objects.