Report of Star Formation Session

Session Leader: Paul Goldsmith

A 10-m submillimeter-wave telescope located at the South Pole offers exciting potential for research on star formation, both in the Milky Way as well as in external galaxies. The primary reason is the access to spectral regions that have heretofore been essentially inaccessible from the ground, and for which studies with air- or space-borne telescopes of relatively small size, have been inadequate. Along with this, there are wavelengths at which observations can be obtained from existing high-altitude sites ( e.g. Mauna Kea), but only in unusually good weather conditions. Thus, while data has been obtained, it is simply not possible to be confident of more than marginal observing. Consequently, high-accuracy calibrations, large-scale studies and the deep integrations necessary to study weak lines and external galaxies, are extremely difficult. Finally, the extremely low water vapor present at South Pole sites means that the atmospheric noise contribution to continuum observations will be much reduced, effectively enhancing sensitivity by a considerable factor. Here, we present some of the highlights of topics discussed in our group. It must be emphasized that there are many, relatively straightforward observations, that can be carried out using this new instrument. These are generally in the categories of observing in S. Hemisphere sources, which are not accessible from alternative sites. The importance of this class of astronomy should not be underestimated. Some of these observations, such as of the Magellanic Clouds, are to a significant degree unique, since these nearest companions to the Milky Way are visible only from the Southern Hemisphere. Several "key" projects emerged that we feel will be exciting, and yield important information about the process of star formation.

Survey of Cloud Cores

An unbiased continuum survey of all nearby molecular regions to identify dense cloud cores, should be able to locate all of these regions within approximately 500 pc of the Sun. This project can best be carried out at a wavelength of ~ 350 micron. This survey would have as its goal to make a complete census of protostars and protostellar groups visible from the S. Pole. This is the type of large-scale, unbiased project that has been found to be invaluable in other areas of astronomy. Only with the high sensitivity afforded by the S. Pole site can we envision it being carried out. And only by going to this relatively long wavelength will we be able to ensure that even the most heavily obscured (deeply embedded) sources will be included. The 10-m telescope will be particularly capable in terms of tracing extended protostellar disks, and connections between these disks and surrounding, placental material. A follow-on to the survey for nearby cores is a spectral line survey of objects detected, in tracers of kinematics of the star-forming gas. This would enable discrimination among the variety of complex motions found in these regions, and in particular, to identify objects in which infall of material is occurring. The 10-meter telescope will be well suited for mapping the structure of the densest regions of molecular clouds in which stars are forming. Furthermore, with its unique capability for routine observations in the 350 and 200 micron bands, the 10 meter can study the spectral energy distribution of protostars in a spectral regime which is particularly discriminating of protostellar models.

Thermal Balance of Star-Forming Regions: Cooling Line Inventory and PDR Studies

A number of theoretical studies have established the importance of molecular cooling in the initial phases of star formation, along with the fact that the cooling for densities > 10⁶ cm^-3 is by a large number of lines of different species. To test in detail these theories, we need to observe those lines, many of which occur in the submillimeter, and for which a 10-m South Pole telescope is the ideal facility. This work also requires the high angular resolution of this instrument, as it is critical to study the distribution and variation of cooling rate within star forming condensations. Some of the important cooling lines can only be observed from ground-based telescopes at exceptional sites such as the S. Pole. A related project that will be enabled by this telescope is a systematic study of the energetics and structure of photon dominated regions (PDRs). These regions, in which intense photon flux produces a chemical composition as well as thermal structure quite distinct from general molecular clouds, are generally thought to be the outermost layers of dense regions whose innards are shielded from UV radiation field. These regions can be probed effectively by a combination of molecular, atomic and ionized transitions. For the latter two states we are dealing with fine structure transitions. The unexcelled site of the 10-m S. Pole telescope opens up the possibility of ground based observations of the ionized nitrogen line near 1900 GHz, which will be a powerful probe in conjunction with observation of lines from ionized and neutral carbon. These observations can be used to help determine the astrophysically important carbon to oxygen ratio, and are relevant for studies of the Milky Way and external galaxies.

Testing Theories of Star Formation via Magnetic Field Mapping

Current theories of star formation distinguish between the formation of ``low mass'' and "high mass" stars. Low mass stars form in relatively quiescent regions, at a rate that is limited by the ambipolar diffusion of gas through the magnetic field. Condensations form and grow slowly, limited by the relative slippage of neutral and ionized material. The morphology of the magnetic field is substantially unaffected by this process, e.g. initially straight field lines remain straight. On the other hand, "high mass" stars form (accompanied by clusters of low mass stars) only when there is supercritical collapse of a condensation. This means that gravity overwhelms the support provided by the magnetic field, and the matter and field lines collapse together. The predicted result is a highly distorted field geometry, for example, one in which there is a strong pinching of the magnetic field lines. It is clearly of great importance for our understanding of the process of star formation, to be able to verify whether this picture is correct or not. Fortunately, a technique exists, namely measurements of the polarized emission from dust grains aligned by the magnetic field, that should make this critical test possible. The detection of a pinched, or hourglass, field configuration in Orion via submillimeter polarimetry from the CSO (and KAO), has provided a tantalizing single example of this predicted field configuration. Polarimetric observations using the SPST will have sufficient sensitivity to map magnetic fields in dozens of Bok globules, where low mass stars form, as well as in numerous high mass star forming regions. Submillimeter polarimetry with the proposed ten meter South Pole telescope will be complementary to far-infrared polarimetry with SOFIA: Far-infrared observations with SOFIA will have the unique capability of mapping fields in the warmer regions that are generally closer to the protostar, and the SPST observations will provide better sensitivity and higher angular resolution for observations of the quiescent, cooler regions.

Tracing Shocks and other Density Structures Associated with Star Formation in Dense Interstellar Clouds

Observations of multiple transitions of carbon monoxide offer a powerful means of tracing shocks in dense interstellar gas. These shocks are critically important in terms of their effect on surrounding material, and thus mediate the regulation of the star formation process. They undoubtedly have a major effect on determining the initial mass function of a cluster of forming stars. The shocks can be driven by winds from deeply embedded young stellar objects or protostars, or from accretion shocks produced by material falling onto disks around the stars that are forming. For this type of project, it is of particular importance that the angular resolution of the 10-m S. Pole telescope, 4" at 200 microns, matches that of the airborne observatory SOFIA at 50 microns. Together, the CO lines from various J-states probe gas over range of 100 K to several thousand K, and will open up a whole new way of examining the interaction of stars and surrounding gas. Another probe of these regions is the use of vibrationally excited molecules, which can be excited either in regions of extremely high density, n(H₂) > 10⁹ cm^-3, or of intense infrared radiation field. The broad access to the submillimeter window afforded by the 10-m S. Pole telescope will enhance the investigation of high rotational as well as vibrationally excited lines of a number of species, and will permit their systematic exploitation in terms of mapping these relatively weak lines throughout star forming regions.