4 Star Formation outside Giant Molecular Clouds
Due to SIRTF's unparalleled sensitivity, the SEDI point-sources catalog will be a gold-mine for studies of many types and, in particular, for star formation research. The SIRTF colors will allow us to easily identify all stars with circumstellar disks as well as young brown dwarfs (Figure 4). At a distance of 150 pc, our 10 sigmas detection limit at 3.6 micron (~100 mJy) corresponds to the photosphere of a 0.03 solar masses Brown Dwarf at an age of ~10 Myr. The census of such stars over a range of cloud mass, densities and column densities provides an original perspective for star formation studies. Thus the SEDI program will give an impetus to an area of star formation research which is often neglected, because of the time consuming task of acquiring highly sensitive data over large areas of the sky.
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Figure 4. Color-magnitude diagram ( F(24 micron)/F(5.8 micron) vs. F(24 micron) ) for low mass (M<1 solar mass) 1 Myr old YSOs. The masses for each model are indicated in solar units near the corresponding photospheric point. The models are calculated for a distance of 150 pc. |
4.1 Dispersed population of young stars
Most galactic star formation is thought to occur within giant molecular clouds. Within these clouds, a large fraction of the observed Young Stellar Objects (YSOs) are found in tight clusters or associations surrounded by molecular gas (Clarke et al. 2000). The stars which we expect to see with SIRTF will mostly have ages of a few million years and hence could have moved away from their place of birth. Indeed, on a timescale of 10E6-10E7 years, one expects a star moving with a velocity of 1 km/s (typical of the stellar velocity dispersion in molecular clouds) to move a distance of 1-10 pc or between 0.3 and 3 degrees at a distance of 150 pc. Searches for YSOs outside molecular clouds have mainly been based on X-ray surveys such as that by ROSAT (Feigelson 1996) and Chandra (Garmire et al. 2000) in conjunction with optical and infrared observations. The X-ray luminosity, however, is variable and without any clear correlation to stellar parameters including age. Infrared fluxes are fundamental tracers of young stars, measuring the total luminosity and infrared excess, which is evidence of a circumstellar disk. The SEDI observations include several fields within molecular complexes (e.g. Rho Halo, Serpens, Taurus filament), where our wide-field coverage will probe a dispersed population of YSOs with disks to unprecedented depth. With the SIRTF colors, we will also be able to compare the evolution of the dispersed population to that of the better studied population of YSOs in clusters and associations. Combined with the ancillary optical observations, the SEDI star catalog will provide complete, luminosity-limited samples and candidates for further study.
4.2 Star Formation in Cirrus Clouds
Many of the SEDI fields target cirrus clouds well separated from star-forming regions, allowing the search for YSOs with disks and young brown dwarfs in low mass clouds spanning a range of column density. Can star formation occur in such clouds, and if so, is the initial mass function (IMF) different from that of large molecular clouds? The situation is still inconclusive, because there is evidence for star formation in some high-latitude molecular clouds (e.g., 11 T Tauri stars in MBM 12; Hearty et al. 2000), whereas lower opacity clouds reveal no evidence for star formation (Hearty et al. 1999). The SEDI observations are sensitive to fainter and cooler stars than could have ever been detected, probing a distinct population extending into the brown dwarf mass range. In addition, the far-IR MIPS observations will detect dense, potentially pre-stellar, cores and possibly `proto-brown-dwarfs' (Pound & Blitz 1993).
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