NEAR-INFRARED SPECTRA AND THE EVOLUTIONARY STATUS OF YOUNG STELLAR OBJECTS - RESULTS OF A 1.1-2.4 MU-M SURVEY

We present the results of a moderate resolution (R similar to 500) 1.1 5-2.42 mu m near-IR spectroscopic survey of young stellar objects (YSO s), EU Ori type stars, and MK spectral standards. The survey sample in cludes approximately 100 mostly low-mass YSOs characterized by a wide range of spectral energy distributions (SEDs) and evolutionary states which are drawn from the Ophiuchus, Taurus, and other nearby star form ing regions. As part of this study we also present the first systemati c, flux-limited spectroscopic survey of an entire population of YSOs e mbedded within a single molecular cloud (Ophiuchus), In general we fin d the shapes of YSO spectra to be correlated with SED class such that the most embedded objects generally have the steepest IR spectra. Many YSOs also show absorption features similar to those of (mostly late-t ype) MK standard stars. For sources in the flux-limited sample, we fin d that the strengths of atomic and CO absorption features are closely related to SED class and evolutionary state. In particular, these line strengths generally decrease from the revealed Class III phase to the Class II phase to the self-embedded Class I phase where absorption fe atures are typically absent at the resolution of our survey, confirmin g trends found in earlier studies. This correlation of absorption stre ngth with SED class can be explained by a systematic increase in the v eiling of an underlying stellar photosphere from Class III to Class I objects, The likely source of this veiling is continuum emission from increasing amounts of luminous circumstellar material surrounding thes e objects. Moreover, it appears that the absence of absorption feature s and the resulting large veilings of Class I sources are likely relat ed to significantly increased levels of accretion/infall compared to C lass II and III sources, Most YSOs in our study appear to have surface gravities which range between those of giant and dwarf stars. Flat-sp ectrum YSOs have the lowest surface gravities of all objects in our fl ux-limited Ophiuchus sample. FU Ori stars have even lower surface grav ities, similar to those of giant or supergiant stars, suggestive of li ne formation in disks rather than stellar photospheres. The FU Ori typ e stars and a few other YSOs show deep and broad H2O absorptions, indi cative of cool (T less than or equal to 3000 K) disks or stellar photo spheres. Many Class I and Class II objects show relatively strong H I emission lines which probably originate in a partially ionized circums tellar region. However, mostly Class I objects show detectable H-2 emi ssion lines. (C) 1996 American Astronomical Society.