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) 1.15-2.42 mu m near-IR spectroscopic survey of young stellar objects (YSOs), EU Ori type stars, and MK spectral standards. The survey sample includes 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 forming regions. As part of this study we also present the first systematic, flux-limited spectroscopic survey of an entire population of YSOs embedded within a single molecular cloud (Ophiuchus), In general we find 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-type) MK standard stars. For sources in the flux-limited sample, we find 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 features are typically absent at the resolution of our survey, confirming trends found in earlier studies. This correlation of absorption strength with SED class can be explained by a systematic increase in the veiling 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 these objects. Moreover, it appears that the absence of absorption features and the resulting large veilings of Class I sources are likely related to significantly increased levels of accretion/infall compared to Class II and III sources, Most YSOs in our study appear to have surface gravities which range between those of giant and dwarf stars. Flat-spectrum YSOs have the lowest surface gravities of all objects in our flux-limited Ophiuchus sample. FU Ori stars have even lower surface gravities, similar to those of giant or supergiant stars, suggestive of line formation in disks rather than stellar photospheres. The FU Ori type stars and a few other YSOs show deep and broad H2O absorptions, indicative of cool (T less than or equal to 3000 K) disks or stellar photospheres. Many Class I and Class II objects show relatively strong H I emission lines which probably originate in a partially ionized circumstellar region. However, mostly Class I objects show detectable H-2 emission lines. (C) 1996 American Astronomical Society.