The physical natures of Class I and flat-spectrum protostellar photospheres: A near-infrared spectroscopic study

We present high-resolution (R similar or equal to 18,000), high signal-to-noise ratio, 2 mu m spectra of 52 IR-selected Class I and flat-spectrum young stellar objects in the Taurus-Auriga, rho Ophiuchi, Serpens, Perseus, and Corona Australis dark clouds. We detect key absorption lines in 41 objects and fit synthetic spectra generated from pre-main-sequence models to deduce the effective temperatures, surface gravities, near-IR veilings, rotation velocities, and radial velocities of each of these 41 sources. We find these objects to span ranges in effective temperature, surface gravity, and stellar luminosity that appear similar to those of late spectral type Class II sources and classical T Tauri stars. However, because of significant but uncertain corrections for scattering and extinction, the derived luminosities for the embedded protostellar objects must be regarded as being highly uncertain. We determine that the mean 2 mu m veiling of Class I and flat-spectrum objects is significantly higher than that of Class II objects in the same region where both types of objects are extensively observed (rho Oph). We find that a significant fraction of our protostellar sample also exhibits emission lines. Twenty-three objects show H-2 emission, which is usually indicative of the presence of energetic outflows. Thirty-four sources show H I Br gamma emission, and a number of these exhibit profile asymmetries consistent with infall. Eight sources show significant Delta(v) = 2 CO emission suggestive of emission from a circumstellar disk. Overall, these observations indicate that Class I and flat-spectrum objects are self-embedded protostars undergoing significant mass accretion, although the objects appear to span a broad range of mass accretion activity.