Detection of Photospheric Features in the Near-Infrared Spectrum of a
  Class 0 Protostar
  We present a near-infrared $K$-band $R \simeq 1500$ Keck spectrum of S68N, a
Class 0 protostar in the Serpens molecular cloud. The spectrum shows a very red
continuum, CO absorption bands, weak or non-existent atomic metal absorptions,
and H$_2$ emission lines. The near-IR H$_2$ emission is consistent with
excitation in shocks or by X-rays but not by UV radiation. We model the
absorption component as a stellar photosphere plus circumstellar continuum
emission with wavelength-dependent extinction. A Markov Chain Monte Carlo
analysis shows that the most likely model parameters are consistent with a
low-temperature, low-gravity photosphere with significant extinction and no
more than modest continuum veiling. Its $T_{\mathrm{eff}} \simeq 3260$ K
effective temperature is similar to that of older, more evolved
pre-main-sequence stars, but its surface gravity log $g \simeq 2.4$ cm s$^{-2}$
is approximately 1 dex lower. This implies that the radius of this protostar is
a factor of $\sim 3$ larger than that of $10^6$ yr old T Tauri stars. Its low
veiling is consistent with a circumstellar disk having intrinsic near-IR
emission that is less than or equal to that of more evolved Class I protostars.
Along with the high extinction, this suggests that most of the circumstellar
material is in a cold envelope, as expected for a Class 0 protostar. This is
the first known detection and analysis of a Class 0 protostar absorption
spectrum.
