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.