2018ApJ...862...85G

We present a near-infrared K-band R ≃ 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 nonexistent atomic metal absorptions, and H₂ emission lines. The near-IR H₂ 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_eff ≃ 3260 K effective temperature is similar to that of older, more evolved pre-main-sequence stars, but its surface gravity log g ≃ 2.4 cm s^–2 is approximately 1 dex lower. This implies that the radius of this protostar is a factor of ∼3 larger than that of 10⁶ year 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.