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 .