Moderate-resolution K-band Spectroscopy of the Substellar Companion VHS 1256 b

We present moderate-resolution (R ∼ 4000) K-band spectra of the planetary-mass companion VHS 1256 b. The data were taken with the OSIRIS integral field spectrograph at the W.M. Keck Observatory. The spectra reveal resolved molecular lines from H₂O and CO. The spectra are compared to custom PHOENIX atmosphere model grids appropriate for young, substellar objects. We fit the data using a Markov chain Monte Carlo forward-modeling method. Using a combination of our moderate-resolution spectrum and low-resolution broadband data from the literature, we derive an effective temperature of 1240 K, with a range of 1200-1300 K, a surface gravity of log g = 3.25, with a range of 3.25-3.75, and a cloud parameter of log P cloud = 6 , with a range of 6.0-6.6. These values are consistent with previous studies, regardless of the new, larger system distance from GAIA EDR3 (21.15 ± 0.22 pc). We derive a C/O ratio of 0.590 − 0.354 + 0.280 for VHS 1256b. Both our OSIRIS data and spectra from the literature are best modeled when using a larger 3 μm grain size for the clouds than used for hotter objects, consistent with other sources in the L/T transition region. VHS 1256 b offers an opportunity to look for systematics in the modeling process that may lead to the incorrect derivation of properties like C/O ratio in the high contrast regime.