摘要：We present moderate-resolution (R ∼ 4000) K-band spectra of the planetary-mass companion VHS 1256 b. Thedata were taken with the OSIRIS integral field spectrograph at the W.M. Keck Observatory. The spectra revealresolved molecular lines from H2O and CO. The spectra are compared to custom PHOENIX atmosphere modelgrids 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 datafrom the literature, we derive an effective temperature of 1240 K, with a range of 1200–1300 K, a surface gravityof log g = 3.25, with a range of 3.25–3.75, and a cloud parameter of P = log 6 cloud , with a range of 6.0–6.6. Thesevalues 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+0.3540.280 for VHS 1256b. Both our OSIRIS data and spectra from theliterature 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 forsystematics in the modeling process that may lead to the incorrect derivation of properties like C/O ratio in thehigh contrast regime.
