Moderate-Resolution $K$-Band Spectroscopy of the Substellar Companion VHS 1256 b

TL;DR

This study presents high-resolution $K$-band spectra of the substellar companion VHS 1256 b, deriving atmospheric properties and C/O ratio through advanced modeling, and compares results with existing data to understand cloud properties and systematics.

Contribution

First detailed moderate-resolution spectra of VHS 1256 b analyzed with MCMC modeling to derive atmospheric parameters and cloud properties, highlighting the importance of cloud grain size in spectral fitting.

Findings

Effective temperature of 1240 K with a range of 1200–1300 K.

Surface gravity $ ext{log}g$ of 3.25–3.75.

C/O ratio of approximately 0.59, with large uncertainties.

Abstract

We present moderate-resolution ($R\sim4000$) $K$ 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 H$_{2}$O 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 (22.2$^{+1.1}_{-1.2}$ 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 $\mu$m 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.