PREPRINT
3B33B439-0AAB-4CA8-9FF3-AE4581D3606A

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

Kielan K. W. Hoch, Quinn M. Konopacky, Travis S. Barman, Christopher A. Theissen, Laci Brock, Marshall D. Perrin, Jean-Baptiste Ruffio, Bruce Macintosh, Christian Marois
arXiv:2207.03819

Submitted on 8 July 2022

## Abstract

We present moderate-resolution ($R\sim 4000$) $K$ 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 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 $\mathrm{log}g=$ 3.25, with a range of 3.25--3.75 and a cloud parameter of $\mathrm{log}{P}_{cloud}=$ 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${}_{-1.2}^{+1.1}$ 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.

## Preprint

Comment: 16 pages, 10 figures. arXiv admin note: text overlap with arXiv:2009.08959

Subject: Astrophysics - Earth and Planetary Astrophysics