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

Submitted on 8 July 2022


We present moderate-resolution (R4000) 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 logg= 3.25, with a range of 3.25--3.75 and a cloud parameter of logPcloud= 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 (22.21.2+1.1 pc). We derive a C/O ratio of 0.5900.354+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.


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

Subject: Astrophysics - Earth and Planetary Astrophysics