Moderate-resolution K-band Spectroscopy of the Substellar Companion VHS 1256 b
Kielan K. W. Hoch, Quinn Konopacky, Travis Barman, Christopher A. Theissen, Laci Brock, Marshall D. Perrin, Jean-Baptiste Ruffio, Bruce Macintosh, Christian Marois
Abstract
Abstract 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 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>cloud</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>6</mml:mn> </mml:math> , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>±</mml:mo> </mml:math> 0.22 pc). We derive a C/O ratio of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.590</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.354</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.280</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> for VHS 1256b. Both our OSIRIS data and spectra from the literature are best modeled when using a larger 3 μ 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.