On the Masses, Age, and Architecture of the VHS J1256-1257AB b System

Abstract

VHS J1256$-$1257 AB is an ultracool dwarf binary that hosts a wide-separation planetary-mass companion that is a key target of the {\sl JWST} Exoplanet Early Release Science (ERS) program. Using Keck adaptive optics imaging and aperture masking interferometry, we have determined the host binary's orbit, $a=1.96\pm 0.03$ au, $P=7.31\pm 0.02$ yr, $e=0.883\pm 0.003$, and measured its dynamical total mass, $0.141\pm 0.008$ $M_{\odot }$. This total mass is consistent with VHS J1256$-$1257 AB being a brown dwarf binary or pair of very low-mass stars. In addition, we measured the orbital motion of VHS J1256$-$1257 b with respect to the barycenter of VHS J1256$-$1257 AB, finding that the wide companion's orbit is also eccentric, $e=0.68_{-0.10}^{+0.11}$, with a mutual inclination of $115\pm 14^{\circ }$ with respect to the central binary. This orbital architecture is consistent with VHS J1256$-$1257 b attaining a significant mutual inclination through dynamical scattering and thereafter driving Kozai-Lidov cycles to pump the eccentricity of VHS J1256$-$1257 AB. We derive a cooling age of $140\pm 20$ Myr for VHS J1256$-$1257 AB from low-mass stellar/substellar evolutionary models. At this age, the luminosity of VHS J1256$-$1257 b is consistent with both deuterium-inert and deuterium-fusing evolutionary tracks. We thus find a bimodal probability distribution for the mass of VHS J1256$-$1257 b, either $12.0\pm 0.1$ $M_{\mathrm{Jup}}$ or $16\pm 1$ $M_{\mathrm{Jup}}$, from these models. Future spectroscopic data to measure isotopologues such as HDO and CH${}_3$D could break this degeneracy and provide a strong test of substellar models at the deuterium-fusion mass boundary.

MNRAS published version; MCMC posteriors now included in source material; AB-b orbital parameters are updated after correcting a typo in the Dec orbital motion of b

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