http://arxiv.org/abs/2208.08448
VHS J1256-1257AB is an ultracool dwarf binary that hosts a wide-separation planetary-mass companion that is a key target of the JWST Exoplanet Early Release Science program. Using Keck adaptive optics imaging and aperture masking interferometry, we have determined the host binary’s orbit ($a=1.96\pm0.03$ au, $P=7.31\pm0.02$ yr, $e=0.8826^{+0.0025}{-0.0024}$) and measured its dynamical total mass ($0.141\pm0.008$ $M{\odot}$). This total mass is consistent with VHS J1256-1257AB 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 J1256-1257AB, finding that the wide companion’s orbit is also eccentric ($e=0.73^{+0.09}{-0.10}$), with a mutual inclination of $116^{\circ}\pm16^{\circ}$ 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 J1256-1257AB. We derive a cooling age of $140\pm20$ Myr for VHS J1256-1257AB 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 $11.8\pm0.2$ $M{\rm Jup}$ or $16\pm1$ $M_{\rm Jup}$, from Saumon & Marley (2008) hybrid 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.
T. Dupuy, M. Liu, E. Evans, et. al.
Fri, 19 Aug 22
2/55
Comments: submitted to MNRAS Letters
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