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

Trent J. Dupuy*, Michael C. Liu, Elise L. Evans, William M. J. Best, Logan A. Pearce, Aniket Sanghi, Mark W. Phillips, Daniella C. Bardalez Gagliuffi

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

VHS J1256−1257 AB 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 programme. Using Keck adaptive optics imaging and aperture masking interferometry, we have determined the host binary’s orbit (a = 1.96 ± 0.03 au, P = 7.31 ± 0.02 yr, e = 0.883 ± 0.003) and measured its dynamical total mass (0.141 ± 0.008 M⊙). 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 barycentre of VHS J1256−1257 AB, finding that the wide companion’s orbit is also eccentric (⁠e=0.68+0.11−0.10⁠), with a mutual inclination of 115○ ± 14○ 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 ± 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 ± 0.1 MJup or 16 ± 1 MJup, from these models. Future spectroscopic data to measure isotopologues such as HDO and CH3D could break this degeneracy and provide a strong test of substellar models at the deuterium-fusion mass boundary.
Original languageEnglish
Pages (from-to)1688-1694
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date6 Dec 2022
Publication statusPublished - 1 Feb 2023

Keywords / Materials (for Non-textual outputs)

  • astrometry
  • planetary systems
  • binaries: visual
  • brown dwarfs


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