WISE J072003.20-084651.2B is a Massive T Dwarf
Abstract
We present individual dynamical masses for the nearby M9.5+T5.5 binary WISE J072003.20-084651.2AB, a.k.a. Scholz’s star. Combining high-precision Canada-France-Hawaii Telescope/WIRCam photocenter astrometry and Keck adaptive optics resolved imaging, we measure the first high-quality parallactic distance ({6.80}-0.06+0.05 pc) and orbit ({8.06}-0.25+0.24yr period) for this system composed of a low-mass star and brown dwarf. We find a moderately eccentric orbit (e={0.240}-0.010+0.009), incompatible with previous work based on less data, and dynamical masses of 99 ± 6 {M}Jup} and 66 ± 4 {M}Jup} for the two components. The primary mass is marginally inconsistent (2.1σ) with the empirical mass-magnitude-metallicity relation and models of main-sequence stars. The relatively high mass of the cold ({T}eff}=1250+/- 40 K) brown dwarf companion indicates an age older than a few gigayears, in accord with age estimates for the primary star, and is consistent with our recent estimate of ≈70 {M}Jup} for the stellar/substellar boundary among the field population. Our improved parallax and proper motion, as well as an orbit-corrected system velocity, improve the accuracy of the system’s close encounter with the solar system by an order of magnitude. WISE J0720-0846AB passed within 68.7 ± 2.0 kau of the Sun 80.5 ± 0.7 kyr ago, passing through the outer Oort cloud where comets can have stable orbits.