WISE J072003.20-084651.2B is a Massive T Dwarf

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)(+0.24) yr 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 +/- 6M(Jup) and 66 +/- 4 M-Jup for the two components. The primary mass is marginally inconsistent (2.1 sigma) 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 approximate to 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.