ADAPTIVE OPTICS IMAGING OF VHS 1256-1257: A LOW MASS COMPANION TO A BROWN DWARF BINARY SYSTEM

摘要

Recently, Gauza et al. reported the discovery of a companion to the late M-dwarf, VHS. J125601.92-125723.9 (VHS 1256-1257). The companion's absolute photometry suggests its mass and atmosphere are similar to the HR 8799 planets. However, as a wide companion to a late-type star, it is more accessible to spectroscopic characterization. We discovered that the primary of this system is an equal-magnitude binary. For an age similar to 300 Myr the A and B components each have a mass of 64.6(-2.0)(+0.8)M(Jup), and the b component has a mass of 11.2(-1.8)(+9.7), making VHS. 1256-1257 only the third brown dwarf triple system. There exists some tension between the spectrophotometric distance of 17.2 +/- 2.6 pc and the parallax distance of 12.7 +/- 1.0 pc. At 12.7 pc VHS. 1256-1257 A and B would be the faintest known M7.5 objects, and are even faint outliers among M8 types. If the larger spectrophotmetric distance is more accurate than the parallax, then the mass of each component increases. In particular, the mass of the b component increases well above the deuterium burning limit to similar to 35 M-Jup and the mass of each binary component increases to 73(-17)(+20)M(Jup). At 17.1 pc, the UVW kinematics of the system are consistent with membership in the AB. Dor moving group. The architecture of the system resembles a hierarchical stellar multiple suggesting it formed via an extension of the star formation process to low masses. Continued astrometric monitoring will resolve this distance uncertainty and will provide dynamical masses for a new benchmark system.