2012ApJ...756...69B

We present the discovery of a 0".2 companion to the young M dwarf GJ 3629 as part of our high-contrast adaptive optics imaging search for giant planets around low-mass stars with the Keck-II and Subaru telescopes. Two epochs of imaging confirm that the pair is comoving and reveal signs of orbital motion. The primary exhibits saturated X-ray emission which, together with its UV photometry from GALEX, points to an age younger than ∼300 Myr. At these ages the companion lies below the hydrogen burning limit with a model-dependent mass of 46±16 M_Jup based on the system's photometric distance of 22 ±3 pc. Resolved YJHK photometry of the pair indicates a spectral type of M7±2 for GJ 3629 B. With a projected separation of 4.4±0.6 AU and an estimated orbital period of 21±5 yr, GJ 3629 AB is likely to yield a dynamical mass in the next several years, making it one of only a handful of brown dwarfs to have a measured mass and an age constrained from the stellar primary.