Planets around low-mass stars (Palms). II. A low-mass companion to the young Mdwarf GJ3629 separated by 02

We present the discovery of a 02 companion to the young Mdwarf GJ3629 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 ∼300Myr. 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 ± 3pc. Resolved YJHK photometry of the pair indicates a spectral type of M7 ± 2 for GJ3629B. With a projected separation of 4.4 ± 0.6AU and an estimated orbital period of 21± 5yr, GJ3629AB 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.