BANYAN. VI. DISCOVERY OF A COMPANION AT THE BROWN DWARF/PLANET-MASS LIMIT TO A TUCANA-HOROLOGIUM M DWARF

We report the discovery of a substellar companion to 2MASS J02192210-3925225, a young M6. candidate member of the Tucana-Horologium association (30-40 Myr). This L4 gamma companion has been discovered with seeing-limited direct imaging observations; at a 4 '' separation (160 AU) and a modest contrast ratio, it joins the very short list of young low-mass companions amenable to study without the aid of adaptive optics, enabling its characterization with a much wider suite of instruments than is possible for companions uncovered by highcontrast imaging surveys. With a model-dependent mass of 12-15 M-Jup, it straddles the boundary between the planet and brown dwarf mass regimes. We present near-infrared spectroscopy of this companion and compare it to various similar objects uncovered in the last few years. The J0219-3925 system falls in a sparsely populated part of the host mass versus mass ratio diagram for binaries; the dearth of known similar companions may be due to observational biases in previous low-mass companion searches.