DISCOVERY OF A PLANETARY-MASS COMPANION TO A BROWN DWARF IN TAURUS

Abstract

We have performed a survey for substellar companions to young brown dwarfs in\nthe Taurus star-forming region using the Wide Field Planetary Camera 2 on board\nthe Hubble Space Telescope. In these data, we have discovered a candidate\ncompanion at a projected separation of 0.105" from one of the brown dwarfs,\ncorresponding to 15 AU at the distance of Taurus. To determine if this object\nis a companion, we have obtained images of the pair at a second epoch with the\nadaptive optics system at Gemini Observatory. The astrometry from the Hubble\nand Gemini data indicates that the two objects share similar proper motions and\nthus are likely companions. We estimate a mass of 5-10 Mjup for the secondary\nbased on a comparison of its bolometric luminosity to the predictions of\ntheoretical evolutionary models. This object demonstrates that planetary-mass\ncompanions to brown dwarfs can form on a timescale of <=1 Myr. Companion\nformation on such a rapid timescale is more likely to occur via gravitational\ninstability in a disk or fragmentation of a cloud core than through core\naccretion. The Gemini images also reveal a possible substellar companion\n(rho=0.23") to a young low-mass star that is 12.4" from the brown dwarf\ntargeted by Hubble. If these four objects comprise a quadruple system, then its\nhierarchical configuration would suggest that the fragmentation of molecular\ncloud cores can produce companions below 10 Mjup.\n