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