Hubble Space Telescope observations of binary very low mass stars and brown dwarfs

We present analysis of Hubble Space Telescope (HST) images of 82 nearby field late M and L dwarfs. We resolve 13 of these systems into double M/L dwarf systems and identify an additional possible binary. Combined with previous observations of 20 L dwarfs, we derive an observed binary fraction for ultracool dwarfs of 17(-3)(4)%, where the statistics included systems with separations in the range 1.6 - 16 AU. We argue that accounting for biases and incompleteness leads to an estimated binary fraction 15% +/- 5% in the range 1.6 - 16 AU. No systems wider than 16 AU are seen, implying that the wide companion frequency is less than 1.7%; the distribution of orbital separation is peaked at similar to2 - 4 AU and differs greatly from the G dwarf binary distribution. Indirect evidence suggests that the binary fraction is similar to5% +/- 3% for separations less than 1.6 AU. We find no evidence for differences in the binary fraction between stellar late M and L dwarfs and substellar L dwarfs. We note, however, that the widest ( greater than 10 AU) systems in our sample are all of earlier (M8 - L0) spectral type; a larger sample is needed to determine if this is a real effect. One system with a spectral type of L7 has a secondary that is fainter in the HST F814W filter but brighter in F1042M; we argue that this secondary is an early T dwarf.