THE SEARCH FOR PLANETARY MASS COMPANIONS TO FIELD BROWN DWARFS WITH HST/NICMOS

We present the results of a high-resolution spectral differential imaging survey of 12 nearby, relatively young field L dwarfs (<= 1 Gyr) carried out with the Hubble Space Telescope/NICMOS to search for planetary mass companions at small physical separations from their host. The survey resolved two brown dwarf binaries: the L dwarf system Kelu-1 AB and the newly discovered L/T transition system 2MASS 031059+164815 AB. For both systems, common proper motion has already been confirmed in follow-up observations which have been published elsewhere. The derived separations of the binaries are smaller than 6 AU and consistent with previous brown dwarf binary statistics. Their mass ratios of q >= 0.8 confirm the preference for equal-mass systems similar to a large number of other surveys. Furthermore, we found tentative evidence for a companion to the L4 dwarf 2MASSW 033703-175807, straddling the brown dwarf/planetary mass boundary and revealing an uncommonly low-mass ratio system (q approximate to 0.2) compared to the vast majority of previously found brown dwarf binaries. With a derived minimum mass of 10-15 M-Jup a planetary nature of the secondary cannot be ruled out yet. However, it seems more likely to be a very low mass brown dwarf secondary at the border of the spectral T/Y transition regime, primarily due to its similarities to recently found very cool T dwarfs. This would make it one of the closest resolved brown dwarf binaries (0 ''.087 +/- 0 ''.015, corresponding to 2.52 +/- 0.44 AU at a distance of 29 pc) with the coolest (T-eff approximate to 600-630 K) and least massive companion to any L or T dwarf.