The search for planetary mass companions to field brown dwarfs with HST/NICMOS

Abstract: 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 HST/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-1AB and the newly discovered L/T transition system 2MASS J031059+164815AB. 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 2MASS W033703-175807, straddling the brown dwarf/planetary mass boundary and revealing an uncommonly low mass ratio system (q ~ 0.2) compared to the vast majority of previously found brown dwarf binaries. With a derived minimum mass of 10 - 15 Mjup, 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" $/pm$ 0.015", corresponding to 2.52 $\pm$ 0.44 AU at a distance of 29 pc) with the coolest (Teff ~ 600-630 K) and least massive companion to any L or T dwarf.