Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition

We present the results of a Hubble Space Telescope NICMOS imaging survey of22 T-type field brown dwarfs. Five are resolved as binary systems with angularseparations of 0"05-0"35, and companionship is established on the basis ofcomponent F110W-F170M colors (indicative of CH4 absorption) and lowprobabilities of background contamination. Prior ground-based observations show2MASS 1553+1532AB to be a common proper motion binary. The properties of thesesystems - low multiplicity fraction (11[+7][-3]% resolved, as corrected forsample selection baises), close projected separations (a = 1.8-5.0 AU) andnear-unity mass ratios - are consistent with previous results for field browndwarf binaries. Three of the binaries have components that span thepoorly-understood transition between L dwarfs and T dwarfs. Spectraldecomposition analysis of one of these, SDSS 1021-0304AB, reveals a peculiarflux reversal between its components, as its T5 secondary is ~30% brighter at1.05 and 1.27 micron than its T1 primary. This system, 2MASS 0518-2828AB andSDSS 1534+1615AB all demonstrate that the J-band brightening observed betweenlate-type L to mid-type T dwarfs is an intrinsic feature of this spectraltransition, albeit less pronounced than previously surmised. We also find thatthe resolved binary fraction of L7 to T3.5 dwarfs is twice that of other L andT dwarfs, an anomaly that can be explained by a relatively rapid evolution ofbrown dwarfs through the L/T transition, perhaps driven by dynamic(nonequilibrium) depletion of photospheric condensates.Comment: ~40 pages, 17 figures, accepted for publication to ApJ. Note that emulateapj style file cuts off part of Table