Binarity in brown dwarfs:: T dwarf binaries discovered with the Hubble Space Telescope Wide Field Planetary Camera 2

We present the discovery of two T dwarf binaries, 2MASS 1225 2739AB and 2MASS 1534-2952AB, identified in a sample of 10 T dwarfs imaged with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2. Companionship is established by the uniquely red F814W-F1042M colors of the binary components, caused by heavily pressure-broadened K I absorption centered at 7665 and 7699 Angstrom. The separations of the two binary systems are 0."282 +/- 0."005 and 0."065 +/- 0."007, implying projected separations of 3.17 +/- 0.14 and 1.0 +/- 0.3 AU, respectively. These close separations are similar to those found in previous brown dwarf binary searches and permit orbital mapping over the coming decade. 2MASS 1225 2739AB has a substantially fainter secondary, with DeltaM(F814W) = 1.59 +/- 0.04 and DeltaM(F1042M) = 1.05 +/- 0.03; this system is likely composed of a T6 primary and T8 secondary with mass ratio 0.7-0.8. The observed binary fraction of our HST sample, 20(-7)(+17) %, is consistent with results obtained for late-type M and L field dwarfs and implies a bias-corrected binary fraction of 9(-4)(+15)% for a greater than or similar to1 AU and qgreater than or similar to0.4, significantly lower than the binary fractions of F-G and early-type M dwarf stars. Neither of the T binaries have separations agreater than or similar to10 AU, consistent with results from other brown dwarf binary searches. Using the statistical models of Weinberg, Shapiro, & Wasserman, we conclude that tidal disruption by passing stars or giant molecular clouds, which limits the extent of wide stellar binaries, plays no role in eliminating wide brown dwarf binaries, implying either disruption very early in the formation process (ages less than or similar to1-10 Myr) or a formation mechanism that precludes such systems. We find that the maximum binary separation in the brown dwarf regime appears to scale as M-total(2), a possible clue to the physical mechanism that restricts wide substellar systems.