Binarity in Brown Dwarfs: T Dwarf Binaries Discovered with theHubble Space TelescopeWide Field Planetary Camera 2

(abridged) We present the discovery of two T dwarf binaries, 2MASS1225-2739AB and 2MASS 1534-2952AB, identified in a sample of ten T dwarfsimaged with the Hubble Space Telescope Wide Field Planetary Camera 2. Theseparations of the two binary systems are 0{\farcs}282$\pm$0{\farcs}005 and0{\farcs}065$\pm$0{\farcs}007, implying projected separations of 3.17$\pm$0.14and 1.0$\pm$0.3 AU, respectively. The observed binary fraction of our HSTsample, 20$^{+17}_{-7}$%, is consistent with results obtained for late-M and Lfield dwarfs, and implies a bias-corrected binary fraction of 9$^{+15}_{-4}$%for $a \gtrsim 1$ AU and $q \gtrsim 0.4$, significantly lower than the binaryfractions of F--G and early-type M dwarf stars. Neither of the T binaries haveseparations $a \gtrsim 10$ AU, consistent with results from other brown dwarfbinary searches. We conclude that tidal disruption by passing stars or GiantMolecular Clouds, which limits the extent of wide stellar binaries, plays norole in eliminating wide brown dwarf binaries, implying either disruption veryearly in the formation process (ages $\lesssim 1-10$ Myr) or a formationmechanism which precludes such systems. We find that the maximum binaryseparation in the brown dwarf regime appears to scale as M$_{total}^2$, apossible clue to the physical mechanism which restricts wide substellarsystems.Comment: 21 pages including 10 figures, accepted to Ap