We present the discovery of two T dwarf binaries , 2MASS 1225 - 2739AB and 2MASS 1534 - 2952AB , identified in a sample of ten T dwarfs imaged with the Hubble Space Telescope 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 & 7699 Å .
The separations of the two binary systems are 0 \farcs 282 \pm 0 \farcs 005 and 0 \farcs 065 \pm 0 \farcs 007 , implying projected separations of 3.17 \pm 0.14 and 1.0 \pm 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 \Delta M _ { F 814 W } = 1.59 \pm 0.04 and \Delta M _ { F 1042 M } = 1.05 \pm 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 ^ { +17 } _ { -7 } % , is consistent with results obtained for late-M and L field 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 binary fractions of F–G and early-type M dwarf stars .
Neither of the T binaries have separations a \gtrsim 10 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 \lesssim 1 - 10 Myr ) or a formation mechanism which 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 which restricts wide substellar systems .