Binarity in Brown Dwarfs: T Dwarf Binaries Discovered with the<i>Hubble Space Telescope</i>Wide Field Planetary Camera 2

Abstract

We present the discovery of two T dwarf binaries, 2MASS 12252739AB and 2MASS 15342952AB, 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 F814WF1042M colors of the binary components, caused by heavily pressure-broadened K i absorption centered at 7665 and 7699 Å. The separations of the two binary systems are 0&amp;gt;282 0&amp;gt;005 and 0&amp;gt;065 0&amp;gt;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 12252739AB has a substantially fainter secondary, with DMF814W 1:59 0:04 and DMF1042M 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 ourHST sample, 20þ177 %, is consistent with results obtained for late-typeM and L field dwarfs and implies a bias-corrected binary fraction of 9þ154 % for ae1 AU and qe0:4, significantly lower than the binary fractions of F–G and early-type M dwarf stars. Neither of the T binaries have separations ae10 AU, consistent with results from other brown dwarf binary searches. Using the statistical models of Weinberg, Shapiro, &amp;amp; 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 d1–10 Myr) or a formation mechanism that precludes such systems. We find that the maximum binary separation in the brown dwarf regime appears to scale asM2total, a possible clue to the physical mechanism that restricts wide substellar systems.

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Affiliated Institutions

• University of California, Los Angeles US
• Infrared Processing and Analysis Center US
• Space Telescope Science Institute US
• California Institute of Technology US
• Goddard Space Flight Center US
• University of Delaware US
• Université Bourgogne Franche-Comté FR
• Catholic University of America US

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