Discovery of a Visual T-Dwarf Triple System and Binarity at the L/T Transition

We present new high contrast imaging of 8 L/T transition brown dwarfs using the NIRC2 camera on the Keck II telescope. One of our targets, the T3.5 dwarf 2MASS J08381155 + 1511155, was resolved into a hierarchal triple with projected separations of 2.5+/-0.5 AU and 27+/-5 AU for the BC and A(BC) components respectively. Resolved OSIRIS spectroscopy of the A(BC) components confirm that all system members are T dwarfs. The system therefore constitutes the first triple T-dwarf system ever reported. Using resolved photometry to model the integrated-light spectrum, we infer spectral types of T3, T3, and T4.5 for the A, B, and C components respectively. The uniformly brighter primary has a bluer J-Ks color than the next faintest component, which may reflect a sensitive dependence of the L/T transition temperature on gravity, or alternatively divergent cloud properties amongst components. Relying on empirical trends and evolutionary models we infer a total system mass of 0.034-0.104 Msun for the BC components at ages of 0.3-3 Gyr, which would imply a period of 12-21 yr assuming the system semi-major axis to be similar to its projection. We also infer differences in effective temperatures and surface gravities between components of no more than ~150 K and ~0.1 dex. Given the similar physical properties of the components, the 2M0838+15 system provides a controlled sample for constraining the relative roles of effective temperature, surface gravity, and dust clouds in the poorly understood L/T transition regime. Combining our imaging survey results with previous work we find an observed binary fraction of 4/18 or 22_{-8}^{+10}% for unresolved spectral types of L9-T4 at separations >~0.1 arcsec. This translates into a volume-corrected frequency of 13^{-6}_{+7}%, which is similar to values of ~9-12% reported outside the transition. (ABRIDGED)