Keck Laser Guide Star Adaptive Optics Monitoring of 2MASS J1534-2952AB: First Dynamical Mass Determination of a Binary T Dwarf

(Abridged) We present multi-epoch imaging of the T5.0+T5.5 binary 2MASS J1534-2952AB obtained with the Keck laser guide star adaptive optics system. Combined with an extensive (re-)analysis of archival HST imaging, we find a total mass of 0.056+/-0.003 Msun (59+/-3 Mjup). This is the first field binary for which both components are directly confirmed to be substellar. This is also the coolest and lowest mass binary with a dynamical mass determination to date. Using evolutionary models, we derive an age of 0.78+/-0.09 Gyr for the system, and we find Teff = 1028+/-17 K and 978+/-17 K and masses of 0.0287+/-0.0016 Msun (30.1+/-1.7 Mjup) and 0.0269+/-0.0016 Msun (28.2+/-1.7 Mjup) for the individual components. These precise measurements generally agree with previous studies of T dwarfs and affirm the current theoretical models. However, (1) the temperatures are about 100 K cooler than derived for similar objects and suggest that the ages of field brown dwarfs may be overestimated. Also, (2) the H-R diagram positions are discrepant with current predictions. While this may arise from large errors in the evolutionary models, the likely cause is a modest (~100 K) overestimate in temperatures determined from model atmospheres. We discuss future tests of theory as the sample of substellar dynamical masses increases. In particular, we suggest that low-mass field binaries with dynamical masses ("mass benchmarks") can serve as reference points for Teff and log(g) and thereby constrain ultracool atmosphere models, as good as or even better than single brown dwarfs with age estimates ("age benchmarks").
Comment: ApJ, in press. Typo in Eqn. 6 corrected in v2