KECK LASER GUIDE STAR ADAPTIVE OPTICS MONITORING OF 2MASS J15344984-2952274AB: FIRST DYNAMICAL MASS DETERMINATION OF A BINARY T DWARF

We present multiepoch imaging of the T5.0+T5.5 binary 2MASS J15344984-2952274AB obtained with the Keck laser guide star adaptive optics system. Combined with archival HST imaging, our total data span similar to 50% of the orbital period. We use a Markov chain Monte Carlo analysis to determine a period of 15.1(-1.6)(+2.3) yr and a total mass of 0.056 +/- 0.003 M-circle dot (59 +/- 3 M-J). This is the first field binary for which both components are confirmed to be substellar. This is also the coolest and lowest mass binary with a dynamical mass to date. Using evolutionary models and accounting for the measurement covariances, we derive an age of 0.78 +/- 0.09 Gyr and a mass ratio of 0.936(-0.008)(+0.012) The relatively youthful age is consistent with the low tangential velocity of this system. For the individual components, we find T-eff = 1028 +/- 17 and 978 +/- 17 K and masses of 0.0287 +/- 0.0016 M-circle dot (30.1 +/- 1.7 M-J) and 0.0269 +/- 0.0016 M-circle dot (28.2 +/- 1.7 M-J). These values generally agree with previous studies of T dwarfs and affirm current theoretical models. However, (1) the temperatures are about 100 K cooler than derived for similar objects and suggest that the representative ages of field brown dwarfs may be overestimated. Similarly, (2) the H-R diagram positions are discrepant with current models and taken at face value would overestimate the masses. While this may arise from errors in the luminosities and/or radii predicted by evolutionary models, the likely cause is a modest (approximate to 100 K) overestimate in temperature determined from model atmospheres. We elucidate future tests of theory as the sample of dynamical masses grows. In particular, we suggest that low-mass field binaries with dynamical masses (mass benchmarks) can serve as reference points for T-eff and log g to constrain atmospheric models, as good as or even better than single brown dwarfs with age estimates (age benchmarks).