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 50% of the orbital period. We use a Markov chain Monte Carlo analysis to determine a period of 15:1þ2:3 1:6 yr and a total mass of 0:056 0:003 M (59 3 MJ). 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 0:936þ0:012 0:008. The relatively youthful age is consistent with the low tangential velocity of this system. For the individual components, we find TeA 1⁄4 1028 17 and 978 17 K and masses of 0:0287 0:0016 M (30:1 1:7 MJ) and 0:0269 0:0016 M (28:2 1:7 MJ). 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 ( 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 dynamicalmasses (‘‘mass benchmarks’’) can serve as reference points for TeA and log g to constrain atmospheric models, as good as or even better than single brown dwarfs with age estimates (‘‘age benchmarks’’). Subject headinggs: binaries: close — binaries: visual — infrared: stars — stars: fundamental parameters — stars: low-mass, brown dwarfs — techniques: high angular resolution