Keck laser guide star adaptive optics monitoring of 2MASS J15344984-2952274AB: first dynamical mass determination of a binary T Dwarf.
LIU M.C., DUPUY T.J. and IRELAND M.J.
Abstract (from CDS):
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 15.1+2.3–1.6 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 0.936+0.012–0.008. The relatively youthful age is consistent with the low tangential velocity of this system. For the individual components, we find Teff=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 dynamical masses (``mass benchmarks'') can serve as reference points for Teff and logg to constrain atmospheric models, as good as or even better than single brown dwarfs with age estimates (``age benchmarks'').
Stars: Binaries: Close - Stars: Binaries: Visual - Infrared: Stars - Stars: Fundamental Parameters - Stars: Low-Mass, Brown Dwarfs - Techniques: High Anular Resolution