We present multi-epoch , near-infrared imaging of the binary T5.0+T5.5 dwarf 2MASS J15344984 - 2952274AB obtained with the Keck laser guide star adaptive optics system .
Our Keck data achieve sub-milliarcsecond relative astrometry and combined with an extensive ( re- ) analysis of archival HST imaging , the total dataset spans \sim 50 % the orbital period .
We use a Markov Chain Monte Carlo analysis to determine an orbital period of 15.1 _ { -1.6 } ^ { +3.1 } yr and a semi-major axis of 2.3 _ { -0.2 } ^ { +0.3 } AU .
We measure a total mass of 0.056 \pm 0.003 M _ { \sun } ( 59 \pm 3 M _ { Jup } ) , where the largest uncertainty arises from the parallax .
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 determination to date .
Using evolutionary models and accounting for the measurement covariances , we derive an age of 0.78 \pm 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 \mbox { $T _ { eff } $ } = 1028 \pm 17 K and 978 \pm 17 K , \mbox { $ \log ( g ) $ } = 4.91 \pm 0.04 and 4.87 \pm 0.04 ( cgs ) , and masses of 0.0287 \pm 0.0016 ~ { } \mbox { $M _ { \sun } $ } ( 30.1 \pm 1.7 M _ { Jup } ) and 0.0269 \pm 0.0016 ~ { } \mbox { $M _ { \sun } $ } ( 28.2 \pm 1.7 M _ { Jup } ) .
These precise 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 field 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 model predictions and taken at face value would overestimate the masses .
While this may arise from large errors in the luminosities and/or radii predicted by evolutionary models , the likely cause is a modest ( \approx 100 K ) overestimate in temperature of T dwarfs determined from model atmospheres .
We elucidate future tests of theory as the sample of substellar dynamical masses increases .
In particular , we suggest that field brown dwarf binaries with dynamical masses ( “ mass benchmarks ” ) can serve as reference points for T _ { eff } and \log ( g ) and thereby constrain ultracool atmosphere models , as good as or even better than single brown dwarfs with age estimates ( “ age benchmarks ” ) .