HIGH-PRECISION RADIO AND INFRARED ASTROMETRY OF LSPM J1314+1320AB. II. TESTING PREMAIN-SEQUENCE MODELS AT THE LITHIUM DEPLETION BOUNDARY WITH DYNAMICAL MASSES

We present novel tests of pre-main-sequence models based on individual dynamical masses for the M7 binary LSPM J1314+1320AB. Joint analysis of Keck adaptive optics astrometric monitoring along with Very Long Baseline Array radio data from a companion paper yield component masses of 92.8 +/- 0.6 M-Jup (0.0885 +/- 0.0006 M-circle dot) and 91.7 +/- 1.0 M-Jup (0.0875 +/- 0.0010 M-circle dot) and a parallactic distance of 17.249 +/- 0.013 pc. We find component luminosities consistent with the system being coeval at 80.8 +/- 2.5 Myr, according to BHAC15 evolutionary models. The presence of lithium is consistent with model predictions, marking the first test of the theoretical lithium depletion boundary using ultracool dwarfs of known mass. However, we find that the evolutionary model-derived average effective temperature (2950 +/- 5K) is 180 K hotter than that given by a spectral type-T-eff relation based on BT-Settl models (2770 +/- 100 K). We suggest that the dominant source of this discrepancy is model radii being too small by approximate to 13%. In a test mimicking the typical application of models by observers, we derive masses on the H-R diagram using luminosity and BT-Settl temperature. The estimated masses are lower by 46(-19)(+16)% 16 (2.0 sigma) than we measure dynamically and would imply that this is a system of approximate to 50M(Jup) brown dwarfs, highlighting the large systematic errors possible in H-R diagram properties. This is the first time masses have been measured for ultracool (>= M6) dwarfs displaying spectral signatures of low gravity. Based on features in the infrared, LSPM J1314+1320AB appears to have higher gravity than typical Pleiades and AB. Dor members, opposite the expectation given its younger age. The components of LSPM J1314+1320AB are now the nearest, lowest mass pre-main-sequence stars with direct mass measurements.