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We present novel tests of pre$-$main-sequence models based on individualdynamical masses for the M7 binary LSPM J1314+1320AB. Joint analysis of ourKeck adaptive optics astrometric monitoring along with Very Long Baseline Arrayradio data from a companion paper yield component masses of $0.0885\pm0.0006$$M_{\odot}$ and $0.0875\pm0.0010$ $M_{\odot}$ and a parallactic distance of$17.249\pm0.013$ pc. We also derive component luminosities that are consistentwith the system being coeval at an age of $80.8\pm2.5$ Myr, according to BHAC15evolutionary models. The presence of lithium is consistent with modelpredictions, marking the first time the theoretical lithium depletion boundaryhas been tested with ultracool dwarfs of known mass. However, we find that theaverage evolutionary model-derived effective temperature ($2950\pm5$ K) is 180K hotter than we derive from a spectral type$-$$T_{\rm eff}$ relation based onBT-Settl models ($2770\pm100$ K). We suggest that the dominant source of thisdiscrepancy is model radii being too small by $\approx$13%. In a test thatmimics the typical application of evolutionary models by observers, we derivemasses on the H-R diagram using the luminosity and BT-Settl temperature. Theestimated masses are $46^{+16}_{-19}$% (2.0$\sigma$) lower than we measuredynamically and would imply that this is a system of $\approx$50 $M_{\rm Jup}$brown dwarfs, highlighting the large systematic errors possible when inferringmasses from the H-R diagram. This is first time masses have been measured forultracool ($\geq$M6) dwarfs displaying spectral signatures of low gravity.Based on features in the infrared, LSPM J1314+1320AB appears higher gravitythan typical Pleiades and AB Dor members, opposite the expectation given itsyounger age. The components of LSPM J1314+1320AB are now the nearest, lowestmass pre$-$main-sequence stars with direct mass measurements.

astrophysical journal/the astrophysical journal

High-Precision Radio and Infrared Astrometry of LSPM J1314+1320AB - II:  Testing Pre--Main-Sequence Models at the Lithium Depletion Boundary with  Dynamical Masses