We present Keck laser guide star adaptive optics imaging and aperture masking observations of the M8+L7 binary LHS 2397aAB .
Together with archival HST , Gemini-North , and VLT data , our observations span 11.8 years of the binary ’ s 14.2-year orbital period .
We determine a total dynamical mass of 0.146 ^ { +0.015 } _ { -0.013 } M _ { \sun } ( 153 ^ { +16 } _ { -14 } M _ { Jup } ) .
Using the combined observational constraints of the total mass and individual luminosities , the Tucson ( Lyon ) evolutionary models give an age for the system of 1.5 ^ { +4.1 } _ { -0.6 } Gyr ( 1.8 ^ { +8.2 } _ { -0.8 } Gyr ) , which is consistent with its space motion based on a comparison to the Besançon Galactic structure model .
We also use these models to determine the mass ratio , giving individual masses of 0.0839 ^ { +0.0007 } _ { -0.0015 } M _ { \sun } ( 0.0848 ^ { +0.0010 } _ { -0.0012 } M _ { \sun } ) for LHS 2397aA and 0.061 ^ { +0.014 } _ { -0.011 } M _ { \sun } ( 0.060 ^ { +0.008 } _ { -0.012 } M _ { \sun } ) for LHS 2397aB .
Because LHS 2397aB is very close to the theoretical mass-limit of lithium burning , which remains untested by dynamical masses , measuring its lithium depletion would uniquely test substellar models .
We estimate a spectral type of L7 \pm 1 for LHS 2397aB , making it the first L/T transition object with a dynamical mass determination .
This enables a precise estimate of its effective temperature from Tucson ( Lyon ) evolutionary models of 1450 \pm 40 K ( 1430 \pm 40 K ) , which is 200 K higher than estimates for young late-L companions but consistent with older late-L field dwarfs , supporting the idea that the temperature of the L/T transition is surface gravity dependent .
Comparing our temperature estimate for LHS 2397aB to those derived from spectral synthesis modeling for similar objects reveals consistency between evolutionary and atmospheric models at the L/T transition , despite the currently limited understanding of this phase of substellar evolution .
Future dynamical masses for L/T binaries spanning a range of surface gravity , age , and mass will provide the next critical tests of substellar models at the L/T transition .