* Astronomy

Title: Keck Laser Guide Star Adaptive Optics Monitoring of the M8+L7 Binary LHS 2397aAB: First Dynamical Mass Benchmark at the L/T Transition
Authors: Trent J. Dupuy, Michael C. Liu (IfA/Hawaii), Michael J. Ireland (Sydney)

We present Keck LGS AO 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±0.014 MSun (153±15 MJup). Using the combined observational constraints of the total mass and individual luminosities, the Tucson (Lyon) evolutionary models give an age for the system of 1.6 Gyr (1.8 Gyr), consistent with its space motion. We also use these models to determine the mass ratio, giving individual masses of 0.0839±0.0011 MSun (0.0848±0.0011 MSun) for LHS 2397aA and 0.061±0.013 MSun (0.060±0.010 MSun) for LHS 2397aB. Because LHS 2397aB is very close to the theoretical mass-limit of lithium burning, measuring its lithium depletion would uniquely test substellar models. We estimate a spectral type of L7±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) models of 1460±40 K (1440±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 modelling 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 and age will provide the next critical tests of substellar models at the L/T transition.

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