Keck Laser Guide Star Adaptive Optics Monitoring of the M8+L7 Binary LHS 2397aAB: First Dynamical Mass Benchmark at the L/T Transition

Abstract

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 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 and age will provide the next critical tests of substellar models at the L/T transition.

ApJ, in press

References in corpus (16)

• Direct Imaging of Multiple Planets Orbiting the Star HR 8799
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
• Constraining the Age-Activity Relation for Cool Stars: The SDSS DR5 Low-Mass Star Spectroscopic Sample
• The Solar Neighborhood XVII: Parallax Results from the CTIOPI 0.9m Program -- Twenty New Members of the RECONS 10 Parsec Sample
• Discovery of Two T Dwarf Companions with the Spitzer Space Telescope
• The Disk Around CoKu Tau/4: Circumbinary, not Transitional
• Mapping the Shores of the Brown Dwarf Desert I.: Upper Scorpius
• Subtle Signatures of Multiplicity in Late-type Dwarf Spectra: The Unresolved M8.5 + T5 Binary 2MASS J03202839-0446358
• HD 203030B: an Unusually Cool Young Sub-Stellar Companion near the L/T Transition
• A Survey for Spectroscopic Binaries Among Very Low-Mass Stars
• Keck Laser Guide Star Adaptive Optics Monitoring of 2MASS J1534-2952AB: First Dynamical Mass Determination of a Binary T Dwarf
• On the Presence of Water and Global Circulation in the Transiting Planet HD 189733b
• Space Velocities of L- and T-type Dwarfs
• Dynamical Mass of GJ 802B: a brown dwarf in a triple system
• Improved orbital solution and masses for the very low-mass multiple system LHS 1070

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• A Search for L/T Transition Dwarfs With Pan-STARRS1 and WISE. II. L/T Transition Atmospheres and Young Discoveries
• The Hawaii Infrared Parallax Program. IV. A Comprehensive Parallax Survey of L0-T8 dwarfs with UKIRT
• New Evidence for a Substellar Luminosity Problem: Dynamical Mass for the Brown Dwarf Binary Gl 417BC
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