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High-precision Dynamical Masses of Very Low Mass Binaries
We present the results of a three year monitoring program of a sample ofvery low mass (VLM) field binaries using both astrometric andspectroscopic data obtained in conjunction with the laser guide staradaptive optics system on the W. M. Keck II 10 m telescope. Among the 24systems studied, 15 have undergone sufficient orbital motion, allowingus to derive their relative orbital parameters and hence their totalsystem mass. These measurements more than double the number of massmeasurements for VLM objects, and include the most precise massmeasurement to date (<2%). Among the 11 systems with both astrometricand spectroscopic measurements, six have sufficient radial velocityvariations to allow us to obtain individual component masses. This isthe first derivation of the component masses for five of these systems.Altogether, the orbital solutions of these low mass systems show acorrelation between eccentricity and orbital period, consistent withtheir higher mass counterparts. In our primary analysis, we find thatthere are systematic discrepancies between our dynamical massmeasurements and the predictions of theoretical evolutionary models(TUCSON and LYON) with both models either underpredicting oroverpredicting the most precisely determined dynamical masses. Thesediscrepancies are a function of spectral type, with late-M through mid-Lsystems tending to have their masses underpredicted, while one T-typesystem has its mass overpredicted. These discrepancies imply that eitherthe temperatures predicted by evolutionary and atmosphere models areinconsistent for an object of a given mass, or the mass-radiusrelationship or cooling timescales predicted by the evolutionary modelsare incorrect. If these spectral-type trends are correct and hold intothe planetary mass regime, the implication is that the masses ofdirectly imaged extrasolar planets are overpredicted by the evolutionarymodels.
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