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Table · dataset · 2015

BDKP IV. RVs of late-M and L dwarfs with MagE

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Radial velocity measurements are presented for 85 late M- and L-type very low-mass stars and brown dwarfs obtained with the Magellan Echellette spectrograph.

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Targets primarily have distances within 20pc of the Sun, with more distant sources selected for their unusual spectral energy distributions. We achieved precisions of 2-3km/s, and combined these with astrometric and spectrophotometric data to calculate UVW velocities.

Most are members of the thin disk of the Galaxy, and velocity dispersions indicate a mean age of 5.2+/-0.2Gyr for sources within 20pc. We find significantly different kinematic ages between late-M dwarfs (4.0+/-0.2Gyr) and L dwarfs (6.5+/-0.4Gyr) in our sample that are contrary to predictions from prior simulations. This difference appears to be driven by a dispersed population of unusually blue L dwarfs which may be more prevalent in our local volume-limited sample than in deeper magnitude-limited surveys.

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The L dwarfs exhibit an asymmetric U velocity distribution with a net inward flow, similar to gradients recently detected in local stellar samples. Simulations incorporating brown dwarf evolution and Galactic orbital dynamics are unable to reproduce the velocity asymmetry, suggesting non-axisymmetric perturbations or two distinct L dwarf populations. We also find the L dwarfs to have a kinematic age-activity correlation similar to more massive stars.

We identify several sources with low surface gravities, and two new substellar candidate members of nearby young moving groups: the astrometric binary DENIS J08230313-4912012AB, a low-probability member of the {beta} Pictoris Moving Group; and 2MASS J15104786-2818174, a moderate-probability member of the 30-50Myr Argus Association.

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