Proper Motions and Tangential Velocities for aLarge Sample of Late-type M, L and T Dwarfs

摘要

We report proper motion measurements for 427 late-type M, L and T dwarfs, 332 ofwhich have been measured for the first time. Combining these new proper motions with previouslypublished measurements yields a sample of 841 M7-T8 dwarfs. We combined parallax measure-ments or calculated spectrophotometric distances and computed tangential velocities for the entiresample. We find that kinematics for the full and volume-limited 20 pc samples are consistent withthose expected for the Galactic thin disk, with no significant differences between late-type M, L, andT dwarfs. Applying an age-velocity relation we conclude that the average kinematic age of the 20pc sample of ultracool dwarfs is older than recent kinematic estimates and more consistent with ageresults calculated with population synthesis models. We isolate subsets of the entire sample, includ-ing low surface-gravity dwarfs, unusually blue L dwarfs, and photometric outliers in J — IC, colorand investigate their kinematics. We find that the spectroscopically distinct class of unusually blue Ldwarfs has kinematics clearly consistent with old age, implying that high surface-gravity and/or lowmetallicity may be relevant to their spectral properties. The low surface-gravity dwarfs are kinemat-ically younger than the overall population, and the kinematics of the red and blue ultracool dwarfssuggest ages that are younger and older than the full sample, respectively.