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In a volume-limited sample of 63 ultracool dwarfs of spectral type M7--M9.5,we have obtained high-resolution spectroscopy with UVES at the Very LargeTelescope and HIRES at Keck Observatory. In this first paper we introduce ourvolume-complete sample from DENIS and 2MASS targets, and we derive radialvelocities and space motion. Kinematics of our sample are consistent with thestars being predominantly members of the young disk. The kinematic age of thesample is 3.1 Gyr. We find that six of our targets show strong Li linesimplying that they are brown dwarfs younger than several hundred million years.Five of the young brown dwarfs were unrecognized before. Comparing the fractionof Li detections to later spectral types, we see a hint of an unexpected localmaximum of this fraction at spectral type M9. It is not yet clear whether thismaximum is due to insufficient statistics, or to a combination of physicaleffects including spectral appearance of young brown dwarfs, Li line formation,and the star formation rate at low masses.

A Volume-limited Sample of 63 M7--M9.5 Dwarfs I. Space Motion, Kinematic Age, and Lithium