On the kinematic age of brown dwarfs: Radial velocities and space
  motions of 43 nearby L dwarfs
  We present radial velocity measurements of a sample of L0-L8 dwarfs observed
with VLT/UVES and Keck/HIRES. We combine these measurements with distance and
proper motion from the literature to determine space motions for 43 of our
targets. We identify nine candidate members of young moving groups, which have
ages of 50-600 Myr according to their space motion. From the total velocity
dispersion of the 43 L dwarfs, we calculate a kinematic age of ~5 Gyr for our
sample. This age is significantly higher than the ~3 Gyr age known for late M
dwarfs in the solar neighbourhood. We find that the distributions of the U and
V velocity components of our sample are clearly non-Gaussian, placing the age
estimate inferred from the full space motion vector into question. The
W-component exhibits a distribution more consistent with a normal distribution,
and from W alone we derive an age of ~3 Gyr, which is the same age found for
late-M dwarf samples. Our brightness-limited sample is probably contaminated by
a number of outliers that predominantly bias the U and V velocity components.
The origin of the outliers remain unclear, but we suggest that these brown
dwarfs may have gained their high velocities by means of ejection from multiple
systems during their formation.
