On the kinematic age of brown dwarfs: Radial velocities and space motions of 43 nearby L dwarfs

摘要

We present radial velocity measurements of a sample of L0-L8 dwarfs observedwith VLT/UVES and Keck/HIRES. We combine these measurements with distance andproper motion from the literature to determine space motions for 43 of ourtargets. We identify nine candidate members of young moving groups, which haveages of 50-600 Myr according to their space motion. From the total velocitydispersion of the 43 L dwarfs, we calculate a kinematic age of ~5 Gyr for oursample. This age is significantly higher than the ~3 Gyr age known for late Mdwarfs in the solar neighbourhood. We find that the distributions of the U andV velocity components of our sample are clearly non-Gaussian, placing the ageestimate inferred from the full space motion vector into question. TheW-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 forlate-M dwarf samples. Our brightness-limited sample is probably contaminated bya number of outliers that predominantly bias the U and V velocity components.The origin of the outliers remain unclear, but we suggest that these browndwarfs may have gained their high velocities by means of ejection from multiplesystems during their formation.