Abstract
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.
Highlights
Early- and mid-L dwarfs encompass a mass range that includes very low mass stars as well as more massive brown dwarfs around the hydrogen-burning minimum mass (HBMM ≈ 0.07– 0.08 M, Chabrier & Baraffe 1997)
We present radial velocity measurements of a sample of L0–L8 dwarfs observed with VLT/UVES and Keck/HIRES
We are unable to confirm this finding. Both objects belong to the young disk population, their space motions are inconsistent with the space vector of the Hyades in V and W (2MASS 0036+18) and U and W (2MASS 0825+21), respectively. – 2MASS 0255-47: we report a radial velocity for this object of vrad = 25 ± 4.1 km s−1, which is inconsistent with vrad = 17.5 ± 2.8 km s−1 reported in Zapatero Osorio et al (2007), and vrad = 13.0 ± 3.0 km s−1 in Basri et al (2000)
Summary
Early- and mid-L dwarfs encompass a mass range that includes very low mass stars as well as more massive brown dwarfs around the hydrogen-burning minimum mass (HBMM ≈ 0.07– 0.08 M , Chabrier & Baraffe 1997). The kinematics of these stars, especially within the solar neighbourhood, are important because the intrinsic velocity dispersion of the stars can be used to determine their age (Wielen 1977; Fuchs et al 2001).
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