Improved kinematics for brown dwarfs and very low - mass stars in Cha I and a discussion of brown dwarf formation ⋆

We present a precise kinematic study of very young brown dwarfs in the Cha I cloud based on radial velocities (RVs) measured with UVES at the VLT. The kinematics of the brown dwarfs in Cha I are compared to the kinematics of T Tauri stars in the same field based on UVES measurements for very low-mass ones and based on RVs from the literature. In comparison with a former paper (Joergens & Guenther 2001), additional UVES spectra have been taken, the reduction of the spectra was improved and a re-study of the literature for RVs of T Tauri stars in Cha I lead to a cleaned and enlarged sample for the T Tauri stars. The result is an improved empirical RV distribution of the brown dwarfs as well as of the T Tauri stars in Cha I. We find that the RVs of the studied nine brown dwarfs and very low-mass stars (M6–M8) in ChaI have a mean value of 15.7 km s −1 , a dispersion measured in terms of standard deviation of 0.9 km s −1 , and they cover a total range of 2.6 km s −1. The standard deviation is consistent with the dispersion measured in terms of fwhm of 2.1 km s −1 by Joergens & Guenther (2001). The studied sample of 25 T Tauri stars (G2–M5) has a mean RV of 14.7 km s −1 , a dispersion in terms of standard deviation of 1.3 km s −1 and in terms of fwhm of 3.0 km s −1 and a total range of 4.5 km s −1. The RV dispersion of the brown dwarfs is consistent within the errors with that of the T Tauri stars, which is in line with the finding of no mass dependence in some theoretical models of the ejection-scenario for the formation of brown dwarfs. In contrast to current N-body simulations, we do not find a high-velocity tail for the brown dwarfs RVs. We find suggestive hints for different kinematics of binaries compared to predominantly single objects in Cha I, as suggested by some models. The global RV dispersion for Cha I members (1.24 km s −1) is significantly lower than for Taurus members (2.0 km s −1), despite a larger stellar density in Cha I showing that a fundamental increase of velocity dispersion with stellar density of the star forming region is observationally not established. The observed RVs …