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

Abstract

We present a precise kinematic study of very young brown dwarfs in the Cha I cloud based on radial velocities ( RV s) 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 both UVES measurements for very low-mass ones and on RV s from the literature. More UVES spectra were taken compared with a former paper (Joergens & Guenther 2001, A&A, 379, L9), and the reduction of the spectra was improved, while studying the literature for RV s of T Tauri stars in Cha I led to a cleaned and enlarged sample of T Tauri stars. The result is an improved empirical RV distribution of brown dwarfs, as well as of T Tauri stars in Cha I. We found that the RV s of the nine brown dwarfs and very low-mass stars (M 6-M 8) in ChaI that were studied have a mean value of 15.7 km s -1 and a dispersion measured in terms of a 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 earlier in terms of fwhm of 2.1 km s -1 . The studied sample of 25 T Tauri stars (G2-M 5) 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 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 did not find a high-velocity tail for the brown dwarfs RV s. We found hints suggesting different kinematics for 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 higher stellar density in Cha I showing that a fundamental increase in velocity dispersion with stellar density of the star-forming region is not established observationally. The RV s of brown dwarfs observed in Cha I are less dispersed than predicted by existing models for the ejection-scenario.