Paper: astro-ph/0509462
Date: Thu, 15 Sep 2005 16:05:01 GMT (34kb)

Title: Improved kinematics for brown dwarfs and very low-mass stars in ChaI and
a discussion of brown dwarf formation

Authors: V. Joergens

Comments: accepted for publication in A&A, 9 pages, 4 figures
\\
We present a precise kinematic study of very young brown dwarfs (BDs) in the
ChaI cloud based on radial velocities (RVs) measured with UVES / VLT. This is
compared to the kinematics of T Tauri stars (TTS) in the same field based on
own measurements and 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 TTS in ChaI was done. The result is an improved empirical RV distribution of
the BDs as well as of the TTS in ChaI. We find that nine BDs/VLMSs (M6--M8) in
ChaI have a RV dispersion of 0.9 km/s measured in terms of standard deviation.
This is consistent with the dispersion measured in terms of fwhm of 2.1 km/s by
Joergens & Guenther (2001). The studied sample of 25 TTS (G2-M5) has a
dispersion of 1.3 km/s (standard deviation). The RV dispersion of the BDs is
consistent within the errors with that of the TTS, 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 BDs RVs. We
find suggestive hints for different kinematics of binaries compared to
predominantly single objects in ChaI. The global RV dispersion for ChaI members
(1.24 km/s) is significantly lower than for Taurus members (2.0 km/s), despite
a larger stellar density in ChaI showing that a fundamental increase of
velocity dispersion with stellar density of the star-forming region is
observationally not established. The observed RVs of BDs in ChaI are less
dispersed than predicted by any of the models for the ejection-scenario.

\\ ( http://arXiv.org/abs/astro-ph/0509462 , 34kb)