8 Summary

Based on high-resolution UVES spectra of bona fide and candidate brown dwarfs and of T Tauri stars in ChaI we determined RVs with a precision of $\sim$200 ms^-1.

We found that the RV dispersion of nine of the twelve bona fide and candidate brown dwarfs in ChaI is 2.0 kms^-1, i.e. significantly smaller than the RV dispersion of T Tauri stars (3.6 kms^-1) in this cloud and slightly larger than the dispersion of the surrounding molecular gas (1.2 kms^-1) (Mizuno et al. 1999). This result indicates that the majority of the bona fide and candidate brown dwarfs in ChaI are not ejected with high velocities out of a dense region as proposed in some formation scenarios (Sterzik & Durisen 1999; Reipurth & Clarke 2001). Some or all of the brown dwarfs may still have been thrown with less than escape velocity into an extended orbit around another component of a multiple system.

The kinematic study of the T Tauri stars in ChaI was based on a compilation of all T Tauri stars located in ChaI where RVs were known to better than 2 kms^-1(including our UVES data for three T Tauri stars as well as unpublished data taken with FEROS). The mean RV of the T Tauri stars is 14.9 kms^-1, i.e. the same as for the brown dwarfs. The larger RV dispersion of the T Tauri stars can at least partly be attributed to "RV noise'' caused by stellar activity (Guenther et al. 2000).

Time-resolved spectroscopy revealed significant RV variations for five of the bona fide and candidate brown dwarfs in ChaI, which may be caused by orbiting planets or spots on the surface (Joergens et al. 2001).

Determination of $v\,\sin\,i$ showed that the brown dwarfs do not rotate crucially faster than the T Tauri stars in the same cloud. The $v\,\sin\,i$ values together with radii derived from the literature constrained the maximum rotational periods for the individual brown dwarfs to one to three days. Last not least we detected lithium in absorption for all studied objects and measured the equivalent width - for ChaH$\alpha$12 this is the first lithium detection.

The data presented in this paper showed once more that the brown dwarfs in ChaI form a very homogenous sample and are highly interesting astrophysical objects.

Acknowledgements

We acknowledge helpful discussions on the topic of this paper with R. Neuhäuser, R. Durisen, K. Tachihara and F. Comerón. Furthermore we like to acknowledge the brilliant work of the ESO staff at Paranal. V.J. acknowledges grant from the Deutsche Forschungsgemeinschaft (Schwerpunktprogramm "Physics of star formation'').