A study of the Chamaeleon star-forming region from the ROSAT all-sky survey .3. High resolution spectroscopic study

We present the results of a high-resolution spectroscopic study on some 70 stars, discovered recently on the basis of the ROSAT all-sky survey spread over a wide area in the Chamaeleon star forming region and classified as new weak-line T Tauri stars. We refine the previous spectral type classification, based on low-resolution spectra, and characterize each star in the sample according to the Her line profile. We use the strength of Li I 6708, compared to Pleiades stars of the same spectral type, as a youth discriminator in order to recognize bona-fide pre-main sequence stars. According to the adopted ''lithium criterion'', more than 50% of the stars in our sample are confirmed to be truly young, PMS stars (most having age less than 5 x 10(6) yr), while the remaining part seems mostly composed by active, young, foreground main-sequence stars (possibly Pleiades-like), which contaminate the original sample. We confirm the existence of some very young stars far from the main Chamaeleon clouds, while we do not find clear evidence for the presence of post-T Tauri stars in our sample. We find that 5 stars in the sample are spectroscopic binaries and 1 is a spectroscopic triple system. We derive radial and rotational velocities for all the stars in sample and analyse their distributions for different spectral type intervals. The radial velocity distribution shows a clear peak at about 15 km s(-1), which coincides with the radial velocity of stars and gas in the Cha I cloud. However, the velocity dispersion of the weak-line T Tauri stars appears much broader and, possibly, a second peak is present around 16-18 km s(-1). A clear segregation in radial velocity is observed between the strong-lithium and the weak-lithium stars, with the former showing radial velocities which, in most cases, fall in the interval 12<RV [km/s] < 18, consistent with the radial velocity peak observed for the Cha I dark cloud, and the latter having somewhat different and more widely spread radial velocities. Some strong-lithium stars having radial velocities outside the aforementioned interval might be unrecognized spectroscopic binaries, but they may also be considered good candidates to be run-away T Tauri stars. The nu sini distribution of the confirmed WTTS is found to resemble very closely that of other pre-ROSAT PMS stars. We conclude that the whole star forming region has a common origin, possibly related to the impact of a high-velocity cloud with the galactic plane.