On the evolutionary status of X-ray selected weak-line T Tauri star candidates in Taurus-Auriga

We present lithium observations of 35 stars previously reported by Wichmann et al. (1996) to be possible new weak T Tauri stars (WTTS) discovered by R OSAT in the Taurus-Auriga star-forming region. These stars were identified on the basis of low-resolution optical spectra. We have used our higher res olution spectra for measuring the equivalent widths of the Li I 670.8 nm re sonance line, and for revisiting the evolutionary status of these stars. Mo st (similar to 85%) of the stars in our sample coming from ROSAT pointed ob servations are indeed confirmed to be new WTTS, but only a minority (simila r to 22%) of the stars coming from the ROSAT all-sky survey are confirmed a s WTTS. There are two reasons why we reject some stars as WTTS. One is that seven of the stars do not have a detectable lithium line at all. The other is that we use a definition different from that Wichmann et al. (1996) for classifying stars as WTTS. In particular, we identify eight stars as post T Tauri stars (PTTS) on the basis of their moderate lithium depletion. Our results confirm that the widely dispersed RASS-selected candidate WITS tend to be older than the T Tauri stars associated with dark molecular clouds. The presence of PTTS around central Taurus suggests that the clouds may hav e been forming stars for more than similar to 10 Myr, although at a very lo w rate. On the basis of the PTTS identified in this work we discuss possibl e differences between them and the WTTS. We find that PTTS seem to have sli ghtly lower H alpha emission equivalent width than WTTS, but the small numb er of known PTTS prevent us from making a strong conclusion.