HIGH-RESOLUTION SPECTROSCOPY OF ROSAT DETECTED LATE-TYPE STARS SOUTH OF THE TAURUS MOLECULAR CLOUDS

We study 111 late-type stars found with optical follow-up observations of ROSAT AII-S ky Survey sources south of the Taurus molecular clouds . Some 30 of them have been claimed to be weak-line T Tauri stars, low -mass pre-main sequence (PMS) stars, based mainly on the presence of s trong lithium 6708 Angstrom absorption in intermediate-resolution (sim ilar to 1 Angstrom) spectra. We obtained single-order echelle spectra at blue wavelengths for 106 of these stars, in order to measure their radial and rotational velocities, and investigate their angular moment um evolution and kinematic membership to the Taurus clouds. In additio n, we obtained echelle spectra with high (similar to 0.25 Angstrom) re solution for seven stars to measure precisely the lithium equivalent w idth W-lambda(Li). We find that similar to 1 Angstrom resolution is su fficient in order not to overestimate W-lambda(Li), e.g. due to blendi ng. Of our 111 stars, 19 are located on the lambda Ori cloud, nine of which are K-type stars with lithium in excess of the zero-age-main-seq uence (ZAMS) level, i.e. are PMS stars. At least 40 of the remaining 9 2 off-cloud stars display detectable lithium, 24 of which are lithium- excess stars, i.e. show lithium at least as strong as IC 2602 stars wi th the same spectral types. Of those 24 stars, nine (25% of the off-cl oud stars with detectable lithium) are PMS stars isolated from cloud m aterial; all have spectral type K, and three of them are spectroscopic binaries. 15 off-cloud stars have spectral type G and lithium compara ble to IC 2602 stars, i.e. may have already arrived on the ZAMS just l ike G-type IC 2602 stars. However, all these 24 off-cloud lithium-exce ss stars are probably not older than IC 2602 (similar to 3.10(7) yrs). We discuss possible sites and modes of origin of the isolated PMS sta rs south of Taurus.