1997A&A...325..647N

We study 111 late-type stars found with optical follow-up observations of ROSAT All-Sky 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 strong lithium 6708Å absorption in intermediate-resolution (∼1Å) 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 momentum evolution and kinematic membership to the Taurus clouds. In addition, we obtained echelle spectra with high (∼0.25Å) resolution for seven stars to measure precisely the lithium equivalent width W_λ(Li). We find that ∼1Å resolution is sufficient in order not to overestimate W_λ(Li), e.g. due to blending. Of our 111 stars, 19 are located on the λ Ori cloud, nine of which are K-type stars with lithium in excess of the zero-age-main-sequence (ZAMS) level, i.e. are PMS stars. At least 40 of the remaining 92 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 with the same spectral types. Of those 24 stars, nine (25% of the off-cloud stars with detectable lithium) are PMS stars isolated from cloud material; all have spectral type K, and three of them are spectroscopic binaries. 15 off-cloud stars have spectral type G and lithium comparable to IC 2602 stars, i.e. may have already arrived on the ZAMS just like G-type IC 2602 stars. However, all these 24 off-cloud lithium-excess stars are probably not older than IC 2602 (∼3x10⁷yrs). We discuss possible sites and modes of origin of the isolated PMS stars south of Taurus.