Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. VI. K to M-type stars south of Taurus-Auriga

We present first results of a spectroscopic study of lithium absorption lines in late-type stars at high galactic latitude. The investigated stars were identified previously as optical counterparts of ROSAT All-Sky Survey sources. They are contained in a complete X-ray count-rate limited representive subsample of the RASS which was completely optically identified by our group. No selection criteria other than requiring RASS count rates larger than 0.03 cts s(-1) were applied. The stars are located at galactic latitudes |b(II) | between 28°\ and 45°\ in an area ~ 20°\ south of the Taurus-Auriga star forming region. In the sample of 35 stars with spectral types between K0 and M5 we found 9 stars with significant Li i<~mbda6708 absorption lines, 8 K stars and 1 M0 star. No lithium was found in stars with spectral types later than M0. The Li i equivalent widths (corrected for the contribution of Fe i ) are between ~ 150 m Angstroms and ~ 400 m Angstroms . The lithium strength of these stars is consistent with ages on the order of 10-30 Myr. These stars are still in a pre-main sequence evolutionary stage, and belong to the weak-line Tauri stars.