PHOTOMETRIC AND SPECTROSCOPIC STUDIES OF COOL STARS DISCOVERED IN EXOSAT X-RAY IMAGES .2. LITHIUM ABUNDANCES

We present high-resolution spectra in the Li I 6708 angstrom region, a nd multicolor photometry, for a sample of active stars detected serend ipitously by the EXOSAT satellite. We analyse the spectral data with a spectral synthesis code and derive Li abundances, rotation rates and metallicities. We use our multicolor photometry to infer the spectral classification and distance of the stars in the sample. The derived Li abundances are compared with those of various samples including rando mly selected field stars, RS CVn binaries, and clusters and associatio ns of different ages. We show that Li per se is not sufficient to dete rmine unambiguously the nature and evolutionary state of the sample st ars, but can provide useful constraints if used in conjunction with ot her diagnostics like binarity, spectral class, rotation rate and X-ray luminosity. We show in particular that at least one third of the EXOS AT sample is constituted by young stars, with ages comparable to or yo unger than the Pleiades (approximately 7 x 10(7) yr). Another third co nsists of RS CVn binaries, while the classification of the remaining o bjects is uncertain. They could be either young stars or evolved activ e binaries. We argue that a similar composition also applies to the sa mple of X-ray stellar sources detected serendipitously by the Einstein Observatory.