Lithium in X-Ray Selected Active Cool Stars

Abstract

As part of a larger program to study in the optical region the physical characteristics of X-ray selected cool active stars, we have analysed in the LiI 6708 A line region 56 stars of spectral types F5–K8 detected by EXOSAT and ROSAT surveys. High resolution spectra were taken with the McMath telescope at Kitt Peak. Lithium abundances have been computed with n-LTE models and rotation velocities derived by using the fxcor IRAF package with a sample of standard stars; colour indices B — V have been taken from the Hipparcos and Ticho Catalogues or obtained from FeI equivalent widths; X-ray luminosities have been computed using parallaxes from Hipparcos Catalogue. The ages have been estimated comparing Li EW with those of stars belonging to the Hyades, Pleiades and IC 2602. Figure 1 compares our values with the typical Li values in the Hyades, the Li upper envelope of the rapid rotating Pleiades stars, the Li upper envelope of IC 2602 stars (~ 30 Myr old), and the Li upper envelope for T Tauri stars (see also Figure 2 by Pastori et al., 1998). A large fraction of stars is found in the interval 70–300 Myr. We find good correlations between Li abundances and B-V indices for single stars (excluding BY Dra stars) and components of visual binaries, and for different ages. Of course, the correlation is less satisfactory for the whole sample, as expected. For the single stars we find also a weak correlation between Li abundances and L x (Figure 2). No clear trends are shown by the diagrams log N(Li), L x vs. Vsini for the single stars (Figures 3 and 4), with a general increase of these quantities for V sin i > 10 km sec-1. The increase of L x shows a more regular trend for stars younger than the Pleiades. Our sample includes also EK Dra which is considered an ideal and very active analog of the young Sun at a time when it had just arrived on the ZAMS and was forming its planetary system. We find for this star: log(N Li /N H ) = 3.105, V sin i = 15.9 km sec-1, L x = 1.03 × 1030 erg sec-1. The age is estimated to be between that of the Pleiades and IC 2602.