Lithium abundances for 185 main-sequence stars - Galactic evolution and stellar depletion of lithium

We present a survey of lithium abundances in 185 main-sequence field stars with 5600 less than or similar to T-eff less than or similar to 6600 K and -1.4 less than or similar to [Fe/H] less than or similar to +0.2 based on new measurements of the equivalent width of the lambda 6708 Li I line in high-resolution spectra of 130 stars and a reanalysis of data for 55 stars from Lambert et al. (1991). The survey takes advantage of improved photometric and spectroscopic determinations of effective temperature and metallicity as well as mass and age derived from Hipparcos absolute magnitudes, offering an opportunity to investigate the behaviour of Li as a function of these parameters. An interesting result from this study is the presence of a large gap in the log epsilon (Li) T-eff plane, which distinguishes Li-dip stars like those first identified in the Hyades cluster by Boesgaard & Tripicco (1986) from other stars with a much higher Li abundance. The Li-dip stars concentrate on a certain mass, which decreases with metallicity from about 1.4 M-. at solar metallicity to 1.1 M-. at [Fe/H] similar or equal to -1.0. Excluding the Li-dip stars and a small group of lower mass stars with T-eff < 5900 K and log (Li) < 1.5, the remaining stars, when divided into four metallicity groups, may show a correlation between Li abundance and stellar mass. The dispersion around the log (Li)-mass relation is about 0.2 dex below [Fe/H] similar or equal to -0.4 and 0.3 dex above this metallicity, which cannot be explained by observational errors or differences in metallicity. Furthermore, there is no correlation between the residuals of the log epsilon (Li)-mass relations and stellar age, which ranges from 1.5 Gyr to about 15 Gyr. This suggests that Li depletion occurs early in stellar life and that parameters other than stellar mass and metallicity affect the degree of depletion, e.g. initial rotation velocity and/or the rate of angular momentum loss. It cannot be excluded, however, that a cosmic scatter of the Li abundance in the Galaxy at a given metallicity contributes to the dispersion in Li abundance. These problems make it difficult to determine the Galactic evolution of Li from the data, but a comparison of the upper envelope of the distribution of stars in the log epsilon (Li) [Fe/H] plane with recent Galactic evolutionary models by Romano et al. (1999) suggests that novae are a major source for the Li production in the Galactic disk; their occurrence seems to be the explanation for the steep increase of Li abundance at [Fe/H] similar or equal to -0.4.