Two distinct halo populations in the solar neighborhood IV. Lithium abundances

Context. A previous study of F and G main-sequence stars in the solar neighborhood has revealed the existence of two distinct halo populations with a clear separation in [alpha/Fe] for the metallicity range -1.4 < [Fe/H] < -0.7. Taking into account the kinematics and ages of the stars, some Galactic formation models suggest that the high-alpha halo stars were formed in situ, whereas the low-alpha stars have been accreted from satellite galaxies. Aims. In this paper we investigate if there is a systematic difference in the lithium abundances of stars belonging to the high- and low-alpha halo populations. Methods. Equivalent widths of the Li I 6707.8 angstrom resonance line are measured from high resolution VLT/UVES and NOT/FIES spectra and used to derive Li abundances on the basis of MARCS model atmospheres. Furthermore, masses of the stars are determined from the log T-eff - log g diagram by interpolating between evolutionary tracks based on Yonsei-Yale models. Results. There is no significant systematic difference in the lithium abundances of high-and low-alpha stars. For the large majority of stars with masses 0.7 < M/M-circle dot < 0.9 and heavy-element mass fractions 0.001 less than or similar to Z < 0.006, the lithium abundance is well fitted by a relation A(Li) = a(0) + a(1) M + a(2) Z + a(3) M Z, where a(0), a(1), a(2), and a(3) are constants. Extrapolating this relation to Z = 0 leads to a lithium abundance close to the primordial value predicted from standard Big Bang nucleosynthesis calculations and the WMAP baryon density. The relation, however, does not apply to stars with metallicities below [Fe/H] similar or equal to -1.5. Conclusions. We suggest that metal-rich halo stars were formed with a lithium abundance close to the primordial value, and that lithium in their atmospheres has been depleted in time with an approximately linear dependence on stellar mass and Z. The lack of a systematic difference in the Li abundances of high-and low-alpha stars indicates that an environmental effect is not important for the destruction of lithium.