Lithium abundances in metal-poor stars

Aims. Lithium abundances for 19 metal-poor stars are determined using high-resolution spectroscopy. The abundances of stars on the lithium plateau are discussed. Methods. All abundance results are derived from NLTE statistical equilibrium calculations and spectrum synthesis methods. Results. In agreement with previous analyses it is found that excitation and de-excitation due to hydrogen collisions are negligible for the lithium line formation process, while charge transfer reactions are an important source of thermalization. However, the resulting NLTE effects on the determination of lithium abundances for metal-poor stars are negligible (< 0.06 dex). Conclusions. The mean lithium abundance for stars on the lithium plateau determined from NLTE analyses is A(Li) similar to 2.26, while it is 2.21 dex when charge transfer reactions are included. The latter result enhances the discrepancy between the observed lithium abundances and the primordial lithium abundance as inferred by the WMAP analysis of the cosmic microwave background. This discrepancy may be explained by metal diffusion.