Multi-level 3D non-LTE computations of lithium lines in the metal-poor halo stars HD 140283 and HD 84937

The lithium abundances in metal-poor halo stars are of importance for cosmology, galaxy evolution and stellar structure. In an attempt to study possible systematic errors in the derived Li abundances, the line formation of Li I lines has been investigated by means of realistic 3D hydrodynamical model atmospheres of halo stars and 3D non-LTE radiative transfer calculations. These are the first detailed 3D non-LTE computations reported employing a multi-level atomic model showing that such problems are now computationally tractable. The detailed computations reveal that the Li I population has a strong influence from the radiation field rather than the local gas temperature, indicating that the low derived Li abundances found by Asplund et al. (1999) are an artifact of their assumption of LTE. Relative to 3D LTE, the detailed calculations show pronounced over-ionization. In terms of abundances the 3D non-LTE values are within 0.05 dex of the ID non-LTE results for the particular cases of HD 140283 and HD 84937, which is a consequence of the dominance of the radiation in determining the population density of Li I. Although 3D non-LTE can be expected to give results rather close (approximate to+/-0.1 dex) to 1D non-LTE for this reason, there may be systematic trends with metallicity and effective temperature.