Inelastic H+Li and H^-+Li⁺ collisions and non-LTE Li I line formation in stellar atmospheres

Rate coefficients for inelastic collisions between Li and H atoms covering all transitions between the asymptotic states Li(2s,2p,3s,3p,3d,4s,4p,4d,4f)+H(1s) and Li⁺+H^- are presented for the temperature range 2000-8000 K based on recent cross-section calculations. The data are of sufficient completeness for non- LTE modelling of the Li I 670.8 nm and 610.4 nm features in late-type stellar atmospheres. Non-LTE radiative transfer calculations in both 1D and 3D model atmospheres have been carried out for test cases of particular interest. Our detailed calculations show that the classical modified Drawin-formula for collisional excitation and de-excitation (Li* + H ⇌ Li*′ + H) over-estimates the cross-sections by typically several orders of magnitude and consequently that these reactions are negligible for the line formation process. However, the charge transfer reactions collisional ion-pair production and mutual neutralization (Li* + H ⇌ Li⁺ + H^-) are of importance in thermalizing Li. In particular, 3D non-LTE calculations of the Li 1670.8 nm line in metal- poor halo stars suggest that ID non-LTE results over-estimate the Li abundance by up to about 0.1 dex, aggrevating the discrepancy between the observed Li abundances and the primordial Li abundance as inferred by the WMAP analysis of the cosmic microwave background.