The non-LTE line formation of neutral carbon in late-type stars

Aims. We investigate the non-Local Thermodynamic Equilibrium (non-LTE) line formation of neutral carbon in late-type stars in order to remove some of the potential systematic errors in stellar abundance analyses employing CI features. Methods. The statistical equilibrium code MULTI was used on a grid of plane-parallel ID MARCS atmospheric models. Results. Within the parameter space explored, the high-excitation CI lines studied are stronger in non-LTE due to the combined effect of line source function drop and increased line opacity due to overpopulation of the lower level for the transitions considered; the relative importance of the two effects depends on the particular combination of T_eff, log g, [Fe/H] and [C/Fe] and on the analysed CI line. As a consequence, the non-LTE abundance corrections are negative and can be substantially so, for example ∼-0.4 dex in halo turn-off stars at [Fe/H] ∼ -3. The magnitude of the non-LTE corrections is rather insensitive to whether inelastic H collisions are included or not. Conclusions. Our results have implications on studies of nucleosynthetic processes and on Galactic chemical evolution models. When applying our calculated corrections to recent observational data, the upturn in [C/O] at low metallicity might still be present (thus apparently still necessitating contributions from massive Pop. Ill stars for the carbon production), but at a lower level and possibly with a rather shallow trend of ∼-0.2 dex/dex below [O/H] ∼ -1.