1998A&A...338..637B

Systematic effects in the statistical equilibrium of sodium in cool metal-poor stars are analyzed using full NLTE line formation. To determine the important influence of collision processes and of the atomic model, NLTE effects of neutral sodium are first evaluated in the solar photosphere where the statistical equilibrium of Na I can be followed by examination of a relatively large number of line transitions. In agreement with previous analyses it is found that even very simple atomic models are sufficient to describe the most important interactions. In the solar atmosphere the inner cores of the lines are most affected by deviations from LTE, but the corresponding abundance corrections due to NLTE populations are small. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores. The profile analysis depends sensitively on the appropriate choice of velocity amplitudes and its variation with depth. The resulting solar sodium abundance is obtained with small scatter, logε_Na, ☉_=6.30±0.03. In metal-poor stars NLTE effects are more pronounced since the statistical equilibrium is dominated by collisions in which at least the electronic component is substantially reduced. The resulting influence on the determination of Na abundances is in a direction opposite to that found previously for Al. Stars determined in LTE analyses to have a solar [Na/Fe] ratio reveal a lower [Na/Fe] when NLTE line formation is taken into account. As for aluminium, the extremely metal-poor and the hotter subdwarfs are affected most strongly by Na abundance corrections reaching -0.5dex for the D lines. The resulting Galactic evolution of the Na/Fe and Na/Mg ratios is not adequately described by current chemical evolution scenarios.