Non-LTE analysis of the sodium abundance of metal-poor stars in the galactic disk and halo

We performed an extensive non-LTE analysis of the neutral sodium lines of Na 1 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halo stars of types F-K covering a wide metallicity range (-4 less than or similar to [Fe/H] less than or similar to +0.4), using our own data as well as data collected from the literature. For comparatively metal-rich disk stars (-1 less than or similar to [Fe/H] less than or similar to +0.4) where the weaker 6154/6161 lines are the best abundance indicators, we confirmed [Na/Fe] similar to 0 with an upturn (i.e., a shallow/broad dip around -0.5 less than or similar to [Fe/H] less than or similar to 0) as already reported in previous studies. For the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines subject to considerable (negative) non-LTE corrections amounting to 0.5 dex have to be used, our analysis suggests mildly subsolar [Na/Fe] values down to similar to -0.4 (with a somewhat large scatter of similar to +/-0.2 dex) on the average at the typical halo metallicity of [Fe/H] similar to -2, followed by a rise again to a near-solar ratio of [Na/Fe] similar to 0 at the very metal-poor regime [Fe/H] similar to -3 to -4. These results are discussed in comparison with the previous observational studies along with the theoretical predictions from the available chemical evolution models.