Abstract We performed an extensive non-LTE analysis of the neutral sodium lines of Na I 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halo stars of types F--K covering a wide metallicity range (–4 ≲[Fe/H]≲ +0.4), using our own data as well as data collected from the literature. For comparatively metal-rich disk stars (–1 ≲[Fe/H]≲ +0.4) where the weaker 6154/6161 lines are the best abundance indicators, we confirmed [Na/Fe] ∼ 0 with an ``upturn'' (i.e., a shallow/broad dip around –0.5 ≲ [Fe/H] ≲ 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 ∼ –0.4 (with a somewhat large scatter of ∼±0.2dex) on the average at the typical halo metallicity of [Fe/H] ∼ –2, followed by a rise again to a near-solar ratio of [Na/Fe] ∼ 0 at the very metal-poor regime [Fe/H] ∼ –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.

Keywords Galaxy: evolution --- radiative transfer --- stars: abundances --- stars: atmospheres --- stars: late-type

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