Non-LTE Analysis for the Sodium Abundances of Metal-Poor Stars in the Galactic Disk and Halo

We performed an extensive non-LTE analysis of the neutral sodium lines of Na i 5683/5688, 5890/5896, 6154/6161, and 8183/8195 for disk/halo stars of F– K type covering a wide metallicity range (−4 ∼< [Fe/H] ∼< 0.4), based on our own data as well as those collected from the literature. For comparatively metal-rich disk stars (−1 ∼< [Fe/H] ∼< 0) where the weaker 6154/6161 lines are 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 by previous studies. Regarding the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines suffering 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 ∼ ±0.2 dex) on the average at the typical halo metallicity of [Fe/H] ∼ −2, while they appear to rise again toward a very metal-poor regime recovering a near-solar ratio of [Na/Fe] ∼ 0 ([Fe/H] ∼ −3 to −4). These results ⋆ E-mail: [email protected]