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

TLDR: In this article, 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 was performed.

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 ± 0.2 dex) 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.