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

Abstract

We performed an extensive non-LTE analysis of the neutral sodium lines of Na {\\sc i} 5683/5688, 5890/5896, 6154/6161, and 8183/8195 for disk/halo stars of F--K type covering a wide metallicity range ($-4 \\la$ [Fe/H] $\\la 0.4$), based on our own data as well as those collected from the literature. For comparatively metal-rich disk stars ($-1 \\la$ [Fe/H] $\\la 0$) where the weaker 6154/6161 lines are best abundance indicators, we confirmed [Na/Fe] $\\sim$ 0 with an ``upturn'' (i.e., a shallow/broad dip around $-0.5 \\la$ [Fe/H] $\\la 0$) 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 $\\sim -0.4$ (with a somewhat large scatter of $\\sim \\pm 0.2$ dex) on the average at the typical halo metallicity of [Fe/H] $\\sim -2$, while they appear to rise again toward a very metal-poor regime recovering a near-solar ratio of [Na/Fe] $\\sim$ 0 ([Fe/H] $\\sim -3$ to -4). These results are discussed in comparison with the previous observational studies along with the theoretical predictions from the available chemical evolution models.