Non-LTE Sodium Abundance in Galactic Thick- and Thin-Disk Red Giants

TL;DR

This study investigates sodium abundances in various types of red giant stars using non-LTE modeling, finding no significant difference between galactic disk populations and confirming sodium enrichment in BaII stars.

Contribution

It provides the first non-LTE sodium abundance analysis for a diverse sample of red giants across different galactic populations.

Findings

Non-LTE corrections range from -0.06 to -0.24 dex.

No significant [Na/Fe] difference between thin and thick disk stars.

Na abundance in BaII stars shows a weak excess.

Abstract

We evaluate non-local thermodynamical equilibrium (non-LTE) line formation for neutral sodium in model atmospheres of the 79 red giants using the model atom that incorporates the best available atomic data. The non-LTE abundances of Na were determined from Na I 6154, 6161 \AA\ for the 38 stars of the thin disk (15 of them the BaII stars), 15 stars of the thick disk, 13 stars of Hercules stream and 13 transition stars which can be identified with neither thin disk nor thick disk. For Na I 6154, 6161 \AA\ non-LTE abundance corrections amount to -0.06 to -0.24 dex depending on stellar parameters. We found no difference in [Na/Fe] abundance between the thick disk and thin disk and the obtained abundances are close to the solar one. We confirmed a weak excess of [Na/Fe] in BaII stars. Stars of the Hercules stream reveal [Na/Fe] abundances close to the solar one. The obtained results can be used to constrain the nucleosynthesis models for Na.