On the sodium overabundance of giants in open clusters: The case of the Hyades

TL;DR

This study carefully measures sodium abundances in Hyades giants, finding a modest overabundance consistent with stellar evolution models, and clarifies previous conflicting results by standardizing analysis methods.

Contribution

It provides a precise NLTE sodium abundance measurement for Hyades giants, addressing discrepancies in prior studies and linking sodium overabundance to stellar rotation and evolution.

Findings

Average [Na/Fe] = +0.30 in NLTE

Na overabundance explained by high initial rotation

No strong evidence of excess Na beyond evolutionary predictions

Abstract

Sodium abundances have been determined in a large number of giants of open clusters but conflicting results, ranging from solar values to overabundances of up to five orders of magnitude, have been found. The reasons for this disagreement are not well-understood. As these Na overabundances can be the result of deep mixing, their proper understanding has consequences for models of stellar evolution. As discussed in the literature, part of this disagreement comes from the adoption of different corrections for non-LTE effects and from the use of different atomic data for the same set of lines. However, a clear picture of the Na behaviour in giants is still missing. To contribute in this direction, this work presents a careful redetermination of the Na abundances of the Hyades giants, motivated by the recent measurement of their angular diameters. An average of [Na/Fe] = +0.30, in NLTE, has been found. This overabundance can be explained by hydrodynamical models with high initial rotation velocities. This result, and a trend of increasing Na with increasing stellar mass found in a previous work, suggests that there is no strong evidence of Na overabundances in red giants beyond those values expected by evolutionary models of stars with more than ~ 2 Msun.