Galactic evolution of Copper in the light of NLTE computations

TL;DR

This paper introduces a new NLTE model atom for copper that improves the accuracy of stellar copper abundance measurements, challenging previous LTE-based conclusions about galactic copper evolution.

Contribution

The study develops and validates a NLTE model atom for Cu, providing more accurate stellar abundance measurements across various stars and prompting a revision of galactic copper evolution models.

Findings

NLTE abundances differ significantly from LTE results, up to 1 dex.

The mean [Cu/Fe] ratio in metal-poor stars is -0.22 dex.

Good agreement among Cu I lines across different stellar types.

Abstract

We have developed a model atom for Cu with which we perform statistical equilibrium computations that allow us to compute the line formation of Cu I lines in stellar atmospheres without assuming Local Thermodynamic Equilibrium (LTE). We validate this model atom by reproducing the observed line profiles of the Sun, Procyon and eleven metal-poor stars. Our sample of stars includes both dwarfs and giants. Over a wide range of stellar parameters we obtain excellent agreement among different Cu I lines. The eleven metal-poor stars have iron abundances in the range -4.2 <= [Fe/H] <= -1.4, the weighted mean of the [Cu/Fe] ratios is -0.22 dex, with a scatter of -0.15 dex. This is very different from the results from LTE analysis (the difference between NLTE and LTE abundances reaches 1 dex) and in spite of the small size of our sample it prompts for a revision of the Galactic evolution of Cu.