Copper abundance from Cu I and Cu II lines in metal-poor star spectra: NLTE vs LTE

TL;DR

This study compares LTE and NLTE methods for determining copper abundance in metal-poor stars, confirming NLTE's reliability and suggesting copper acts as a primary element at low metallicities, impacting nucleosynthesis models.

Contribution

It introduces a reliable NLTE copper atomic model and demonstrates its consistency with LTE results in metal-poor stars, challenging previous LTE-based assumptions.

Findings

Good agreement between Cu I and Cu II lines supports NLTE model validity.

Copper behaves as a primary element at low metallicities, not secondary.

Reevaluation of copper yields in explosive nucleosynthesis is needed.

Abstract

We checked consistency between the copper abundance derived in six metal-poor stars using UV Cu II lines (which are assumed to form in LTE) and UV Cu I lines (treated in NLTE). Our program stars cover the atmosphere parameters which are typical for intermediate temperature dwarfs (effective temperature is in the range from approximately 5800 to 6100 K, surface garvity is from 3.6 to 4.5, metallicity is from about -1 to -2.6 dex). We obtained a good agreement between abundance from these two sets of the lines, and this testifies about reliability of our NLTE copper atomic model. We confirmed that no underabundace of this element is seen at low metallicities (the mean [Cu/Fe] value is about -0.2 dex, while as it follows from the previous LTE studies copper behaves as a secondary element and [Cu/Fe] ratio in the range of [Fe/H from -2 to -3 dex should be about -1 dex). According to our NLTE data the copper behaves as a primary element at low metallicity regime. We also conclude that our new NLTE copper abundance in metal-poor stars requires significant reconsideration of this element yields in the explosive nucleosynthesis.