NLTE analysis of copper lines in different stellar populations

TL;DR

This study investigates the non-local thermodynamic equilibrium (NLTE) effects on copper line formation in 29 metal-poor stars across different Galactic populations, revealing significant NLTE corrections especially in very metal-poor stars.

Contribution

It provides detailed NLTE calculations for copper lines using an extensive atomic model and demonstrates the importance of NLTE effects in accurately determining copper abundances in metal-poor stars.

Findings

NLTE effects are significant for Cu I lines in metal-poor stars.

NLTE abundance corrections can reach up to +0.5 dex.

[Cu/Fe] is under-abundant (~ -0.5 dex) in metal-poor stars when NLTE is considered.

Abstract

The copper abundances of 29 metal-poor stars are determined based on the high resolution, high signal-to-noise ratio spectra from the UVES spectragraph at the ESO VLT telescope. Our sample consists of the stars of the Galactic halo, thick- and thin-disk with [Fe/H] ranging from ~ -3.2 to ~ 0.0 dex. The non-local thermodynamic equilibrium (NLTE) effects of Cu I lines are investigated, and line formation calculations are presented for an atomic model of copper including 97 terms and 1089 line transitions. We adopted the recently calculated photo-ionization cross-sections of Cu I, and investigated the hydrogen collision by comparing the theoretical and observed line profiles of our sample stars. The copper abundances are derived for both local thermodynamic equilibrium (LTE) and NLTE based on the spectrum synthesis methods. Our results show that the NLTE effects for Cu I lines are important for metal-poor stars, in particular for very metal-poor stars, and these effects depend on the metallicity. For very metal-poor stars, the NLTE abundance correction reaches as large as ~ +0.5 dex compared to standard LTE calculations. Our results indicate that [Cu/Fe] is under-abundant for metal-poor stars (~ -0.5 dex) when the NLTE effects are included.