Non-LTE analysis of neutral copper in the late-type metal-poor stars

TL;DR

This study performs a detailed non-LTE analysis of copper in late-type, metal-poor stars, revealing significant non-LTE effects and a metallicity-dependent copper abundance trend that refines previous understandings.

Contribution

It introduces the first non-LTE copper abundance analysis for late-type stars, highlighting the importance of non-LTE effects and their impact on abundance trends.

Findings

Non-LTE effects cause ~0.17 dex enhancement at [Fe/H] ~ -1.5.

Copper abundances increase with decreasing metallicity in non-LTE calculations.

The [Cu/Fe] distribution is flatter than previously thought, with possible differences between disk populations.

Abstract

We investigated the copper abundances for $64$ late-type stars in the Galactic disk and halo with effective temperatures from $5400$ K to $6700$ K and [Fe/H] from $-1.88$ to $-0.17$. For the first time, the copper abundances are derived using both local thermodynamic equilibrium (LTE) and non-local thermodynamic equilibrium (non-LTE) calculations. High resolution ($R > 40,000$), high signal-to-noise ratio ($S/N > 100$) spectra from the FOCES spectrograph are used. The atmospheric models are calculated based on the MAFAGS opacity sampling code. All the abundances are derived using the spectrum synthesis methods. Our results indicate that the non-LTE effects of copper are important for metal-poor stars, showing a departure of $\sim 0.17$ dex at the metallicity $\sim -1.5$. We also find that the copper abundances derived from non-LTE calculations are enhanced compared with those from LTE. The enhancements show clear dependence on the metallicity, which gradually increase with decreasing [Fe/H] for our program stars, leading to a flatter distribution of [Cu/Fe] with [Fe/H] than previous work. There is a hint that the thick- and thin-disk stars have different behaviors in [Cu/Fe], and a bending for disk stars may exist.