NLTE Analysis of Copper Abundances in the Galactic Bulge Stars

TL;DR

This study investigates copper abundances in 129 Galactic bulge stars using NLTE spectral analysis, revealing that NLTE effects significantly influence copper measurements across a wide metallicity range, and the [Cu/Fe] trend aligns with that of the Galactic disk.

Contribution

It provides the first detailed NLTE analysis of copper in bulge stars, highlighting the importance of NLTE effects for accurate abundance determinations.

Findings

NLTE effects increase with decreasing [Fe/H]

LTE underestimates Cu abundances especially in metal-poor stars

[Cu/Fe] trend similar to Galactic disk stars

Abstract

Based on the medium-high resolution (R~ 20,000), modest signal-to-noise ratio (S/N > 70) FLAMES-GIRAFFE spectra, we investigated the copper abundances of 129 red giant branch stars in the Galactic bulge with [Fe/H] from -1.14 to 0.46 dex. The copper abundances are derived from both local thermodynamic equilibrium (LTE) and nonlocal thermodynamic equilibrium (NLTE) with the spectral synthesis method. We find that the NLTE effects for Cu I lines show a clear dependence on metallicity, and they gradually increase with decreasing [Fe/H] for our sample stars. Our results indicate that the NLTE effects of copper are important not only for metal-poor stars but also for supersolar metal-rich ones and the LTE results underestimate the Cu abundances. We note that the [Cu/Fe] trend of the bulge stars is similar to that of the Galactic disk stars spanning the metallicity range of -1.14 < [Fe/H] < 0.0 dex and the [Cu/Fe] ratios increase with increasing metallicity when [Fe/H] is from~-1.2 to~-0.5 dex, favoring a secondary (metallicity-dependent) production of Cu.