NLTE analysis of high-resolution H-band spectra IV: neutral copper

Abstract

To obtain reliable Cu abundances with the APOGEE H-band spectra, it is important to investigate the non-local thermodynamic equilibrium (NLTE) effects on the formation of the H-band Cu I lines. In addition, the Cu atomic model needs to be tested. Based on both the high-resolution and high signal-to-noise ratio H-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and optical data, we derived the LTE and NLTE copper abundances of 13 FGK sample stars with the spectral synthesis method. We find that the NLTE effects are negligible for the Cu I 16005.7Å line in the IR H-band. Consistent copper abundances within the uncertainties from these two sets of lines have been obtained, which indicates the reliability of our NLTE copper atomic model. We note that the [Cu/Fe] ratios increase with increasing metallicity when ∼ – 1.4 dex < [Fe/H] < ∼ – 0.5 dex, favoring a secondary (metallicity-dependent) copper production.