Determination of robust metallicities for metal-rich red giant branch stars

Abstract

The study of the Milky Way relies on our ability to interpret the light from stars correctly. This calls for a reinvestigation of how reliably we can determine, e.g., iron abundances in such stars and how well they reproduce those of dwarf stars. Here we explore robust ways to determine the iron content of metal-rich giant stars. We aim to understand what biases and shortcomings widely applied methods suffer from. In this study we are mainly concerned with standard methods to analyse stellar spectra. This includes the analysis of individual lines to determine stellar parameters, analysis of the broad wings of certain lines (e.g., H$\alpha$ and calcium lines) to determine effective temperature and surface gravity for the stars. For NGC 6528 we find that [Fe/H] = $+0.04$ dex with a scatter of $\sigma=0.07$ dex, which gives an error in the derived mean abundance of 0.02 dex. Our work has two important conclusions for analysis of metal-rich red giant branch stars. 1) For spectra with S/N below about 35 per reduced pixel [Fe/H] become too high, 2) Determination of $T_{\rm eff}$ using the wings of the H$\alpha$ line results in [Fe/H] values about 0.1 dex higher than if excitational equilibrium is used. The last conclusion is perhaps not surprising as we expect NLTE effect to become more prominent in cooler stars and we can not use the the wings of the H$\alpha$ line to determine $T_{\rm eff}$ for the cool stars in our sample. We therefore recommend that in studies of metal-rich red giant stars care needs to be taken to obtain sufficient calibration data in order to be able to also use the cooler stars.