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

Abstract

Context. The study of the Milky Way relies on our ability to interpret the light from stars correctly. With the advent of the astrometric ESA mission Gaia we will enter a new era where the study of the Milky Way can be undertaken on much larger scales than currently possible. In particular we will be able to obtain full 3D space motions of red giant stars at large distances. 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. Aims. 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. Methods. 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., Hα and calcium lines) to determine effective temperature and surface gravity for the stars. Results. For NGC 6528 we find that [Fe/H] = +0.04 dex with a scatter of σ = 0.07 dex, which gives an error in the derived mean abundance of 0.02 dex. Conclusions. 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 eff using the wings of the Hα 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 Hα line to determine T 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.

Chemical analysis of NGC 6528: one of the most metal-rich bulge globular clusters

“…Zoccali et al (2004) derived a value of -0.10 dex in three stars and Harris (1996Harris ( , 2010 quotes a metallicity of -0.11 dex for NGC 6528. Liu et al (2017) found a metallicity of 0.04 dex with a scatter of 0.07 dex, their S/N is similar to ours. Moreover, we have three stars in common (see table 2), with good agreement for star #3 and #6.…”

“…Table 1 lists the stellar parameters of our sample: ID, the J2000 coordinates (RA and Dec), J, H, K s magnitudes from VVV PSF photometry (de-reddened), calibrated on the system of 2MASS (Mauro et al 2014;Cohen et al 2017), heliocentric radial velocity, Teff, log(g), metallicity and micro-turbulent velocity (v t ). Moreover, Table 2 shows the metallicity values from Saviane et al (2012), Mauro et al (2014) and Liu et al (2017).…”