An empirical calibration of Lick indices using Milky Way Globular Clusters

TL;DR

This paper empirically calibrates Lick indices using Milky Way Globular Clusters to improve the conversion of spectral features into stellar abundances across a wide metallicity range.

Contribution

It provides new empirical calibration relations for Lick indices based on real stellar abundance patterns, replacing previous models that relied on metallicity scales from colors.

Findings

Fe indices increase linearly with [Fe/H] for [Fe/H]>-2

Mg indices respond similarly to [Mg/H], but Mgb is less reliable at low metallicity

Empirical calibrations for Ca, TiO, Ba, Eu indices and H_alpha, NaD are presented

Abstract

To provide an empirical calibration relation in order to convert Lick indices into abundances for the integrated light of old, simple stellar populations for a large range in the observed [Fe/H] and [alpha/Fe]. This calibration supersedes the previously adopted ones because it is be based on the real abundance pattern of the stars instead of the commonly adopted metallicity scale derived from the colours. We carried out a long-slit spectroscopic study of 23 Galactic globular cluster for which detailed chemical abundances in stars have been recently measured. The line-strength indices, as coded by the Lick system and by Serven et al. (2005), were measured in low-resolution integrated spectra of the GC light. The results were compared to average abundances and abundance ratios in stars taken from the compilation by Pritzl et al. (2005) as well as to synthetic models. Fe-related indices grow linearly as a function of [Fe/H] for [Fe/H]>-2. Mg-related indices respond in a similar way to [Mg/H] variations, however Mgb turns out to be a less reliable metallicity indicator for [Z/H]<-1.5 . Despite the known Mg overabundance with respect to Fe in GC stars, it proved impossible to infer a mean [Mg/Fe] for integrated spectra that correlates with the resolved stars properties, because the sensitivity of the indices to [Mg/Fe] is smaller at lower metallicities. We present empirical calibrations for Ca, TiO, Ba and Eu indices as well as the measurements of H_alpha and NaD.