Intrinsic iron spread and a new metallicity scale for Globular Clusters

TL;DR

This study provides a precise, homogeneous measurement of iron abundances in 19 Galactic globular clusters, revealing most are nearly mono-metallic with minimal intrinsic scatter, and introduces an updated metallicity scale.

Contribution

It offers a new, accurate metallicity scale for globular clusters based on a large, homogeneous dataset of Fe abundances, improving previous calibrations.

Findings

Most GCs are nearly mono-metallic with less than 0.05 dex Fe scatter.

Fe scatter correlates with cluster luminosity and mass.

The new scale refines existing metallicity measurements for GCs.

Abstract

We have collected spectra of about 2000 red giant branch (RGB) stars in 19 Galactic globular clusters (GC) using FLAMES@VLT (about 100 star with GIRAFFE and about 10 with UVES, respectively, in each GC). These observations provide an unprecedented, precise, and homogeneous data-set of Fe abundances in GCs. We use it to study the cosmic scatter of iron and find that, as far as Fe is concerned, most GCs can still be considered mono-metallic, since the upper limit to the scatter in iron is less than 0.05 dex, meaning that the degree of homogeneity is better than 12%. The scatter in Fe we find seems to have a dependence on luminosity, possibly due to the well-known inadequacies of stellar atmospheres for upper-RGB stars and/or to intrinsic variability. It also seems to be correlated with cluster properties, like the mass, indicating a larger scatter in more massive GCs which is likely a (small) true intrinsic scatter. The 19 GCs, covering the metallicity range of the bulk of Galactic GCs, define an accurate and updated metallicity scale. We provide transformation equations for a few existing scales. We also provide new values of [Fe/H], on our scale, for all GCs in the Harris' catalogue.