On the metallicity distribution of the peculiar globular cluster M22

TL;DR

This study confirms that the globular cluster M22 has a metallicity spread, supporting the idea that it formed from the merger of two clusters with different chemical compositions, contrary to some recent claims.

Contribution

The paper introduces new LTE abundance analysis methods that reveal a metallicity spread in M22, challenging previous findings of uniform metallicity.

Findings

M22 exhibits a spread in iron abundance.

New analysis methods detect chemical diversity.

Supports a merger origin for M22.

Abstract

In our previous study, we showed that the peculiar globular cluster (GC) M22 contains two distinct stellar populations, namely the Ca-w and Ca-s groups with different physical properties, having different chemical compositions, spatial distributions and kinematics. We proposed that M22 is most likely formed via a merger of two GCs with heterogeneous metallicities in a dwarf galaxy environment and accreted later to our Galaxy. In their recent study, Mucciarelli et al. claimed that M22 is a normal mono-metallic globular cluster without any perceptible metallicity spread among the two groups of stars, which challenges our results and those of others. We devise new strategies for the local thermodynamic equilibrium (LTE) abundance analysis of red giant branch (RGB) stars in GCs and show there exists a spread in the iron abundance distribution in M22.