The two metallicity groups of the globular cluster M22: a chemical perspective

TL;DR

This study analyzes the chemical compositions of 35 red giant stars in globular cluster M22, revealing two distinct stellar groups with different metallicities and neutron-capture element abundances, shedding light on its complex enrichment history.

Contribution

It provides a detailed chemical analysis of M22's stellar populations, highlighting the existence of two groups with different chemical signatures and their implications for cluster formation.

Findings

Identification of two stellar groups with different iron content.

Correlation between neutron-capture element ratios and metallicity.

Presence of Na-O and C-N anticorrelations in both groups.

Abstract

We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, alpha, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and alpha element Ca. The principal results of this study are: (i) substantial star-to-star metallicity scatter (-2.0<[Fe/H]<-1.6); (ii) enhancement of s-process/r-process neutron-capture abundance ratios in a fraction of giants, positively correlated with metallicity; (iii) sharp separation between the s-process rich and s-process poor groups by [La/Eu] ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H], suggesting that this element also has a significant s-process component; and (v) presence of Na-O and C-N anticorrelations in both the stellar groups.