Horizontal-branch morphology and multiple stellar populations in the anomalous globular cluster M22

TL;DR

This study investigates the chemical composition of stars in the globular cluster M22, revealing that HB morphology is linked to multiple stellar populations with distinct chemical signatures, indicating a complex star-formation history.

Contribution

It provides the first detailed chemical abundance analysis of HB stars in M22, highlighting the relationship between chemical composition and HB position, considering NLTE effects.

Findings

Stars are barium-poor and sodium-poor.

HB morphology correlates with chemical composition.

Multiple stellar populations influence HB structure.

Abstract

M22 is an anomalous globular cluster that hosts two groups of stars with different metallicity and s-element abundance. The star-to-star light-element variations in both groups, with the presence of individual Na-O and C-N anticorrelations, demonstrates that this Milky-Way satellite has experienced a complex star-formation history. We have analysed FLAMES/UVES spectra for seven stars covering a small color interval, on the reddest horizontal-branch (HB) portion of this cluster and investigated possible relations between the chemical composition of a star and its location along the HB. Our chemical abundance analysis takes into account effects introduced by deviations from the local-thermodynamic equilibrium (NLTE effects), that are significant for the measured spectral lines in the atmospheric parameters range spanned by our stars. We find that all the analysed stars are barium-poor and sodium-poor, thus supporting the idea that the position of a star along the HB is strictly related to the chemical composition, and that the HB-morphology is influenced by the presence of different stellar populations.