The double sub-giant branch of NGC 6656 (M22): a chemical characterization

TL;DR

This study analyzes the chemical composition of subgiant stars in globular cluster M22, revealing a correlation between chemical abundances and the split in the subgiant branch, which constrains their relative ages.

Contribution

It provides the first detailed chemical characterization of the double subgiant branch in M22, linking chemical differences to photometric splits and age constraints.

Findings

Stars with higher metallicity and s-process elements are on the fainter SGB.

Stars with lower metallicity and s-process elements are on the brighter SGB.

The age difference between the two stellar groups is less than 300 million years.

Abstract

We present an abundance analysis of 101 subgiant branch (SGB) stars in the globular cluster M22. Using low resolution FLAMES/GIRAFFE spectra we have determined abundances of the neutron-capture strontium and barium and the light element carbon. With these data we explore relationships between the observed SGB photometric split in this cluster and two stellar groups characterized by different contents of iron, slow neutron-capture process (s-process) elements, and the alpha element calcium, that we previously discovered in M22's red-giant stars. We show that the SGB stars correlate in chemical composition and color-magnitude diagram position: the stars with higher metallicity and relative s-process abundances define a fainter SGB, while stars with lower metallicity and s-process content reside on a relatively brighter SGB. This result has implications for the relative ages of the two stellar groups of M22. In particular, it is inconsistent with a large spread in ages of the two SGBs. By accounting for the chemical content of the two stellar groups, isochrone fitting of the double SGB suggests that their ages are not different by more than 300 Myr.