The complex stellar system M 22: constraining the chemical enrichment from AGB stars using magnesium isotope ratios

The complex star cluster M 22 (NGC 6656) provides a unique opportunity for studying the slow neutron capture (s-)process nucleosynthesis at low metallicity due to its two stellar groups with distinct iron-peak and neutron capture element abundances. Previous studies attribute these abundance differences to pollution from asymptotic giant branch (AGB) stars which produce significant quantities of the neutron-rich Mg isotopes ²⁵Mg and ²⁶Mg. We report the first-ever measurements of Mg isotopic abundance ratios at in a globular cluster-like system using very high-resolution and signal-to-noise spectra (R = 110 000, S/N = 300 per pixel at 514 nm) from the VLT/UVES spectrograph for six stars; three in each s-process group. Despite the presence of star-to-star variations in ²⁴Mg, ²⁵Mg, and ²⁶Mg, we find no correlation with heavy element abundances, implying that the nucleosynthetic source of s-process enrichment must not influence Mg isotope ratios. Instead, a key result of this work is that we identify correlations between ²⁶Mg/²⁴Mg and some light elements. Using a custom suite of AGB nucleosynthesis yields tailored to the metallicity of M 22, we find that low mass () AGB stars are capable of reproducing the observed s-process abundances of M 22 and that the absence of any difference in Mg isotope ratios between the two s-process groups precludes AGBs with masses above. This places tighter constraints on possible formation scenarios and suggests an age difference of at least between the two populations which is independent of isochrone fitting.