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 3 - 6 M_⊙ 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 [Fe/H] ∼−2 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 (∼ 1−3 M_⊙) 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 ∼ 3 M_⊙. This places tighter constraints on possible formation scenarios and suggests an age difference of at least ∼ 280-480 Myr between the two populations which is independent of isochrone fitting.