Tracing M22's origins: Spatial and chemical constraints on its formation history
Le résumé fourni par la source
Globular clusters (GCs) are well known to host stellar populations characterized by light-element variations. A subset of Galactic GCs, beyond such 'canonical' populations, contains an additional group enriched in iron, s-process elements, and total C+N+O abundance (i.e., the anomalous stars). We combine multi-facility photometry with APOGEE spectroscopy to investigate the spatial distribution, chemical properties, and formation history of the stellar populations in M22, with particular focus on its anomalous component. We trace the canonical and anomalous populations, together with their subpopulations, from the cluster center out to eight half-mass radii. The anomalous population becomes increasingly dominant in the outermost regions, whereas no significant radial gradients are detected among its subpopulations. Our chemical analysis reveals light-element anticorrelations within both the canonical and anomalous components, although the latter are shifted toward higher C, N, and Al abundances. For the first time, we show that the Fe, s-process, and C+N+O enhancements among anomalous stars are not uniform but correlate with their light-element composition: the most chemically extreme anomalous stars are also the most Fe-, Ce-rich, and C+N+O-poor. We identify a distinct red overdensity on the horizontal branch, likely populated by the most He-poor stars, and tentatively associate the extreme horizontal branch with the most chemically enriched anomalous population. These observations are difficult to reconcile with M22 being a merger between two GCs. Instead, they qualitatively favor a self-enrichment scenario regulated by dilution, similar to that recently proposed for $ω$Centauri, with their close chemical correspondence suggesting that they experienced analogous formation histories.
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