Tracing M22’s Origins: Spatial and Chemical Constraints on Its Formation History
9 affiliations
- 1American University of Sharjah
- 2Istituto Nazionale di Astrofisica—Osservatorio Astronomico di Padova
- 3Univ. di Padova
- 4Università degli Studi di Padova
- 5NSF’s NOIRLab
- 6Space Telescope Science Institute
- 7Australian National University
- 8Yunnan University
- 9Universitá di Bologna
Abstract
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 multifacility 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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BibTeX
@article{dondoglio2026tracing,
title = {{Tracing M22’s Origins: Spatial and Chemical Constraints on Its Formation History}},
author = {Emanuele Dondoglio and Antonino P. Milone and Randa Asa’d and Anna F. Marino and Alessandra Mastrobuono-Battisti and Fabrizio Muratore and Tuila Ziliotto and Emanuele Bortolan and Giacomo Cordoni and Maria Vittoria Legnardi and Edoardo P. Lagioia and Marco Tailo},
journal = {The Astronomical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-3881/aea564},
url = {https://iopscience.iop.org/article/10.3847/1538-3881/aea564},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Tracing M22’s Origins: Spatial and Chemical Constraints on Its Formation History
AU - Emanuele Dondoglio
AU - Antonino P. Milone
AU - Randa Asa’d
AU - Anna F. Marino
AU - Alessandra Mastrobuono-Battisti
AU - Fabrizio Muratore
AU - Tuila Ziliotto
AU - Emanuele Bortolan
AU - Giacomo Cordoni
AU - Maria Vittoria Legnardi
AU - Edoardo P. Lagioia
AU - Marco Tailo
PY - 2026
DA - 2026/10/07
JO - The Astronomical Journal
DO - 10.3847/1538-3881/aea564
UR - https://iopscience.iop.org/article/10.3847/1538-3881/aea564
AB - 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 multifacility 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.
SN - 0004-6256
SN - 1538-3881
ER -