Spatial mixing of stellar populations in globular clusters via binary–single star scattering
Year: 2025
Type: article
Abstract: Context. The majority of Galactic globular star clusters (GCs) have been reported to contain at least two populations of stars (hereafter, we use P1 for the primordial and P2 for the chemically enriched population). Recent observational studies found that dynamically old GCs have P1 and P2 spatially mixed due to relaxation processes. However, in dynamically young GCs, where P2 is expected to be more centrally concentrated from birth, the spatial distributions of P1 and P2 are sometimes very different from system to system. This suggests that more complex dynamical processes specific to certain GCs might have shaped those distributions. Aims. We aim to investigate the discrepancies between the spatial concentration of P1 and P2 stars in dynamically young GCs. Our main focus is to evaluate whether massive binary stars (e.g. black holes) can cause the expansion of the P2 stars through binary–single interactions in the core, and whether they can mix or even radially invert the P1 and P2 distributions. Methods. We use a set of theoretical and empirical arguments to evaluate the effectiveness of binary–single star scattering. We then construct a set of direct N -body models with massive primordial binaries to verify our estimates further and gain more insights into the dynamical processes in GCs. Results. We find that binary–single star scatterings can push the central P2 stars outwards within a few relaxation times. While we do not produce radial inversion of P1 and P2 for any initial conditions we tested, this mechanism systematically produces clusters where P1 and P2 look fully mixed even in projection. The mixing is enhanced (1) in denser GCs, (2) in GCs containing more binary stars, and (3) when the mass ratio between the binary components and the cluster members is higher. Conclusions. Binary–single star interactions seem able to explain the observable properties of some dynamically young GCs (e.g. NGC 4590 or NGC 5904) where P1 and P2 are fully radially mixed.
Source: Astronomy and Astrophysics
Institutions: Czech Academy of Sciences, Astronomical Institute, Indiana University, Lund University, European Southern Observatory, European Southern Observatory, University of Bologna, The University of Queensland, Technion – Israel Institute of Technology, ARC Centre of Excellence for Gravitational Wave Discovery, Monash University, Centre National de la Recherche Scientifique, Queen Mary University of London, Université Côte d'Azur, Max Planck Institute for Astronomy, Lagrange Laboratory, Observatoire de la Côte d’Azur
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: Royal Society, European Commission, Deutsche Forschungsgemeinschaft, Akademie Věd České Republiky, HORIZON EUROPE Framework Programme, European Research Council, Australian Research Council, H2020 European Research Council, H2020 Marie Skłodowska-Curie Actions, HORIZON EUROPE Marie Sklodowska-Curie Actions
Awards: G1137327261, G1586188551, Galactic archeology with high temporal resolution, G1897041155, G2361505538, G2368947799, Observational Signatures of planet formation in externally IRradiated dIScs, European Project on Mendelian Forms of Parkinson’s Disease, G3550382679, G4066435496, EXC 2094: ORIGINS: Vom Ursprung des Universums bis zu den ersten Bausteinen des Lebens, G4420011367, G4576739527, G4765345657, G5157879570, G5244800852, Central Bohemia Mobility Programme for Excellence in Research, Innovation and Technology, G601392416, ARC Centre of Excellence for Gravitational Wave Discovery, G6998206882, G7697091281, G8696512317, G941802975
OpenAlex ID: W4415123367