Is the overconcentration of pristine populations in Galactic globular clusters real?

An N-body approach to the problem

¹ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
² Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
³ Fesenkov Astrophysical Institute, Observatory 23, 050020 Almaty, Kazakhstan
⁴ Department of Physics, New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁵ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁶ Dipartimento di Fisica, Sapienza, Universitá di Roma, P.le Aldo Moro, 5, 00185 Rome, Italy
⁷ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Rd., Chaoyang District, 100101 Beijing, China
⁸ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yi He Yuan Road, Beijing 100871, China
⁹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12–14, 69120 Heidelberg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 February 2026
Accepted: 1 May 2026

Abstract

Aims. Recent observations indicate that in some Milky Way globular clusters, pristine red giant branch (RGB) stars are more centrally concentrated than enriched ones. This contradicts most multiple stellar population (MSP) formation scenarios, which predict that the enriched (second) population (2P) should initially be more concentrated than the pristine (first) population (1P). Previous Monte Carlo Cluster Simulator (MOCCA) simulations suggested that this apparent overconcentration is a transient effect arising in clusters that have lost a large fraction of their initial mass and host an active black hole subsystem (BHS), and is visible only when RGB stars are used as tracers. We tested this interpretation using tailored NBODY6++GPU models evolved with direct N-body simulations and provide an independent validation that does not rely on a statistical treatment of relaxation.

Methods. We performed direct N-body simulations with the NBODY6++GPU code, adopting initial conditions designed to reproduce the dynamical regime relevant to the proposed mechanism. The simulations include updated stellar and binary evolution, dynamical interactions, and the Galactic tidal field, enabling a direct comparison with MOCCA results.

Results. The simulations confirm that the spatial distributions and kinematics inferred from RGB stars can be strongly affected by stochastic fluctuations and interactions with the BHS. Preferential ejection of 2P RGB stars and their progenitors from the cluster centre leads to a transient apparent overconcentration of 1P RGB stars, in agreement with earlier MOCCA predictions. We show that this effect does not reflect the global MSP structure and that analyses based solely on RGB tracers may yield biased interpretations. These results support the view that dynamical evolution within the current MSP formation scenarios in our model can explain the apparent 1P overconcentration inferred in real clusters such as NGC 3201 and NGC 6101.