Is the overconcentration of pristine populations in Galactic globular clusters real? An N-body approach to the problem

TL;DR

This study uses direct N-body simulations to validate that the observed overconcentration of pristine stars in some globular clusters is a transient effect caused by black hole subsystem interactions, aligning with previous Monte Carlo results.

Contribution

It provides an independent validation of the transient overconcentration phenomenon using detailed N-body simulations, confirming the role of black hole subsystems.

Findings

Transient overconcentration of 1P RGB stars explained by black hole interactions.

Simulations match previous Monte Carlo predictions.

Stochastic fluctuations influence observed stellar distributions.

Abstract

Recent observations indicate that in some Milky Way globular clusters (GCs) pristine red giant branch (RGB) stars appear 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 MOCCA Monte Carlo 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. In this letter, we test this interpretation using tailored NBODY6++GPU models evolved with direct N-body simulations, providing an independent validation that does not rely on a statistical treatment of relaxation. 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. 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 and their progenitors from the cluster center leads to a transient apparent overconcentration of 1P RGB stars, in agreement with earlier MOCCA predictions.