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

TL;DR

This work addresses the puzzling observation that pristine RGB stars in some Milky Way globular clusters can appear more centrally concentrated than enriched ones, challenging MSP formation theories. By performing tailored direct $N$-body simulations with NBODY6++GPU and comparing to MOCCA Monte Carlo results, the authors test whether this inversion is a real global feature or a transient, tracer-dependent effect. They find that RGB-based overconcentration is transient and driven by stochastic dynamics and interactions with a black hole subsystem, while MS stars show near-complete mixing; the NBODY6++GPU results align with MOCCA, supporting the proposed dynamical explanation. The study emphasizes the dangers of drawing global MSP conclusions from RGB tracers alone and highlights the need for multi-tracer, complementary diagnostics to robustly interpret MSP structure in globular clusters, especially those close to dissolution.

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.

Figures (5)

• Figure 1: Cumulative number distributions of RGB stars for 1P (green) and 2P (blue) around 1.2 Gyr as a function of projected distance, normalised by the 2D half-light radius $R_{\rm hl}$. Solid line: $N$-body model (average of 33 snapshots). Dashed line: MOCCA model (average of 5 snapshots). Snapshots span 1.18--1.22 Gyr.
• Figure 2: The time evolution of the A$^+$ parameter for different type of stars and for $X-Y$ projection of 3D snapshots. The left panel shows RGB stars, the middle panel -- evolved luminous stars and the right panel -- MS stars. The red, green and blue lines are averaged over 9, 17 and 33 snapshots, respectively.
• Figure 3: Time evolution of the number of RGB stars in the 1P, 2P, and total population for different projections of the 3D snapshots. The left panel shows the $X-Y$ projection, the middle panel the $X-Z$ projection, and the right panel the $Y-Z$ projection. The red, green, and blue lines correspond to averages over 9, 17, and 33 snapshots centred on the selected time, respectively.
• Figure 4: Time evolution of the A$^+$ parameter for RGB stars for projections of the 3D snapshots onto the $X-Y$, $X-Z$, and $Y-Z$ planes (left to right panels). The red, green, and blue lines show averages over 9, 17, and 33 snapshots, respectively.
• Figure 5: NBODY6++GPU evolution of the cluster mass normalised to its initial value $M(0) = 56953.8\rm\;M_{\odot}$ (red line) and of the BHS mass normalised to the current cluster mass (green line).