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

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

Initial model and snapshot data.  Paper: AA59469-26. 

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 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. In this letter, we test this interpretation using tailored \nb 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 \nb 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 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 can support the view that dynamical evolution within current MSP formation scenarios in our model can explain the apparent 1P overconcentration inferred in real clusters such as NGC~3201 and NGC~6101.