Letter to the Editor

Multiple stellar populations in MOCCA globular cluster models: Transient spatial overconcentration of pristine red giant stars driven by strong dynamical encounters

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, PL-00-716 Warsaw, Poland
² Faculty of Mathematics and Computer Science, A. Mickiewicz University, Uniwersytetu Poznańskiego 4, 61-614 Poznań, Poland
³ Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea

^⋆ Corresponding authors: mig@camk.edu.pl, askar@camk.edu.pl

Received: 23 February 2025
Accepted: 14 May 2025

Abstract

Recent findings show that, in some Milky Way globular clusters (GCs), pristine red giant branch (RGB) stars are more centrally concentrated than enriched ones. This contradicts most multiple-population formation scenarios, which predict that the enriched population (2P) should initially be more concentrated than the pristine population (1P). We analyzed a MOCCA GC model that exhibits a higher spatial concentration of 1P RGB stars than 2P RGB stars at 13 Gyr. The MOCCA models assume the asymptotic giant branch (AGB) scenario, in which 2P stars are initially more concentrated than 1P stars. Our results indicate that the observed spatial distributions of multiple populations, and possibly their kinematics, are significantly shaped by dynamical interactions. These interactions preferentially eject 2P RGB progenitors from the central regions, leading to a transient overconcentration of 1P RGB stars at late times. This effect is particularly relevant for GCs with present-day masses of a few 10⁵ M_⊙, which have retained only about 10–20% of their initial mass. Such clusters may appear dynamically young due to heating from a black hole subsystem, even if they have undergone significant mass loss and dynamical evolution. Additionally, the relatively small number of RGB stars in these clusters suggests that interpreting the spatial distributions of multiple populations solely from RGB stars may lead to biased conclusions about the overall distribution of 1P and 2P. The apparent overconcentration of the 1P relative to the 2P is likely a transient effect driven by the preferential removal of 2P RGB progenitors via strong dynamical encounters. MOCCA models of multiple stellar populations based on the AGB scenario may explain anomalous features observed in some Galactic GCs, such as NGC 3201 and NGC 6101.