Evolution of the kinematic properties of rotating, multiple-population globular clusters

¹ Department of Astronomy, Indiana University, Bloomington, Swain West, 727 E. 3rd Street, IN 47405, USA
² INAF – Astrophysics and Space Science Observatory Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ School of Mathematics and Maxwell Institute for Mathematical Sciences, University of Edinburgh, University of Edinburgh, Kings Buildings, Edinburgh EH9 3FD, UK

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

Received: 11 July 2025
Accepted: 30 September 2025

Abstract

Globular clusters host multiple stellar populations differing in their chemical and dynamical properties. A number of models for the formation of multiple populations predict that the subsystem of second-generation (SG) stars (those with anomalous chemical abundances) is characterised by a more centrally concentrated spatial distribution and a more rapid rotation than the system of firstgeneration (FG) stars (those with chemical properties similar to field stars). In this paper, we present the results of a suite of N-body simulations aimed at exploring the long-term dynamical evolution of rotating, multiple-population globular clusters. We studied the evolution of systems starting with four different orientations of the cluster’s total internal angular momentum vector relative to the orbital angular momentum. This allows us not only to explore the internal evolution driven by two-body relaxation, but also the effects of the cluster’s interaction with the galactic tidal field and how this interaction affects the cluster’s internal rotation over time. We focused our attention on the kinematic differences between the two generations, and we quantify these differences by exploring the FG and SG stars’ rotational velocity and angular momenta. We find that kinematic differences between the generations persist for a majority of the simulations’ lifetimes, although the strength of these differences rapidly decreases after a few relaxation times. The differences can be seen most clearly in the lowest mass stars in the models. We find that the clusters’ internal angular momentum gradually aligns with the orbital angular momentum over time, although there is little difference in this alignment between the FG and SG systems. We also find that stars in the cluster’s outer regions align with the orbital angular momentum vector more rapidly than those in the inner regions leading to a variation of the orientation of the internal angular momentum with the clustercentric distance. The alignment between internal angular momentum and orbital angular momentum occurs more rapidly for low-mass stars. We also studied the evolution of the anisotropy in the velocity distribution and, in agreement with previous results, find the SG to be characterised by a stronger radial anisotropy than the FG. Overall, our results show that the kinematic properties of multiple populations provide key information related to their formation and dynamical evolution.