Second generation stellar disks in Dense Star Clusters and cluster ellipticities

TL;DR

This study investigates how second-generation stellar disks in dense star clusters influence their shape and kinematic properties, revealing correlations between cluster ellipticity, rotation, and the properties of these disks through N-body simulations.

Contribution

The paper demonstrates that second-generation stellar disks can cause globular clusters to become more elliptical and rotate faster, linking cluster shape to the formation and evolution of these disks.

Findings

Second-generation disks lead to increased ellipticity in clusters.

Clusters with larger SG populations show more rotation.

Ellipticity correlates with relaxation time and SG fraction.

Abstract

Globular Clusters (GCs) and Nuclear Star Clusters (NSCs) are typically composed by several stellar populations, characterized by different chemical compositions. Different populations show different ages in NSCs but not necessarily in GCs. The youngest populations in NSCs appear to reside in disk-like structures, as observed in our Galaxy and in M31. Gas infall followed by formation of second generation (SG) stars in GCs may similarly form disk-like structures in the clusters nuclei. Here we explore this possibility and follow the long term evolution of stellar disks embedded in GCs, and study their effects on the evolution of the clusters. We study disks with different masses by means of detailed N-body simulations and explore their morphological and kinematic signatures on the GC structures. We find that as a second generation disk relaxes, the old, first generation, stellar population flattens and becomes more radially anisotropic, making the GC structure become more elliptical. The second generation stellar population is characterized by a lower velocity dispersion, and a higher rotational velocity, compared with the primordial older population. The strength of these kinematic signatures depends both on the relaxation time of the system and on the fractional mass of the second generation disk. We therefore conclude that SG populations formed in flattened configurations will give rise to two systematic trends: (1) Positive correlation between GC ellipticity and fraction of SG population (2) Positive correlation between GC relaxation time and ellipticity. Thereby GC ellipticities and rotation could be related to the formation of SG stars and their initial configuration.