Central Dynamics of Multi-mass Rotating Star Clusters

TL;DR

This study uses N-body simulations to explore how rotation and mass spectrum influence the shape and internal dynamics of star clusters, revealing rapid formation of oblate cores and their long-lasting nature.

Contribution

It demonstrates that rotation combined with a multi-mass spectrum leads to persistent oblate cores, a novel insight into the morphology of star clusters.

Findings

Multi-mass rotating systems form long-lasting oblate cores.

Flattening correlates with initial rotation levels.

Rotation influences orbital inclinations of massive stars.

Abstract

We investigate the evolutionary nexus between the morphology and internal kinematics of the central regions of collisional, rotating, multi-mass stellar systems, with special attention to the spatial characterisation of the process of mass segregation. We report results from idealized, purely $N$-body simulations that show multi-mass, rotating, and spherical systems rapidly form an oblate, spheroidal massive core, unlike single-mass rotating or multi-mass non-rotating configurations with otherwise identical initial properties, indicating that this evolution is a result of the interplay between the presence of a mass spectrum and angular momentum. This feature appears to be long-lasting, preserving itself for several relaxation times. The degree of flattening experienced by the systems is directly proportional to the initial degree of internal rotation. In addition, this morphological effect has a clear characterisation in terms of orbital architecture, as it lowers the inclination of the orbits of massive stars. We offer an idealised dynamical interpretation that could explain the mechanism underpinning this effect and we highlight possible useful implications, from kinematic hysteresis to spatial distribution of dark remnants in dense stellar systems.