Mass segregation in star clusters is not energy equipartition

TL;DR

Mass segregation in star clusters does not necessarily indicate energy equipartition, as simulations show no velocity differences across masses despite significant mass segregation.

Contribution

This study demonstrates through N-body simulations that mass segregation does not imply energy equipartition in star clusters.

Findings

Mass segregation occurs without energy equipartition.

Kinetic energies decrease uniformly across stellar masses.

Massive stars do not have significantly lower velocities.

Abstract

Mass segregation in star clusters is often thought to indicate the onset of energy equipartition, where the most massive stars impart kinetic energy to the lower-mass stars and brown dwarfs/free floating planets. The predicted net result of this is that the centrally concentrated massive stars should have significantly lower velocities than fast-moving low-mass objects on the periphery of the cluster. We search for energy equipartition in initially spatially and kinematically substructured N-body simulations of star clusters with N = 1500 stars, evolved for 100 Myr. In clusters that show significant mass segregation we find no differences in the proper motions or radial velocities as a function of mass. The kinetic energies of all stars decrease as the clusters relax, but the kinetic energies of the most massive stars do not decrease faster than those of lower-mass stars. These results suggest that dynamical mass segregation -- which is observed in many star clusters -- is not a signature of energy equipartition from two-body relaxation.