The effect of orbital eccentricity on the dynamical evolution of star clusters

Abstract

We use N-body simulations to explore the influence of orbital eccentricity on the dynamical evolution of star clusters. Specifically, we compare the mass-loss rate, velocity dispersion, relaxation time, and the mass function of star clusters on circular and eccentric orbits. For a given perigalactic distance, increasing orbital eccentricity slows the dynamical evolution of a cluster due to a weaker mean tidal field. However, we find that perigalactic passes and tidal heating due to an eccentric orbit can partially compensate for the decreased mean tidal field by energizing stars to higher velocities and stripping additional stars from the cluster, accelerating the relaxation process. We find that the corresponding circular orbit which best describes the evolution of a cluster on an eccentric orbit is much less than its semi-major axis or time-averaged galactocentric distance. Since clusters spend the majority of their lifetimes near apogalacticon, the properties of clusters which appear very dynamically evolved for a given galactocentric distance can be explained by an eccentric orbit. Additionally, we find that the evolution of the slope of the mass function within the core radius is roughly orbit independent, so it could place additional constraints on the initial mass and initial size of globular clusters with solved orbits. We use our results to demonstrate how the orbit of Milky Way globular clusters can be constrained given standard observable parameters like galactocentric distance and the slope of the mass function. We then place constraints on the unsolved orbits of NGC 1261, NGC 6352, NGC 6496, and NGC 6304 based on their positions and mass functions. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.