Measuring energy equipartition in globular clusters

Abstract

Globular clusters (GCs) evolve toward a state of partial energy equipartition over their long-term evolution, due to relaxation processes that induce exchange of energy between stars. This establishes a mass-dependent internal kinematics: more-massive stars lose kinetic energy and sink toward the center while less-massive stars gain kinetic energy and move outward. In this contribution, we show how the mass-dependence of the velocity dispersion can be efficiently described, both in simulations and observations, with an exponential function characterized by one parameter indicating how close to full energy equipartition a system is. Moreover, we show that the degree of equipartition reached by a GC tightly correlates with its dynamical state and therefore we propose this equipartition-dynamical state relation as a tool to characterize the relaxation condition of a cluster, based on kinematic measurements.