A comprehensive Nbody study of mass segregation in star clusters: Energy equipartition and escape

Abstract

We address the dynamical evolution of an isolated self--gravitating system with two stellar mass groups. We vary the individual ratio of the heavy to light bodies, $\mu$ from 1.25 to 50 and alter also the fraction of the total heavy mass $\MH$ from 5% to 40% of the whole cluster mass. Clean-cut properties of the cluster dynamics are examined, like core collapse, the evolution of the central potential, as well as escapers. We present in this work collisional $N$-body simulations, using the high--order integrator NBODY6++ with up to ${\cal N}_{\star}=2\cdot 10^4$ particles improving the statistical significancy of the lower--${\cal N}_{\star}$ simulations by ensemble averages. Equipartition slows down the gravothermal contraction of the core slightly. Beyond a critical value of $\mu \approx 2$, no equipartition can be achieved between the different masses; the heavy component decouples and collapses. For the first time the critical boundary between Spitzer--stable and --unstable systems is demonstrated in direct $N$-body models.