A comprehensive NBODY study of mass segregation in star clusters: energy equipartition and escape

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 M-h from 5 to 40 per cent 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 N-* = 2 x 10(4) particles improving the statistical significancy of the lower-N-* simulations by ensemble averages. Equipartition slows down the gravothermal contraction of the core slightly. Beyond a critical value of mu approximate to 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 Spitzer-unstable systems is demonstrated in direct N-body models. We also present the measurements of the Coulomb logarithm and discuss the relative importance of the evaporation and ejection of escapers.