Long-term evolution of isolated N-body systems

Abstract

We report results of N-body simulations of isolated star clusters, performed up to the point where the clusters are nearly completely dissolved. Our main focus is on the post-collapse evolution of these clusters. We find that, after core collapse, isolated clusters evolve along nearly a single sequence of models, the properties of which are independent of the initial density profile and particle number. Because of the slower expansion of high-N clusters, relaxation times become almost independent of the particle number after several core collapse times, at least for the particle range of our study. As a result, the dissolution times of isolated clusters exhibit a surprisingly weak dependence on N. We find that most stars escape as a result of encounters between single stars inside the half-mass radius of the cluster. Encounters with binaries take place mostly in the cluster core and account for roughly 15 per cent of all escapers. Encounters between single stars at intermediate radii are also responsible for the build-up of a radial anisotropic velocity distribution in the halo. For clusters undergoing core oscillations, escape owing to binary stars is efficient only when the cluster centre is in a contracted phase. Our simulations show that it takes about 105N-body time-units until the global anisotropy reaches its maximum value. The anisotropy increases with particle number, and it seems conceivable that isolated star clusters become vulnerable to radial orbit instabilities for large enough N. However, no indication for the onset of such instabilities was seen in our runs.