ON THE DISSOLUTION OF EVOLVING STAR-CLUSTERS

Using direct N-body simulations which include both the evolution of si ngle stars and the tidal field of the parent galaxy, we study the dyna mical evolution of globular clusters and rich open clusters. We compar e our results with other N-body simulations and Fokker-Planck calculat ions. Our simulations, performed on the GRAPE-4, employ up to 32.768 s tars. The results are not in agreement with Fokker-Planck models, in t he sense that the lifetimes of stellar systems derived using the latte r are an order of magnitude smaller than those obtained in our simulat ions. For our standard run, Fokker-Plank calculations obtained a lifet ime of 0.28 Gyr, while our equivalent N-body calculations find similar to 4 Gyr. The principal reason for the discrepancy is that a basic as sumption of the Fokker-Plank approach is not valid for typical cluster parameters. The stellar evolution timescale is comparable to the dyna mical timescale, and therefore the assumption of dynamical equilibrium leads to an overestimate of the dynamical effects of mass loss. Our r esults suggest that the region in parameter space for which Fokker-Pla nck studies of globular cluster evolution, including the effects of bo th stellar evolution and the galactic tidal field, are valid is limite d. The discrepancy is largest for clusters with short lifetimes.