BASIC N-BODY MODELING OF THE EVOLUTION OF GLOBULAR-CLUSTERS - I - TIME SCALING

We consider the use of N-body simulations for studying the evolution o f rich star clusters (i.e. globular clusters).The dynamical processes included in this study are restricted to gravitational (point-mass) in teractions, the steady tidal field of a galaxy, and instantaneous mass loss resulting from stellar evolution. With evolution driven by these mechanisms, it is known that clusters fall roughly into two broad cla sses: those that dissipate promptly in the tidal field, as a result of mass loss; and those that survive long enough for their evolution to become dominated by two-body relaxation. The time-scales of the proces ses we consider scale in different ways with the number of stars in th e simulation, and the main aim of the paper is to suggest how the scal ing of a simulation should be done so that the results are representat ive of the evolution of a 'real' cluster. We investigate three differe nt ways of scaling time. One of these is appropriate to the first type of cluster, i.e. those that dissipate rapidly; similarly, a second sc aling is appropriate only to the second (relaxation-dominated) type. W e also develop a hybrid scaling, which is a satisfactory compromise fo r both types of cluster. Finally we present evidence that the widely u sed Fokker-Planck method produces models that are in good agreement wi th N-body models of those clusters that are relaxation-dominated, at l east for N-body models with several thousand particles, but that the F okker-Planck models evolve too fast for clusters that dissipate prompt ly.