On encounter rates in star clusters

Close encounters between stars in star-forming regions are important as they can perturb or destroy protoplanetary disks, young planetary systems, and multiple-star systems. We simulate simple, virialized, equal-mass N-body star clusters and find that both the rate and the total number of encounters between stars vary by factors of several in statistically identical clusters due to the stochastic/chaotic details of the orbits and stellar dynamics. Encounters tend to "saturate" rapidly in the core of a cluster, with stars there each having many encounters, while more distant stars have none. However, we find that the fraction of stars that has had at least one encounter within a particular distance grows in the same way (scaling with the crossing time and half-mass radius) in all clusters, and we present a new (empirical) way of estimating the fraction of stars that has had at least one encounter at a particular distance.