Ejected and Rejected: Strong Scatterings in Dense Star Clusters

Abstract

Understanding the evolution of collisional stellar systems is key to predicting both the bulk evolution of dense star clusters and also the rates of tidal disruption events, extreme mass ratio inspirals (EMRIs), stellar ejections, and other loss cone phenomena. Although a majority of the evolution is dictated by weak, small-angle scatterings, rare close encounters resulting in strong scatterings can sometimes produce qualitatively different outcomes. To systematically determine the astrophysical importance of strong scatterings, we derive the exact differential rate distribution of velocity kicks in isotropic and spherically symmetric distribution functions representing globular and nuclear star clusters, while also deriving the distribution boundaries and approximate rate scalings. We then extend this formalism to derive approximate rates of energy and angular momentum kicks for objects in the nuclear star cluster, and then apply this framework to study (i) ejections and (ii) ``cancellation'' of extreme mass ratio inspirals. We find that ejection rates are complicated in some regimes due to the saturation of the strong kick distribution and the breakdown of the weak limit, likely enhancing the effect of loss cone shielding. We also determine that a sizable fraction of proto-EMRIs will likely be canceled by strong scatterings, highlighting the need for future work to determine to what degree strong scatterings impact the true EMRI rate.