Hyper Velocity Stars and the Restricted Parabolic 3-body Problem

TL;DR

This paper models the disruption of binary stars near a supermassive black hole using a restricted parabolic three-body approximation, revealing new insights into star ejection velocities and probabilities of binary survival.

Contribution

It introduces a simplified, scalable model for binary disruption near black holes, challenging previous assumptions about star retention and providing detailed probability distributions.

Findings

Lighter stars are ejected in 50% of disruptions.

Higher ejection velocities are associated with lighter objects.

Deep penetrations into the tidal sphere do not always lead to disruption.

Abstract

Motivated by detections of hypervelocity stars that may originate from the Galactic Center, we revist the problem of a binary disruption by a passage near a much more massive point mass. The six order of magnitude mass ratio between the Galactic Center black hole and the binary stars allows us to formulate the problem in the restricted parabolic three-body approximation. In this framework, results can be simply rescaled in terms of binary masses, its initial separation and binary-to-black hole mass ratio. Consequently, an advantage over the full three-body calculation is that a much smaller set of simulations is needed to explore the relevant parameter space. Contrary to previous claims, we show that, upon binary disruption, the lighter star does not remain preferentially bound to the black hole. In fact, it is ejected exactly in 50% of the cases. Nonetheless, lighter objects have higher ejection velocities, since the energy distribution is independent of mass. Focusing on the planar case, we provide the probability distributions for disruption of circular binaries and for the ejection energy. We show that even binaries that penetrate deeply into the tidal sphere of the black hole are not doomed to disruption, but survive in 20% of the cases. Nor do these deep encounters produce the highest ejection energies, which are instead obtained for binaries arriving to 0.1-0.5 of the tidal radius in a prograde orbit. Interestingly, such deep-reaching binaries separate widely after penetrating the tidal radius, but always approach each other again on their way out from the black hole.[shortened]