Stellar Escape from Globular Clusters. I. Escape Mechanisms and Properties at Ejection

TL;DR

This paper reviews stellar escape mechanisms from globular clusters, emphasizing the role of three-body binary formation and black holes, and sets the stage for future comparisons between models and Gaia data to understand stellar streams.

Contribution

It provides the first comprehensive review of escape mechanisms from GCs, highlighting the significance of three-body binary formation and black holes in stellar ejection processes.

Findings

Three-body binary formation may dominate high-speed ejections.

Black holes catalyze binary formation, affecting escape rates.

Escaping binaries exhibit nonthermal eccentricity distributions.

Abstract

The theory of stellar escape from globular clusters (GCs) dates back nearly a century, especially the gradual evaporation of GCs via two-body relaxation coupled with external tides. More violent ejection can also occur via strong gravitational scattering, supernovae, gravitational wave-driven mergers, tidal disruption events, and physical collisions, but comprehensive study of the many escape mechanisms has been limited. Recent exquisite kinematic data from the Gaia space telescope has revealed numerous stellar streams in the Milky Way (MW) and traced the origin of many to specific MWGCs, highlighting the need for further examination of stellar escape from these clusters. In this study, the first of a series, we lay the groundwork for detailed follow-up comparisons between Cluster Monte Carlo (CMC) GC models and the latest Gaia data on the outskirts of MWGCs, their tidal tails, and associated streams. We thoroughly review escape mechanisms from GCs and examine their relative contributions to the escape rate, ejection velocities, and escaper demographics. We show for the first time that three-body binary formation may dominate high-speed ejection from typical MWGCs, potentially explaining some of the hypervelocity stars in the MW. Due to their mass, black holes strongly catalyze this process, and their loss at the onset of observable core collapse, characterized by a steep central brightness profile, dramatically curtails three-body binary formation, despite the increased post-collapse density. We also demonstrate that even when born from a thermal eccentricity distribution, escaping binaries have significantly nonthermal eccentricities consistent with the roughly uniform distribution observed in the Galactic field.