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Escape from Globular Clusters: Untangling Causes and their Observable Signatures

Presentation #116.51 in the session Stellar/Compact Objects.

Published onJul 01, 2023
Escape from Globular Clusters: Untangling Causes and their Observable Signatures

Stellar bodies escape from globular clusters (GCs) in numerous ways. Two-body relaxation coupled with external tides especially dominates low-speed escape, driving the gradual evaporation of GCs and their formation of tidal tails. High-speed escape at up to hundreds or even >1,000 km/s (e.g., ‘hyper-velocity stars’) can occur through more violent ejection mechanisms, including strong gravitational scattering, supernovae, gravitational wave-driven mergers, tidal disruption events, and physical collisions. Yet many nuances of collisional dynamics within a tidal field and long-time dearth of data on the outskirts of Galactic GCs have discouraged comprehensive study of escape mechanisms, their combined observable properties, or detailed comparison between models and observations. The opportunity and demand for such study has recently bloomed, however; the 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. In this presentation, we discuss our new and ongoing series of studies exploring escape from GCs simulated with the Cluster Monte Carlo (CMC) code, including current work comparing our results to Gaia data on GC outskirts. We discuss the relative contributions of various escape mechanisms to the overall escape rate and to production of tidal tails and high-speed ejecta. We emphasize the surprisingly strong role binary formation (from three independent bodies) may play in high-speed escape in typical, non-core-collapsed GCs. Black holes are an essential catalyst in this process and their loss at the onset of observable core collapse (i.e., as measured from the brightness profile), dramatically curtails three-body binary formation and ejection through this channel, contrary to intuition based solely on cluster density. Secondarily, we demonstrate that even when born from a thermal eccentricity distribution, escaping binaries have significantly non-thermal eccentricities consistent with the roughly uniform distribution observed in the Galactic field. We also touch on preliminary results on escaper trajectories integrated in the full combined potential of the cluster and host galaxy, including impacts on observable proper motions beyond the tidal boundary, and identification of escaped cluster members with Gaia.

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