The properties of energetically unbound stars in stellar clusters

TL;DR

This study investigates the properties of energetically unbound stars, potential escapers, in stellar clusters through N-body simulations, revealing their impact on cluster kinematics and implications for dark matter profiles.

Contribution

It provides a new model predicting the velocity dispersion scaling due to potential escapers and links their properties to the galactic dark matter profile.

Findings

Potential escapers can constitute up to 40% of stars in cluster outskirts.

Velocity dispersion at the Jacobi surface depends on cluster mass and orbital velocity.

PE fraction and anisotropy are sensitive to the galactic mass profile.

Abstract

Several Milky Way star clusters show a roughly flat velocity dispersion profile at large radii, which is not expected from models with a tidal cut-off energy. Possible explanations for this excess velocity include: the effects of a dark matter halo, modified gravity theories and energetically unbound stars inside of clusters. These stars are known as potential escapers (PEs) and can exist indefinitely within clusters which are on circular orbits. Through a series of N-body simulations of star cluster systems, where we vary the galactic potential, orbital eccentricity and stellar mass function, we investigate the properties of the PEs and their effects on the kinematics. We derive a prediction for the scaling of the velocity dispersion at the Jacobi surface due to PEs, as a function of cluster mass, angular velocity of the cluster orbit, and slope of the mass profile of the host galaxy. We see a tentative signal of the mass and orbital velocity dependence in kinematic data of globular clusters from literature. We also find that the fraction of PEs depends sensitively on the galactic mass profile, reaching as high as 40% in the cusp of a Navarro-Frenk-White profile and as the velocity anisotropy also depends on the slope of the galactic mass profile, we conclude that PEs provide an independent way of inferring the properties of the dark matter mass profile at the galactic radius of (globular) clusters in the Gaia era.