The properties of energetically unbound stars in stellar clusters
  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.
