Taking apart the dynamical clock. Fat-tailed dynamical kicks shape the
  blue-straggler star bimodality
  In globular clusters, blue straggler stars are heavier than the average star,
so dynamical friction strongly affects them. The radial distribution of BSS,
normalized to a reference population, appears bimodal in a fraction of Galactic
GCs, with a density peak in the core, a prominent zone of avoidance at
intermediate radii, and again higher density in the outskirts. The zone of
avoidance appears to be located at larger radii the more relaxed the host
cluster, acting as a sort of dynamical clock. We use a new method to compute
the evolution of the BSS radial distribution under dynamical friction and
diffusion. We evolve our BSS in the mean cluster potential under dynamical
friction plus a random fluctuating force, solving the Langevin equation with
the Mannella quasi symplectic scheme. This amounts to a new simulation method
which is much faster and simpler than direct N-body codes but retains their
main feature: diffusion powered by strong, if infrequent, kicks. We compute the
radial distribution of initially unsegregated BSS normalized to a reference
population as a function of time. We trace the evolution of its minimum,
corresponding to the zone of avoidance. We compare the evolution under kicks
extracted from a Gaussian distribution to that obtained using a Holtsmark
distribution. The latter is a fat tailed distribution which correctly models
the effects of close gravitational encounters. We find that the zone of
avoidance moves outwards over time, as expected based on observations, only
when using the Holtsmark distribution. Thus the correct representation of near
encounters is crucial to reproduce the dynamics of the system. We confirm and
extend earlier results that showed how the dynamical clock indicator depends
both on dynamical friction and effective diffusion powered by dynamical
encounters.
