Tidal mass loss in star clusters and treatment of escapers in
  Fokker-Planck models
  This paper presents a new scheme to treat escaping stars in the
orbit-averaged Fokker-Planck models of globular star clusters in a galactic
tidal field. The existence of a large number of potential escapers, which have
energies above the escape energy but are still within the tidal radius, is
taken into account in the models. The models allow potential escapers to
experience gravitational scatterings before they leave clusters and thus some
of them may lose enough energy to be bound again. It is shown that the mass
evolution of the Fokker-Planck models are in good agreement with that of N-body
models including the full tidal-force field. The mass-loss time does not simply
scale with the relaxation time due to the existence of potential escapers; it
increases with the number of stars more slowly than the relaxation time, though
it tends to be proportional to the relaxation time in the limit of a weak tidal
field. The Fokker-Planck models include two parameters, the coefficient gamma
in the Coulomb logarithm ln(gamma N) and the coefficient nu_e controlling the
efficiency of the mass loss. The values of these parameters are determined by
comparing the Fokker-Planck models with the N-body models. It is found that the
parameter set (gamma, nu_e)=(0.11, 7) works well for both single-mass and
multi-mass clusters, but that the parameter set (gamma, nu_e)=(0.02, 40) is
another possible choice for multi-mass clusters.
