The mass and radius evolution of globular clusters in tidal fields
  We present a simple theory for the evolution of initially compact clusters in
a tidal field. The fundamental ingredient of the model is that a cluster
conducts a constant fraction of its own energy through the half-mass radius by
two-body interactions every half-mass relaxation time. This energy is produced
in a self-regulative way in the core by an (unspecified) energy source. We find
that the half-mass radius increases during the first part (roughly half) of the
evolution and decreases in the second half, while the escape rate is constant
and set by the tidal field. We present evolutionary tracks and isochrones for
clusters in terms of cluster half-mass density, cluster mass and
galacto-centric radius. We find substantial agreement between model isochrones
and Milky Way globular cluster parameters, which suggests that there is a
balance between the flow of energy and the central energy production for almost
all globular clusters. We also find that the majority of the globular clusters
are still expanding towards their tidal radius. Finally, a fast code for
cluster evolution is presented.
