Effects of tidal shocks on the evolution of globular clusters

We present new Fokker-Planck models of the evolution of globular clusters, including gravitational tidal shocks. We extend our calculations beyond the core collapse by adopting three-body binary heating. Effects of the shocks are included by adding the tidal shock diffusion coefficients to the ordin ary Fokker-Planck equation: the first-order heating term, [Delta E], and th e second-order energy dispersion term, [Delta E-2]. As an example, we inves tigate the evolution of models for the globular cluster NGC 6254. Using the Hipparcos proper motions, we are now able to construct orbits of this clus ter in the Galaxy. Tidal shocks accelerate significantly both core collapse and the evaporation of the cluster and shorten the destruction time from 2 4 to 18 Gyr. We examine various types of adiabatic corrections and find tha t they are critical for accurate calculation of the evolution. Without adia batic corrections, the destruction time of the cluster is twice as short. W e examine duster evolution for a wide range of the concentration and tidal shock parameters and determine the region of the parameter space where tida l shocks dominate the evolution. We present fitting formulae for the core c ollapse time and the destruction time, covering all reasonable initial cond itions. In the limit of strong shocks, the typical value of the core collap se time decreases from 10t(rh) to 3t(rh) or less, while the destruction tim e is just twice that number. The effects of tidal shocks are rapidly self-l imiting: as clusters lose mass and become more compact, the importance of t he shocks diminishes. This implies that tidal shocks were more important in the past.