The Evolution of Globular Clusters in the Galaxy

Abstract

We investigate the evolution of globular clusters using N-body calculations and anisotropic Fokker-Planck calculations. The models include a mass spectrum, mass loss due to stellar evolution, and the tidal field of the parent galaxy. Recent N-body calculations revealed a serious discrepancy between the results of N-body calculations and isotropic Fokker-Planck calculations. It has been turned out that the main reason for the discrepancy is an oversimplified treatment of the tidal field employed in the isotropic Fokker-Planck models. We perform a series of calculations with anisotropic Fokker-Planck models with a better treatment of the tidal boundary and compare these with N-body calculations. The new tidal boundary condition in our Fokker-Planck model includes one free parameter. We find that a single value of this parameter gives satisfactory agreement between the N-body and Fokker-Planck models over a wide range of initial conditions. With this improved Fokker-Planck model we also carry out an extensive survey of the evolution of globular clusters over a wide range of initial conditions; the slope of the mass function, the central concentration and the relaxation time. The clusters are evolved up to the moment of disruption or core collapse. In general, our model clusters, calculated with the anisotropic Fokker-Planck model with the improved treatment for the tidal boundary, live considerably longer (up to an order of magnitude) than the isotropic models. The differences in the lifetime between the isotropic and anisotropic models are smallest for clusters with a steep initial mass function and with a high initial central concentration.