Comparison between Fokker-Planck and gaseous models of star clusters in the multi-mass case revisited
Numerical models for the secular evolution of spherical star clusters are compared. Solutions of the orbit-averaged Fokker-Planck equation for isotropic systems are confronted with anisotropic gaseous models of star clusters for the special case of a system with two distinct individual stellar masses (fixed mass ratio) but varying total mass ratios (i.e. particle numbers per component) and total particle numbers of N=1000 and N=10000. It is shown that with numerically and physically improved models the agreement between the two methods is much better than previously reported. In a comparison with a direct averaged N-body calculation, the relevance of the statistical models for real star clusters is discussed. Whereas the gaseous model has deficiencies in the local determination of the effects of gravitational encounters, the Fokker-Planck (henceforth FP) models suffer from their restriction to isotropy. Although in many aspects the present models provide a fairly reliable description of star clusters (they have to be used for particle numbers as large as N=10^5 for global clusters), a quantitative agreement between N-body results and statistical models in all respects could probably be expected for a 2D FP model which is not yet available.