Fokker-Planck models of star clusters with anisotropic velocity distributions .3. Multi-mass clusters

The evolution of globular clusters driven by two-body relaxation was investigated by means of a numerical integration of the two-dimensional Fokker-Planck equation in energy-angular momentum space. The two-dimensional Fokker-Planck equation allows the development of velocity anisotropy. We include a spectrum of stellar masses in this paper. The radial anisotropy develops,that is, the radial velocity dispersion exceeds the tangential one, in the outer halo of multi-mass clusters as in single-mass clusters. However, the evolution of the velocity anisotropy significantly depends on the stellar mass in some cases. In fact, the tangential-velocity dispersion becomes dominant around the half-mass radius for massive components in clusters with a steep mass function. The development of this tangential anisotropy is closely related to the initial cooling of the massive components toward energy equipartition. Our simulation results suggest that multi-mass anisotropic King-Michie models are not always appropriate for describing the velocity anisotropy in globular clusters.