Dynamical evolution of rotating stellar systems: I pre-collapse, equal mass systems

Abstract

The influence of rotation on the dynamical evolution of collisional stellar systems is investigated by solving the orbit-averaged Fokker-Planck equation in (E,J_z)-space. We find that large amounts of initial rotation drive the system into a phase of strong mass loss while it is moderately contracting. The core is rotating even faster than before although angular momentum is transported outwards. At the same time the core is heating. Given these features this phase can be associated with the gravo-gyro 'catastrophe' found by Hachisu (1979). The increase in central angular momentum levels off after about 2-3 initial half-mass relaxation times indicating that the source of this `catastrophe' is depleted. Finally, the central angular velocity increases again, but with a rather small power of the central density -- the same power as for the central velocity dispersion during self-similar contraction towards the collapse. The rotation curves flatten and the ellipticity variations decrease with time, but their shapes are very similar. These results suggest the existence of a self-similar solution for a rotating cluster as well. The maximum values of rotational velocity and ellipticity occur at about the half mass radius.