Dynamical evolution of rotating stellar systems - III. The effect of the mass spectrum

Abstract

We have studied the dynamical evolution of rotating star clusters with mass spectrum using a Fokker-Planck code. As the simplest multimass model, we first investigated two-component clusters. Rotation is found to accelerate the dynamical evolution through the transfer of angular momentum outward, as well as from high masses to low masses. However, the degree of acceleration depends sensitively on the assumed initial mass function since dynamical friction, which generates mass segregation, also tends to accelerate the evolution, and the combined effect of both is not linear or multiplicative. As long as dynamical friction dominates in the competition with angular momentum exchange, the heavy masses lose random energy and angular momentum and sink towards the centre, but their remaining angular momentum is sufficient to speed them up rotationally. This is gravo-gyro instability. As a consequence, we find that the high-mass stars in the central parts rotate faster than low-mass stars. This leads to the suppression of mass segregation compared with the non-rotating clusters. From the study of multicomponent models, we observe similar trends to the two-component models in almost all aspects. The mass function changes less drastically for clusters with rotation. Unlike non-rotating clusters, the mass function depends on R and z. Our models are the only ones that can predict mass function and other quantities to be compared with new observations.