Evolution towards energy equipartition in star clusters: effects of the tidal field, primordial binaries, and internal velocity anisotropy

Abstract

ABSTRACT This paper is the second in a series investigating the evolution of star clusters towards energy equipartition (EEP). Here, we focus on the effects of the external tidal field of the host galaxy, initial anisotropy in the velocity distribution, and primordial binary star population. The results of our N-body simulations show that regardless of the strength of the tidal field or the fraction of primordial binaries: (i) the evolution towards EEP in the intermediate and outer regions of initially anisotropic systems is more rapid than for isotropic systems; (ii) this evolution also proceeds at different rates for the tangential and radial components of the velocity dispersion; and (iii) the outer regions of the initially isotropic systems show a tendency to evolve towards a state of ‘inverted’ EEP in which low-mass stars have smaller velocity dispersion than high-mass stars. We also find that the clusters with primordial binaries stay even farther from EEP than systems containing only single stars. Finally, we show that all these results also hold when the degree of EEP is calculated using quantities measured in projection as it is done in observational studies, and that our findings could be tested with current and upcoming observational data.