Partial suppression of the radial orbit instability in stellar systems

Abstract

It is well known that the simple criterion originally proposed by Polyachenko and Shukhman in their 1981 paper for the onset of the radial orbit instability, although generally a useful tool, faces significant exceptions for both mildly anisotropic systems (with some that can be proved to be unstable) and strongly anisotropic models (with some that can be shown to be stable). In this paper we address two issues: Are there processes of collisionless collapse that can lead to equilibria of the exceptional type? And what is the intrinsic structural property that is responsible for the sometimes-noted exceptional stability behavior? To clarify these issues, we have performed a series of simulati..