Interaction between stars and an inactive accretion disc in a galactic core

Abstract

We discuss the structure of a central stellar cluster whose dynamics is influenced by gravitation of a supermassive black hole and by the dissipative interaction of orbiting stars with an accretion disc. We also take the effect of disc self-gravity into account. We show that the cluster properties are determined predominantly by the radial profile of the disc surface density. To this aim we develop a simple steady-state model of the central cluster and we estimate the rate at which stars migrate to the centre. This model is relevant for central regions of an active galactic nucleus (AGN) containing a rather dense accretion medium. In passing we also briefly mention a possibility that a fossil (inactive) disc could exist in the centre of our own Galaxy. Such a hypothetical disc could perturb the motion of stars and set them on highly elliptic trajectories with small pericentre distances. The required mass of the disc is less than one percent of the central black hole mass, i.e. below the upper limit permitted by present accuracy of the orbital parameters of S-stars in the Galactic Centre.