The theory of circumstellar accretion disks

Abstract

Recent theoretical advances in our understanding of the dynamics of circumstellar accretion disks are reviewed. Of particular importance are developments regarding angular momentum transport processes in disks. It has recently been shown through direct numerical simulation that vertical convection results in inward transport, and thus is unlikely to serve as a source of anomalous viscosity. More importantly, these results have been generalized to demonstrate that any form of hydrodynamic turbulence in which velocity fluctuations do not extract both energy and angular momentum from the mean shear flow will be associated with inward angular momentum transport. In fact, the analysis predicts that nonlinear shear instabilities are absent in Keplerian disks, a result which has been confirmed through direct numerical simulation. The important role that MHD turbulence driven by the Balbus-Hawley (BH) instability plays in outward angular momentum transport is emphasized, especially regarding the implications for the production of MHD winds from disks. A detailed understanding of the dynamical interaction between an accretion disk and the magnetosphere of the central object will be crucial to understanding accretion from disks in general.