Angular momentum transport in quasi-Keplerian accretion disks

Abstract

We reexamine arguments advanced by Hayashi &Matsuda (2001), who claim that several simple, physically motivated deriva- tions based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the na- ture of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expres- sion for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The deriva- tion is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.