THEORY OF ACCRETION DISKS .1. ANGULAR-MOMENTUM TRANSPORT PROCESSES

Accretion disk flow is a common phenomenon in astrophysics. It provide s the nursery for planetary system formation and the channel for mass transfer in interacting binary stars. Such flows are also associated w ith the central engine for active galactic nuclei. Mass is redistribut ed in accretion disks as a consequence of angular momentum transfer. T he identification of the dominant process involved is an important tas k in the development of accretion disk theory. Here, we review recent theoretical investigations on several important physical processes, in cluding: 1. the removal of angular momentum from disks through hydroma gnetic winds, 2. the amplification of local viscous stress through the onset of turbulence resulting from possible hydromagnetic, convective , or shear flow instabilities, 3. the transport of angular momentum ca rried by propagating waves, and 4. torque resulting from the presence of nonaxisymmetric unstable modes in self-gravitating and geometricall y thick disks. Because of the technical nature of this subject, we pre sent some of the mathematical formalisms in a pedagogical manner. We f ocus our attention on the physical discussion of the necessary conditi ons for each process to operate and the efficiency of angular momentum transfer to be expected. In Part II of this review, we shall present observational evidences and discuss applications of theoretical result s in different astrophysical contexts.