Models of astrophysical decretion-accretion diffusional disks

The formation of gaseous diffusional accretion-decretion disks is an important stage in the evolution of numerous astronomical objects. Matter is accreted onto the object in the accretion part of these disks, while the angular momentum of the accreted matter is transported from the central region to the periphery in the decretion part. Here, we consider general questions connected with the formation and evolution of diffusive accretion-decretion disks in various astrophysical objects. Such disks can be described using nonstationary diffusion models. The phenomenological parameters of these models are the coefficients in the relations for the characteristic turbulent velocity and mean free path of diffusion elements in the disk. We have developed a numerical technique to compute the disk evolution for a number of models (a massive disk, a disk with continuous accretion, a purely decretion disk). Analytical expressions estimating the basic parameters of accretion-decretion disks are presented. We discuss the relationship between the models considered and the classical α model of an accretion disk.