Self-similar models of magnetized accretion disks

Abstract

Steady state models of magnetized accretion disks that power centrifugally driven outflows are presented. In these models, most of the excess angular momentum of the accreted matter is carried away by the wind, and the shearing and advection of the magnetic field lines inside the disk are balanced either by ambipolar diffusion or by Ohmic diffusivity. Radially self-similar solutions for the magnetic field configuration in a Keplerian, nonmagnetically dominated disk that is threaded by open field lines are constructed using both of these field diffusion mechanisms. The derived disk solutions are required to match smoothly onto the self-similar, cold wind solutions obtained by Blandford and Payne. Illustrative examples are presented of nonviscous accretion disks in protostellar environments and in active galactic nuclei in which most of the liberated gravitational energy is used to power an MHD wind that becomes super-Alfvenic and magnetically collimated at a finite distance from the origin. The application of these results to cosmic jet sources, and in particular to bipolar outflow sources associated with young stellar objects, is considered. It is suggested that the strong correlation that appears to exist between the presence of circumstellar disks and the occurrence of energetic winds in these objects could be interpreted, in the context of the present model, as a reflection of the fact that the outflows regulate the angular momentum transport in the disk and hence are a necessary ingredient in the accretion process.