  We consider the accretion process in a disk with magnetic fields that are
dragged in from the interstellar medium by gravitational collapse. Two
diffusive processes are at work in the system: (1) "viscous" torques exerted by
turbulent and magnetic stresses, and (2) "resistive" redistribution of mass
with respect to the magnetic flux arising from the imperfect conduction of
current. In steady state, self-consistency between the two rates of drift
requires that a relationship exists between the coefficients of turbulent
viscosity and turbulent resistivity. Ignoring any interactions with a stellar
magnetosphere, we solve the steady-state equations for a magnetized disk under
the gravitational attraction of a mass point and threaded by an amount of
magnetic flux consistent with calculations of magnetized gravitational collapse
in star formation. Our model mean-field equations have an exact analytical
solution that corresponds to magnetically diluted Keplerian rotation about the
central mass point. The solution yields the strength of the magnetic field and
the surface density as functions of radial position in the disk and their
connection with the departure from pure Keplerian rotation in representative
cases. We compare the predictions of the theory with the available observations
concerning T Tauri stars, FU Orionis stars, and low- and high-mass protostars.
Finally, we speculate on the physical causes for high and low states of the
accretion disks that surround young stellar objects. One of the more important
results of this study is the physical derivation of analytic expressions for
the turbulent viscosity and turbulent resistivity.
