  We show that the standard model for geometrically thin accretion disks
(alpha-disks) leads to inconsistencies if selfgravity plays a role. This
problem arises from the parametrization of viscosity in terms of local sound
velocity and vertical disk scale height. A viscosity prescription based on
turbulent flows at the critical effective Reynolds number allows for consistent
models of thin selfgravitating disks, and recovers the alpha-disk solution as
the limiting case of negligible selfgravity. We suggest that such
selfgravitating disks may explain the observed spectra of protoplanetary disks
and yield a natural explanation for the radial motions inferred from the
observed metallicity gradients in disk galaxies.
