Dust diffusion, sedimentation, and gravitational instabilities in protoplanetary disks

We derive the diffusion coefficient D of dust under the conditions of a protostellar disk. A reliable estimate of this quantity is essential for the transport, especially the sedimentation, of dust particles in the disk environment. In contrast to earlier treatments, the diffusion coefficient is derived on the basis of a mean field theory that allows a reliable determination of D. In the course of this derivation, we used two different turbulence models. We applied the diffusion coefficient in order to calculate how dust grains sediment and how a dust subdisk develops. Starting from the physical conditions in the subdisk, we discuss gravitational instabilities of this structure.