Presentation #102.04 in the session Formation of Planets and Satellites.
Various physical processes that ensue within protoplanetary disks — including vertical settling of icy/rocky grains, radial drift of solids, planetesimal formation, as well as planetary accretion itself — are facilitated by hydrodynamic interactions between H/He gas and solid dust. The Stokes number, which quantifies the strength of dust-gas coupling, thus plays a central role in protoplanetary disk evolution. However, because the Stokes number depends both upon local disk properties and the particle size, it is notoriously difficult to compute, and its value is typically assumed rather than calculated. In this talk, I will present a simple, but self-consistent model for dust-gas coupling, and demonstrate that for a specified combination of the nebular accretion rate, and turbulent viscosity, the radial profile of the Stokes number can be calculated uniquely. For fiducial protoplanetary disk parameters, our theory yields characteristic values of the Stokes number that translate to grains sizes of s ~ mm in the inner nebula and s ~ few-cm, in the outer regions of the disk. Accordingly, solids are expected to settle into a thin sub-disk at large stellocentric distances, while remaining vertically well-mixed inside the ice line.