THE DUST SUBDISK IN THE PROTOPLANETARY NEBULA

We present a self-consistent computation of the structure of the dust subdisk in the protoplanetary nebula. The main physical processes governing the dynamics of the dust disk are reviewed. A (nonlinear) vertical diffusion equation for the transport of dust particles is derived. It is based on a competition between sedimentation processes due to gravity and diffusion due to turbulence. The vertical structure of the subdisk is computed by solving numerically the diffusion equation. The influence of both the particle size and the strength of the turbulence is studied. Large particles are found to settle down toward their equilibrium distribution in a turbulent diffusive time scale. Small particles remain mixed throughout the whole gas disk. Simple analytical estimates of the dust scale height are given. They are found to agree closely with the exact numerical solutions. The implications of our results for cosmochemistry and the structure of the solar nebula are discussed. (C) 1995 Academic Press, Inc.