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 gove rning the dynamics of the dust disk are reviewed. A (nonlinear) vertic al diffusion equation for the transport of dust particles is derived. It is based on a competition between sedimentation processes due to gr avity and diffusion due to turbulence. The vertical structure of the s ubdisk 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 equ ilibrium distribution in a turbulent diffusive time scale. Small parti cles remain mixed throughout the whole gas disk. Simple analytical est imates of the dust scale height are given. They are found to agree clo sely with the exact numerical solutions. The implications of our resul ts for cosmochemistry and the structure of the solar nebula are discus sed. (C) 1995 Academic Press, Inc.