  We study the radial migration of dust particles in accreting protostellar
disks analogous to the primordial solar nebula. This study takes account of the
two dimensional (radial and normal) structure of the disk gas, including the
effects of the variation in the gas velocity as a function of distance from the
midplane. It is shown that the dust component of disks accretes slower than the
gas component. At high altitude from the disk midplane, the gas rotates faster
than particles because of the inward pressure gradient force, and its drag
force causes particles to move outward in the radial direction. Viscous torque
induces the gas within a scale height from the disk midplane to flow outward,
carrying small (size < 100 micron at 10 AU) particles with it. Only particles
at intermediate altitude or with sufficiently large sizes (> 1 mm at 10 AU)
move inward. When the particles' radial velocities are averaged over the entire
vertical direction, particles have a net inward flux. At large distances from
the central star, particles migrate inward with a velocity much faster than the
gas accretion velocity. However, their inward velocity is reduced below that of
the gas in the inner regions of the disk. The rate of velocity decrease is a
function of the particles' size. While larger particles retain fast accretion
velocity until they approach closer to the star, 10 micron particles have
slower velocity than the gas in the most part of the disk (r < 100 AU). This
differential migration of particles causes the size fractionation. Dust disks
composed mostly of small particles (size < 10 micron) accrete slower than gas
disks, resulting in the increase in the dust-gas ratio during the gas accretion
phase.
