Radial Flow of Dust Particles in Accretion Disks

We study the radial migration of dust particles in accreting protostellardisks analogous to the primordial solar nebula. This study takes account of thetwo dimensional (radial and normal) structure of the disk gas, including theeffects of the variation in the gas velocity as a function of distance from themidplane. It is shown that the dust component of disks accretes slower than thegas component. At high altitude from the disk midplane, the gas rotates fasterthan particles because of the inward pressure gradient force, and its dragforce causes particles to move outward in the radial direction. Viscous torqueinduces 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 particlesat intermediate altitude or with sufficiently large sizes (> 1 mm at 10 AU)move inward. When the particles' radial velocities are averaged over the entirevertical direction, particles have a net inward flux. At large distances fromthe central star, particles migrate inward with a velocity much faster than thegas accretion velocity. However, their inward velocity is reduced below that ofthe gas in the inner regions of the disk. The rate of velocity decrease is afunction of the particles' size. While larger particles retain fast accretionvelocity until they approach closer to the star, 10 micron particles haveslower velocity than the gas in the most part of the disk (r < 100 AU). Thisdifferential migration of particles causes the size fractionation. Dust diskscomposed mostly of small particles (size < 10 micron) accrete slower than gasdisks, resulting in the increase in the dust-gas ratio during the gas accretionphase.Comment: ApJ, accepted, 17 pages, 14 figure