Surface Outflow in Optically Thick Dust Disks by the Radiation Pressure

We study the outflow of dust particles on the surface layers of optically thick disks. At the surface of disks around young stars, small dust particles (size < 10 micron) experience stellar radiation pressure support and orbit more slowly than the surrounding gas. The resulting tail-wind imparts energy and angular momentum to the dust particles, moving them outward. This outflow occurs in the thin surface layer of the disk that is exposed to starlight, and the outward mass flux is carried primarily by particles of size ~0.1 micron. Beneath the irradiated surface layer, dust particles experience a head-wind, which drives them inward. For the specific case of a minimum-mass-solar-nebula, less than a thousandth of the dust mass experiences outward flow. If the stellar luminosity is 15 times brighter than the sun, however, or if the gas disk mass is as small as ~100 M_earth, then the surface outflow can dominate the inward flux in certain radial ranges, leading to the formation of rings or gaps in the dust disks.