Dust Migration and Morphology in Optically Thin Circumstellar Gas Disks

We analyze the dynamics of gas-dust coupling in the presence of stellarradiation pressure in circumstellar gas disks, which are in a transitionalstage between the gas-dominated, optically thick, primordial nebulae, and thedust-dominated, optically thin Vega-type disks. Dust undergo radial migration,seeking a stable equilibrium orbit in corotation with gas. The migration ofdust gives rise to radial fractionation of dust and creates a variety ofpossible observed disk morphologies, which we compute by considering theequilibrium between the dust production and the dust-dust collisions removingparticles from their equilibrium orbits. Sand-sized and larger grains aredistributed throughout most of the gas disk, with concentration near the gaspressure maximum in the inner disk. Smaller grains (typically in the range of10 to 200 micron) concentrate in a prominent ring structure in the outer regionof the gas disk (presumably at radius 100 AU), where gas density is rapidlydeclining with radius. The width and density, as well as density contrast ofthe dust ring with respect to the inner dust disk depend on the distribution ofgas. Our results open the prospect for deducing the distribution of gas incircumstellar disks by observing their dust. We have qualitatively compared ourmodels with two observed transitional disks around HR 4796A and HD 141569A.Dust migration can result in observation of a ring or a bimodal radial dustdistribution, possibly very similar to the ones produced by gap-openingplanet(s) embedded in the disk, or shepherding it from inside or outside. Weconclude that a convincing planet detection via dust imaging should includespecific non-axisymmetric structure following from the dynamical simulations ofperturbed disks.Comment: 27 pages, 16 figures, submitted to Ap