Dust migration and morphology in optically thin circumstellar gas disks

We analyze the dynamics of gas-dust coupling in the presence of stellar radiation pressure in circumstellar disks, which are in a transitional stage between the gas-dominated, optically thick, primordial nebulae, and the dust-dominated, optically thin Vega-type disks. Dust grains undergo radial migration, either leaving the disk owing to a strong radiation pressure or seeking a stable equilibrium orbit in corotation with gas. In our models of A-type stars surrounded by a total gas mass from a fraction to dozens of Earth masses, the outward migration speed of dust is comparable with the gas sound speed. Equilibrium orbits are circular, with exception of those significantly affected by radiation pressure, which can be strongly elliptic with apocenters extending beyond the bulk of the gas disk. The migration of dust gives rise to radial fractionation of dust and creates a variety of possible observed disk morphologies, which we compute by considering the equilibrium between the dust production and the dust-dust collisions removing particles from their equilibrium orbits. Large grains (typically greater than or similar to 200 mum) are distributed throughout most of the gas disk. Smaller grains (in the range of 10-200 mum) concentrate in a prominent ring structure in the outer region of the gas disk (presumably at radius similar to 100 AU), where gas density is rapidly declining with radius. The width and density, as well as density contrast of the dust ring with respect to them inner dust disk, depend on the distribution of gas and the mechanical strength of the particles. Our results open the prospect for deducing the distribution of gas in circumstellar disks by observing their dust. We have qualitatively compared our models with two observed transitional disks around HR 4796A and HD 141569A. Dust migration can result in observation of a ring or a bimodal radial dust distribution, possibly very similar to the ones produced by gap-opening planets embedded in the disk, or shepherding it from inside or outside. We conclude that a convincing planet detection via dust imaging should include specific nonaxisymmetric structure (spiral waves, streamers, resonant arcs) following from the dynamical simulations of perturbed disks.