Dust migration and morphology in optically thin circumstellar gas disks

We analyze the dynamics of gas-dust coupling in the presence of stellar rad iation pressure in circumstellar disks, which are in a transitional stage b etween the gas-dominated, optically thick, primordial nebulae, and the dust -dominated, optically thin Vega-type disks. Dust grains undergo radial migr ation, either leaving the disk owing to a strong radiation pressure or seek ing a stable equilibrium orbit in corotation with gas. In our models of A-t ype 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 sou nd speed. Equilibrium orbits are circular, with exception of those signific antly affected by radiation pressure, which can be strongly elliptic with a pocenters extending beyond the bulk of the gas disk. The migration of dust gives rise to radial fractionation of dust and creates a variety of possibl e observed disk morphologies, which we compute by considering the equilibri um between the dust production and the dust-dust collisions removing partic les from their equilibrium orbits. Large grains (typically greater than or similar to 200 mum) are distributed throughout most of the gas disk. Smalle r grains (in the range of 10-200 mum) concentrate in a prominent ring struc ture in the outer region of the gas disk (presumably at radius similar to 1 00 AU), where gas density is rapidly declining with radius. The width and d ensity, as well as density contrast of the dust ring with respect to them i nner dust disk, depend on the distribution of gas and the mechanical streng th of the particles. Our results open the prospect for deducing the distrib ution of gas in circumstellar disks by observing their dust. We have qualit atively 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 prod uced by gap-opening planets embedded in the disk, or shepherding it from in side or outside. We conclude that a convincing planet detection via dust im aging should include specific nonaxisymmetric structure (spiral waves, stre amers, resonant arcs) following from the dynamical simulations of perturbed disks.