2010ApJ...719.1633H

Samples returned from comet 81P/Wild 2 by the Stardust mission confirm that substantial quantities of crystalline silicates were incorporated into the comet at the time of its formation. We investigate the constraints that this observation places upon protoplanetary disk physics, under the assumption that outward transport of particles processed at high temperatures occurs via a combination of advection and turbulent diffusion in an evolving disk. We also look for possible constraints on the formation locations of such particles. Our results are based upon one-dimensional disk models that evolve with time under the action of viscosity and photoevaporative mass loss, and track solid transport using an ensemble of individual particle trajectories. We find that two broad classes of the disk model are consistent with the Stardust findings. One class of models features a high particle diffusivity (a Schmidt number, Sc < 1), which suffices to diffuse particles up to 20 µm in size outward against the mean gas flow. For Sc ≥ 1 such models are unlikely to be viable and significant outward transport appears to require that the particles of interest settle into a midplane layer that experiences an outward gas flow. In either class of models, the mass of inner disk material that reaches the outer disk is a strong function of the initial compactness of the disk. Hence, models of grain transport within steady-state disks underestimate the efficiency of outward transport. Neither model results in sustained outward transport of very large particles exceeding a millimeter in size. We show that in most circumstances, the transport efficiency falls off rapidly with time. Hence, high-temperature material must be rapidly incorporated into icy bodies to avoid fallback to small radii. We suggest that significant radial transport may only occur during the initial phase of rapid disk evolution. It may also vary substantially between disks depending upon their initial mass distributions. We discuss how our model may inform recent Spitzer observations of crystalline silicates in T Tauri star-disk systems.