2019ApJ...882...33G

There is evidence that protoplanetary disks-including the protosolar one-contain crystalline dust grains on spatial scales where the dust temperature is lower than the threshold value for their formation through thermal annealing of amorphous interstellar silicates. We interpret these observations in terms of an extended, magnetocentrifugally driven disk wind that transports grains from the inner disk-where they are thermally processed by the stellar radiation after being uplifted from the disk surfaces-to the outer disk regions. For any disk radius r, there is a maximum grain size a_max(r) that can be uplifted from that location: grains of size a ≪ a_max are carried away by the wind, whereas those with a <= a_max reenter the disk at larger radii. A significant portion of the reentering grains converge to-and subsequently accumulate in-a narrow region just beyond r_max(a), the maximum radius from which grains of size a can be uplifted. We show that this model can account for the inferred crystallinity fractions in classical T Tauri and Herbig Ae disks and for their indicated near constancy after being established early in the disk evolution. It is also consistent with the reported radial gradients in the mean grain size, crystallinity, and crystal composition. In addition, this model yields the properties of the grains that remain embedded in the outflows from protoplanetary disks and naturally explains the inferred persistence of small grains in the surface layers of these disks.