Dust coagulation and settling in layered protoplanetary disks

Previous models of dust growth in protoplanetary disks considered either uniformly laminar or turbulent disks. This Letter explores how dust growth occurs in a layered protoplanetary disk in which the magnetorotational instability (MRI) generates turbulence only on the surface layers of a disk. Two cases are considered: a completely laminar dead zone and a dead zone in which turbulence is stirred up from the MRI acting above. It is found that dust is depleted from high altitudes in layered disks faster than in uniformly laminar or turbulent disks. This is a result of the accelerated growth of particles in the turbulent regions and their storage in the laminar lower levels where they escape energetic collisions that would result in disruption. Thus, the regions of a protoplanetary disk above a dead zone would become rapidly depleted in small dust grains, whereas the outer regions will maintain a small dust population at all heights due to the disruptive collisions and vertical mixing from turbulence. This structure is similar to that inferred for disks around TW Hydra, GM Auriga, DM Tau, and CoKu Tau/4, which are depleted in dust close to the star but are optically thick at larger heliocentric distances.