2016ApJ...829..111H

Observations of pre-transitional disks show a narrow inner dust ring and a larger outer one. They are separated by a cavity with no or only little dust. We propose an efficient recycling mechanism for the inner dust ring which keeps it in a steady state. No major particle sources are needed for replenishment. Dust particles and pebbles drift outwards by radiation pressure and photophoresis. The pebbles grow during outward drift until they reach a balanced position where residual gravity compensates photophoresis. While still growing larger they reverse their motion and drift inward. Eventually, their speed is fast enough for them to be destroyed in collisions with other pebbles and drift outward again. We quantify the force balance and drift velocities for the disks LkCa15 and HD 135344B. We simulate single-particle evolution and show that this scenario is viable. Growth and drift timescales are on the same order and a steady state can be established in the inner dust ring.