Can Photoevaporation Trigger Planetesimal Formation?

We propose that UV radiation can stimulate the formation of planetesimals in externally illuminated protoplanetary disks. We present a numerical model of disk evolution including vertical sedimentation and photoevaporation by an external O or B star. As solid material grows and settles toward the disk midplane, the outer layers of the disk become dust-depleted. When such a disk is exposed to UV radiation, heating drives photoevaporative mass loss from its surface, generating a dust-depleted outflow. The dust-to-gas surface density ratio in the disk interior grows until dust in the disk midplane becomes gravitationally unstable. Thus, UV radiation fields may induce the rapid formation of planetesimals in disks where sedimentation has occurred.