  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 photo-evaporation 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 photo-evaporative mass-loss from its
surface, generating a dust-depleted outflow. The dust: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.
