  Planet-forming disks turn from gas-rich, massive disks made of dust and gas
into planetary systems containing only small amounts dust produced by
collisions between smaller planetary objects like planetesimals, asteroids, or
comets. Traditionally we talk about protoplanetary (age $\sim$1 Myr),
transitional ($\sim$ 5-10 Myr), and debris disks ($\sim$ 10-hundreds of Myr),
even though the overlap between these phases may be relevant. We aim to show
that in the transition phase of a disk, when gas surface densities are reduced
but not yet negligible, a seemingly small amount of collisional activity may
lead to the production of dust on a level that is observationally relevant by
creating regions of optical depth 1 or above. In particular, we aim to show
that the hot dust emission component of transitional disks may in fact be
debris dust produced in such collisions. We develop an analytical model to
derive the conditions under which observationally relevant amounts of dust can
be produced. We focus on the effect of the gas surface density during the
transition phase of a disk from fully gas-dominated to the gas-poor debris
stage. We show that the decrease of the gas surface density has an important
effect. It allows smaller planetesimals to become collisional, starting a
cascade. At the same time, small particles are not destroyed by collisions.
That interrupted cascade is critical to preserve the produced dust for
significant time intervals, allowing a seemingly minor amount of planetesimal
collisions to be effective in producing detectable amounts of dust. The warm
emission of low amounts of dust in many transitional planet-forming disks might
be caused entirely by second-generation dust, exposing planetary material at a
time much earlier than the age of what are traditionally considered as debris
disks.
