  Debris rings of dust are found around young luminous stars such as HR4796A
and HD141569. Some of these entities have sharp edges and gaps which have been
interpreted as evidence for the presence of shepherding and embedded planets.
Here we show that gaps and sharp edges in the debris disks of dust can also be
spontaneously self generated if they are embedded in optically thin regions of
gaseous disks. This clumping instability arises in regions where an enhancement
in the dust density leads to local gas temperature and pressure increases.
Consequently, the relative motion between the gas and the dust is modified. The
subsequent hydrodynamic drag on the dust particles leads to further enhancement
of their concentration. We show that this process is linearly unstable and
leads to the formation of ring-like structures within the estimated life time
of such young objects. Once the gas is removed (e.g. by photo evaporation) the
structures are ``frozen'' and will persist, even when the gas might not be
observable anymore.
