  Context: While sub-micron- and micron-sized dust grains are generally well
mixed with the gas phase in protoplanetary disks, larger grains will be
partially decoupled and as a consequence have a different distribution from
that of the gas. This has ramifications for predictions of the observability of
protoplanetary disks, for which gas-only studies will provide an inaccurate
picture. Specifically, criteria for gap opening in the presence of a planet
have generally been studied for the gas phase, whereas the situation can be
quite different in the dust layer once grains reach mm sizes, which is what
will be observed by ALMA. Aims: We aim to investigate the formation and
structure of a planetary gap in the dust layer of a protoplanetary disk with an
embedded planet. Methods: We perform 3D, gas+dust SPH simulations of a
protoplanetary disk with a planet on a fixed circular orbit at 40 AU to study
the evolution of both the gas and dust distributions and densities in the disk.
We run a series of simulations in which the planet mass and the dust grain size
varies. Results: We show that the gap in the dust layer is more striking than
in the gas phase and that it is deeper and wider for more massive planets as
well as for larger grains. For a massive enough planet, we note that cm-sized
grains remain inside the gap in corotation and that their population in the
outer disk shows an asymmetric structure, a signature of disk-planet
interactions even for a circular planetary orbit, which should be observable
with ALMA.
