[Abridged] Aims: We provide predictions for ALMA observations of planet gaps
that account for the specific spatial distribution of dust that results from
consistent gas+dust dynamics. Methods: In a previous work, we ran full 3D,
two-fluid Smoothed Particle Hydrodynamics (SPH) simulations of a planet
embedded in a gas+dust T Tauri disk for different planet masses and grain
sizes. In this work, the resulting dust distributions are passed to the Monte
Carlo radiative transfer code MCFOST to construct synthetic images in the ALMA
wavebands. We then use the ALMA simulator to produce images that include
thermal and phase noise for a range of angular resolutions, wavelengths, and
integration times, as well as for different inclinations, declinations and
distances. We also produce images which assume that gas and dust are well mixed
with a gas-to-dust ratio of 100 to compare with previous ALMA predictions, all
made under this hypothesis. Results: Our findings clearly demonstrate the
importance of correctly incorporating the dust dynamics. We show that the gap
carved by a 1 M_J planet orbiting at 40 AU is visible with a much higher
contrast than the well-mixed assumption would predict. In the case of a 5 M_J
planet, we clearly see a deficit in dust emission in the inner disk, and point
out the risk of interpreting the resulting image as that of a transition disk
with an inner hole if observed in unfavorable conditions. Planet signatures are
fainter in more distant disks but declination or inclination to the
line-of-sight have little effect on ALMA's ability to resolve the gaps.
Conclusions: ALMA has the potential to see signposts of planets in disks of
nearby star-forming regions. We present optimized observing parameters to
detect them in the case of 1 and 5 M_J planets on 40 AU orbits.Comment: 15 pages, 21 figures, accepted by Astronomy & Astrophysics, a higher
resolution version of the paper is available at
http://www-obs.univ-lyon1.fr/labo/perso/jean-francois.gonzalez/Papers/Gaps_ALMA.pd
