Gas and dust structures in protoplanetary disks hosting multiple planets

Abstract

Transition disks have dust-depleted inner regions and may represent an intermediate step of an on-going disk dispersal process, where planet formation is probably in progress. Recent millimetre observations of transition disks reveal radially and azimuthally asymmetric structures, where micron- and millimetre-sized dust particles may not spatially coexist. These properties can be the result of particle trapping and grain growth in pressure bumps originating from the disk interaction with a planetary companion. The multiple features observed in some transition disks, such as SR 21, suggest the presence of more than one planet. We study the gas and dust distributions of a disk hosting two massive planets as function of different disk and dust parameters. Observational signatures, such as the spectral energy distribution, sub-millimetre, and polarised images are simulated for the various parameters. We confirm that planets can lead to particle trapping, although for a disk with high viscosity ($\alpha_{\rm{turb}}=10^{-2}$), the planet should be more massive than $5 M_{\rm{Jup}}$ and dust fragmentation should occur with low efficiency ($v_{f}\sim30\rm{m s}^{-1}$). This will lead to a ring-like feature as observed in transition disks in the millimetre. When trapping occurs, we find that a smooth distribution of micron sized grains throughout the disk, sometimes observed in scattered light, can only happen if the combination of planet mass and turbulence is such that small grains are not fully filtered out. A high disk viscosity ($\alpha_{\rm{turb}}=10^{-2}$) ensures a replenishment of the cavity in micron-sized dust, while for lower viscosity ($\alpha_{\rm{turb}}=10^{-3}$), the planet mass is constrained to be less than $5 M_{\rm{Jup}}$. In these cases, the gas distribution is likely to show low-amplitude azimuthal asymmetries caused by disk eccentricity rather than by long-lived vortices.