Dust Filtration by Planet-Induced Gap Edges: Implications for Transitional Disks

Abstract

By carrying out two-dimensional two-fluid global simulations, we have studied the response of dust to gap formation by a single planet in the gaseous component of a protoplanetary disk-the so-called dust filtration mechanism. We have found that a gap opened by a giant planet at 20 AU in an α = 0.01,Ṁ = 10 −8 M yr −1 disk can effectively stop dust particles larger than 0.1 mm drifting inward, leaving a submillimeter (submm) dust cavity/hole. However, smaller particles are difficult to filter by a gap induced by a several M J planet due to (1) dust diffusion and (2) a high gas accretion velocity at the gap edge. Based on these simulations, an analytic model is derived to understand what size particles can be filtered by the planet-induced gap edge. We show that a dimensionless parameter T s /α, which is the ratio between the dimensionless dust stopping time and the disk viscosity parameter, is important for the dust filtration process. Finally, with our updated understanding of dust filtration, we have computed Monte Carlo radiative transfer models with variable dust size distributions to generate the spectral energy distributions of disks with gaps. By comparing with transitional disk observations (e.g., GM Aur), we have found that dust filtration alone has difficulties depleting small particles sufficiently to explain the near-IR deficit of moderateṀ transitional disks, except under some extreme circumstances. The scenario of gap opening by multiple planets studied previously suffers the same difficulty. One possible solution is to invoke both dust filtration and dust growth in the inner disk. In this scenario, a planet-induced gap filters large dust particles in the disk, and the remaining small dust particles passing to the inner disk can grow efficiently without replenishment from fragmentation of large grains. Predictions for ALMA have also been made based on all these scenarios. We conclude that dust filtration with planet(s) in the disk is a promising mechanism to explain submm observations of transitional disks but it may need to be combined with other processes (e.g., dust growth) to explain the near-IR deficit of some systems.

Fingerprints of giant planets in the photospheres of Herbig stars

“…A gap-opening giant planet reduces the surface density of large (millimetrescale) dust by a larger factor than that of the gas (Rice et al 2006;Zhu et al 2012). This offers a natural explanation for a decreased photospheric abundance of refractory elements, because a large portion of the dust mass can remain trapped farther out in the disk while gas and smaller grains continue to accrete.…”

Gas structure inside dust cavities of transition disks: Ophiuchus IRS 48 observed by ALMA

“…The dust trap found towards IRS 48, preventing the dust to drift inwards, could filter the gas from the dust (Zhu et al 2012). Assuming an initial gasto-dust ratio of 100, the amount of gas that has been removed corresponds to ∼1.5 M Jupiter , which can easily be bound into one companion or giant planet.…”

The dispersal of protoplanetary discs

“…However further modelling of the dust processes is needed to be able to fully exploit the observational consequences of this process. (iii) Assuming that the planet is able to filter dust completely (Rice et al 2006;Pinilla et al 2012;Zhu et al 2012), large hole transition discs could be produced. PIPE may instigate the premature shutting down of accretion.…”