A tunnel and a traffic jam: How transition disks maintain a detectable warm dust component despite the presence of a large planet-carved gap

Abstract

Context. Transition disks are circumstellar disks that show evidence of a dust cavity, which may be related to dynamical clearing by embedded planet(s). Most of these objects show signs of significant accretion, indicating that the inner disks are not truly empty, but that gas is still streaming through to the star. A subset of transition disks, sometimes called pre-transition disks, also shows a strong near-infrared excess, interpreted as an optically thick dusty belt located close to the dust sublimation radius within the first astronomical unit. Aims. We study the conditions for the survival and maintenance of such an inner disk in the case where a massive planet opens a gap in the disk. In this scenario, the planet filters out large dust grains that are trapped at the outer edge of the gap, while the inner regions of the disk may or may not be replenished with small grains. Methods. We combined hydrodynamical simulations of planet-disk interactions with dust evolution models that include coagulation and fragmentation of dust grains over a large range of radii and derived observational properties using radiative transfer calculations. We studied the role of the snow line in the survival of the inner disk of transition disks. Results. Inside the snow line, the lack of ice mantles in dust particles decreases the sticking efficiency between grains. As a consequence, particles fragment at lower collision velocities than in regions beyond the snow line. This effect allows small particles to be maintained for up to a few Myr within the first astronomical unit. These particles are closely coupled to the gas and do not drift significantly with respect to the gas. For lower mass planets (1 M Jup ), the pre-transition appearance can be maintained even longer because dust still trickles through the gap created by the planet, moves invisibly and quickly in the form of relatively large grains through the gap, and becomes visible again as it fragments and gets slowed down inside of the snow line. Conclusions. The global study of dust evolution of a disk with an embedded planet, including the changes of the dust aerodynamics near the snow line, can explain the concentration of millimetre-sized particles in the outer disk and the survival of the dust in the inner disk if a large dust trap is present in the outer disk. This behaviour solves the conundrum of the combination of both near-infrared excess and ring-like millimetre emission observed in several transition disks.

Pebble-isolation mass: Scaling law and implications for the formation of super-Earths and gas giants

“…Inside the pressure bump generated by a Jupiter-mass planet in 2D simulations of a disc with α = 0.001 and H/r = 0.05, the maximum ∆v/c s = 0.66. This leads to a critical Stokes number of τ f = 7.5 × 10 −4 , in agreement with previous studies including diffusion, see for example Pinilla et al (2016), where very small particles can drift through the pressure bump generated by large planets. Adding the effects of particle diffusion in 2D disc simulations, Ataiee et al (2017, in prep) found that diffusion can increase the pebble-isolation mass, in agreement with Pinilla et al (2012) and our estimates.…”

“…This mechanism allowed the small particles to move across the pressure bump generated by the planet in Pinilla et al (2016), while our simulations show an effective trapping of small particles as a result of the lack of diffusion. The difference of Pinilla et al (2016) to is probably related to different prescriptions of diffusion. We therefore estimate the effects of diffusion in the following for the pebble-isolation mass.…”

“…included diffusion into the motion of dust particles in gas discs in the presence of 30 M E planets and estimated this effect to be of the order of 1%, indicating that diffusion of dust particles does not play a role in opening a gap in dust distribution of protoplanetary discs. Pinilla et al (2016), on the other hand, studied dust filtration by giant planets in the context of transition discs. Giant planets open deep gaps in protoplanetary discs that prevent dust from drifting through.…”

“…10, where the pebble-isolation mass is reached in 2D disc simulations at lower masses (see Appendix B) than in 3D simulations. Pinilla et al (2016) considered turbulent mixing of dust particles, where the dust diffusivity follows the prescriptions by Youdin & Lithwick (2007), which depend on the Stokes number A&A proofs: manuscript no. Pebbleisolation and the gas diffusivity (assumed to be equal to the disc viscosity).…”

The Influence of Forming Companions on the Spectral Energy Distributions of Stars with Circumstellar Discs

“…Zakhozhay ing of the vertical disc wall at the outer edge of the gap. Pinilla et al (2016) show that discs with massive planets lose the near infrared excess after a few Myrs and their SEDs look like true transition discs, while SEDs of the discs with less massive planets look like pre-transition disc SEDs for most of their remaining lifetimes.…”