Vortex instabilities triggered by low-mass planets in pebble-rich, inviscid protoplanetary discs

Abstract

In the innermost regions of protoplanerary discs, the solid-to-gas ratio can be increased considerably by a number of processes, including photoevaporative and particle drift. MHD disc models also suggest the existence of a dead-zone at R 10 AU, where the regions close to the midplane remain laminar. In this context, we use two-fluid hydrodynamical simulations to study the interaction between a low-mass planet (∼ 1.7 M ⊕ ) on a fixed orbit and an inviscid pebble-rich disc with solid-to-gas ratio 0.5. For pebbles with Stokes numbers St = 0.1, 0.5, multiple dusty vortices are formed through the Rossby Wave Instability at the planet separatrix. Effects due to gas drag then lead to a strong enhancement in the solid-to-gas ratio, which can increase by a factor of ∼ 10 3 for marginally coupled particles with St = 0.5. As in streaming instabilities, pebble clumps reorganize into filaments that may plausibly collapse to form planetesimals. When the planet is allowed to migrate in a MMSN disc, the vortex instability is delayed due to migration but sets in once inward migration stops due a strong positive pebble torque. Again, particle filaments evolving in a gap are formed in the disc while the planet undergoes an episode of outward migration. Our results suggest that vortex instabilities triggered by low-mass planets could play an important role in forming planetesimals in pebble-rich, inviscid discs, and may significantly modify the migration of low-mass planets. They also imply that planetary dust gaps may not necessarily contain planets if these migrated away.

Influence of planetary gas accretion on the shape and depth of gaps in protoplanetary discs

“…This instability generates vortices at the location of steep density gradients. Such steep gradients are induced by planetary gaps formed at low viscosity (Hammer et al 2017;Pierens et al 2019), as we can see on the radial density profiles in the lower panel of Fig. 9.…”

Meso-scale Instability Triggered by Dust Feedback in Dusty Rings: Origin and Observational Implications

“…Because gap edges are also pressure bumps, similar instabilities could happen at dusty rings induced by planets including dust feedback. Pierens et al (2019) showed interactions between a low-mass planet and an inviscid pebble-rich disk. They found that a RWI-liked instability happens at gap edges of planet and a lot of dusty vortices emerged in their simulations.…”

Migrating low-mass planets in inviscid dusty protoplanetary discs

“…separatrix in the disc with St = 10 −2 . This results from the scattering of librating, co-orbital dust by the planet whenever it undergoes horseshoe turns, leading to the formation of overdense dust flow at the downstream separatrix (Morbidelli & Nesvorny 2012; Benítez-Llambay & Pessah 2018; Pierens et al 2019). When the metallicity in the co-orbital ring is high, feedback from the scattered, overdense flow can significantly modify the local gas azimuthal velocity, as shown in the top panels of Fig.…”

“…The formation of planet-induced dust vortices via RWI-like processes (i.e. edge instabilities) can depend on resolution and viscosity (Chen & Lin 2018;Pierens et al 2019). Here, we perform additional simulations with different resolutions and viscosities to study their effects on planet migration.…”

“…However, this did not significantly affect the overall migration behaviour, especially when either dynamical corotation torques or vortex-planet interactions dominate. On the other hand, a fully self-gravitating disc may provide an effective viscosity that suppresses vortex formation and prevents stochastic migration (Pierens et al 2019) -an effect that will depend on the disc mass and should be examined further.…”