Planet filtering at the inner edges of dead zones in protoplanetary disks

This research was supported by an STFC Consolidated grant awarded to the QMUL Astronomy Unit 2012-2016. This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure

Vortex formation in protoplanetary discs induced by the vertical shear instability

This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC Operations grant ST/K0003259/1. DiRAC is part of the national E-Infrastructure

Linear analysis of the vertical shear instability: outstanding issues and improved solutions

This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure

Buoyancy torques prevent low-mass planets from stalling in low-turbulence radiative discs

Low-mass planets migrating inwards in laminar protoplanetary discs (PPDs) experience a dynamical corotation torque (DCT), which is expected to slow down migration to a stall. However, baroclinic effects can reduce or even reverse this effect, leading to rapid inward migration. In the radiatively inefficient inner disc, one such mechanism is the buoyancy response of the disc to an embedded planet. Recent work has suggested that radiative cooling can quench this response, but for parameters that are not necessarily representative of the inner regions of PPDs. We perform global 3D inviscid radiation hydrodynamics simulations of planet-disc interaction to investigate the effect of radiative cooling on the buoyancy-driven torque in a more realistic disc model. We find that the buoyancy response exerts a negative DCT - albeit partially damped due to radiative cooling - resulting in sustained, rapid inward migration. Models that adopt a local cooling prescription significantly overestimate the impact of the buoyancy response, highlighting the importance of a realistic treatment of radiation transport that includes radiative diffusion. Our results suggest that low-mass planets should migrate inwards faster than has been previously expected in radiative discs, with implications for the formation and orbital distribution of super-Earths and sub-Neptunes at intermediate distances from their host stars, unless additional physical processes that can slow down migration are considered

Parking planets in circumbinary discs

The Kepler space mission has discovered about a dozen planets orbiting around binary stars systems. Most of these circumbinary planets lie near their instability boundaries, at about three to five binary separations. Past attempts to match these final locations through an inward migration process were only shown to be successful for the Kepler-16 system. Here, we study ten circumbinary systems and attempt to match the final parking locations and orbital parameters of the planets with a disc-driven migration scenario. We performed 2D locally isothermal hydrodynamical simulations of circumbinary discs with embedded planets and followed their migration evolution using different values for the disc viscosity and aspect ratio. We found that for the six systems with intermediate binary eccentricities (0.1 ≤ ebin ≤ 0.21), the final planetary orbits matched the observations closely for a single set of disc parameters, specifically, a disc viscosity of α = 10−4 and an aspect ratio of H∕r ~ 0.04. For these systems the planet masses are large enough to open at least a partial gap in their discs as they approach the binary, forcing the discs to become circularised and allowing for further migration towards the binary – ultimately leading to a good agreement with the observed planetary orbital parameters. For systems with very small or large binary eccentricities, the match was not as good as the very eccentric discs and the large inner cavities in these cases prevented close-in planet migration. In test simulations with higher than observed planet masses, a better agreement was found for those systems. The good agreement for six out of the ten modelled systems, where the relative difference between observed and simulated final planet orbit is ≤10% strongly supports the idea that planet migration in the disc brought the planets to their present locations

Hydrodynamic turbulence in disks with embedded planets

The vertical shear instability (VSI) is a source of hydrodynamic turbulence that can drive vigorous vertical mixing and moderate levels of accretion in protoplanetary disks, and it could be observable in the near future. With high-resolution three-dimensional numerical hydrodynamics simulations, we modeled the behavior of the VSI in protoplanetary disks with and without embedded planets. We then measured its accretion and mixing capabilities by comparing the full Reynolds stress, which includes the contribution of nonaxisymmetric features, such as spiral arms and vortices, to the Reynolds stress due to the azimuthally averaged velocity field, which can be attributed to good approximation to the VSI. We verified that the VSI can contribute to the accretion stress and showed that, depending on disk conditions, an embedded planet can coexist with or suppress VSI turbulent stress. Specifically, the presence of spiral shocks launched by a planet or planet-generated vortices can interfere with the VSI near the planet’s vicinity, with the instability recovering at large enough distances from the planet or vortex. Our results suggest that observations of VSI signatures are unlikely in disks that contain massive, nonaxisymmetric features

Hydrodynamical turbulence in eccentric circumbinary discs and its impact on the in situ formation of circumbinary planets

Eccentric gaseous discs are unstable to a parametric instability involving the resonant interaction between inertial-gravity waves and the eccentric mode in the disc. We present 3D global hydrodynamical simulations of inviscid circumbinary discs that form an inner cavity and become eccentric through interaction with the central binary. The parametric instability grows and generates turbulence that transports angular momentum with stress parameter $\alpha \sim 5 \times 10^{-3}$ at distances $\lesssim 7 \;a_{bin}$, where $a_{bin}$ is the binary semi-major axis. Vertical turbulent diffusion occurs at a rate corresponding to $\alpha_{diff}\sim 1-2\times 10^{-3}$. We examine the impact of turbulent diffusion on the vertical settling of pebbles, and on the rate of pebble accretion by embedded planets. In steady state, dust particles with Stokes numbers ${\it St} \lesssim 0.1$ form a layer of finite thickness $H_d \gtrsim 0.1 H$, where $H$ is the gas scale height. Pebble accretion efficiency is then reduced by a factor $r_{acc}/H_d$, where $r_{acc}$ is the accretion radius, compared to the rate in a laminar disc. For accreting core masses with $m_p \lesssim 0.1\; M_\oplus$, pebble accretion for particles with ${\it St} \gtrsim 0.5$ is also reduced because of velocity kicks induced by the turbulence. These effects combine to make the time needed by a Ceres-mass object to grow to the pebble isolation mass, when significant gas accretion can occur, longer than typical disc lifetimes. Hence, the origins of circumbinary planets orbiting close to their central binary systems, as discovered by the Kepler mission, are difficult to explain using an in situ model that invokes a combination of the streaming instability and pebble accretion.Comment: Accepted in MNRA