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Register a new userInterpreting Brightness Asymmetries in Transition Disks: Vortex at Dead Zone or Planet Carved Gap Edges?
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Recent sub-millimeter observations show non-axisymmetric brightness distributions with a horseshoe-like morphology for more than a dozen transition disks. The most accepted explanation for the observed asymmetries is the accumulation of dust in large-scale vortices. Protoplanetary disks vortices can form by the excitation of Rossby-wave instability in the vicinity of a steep pressure gradient, which can develop at the edges of a giant planet carved gap or at the edges of an accretionally inactive zone. We studied the formation and evolution of vortices formed in these two distinct scenarios by means of two-dimensional locally isothermal hydrodynamic simulations. We found that the vortex formed at the edge of a planetary gap is short-lived, unless the disk is nearly inviscid. In contrast, the vortex formed at the outer edge of a dead zone is long-lived. The vortex morphology can be significantly different in the two scenarios: the vortex radial and azimuthal extensions are ~1.5 and ~3.5 times larger for the dead zone edge compared to gap models. In some particular cases, the vortex aspect ratios can be similar in the two scenarios, however, the vortex azimuthal extensions can be used to distinguish the vortex formation mechanisms. We calculate predictions for vortex observability in the sub-millimeter continuum with ALMA. We found that the azimuthal and radial extent of brightness asymmetry correlates with vortex formation process, within the limitations of alpha-viscosity prescription.
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In this paper the migration of a 10 Earth-mass planetary core is investigated at the outer boundary of the dead zone of a protoplanetary disc by means of 2D hydrodynamic simulations done with the graphics processor unit version of the FARGO code. In the dead zone, the effective viscosity is greatly reduced due to the disc self-shielding against stellar UV radiation, X-rays from the stellar magnetosphere and interstellar cosmic rays. As a consequence, mass accumulation occurs near the outer dead zone edge, which is assumed to trap planetary cores enhancing the efficiency of the core-accretion scenario to form giant planets. Contrary to the perfect trapping of planetary cores in 1D models, our 2D numerical simulations show that the trapping effect is greatly dependent on the width of the region where viscosity reduction is taking place. Planet trapping happens exclusively if the viscosity reduction is sharp enough to allow the development of large-scale vortices due to the Rossby wave instability. The trapping is only temporarily, and its duration is inversely proportional to the width of the viscosity transition. However, if the Rossby wave instability is not excited, a ring-like axisymmetric density jump forms, which cannot trap the 10 Earth-mass planetary cores. We revealed that the stellar torque exerted on the planet plays an important role in the migration history as the barycentre of the system significantly shifts away from the star due to highly non-axisymmetric density distribution of the disc. Our results still support the idea of planet formation at density/pressure maximum, since the migration of cores is considerably slowed down enabling them further growth and runaway gas accretion in the vicinity of an overdense region.
We perform a systematic study of the dynamics of dust particles in protoplanetary disks with embedded planets using global 2-D and 3-D inviscid hydrodynamic simulations. Lagrangian particles have been implemented into magnetohydrodynamic code Athena with cylindrical coordinates. We find two distinct outcomes depending on the mass of the embedded planet. In the presence of a low mass planet ($8 M_{oplus}$), two narrow gaps start to open in the gas on each side of the planet where the density waves shock. These shallow gaps can dramatically affect particle drift speed and cause significant, roughly axisymmetric dust depletion. On the other hand, a more massive planet ($>0.1 M_{J}$) carves out a deeper gap with sharp edges, which are unstable to the vortex formation. Particles with a wide range of sizes ($0.02<Omega t_{s}<20$) are trapped and settle to the midplane in the vortex, with the strongest concentration for particles with $Omega t_{s}sim 1$. The dust concentration is highly elongated in the $phi$ direction, and can be as wide as 4 disk scale heights in the radial direction. Dust surface density inside the vortex can be increased by more than a factor of 10$^2$ in a very non-axisymmetric fashion. For very big particles ($Omega t_{s}gg 1$) we find strong eccentricity excitation, in particular around the planet and in the vicinity of the mean motion resonances, facilitating gap opening there. Our results imply that in weakly turbulent protoplanetary disk regions (e.g. the dead zone) dust particles with a very wide range of sizes can be trapped at gap edges and inside vortices induced by planets with $M_{p}<M_{J}$, potentially accelerating planetesimal and planet formation there, and giving rise to distinctive features that can be probed by ALMA and EVLA.
The dynamical evolution of protoplanetary disks is of key interest for building a comprehensive theory of planet formation and to explain the observational properties of these objects. Using the magnetohydrodynamics code Athena++, with an isothermal shearing box setup, we study the boundary between the active and dead zone, where the accretion rate changes and mass can accumulate. We quantify how the turbulence level is affected by the presence of a non uniform ohmic resistivity in the radial-x direction that leads to a region of inhibited turbulence (or dead zone). Comparing the turbulent activity to that of ideal simulations, the turbulence inhibited area shows density fluctuations and magnetic activity at its boundaries, driven by energy injection from the active (ideal) zone boundaries. We find magnetic dissipation to be significantly stronger in the ideal regions, and the turbulence penetration through the boundary of the dead zone is determined by the value of the resistivity itself, through the ohmic dissipation process, though the thickness of the transition does not play a significant role in changing the dissipation. We investigate the 1D spectra along the shearing direction: magnetic spectra appear flat at large scales both in ideal as well as resistive simulations, though a Kolmogorov scaling over more than one decade persists in the dead zone, suggesting the turbulent cascade is determined by the hydrodynamics of the system: MRI dynamo action is inhibited where sufficiently high resistivity is present.
One possible explanation of the cavity in debris discs is the gravitational perturbation of an embedded giant planet. Planetesimals passing close to a massive body are dynamically stirred resulting in a cleared region known as the chaotic zone. Theory of overlapping mean-motion resonances predicts the width of this cavity. To test whether this cavity is identical to the chaotic zone, we investigate the formation of cavities by means of collisionless N-body simulations assuming a 1.25-10 Jupiter mass planet with eccentricities of 0-0.9. Synthetic images at millimetre wavelengths are calculated to determine the cavity properties by fitting an ellipse to 14 percent contour level. Depending on the planetary eccentricity, e_pl, the elliptic cavity wall rotates as the planet orbits with the same (e_pl<0.2) or half (e_pl>0.2) period that of the planet. The cavity centre is offset from the star along the semi-major axis of the planet with a distance of d=0.1q^-0.17e_pl^0.5 in units of cavity size towards the planets orbital apocentre, where q is the planet-to-star mass ratio. Pericentre (apocentre) glow develops for e_pl<0.05 (e_pl>0.1), while both are present for 0.05<=e_pl<=0.1. Empirical formulae are derived for the sizes of the cavities: da_cav=2.35q^0.36 and da_cav=7.87q^0.37e_pl^0.38 for e_pl<=0.05 and e_pl>0.05, respectively. The cavity eccentricity, e_cav, equals to that of the planet only for 0.3<=e_pl<=0.6. A new method based on ALMA observations for estimating the orbital parameters and mass of the planet carving the cavity is also given.
We present global hydrodynamic and magnetohydrodynamic (MHD) simulations with mesh refinement of accreting planets embedded in protoplanetary disks (PPDs). The magnetized disk includes Ohmic resistivity that depends on the overlying mass column, leading to turbulent surface layers and a dead zone near the midplane. The main results are: (i) The accretion flow in the Hill sphere is intrinsically 3D for hydrodynamic and MHD models. Net inflow toward the planet is dominated by high latitude flows. A circumplanetary disk (CPD) forms. Its midplane flows outward in a pattern whose details differ between models. (ii) Gap opening magnetically couples and ignites the dead zone near the planet, leading to stochastic accretion, a quasi-turbulent flow in the Hill sphere and a CPD whose structure displays high levels of variability. (iii) Advection of magnetized gas onto the rotating CPD generates helical fields that launch magnetocentrifugally driven outflows. During one specific epoch a highly collimated, one-sided jet is observed. (iv) The CPDs surface density $sim30{rm,g,cm^{-2}}$, small enough for significant ionization and turbulence to develop. (v) The accretion rate onto the planet in the MHD simulation reaches a steady value $8 times 10^{-3} {rm M_oplus yr^{-1}}$, and is similar in the viscous hydrodynamic runs. Our results suggest that gas accretion onto a forming giant planet within a magnetized PPD with dead zone allows rapid growth from Saturnian to Jovian masses. As well as being relevant for giant planet formation, these results have important implications for the formation of regular satellites around gas giant planets.
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