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Register a new userComments on Type II migration strikes back -- An old paradigm for planet migration in discs by Scardoni et al
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Kazuhiro Kanagawa D
Publication date
2020
fields
Physics
and research's language is
English
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In the conventional view of type II migration, a giant planet migrates inward in the viscous velocity of the accretion disc in the so-call disc-dominate case. Recent hydrodynamic simulations, however, showed that planets migrate with velocities much faster than the viscous one in massive discs. Such fast migration cannot be explained by the conventional picture. Scardoni et al. (2020) has recently argued this new picture. By carrying out similar hydrodynamic simulations, they found that the migration velocity slows down with time and eventually reaches the prediction by the conventional theory. They interpreted the fast migration as an initial transient one and concluded that the conventional type II migration is realised after the transient phase. We show that the migration velocities obtained by Scardoni et al. (2020) are consistent with the previous simulations even in the transient phase that they proposed. We also find that the transient fast migration proposed by Scardoni et al. (2020) is well described by a new model of Kanagawa et al. (2018). The new model can appropriately describe significant inward migration during the initial transient phase that Scardoni et al. (2020) termed. Hence, we conclude that the time-variation of the transient migration velocity is due to the changes of the orbital radius of the planet and its background surface density during the migration.
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Context. Giant planets open gaps in their protoplanetary and subsequently suffer so-called type II migration. Schematically, planets are thought to be tightly locked within their surrounding disks, and forced to follow the viscous advection of gas onto the central star. This fundamental principle however has recently been questioned, as migrating planets were shown to decouple from the gas radial drift. Aims. In this framework, we question whether the traditionally used linear scaling of migration rate of a giant planet with the disks viscosity still holds. Additionally, we assess the role of orbit-crossing material as part of the decoupling mechanism. Methods. We have performed 2D (r, {theta}) numerical simulations of point-mass planets embedded in locally isothermal {alpha}-disks in steady-state accretion, with various values of {alpha}. Arbitrary planetary accretion rates were used as a means to diminish or nullify orbit-crossing flows. Results. We confirm that the migration rate of a gap-opening planet is indeed proportional to the disks viscosity, but is not equal to the gas drift speed in the unperturbed disk. We show that the role of gap-crossing flows is in fact negligible. Conclusions. From these observations, we propose a new paradigm for type II migration : a giant planet feels a torque from the disk that promotes its migration, while the gap profile relative to the planet is restored on a viscous timescale, thus limiting the planet migration rate to be proportional to the disks viscosity. Hence, in disks with low viscosity in the planet region, type II migration should still be very slow. Key words. protoplanetary disks; planet-disk interactions; planets and satellites: formation
We carry out three-dimensional SPH simulations to study whether planets can survive in self-gravitating protoplanetary discs. The discs modelled here use a cooling prescription that mimics a real disc which is only gravitationally unstable in the outer regions. We do this by modelling the cooling using a simplified method such that the cooling time in the outer parts of the disc is shorter than in the inner regions, as expected in real discs. We find that both giant (> M_Sat) and low mass (< M_Nep) planets initially migrate inwards very rapidly, but are able to slow down in the inner gravitationally stable regions of the disc without needing to open up a gap. This is in contrast to previous studies where the cooling was modelled in a more simplified manner where regardless of mass, the planets were unable to slow down their inward migration. This shows the important effect the thermodynamics has on planet migration. In a broader context, these results show that planets that form in the early stages of the discs evolution, when they are still quite massive and self-gravitating, can survive.
Planet migration originally refers to protoplanetary disks, which are more massive and dense than typical accretion disks in binary systems. We study planet migration in an accretion disk in a binary system consisting of a solar-like star hosting a planet and a red giant donor star. The accretion disk is fed by a stellar wind. %, disk self-gravity is neglected. We use the $alpha$-disk model and consider that the stellar wind is time-dependent. Assuming the disk is quasi-stationary we calculate its temperature and surface density profiles. In addition to the standard disk model, when matter is captured by the disk at its outer edge, we study the situation when the stellar wind delivers matter on the whole disc surface inside the accretion radius with the rate depending on distance from the central star. Implying that a planet experiences classical type I/II migration we calculate migration time for a planet on a circular orbit coplanar with the disk. Potentially, rapid inward planet migration can result in a planet-star merger which can be accompanied by an optical or/and UV/X-ray transient. We calculate timescale of migration for different parameters of planets and binaries. Our results demonstrate that planets can fall on their host stars within the lifetime of the late-type donor for realistic sets of parameters.
Earth-mass planets embedded in gaseous protoplanetary disks undergo Type I orbital migration. In radiative disks an additional component of the corotation torque scaling with the entropy gradient across the horseshoe region can counteract the general inward migration, Type I migration can then be directed either inward or outward depending on the local disk properties. Thus, special locations exist in the disk toward which planets migrate in a convergent way. Here we present N-body simulations of the convergent migration of systems of low-mass (M=1-10 m_earth) planets. We show that planets do not actually converge in convergence zones. Rather, they become trapped in chains of mean motion resonances (MMRs). This causes the planets eccentricities to increase to high enough values to affect the structure of the horseshoe region and weaken the positive corotation torque. The zero-torque equilibrium point of the resonant chain of planets is determined by the sum of the attenuated corotation torques and unattenuated differential Lindblad torques acting on each planet. The effective convergence zone is shifted inward. Systems with several planets can experience stochastic migration as a whole due to continuous perturbations from planets entering and leaving resonances.
Transition discs are expected to be a natural outcome of the interplay between photoevaporation (PE) and giant planet formation. Massive planets reduce the inflow of material from the outer to the inner disc, therefore triggering an earlier onset of disc dispersal due to PE through a process known as Planet-Induced PhotoEvaporation (PIPE). In this case, a cavity is formed as material inside the planetary orbit is removed by PE, leaving only the outer disc to drive the migration of the giant planet. We investigate the impact of PE on giant planet migration and focus specifically on the case of transition discs with an evacuated cavity inside the planet location. This is important for determining under what circumstances PE is efficient at halting the migration of giant planets, thus affecting the final orbital distribution of a population of planets. For this purpose, we use 2D FARGO simulations to model the migration of giant planets in a range of primordial and transition discs subject to PE. The results are then compared to the standard prescriptions used to calculate the migration tracks of planets in 1D planet population synthesis models. The FARGO simulations show that once the disc inside the planet location is depleted of gas, planet migration ceases. This contradicts the results obtained by the impulse approximation, which predicts the accelerated inward migration of planets in discs that have been cleared inside the planetary orbit. These results suggest that the impulse approximation may not be suitable for planets embedded in transition discs. A better approximation that could be used in 1D models would involve halting planet migration once the material inside the planetary orbit is depleted of gas and the surface density at the 3:2 mean motion resonance location in the outer disc reaches a threshold value of $0.01,mathrm{g,cm^{-2}}$.
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