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Register a new userSlowing Down Type II Migration of Gas Giants to Match Observational Data
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The mass and semimajor axis distribution of gas giants in exoplanetary systems obtained by radial velocity surveys shows that super-jupiter-mass planets are piled up at > 1 au, while jupiter/sub-jupiter-mass planets are broadly distributed from ~0.03 au to beyond 1 au. This feature has not been explained by theoretical predictions. In order to reconcile this inconsistency, we investigate evolution of gas giants with a new type II migration formula by Kanagawa et al. (2018), by comparing the migration, growth timescales of gas giants, and disk lifetime and by population synthesis simulation. While the classical migration model assumes that a gas giant opens up a clear gap in the protoplanetary disk and the planet migration tied to the disk gas accretion, recent high-resolution simulations show that the migration of gap-opening planets is decoupled from the disk gas accretion and Kanagawa et al. (2018) proposed that type II migration speed is no other than type I migration speed with the reduced disk gas surface density in the gap. We show that with this new formula, type II migration is significantly reduced for super-jupiter-mass planets, if the disk accretion is driven by the disk wind as suggested by recent MHD simulations. Population synthesis simulations show that super-jupiter-mass planets remain at > 1 au without any additional ingredient such as disk photoevaporation. Therefore, the mystery of the pile-up of gas giants at > 1 au would be theoretically solved, if the new formula is confirmed and wind-driven disk accretion dominates.
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During their formation, emerging protoplanets tidally interact with their natal disks. Proto-gas-giant planets, with Hills radius larger than the disk thickness, open gaps and quench gas flow in the vicinity of their orbits. It is usually assumed that their type II migration is coupled to the viscous evolution of the disk. Although this hypothesis provides an explanation for the origin of close-in planets, it also encounter predicament on the retention of long-period orbits for most gas giant planets. Moreover, numerical simulations indicate that planets migrations are not solely determined by the viscous diffusion of their natal disk. Here we carry out a series of hydrodynamic simulations combined with analytic studies to examine the transition between different paradigms of type II migration. We find a range of planetary mass for which gas continues to flow through a severely depleted gap so that the surface density distribution in the disk region beyond the gap is maintained in a quasi-steady state. The associated gap profile modifies the location of corotation & Lindblad resonances. In the proximity of the planets orbit, high-order Lindblad & corotation torque are weakened by the gas depletion in the gap while low-order Lindblad torques near the gap walls preserves their magnitude. Consequently, the intrinsic surface density distribution of the disk determines delicately both pace and direction of planets type II migration. We show that this effect might stall the inward migration of giant planets and preserve them in disk regions where the surface density is steep.
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
In a recent paper we proposed that the giant planets primordial orbits may have been eccentric (~0.05), and used a suite of dynamical simulations to show outcomes of the giant planet instability that are consistent with their present-day orbits. In this follow-up investigation, we present more comprehensive simulations incorporating superior particle resolution, longer integration times, and eliminating our prior means of artificially forcing instabilities to occur at specified times by shifting a planets position in its orbit. While we find that the residual phase of planetary migration only minimally alters the the planets ultimate eccentricities, our work uncovers several intriguing outcomes in realizations where Jupiter and Saturn are born with extremely large eccentricities (~0.10 and ~0.25, respectively). In successful simulations, the planets orbits damp through interactions with the planetesimal disk prior to the instability, thus loosely replicating the initial conditions considered in our previous work. Our results therefore suggest an even wider range of plausible evolutionary pathways are capable of replicating Jupiter and Saturns modern orbital architecture.
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