Toward a new paradigm for Type II migration

TL;DR

This paper challenges the traditional view of Type II migration, demonstrating that a giant planet's migration rate is proportional to disk viscosity and not solely dictated by gas flow, leading to a new paradigm.

Contribution

It introduces a revised model for Type II migration, emphasizing the role of viscous timescales over gas advection and dismissing the significance of orbit-crossing flows.

Findings

Migration rate proportional to disk viscosity

Gap-crossing flows are negligible

Migration limited by viscous timescale

Abstract

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 disk's 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 disk's 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 disk's 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