Halting Migration: Numerical Calculations of Corotation Torques in the Weakly Nonlinear Regime

TL;DR

This paper numerically investigates how intermediate-mass planets in isothermal disks experience a positive corotation torque that can slow or reverse inward migration, especially during the transition to gap-opening.

Contribution

It provides the first explicit numerical calculations of corotation torques in the weakly nonlinear regime for intermediate-mass planets in isothermal disks.

Findings

Positive nonlinear corotation torque can counteract linear inward torque.

Outward migration is possible for planets with 20-100 Earth masses.

Empirical scalings for migration rates in the weakly nonlinear regime are presented.

Abstract

Planets in their formative years can migrate due to the influence of gravitational torques in the protoplanetary disk they inhabit. For low-mass planets in an isothermal disk, it is known that there is a strong negative torque on the planet due to its linear perturbation to the disk, causing fast inward migration. The current investigation demonstrates that in these same isothermal disks, for intermediate-mass planets, there is a strong positive nonlinear corotation torque due to the effects of gas being pulled through a gap on horseshoe orbits. For intermediate-mass planets, this positive torque can partially or completely cancel the linear (Type I) torque, leading to slower or outward migration, even in an isothermal disk. The effect is most significant for Super-Earth and Sub-Jovian planets, during the transition from a low-mass linear perturber to a non-linear gap-opening planet, when the planet has opened a so-called 'partial gap'. In this study, numerical calculations of planet-disk interactions calculate these torques explicitly, and scalings are empirically constructed for migration rates in this weakly nonlinear regime. These results find outward migration is possible for planets with masses in the 20 - 100 Earth Mass range.