Effect of wind-driven accretion on planetary migration

Abstract

Planetary migration is a key link between planet formation models and observed exoplanet statistics. So far the theory of migration has focused on the interaction of planets with an inviscid or viscously evolving disk. Turbulent viscosity is thought to be the main driver of disk evolution and is known to affect the migration process. Recently, the topic of wind-driven accretion is experiencing a renaissance, as evidence is mounting that PPDs may be less turbulent than previously thought, and 3-D non-ideal MHD modeling of the wind-launching process is maturing. Aim: We wish to investigate how wind-driven accretion may affect migration. We aim for a qualitative exploration of the main effects, rather than a quantitative prediction. Methods: We perform 2-D hydrodynamic simulations with the FARGO3D code in the $(r,\phi)$-plane. The vertical coordinate and the launching of the wind are not treated explicitly. Instead, the torque of the wind onto the disk is treated using a simple 2-parameter formula treating the wind mass loss rate and the lever arm. Results: We find that the wind-driven accretion process has a different way of replenishing the co-orbital region than the viscous accretion. The former always injects mass from the outer edge of the co-orbital region and removes mass from the inner edge, while the latter injects or removes mass from the co-orbital region depending on the radial density gradients in the disk. The migration behavior can differ very much and under certain conditions it can drive rapid type-III-like outward migration. We derive an analytic expression for the parameters under which this outward migration occurs. Conclusion: If wind-driven accretion plays a role in the secular evolution of PPDs, migration studies have to include this process as well, because it can strongly affect the resulting migration rate and direction.