Trapping (sub-)Neptunes similar to TOI-216b at the inner disk rim: Implications for the disk viscosity and the Neptunian desert

TL;DR

This study investigates how sub-Neptune planets like TOI-216b can be trapped at the inner disk rim, considering the effects of disk viscosity transitions and gap opening, with implications for the Neptunian desert.

Contribution

The paper introduces hydrodynamic simulations showing conditions under which gap-opening sub-Neptunes can be trapped at the inner disk rim, highlighting the role of viscosity contrasts.

Findings

Trapping depends on the formation of a gas overdensity ahead of the planet.

Efficient trapping requires high viscosity in the active zone, with specific alpha values.

Gap opening reshapes the density profile, affecting migration torques.

Abstract

[Abridged] The occurrence rate of observed sub-Neptunes has a break at 0.1 au, which is often attributed to a migration trap at the inner rim of protoplanetary disks where a positive co-rotation torque prevents inward migration. We argue that conditions in inner disk regions are such that sub-Neptunes are likely to open gaps, lose the support of the co-rotation torque as their co-rotation regions become depleted, and the trapping efficiency then becomes uncertain. We study what it takes to trap such gap-opening planets at the inner disk rim. We performed 2D locally isothermal and non-isothermal hydrodynamic simulations of planet migration. A viscosity transition was introduced in the disk to (i) create a density drop and (ii) mimic the viscosity increase as the planet migrated from a dead zone towards a region with active magneto-rotational instability (MRI). We chose TOI-216b as a Neptune-like upper-limit test case, but we also explored different planetary masses, both on fixed and evolving orbits. For planet-to-star mass ratios $q\simeq(4$-$8)\times10^{-5}$, the density drop at the disk rim becomes reshaped due to a gap opening and is often replaced with a small density bump centered on the planet's corotation. Trapping is possible only if the bump retains enough gas mass and if the co-rotation region becomes azimuthally asymmetric, with an island of librating streamlines that accumulate a gas overdensity ahead of the planet. The overdensity exerts a positive torque that can counteract the negative torque of spiral arms. In our model, efficient trapping depends on the $\alpha$ viscosity and its contrast across the viscosity transition. In order to trap TOI-216b, $\alpha_{\mathrm{DZ}}=10^{-3}$ in the dead zone requires $\alpha_{\mathrm{MRI}}\gtrsim5\times10^{-2}$ in the MRI-active zone. If $\alpha_{\mathrm{DZ}}=5\times10^{-4}$, $\alpha_{\mathrm{MRI}}\gtrsim7.5\times10^{-2}$ is needed.