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

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 $q\simeq (4$-$8)\times 10^{-5}$, 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 ${\alpha }_{\mathrm{MRI}}\gtrsim 5\times 10^{-2}$ in the MRI-active zone. If ${\alpha }_{\mathrm{DZ}}=5\times 10^{-4}$, ${\alpha }_{\mathrm{MRI}}\gtrsim 7.5\times 10^{-2}$ is needed.

Accepted for publication in A&A, 21 pages, 16 figures; v2 with minor text corrections

References in corpus (36)

• A comparative study of disc-planet interaction
• Migration and the formation of systems of hot super-Earths and Neptunes
• Formation of planetary systems by pebble accretion and migration: How the radial pebble flux determines a terrestrial-planet or super-Earth growth mode
• The Migration of Gap-Opening Planets is not Locked to Viscous Disk Evolution
• Migration of massive planets in accreting disks
• Type I planetary migration in a self-gravitating disk
• Convective Overstability in radially stratified accretion disks under thermal relaxation
• Reduced gas accretion on super-Earths and ice giants
• Treating gravity in thin disk simulations
• Dynamical corotation torques on low-mass planets
• 3D Radiation Non-ideal Magnetohydrodynamical Simulations Of The Inner Rim In Protoplanetary Disks
• Improved torque formula for low and intermediate mass planetary migration
• Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
• Influence of an inner disc on the orbital evolution of massive planets migrating in resonance
• Accretion and Evolution of ~2.5 Earth-mass Planets with Voluminous H/He Envelopes
• Planet formation and migration near the silicate sublimation front in protoplanetary disks
• Numerical simulations of type III planetary migration: III. Outward migration of massive planets
• The sub-Neptune desert and its dependence on stellar type: Controlled by lifetime X-ray irradiation
• Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses
• Numerical simulations of the type III migration:I. Disc model and convergence tests
• Influence of planetary gas accretion on the shape and depth of gaps in protoplanetary discs
• Eccentricity excitation and merging of planetary embryos heated by pebble accretion
• A resonant pair of warm giant planets revealed by TESS
• Migration of Jupiter mass planets in low viscosity discs
• Pushing planets into an inner cavity by a resonant chain
• Precise transit and radial-velocity characterization of a resonant pair: a warm Jupiter TOI-216c and eccentric warm Neptune TOI-216b
• TOI-216b and TOI-216c: Two warm, large exoplanets in or slightly wide of the 2:1 orbital resonance
• Sculpting the sub-Saturn Occurrence Rate via Atmospheric Mass Loss
• The effect of planetary migration on the corotation resonance
• Analytic Expressions for the Inner-Rim Structure of Passively Heated Protoplanetary Disks
• Unified simulations of planetary formation and atmospheric evolution II: Rapid disk clearing by photoevaporation yields low-mass super-Earth atmospheres
• Close-in Super-Earths: The first and the last stages of planet formation in an MRI-accreting disc
• Planet-vortex interaction:How a vortex can shepherd a planetary embryo
• Type II migration strikes back -- An old paradigm for planet migration in discs
• Constraining the masses of planets in protoplanetary discs from the presence or absence of vortices -- Comparison with ALMA observations
• Comments on "Type II migration strikes back -- An old paradigm for planet migration in discs" by Scardoni et al

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• Apsidal Alignment and Anti-Alignment of Planets in Mean-Motion Resonance: Disk-Driven Migration and Eccentricity Driving
• Accreting luminous low-mass planets escape from migration traps at pressure bumps
• Transit Depth Variations Reveal TOI-216 b to be a Super-Puff
• The New Generation Planetary Population Synthesis (NGPPS). VII. Statistical comparison with the HARPS/Coralie survey
• Multi dust species inner rim in magnetized protoplanetary disks
• The role of detailed gas and dust opacities in shaping the evolution of the inner disc edge subject to episodic accretion