Inside-Out Planet Formation. III. Planet-disk interaction at the dead zone inner boundary

TL;DR

This paper uses numerical simulations to study planet-disk interactions at the dead zone inner boundary, focusing on planet migration, gap opening, and implications for the Inside-Out Planet Formation model.

Contribution

It provides new insights into how planets interact with the disk at the DZIB, including migration behavior and gap formation thresholds, supporting the IOPF scenario.

Findings

Planets tend to stay near their formation pressure maximum due to disk torques.

A specific planet mass threshold for significant gap opening is identified.

The model for DZIB retreat explains the formation of multiple planets at different locations.

Abstract

The Kepler mission has discovered more than 4000 exoplanet candidates. Many are in systems with tightly packed inner planets. Inside-Out Planet Formation (IOPF) has been proposed to explain these systems. It involves sequential in situ planet formation at the local pressure maximum of a retreating dead zone inner boundary (DZIB). Pebbles accumulate at this pressure trap, which builds up a ring, and then a planet. The planet is expected to grow until it opens a gap, which helps to both truncate pebble accretion and induce DZIB retreat that sets the location of formation of the next planet. This simple scenario may be modified if the planet migrates significantly from its formation location. Thus planet-disk interactions play a crucial role in the IOPF scenario. We present numerical simulations that first assess migration of planets of various masses that are forming at the DZIB of an active accretion disk, where the effective viscosity rapidly increases in the radially inward direction. We find that the disk's torques on the planet tend to trap the planet at a location very close to the initial pressure maximum where it formed. We then study gap opening by these planets to assess at what mass a significant gap is created. Finally we present a simple model for DZIB retreat due to penetration of X-rays from the star to the disk midplane. Overall, these simulations help to quantify both the mass scale of first,"Vulcan," planet formation and the orbital separation to the location of second planet formation.