Making giant planet cores: convergent migration and growth of planetary embryos in non-isothermal discs

TL;DR

This study investigates how multiple planetary embryos in non-isothermal discs migrate and grow through convergent migration and giant impacts, leading to the formation of giant planet cores within about a million years.

Contribution

It demonstrates that convergent migration zones can facilitate the growth of giant planet cores via collisions, especially with initial protoplanets of around 2-3 Earth masses, in non-isothermal disc conditions.

Findings

Stable resonant chains form with few embryos without stochastic forces.

Stochastic forces disrupt resonant configurations, promoting collisions.

Giant planet cores of 10 Earth masses form in about half of the simulations with 3 Earth mass embryos.

Abstract

Earth-mass bodies are expected to undergo Type I migration directed either inward or outward depending on the thermodynamical state of the protoplanetary disc. Zones of convergent migration exist where the Type I torque cancels out. We study the evolution of multiple protoplanets of a few Earth masses embedded in a non-isothermal protoplanetary disc. The protoplanets are located in the vicinity of a convergence zone located at the transition between two different opacity regimes. Inside the convergence zone, Type I migration is directed outward and outside the zone migration is directed inward. We used a grid-based hydrodynamical code that includes radiative effects. We performed simulations varying the initial number of embryos and tested the effect of including stochastic forces to mimic the effects resulting from turbulence. We also performed N-body runs calibrated on hydrodynamical calculations to follow the evolution on Myr timescales. For a small number of initial embryos (N = 5-7) and in the absence of stochastic forcing, the population of protoplanets migrates convergently toward the zero-torque radius and forms a stable resonant chain that protects embryos from close encounters. In systems with a larger initial number of embryos, or in which stochastic forces were included, these resonant configurations are disrupted. This in turn leads to the growth of larger cores via a phase of giant impacts, after which the system settles to a new stable resonant configuration. Giant planets cores with masses of 10 Earth masses formed in about half of the simulations with initial protoplanet masses of m_p = 3 Earth masses but in only 15% of simulations with m_p = 1 Earth mass. This suggests that if ~2-3 Earth mass protoplanets can form in less than ~1 Myr, convergent migration and giant collisions can grow giant planet cores at Type I migration convergence zones.