Hydrodynamics of embedded planets' first atmospheres - III. The role of radiation transport for super-Earth planets

TL;DR

This study uses 3D radiation hydrodynamics to examine how recycling of gas from the protoplanetary disc influences the atmospheres of super-Earths, suggesting it can prevent runaway gas accretion and maintain higher entropy atmospheres.

Contribution

It provides the first detailed 3D radiation hydrodynamics analysis of atmospheric recycling in super-Earths, comparing with 1D models and identifying an inner core region with distinct properties.

Findings

3D atmospheres maintain higher entropy than 1D models.

Recycling operates vigorously, slowing down envelope contraction.

An inner core region with lower entropy may become hydrodynamically isolated.

Abstract

The population of close-in super-Earths, with gas mass fractions of up to 10% represents a challenge for planet formation theory: how did they avoid runaway gas accretion and collapsing to hot Jupiters despite their core masses being in the critical range of $M_\mathrm{c} \simeq 10 M_\mathrm{\oplus}$? Previous three-dimensional (3D) hydrodynamical simulations indicate that atmospheres of low-mass planets cannot be considered isolated from the protoplanetary disc, contrary to what is assumed in 1D-evolutionary calculations. This finding is referred to as the recycling hypothesis. In this Paper we investigate the recycling hypothesis for super-Earth planets, accounting for realistic 3D radiation hydrodynamics. Also, we conduct a direct comparison in terms of the evolution of the entropy between 1D and 3D geometries. We clearly see that 3D atmospheres maintain higher entropy: although gas in the atmosphere loses entropy through radiative cooling, the advection of high entropy gas from the disc into the Bondi/Hill sphere slows down Kelvin-Helmholtz contraction, potentially arresting envelope growth at a sub-critical gas mass fraction. Recycling, therefore, operates vigorously, in line with results by previous studies. However, we also identify an "inner core" -- in size $\approx$ 25% of the Bondi radius -- where streamlines are more circular and entropies are much lower than in the outer atmosphere. Future studies at higher resolutions are needed to assess whether this region can become hydrodynamically-isolated on long time-scales.