Suppression of atmospheric recycling of planets embedded in a protoplanetary disc by buoyancy barrier

TL;DR

This study uses 3D hydrodynamical simulations to show that buoyancy barriers in non-isothermal conditions can inhibit atmospheric recycling in protoplanets, potentially enabling gas accretion and planet growth.

Contribution

It demonstrates that buoyancy barriers in non-isothermal environments prevent deep envelope recycling, a novel insight into planet formation processes.

Findings

Recycling flows reach the envelope in isothermal cases.

Buoyancy prevents deep envelope intrusion in non-isothermal cases.

Buoyancy barriers may enable runaway gas accretion.

Abstract

The ubiquity of super-Earths poses a problem for planet formation theory to explain how they avoided becoming gas giants. Rapid recycling of the envelope gas of planets embedded in a protoplanetary disc has been proposed to delay the cooling and following accretion of disc gas. We compare isothermal and non-isothermal 3D hydrodynamical simulations of the gas flow past a planet to investigate the influence on the feasibility of the recycling mechanism. Radiative cooling is implemented by using the beta cooling model. We find that, in either case, gas enters the Bondi sphere at high latitudes and leaves through the midplane regions, or vice versa when disc gas rotates sub-Keplerian. However, in contrast to the isothermal case where the recycling flow reaches the deeper part of the envelope, the inflow is inhibited from reaching the deep envelope in the non-isothermal case. Once the atmosphere starts cooling, buoyant force prevents the high-entropy disc gas from intruding the low-entropy atmosphere. We suggest that the buoyancy barrier isolates the lower envelope from the recycling and allows further cooling, which may lead runaway gas accretion onto the core.