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

Abstract

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