Exploring the conditions required to form giant planets via gravitational instability in massive protoplanetary discs

TL;DR

This study uses 3D radiation hydrodynamical simulations to identify conditions, such as low opacity and weak stellar irradiation, under which massive protoplanetary discs can fragment to form giant planets, especially relevant for systems like HR 8799.

Contribution

It demonstrates how disc opacity and stellar irradiation influence gravitational fragmentation, providing new insights into giant planet formation via instability.

Findings

Low opacities promote disc fragmentation at small radii.

Stellar irradiation generally inhibits fragmentation.

Fragmentation is possible with weak irradiation allowing low Toomre parameter.

Abstract

We carry out global three-dimensional radiation hydrodynamical simulations of self-gravitating accretion discs to determine if, and under what conditions, a disc may fragment to form giant planets. We explore the parameter space (in terms of the disc opacity, temperature and size) and include the effect of stellar irradiation. We find that the disc opacity plays a vital role in determining whether a disc fragments. Specifically, opacities that are smaller than interstellar Rosseland mean values promote fragmentation (even at small radii, R < 25AU) since low opacities allow a disc to cool quickly. This may occur if a disc has a low metallicity or if grain growth has occurred. With specific reference to the HR 8799 planetary system, given its star is metal-poor, our results suggest that the formation of its imaged planetary system could potentially have occurred by gravitational instability. We also find that the presence of stellar irradiation generally acts to inhibit fragmentation (since the discs can only cool to the temperature defined by stellar irradiation). However, fragmentation may occur if the irradiation is sufficiently weak that it allows the disc to attain a low Toomre stability parameter.