The formation of circumbinary planets through disc fragmentation

TL;DR

This study uses hydrodynamic simulations to explore how circumbinary discs can fragment via gravitational instability, potentially forming gas giant planets at wide orbits around binary stars.

Contribution

It demonstrates that realistic circumbinary discs are more prone to fragmentation and can produce multiple protoplanets, favoring gas giant formation over brown dwarfs.

Findings

Wider binary separations lead to earlier and more efficient disc fragmentation.

Realistic circumbinary discs form more protoplanets than fiducial or circumstellar discs.

Protoplanets in circumbinary discs tend to be lower mass and located beyond 50AU, favoring gas giant formation.

Abstract

Over 50 circumbinary exoplanets have been discovered in recent years, with several of them being gas giants on wide orbits ($>10$AU). The aim of this work is to investigate whether these planets can form through circumbinary disc fragmentation due to gravitational instability. We perform hydrodynamic simulations of marginally unstable (i) circumstellar discs, (ii) circumbinary discs with the same temperature profile as the circumstellar discs (fiducial model), and (iii) realistic circumbinary discs heated individually by each star of the binary. We find that discs around binaries with wider separations fragment earlier and more efficiently than those around closer binaries, and earlier than circumstellar discs. Realistic circumbinary discs form a larger number of protoplanets ($9\pm0.9$ protoplanets per disc), than fiducial circumbinary ($6.5\pm0.6$), and circumstellar discs ($7.5\pm0.8$). In realistic circumbinary discs, initial protoplanet masses are lower than those formed in circumstellar discs, and a larger fraction of them lie in the planetary-mass regime, favouring the formation of gas giant planets over brown dwarfs or low-mass stars. Fragmentation occurs predominantly beyond a binary-imposed forbidden region of $\sim50$AU, leading to final orbital radii peaking at $\sim100$AU. We also find that in circumbinary discs dynamical interactions eject a higher fraction of protoplanets than in circumstellar discs, producing free-floating objects, with ejection velocities on the order of $2-6~{\rm km s^{-1}}$. We conclude that gravitational fragmentation of circumbinary discs is a viable and potentially significant formation pathway for circumbinary gas giant planets.