On the lower mass limit for circumbinary disc fragmentation

Abstract: In recent years, many wide orbit circumbinary (CB) giant planets have been discovered; some of these may have formed by gravitational fragmentation of circumbinary discs. The aim of this work is to investigate the lower mass limit for circumbinary disc fragmentation. We use the Smoothed Particle Hydrodynamics (SPH) code SEREN, which employs an approximate method for the radiative transfer, to perform 3 sets of simulations of gravitationally unstable discs. The first set of simulations covers circumstellar discs heated by a single 0.7$\,{\rm M}_{\odot}$ star (CS model), the second set covers binaries with the same total stellar mass as the CS model, attended by circumbinary discs with the same temperature profile (CB fiducial model), and the third set covers circumbinary discs heated by each individual star (CB realistic model). We vary the binary separation, mass ratio and eccentricity to see their effect on disc fragmentation. For the circumstellar disc model, we find a lower disc-to-star mass ratio for fragmentation of $\sim\,$0.31. For the circumbinary fiducial disc model we find the same disc-to-star mass ratio for fragmentation (but slightly lower for more eccentric, equal-mass binaries; 0.26). On the other hand, realistic circumbinary discs fragment at a lower mass limit (disc-to-star mass ratio of 0.17\,-\,0.26), depending on the binary properties. We conclude that circumbinary discs fragment at a lower disc mass (by $\sim 45\%$) than circumstellar discs. Therefore, gas giant planet around binaries may be able to form by gravitational instability easier than around single stars.