On the formation and migration of giant planets in circumbinary discs

Aims. We present the results of hydrodynamic simulations of the formation and subsequent orbital evolution of giant planets embedded in a circumbinary disc. The aim is to examine whether or not giant planets can be found to orbit stably in close binary systems. Methods. We performed numerical simulations using a grid-based hydrodynamics code. We assume that a 20 M. core has migrated to the edge of the inner cavity formed by the binary where it remains trapped by corotation torques. This core is then allowed to accrete gas from the disc, and we study its orbital evolution as it grows in mass. For each of the two accretion time scales we considered, we performed three simulations. In two of the three simulations, we stopped the accretion onto the planet once its mass became characteristic of that of Saturn or Jupiter. In the remaining case, the planet accreted disc material freely in such a way that its mass became higher than Jupiter's. Results. The simulations show different outcomes depending on the final mass m(p) of the giant. For m(p) = 1 M(S) (where M(S) is Saturn's mass), we find that the planet migrates inward through its interaction with the disc until its eccentricity becomes high enough to induce a torque reversal. The planet then migrates outward, and the system remains stable on long time scales. For m(p) >= 1 M(J) (where MJ is Jupiter's mass) we observed two different outcomes. In each case the planet enters the 4: 1 resonance with the binary, and resonant interaction drives up the eccentricity of the planet until it undergoes a close encounter with the secondary star, leading to scattering. The result can either be ejection from the system or scattering out into the disc followed by a prolonged period of outward migration. These results suggest that circumbinary planets are more likely to be quite common in the Saturn-mass range. Jupiter-mass circumbinary planets are likely to be less common because of their less stable evolution, but if present are likely to orbit at large distances from the central binary.