Circumbinary discs with radiative cooling and embedded planets

Abstract

Context. As of today, ten circumbinary planets orbiting solar type main sequence stars have been discovered. Nearly all orbit around the central binary very closely to the region of instability where it is difficult to form them in situ. Hence, it is assumed that they formed further out and then migrated to their observed position, which is determined by binary, disc and planet properties. Aims. We extend previous studies to a more realistic thermal disc structure and determine what parameter influence the final parking location of a planet around a binary star. Methods. We performed two-dimensional numerical simulations of viscous accretion discs around a central binary. These simulations include viscous heating and radiative cooling from the disc surfaces. We vary the binary eccentricity as well as disc viscosity and mass. Results. Concerning the disc evolution, we find that it can take well over 100 000 binary orbits until an equilibrium state is reached. As seen previously, we find that the central cavity opened by the binary becomes eccentric and precesses slowly in a prograde sense. Embedded planets migrate to the inner edge of the disc. In cases of lower disc viscosity they migrate further in maintaining a circular orbit, while for high viscosity they are parked further out on an eccentric orbit. Conclusions. Discs around binary stars are eccentric, and precess very slowly around the binary. The final location of an embedded planet is linked to its ability to open a gap in the disc. Gap-opening planets separate inner from outer disc, preventing eccentricity excitation in the latter and making it more circular. This allows embedded planets to migrate closer to the binary, in agreement with the observations. The necessary conditions for gap opening and the final planet position depend on the planet mass and disc viscosity.