On the evolution of multiple low mass planets embedded in a circumbinary disc

Context.Previous work has shown that the tidal interaction between a binary system and a circumbinary disc leads to the formation of a large inner cavity in the disc. Subsequent formation and inward migration of a low mass planet causes it to become trapped at the cavity edge, where it orbits until further mass growth or disc dispersal. The question of how systems of multiple planets in circumbinary discs evolve has not yet been addressed. Aims. We present the results of hydrodynamic simulations of multiple low mass planets embedded in a circumbinary disc. The aim is to examine their long term evolution as they approach and become trapped at the edge of the tidally truncated inner cavity. Methods.A grid-based hydrodynamics code was used to compute simulations of 2D circumbinary disc models with embedded planets. The 3D evolution of the planet orbits was computed, and inclination damping due to the disc was calculated using prescribed forces. We present a suite of simulations which study the evolution of pairs of planets migrating in the disc. We also present the results of hydrodynamic simulations of five-planet systems, and study their long term evolution after disc dispersal using a N-body code. Results. For the two-planet simulations we assume that the innermost planet has migrated to the edge of the inner cavity and remains trapped there, and study the subsequent evolution of the system as the outermost planet migrates inward. We find that the outcomes largely depend on the mass ratio $q=m_{\rm i}/m_{\rm o}$, where $m_{\rm i} (m_{\rm o})$ is the mass of the innermost (outermost) planet. For $q < 1$, planets usually undergo dynamical scattering or orbital exchange. For values of $q > 1$ the systems reach equilibrium configurations in which the planets are locked into mean motion resonances, and remain trapped at the edge of the inner cavity without further migration. Most simulations of five-planet systems we performed resulted in collisions and scattering events, such that only a single planet remained in orbit about the binary. In one case however, a multiplanet resonant system was found to be dynamically stable over long time scales, suggesting that such systems may be observed in planet searches focussed on close binary systems.