Building giant-planet cores at a planet trap

Context. A well-known bottleneck for the core-accretion model of giant-planet formation is the loss of the cores into the star by type I migration, due to the tidal interactions with the gas disk. It has been shown that a steep surface-density gradient in the disk, such as the one expected at the boundary between an active and a dead zone, acts as a planet trap and prevents isolated cores from migrating down to the central star. Aims. We study the relevance of the planet trap concept for the accretion and evolution of systems of multiple planetary embryos/cores. Methods. We performed hydrodynamical simulations of the evolution of systems of multiple massive objects in the vicinity of a planet trap. The planetary embryos evolve in 3 dimensions, whereas the disk is modeled with a 2D grid. Synthetic forces are applied onto the embryos to mimic the damping effect that the disk has on their inclinations. Results. Systems with two embryos tend to acquire stable, separated and non-migrating orbits, with the more massive embryo placed at the planet trap and the lighter one farther out in the disk. Systems of multiple embryos are intrinsically unstable. Consequently, a long phase of mutual scattering can lead to accreting collisions among embryos; some embryos are injected into the inner part of the disk, where they can be evacuated into the star by type I migration. The system can resume a stable, non-migrating configuration only when the number of surviving embryos decreases to a small value (similar to 2-4). This can explain the limited number of giant planets in our solar system. These results should apply in general to any case in which the type I migration of the inner embryo is prevented by some mechanism, and not solely to the planet trap scenario.