  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. We study the relevance of the
planet trap concept for the accretion and evolution of systems of multiple
planetary embryos/cores. 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. 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 (~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.
