Planetesimal accretion in binary star systems

Numerical simulations of planetesimal accretion in circumprimary and circumbinary orbits are described. The secular perturbations by the companion star and gas drag are included in our models. We derive limits on the parameters of the binary system for which accretion and then planetary formation are possible. In the circumbinary case we also outline the radial distance from the baricenter of the stars beyond which accumulation always occurs. Hydrodynamical simulations are also presented to validate our N–body approach based on the axisymmetric approximation for the gas of the disk. 1. Formation of planets by core–accretion The formation of terrestrial planets and cores of giant planets within circumstellar disks involves the accumulation of a large number of planetesimals, solid bodies with initial sizes of roughly several kilometers (Lissauer 1993; Wetherill & Stewart 1993). The initial growth of the planetesimals can follow different paths depending on the their mutual velocities. If runaway growth occurs, a limited number of large planetary embryos form on a short timescale (about 10 − 10 years) followed by a period of violent mutual collisions until the planets reach their final mass. If the encounter velocities exceeds the planetesimal’s escape velocities, the size distribution of the entire population exhibits an orderly growth until larger bodies are formed on a much longer timescale. Most observed extrasolar planets are believed to have formed from planetesimals. The core–accretion model (Pollack et al., 1996) seems to explain a large fraction of the observed physical and dynamical properties of extrasolar gaseous giants in particular after the inclusion of migration by interaction with an evolving disk and gap formation (Alibert et al. (2005)). Neptune–size extrasolar planets possibly formed directly by planetesimal accumulation without reaching the critical mass to accrete a massive gaseous envelope. Around single stars the efficiency of planetesimal accumulation is very high, leading easily to planet formation. The influence of collective perturbations like stirring by