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H. Scholl (Observatoire de Nice), F. Marzari (University of Padova)
We investigate planetary accretion in binary star systems by integrating planetesimal orbits around each of the stars and around their centers of mass. We include the effects of gas drag and planetesimal mutual collisions. For the Alpha Centauri system, Wiegert and Holman (1997) found that stable planetary orbits may exist within 3 Au from the primary star. We show that planetesimal accretion is possible within this range since the relative velocities between planetesimals are lower than the threshold velocity for fragmentation. Gas drag is the key mechanism for achieving low impact velocities: it damps the eccentricities and it induces the alignment of perihelia of the planetesimal orbits. Mutual collisions further damp the eccentricity of the planetesimal orbits. Another important effect due to gas drag is the differential drifting of planetesimals towards the star: planetesimals in the outer part of the accretion disk drift faster since their eccentricities are higher due to the secular perturbations of the companion star.