Relative velocities among accreting planetesimals in binary systems: the circumbinary case

Abstract

We numerically investigate the possibility of planetesimal accretion in circumbinary discs, under the coupled influence of both stars' secular perturbations and friction due to the gaseous component of the protoplanetary disc. We focus on one crucial parameter: the distribution of encounter velocities between planetesimals in the 0.5-100 km size range. An extended range of binary systems with differing orbital parameters is explored. The resulting encounter velocities are compared to the threshold velocities below which the net outcome of a collision is accumulation into a larger body instead of mass erosion. For each binary configuration, we derive the critical radial distance from the binary barycentre beyond which planetesimal accretion is possible. This critical radial distance is smallest for equal-mass binaries on almost circular orbits. It shifts to larger values for increasing eccentricities and decreasing mass ratio. The importance of the planetesimals' orbital alignments of planetesimals due to gas drag effects is discussed.