We investigate classical planetesimal accretion in a binary star system of
separation ab<50AU by numerical simulations, with particular focus on the
region at a distance of 1 AU from the primary. The planetesimals orbit the
primary, are perturbed by the companion and are in addition subjected to a gas
drag force. We concentrate on the problem of relative velocities dv among
planetesimals of different sizes. For various stellar mass ratios and binary
orbital parameters we determine regions where dv exceed planetesimal escape
velocities v_esc (thus preventing runaway accretion) or even the threshold
velocity v_ero for which erosion dominates accretion. Gaseous friction has two
crucial effects on the velocity distribution: it damps secular perturbations by
forcing periastron alignment of orbits, but at the same time the
size--dependence of this orbital alignment induces a significant dv increase
between bodies of different sizes. This differential phasing effect proves very
efficient and almost always increases dv to values preventing runaway
accretion, except in a narrow domain of almost circular companion orbits. The
erosion threshold dv>v_ero is reached in a wide (ab,eb) space for small (<10km)
planetesimals, but in a much more limited region for bigger ~50km objects. In
the intermediate v_esc<dv < v_ero domain, a possible growth mode would be the
type II runaway growth identified by Kortenkmap et al.(2001)Comment: to appear in Icarus (accepted 30 january 2006
