Relative velocities among accreting planetesimals in binary systems:: The circumprimary case

We investigate classical planetesimal accretion in a binary star system of separation c(b) <= 50 AU by numerical simulations, with particular focus oil 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 Delta v among planetesimals of different sizes. For various stellar mass ratios and binary orbital parameters we determine regions where Delta v exceed planetesimal escape velocities v(esc) (thus preventing runaway accretion) or even the threshold velocity Vero for which erosion dominates accretion. Gaseous friction has two crucial effects oil 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 Delta v increase between bodies of different sizes. This differential phasing effect proves very efficient and almost always increases Delta v to values preventing runaway accretion, except in a narrow e(b) similar or equal to 0 domain. The erosion threshold Delta v > v(ero) is reached in a wide (C-b, e(b)) space for small < 10-km planetesimals, but in a much more limited region for bigger similar or equal to 50-km objects. In the intermediate v(esc) < Delta v < v(ero) domain. a possible growth mode would be the type II runaway growth identified by Kortenkamp et al. [Kortenkamp, S., Wetherill, G., Inaba. S., 2001, Science 293, 1127-1129]. (c) 2006 Elsevier Inc. All rights reserved.