A new mechanism relevant to the formation of planetesimals in the solar nebula

Current models of the formation of km-sized planetary building blocks, or p lanetesimals, by collisional accretion require unrealistically low collisio n velocities or ad hoc assumptions about sticking in order for growth to oc cur. Collision velocities in the protoplanetary nebula increase with increa sing particle size, leading to bouncing and fragmentation of colliding bodi es rather than growth. We describe a new and efficient mechanism in planete simal accretion which leads to net growth in spite of fragmentation, at lea st for small planetesimals. Our experimental results show that nebular gas flow returns protoplanetary dust grains to a growing planetesimal, resultin g in accretion for impact speeds up to at least 10-15 ms(-1). This new thre shold velocity is three orders of magnitude higher than the threshold veloc ity for accretion between macroscopic ice spheres and ten times higher than the sticking velocity for micron-sized dust particles. (C) 2001 Academic P ress.