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 planetesimals, by collisional accretion require unrealistically low collision velocities or ad hoc assumptions about sticking in order for growth to occur. Collision velocities in the protoplanetary nebula increase with increasing particle size, leading to bouncing and fragmentation of colliding bodies rather than growth. We describe a new and efficient mechanism in planetesimal accretion which leads to net growth in spite of fragmentation, at least for small planetesimals. Our experimental results show that nebular gas flow returns protoplanetary dust grains to a growing planetesimal, resulting in accretion for impact speeds up to at least 10-15 ms(-1). This new threshold velocity is three orders of magnitude higher than the threshold velocity for accretion between macroscopic ice spheres and ten times higher than the sticking velocity for micron-sized dust particles. (C) 2001 Academic Press.