Dust coagulation assisted by streaming instability in protoplanetary disks

The streaming instability is a promising mechanism for planetesimal formation. The instability can rapidly form dense clumps that collapse self-gravitationally, which is efficient for large dust grains with the Stokes number on the order of 0.1. However, dust growth models predict that collisional fragmentation prevents dust grains from growing to such sizes. We perform local simulations of the streaming instability and measure characteristic collision velocities and collision rates of dust grains based on their trajectories in moderate clumping. The collision velocities are on the order of 0.1 percent of the sound speed or lower, implying that dust grains can overcome the fragmentation barrier via the clumping. We also find that the collision rates are appreciably high regardless of the low collision velocities. Corresponding timescales are on the order of ten Keplerian periods or shorter, suggesting that dust grains can overcome the drift barrier as well. This streaming-instability-assisted (SI-assisted) coagulation greatly relaxes the conditions for planetesimal formation as recently implied.