Rapid Dust Growth during Hydrodynamic Clumping due to Streaming Instability

Abstract

Abstract Streaming instability is considered to be one of the dominant processes in promoting planetesimal formation by the gravitational collapse of dust clumps. The development of streaming instability is expected to form dust clumps in which the local dust density is strongly enhanced and even greater than the Roche density. The resulting clumps can collapse to form planetesimals. Recent simulations showed that such strong clumping occurs in a wider parameter space than previously expected. However, the indicated timescale for strong clumping can be on the order of tens to hundreds of Keplerian periods. In this paper, we estimate the growth time of dust grains during the pre-clumping phase. We find that the dust growth proceeds considerably before the strong clumping because even the moderate clumping due to streaming instability increases the local dust-to-gas ratio to ≳10. Depending on the sound speed, the dust collision velocity can be kept below ∼1 m s−1 once sufficiently strong dust clumping occurs. Thus, even silicate grains might have the potential to grow safely toward the size whose Stokes number is unity during the clumping. Our results demonstrate the importance of local dust coagulation during dust clumping due to streaming instability.