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The streaming instability (SI) provides a promising mechanism for planetesimal formation because of its ability to concentrate solids into dense clumps. The degree of clumping strongly depends on the height-integrated solid to gas mass ratio Z in protoplanetary disks (PPDs). In this letter, we show that the magnitude of the radial pressure gradient (RPG) which drives the SI (characterized by qK/cₛ, where vK is the reduction of Keplerian velocity due to the RPG and cₛ is the sound speed) also strongly affects clumping. We present local two-dimensional hybrid numerical simulations of aerodynamically coupled particles and gas in the midplane of PPDs. Magnetic fields and particle self-gravity are ignored. We explore three different RPG values appropriate for typical PPDs: q=0. 025, 0. 05 and 0. 1. For each q value, we consider four different particle size distributions ranging from sub millimeter to meter sizes and run simulations with solid abundance from Z=0. 01 up to Z=0. 07. We find that a small RPG strongly promotes particle clumping in that: 1) At fixed particle size distribution, the critical solid abundance Z ₂ₑ₈ₓ above which particle clumping occurs monotonically increases with q; 2) At fixed Z, strong clumping can occur for smaller particles when q is smaller. Therefore, we expect planetesimals to form preferentially in regions of PPDs with a small RPG.
Bai et al. (Mon,) studied this question.