Probing Conditions for Strong Clumping by the Streaming Instability: Small Dust Grains and Low Dust-to-gas Density Ratio

The streaming instability (SI) is a leading mechanism for concentrating solid particles into regions dense enough to form planetesimals. Its efficiency in clumping particles depends primarily on the dimensionless stopping time (tau s, a proxy for particle size) and dust-to-gas surface density ratio (Z). Previous simulations identified a critical Z (Zcrit) above which strong clumping occurs, where particle densities exceed the Hill density (thus satisfying a condition for gravitational collapse), over a wide range of tau s. These works found that, for tau s <= 0.01, Zcrit was above the interstellar medium value (similar to 0.01). In this work, we reexamine the clumping threshold using 2D axisymmetric, stratified simulations at high resolution and with relatively large (compared to many previous simulations) domain sizes. Our main results are as follows: First, when tau s = 0.01, strong clumping occurs even at Z less than or similar to 0.01, lower than Zcrit found in all previous studies. Consequently, we revise a previously published fit to the Zcrit curve to account for this updated Zcrit. Second, higher resolution results in a thicker dust layer, which may result from other instabilities manifesting, such as the vertically shearing SI. Third, despite this thicker layer, higher resolution can lead to strong clumping even with a lower midplane dust-to-gas density ratios (which results from the thicker particle layer) so long as Z greater than or similar to Zcrit. Our results demonstrate the efficiency of the SI in clumping small particles at Z similar to 0.01, which is a significant refinement of the conditions for planetesimal formation by the SI.