Azimuthal and Vertical Streaming Instability at High Dust-to-gas Ratios and on the Scales of Planetesimal Formation

The collapse of dust particle clouds directly to kilometer-sized planetesimals is a promising way to explain the formation of planetesimals, asteroids, and comets. In the past, this collapse has been studied in stratified shearing box simulations with super-solar dust-to-gas ratio epsilon, allowing for streaming instability (SI) and gravitational collapse. This paper studies the non-stratified SI under dust-to-gas ratios from epsilon = 0.1 up to epsilon = 1000 without self-gravity. The study covers domain sizes of L = 0.1 H, 0.01 H, and 0.001 H in terms of the gas-disk scale height H using the PENCILCODE. They are performed in radial-azimuthal (2D) and radial-vertical (2.5D) extents. The used particles of St = 0.01 and 0.1 mark the upper end of the expected dust growth. SI activity is found up to very high dust-to-gas ratios, providing fluctuations in the local dust-to-gas ratios and turbulent particle diffusion delta. We find an SI-like instability that operates in r-phi, even when vertical modes are suppressed. This new azimuthal streaming instability (aSI) shows similar properties and appearance as the SI. Both, SI and aSI show diffusivity at epsilon = 100 only to be two orders of magnitude lower than at epsilon = 1, suggesting a delta similar to epsilon(-1). relation that is shallow around epsilon approximate to 1. The (a) SI ability to concentrate particles is found to be uncorrelated with its strength in particle turbulence. Finally, we performed a resolution study to test our findings of the aSI. This paper stresses the importance of properly resolving the (a) SI at high dust-to-gas ratios and planetesimal collapse simulations, leading otherwise to potentially incomplete results.