Bridging Unstratified and Stratified Simulations of the Streaming Instability for $\tau_s=0.1$ Grains

TL;DR

This study compares stratified and unstratified simulations of the streaming instability at $ au_s=0.1$, finding that unstratified models accurately replicate midplane dust-gas dynamics before clumping occurs.

Contribution

It demonstrates that unstratified simulations can effectively model midplane dust-gas behavior in stratified disks for specific parameters, validating their use in certain regimes.

Findings

Midplane velocities are consistent between stratified and unstratified simulations.

Gas and dust density distributions near the midplane are similar in both simulation types.

Dust filament formation shows morphological similarities across models.

Abstract

The streaming instability (SI), driven by aerodynamic coupling between solids and the gas under a global radial pressure gradient, concentrates solids and facilitates planetesimal formation. Unstratified simulations are commonly used to study the SI, based on the assumption that they approximate conditions near the disk midplane. However, it remains unclear how accurately these unstratified simulations capture the midplane dust-gas dynamics in stratified disks. To address this, we examine the saturated state of the SI in stratified simulations and compare dust-gas dynamics to those in unstratified simulations across various radial pressure gradients. To this end, we consider a dimensionless dust stopping time ($\tau_s$) of 0.1 and perform 2D axisymmetric, stratified simulations. We find that the formation of dust filaments during dust settling exhibits morphological similarities to those in unstratified simulations. Vertical gravity acts to redistribute momentum vertically in response to momentum flux, resulting in midplane velocities in the center-of-mass frame that are consistent with those from unstratified models at any given pressure gradient. Furthermore, the velocity dispersions and density distributions of the gas and dust near the midplane of our stratified simulations closely match those in unstratified simulations. While further exploration across the parameter space is needed, our results suggest that, for $\tau_s=0.1$, unstratified simulations represents well the midplane dust--gas dynamics in stratified disks before any strong clumping occurs. Consequently, our results confirm that in the saturated state, the streaming turbulence in stratified simulations behaves similarly to that in unstratified simulations for the parameter values explored here.