New Insights on Streaming Instability in a Laminar Disk

The evolution of the solar system’s first few million years is shaped by the formation of its planetesimals. The meteoritic record tells us that terrestrial-region planetesimals started forming a fraction of a Myr after the first solids condensed and went on forming for 2-3 Myr. Our understanding of how these ~100 km size objects form is still in its infancy. The streaming instability (SI), which stems from the dynamical interaction of particles and gas, is regarded as the leading candidate for this formation process. We revisit the SI from a more basic standpoint with an eye toward better understand how turbulence acts to mitigate the SI while possibly promoting other proposed accumulation processes like turbulent concentration. We perform both 2D axisymmetric and 3D hydrodynamic simulations with varied resolution and a wide suite of particle Stokes number (stopping time, St). We identify the maintenance of turbulent jets sandwiching the settled dust layer, which appears to play an essential role in the dispersal/mixing of the solid component. We find that it is primarily the radial velocity fluctuations in the dust-gas midplane layer, as opposed to the azimuthal ones as previously thought, that drive the observed transition to a turbulent state. We also report the energy spectrum for the dust-gas system which hovers around a k^-2.2 power law and falls under the category of a stratified rotating flow for which no completely known turbulence phenomenology exists so far. Finally, we discuss the transition to turbulence in the 2D axisymmetric case.