Streaming Instability and Turbulence: Conditions for Planetesimal Formation

• Published in: The Astrophysical journal Vol. 969; no. 2; pp. 130 - 150
• Main Authors: Lim, Jeonghoon, Simon, Jacob B., Li, Rixin, Armitage, Philip J., Carrera, Daniel, Lyra, Wladimir, Rea, David G., Yang, Chao-Chin, Youdin, Andrew N.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.07.2024; IOP Publishing
• Subjects: Density ratio; Eddy diffusion; Gas density; Hydrodynamical simulations; Hydrodynamics; Parameters; Planet formation; Planetesimals; Protoplanetary disks; Scale height; Shearing; Turbulence; Turbulent diffusion
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad47a2

The streaming instability (SI) is a leading candidate for planetesimal formation, which can concentrate solids through two-way aerodynamic interactions with the gas. The resulting concentrations can become sufficiently dense to collapse under particle self-gravity, forming planetesimals. Previous studies have carried out large parameter surveys to establish the critical particle to gas surface density ratio ( Z ), above which SI-induced concentration triggers planetesimal formation. The threshold Z depends on the dimensionless stopping time ( τ s , a proxy for dust size). However, these studies neglected both particle self-gravity and external turbulence. Here, we perform 3D stratified shearing box simulations with both particle self-gravity and turbulent forcing, which we characterize via a turbulent diffusion parameter, α D . We find that forced turbulence, at amplitudes plausibly present in some protoplanetary disks, can increase the threshold Z by up to an order of magnitude. For example, for τ s = 0.01, planetesimal formation occurs when Z ≳ 0.06, ≳0.1, and ≳0.2 at α D = 10 −4 , 10 −3.5 , and 10 −3 , respectively. We provide a single fit to the critical Z required for the SI to work as a function of α D and τ s (although limited to the range τ s = 0.01–0.1). Our simulations also show that planetesimal formation requires a mid-plane particle-to-gas density ratio that exceeds unity, with the critical value being largely insensitive to α D . Finally, we provide an estimation of particle scale height that accounts for both particle feedback and external turbulence.