Requirements for gravitational collapse in planetesimal formation --- the impact of scales set by Kelvin-Helmholtz and nonlinear streaming instability

Abstract

The formation of planetesimals is an unsolved problem in planet formation theory. A prominent scenario for overcoming dust growth barriers in dead zones is the gravitational collapse of locally overdense regions, shown to robustly produce ∼100 km sized objects. Still, the conditions under which planetesimal formation occurs remain unclear. For collapse to proceed, the self-gravity of an overdensity must overcome stellar tidal disruption on large scales and turbulent diffusion on small scales. Here, we relate the scales of streaming and Kelvin-Helmholtz instability, which both regulate particle densities on the scales of gravitational collapse, directly to planetesimal formation. We support our analytic findings by performing 3D hydrodynamical simulations of streaming and Kelvin-Helmholtz instability and planetesimal formation.We find that the vertical extent of the particle mid-plane layer and the radial width of streaming instability filaments are set by the same characteristic length scale, thus governing the strength of turbulent diffusion on the scales of planetesimal formation. We present and successfully test a collapse criterion: 0.1Qβ −1 Z −1 1 and show that even for Solar metallicities, planetesimals can form in dead zones of sufficiently massive disks. For a given gas Toomre-parameter Q, pressure gradient β, metallicity Z and local particle enhancement , the collapse criterion also provides a range of unstable scales, instituting a promising path for studying initial planetesimal mass distributions. Streaming instability is not required for planetesimal collapse, but by increasing , can evolve a system to instability.

A re-assessment of the Kuiper belt size distribution for sub-kilometer objects, revealing collisional equilibrium at small sizes

“…The streaming instability (Youdin and Goodman, 2005;Johansen et al 2007) is today the most credited mechanism for the formation of planetesimals, particularly since it has been shown to explain the orientation of KBO binaries (Nesvorny et al, 2019a) and the structure of Arrokoth itself (McKinnon et al, 2020). Bodies of 10-30 m have such a low gravity that is inconceivable they could form by the streaming instability, which is a gravitational instability process (Gerbig et al, 2020;Klahr and Schreiber, 2020). Instead, they are likely to be fragments of larger bodies produced in collisions.…”

Streaming instability on different scales. I. Planetesimal mass distribution variability

“…The non-linear physics that produces these filaments makes a priori predictions from analytical theory difficult. A few studies have empirically investigated these length scales (Yang & Johansen 2014;Gerbig et al 2020). Of particular interest is whether scales significantly larger than typical simulation boxes could affect filaments and consequent planetesimal formation.…”