Minimum Dust Abundances for Planetesimal Formation via Secular Gravitational Instabilities

TL;DR

This paper estimates the minimum dust abundance needed for secular gravitational instability to form planetesimals in protoplanetary disks, highlighting conditions under which dust can directly collapse into planetesimals.

Contribution

It introduces a method to estimate the critical dust abundance for SGI, considering turbulence driven by Kelvin-Helmholtz or streaming instabilities, and relates it to disk parameters.

Findings

Critical dust abundance Z_min is less than 0.1 in many cases.

Higher dust abundance can enable direct collapse into planetesimals.

Growth time of instability is shorter than radial drift time under certain conditions.

Abstract

We estimate minimum dust abundances required for secular gravitational instability (SGI) to operate at the midplane dust layer of protoplanetary disks. For SGI to be a viable process, the growth time of the instability T_grow must be shorter than the radial drift time of the dust T_drift. The growth time depends on the turbulent diffusion parameter alpha, because the modes with short wavelengths are stabilized by turbulent diffusion. Assuming that turbulence is excited via the Kelvin-Helmholtz or streaming instabilities in the dust layer, and that its strength is controlled by the energy supply rate from dust accretion, we estimate the diffusion parameter and the growth time of the instability. The condition T_grow < T_drift requires that the dust abundance must be greater than a critical abundance Z_min, which is a function of the Toomre parameter Q_g and aspect ratio h_g / r of the gas disk. For a wide range of parameter space, the required dust abundance is less than 0.1. A slight increase in dust abundance opens a possible route for the dust to directly collapse to planetesimals.