Collision velocity of dust grains in self-gravitating protoplanetary discs

TL;DR

This study uses numerical simulations to analyze dust grain collision velocities in self-gravitating protoplanetary discs, assessing the potential for planetesimal formation via direct collapse, especially focusing on icy versus silicate particles.

Contribution

It provides the first detailed numerical analysis of dust collision velocities in self-gravitating discs, highlighting conditions for growth to planetesimal sizes and the effects of different dust compositions.

Findings

Gravitational perturbations drive velocities effectively only for St > 3.

Radial drift causes high relative velocities, limiting silicate growth beyond small sizes.

Icy solids may grow to larger sizes and concentrate in spiral features, enabling potential planetesimal formation.

Abstract

We have conducted the first comprehensive numerical investigation of the relative velocity distribution of dust particles in self-gravitating protoplanetary discs with a view to assessing the viability of planetesimal formation via direct collapse in such environments. The viability depends crucially on the large sizes that are preferentially collected in pressure maxima produced by transient spiral features (Stokes numbers, $St \sim 1$); growth to these size scales requires that collision velocities remain low enough that grain growth is not reversed by fragmentation. We show that, for a single sized dust population, velocity driving by the disc's gravitational perturbations is only effective for $St > 3$, while coupling to the gas velocity dominates otherwise. We develop a criterion for understanding this result in terms of the stopping distance being of order the disc scale height. Nevertheless, the relative velocities induced by differential radial drift in multi-sized dust populations are too high to allow the growth of silicate dust particles beyond $St \sim 10^{-2}$ or $10^{-1}$ ($10\,\mathrm{cm}$ to $\mathrm{m}$ sizes at $30\,\mathrm{au}$), such Stokes numbers being insufficient to allow concentration of solids in spiral features. However, for icy solids (which may survive collisions up to several $10\,\mathrm{m\,s}^{-1}$), growth to $St \sim 1$ ($10\,\mathrm{m}$ size) may be possible beyond $30\,\mathrm{au}$ from the star. Such objects would be concentrated in spiral features and could potentially produce larger icy planetesimals/comets by gravitational collapse. These planetesimals would acquire moderate eccentricities and remain unmodified over the remaining lifetime of the disc.