A coagulation-fragmentation model for the turbulent growth and destruction of preplanetesimals

TL;DR

This paper investigates a simplified coagulation-fragmentation model for protoplanetary disc solids, revealing that turbulence-driven diffusion hampers growth of meter-sized objects, confirming that this process is inefficient across the meter barrier.

Contribution

It introduces an analytical and simulation-based study of coagulation-fragmentation dynamics in turbulent protoplanetary discs, emphasizing the impact of turbulent diffusion on planetesimal growth.

Findings

Growth by coagulation-fragmentation is efficient within certain conditions.

Turbulent diffusion prevents dust fragments from accumulating in the mid-plane.

Coagulation-fragmentation is not effective for crossing the meter barrier in turbulent discs.

Abstract

To treat the problem of growing protoplanetary disc solids across the meter barrier, we consider a very simplified two-component coagulation-fragmentation model that consists of macroscopic boulders and smaller dust grains, the latter being the result of catastrophic collisions between the boulders. Boulders in turn increase their radii by sweeping up the dust fragments. An analytical solution to the dynamical equations predicts that growth by coagulation-fragmentation can be efficient and allow agglomeration of 10-meter-sized objects within the time-scale of the radial drift. These results are supported by computer simulations of the motion of boulders and fragments in 3-D time-dependent magnetorotational turbulence. Allowing however the fragments to diffuse freely out of the sedimentary layer of boulders reduces the density of both boulders and fragments in the mid-plane, and thus also the growth of the boulder radius, drastically. The reason is that the turbulent diffusion time-scale is so much shorter than the collisional time-scale that dust fragments leak out of the mid-plane layer before they can be swept up by the boulders there. Our conclusion that coagulation-fragmentation is not an efficient way to grow across the meter barrier in fully turbulent protoplanetary discs confirms recent results by Brauer, Dullemond, & Henning who solved the coagulation equation in a parameterised turbulence model with collisional fragmentation, cratering, radial drift, and a range of particle sizes. We find that a relatively small population of boulders in a sedimentary mid-plane layer can populate the entire vertical extent of the disc with small grains and that these grains are not first generation dust, but have been through several agglomeration-destruction cycles during the simulations.