Diffusion and Concentration of Solids in the Dead Zone of a Protoplanetary Disk

TL;DR

This study uses simulations to show that solid particle concentration and clumping in protoplanetary disk dead zones occur even without significant sedimentation, challenging previous assumptions about planetesimal formation conditions.

Contribution

It demonstrates that particle sedimentation is not essential for planetesimal formation via streaming instability in dead zones, using detailed MHD simulations with varying conditions.

Findings

Particles do not sediment more in dead zones than in turbulent disks.

Strong solid clumping occurs even with minimal sedimentation.

Radial diffusion near the mid-plane is weak, facilitating clumping.

Abstract

The streaming instability is a promising mechanism to drive the formation of planetesimals in protoplanetary disks. To trigger this process, it has been argued that sedimentation of solids onto the mid-plane needs to be efficient and therefore that a quiescent gaseous environment is required. It is often suggested that dead-zone or disk-wind structure created by non-ideal magnetohydrodynamical (MHD) effects meets this requirement. However, simulations have shown that the midplane of a dead zone is not completely quiescent. In order to examine the concentration of solids in such an environment, we use the local-shearing-box approximation to simulate a particle-gas system with an Ohmic dead zone including mutual drag force between the gas and the solids. We systematically compare the evolution of the system with ideal or non-ideal MHD, with or without back-reaction drag force from particles on gas, and with varying solid abundances. Similar to previous investigations of dead zone dynamics, we find that particles of dimensionless stopping time $\tau_s=0.1$ do not sediment appreciably more than those in ideal magneto-rotational turbulence, resulting in a vertical scale height an order of magnitude larger than in a laminar disk. Contrary to the expectation that this should curb the formation of planetesimals, we nevertheless find that strong clumping of solids still occurs in the dead zone when solid abundances are similar to the critical value for a laminar environment. This can be explained by the weak radial diffusion of particles near the mid-plane. The results imply that the sedimentation of particles to the mid-plane is not a necessary criterion for the formation of planetesimals by the streaming instability.