On vertical variations of gas flow in protoplanetary disks and their impact on the transport of solids

TL;DR

This study compares viscous and MHD turbulence prescriptions in protoplanetary disks, finding that net solid transport is insensitive to flow structure, challenging the use of high-temperature material in comets as a meridional circulation indicator.

Contribution

It introduces a turbulence model with parameters matching viscous prescriptions and demonstrates that radial solid transport is largely unaffected by flow structure, impacting interpretations of disk dynamics.

Findings

Net radial flux of solids is insensitive to flow structure for a given turbulence parameter.

High-temperature materials in comets are not definitive evidence of meridional circulation.

Turbulent diffusion, not mean flows, likely explains outward transport in disks.

Abstract

A major uncertainty in accretion disk theory is the nature and properties of gas turbulence, which drives transport in protoplanetary disks. The commonly used viscous prescription for the Maxwell-Reynolds stress tensor gives rise to a meridional circulation where flow is outward near the midplane and inward away from it. This meridional circulation has been proposed as an explanation for the presence of high-temperature minerals (believed to be of inner solar system provenance) in comets. However, it has not been observed in simulations of magnetohydrodynamical (MHD) turbulence so far. In this study, we evaluate the extent to which the net transport of solids can be diagnostic of the existence of meridional circulation. To that end, we propose and motivate a prescription for MHD turbulence which has the same free parameters as the viscous one. We compare the effects of both prescriptions on the radial transport of small solid particles and find that their net, vertically integrated radial flux is actually quite insensitive to the flow structure for a given vertical average of the turbulence parameter $\alpha$, which we explain. Given current uncertainties on disk turbulence, one-dimensional models are thus most appropriate to investigate radial transport of solids. A corollary is that the presence of high-temperature material in comets cannot be considered an unequivocal diagnostic of meridional circulation. In fact, we argue that outward transport in viscous disk models is more properly attributed to turbulent diffusion rather than to the mean flows of the gas.