Gas and multi-species dust dynamics in viscous protoplanetary discs: the importance of the dust back-reaction

TL;DR

This paper investigates how dust particles of various sizes influence the gas dynamics in viscous protoplanetary discs, revealing that dust back-reaction significantly alters gas flow and observable structures, even at low dust-to-gas ratios.

Contribution

It provides the first comprehensive analysis of multi-size dust back-reaction effects on gas dynamics in viscous protoplanetary discs, highlighting their importance in disc evolution models.

Findings

Dust back-reaction reduces gas accretion flow in typical discs.

Outer disc dust back-reaction can drive outward gas flow.

Large grains' inward drift is slowed, small grains follow gas dynamics.

Abstract

We study the dynamics of a viscous protoplanetary disc hosting a population of dust grains with a range of sizes. We compute steady-state solutions, and show that the radial motion of both the gas and the dust can deviate substantially from those for a single-size dust population. Although the aerodynamic drag from the dust on the gas is weaker than in the case where all grains are optimally coupled to the gas, the cumulative "back-reaction" of the dust particles can still alter the gas dynamics significantly. In typical protoplanetary discs, the net effect of the dust back-reaction decreases the gas accretion flow compared to the dust-free (viscous) case, even for dust-to-gas ratios of order $1\%$. In the outer disc, where dust grains are typically less strongly coupled to the gas and settle towards the midplane, the dust back-reaction can even drive outward gas flow. Moreover, the radial inward drift of large grains is reduced below the gas motion in the inner disc regions, while small dust grains follow the gas dynamics over all the disc extent. The resulting dust and gas dynamics can give rise to observable structures, such as gas and dust cavities. Our results show that the dust back-reaction can play a major role in both the dynamics and observational appearance of protoplanetary discs, and cannot be ignored in models of protoplanetary disc evolution.