Dust-driven viscous ring-instability in protoplanetary disks

TL;DR

This paper proposes a dust-driven viscous ring-instability mechanism in protoplanetary disks, suggesting dust accumulation can create pressure maxima that form rings, independent of planetary influences.

Contribution

It introduces a new non-planetary explanation for ring formation, based on dust-gas interactions affecting disk viscosity and stability.

Findings

Dust reduces gas conductivity, inhibiting turbulence.

A feedback loop can amplify dust and gas density perturbations.

Small dust grains can trigger the instability leading to ring formation.

Abstract

Protoplanetary disks often appear as multiple concentric rings in dust continuum emission maps and scattered light images. These features are often associated with possible young planets in these disks. Many non-planetary explanations have also been suggested, including snow lines, dead zones and secular gravitational instabilities in the dust. In this paper we suggest another potential origin. The presence of copious amounts of dust tends to strongly reduce the conductivity of the gas, thereby inhibiting the magneto-rotational instability, and thus reducing the turbulence in the disk. From viscous disk theory it is known that a disk tends to increase its surface density in regions where the viscosity (i.e. turbulence) is low. Local maxima in the gas pressure tend to attract dust through radial drift, increasing the dust content even more. We investigate mathematically if this could potentially lead to a feedback loop in which a perturbation in the dust surface density could perturb the gas surface density, leading to increased dust drift and thus amplification of the dust perturbation and, as a consequence, the gas perturbation. We find that this is indeed possible, even for moderately small dust grain sizes, which drift less efficiently, but which are more likely to affect the gas ionization degree. We speculate that this instability could be triggered by the small dust population initially, and when the local pressure maxima are strong enough, the larger dust grains get trapped and lead to the familiar ring-like shapes. We also discuss the many uncertainties and limitations of this model.