Survival of ALMA Rings in the Absence of Pressure Maxima

TL;DR

This paper explores an alternative explanation for ALMA-observed rings in protoplanetary discs, suggesting they are formed by dust-gas interactions in a smooth gas disc, leading to dense, clumpy mid-planes that form planetesimals without pressure maxima.

Contribution

It introduces a model where ALMA rings result from dust-gas interactions in a smooth disc, challenging the pressure bump hypothesis and showing rings can survive and produce planetesimals without pressure support.

Findings

Rings tend to move outward due to diffusion of clumps.

Rings leak material at about 40 Earth masses per million years.

Simulated images match observed ALMA rings of Elias 24 and AS 209.

Abstract

Recent ALMA observations have revealed that a large fraction of protoplanetary discs contain bright rings at (sub)millimeter wavelengths. Dust trapping induced by pressure maxima in the gas disc is a popular explanation for these rings. However, it is unclear whether such pressure bumps can survive for evolutionary time-scales of the disc. In this work, we investigate an alternative scenario, which involves only dust-gas interactions in a smooth gas disc. We postulate that ALMA rings are a manifestation of a dense, clumpy mid-plane that is actively forming planetesimals. The clumpy medium itself hardly experiences radial drift, but clumps lose mass by disintegration and vertical transport and planetesimal formation. Starting from a seed ring, we numerically solve the transport equations to investigate the ring's survival. In general, rings move outward, due to diffusion of the clump component. Without pressure support, rings leak material at rates $\sim$40 $M_\oplus\,\mathrm{Myr}^{-1}$ and in order for rings to survive, they must feed from an external mass reservoir of pebbles. In the case where the pebble size is constant in the disk, a cycle between ring formation and dispersion emerges. Rings produce large quantities of planetesimals, which could be material for planet formation and explain the massive budget inferred debris disc. Mock images of ALMA observations compare well to the rings of Elias 24 and AS 209 from DSHARP's sample.