The DSHARP Rings: Evidence of Ongoing Planetesimal Formation?

TL;DR

This study suggests that ongoing planetesimal formation via streaming instability can explain the observed consistent optical depths in protoplanetary disk rings, indicating a universal process regulating dust in these environments.

Contribution

We demonstrate through simulations and analytical modeling that planetesimal formation can naturally regulate ring optical depths to observed levels in protoplanetary disks.

Findings

Planetesimal formation explains the optical depth regulation in rings.

Streaming instability likely maintains the dust-to-gas ratio at unity.

The model supports a universal process in dust ring evolution.

Abstract

Recent high-resolution interferometric observations of protoplanetary disks at (sub-)millimeter wavelengths reveal omnipresent substructures, such as rings, spirals, and asymmetries. A detailed investigation of eight rings detected in five disks by the DSHARP survey came to the conclusion that all rings are just marginally optically thick with optical depths between 0.2 and 0.5 at a wavelength of 1.25 mm. This surprising result could either be coincidental or indicate that the optical depth in all of the rings is regulated by the same process. We investigated if ongoing planetesimal formation could explain the "fine-tuned" optical depths in the DSHARP rings by removing dust and transforming it into "invisible" planetesimals. We performed a one-dimensional simulation of dust evolution in the second dust ring of the protoplanetary disk around HD 163296, including radial transport of gas and dust, dust growth and fragmentation, and planetesimal formation via gravitational collapse of sufficiently dense pebble concentrations. We show that planetesimal formation can naturally explain the observed optical depths if streaming instability regulates the midplane dust-to-gas ratio to unity. Furthermore, our simple monodisperse analytical model supports the hypothesis that planetesimal formation in dust rings should universally limit their optical depth to the observed range.