Characterization of the dust content in the ring around Sz 91: indications for planetesimal formation?

TL;DR

This study uses multi-wavelength ALMA observations of the Sz 91 disk to analyze dust properties, supporting models of grain growth and planetesimal formation via streaming instability in dust rings.

Contribution

It provides detailed observational constraints on dust size, optical depth, and spectral index in Sz 91, validating recent grain growth and planetesimal formation models.

Findings

Dust ring shows nearly constant spectral index (~3.34).

Maximum grain size estimated at ~0.61 mm.

Results align with models including fragmentation and streaming instability.

Abstract

One of the most important questions in the field of planet formation is how mm-cm sized dust particles overcome the radial drift and fragmentation barriers to form kilometer-sized planetesimals. ALMA observations of protoplanetary disks, in particular transition disks or disks with clear signs of substructures, can provide new constraints on theories of grain growth and planetesimal formation and therefore represent one possibility to progress on this issue. We here present ALMA band 4 (2.1 mm) observations of the transition disk system Sz 91 and combine them with previously obtained band 6 (1.3 mm) and 7 (0.9 mm) observations. Sz 91 with its well defined mm-ring, more extended gas disk, and evidence of smaller dust particles close to the star, is a clear case of dust filtering and the accumulation of mm sized particles in a gas pressure bump. We computed the spectral index (nearly constant at $\sim$3.34), optical depth (marginally optically thick), and maximum grain size ($\sim\,0.61$ mm) in the dust ring from the multi-wavelength ALMA observations and compared the results with recently published simulations of grain growth in disk substructures. Our observational results are in very good agreement with the predictions of models for grain growth in dust rings that include fragmentation and planetesimal formation through the streaming instability.