The Characterization of the Dust Content in the Ring Around Sz 91: Indications of Planetesimal Formation?

Karina Maucó,Carlos Carrasco-González,Claudio Caceres,Anibal Sierra,Aina Palau,Hector Canovas,Matthias R Schreiber,Johan Olofsson,Amelia Bayo

doi:10.3847/1538-4357/ac21d0

Abstract

Abstract One of the most important questions in the field of planet formation is how millimeter- and centimeter-sized dust particles overcome 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 for 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 band 7 (0.9 mm) observations. Sz 91, with its well-defined millimeter ring, more extended gas disk, and evidence of smaller dust particles close to the star, constitutes a clear case of dust filtering and the accumulation of millimeter-sized particles in a gas pressure bump. We compute the spectral index (nearly constant at ∼3.34), optical depth (marginally optically thick), and maximum grain size (∼0.61 mm) in the dust ring from the multi-wavelength ALMA observations, and compare the results with recently published simulations of grain growth in disk substructures. Our observational results are in strong agreement with the predictions of models for grain growth in dust rings that include fragmentation and planetesimal formation through streaming instability.

Highlights

The most critical step in our understanding of the formation of terrestrial planets and giant planet cores is the assembly of kilometer-sized planetesimals from smaller dust particles (e.g. Johansen et al 2014)
We clearly resolved the disk ring structure, with the north side being brighter than the south side, something found in ALMA observations at 0.9 mm in Tsukagoshi et al (2019)
We estimated the radial profiles of the dust emission at different wavelengths by averaging emission in concentric elliptical rings with widths of 15 mas and with an eccentricity given by the disk inclination (49.7◦) and position angle (18.1◦) taken from the ALMA band 7 continuum image from Tsukagoshi et al (2019)

Summary

Introduction

The most critical step in our understanding of the formation of terrestrial planets and giant planet cores is the assembly of kilometer-sized planetesimals from smaller dust particles (e.g. Johansen et al 2014). The ground-breaking image of the HL Tau disk (ALMA Partnership et al 2015) and the results of the DSHARP program (Andrews et al 2018) revealed that ring-like dust sub-structures are ubiquitous in relatively young protoplanetary disks. These findings are complemented by the identification of transition disks that show dense dust rings around dust depleted cavities (e.g. Andrews et al 2011; Pinilla et al 2018; van der Marel et al 2018). In several cases these ring-like structures are accompanied by azimuthal asymmetries (e.g. Casassus et al 2013; van der Marel et al 2021), spiral arms (e.g. Christiaens et al 2014; Huang et al 2018a), or small and possibly inclined inner disks deduced from NIR observations (e.g. Marino et al 2015) or using ALMA data (e.g. Pérez et al 2018; Francis & van der Marel 2020)

Results

Discussion

Conclusion