Turbulence and dust fragility in protoplanetary discs

Abstract

Dust growth from micron- to planet-size in protoplanetary discs involves multiple physical processes, including dust collisions, the streaming instability, and pebble accretion. Disc turbulence and dust fragility matter at almost every stage. Previous studies typically vary one of them while fixing the other, failing to provide a complete picture. Here, we use analytical models and numerical dust evolution models DustPy to study the combinations of gas turbulence and dust fragility that can reproduce multi-wavelength ALMA observables. We find that only appropriate combinations -- fragile dust (fragmentation velocity $v_\mathrm{frag}$= 1-2 m/s) in discs with viscous $\alpha=10^{-4}$ or resilient dust ($v_\mathrm{frag}$= 6-10 m/s) in discs with viscous $\alpha=10^{-3}$ -- can reproduce observations. Our result is robust to two widely used opacities (DSHARP and Ricci opacities). Regardless of the strength of disc turbulence, reproducing observations requires observed dust rings to be optically thick at $\lambda=1.3$ and $3$ mm. As only small dust can be lifted above the midplane to reach the emitting layers, SED analysis probably yields lower limits on the maximum grain sizes. We highlight the challenge of creating detectable dust rings at large radii when incorporating bouncing in models, and the need for earlier formation of dust rings at smaller radii to reproduce the decreasing ring brightness with radius observed across ALMA wavelengths.