Determine whether compact protoplanetary disks contain small-scale substructures

Determine whether compact protoplanetary disks host radial substructures analogous to the rings and gaps observed in larger disks, or whether their dust distributions are primarily shaped by unimpeded radial drift.

Background

Compact protoplanetary disks with millimeter-continuum dust radii below approximately 20 au are common in ALMA surveys, but their physical origin remains poorly understood. Radial-drift models predict that dust grains migrate inward in disks with smooth pressure profiles, whereas rings and gaps in larger disks are commonly interpreted as pressure maxima that trap drifting pebbles.

The paper presents Sz 76 as a case study: high-resolution, sensitive ALMA observations reveal a faint outer dust ring beyond its compact central disk, reducing the inferred gas-to-dust size ratio and weakening the interpretation that compactness necessarily results from unimpeded radial drift. However, this single system does not resolve whether analogous substructures are a general property of compact disks or whether most compact disks are instead dominated by radial drift.