Spirals, rings, and vortices shaped by shadows in protoplanetary discs: from radiative hydrodynamical simulations to observable signatures

ABSTRACT Numerous protoplanetary discs exhibit shadows in scattered light observations. These shadows are typically cast by misaligned inner discs and are associated with observable structures in the outer disc, such as bright arcs and spirals. Investigating the dynamics of the shadowed outer disc is therefore essential in understanding the formation and evolution of these structures. We carry out two-dimensional radiation hydrodynamics simulations that include radiative diffusion and dust–gas dynamics to study the formation of substructure in shadowed discs. We find that spiral arms are launched at shadow edges, permeating the entire disc. The local dissipation of these spirals results in an angular momentum flux, opening multiple gaps, and leading to a series of concentric, regularly spaced rings. We find that ring formation is favoured in weakly turbulent discs where dust growth is taking place. These conditions are met for typical Class II discs, in which bright rings should form well within a fraction of their lifetime (${\sim}$0.1–0.2 Myr). For hotter discs, gap opening is more efficient, such that the gap edges quickly collapse into vortices that can appear as bright arcs in continuum emission before decaying into rings or merging into massive, long-lived structures. Synthetic observations show that these structures should be observable in scattered light and millimetre continuum emission, providing a new way to probe the presence of substructure in protoplanetary discs. Our results suggest that the formation of rings and gaps is a common process in shadowed discs and can explain the rich radial substructure observed in several protoplanetary discs.