2025ApJ...986...86C

Annular substructures serve as ideal venues for planetesimal formation. In this series, we investigate the linear stage of dust growth within rings. The first paper in this series examines the global streaming instability, while this study focuses on the dusty Rossby wave instability (DRWI). To this end, we perform a linear analysis of the two-fluid equations on a background pressure bump, representing annular substructures. The spectral code DEDALUS is used to solve the linear eigenvalue problem. We identify two distinct DRWI modes: Type I, which originates from dust-modified gas RWI, and Type II, which results from dust-gas coupling. Type I and Type II modes never coexist for a given azimuthal wavenumber k_y, but transition between each other as k_y varies. Type I modes are driven by the advection of background vorticity, and Type II modes possess two waves: Rossby waves, driven by advection, and thin waves, driven by dust-gas drag. Finally, we assess the relevance of DRWI in Atacama Large Millimeter/submillimeter Array (ALMA) rings using DSHARP sources. Our findings suggest that Type I modes could explain the absence of azimuthal asymmetries in many ALMA disks, whereas Type II modes are entirely absent in all eight observed rings, implying that unresolved narrow rings or alternative mechanisms may play a role in dust growth within annular substructures.