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Protoplanetary Disk Rings as Sites for Planetesimal Formation

Axisymmetric dust rings are a ubiquitous feature of young protoplanetary disks. These rings are likely caused by pressure bumps in the gas profile; a small bump can induce a traffic jam-like pattern in the dust density, while a large bump may halt radial dust drift entirely. The resulting increase in dust concentration may trigger planetesimal formation by the streaming instability (SI), as the SI itself requires some initial concentration. Here we present the first 3D simulations of planetesimal formation in the presence of a pressure bump modeled specifically after those observed by ALMA. In particular, we place a pressure bump at the center of a large 3D shearing box, along with an initial solid-to-gas ratio of $Z = 0.01$, and we include both particle back-reaction and particle self-gravity. We consider both mm-sized and cm-sized particles separately. For simulations with cm-sized particles, we find that even a small pressure bump leads to the formation of planetesimals via the streaming instability; a pressure bump does {\it not} need to fully halt radial particle drift for the SI to become efficient. Furthermore, pure gravitational collapse via concentration in pressure bumps (such as would occur at sufficiently high concentrations and without the streaming instability) is not responsible for planetesimal formation. For mm-sized particles, we find tentative evidence that planetesimal formation does not occur. This result, if it holds up at higher resolution and for a broader range of parameters, could put strong constraints on where in protoplanetary disks planetesimals can form. Ultimately, however, our results suggest that for cm-sized particles, planetesimal formation in pressure bumps is an extremely robust process.