A ring-like concentration of mm-sized particles in Sz 91

Models of planet formation and disc evolution predict a variety of observables in the dust structure of protoplanetary discs. Here, we present Atacama Large Millimeter/submillimeter Array (ALMA) Band-6 and Band-7 observations of the transition disc Sz 91 showing that the continuum emission at 870 mu m, which is dominated by emission from large dust grains, is localized in an optically thin narrow ring. We find that most of the emission (similar to 95 per cent) is concentrated in a ring located at 110 au from the central star that is only about 44 au wide. In contrast, the (CO)-C-12 (2-1) emission peaks closer to the star and is detected up to similar to 488 au from the star. The concentration of large grains in a ring-like structure while the gas disc extends much further in and further out is in qualitative agreement with predictions of hydrodynamical models of planet-disc interactions including radial drift and gas drag.