A high-resolution survey of protoplanetary disks in Lupus and the nature of compact disks

Aims. Most of the exoplanets discovered in our Galaxy to date orbit low-mass stars, which tend to host small disks in their early stages. To better elucidate the link between planet formation and disk substructures, observational biases should be reduced through observations of these small, faint disks at the highest resolution using the Atacama Large Millimeter Array (ALMA). Methods. We present new high-resolution (0.03–0.04″) ALMA observations at 1.3 mm of 33 disks located in the Lupus star-forming region that have total dust continuum fluxes of <25 mJy. Combining archival data and previously published work, we provide a near-complete high-resolution image library of 73 protoplanetary (Class II) disks in the Lupus. This enabled us to measure dust disk radii down to a limit of 0.6 au and analyze intensity profiles using visibility modeling. Results. We show that 67% of Lupus protoplanetary dust disks have dust radii smaller than 30 au and characterize the newly discovered substructures in 11 disks with some of the shortest separation gaps. The size–luminosity relation, when accounting for the smallest disks, aligns well with a drift-dominated dust evolution scenario for the Class II Lupus disks. For the most compact disks, with radii of less than 30 au, we compared measured sizes and fluxes with a grid of radiative transfer models to derive millimeter-emitting dust masses, which ranged from 0.3 to 26.3 M⊕. Assuming that the detected substructures were dynamical effects of planets, we approximated the results of an interpolation to estimate planet masses and found a range of 20–2000 M⊕ with separations between 2 and 74 au. Conclusions. Our results indicate that two-thirds of the protoplanetary disks in Lupus are smooth, on scales larger than 4 au, and compact, with substructures being more prominent in the few larger disks. These compact disks are consistent with drift-dominated evolution, with their masses and optical depths suggesting that they may have already experienced some planet formation, with most of the small solids converted into planetesimals and planets. This makes them prime candidates, with optimal conditions, for explaining the formation and origin of super-Earths.