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Class III stars are those in star forming regions without largenon-photospheric infrared emission, suggesting recent dispersal of theirprotoplanetary disks. We observed 30 class III stars in the 1-3 Myr Lupusregion with ALMA at ${\sim}856\mu$m, resulting in 4 detections that weattribute to circumstellar dust. Inferred dust masses are$0.036{-}0.093M_\oplus$, ${\sim}1$ order of magnitude lower than any previousmeasurements; one disk is resolved with radius ${\sim}80$ au. Two class IIsources in the field of view were also detected, and 11 other sources,consistent with sub-mm galaxy number counts. Stacking non-detections yields amarginal detection with mean dust mass ${\sim}0.0048M_\oplus$. We searched forgas emission from the CO J=3-2 line, and present its detection to NO Lupinferring a gas mass ($4.9 {\pm} 1.1$) ${\times}10^{-5} M_\oplus$ andgas-to-dust ratio $1.0{\pm}0.4$. Combining our survey with class II sourcesshows a gap in the disk mass distribution from $0.09{-}2M_\oplus$ for${>}0.7M_\odot$ Lupus stars, evidence of rapid dispersal of mm-sized dust fromprotoplanetary disks. The class III disk mass distribution is consistent with apopulation model of planetesimal belts that go on to replenish the debris disksseen around main sequence stars. This suggests that planetesimal belt formationdoes not require long-lived protoplanetary disks, i.e., planetesimals formwithin ${\sim}$2 Myr. While all 4 class III disks are consistent withcollisional replenishment, for two the gas and/or mid-IR emission couldindicate primordial circumstellar material in the final stages ofprotoplanetary disk dispersal. Two class III stars without sub-mm detectionsexhibit hot emission that could arise from ongoing planet formation processesinside ${\sim}1$ au.

ALMA Survey of Lupus Class III Stars: Early Planetesimal Belt Formation and Rapid Disk Dispersal