Compact CO emission and no evidence of radial drift

Context. A large fraction of the planet-forming disks surveyed by ALMA show faint CO emission, which is commonly interpreted as an indication of severe CO depletion. However, disks can be faint for multiple reasons, including by having their emission unresolved spatially, which may result in their size being overestimated, making their flux appear faint. The limited sensitivity of previous observations prevented us from determining whether this scenario can indeed account for the observed faint CO emission for radially compact disks, hindering our understanding of disk evolution and planet formation in most of the disk population. Aims. We present new ALMA observations targeting 12CO (J = 3–2) and 13CO (J = 3–2) in 17 of the faintest planet-forming disks in Lupus. We aim to test the feasibility of the compact disk scenario as a plausible explanation for compact disks with faint CO isotopolog emission. Methods. Our sample contains 17 disks observed with ALMA in Band 7 at the moderate angular resolution of 0⋅′′25 (≈20 au radius at 160 pc, the median distance of the sample), approximately one order of magnitude deeper than the available archival ALMA data where 12CO and 13CO were not detected. We used line stacking techniques to enhance the signal-to-noise ratio and extract the CO fluxes when possible. Finally, we compared the CO line luminosities with a grid of physical-chemical models of extended and compact disks and computed the disk dust and CO sizes. Results. We detected 12CO and 13CO emission in ten disks. Four disks were detected only in 12CO, and three disks were not detected in either of the two isotopologs. Conclusions. The observations indicate that some of these disks are consistent with being intrinsically compact and optically thick, in both 12CO and 13CO. This scenario offers an alternative explanation to the commonly accepted hypothesis of significant CO depletion. The derived gas radii further support this interpretation (RCO ≤ 40 au), suggesting that a significant fraction of disks may be born intrinsically small, as is also indicated by recent Class 0/I surveys. Furthermore, the resulting gas-to-dust size ratios reveal no clear signs of dust evolution, suggesting that these compact disks are not drift-dominated.