Compact or large? CO observations of the faintest planet-forming disks

Abstract

Planet-forming disks observed by ALMA surveys often exhibit surprisingly faint continuum and CO emission, raising doubts about whether these disks contain enough material to account for the known exoplanet population. Despite this, the fainter end of the disk population - which shows compact, unresolved continuum emission and non-detections in CO isotopologues - has received little detailed investigation. It remains unclear whether this is due to faint but spatially extended emission or intrinsically compact disk structures. Distinguishing between these scenarios is crucial: if such disks are indeed compact, including their gaseous components, and optically thick, their inner regions could harbor significant reservoirs of material, potentially capable of forming gas giants within Jupiter's orbital radius. In this poster we present new ALMA data that target 13CO (3-2) and 12CO (3-2) lines in 18 CO-faint Lupus disks, probing the gaseous component of the faintest planet-forming disks. If the observations confirm that these disks are radially compact and optically thick, we could imply a substantial planet-forming capacity within 10 au in a significant fraction of Lupus disks. Furthermore, if these disks are indeed compact, they challenge widely accepted theories of disk evolution, such as viscous evolution and MHD-driven processes, which cannot account for such small outer radii. Since Lupus is a young, low-density star-forming region, external truncation processes are also unlikely to explain these compact gaseous structures, further emphasising the need for a revised understanding of disk evolution.