Thermochemical constraints on a primordial-origin of gas-rich debris disks

Abstract: Recent observations have revealed gas-rich debris disks around intermediate-mass stars at ages of tens of Myr. The origin of this gas remains unclear: it may be primordial, retained from the protoplanetary phase, or secondary, released from volatile-rich solids. Secondary-origin models reproduce CO emission but often overpredict neutral carbon. Recent observations and disk-evolution models suggest that primordial gas may survive longer than previously assumed, motivating thermochemical tests of the primordial-remnant scenario. We test the previously unexplored possibility that primordial-origin disks satisfy the observational constraints on gas-rich debris disks. Specifically, we determine under what conditions a disk around a $2\,M_{\odot}$ star reproduces substantial CO, low CI/CO ratios, and weak HCO+ emission consistent with current non-detections. We post-processed 20-40 Myr structures from 1D disk-evolution models with Cloudy, varying irradiation geometry, dust-to-gas mass ratio (DTG), and cosmic-ray ionisation rate. In the dust-poor models (DTG $=10<sup>{-4}$), CO remains optically thick around $R\sim100$~au. The models yield low disk-integrated CI/CO mass ratios. Our model produces CO radial intensities of the observed order of magnitude, but its CI-emitting region is more extended than observed. The standard CR model overproduces HCO+, whereas the weak CR model brings its predicted luminosity within current observational limits. These results demonstrate that a primordial origin remains chemically viable for CO-rich debris disks. The main remaining tension is the excessive radial extent of the CI emission, although it may reflect our simplified modelling. Further testing of the primordial-origin scenario will require multidimensional, self-consistent modelling, spatially resolved CI observations, and deeper searches for HCO+.