2018ApJ...865..155C

We present new constraints on gas-phase C, N, and O abundances in the molecular layer of the IM Lup protoplanetary disk. Building on previous physical and chemical modeling of this disk, we use new ALMA observations of C₂H to constrain the C/O ratio in the molecular layer to be ∼0.8, i.e., higher than the solar value of ∼0.54. We use archival ALMA observations of HCN and H¹³CN to show that no depletion of N is required (assuming an interstellar abundance of 7.5 x 10^–5 per H). These results suggest that an appreciable fraction of O is sequestered in water ice in large grains settled to the disk midplane. Similarly, a fraction of the available C is locked up in less volatile molecules. By contrast, N remains largely unprocessed, likely as N₂. This pattern of depletion suggests the presence of true abundance variations in this disk, and not a simple overall depletion of gas mass. If these results hold more generally, then combined CO, C₂H, and HCN observations of disks may provide a promising path for constraining gas-phase C/O and N/O during planet-formation. Together, these tracers offer the opportunity to link the volatile compositions of disks to the atmospheres of planets formed from them.