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 C _ { 2 } H to constrain the C/O ratio in the molecular layer to be \sim 0.8 , i.e. , higher than the solar value of \sim 0.54 .
We use archival ALMA observations of HCN and H ^ { 13 } CN to show that no depletion of N is required ( assuming an interstellar abundance of 7.5 \times 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 mid-plane .
Similarly , a fraction of the available C is locked up in less volatile molecules .
By contrast , N remains largely unprocessed , likely as N _ { 2 } .
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 , C _ { 2 } H , 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 .