Molecular lines observed towards protoplanetary disks carry information about physical and chemical processes associated with planet formation .
We present ALMA Band 6 observations of C _ { 2 } H , HCN , and C ^ { 18 } O in a sample of 14 disks spanning a range of ages , stellar luminosities , and stellar masses .
Using C _ { 2 } H and HCN hyperfine structure fitting and HCN/H ^ { 13 } CN isotopologue analysis , we extract optical depth , excitation temperature , and column density radial profiles for a subset of disks .
C _ { 2 } H is marginally optically thick ( \tau \sim 1–5 ) and HCN is quite optically thick ( \tau \sim 5–10 ) in the inner 200 AU .
The extracted temperatures of both molecules are low ( 10–30K ) , indicative of either sub-thermal emission from the warm disk atmosphere or substantial beam dilution due to chemical substructure .
We explore the origins of C _ { 2 } H morphological diversity in our sample using a series of toy disk models , and find that disk-dependent overlap between regions with high UV fluxes and high atomic carbon abundances can explain a wide range of C _ { 2 } H emission features ( e.g .
compact vs. extended and ringed vs. ringless emission ) .
We explore the chemical relationship between C _ { 2 } H , HCN , and C ^ { 18 } O and find a positive correlation between C _ { 2 } H and HCN fluxes , but no relationship between C _ { 2 } H or HCN with C ^ { 18 } O fluxes .
We also see no evidence that C _ { 2 } H and HCN are enhanced with disk age .
C _ { 2 } H and HCN seem to share a common driver , however more work remains to elucidate the chemical relationship between these molecules and the underlying evolution of C , N , and O chemistries in disks .