2019ApJ...876...25B

Molecular lines observed toward protoplanetary disks carry information about physical and chemical processes associated with planet formation. We present ALMA Band 6 observations of C₂H, HCN, and C¹⁸O in a sample of 14 disks spanning a range of ages, stellar luminosities, and stellar masses. Using C₂H and HCN hyperfine structure fitting and HCN/H¹³CN isotopologue analysis, we extract optical depth, excitation temperature, and column density radial profiles for a subset of disks. C₂H is marginally optically thick (τ ∼ 1-5) and HCN is quite optically thick (τ ∼ 5-10) in the inner 200 au. The extracted temperatures of both molecules are low (10-30 K), indicative of either subthermal emission from the warm disk atmosphere or substantial beam dilution due to chemical substructure. We explore the origins of C₂H 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 C₂H emission features (e.g., compact versus extended and ringed versus ringless emission). We explore the chemical relationship between C₂H, HCN, and C¹⁸O and find a positive correlation between C₂H and HCN fluxes but no relationship between C₂H or HCN with C¹⁸O fluxes. We also see no evidence that C₂H and HCN are enhanced with disk age. C₂H 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.