A Survey of C₂H, HCN, and C¹⁸O in Protoplanetary Disks

Abstract

Abstract Molecular lines observed toward 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 ( 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 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 versus extended and ringed versus 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.