SMA Observations Reveal Abundant HNC Chemistry in Transition Disks

The HCN–HNC isomer pair is a promising tracer of the temperature, elemental abundances, and irradiation environment in protoplanetary disks, but HNC's faintness has made it rarely observed, limiting its use. This study presents new SMA observations of five transition disks around T Tauri stars (GM Aur, J1604, LkCa 15, GG Tau, V4046 Sgr) in the J=3-2 and J=4-3 lines of HCN and HNC.

At least one line of both molecules was detected in every source, yielding disk-integrated HNC-to-HCN flux ratios of ≈0.1–0.7 and column density ratios of ≈0.1–0.4. Measured HNC fluxes exceed predictions from full (non-transition) disk models by ≈3–10×, while HCN fluxes appear normal, strongly linking the presence of a cavity to enhanced HNC production. Chemical models of the DM Tau transition disk confirm that HNC production and destruction are tied to the radiation environment.

Across the sample, the HNC-to-HCN column density ratio shows no trend with disk gas temperature but correlates positively with disk mass, attributed to larger reservoirs of cooler gas in more massive disks boosting HNC abundance. These results establish HNC as a valuable observational probe of transition disk structure and chemistry.