An SMA Survey of HNC Chemistry in Transition Disks

Planets are directly influenced by the chemical environments of the protoplanetary disks (PPDs) in which they form. Hydrogen cyanide (HCN) is a commonly-detected molecule in PPDs and has been widely used as a tracer of disk properties. However, its isomer, HNC, has rarely been targeted in PPDs, despite its frequent use in a variety of other astrophysical settings. Here, we present new Submillimeter Array observations of five transition disks, which include both the J=3–2 and J=4–3 lines of HCN and HNC. We detect at least one transition of HNC in four disks, more than doubling the total number of PPDs with HNC detections. From these observations, we measured disk integrated fluxes, which we used to determine the rotational temperatures and column densities for each molecule. We then explored the HNC/HCN column density ratio and how it relates to UV radiation and disk temperature. We compared measured HNC line intensities to previous modeling by Long et al. (2021) and found that our measured fluxes are significantly higher, indicating that an unexpectedly active HNC chemistry may be occurring in transition disks. Although tentative due to the small sample size, these trends provide strong motivation for the continued investigation of HNC beyond our pilot study.