HCN Snow Lines in Protoplanetary Disks: Constraints from Ice Desorption Experiments

HCN is among the most commonly detected molecules in star- and planet-forming regions. It is of broad interest as a tracer of star formation physics, a probe of nitrogen astrochemistry, and an ingredient in prebiotic chemical schemes. Despite this, one of the most fundamental astrochemical properties of HCN remains poorly characterized: its thermal desorption behavior. Here, we present a series of experiments to characterize the thermal desorption of HCN in astrophysically relevant conditions, with a focus on predicting the HCN sublimation fronts in protoplanetary disks. We derive HCN-HCN and HCN-H₂O binding energies of 3207 ± 197 and 4192 ± 68 K, which translate to disk midplane sublimation temperatures around 85 and 103 K. For a typical midplane temperature profile, HCN should only begin to sublimate ~1-2 au exterior to the H₂O snow line. Additionally, in H₂O-dominated mixtures (20:1 H₂O:HCN), we find that the majority of HCN remains trapped in the ice until H₂O crystallizes. Thus, HCN may be retained in disk ices at almost all radii where H₂O-rich planetesimals form. This implies that icy body impacts to planetary surfaces should commonly deliver this potential prebiotic ingredient. A remaining unknown is the extent to which HCN is pure or mixed with H₂O in astrophysical ices, which impacts the HCN desorption behavior as well as the outcomes of ice-phase chemistry. Pure HCN and HCN:H₂O mixtures exhibit distinct IR bands, raising the possibility that the James Webb Space Telescope will elucidate the mixing environment of HCN in star- and planet-forming regions and address these open questions.