  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\pm197 K and 4192\pm68 K, which translate to disk
midplane sublimation temperatures around 85 K 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.
