HCN snowlines in protoplanetary disks: constraints from ice desorption experiments

TL;DR

This study experimentally characterizes the thermal desorption of HCN in protoplanetary disks, revealing its sublimation temperatures and trapping behavior, which has implications for prebiotic chemistry and planet formation.

Contribution

It provides the first detailed measurements of HCN binding energies and sublimation temperatures relevant to protoplanetary disks, enhancing understanding of HCN's astrochemical role.

Findings

HCN binding energies: 3207±197 K (HCN-HCN) and 4192±68 K (HCN-H2O)

HCN sublimation occurs 1-2 au outside the H2O snow line

Most HCN remains trapped in H2O-rich ices until H2O crystallizes

Abstract

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.