Hydrogen cyanide in comet C/1996 B2 Hyakutake

Spectral emission from HCN in C/1996 B2 Hyakutake was detected on UT 1996 March 24.4 (rh = 1.06 AU, Δ = 0.106 AU), using the CSHELL infrared spectrometer at the NASA Infrared Telescope Facility. A Boltzmann analysis of eight ro‐vibrational lines in the ν3 band returned a rotational temperature (83 ± 9 K) for a region centered on the nucleus. The global HCN production rate was (4.50 ± 0.81) × 1026 molecules s−1. The HCN abundance relative to water is then (0.18 ± 0.04)%, based on direct measurements of H2O made on the same night with the same instrument and reduced with the same data processing algorithms. The measured spatial distribution for HCN is consistent with its release at the nucleus; no significant contribution from a distributed source is required within ∼600 km of the nucleus. We use these data to obtain insights regarding the origin of HCN in this comet.