On the origin of HNC in Comet Lee

We have modelled the chemistry occurring in the coma of Comet Lee and have critically evaluated the possible routes leading to HNC. We show that the o bserved levels of HNC cannot be produced by ion-molecule chemistry, or by r eactions of energetic H atoms with HCN. Rather, it appears that HNC is inje cted into the coma following the photodestruction of an unknown precursor. We discuss the possible nature of the parent of HNC and conclude that photo fragmentation of large HCN polymers, such as polyaminocyanomethylene (PACM) , is responsible. The degradation of hydrogen cyanide polymers may constitu te a common source of HNC in comets, accounting for HNC/HCN ratios in the r ange measured in Lee and Hyakutake (approximate to0.06-0.12). The high HNC/ HCN ratio measured in Hale-Bopp (approximate to0.2) and its heliocentric va riation may, however, require an additional source.