Cyanide chemistry in comet Hale-Bopp (C/1995 O1)

Observations of comet Hale-Bopp (C/1995 O1) have been carried out near perihelion (1997 March) at millimeter wavelengths using the NRAO 12 m telescope. The J = 1 → 0, 2 → 1, and 3 → 2 lines of HCN at 88, 177, and 265 GHz were measured in the comet as well as the J = 3 → 2 lines of H¹³CN, HC¹⁵N, and HNC. The N = 2 → 1 transition of CN near 226 GHz was also detected, and an upper limit was obtained for the J = 2 → 1 line of HCNH⁺. From the measurements, column densities and production rates have been estimated. A column density ratio of [HCN]/[HNC] = 7 ± 1 was observed near perihelion, while it was found that [HCN]/[HCNH⁺] ≳ 1. The production rates at perihelion for HCN and CN were estimated to be Q(HCN) ∼ 1 × 10²⁸ s^-1 and Q(CN) ∼ 2.6 × 10²⁷ s^-1, respectively, resulting in a ratio of [HCN]/[CN] ≃ 3. Consequently, HCN is sufficiently abundant to be the parent molecule of CN in Hale-Bopp, and HCNH⁺ could be a source of HNC. Finally, carbon and nitrogen isotope ratios of ¹²C/¹³C = 109 ± 22 and ¹⁴N/¹⁵N = 330 ± 98 were obtained from HCN measurements, in agreement with previous values obtained from J = 4 → 3 data. Such ratios suggest that comet Hale-Bopp formed coevally with the solar system.