The HNC/HCN Ratio in Comet Hale-Bopp

F.P. Schloerb, W.M. Irvine (FCRAO/UMass), D. Jewitt (IfA/UHawaii), H.E. Matthews (JAC), M. Senay (FCRAO/UMass)

The question of whether the early Solar System preserved any chemical memory of its parent molecular cloud has important consequences for understanding the physical and chemical processes which modulated the formation of the Solar Nebula. In announcing the first detection in a comet of hydrogen isocyanide (HNC), in C/1996 B2, Irvine et al. (1996) noted that the similarity in the cometary HNC/HCN abundance ratio to that found in quiescent interstellar molecular clouds raised the possibility that HNC might be a chemical tracer of surviving interstellar material. They also pointed out that, alternatively, the observed HNC might be produced by photochemical processes, in which case the HNC/HCN ratio would be expected to vary with heliocentric distance. In order to discriminate between these possibilities, we have been observing HNC and HCN in Comet Hale-Bopp (C/1995 O1), both pre- and post-perihelion. Observations of the J=4-3 transitions of both isomers, with some observations of the corresponding 13-C isotopomer of HCN, are being carried out at the JCMT, while the J=1-0 line of HCN is being monitored at the FCRAO. Because the rotational constants and electric dipole moments of the closed-shell species HNC and HCN are very similar, the ratio of intensities observed for a given transition will closely approximate the ratio of abundances, apart from possible effects of optical depth. We shall present a progress report on this analysis, including a table of observed line ratios vs. heliocentric distance.

This research was supported in part by NASA grants NAGW-1483 and NAGW-436 and NSF grant AST-9420159.

Irvine, W.M. et al. (1996), Nature, 383, 418.