title: The HNC/HCN ratio in comets.
abstract: The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4-3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced in significant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus.
authors.affiliation: Five College Radio Astronomy Observatory, University of Massachusetts, Amherst 01003, USA.
authors.forename: W M
authors.initials: WM
authors.lastname: Irvine
authors.forename: J E
authors.initials: JE
authors.lastname: Dickens
authors.forename: A J
authors.initials: AJ
authors.lastname: Lovell
authors.forename: F P
authors.initials: FP
authors.lastname: Schloerb
authors.forename: M
authors.initials: M
authors.lastname: Senay
authors.forename: E A
authors.initials: EA
authors.lastname: Bergin
authors.forename: D
authors.initials: D
authors.lastname: Jewitt
authors.forename: H E
authors.initials: HE
authors.lastname: Matthews
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D003486:Cyanides / Q000032:analysis* / Q000737:chemistry; D005118:Extraterrestrial Environment; D006856:Hydrogen Cyanide / Q000032:analysis* / Q000737:chemistry; D007536:Isomerism; D019110:Meteoroids; D008956:Models, Chemical; D010777:Photochemistry; D016082:Solar System
chemical_list: D003486:Cyanides; D006856:Hydrogen Cyanide