Organic synthesis in the coma of Comet Hale-Bopp?

Several organic molecules have now been detected in the coma of Hale-Bopp. These species may either emanate from the nucleus, or, as has been suggested by Bockelée-Morvan et al., could be synthesized in the coma. We have modelled the gas phase chemistry which occurred in the coma of Hale-Bopp, concentrating on the observed organic molecules HCOOH, HCOOCH₃, HC₃N and CH₃CN. We find that gas phase chemical reactions are unable to synthesize the observed abundances of these molecules, so all these species are most probably present in the nuclear ice. We briefly discuss the implications of this result for the connection between cometary and interstellar ices.