Interstellar chemistry: oxygen and its influence on complex molecule formation and complex molecules containing oxygen in dark clouds

Abstract

We extend and update our previous studies of interstellar chemistry, based on our standard model, with particular emphasis on the effect of adding ion–atomic oxygen reactions and also neutral–neutral atomic oxygen reactions. We include the formation of the series CnO and H2CnO up to n = 5, and investigate the formation of the, as yet undetected, cyclic molecule H3C4NO. The effect of adding ion–atomic oxygen reactions to our model is small. However, if the neutral–atomic oxygen reactions proceed efficiently in dark clouds, there is a substantial curtailment of production of complex species, even at early times, where agreement is best in such models. Indeed, if we ignore further addition of radiative associations to our model, we cannot account for almost all species more complex than two heavy atoms at any time in the life of the cloud, given the current estimates of rate constants. (Even with the optimistic inclusion of further radiative associations into the reaction scheme, it is still likely that significant curtailment to the build-up of complex species will occur.) Even if hydrogen atom abstraction reactions (HAARs) do proceed at a significant rate, it is still not possible to account for the observed abundances of complex molecules in dark clouds. We conclude that, if neutral–atomic oxygen reactions proceed efficiently, the production of complex species in observable amounts in dark clouds is not possible unless the addition of further radiative associations is found to be important in the formation of complex species. However, if the neutral–atomic oxygen reactions do not proceed efficiently, the species CnO and H2CnO up to n = 5 should at least be observable in dark clouds. It is of greatest importance to obtain laboratory data on the neutral–atomic oxygen reactions (and radiative association reactions) in order to understand how complex molecules are formed in molecular clouds. Perhaps the difficulties now revealed in explaining the formation of complex molecules by gas-phase reactions are an indirect line of evidence for contributions to interstellar chemistry coming from grain reactions.