Interstellar chemistry: hot-ion reactions

Summary We have explored the possible significance on interstellar chemistry of translationally excited ions (‘hot ions’) produced in exothermic reactions, focusing on weaknesses that remain in existing gas-phase models of cloud chemistry. Particular instances are the lack of success in accounting for observed abundances of NH3, N2H+ and cyanopolyacetylenes. When ‘hot-ion’ reactions are included in the ion-molecule model we obtain predicted abundances in cold clouds like TMC-1, agreeing very well with observations (to better than one order of magnitude) for virtually all smaller molecules included in the model. In particular, the discrepancies for NH3, N2H+ and cyanopolyacetylenes no longer arise. This occurs in the time regime 106.3–106.5 yr [note that this is not the time where the abundance of complex species go through a maximum (˜ 105.5 yr) but somewhat later] and not for very old clouds (age > 107.5 yr). If we use rate constants for hydrogen atom abstraction reactions based on current estimates of their activation energies, then the ‘hot-ion’ reactions do not lead to a noticeable increase in the production of longer chain hydrocarbons. However, for smaller values of these activation energies (for example, those that might make the rate constants around 10−11 cm3 s−1), such hot-ion reactions could dramatically increase the efficiency of carbon-chain building by gas-phase reactions. Therefore, these hot-ion processes may ultimately prove to be the basis of the build-up of these larger species in cold clouds. If the build-up of long chains is to be attributed to the effect of these hot-ion reactions, then the unexpectedly gradual decline in the abundances of CnH, with increasing n, is readily explained. It seems plausible to attribute the irregular variation in these abundances to the enhanced rate of ion-dipolar processes as compared with ion-non-polar reactions, although such influences are more pronounced at greater cloud ages (> 107.5 yr).