Cosmic carbon chemistry interstellar clouds

In this review we have attempted to show that the circumstellar envelopes of cool, late-type stars possess a rich chemistry which is similar in many respects to that occurring in interstellar clouds. In carbon-rich envelopes, cosmic-rays and ultraviolet photons drive a chemistry dominated by ion-molecule reactions and photo-reactions. Such a chemistry has been applied to the envelope of IRC-l-10216 and has been shown to reproduce the observations extremely well. In oxygen-rich envelopes these processes also occur but the presence of large amounts of OH make neutral chemistry more important. In both cases the effects of ion-dipolar collisions has little effect on abundances, with the exception of HC3N and some protonated species (Glassgold et al. 1987, Millar 1987, unpublished). 

What effect do shocks have on the gas phase synthesis of complex interstellar molecules This question has been investigated at least for hydrocarbons through six carbon atoms in complexity by Mitchell (1983, 1984). He has found that if a shock passes through a dense cloud where much of the carbon is already in the form of carbon monoxide, complex hydrocarbons are not formed in high abundance. However, if a shock passes through a diffuse cloud, of density approximately 103 cm-3, where much of the cosmic abundance of carbon is in the form of C+ and to a lesser extent C, a different scenario is present. As the shock cools, the C+ and C, which remain in appreciable abundance for up to 10s yrs after the shock passage, react via many of the reactions discussed above as well as others to produce a rich hydrocarbon chemistry. The net effect is that large abundances of hydrocarbons build up as the cloud cools and eventually reaches a gas density of 3 x 104 cm-3. Do these results bear any relation to the results obtained from ambient gas phase models In both types of calculations, hydrocarbon chemistry appears to require the presence of C+ and/or C both to synthesize one-carbon hydrocarbons such as methane and then, via insertion reactions, to produce more complex hydrocarbon species. Condensation reactions do not appear to be sufficient. 

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