Circumstellar envelopes carbon chemistry

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

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effect on abundances, with the exception of HC3N and some protonated species (Glassgold et al. 1987, Millar 1987, unpublished). 

Many complex polycyanoacetylenes are observed in both circumstellar envelopes and dense molecular clouds, the heaviest being HCnN, one of the cyanopolyynes. As mentioned in the section on carbon chemistry, these... 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

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