Chemistry in a protoplanetary nebula

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Abstract
In this paper we investigate gas-phase chemistry in the remnant ‘superwind’ of a carbon-rich red giant star, during its transition to a planetary nebula. The interacting stellar winds model is used. It is found that during the first few hundred years of transition, significant abundances of a few small molecules and ions |$(\text {e.g}.\text{CH}^{+},\text{CH}_{2}^{+},\text{CH}_{3}^{+},\text{CH}, \text{CH}_{2},\text{NH})$| may occur in the thin, dense, shocked shell of gas predicted by this model, but that most molecules observed in protoplanetary nebulae will be rapidly destroyed, through photodissociation by strong UV from the central star. If dense clumps are present during transition, they may allow the gas-phase formation and/or survival of small amounts of some molecules, such as HCN, CN, C2H2, and HC3N, until about 2000 yr after termination of the superwind; and young, fully developed planetary nebulae may show observable amounts of polyatomic molecules by this means. Such clumping may explain the existence of, e.g., HCN in NGC 7027. Our results support the view that the large abundances of heavy molecules observed in young transition objects are either survivors from the red-giant superwind, or produced by non-gas-phase processes, such as desorption from grains. If they are relics of the red-giant era, we predict that the abundances of such species in CRL 618 will fall, perhaps on a time-scale of decades, and suggest that these be monitored.

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