Complex Molecules in Carbon-Rich Circumstellar Envelopes

Circumstellar molecular envelopes - Spectroscopy and structure

“…However, since the molecule is radiatively excited, one cannot infer a total gas density from the observations so, as for CO, the absolute HCN abundance is not well known. HCN has been detected in more than 20 C-rich CSEs, but in only a few O-rich cases (Olofsson 1987). H 2 0 is found as maser emission (in the 22.3-GHz line) in ~ 150 O-rich envelopes, but in none of the C-rich objects.…”

“…This agreement suggests that the reaction h 2 cn + + c 2 h 2 h 2 c 3 n + + h 2 followed by dissociative recombination to yield HC 3 N may be an important pathway to producing HC 3 N in the inner envelope, where H 2 CN + is produced from reactions initiated by cosmic rays. Since HC 3 N is the simplest of the cyanopolyynes (HC X N), whose remarkably large abundances have been a puzzle (see Olofsson 1987), these results suggest that cosmic-ray-initiated chemistry may be important in forming the longer carbon chains as well.…”

“…This paper reviews our understanding of the circumstellar molecular envelopes of evolved, late-type stars in terms of their envelope chemical composition and physical structure. There have been several good reviews of this general topic in the last few years (e. g., Glassgold and Huggins 1987;Knapp 1987;Masson 1987;Morris 1985;Olofsson 1987;Omont 1985Omont , 1987Zuckerman 1980Zuckerman , 1987 so in this paper I will emphasize the results of recent observations and theoretical work on the subject, especially with the new instruments for millimeter wavelengths which provide significant improvements in sensitivity and angular resolution compared with past work. I will deal largely with the microwave "thermal" line emission of molecules in the envelopes, and will largely neglect masers, which are worth a separate review (see the article by J.…”