Molecular observations of O- and C-rich circumstellar envelopes

Details Molecular observations of O- and C-rich circumstellar envelopes Molecular observations of O- and C-rich circumstellar envelopes

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...285..247b&link_type=abstract

Astronomy and Astrophysics 285, 247-271 (1994)

Astronomy and Astrophysics

Astrophysics

90

Stars: Abundances, Stars: Circumstellar Matter, Stars: Late Type, Stars: Agb, Post-Agb, Radio Lines: Stars

Scientific paper

We present observations of 10 microwave transitions of the molecules SiO, HCN, HNC, SiS, HC_3_N, CS, SO, and ^13^CO in a total sample of 47 evolved stars. The studied sources are mainly O- and C-rich standard AGB stars, as well as O-rich red supergiants, S-type stars, protoplanetary nebulae and detached envelopes. We also take into account observations of these transitions, as well as of ^12^CO J=1-0, from the literature. We find that the molecules can be classified as "O-like" (SiO and SO) and "C-like" (HCN, HNC, SiS, HC_3_N and CS). The emission of O-like molecules is significantly stronger in O-rich stars than in C-rich sources, and the opposite holds for C-like species. The behaviour of ^12^CO and ^13^CO is found to be intermediate between those of O-/C-like species. We have estimated the molecular abundances in most of the studied objects. Our results indicate that the abundance of the carbon-bearing molecules HCN, HNC and CS in O-rich circumstellar envelopes can only be explained assuming the presence of an active photochemistry in their outer layers. In other cases, such as HCN and CS in C-rich AGB stars, the derived abundances are compatible with LTE chemical models. The SiO and SiS abundances in O-rich and C-rich standard sources can also be explained by LTE chemistry if we assume moreover some depletion onto grains. We finally discuss the chemical properties of some individual interesting sources.

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