AAS 201st Meeting, January, 2003
Session 115. Cool Star Atmospheres and Envelopes
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[115.04] Detection of HCN and C2H2 in ISO Spectra of Oxygen-Rich AGB Stars

D.F. Carbon, J.E. Chiar, D. Goorvitch (NASA Ames Research Center)

Cool \emph{oxygen-rich} AGB stars were not expected to have organic molecules like HCN in either their photospheres or circumstellar envelopes (CSEs). The discovery of HCN and CS microwave emission from the shallowest CSE layers of these stars was a considerable surprise and much theoretical effort has been expended in explaining the presence of such organics. To further explore this problem, we have undertaken a systematic search of oxygen-rich AGB stellar spectra in the Infrared Space Observatory (ISO) data archive. Our purposes are to find evidence regarding critical molecular species that could be of value in choosing among the proposed theoretical models, to locate spectral features which might give clues to conditions deeper in the CSEs, and to lay the groundwork for future SIRTF and SOFIA observations.

Using carefully reduced observations, we have detected weak absorption features arising from HCN and possibly C2H2 in a small number of oxygen-rich AGB stars. The most compelling case is NML~Cyg which shows both HCN (14~\mum) and CO2 (15~\mum). VY~CMa, a similar star, shows evidence for HCN, but not CO2. Two S-type stars show evidence for the C-H bending transitions: W~Aql at 14~\mum (HCN) and both W~Aql and S~Cas at 13.7~\mum (C2H2). Both W~Aql and S~Cas as well as S~Lyr, a SC-type star, show 3~\mum absorption which may arise from the C-H stretch of HCN or C2H2.

In the case of NML~Cyg, we show that the HCN and CO2 spectral features are formed in the CSE at temperatures well above those of the outermost CSE layers and derive approximate column densities. In the case of the S-stars, we discuss the evidence for the organic features and their photospheric origin.

We gratefully acknowledge support from NASA ADP NRA-99-01-ADP-073, NASA LTSA NRA-399-20-61-02, and the NASA Advanced Supercomputing Division at NASA Ames Research Center.

The author(s) of this abstract have provided an email address for comments about the abstract: dcarbon@nas.nasa.gov

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