Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aas...20111504c&link_type=abstract
American Astronomical Society, 201st AAS Meeting, #115.04; Bulletin of the American Astronomical Society, Vol. 34, p.1290
Astronomy and Astrophysics
Astronomy
Scientific paper
Cool 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 μ m) and CO2 (15 μ m). 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 μ m (HCN) and both W Aql and S Cas at 13.7 μ m (C2H2). Both W Aql and S Cas as well as S Lyr, a SC-type star, show 3 μ m 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.
Carbon Duane F.
Chiar Jean E.
Goorvitch David
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