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Infrared spectra of carbon stars observed by the ISO SWS *
II. HCN and C2H2 bands at 14 µm
W. Aoki 1,2,
T. Tsuji 3 and
K. Ohnaka 3,4
Received 3 May 1999 / Accepted 5 August 1999
We analyzed the mid-infrared spectra of 4 optical carbon stars and 4 candidates of infrared carbon stars obtained with the ISO SWS. It has been revealed that special care should be taken in determining the continuum levels for the analysis of the emission and absorption bands of HCN and C2H2 at 14 µm; otherwise the SiC emission at 11 µm as well as molecular absorption at 7.5 µm may lead to the misidentification of spectral features.
In the spectra of the two optical carbon stars TX Psc and V CrB, we detected the emission of the HCN 2- band, which is direct evidence for the existence of HCN in their circumstellar envelopes. The excitation is almost due to radiative pumping, i.e. HCN molecules in the ground level are pumped to the 2 level by 7 µm photon from the photosphere or from the inner envelope. Since this emission band was detected in the spectrum of an Lb variable (TX Psc), a Mira variable (V CrB) and an infrared carbon star (IRC+10216, Cernicharo 1998), it is quite common in carbon stars over a wide range of the optical thickness of circumstellar envelopes.
On the other hand, the absorption features due to the C2H2 bands at 13.7 µm were detected in all of the sources except for TX Psc. The absorption features turned out to be quite broad in the spectra of optical carbon stars. This broad absorption is attributed not only to the Q branches at 13.7 µm but also to the P and R branches between 12 and 16 µm. These features in the optical carbon stars are basically explained by the absorption in the photosphere or in the warm envelope close to the star. The detection of the C2H2 absorption in our infrared sources definitely confirms the carbon-rich nature of these objects. These absorption features would be formed in the inner envelope where the mid-infrared radiation originates.
Key words: stars: AGB and post-AGB stars: atmospheres stars: carbon stars: circumstellar matter stars: late-type infrared: stars
* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.
Send offprint requests to: W. Aoki (email@example.com)
Online publication: October 14, 1999