Infrared spectra of carbon stars observed by the ISO SWS: II. HCN and C₂H₂ bands at 14 μm

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 2ν₂⁰-ν₂¹ 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 2ν₂⁰ 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 ν₅ 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.