Infrared spectra of carbon stars observed by the ISO SWS - II. HCN and C2H2 bands at 14 mu m

We analyzed the mid-infrared spectra of 4 optical carbon stars and 4 candid ates of infrared carbon stars obtained with the ISO SWS. It has been reveal ed that special care should be taken in determining the continuum levels fo r the analysis of the emission and absorption bands of HCN and C2H2 at 14 m u m; otherwise the SIC emission at 11 mu m as well as molecular absorption at 7.5 mu m may lead to the misidentification of spectral features. In the spectra of the two optical carbon stars TX Psc and V CrB, we detecte d the emission of the HCN 2 nu(2)(0)-nu(2)(1), band, which is direct eviden ce for the existence of HCN in their circumstellar envelopes. The excitatio n is almost due to radiative pumping, i.e. HCN molecules in the ground leve l are pumped to the 2 nu(2)(0) level by 7 mu m photon from the photosphere or from the inner envelope. Since this emission band was detected in the sp ectrum 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 star s over a wide range of the optical thickness of circumstellar envelopes. On the other hand, the absorption features due to the C2H2 nu(5) bands at 1 3.7 mu m were detected in all of the sources except for TX Pac. The absorpt ion features turned out to be quite broad in the spectra of optical carbon stars. This broad absorption is attributed not only to the a branches at 13 .7 mu m but also to the P and R branches between 12 and 16 mu m. These feat ures 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 car bon-rich nature of these objects. These absorption features would be formed in the inner envelope where the mid-infrared radiation originates.