ISO-SWS spectra of the carbon stars TX Psc, V460 Cyg, and TT Cyg ?

During our open and discretionary time ISO pro- grams we have observed a number of carbon-rich and oxygen- rich AGB stars. We present here a hydrostatic analysis of 3 carbon rich stars, TX Psc, V460 Cyg, and TT Cyg, which show only modest variability. We identify absorption features of the molecules C2, CN, CH, CO, CS, HCN, C2H2, and C3 in the three stars. The relative intensities of the corresponding bands put strict limits on the possible values of Te , log g, and C/O. In particular the ratio of the intensity of the 3 m band (due to HCN and C2H2) and the 5 m band (due to CO and C3) is a sensitive measure of the C/O ratio. We show that our model atmospheres and corresponding synthetic spectra are able to reproduce the observed spectra quite accurately from the visual region through the infrared out to approximately 10 m. Beyond 10m the flux in the observed spectra increases compared to the computed spectra, and the photospheric 14 m band dominated by C2H2 and HCN is al- most absent in the observed spectra whereas it is very strong in the synthetic ones. This discrepancy is not an artefact in the reduction or in the SWS response function, but a physical phe- nomenon in the stars, which does not appear in oxygen-rich giants. We exclude a so-called "warm molecular envelope" as an explanation of the discrepancies, and suggest that the flux excess at long wavelengths can be interpreted as clumps of 500 K dense material which obscures about 10% of the photosphere. Weak spectral features seen around 13.7 m can be understood as even cooler gas, rich in C2H2, above the clumps. Other possi- ble models to explain the long wavelength discrepancy between the computed and observed spectra need to be explored too.