2000A&A...356..253J

During our open and discretionary time ISO programs 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 T_eff, logg, and C/O. In particular the ratio of the intensity of the 3µm band (due to HCN and C₂H₂) and the 5µm 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 10µm. Beyond 10µm the flux in the observed spectra increases compared to the computed spectra, and the photospheric 14µm band dominated by C₂H₂ and HCN is almost 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 phenomenon 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 ≃500K dense material which obscures about 10% of the photosphere. Weak spectral features seen around 13.7µm can be understood as even cooler gas, rich in C₂H₂, above the clumps. Other possible models to explain the long wavelength discrepancy between the computed and observed spectra need to be explored too.