Infrared spectra of carbon stars observed by the ISO SWS - I. Molecular absorption in N-type and SC-type stars

We carried out observations of bright optical carbon stars by the ISO SWS, and detected the absorption features of CO, CS, CH, SiS and HCN in N-type a nd SC-type stars. We found several characteristics of molecular absorption features correspon ding to the spectral types. The CH fundamental bands in 3-4 mu m in N-type stars are stronger than those in SC-type stars. On the other hand, the firs t overtone bands of SIS at 6.6 mu m were detected in WZ Cas (SC-type), whil e none of them were detected in N-type stars. These results can be explaine d well by the lower C/O ratio in SC-type stars than in N-type stars, The ab sorption features of HCN (nu(1), nu(2) + nu(3) and nu(1) - nu(2)) are stron ger in SC-type stars than in N-type stars. This fact can be explained by th e cooler atmospheres of our SC-type stars due to the lower C/O ratio as wel l as to the lower effective temperature, compared with the N-type stars in our sample. The CS first overtone bands and the band heads of the CO fundamental bands are weaker in the observed spectra than in the ones predicted by our model atmospheres. Though the fundamental bands of CS were detected in TX Psc (N- type), the band heads are quite weak, and none of the fundamental bands of CS were identified in the other stars. These results suggest a contribution by the emission of CO and CS in the outer atmosphere.