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 and SC-type stars. We found several characteristics of molecular absorption features corresponding to the spectral types. The CH fundamental bands in 3-4 μm in N-type stars are stronger than those in SC-type stars. On the other hand, the first overtone bands of SiS at 6.6 μm were detected in WZ Cas (SC-type), while none of them were detected in N-type stars. These results can be explained well by the lower C/O ratio in SC-type stars than in N-type stars. The absorption features of HCN (v₁, V₂ + V₃ and v₁ - v₂) are stronger in SC-type stars than in N-type stars. This fact can be explained by the cooler atmospheres of our SC-type stars due to the lower C/O ratio as well 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.