Molecules of non-photospheric origin in red giants and supergiants revealed by the ISO SWS

Overall SEDs based on ISO SWS observations show fair agreement with photospheric model predictions for red (super)giant stars. However, some details of molecular spectra cannot be explained by a photospheric origin. In particular, fine structure in the H₂O 2.μm band can be clearly resolved by the SWS and is identified in an early M giant, whose photosphere will never produce H₂O. This is definite evidence for H₂O of non-photospheric origin in an early M giant. Also, the observed H₂O and CO₂ bands in a late M giant are too strong to be explained by a photospheric origin alone. Further, the H₂O 2.7μm band is found in four early M supergiants in the h + χ Persei clusters (three of which show UIRs) and is especially strong in the M4 supergiant S Per (which also shows a highly peculiar SED). Thus, against a belief that H₂O is found only in the latest M giants such as Miras, the SWS has revealed the presence of H₂O in a wider region of the HR diagram. The origin of this H₂O is unknown but is probably in a non-photospheric extra envelope. Such a H₂O envelope appears to be a general feature through early M (super)giants to cool supergiants such as S Per, where the envelope has finally developed to be optically thick.