The presence of water in the early M giants was once noticed more than 35 years ago (Woolf et al. 1964), but this important discovery has not been understood properly and overlooked for a long time. We had to wait ISO to confirm the presence of water in normal red giant stars including the late K and early M giant stars. By hindsight, this may partly be due to confusion not to have realized the difference of the photosphere and the atmosphere. For example, stellar photosphere could be modeled rather easily but it has often been referred to as a model atmosphere. However, what we had this way is only a model photosphere and we have no self-consistent model atmosphere of red giant stars yet. What Stratoscope II suggested and what ISO finally unveiled is that the infrared spectra of red giant stars involve a new problem that cannot be represented by the photospheric model, and that the atmosphere and the photosphere should be clearly distinguished. Certainly a more unified understanding of the stellar atmosphere should be required to properly interpret the infrared spectra of red giant stars.
Acknowledgements
I thank I. Yamamura for making available his fine tools in applying OSIA, T. Tanabé for helpful advice on OSIA, and the anonymous referee for valuable comments. This work is supported by the Grant-in-Aid for Scientific Research of the Ministry of Education & Science No. 11640227.
Copyright ESO 2001