We have computed a grid of model atmospheres and spectra focusing on the effects and appearance of the SiO bands. It was shown that the latter never have a large influence on the atmospheric structure, at least as long as one stays within the framework of classical hydrostatic calculations using equilibrium chemistry. In addition we have demonstrated that the intensity of the absorption features monotonously increases with lower temperature and gravitational acceleration as well as with higher metallicity. Thus, from the models we expect the strongest bands in the coolest and most extended objects, whereas they will almost disappear in dwarfs with and in giants with , if solar chemical abundances are assumed.
When we compared our results to existing observations of K and early M giants, we found that there is a good agreement indicating the correctness of the adopted line data and atmospheres. Nevertheless, the calculations fail completely in predicting the band intensities of AGB objects. We think that this is caused by the fact that our classical model atmospheres can not always be applied to this kind of stars and some basic assumptions like hydrostatic and chemical equilibrium may not work anymore as it is indicated by the observed strong variability of the SiO absorption. In addition atmospheric inhomogenities (e.g. Olofsson et al. 1996) and SiO emission might become important. Some of the aspects will be discussed in the second paper of this series, where we will focus on our observations of AGB stars and the impact of dynamical phenomena like stellar pulsation, shock waves, mass loss and dust formation based on existing models (e.g. Höfner & Dorfi 1997) and our synthetic spectra.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998