The SiO molecule in the atmospheres and circumstellar envelopes of AGB stars

In order to investigate the behavior of the SiO bands as a function of different stellar parameters and properties, synthetic spectra based on grids of hydrostatic atmospheres as well as on time sequences of dynamical models have been calculated. The theoretical results have been compared to different low, medium and high resolution data of cool giants. In the case of warmer more compact objects the SiO features predicted by hydrostatic atmospheres are in good agreement with the observations, whereas the very weak absorption or emission bands seen in extended cold AGB variables can only be explained by dynamical models. At a high resolution, the SiO spectra of AGB stars show quite complicated line profiles with shifted emission and absorption components produced by the density and velocity structure of the corresponding atmospheres. This behavior could be simulated by dynamical models. We also found that the presence of SiO never has a considerable effect on the atmospheric structure. (author)