SIO ROTATION-VIBRATION BANDS IN COOL GIANTS .1. A GRID OF MODEL SPECTRA FOR DIFFERENT STELLAR PARAMETERS

In order to study the behavior of the SiO bands of cool oxygen-rich st ars we have computed a grid of 138 hydrostatic model atmospheres using an improved version of the MARCS code and new SiO line data. We found that the corresponding opacity never produces large changes of the at mospheric structure, especially if one compares it with the enormous e ffects from TiO or water. Based on the atmospheres, synthetic SiO spec tra have been calculated with a high resolution for the photometric L- band and with a low resolution (opacity sampling) for the large wavele ngth range between 2.0 and 12.5 mu m, which includes the fundamental a s well as the first and second overtone transitions. Thus, we could st udy the behavior of the SiO bands as a function of different stellar p arameters. It turned out that the intensity of the absorption features monotonously increases with lower temperature and gravitational accel eration as well as with higher metallicity. We expect the strongest ba nds in the coolest and most extended objects. When we compared our res ults to observations of K and M giants, we found a good agreement show ing the correctness of the line data and atmospheres. On the other han d the calculations can not reproduce the relatively weak and variable bands of AGE stars indicating that our classical model atmospheres may not always work for this kind of objects. This will be discussed in a second paper, where we will focus on our observations of AGE stars.