1999A&A...342..799A

The first overtone rotation-vibration transitions of SiO give rise to prominent bandheads in the wavelength range between 4.0 and 4.5µm. In order to study the behaviour of these features in AGB stars we observed the 3.94 to 4.12µm spectra for a sample of 23 oxygen-rich late-type variables. In contrast to the SRb objects, the Miras show a very large scatter of the equivalent widths of the SiO bands. Despite their cool temperatures some of them have only weak or no SiO absorption, which seems to be related to their strong pulsations producing a large variability of the features. When comparing the band intensities with photometric data, we found a general decrease with bluer IRAS (12-25) colors. However, this trend may only reflect the different behaviour of the Miras and SRb stars in our sample. We did not discover any correlation of the equivalent widths with the effective temperatures derived from (J-K), or with the (K-12) color and the IRAS-LRS class, both of which can be regarded as a rough measure for the thickness of the circumstellar shell. In Paper I of this series (Aringer et al., 1997A&A...323..202A) we have shown that synthetic spectra calculated from hydrostatic MARCS atmospheres are successful in reproducing the observed band intensities of giants with spectral types earlier than about M5III and M2II. However, they generally predict too strong features for very cool and extended objects, as they are discussed in this work. And they fail completely when it comes to Miras with weak or no SiO absorption. These stars are dominated by dynamical phenomena and, not surprisingly, they can therefore not be described by hydrostatic structures. Thus, we have also computed synthetic spectra based on experimental dynamical models. Although they still have some shortcomings, we demonstrate that, in principle, they are able to explain the whole range of equivalent widths of the observed SiO bandheads and their variations.