SiO rotation-vibration bands in cool giants - II. The behaviour of SiO bands nn AGB stars

The first overtone rotation-vibration transitions of SiO give rise to promi nent bandheads in the wavelength range between 4.0 and 4.5 mu m. In order t o study the behaviour of these features in AGE stars we observed the 3.94 t o 4.12 mu m 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 equ ivalent widths of the SiO bands. Despite their cool temperatures some of th em have only weak or no SiO absorption, which seems to be related to their strong pulsations producing a large variability of the features. When compa ring the band intensities with photometric data, we found a general decreas e 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 di scover any correlation of the equivalent widths with the effective temperat ures derived from (J-K), or with the (K-12) color and the IRAS-LRS class, b oth of which can be regarded as a rough measure for the thickness of the ci rcumstellar shell. In Paper I of this series (Aringer et al. 1997a) we have shown that synthet ic spectra calculated from hydrostatic MARCS atmospheres are successful in reproducing the observed band intensities of giants with spectral types ear lier than about M5 III and M2 II. However, they generally predict too stron g features for very cool and extended objects, as they are discussed in thi s 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 surp risingly they can therefore not be described by hydrostatic structures. Thu s, 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 o f the observed SiO bandheads and their variations.