Among the different molecules appearing in the atmospheres of cool oxygen-rich stars SiO is particularly interesting. Due to its high dissociation energy of 8.26 eV (Sauval & Tatum 1984) it can be found over a wide range of stellar temperatures and compared with other molecular species it is very abundant. It also represents the main condensate for the solid component in the circumstellar shells around late red giants (Gail & Sedlmayr 1986). This silicate dust produces the well known 10 µm emission observed in the IRAS LRS spectra of AGB stars (Joint Iras science working group 1986). Since the absorption features of SiO are mainly situated in the mid infrared, it may work as an efficient cooling agent (e.g. Muchmore et al. 1987) and therefore have a significant influence on the structure of stellar atmospheres. This is especially true for the hotter stars with effective temperatures between 3600 and 4600 K, because SiO is among the first molecules to be formed, whereas in the cooler objects the opacities are dominated by such species as water and TiO. However, the importance of SiO for the atmospheric cooling was recently doubted by Cuntz & Muchmore (1994).
The most important rotation-vibration bands of SiO are situated around 8 µm () and 4 µm (). Especially the first overtone features can be easily accessed by ground based telescopes, since they extend into the photometric L-band. High resolution FTS spectra covering the corresponding spectral region are available for several stars allowing a detailed analysis of single lines (e.g. Ridgway et al. 1984, Tsuji et al. 1994). Nevertheless, this method is restricted to the brightest objects only. Rinsland & Wing (1982) have measured the and bands of a large sample of stars at a very low resolution in order to study the intensity as a function of temperature and atmospheric extension. They found that the SiO absorption is well correlated with these two parameters and may even be used as a spectral classification criterion. As they mention, this is not true for some of the latest giants, where the SiO bands show intense variations, which can not be simply explained by stellar temperature changes. Such variations have also been observed by Hinkle et al. (1976). Aringer et al. (1995) investigated the behavior of the first overtone bands in Semiregular and Mira variables using a cooled grating spectrograph at a medium resolution. They detected that despite their low temperatures many Mira stars have only a very weak or no SiO absorption, whereas all Semiregulars show strong bands. The authors interpret the weakening of the SiO lines as a consequence of strong stellar pulsations. A correlation with temperature or properties of the circumstellar dust shells could not be found.
The purely rotational lines of SiO can be observed in the radio spectra of mass-losing late giants, where they appear in thermal as well as in maser emission (Olofsson 1988). While the thermal emission originates from the outer regions of circumstellar envelopes (e.g. Sahai & Bieging 1993) the maser lines contain information about the base of stellar winds, where the matter is accelerated and dust formation takes place (Elitzur 1992), thus providing valuable informations about these processes.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998