Laboratory measurement of optical constants of solid SiO and application to circumstellar dust

Context. Silicate minerals belong to the most abundant solids that form in cosmic environments. Their formation requires that a sufficient number of oxygen atoms per silicon atom are freely available. For the standard cosmic element mixture this can usually be taken for granted, but it becomes a problem at the transition from the oxygen-rich chemistry of M-stars to the carbon-rich chemistry of C-stars. In the intermediate type S-stars, most of the oxygen and carbon is consumed by formation of CO and SiO molecules, and left-over oxygen to build SiO4-tetrahedrons in solids becomes scarce. Under such conditions SiO molecules from the gas phase may condense into solid SiO. The infrared absorption spectrum of solid SiO differs from that of normal silicates by the absence of Si-O-Si bending modes around 18 μm whereas the absorption band due to Si-O bond stretching modes at about 10 μm is present. Observations show that exactly this particular characteristic can be found in some S-star spectra.