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

Silicate minerals belong to the most abundant solids in space. Their formation becomes difficult 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 oxygen and carbon are consumed by CO and SiO molecule formation, and left-over oxygen to build the SiO4-tetrahedra of silicates becomes scarce. Then SiO molecules may directly condense into solid SiO. The IR absorption spectrum of solid SiO differs from that of silicates by the absence of Si-O-Si bending modes at 18 mum while the absorption by Si-O bond stretching modes at 10 mum is present. Such characteristics are observed in a number of S-star spectra. We suggest that this observation may be explained by formation of solid SiO as a major dust component at C/O abundance ratios close to unity. We determine the IR absorption properties of solid SiO by laboratory transmission measurements of thin SiO films produced by vapour deposition on a Si(111) wafer. From the measured spectra the dielectric function of SiO is derived. The results are used in model calculations of radiative transfer in circumstellar dust shells with solid SiO dust. Comparison of synthetic and observed spectra shows that reasonable agreement is obtained between the main spectral characteristics of emission bands due to solid SiO and an emission band centred on 10 mum, but without accompanying 18 mum band, observed in some S-stars. We propose that solid SiO is the carrier material of this 10 mum spectral feature.