The SiC problem: Astronomical and meteoritic evidence

Presolar grains of silicon carbide, found in meteorites and interpreted as having had an origin around carbon stars from their isotopic composition, h ave all been found to be of the beta-SiC polytype. Yet, to date, fits to th e 11.3 mu m SiC emission band of carbon stars had been obtained only for al pha-SiC grains. We present thin-film infrared (IR) absorption spectra that were measured in a diamond-anvil cell for both the alpha- and beta-polymorp hs of synthetic SiC, and we compare the results with previously published s pectra that were taken using the KBr matrix method. We find that our thin-f ilm spectra have positions nearly identical to those obtained previously fr om finely ground samples in KBr. Hence, we show that this discrepancy has a risen from inappropriate "KBr corrections" having been made to laboratory s pectra of SiC particles dispersed in KBr matrices. We refitted a sample of carbon star mid-IR spectra, using laboratory data with no KBr correction ap plied, and show that beta-SiC grains fitted the observations while alpha-Si C grains did not. The discrepancy between meteoritic and astronomical ident ifications of the SiC type is therefore removed. This work shows that the d iamond-anvil cell, thin-film method can be used to produce mineral spectra that are applicable to cosmic environments without further manipulation.