The S[CLC]i[/CLC]C Problem: Astronomical and Meteoritic Evidence

Pre-solar grains of silicon carbide found in meteorites and interpreted ashaving had an origin around carbon stars from their isotopic composition, haveall been found to be of the beta-SiC polytype. Yet to date fits to the 11.3microns SiC emission band of carbon stars had been obtained only for alpha-SiCgrains. We present thin film infrared (IR) absorption spectra measured in adiamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiCand compare the results with previously published spectra taken using the KBrmatrix method. We find that our thin film spectra have positions nearlyidentical to those obtained previously from finely ground samples in KBr.Hence, we show that this discrepancy has arisen from inappropriate `KBrcorrections' having been made to laboratory spectra of SiC particles dispersedin KBr matrices. We re-fit a sample of carbon star mid-IR spectra, usinglaboratory data with no KBr correction applied, and show that beta-SiC grainsfit the observations, while alpha-SiC grains do not. The discrepancy betweenmeteoritic and astronomical identifications of the SiC-type is thereforeremoved. This work shows that the diamond anvil cell thin film method can beused to produce mineral spectra applicable to cosmic environments withoutfurther manipulation.Comment: to be published in Astrophysical Journal Letter 4 pages, 3 figure