Infrared absorption band at 7 mum of protostar dusts- An explanation with magnesium

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An infrared absorption band at a wavelength of 7 mum was observed in embedded protostars. Differences of the band width are seen in these objects. Up to now, various materials are proposed to the origin of the band, i.e., hydrocarbons, methanol, photolyzed mixtures of CO_2, H_2, NH_3 etc, and carbonates. We examined again infrared absorption bands of carbonate materials, pure carbonate, and mixtures of carbonate with oxides and hydroxides.Pure MgCO_3 and FeCO_3 show a peak at 6.83-6.85mum and 6.93-6.94mum, respectively. The features are very similar tothe 7 mum band shape observed in W33A,however these carbonates show additional subpeaks around 11mum region, which have not observed in protostars.We synthesized carbonate-oxide-hydroxide mixtures by (1) grinding oxides in CO_2 atmosphere, and (2) heating of magnesium carbonate hydroxide under vacuum. The products show a very broad band around 7 mum which is matched to the band observed in Mon R2 #2. In addition, the peak height of the subpeaks around 11mum of these materialsare very small.The main result is summarized as follows. Carbonates are good candidates for the 7 mum band of protostars though their spectra have some additional bands.