We report the detection of a feature at 65 μm and a broad feature around 100 μm in the far-infrared spectra of the diffuse emission from two active star-forming regions, the Carina Nebula and Sharpless 171. The features are seen in the spectra over a wide area of the observed regions, indicating that the carriers are fairly ubiquitous species in the interstellar medium. A similar 65 μm feature has been detected in evolved stars and attributed to diopside, a Ca-bearing crystalline silicate. The present observations indicate the first detection of a crystalline silicate in the interstellar medium if this identification also holds true for the interstellar feature. A similar broad feature around 90 μm reported in the spectra of evolved stars has been attributed to calcite, a Ca-bearing carbonate mineral. The interstellar feature seems to be shifted to longer wavelengths and have a broader width, although the precise estimate of the feature profile is difficult. As a carrier for the interstellar 100 μm feature, we investigate a possibility that the feature originates from carbon onions, grains consisting of curved graphitic shells. Because of the curved graphitic sheet structure, the optical properties in the direction parallel to the graphitic plane interact with those in the vertical direction in the carbon onion grains. This effect enhances the interband transition feature in the direction parallel to the graphitic plane in the carbon onions, which is suppressed in the graphite particles. Simple calculations suggest that carbon onion grains are a likely candidate for the observed 100 μm feature carrier, but the appearance of the feature is sensitive to the assumed optical properties.
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