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| Crystalline silicates have been found in comets, in protoplanetary disks around young pre-main sequence stars, in debris disks around main-sequence stars, and in dust shells around evolved stars. Their infrared (IR) emission spectra indicate that they are predominantly magnesium (Mg)-rich and iron (Fe)-poor or even Fe-free. The Stardust cometary samples of crystalline silicate minerals are also predominantly Fe-poor. The Fe content of crystalline silicates not only affects the peak wavelength, width and strength of their vibrational features in the IR, but also affects their ultraviolet (UV) and visual absorptivities. Taking the young Herbig Ae star HD 142527 as a test case, we quantitatively demonstrate that, as a consequence of the latter effect, the crystallinity degree of silicate dust might have been substantially underestimated in various astrophysical regions, and the non-detection of the crystalline silicate emission features does not necessarily imply the absence of crystalline silicate dust; instead, crystalline silicates could be present but are just not heated sufficiently to emit at their characteristic IR vibrational bands. |
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| Keywords:Astrophysics; Interstellar medium; Interstellar dust; Circumstellar matter |
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