Crystalline Silicates in Circumstellar Dust Shells

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Scientific paper

The discovery of crystalline silicates outside our own Solar System by the infrared space observatory (ISO) in both young (Waelkens et al. 1996, A&A 315, L245), and evolved stars (Waters et al. 1996, A&A 315, L361) brought new inside in the circumstellar dust formation and evolution. We will present here an extensive overview of the solid state bands found in a sample of 17 stars all with oxygen-rich dust around them. For all stars good ISO-SWS (short wavelength spectrometer 2--45 μ m) spectra were available and for 12 stars also reliable ISO-LWS (long wavelength spectrometer 43--195 μ m) spectra were taken. We could identify about 50 different spectral features, most of them clustered into one of the 7 complexes (which we defined). Most bands could be identified with crystalline silicates and crystalline water ice, however still roughly 20% remains unidentified. An important result was that the presence of strong crystalline silicates bands always correlates with the presence of a disk like structure (N.B. The presence of a disk does not necessary imply a high fraction of crystalline silicates)(Molster et al. 1999, Nature 401, 563). We found that not only the strength but also the shape of the crystalline silicate features is different for sources with and without the presence of a disk. Another surprising result of this research is that the crystalline silicates contain no measurable amount of Fe. The main minerals found, are forsterite (Mg2 SiO4) and enstatite (MgSiO3). We have calculated mean crystalline silicate spectra for both the disk and the non-disk sources. By simple model fitting we derived estimates for the (relative) mass and temperature of the amorphous silicates, forsterite and enstatite. Based on these results we drew the conclusion that the crystalline and amorphous silicate grains are two separate grain populations. This work was part of a PhD-thesis and funded by NWO.

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