STEPS TOWARD INTERSTELLAR SILICATE MINERALOGY - IV - THE CRYSTALLINE REVOLUTION

Mid- and far-infrared spectra gained by the Short Wavelength Spectrome ter (SWS) of the Infrared Space Observatory (ISO) satellite have provi ded striking evidence for the presence of crystalline silicates in com ets, circumstellar envelopes around young stars and, most of all, evol ved stars and planetary nebulae. Since optical properties of astrophys ically relevant crystalline silicates are lacking in the literature, i n this paper mass absorption coefficients (MACs) of olivines and pyrox enes for a wide range of Mg/Fe ratios are presented, which cover the w hole ISO wavelength range. The MAC have been derived from transmission spectra of small grains embedded in potassium bromide and polyethylen e pellets. Only in the case of natural enstatite (MgSiO3), was a monoc rystalline sample available, which allowed the measurement of optical constants for the different crystallographic orientations of this anis otropic silicate. Since not all Mg/Fe ratios are represented among the natural minerals, we supplemented the series by synthetic products pr epared in our lab. We also included two inhomogeneous synthetic materi als, one of olivine and the other one of pyroxene composition, which a re expected to be similar to the primary condensate in cosmic environm ents. For all samples the chemical composition, the purity, and the ho mogeneity have been determined by energy-dispersive X-ray analysis and by scanning electron microscopy. Especially for the minerals, it is i mportant to exclude the spectral influence of differently composed inc lusions. The peak positions of the samples are influenced by different factors which are discussed: chemical composition (FeO content), size and shape distribution of the grains, and the matrix in which the gra ins are embedded for spectroscopy. The continuum-subtracted ISO SWS sp ectrum of the source AFGL 4106 has been compared with simple optically thin model spectra calculated for our olivine and pyroxene samples. T he main result was that a combination of the pure magnesium silicates (forsterite and enstatite) gives a good agreement between observations and laboratory measurements.