Introduction

One of the aims of this thesis was to compile a database of mineral spectra relevant to stardust. These spectra come from a variety of sources and have been adjusted so that they are equivalent to extinction coefficient spectra, K. The minerals of interest can be divided into two groups, like the stars that produce them, carbon- and oxygen- rich. The carbon-rich species are mostly organic molecules and inorganic carbides. The most important species are amorphous carbon, graphite and SiC and are discussed in both section 3.2 and chapter 6. The oxygen rich minerals, discussed here, are far more varied. The types of minerals expected to form around oxygen-rich stars can be divided roughly into oxides and silicates. Silicates can be further divided into olivines, pyroxenes and miscellaneous silicates for the purposes of this study.

The laboratory spectra of oxygen-rich minerals in this catalogue have been divided into three groups: olivines, pyroxenes, and oxides. The miscellaneous silicates were deemed to be of little use in the identification of circumstellar dust as many were hydrated or were minerals expected due to terrestrial metamorphism. Olivines and pyroxenes are the principal forms of silicates. The fundamental unit in the construction of a silicate is the SiO₄-tetrahedron in which the silicon atom (or more strictly, ion) is situated at the centre of a tetrahedron whose corners are occupied by four oxygen atoms. Classification of silicates is based on the different ways in which the SiO₄-tetrahedra occur, either separately or linked together.

From the work of Gilman (1969), Hackwell (1971), Tielens (1990) and Pégourié & Papoular (1985) the expected condensation sequence of dust around oxygen-rich AGB stars is approximately:

Anorthite is not expected to form in circumstellar shells due to the unlikelihood of the required density and temperature conditions being met. Forsterite is expected to use corundum and augite as nucleation centres as the high density, low temperature conditions required for forsterite nucleation are improbable. Whether the transformation of forsterite into enstatite by gas-grain reactions occurs depends on the ambient conditions, i.e. whether there is enough time for the reactions to take place and a solid-gas interface for the reactions to take place on. (Grossman 1972; Larimer 1979).

Given that we have theoretical models for the dust types we expect to find around oxygen-rich M stars, it seems judicious to look at the laboratory spectra of these minerals. One of the problems in identifying which minerals are contributing to the astronomical spectra is that we cannot be sure of the crystalline/amorphous state of the dust grains. A ``smooth'' 10µm feature was discovered in the spectra of oxygen-rich stars and was attributed to amorphous silicate dust around those stars (e.g. Woolf & Ney 1969; see Chapter 8). Likewise, more recently evidence has been found for crystalline silicates around such stars in ISO-SWS spectra (e.g. Waters et al. 1996; Waelkens et al. 1996). The current discussion of laboratory spectra therefore includes both crystalline and amorphous samples. There have been various published laboratory spectra of the relevant materials. Lists of the laboratory spectra we have collated can be found in Tables 5.1,  5.2 &  5.3. Spectra for the 7.5-13.5µm region (except Figs. 5.17 and  5.18 which cover the 7.5-24.5µm region) are shown in figs. 5.8 to  5.20

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