Astron. Astrophys. 339, 904-916 (1998)

1. Introduction

Before the Infrared Space Observatory (ISO, Kessler et al. 1996) opened the mid- and far-infrared range for high-resolution spectroscopy, it was generally assumed that cosmic dust silicates were of amorphous structure. Exceptions were the cometary dust (Hanner et al. 1994, Hanner 1996), interplanetary dust particles (IDPs, Mackinnon & Rietmeijer 1987, Bradley et al. 1992), dust disks of Pictoris-type around main-sequence stars (Knacke et al. 1993, Fajardo-Acosta & Knacke 1995), and the deeply embedded young stellar object AFGL 2591 (Aitken et al. 1988). This assumption was apparently confirmed by the shapes of the observed silicate vibrational bands at about 10 and 19 µm (Dorschner & Henning 1986, 1995, Dorschner 1997) as well as by non-equilibrium condensation experiments providing "chaotic silicates" (Nuth 1996). Therefore, laboratory studies were mainly concentrated on the glassy silicates of supposed cosmic abundances, e.g. in the Jena Laboratory Astrophysics Group efforts have been focused on producing, characterizing, and measuring glasses with the aim to provide their optical constants over the whole astrophysically relevant wavelength range (Jäger et al. 1994, Dorschner et al. 1995, Mutschke et al. 1997). In addition, measurements of optical properties at low temperatures were performed (Henning & Mutschke 1997).

However, when the first ISO SWS spectra became available, one of the most surprising discoveries was the finding of clear evidence of crystalline silicates. The presence of at least a portion of crystalline silicates was found in the spectra of comet Hale-Bopp (Crovisier et al. 1997), circumstellar dust around Herbig Ae/Be stars (Waelkens et al. 1996), low-mass evolved stars with high mass loss including planetary nebulae (Waters et al. 1996; Justtanont et al. 1996) and Luminous Blue Variables (LBVs) (Waters et al. 1997).

In contrast to chaotic silicates and silicate glasses, crystalline silicates show a lot of diagnostic bands due to metal-oxygen vibrations in the mid-infrared (MIR) range beyond 17 µm. This promises at least the possibility to derive values of the Mg/Fe ratio of crystalline silicates. Generally, we would learn more about the formation process of the dust grains and the connection between amorphousness and crystallinity of stardust silicates.

The crystalline silicates mark their presence by narrow features on top of the continuum. Prominent emission and absorption features have been found in ISO spectra around 10.1, 11.2, 13.8, 16.3, 19.5, 21.5, 23.7, 27.9, 33.6, 35.5, 36.5, 40.5, 43.0, and 69.5 µm (Molster et al. in prep.), marking the presence of crystalline olivines and pyroxenes. Not in every case are all of these features visible, which may be explained by temperature effects, i.e. the convolution of the absorption coefficient with a Planck function can suppress features in spectral regions where the emitted radiation is low. The wavelengths of the peak positions can slightly change from source to source by 0.1-0.3 µm, while the widths of the bands vary more drastically. Both types of variation may be due to varying Fe/Mg-ratios within the same silicate type, blending of different silicates, the degree of crystallization and size/shape effects of the particles.

The spectral resolution of the ISO-SWS AOT01 spectra is relatively high for dust spectroscopy (). In order to fully exploit the information on cosmic dust silicates contained in these ISO spectra, laboratory data of adequate spectral resolution covering the entire ISO wavelength range are urgently needed. Unfortunately, such spectral data of Mg-Fe silicates are very rare in the literature (see, e.g., Steyer 1974, Koike et al. 1993, Hofmeister 1997). It is the aim of this paper to provide additional data of crystalline olivine- and pyroxene-type silicates, which cover the whole wavelength range important for the interpretation of ISO spectra, and to study the influence of the Fe/Mg ratio on band positions and strength ratios. This investigation contains synthetic samples of the series end members MgSiO₃, Mg₂SiO₄, and Fe₂SiO₄ as well as natural orthopyroxenes and olivines (enstatite, bronzite, hypersthene, olivine, and hortonolite). We have measured the whole Mg/Fe compositional range for the olivines, but were only able to cover a part of this range for the pyroxenes. The spectroscopy is supplemented by a careful analytical characterization of the samples.

In the course of the characterization procedure of the samples, we found several pure and homogeneous silicates that can be used as reference materials for spectroscopy. Natural minerals often contain phase separations and inclusions of completely different materials which, in extreme cases, could dominate the "silicate" spectrum.

We also investigated synthetic silicates that were effectively quenched so that inhomogeneities developed. Rapid cooling of homogeneous melts of olivine and pyroxenes provides phase separation consisting of an SiO₂-rich and a metal oxide-rich component, comparable to the aforementioned "chaotic" silicates (Rietmeijer et al. 1986, Nuth 1996) suggested to be the primary nucleation products in the multi-component gas of oxygen-rich circumstellar envelopes.

Sect. 2 gives an overview about the composition and structure of crystalline silicates in the solid solution series of pyroxenes and olivines. Sect. 3 describes the exact chemical composition of the synthetic and natural silicates and other results of analytical studies of the samples. Sect. 4 deals with the results of the sample spectroscopy in the MIR and far infrared (FIR), discusses the spectral features and compares the results with previous measurements. We will also present optical constants of the natural enstatite determined from reflection measurements. In Sect. 5 the laboratory data are compared to the new ISO results on "crystalline" features, and other astrophysical implications are discussed.

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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