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Astron. Astrophys. 328, 419-425 (1997)

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1. Introduction

A variety of IRAS sources have been reported recently having an, as yet unidentified, emission feature near 21 microns (Kwok et al. 1989; Chan et al. 1995). Many of these sources are intermediate-mass, carbon-rich stars in a post-AGB evolutionary stage on their way to becoming planetary nebulae (Hrivnak, Kwok, and Volk 1989). Suggested carriers of the 21 micron feature include [FORMULA] (Goebel 1993), polycyclic aromatic hydrocarbons (PAHs) (Buss et al. 1990; Hrivnak et al. 1989) and iron oxides (Cox, 1990). Although the spectra of a wide range of PAHs and several forms of iron oxide are available in the literature from the UV to the IR (e.g. Salisbury et al. 1988; Donn et al. 1989; Clar, 1964 for PAHs or Steyer, 1974; Digiorgi et al. 1987; Henning et al. 1995 for iron oxides), the spectrum of [FORMULA] is only available in the range from 12.5-120 microns (Nuth et al. 1985; Begemann et al. 1996). Within the range of the aforementioned studies [FORMULA] exhibits one major peak at a wavelength near 21 microns, as well as a moderately strong feature near 17 microns.

The purpose of the present study is to extend the range of the previous measurements of [FORMULA] into the mid-infrared and to investigate possible variations in the spectra of samples due to different morphology or crystal structure and temperature. We will present spectra covering the range from 5-25 microns of [FORMULA] samples obtained from three different suppliers having three distinct appearances and two distinct crystallographic natures measured at both room temperature and 600 K. In addition to the previously known features near 21 and 17 microns, we have now characterized several additional features in the interval 7-13 microns which may serve to test the hypothesis that [FORMULA] solids are present in the spectra of some carbon-rich infrared sources.

In what follows we will first describe the natures of the materials used in this study and the methods used to obtain the reported spectra. We will then present the spectra and discuss the variations observed in those spectra in the context of diagnostic indicators suitable for comparison with observation. These discussions naturally lead to suggestions for additional observations.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998