Astron. Astrophys. 363, 1115-1122 (2000)

1. Introduction

The spectra of both evolved and pre-main-sequence stars, observed by ISO, revealed the presence of crystalline silicates such as olivines and pyroxenes in their circumstellar environments (Waters et al. 1996; Justtanont et al. 1996; Waters et al. 1998; Waelkens et al. 1996; Malfait et al. 1998; Molster et al. 1999a,b). The crystalline silicates were also found in many comets. The strong 11.25 emission peak, a characteristic feature of crystalline olivine, has been detected in comets, e.g. comet Halley (Bregman et al. 1987; Campins & Ryan 1989), Bradfield 1987S (Hanner et al. 1990), Levy 1990XX (Lynch et al. 1992), Mueller 1993a (Hanner et al. 1994b) and comet Hale Bopp (Crovisier et al. 1997; Wooden et al. 1999). In addition to the crystalline olivines, the presence of Mg-rich crystalline pyroxene has been confirmed by the detection of the 9.3  feature on top of the broad amorphous silicate spectrum in Comet Hale-Bopp (C/1995 01) (Wooden et al. 1999). In the far infrared region, the ISO spectrum also showed many crystalline silicate features (Crovisier et al. 1997) in comet Hale-Bopp. These spectra are very similar to those observed in the young star HD 100546 (Malfait et al. 1998), and in the evolved star AC Her (Molster et al. 1999b).

These detections provide important clues about the origin and evolution of crystalline dust in the circumstellar shells around young and evolved stars. This opens the possibility to detect various kinds of solid materials such as melilite, diopside and anorthite, which are expected from condensation theory (Grossman 1972; Tielens et al. 1998).

The observations of ISO show that amorphous silicates coexist with crystalline silicates in dust shells around young and evolved stars (Malfait et al. 1998; Molster et al. 1999a). Amorphous silicates are characterized by two broad bands at about 10 and 18 . Their shape and position is not constant. The silicate emission feature, which has been observed in the spectrum of the Taurus dark cloud source Elias 1 is broader, the FWHM is 4.3 , and peaks at a longer wavelength (10.6 ) than the feature reproduced with the Trapezium emissivity (Hanner et al. 1994a).

In contrast to many observed spectra, laboratory data of crystalline silicates are rare in the literature (Steyer 1974; Koike et al. 1993; Koike & Shibai 1998; Jäger et al. 1998). In comparison with the measured spectra of olivine group with different Mg/Fe ratios, only two natural pyroxenes with complicated Ca-poor and Ca-rich compositions were measured (orthopyroxene and augite from Ichinome-gata; Koike et al. 1993). Interestingly, the spectra of the two natural pyroxenes show several sharp peaks around 9-11  and in the far-infrared region. Particularly in the far- infrared region the peak wavelengths are different from each other (Koike et al. 1993).

In this paper we present the absorption spectra of various pyroxenes with Ca-poor and Ca-rich compositions including synthetic and natural pyroxenes to study whether the 9.3  peak is generally common or not in pyroxenes. We also investigated systematic changes in the peak wavelengths according to the crystalline state and/or chemical composition.

With regard to synthetic pyroxenes, three kinds of pyroxenes, i.e. orthoenstatite , clinoenstatite , and diopside , were synthesized. These pure and high-quality crystalline samples are very important to study the correlation between the band shifts and chemical composition.

As for amorphous silicates, the measured spectra of the amorphous pyroxenes show different features; amorphous enstatite (Stephens & Russell 1979), amorphous bronzite (Dorschner et al. 1988), pyroxene glass (Jäger et al. 1994), and amorphous enstatite (Brucato et al. 1999) have broad bands at respectively 9.1 , 9.5 , 9.5 , and 9.89 . An amorphous pyroxene measured by us also shows a broad band at 9.5-9.7  (Koike & Tsuchiyama 1992). The feature in the spectrum of the pyroxene glass satisfactorily matches those observed in the spectra of the Orion Trapezium and massive young stellar objects (YSOs) in molecular clouds (Jäger et al. 1994). These amorphous pyroxene samples, except ours, show a broad band at about 17-19 . Our amorphous pyroxenes show a broad band at bout 22-23 .

In this work, we obtained new amorphous pyroxenes, synthesized by the quenching method. We also investigated the shift of the peak position of the 18  band after hydration of the amorphous pyroxenes. This study is important if we consider the history of cometary dust or interstellar/circumstellar dust especially based on the fact that mixture of amorphous silicates, crystalline silicates and water ice exist around young (Malfait et al. 1998) and evolved stars (Molster et al. 1999a; Lim et al. 2000).

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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