1994A&A...292..641J

Although extrasolar silicates were detected more than 25 years ago, important questions concerning chemical composition, material properties, and grain structure are still without reliable answers. The most important of these questions are listed at the beginning of this paper because they play decisive roles as guide-posts for the silicate research program of the Jena laboratory astrophysics group. This paper communicates the first results of this program aimed at a closer mineralogical characterization of the interstellar/circumstellar silicates that have been observed in different types of objects. In this first approach, pyroxene glass samples, the cation content of which reflects mean cosmic proportions of the four most abundant metals, have been prepared and analytically characterized. They are expected to be good candidates for matching the silicate spectra of star-forming regions and young stellar objects (YSOs). For the pyroxene glass, optical constants from 250nm to 500µm have been determined. Particles having sizes within the Rayleigh limit show broad bands peaking at 9.5 and 18.8µm. For the sake of comparison, a crystalline sample of the same composition was also measured. Its narrow bands are positioned at 9.4, 10.5, 11.1, 13.7, 15.6, 18.1, 19.5, 26.5, 29.5, 37.5, and 49µm in agreement with expectations for a chemical composition corresponding to hypersthene. In addition to the vibration bands weak crystal field bands at 1 and 2µm due to Fe²⁺ have also been detected for the pyroxene glass. If these bands were detectable in interstellar and circumstellar sources they would offer a unique possibility of discriminating the pyroxene-type from the olivine-type silicates. The FIR absorption coefficient measured for the glass sample turned out to be proportional to λ^–2. The centroids of the 10 and 19µm bands of the pyroxene glass satisfactorily match those observed in the Orion Trapezium and massive YSOs in molecular clouds. While the mean 10µm band profile derived from observations of six massive YSOs is excellently fitted by profiles calculated with the pyroxene glass data for Rayleigh grains, the Orion Trapezium profile cannot be satisfactorily represented by analogously calculated profiles. Broadening mechanisms that could give an explanation for the great width of the Orion profile are discussed. In contrast to former conclusions, the new pyroxene glasses do not satisfactorily fit silicate emission profiles observed with less massive YSOs (Herbig Ae/Be stars, T Tauri stars). This suggests that the properties of silicate grains present in circumstellar envelopes of massive YSOs could be considerably different from those of the less massive ones. Finally, the new data are compared with former results on pyroxene glasses of different composition which were derived from transmission spectra. The comparison underscores the point that the kind of preparation sensitively influences the resulting optical data.