• Kemper, Francisca
• Tielens, A. G. G. M.

Abstract

One of the most interesting discoveries in the field of astromineralogy in the last decade is the discovery of crystalline silicates in circumstellar environments, first reported by Knacke et al. (1993, apj, 418, 440). It has become clear that a significant fraction of the silicates in several post- and pre-main-sequence stars have a crystalline lattice structure (Waters et al. 1996,Åp, 315, L361; Waelkens et al. 1996,Åp, 315, L245). Considering that the dust shells of evolved stars and young stars represent the initial and final stages in the life cycle of dust, it is striking that the silicates in the interstellar medium, which is believed to be the intermediate phase in the dust cycle, seem to be highly amorphous. There are only a few reports on crystalline silicates in star forming regions (e.g. Cesarsky et al. 2000,Åp, 358, 708; Onaka &amp; Okada 2003, in press), but these detections are only tentative, and are either based on one far-infrared resonance or on erroneous interpretation of instrumental features. <P />Here we present a more detailed study of the degree of crystallinity in the interstellar medium, using infrared spectroscopy obtained with the short wavelength spectrometer (SWS) on board of the infrared space observatory (ISO). We focus on the line-of-sight toward the galactic centre (Sgr A) which is thought to be representative of the galactic interstellar medium. <P />The silicate dust in the diffuse interstellar medium causes an absorption feature around 10 microns toward Sgr A. The broad shape of this feature, which is due to the Si-O stretching mode within the silica tetrahedras, suggests that the silicate by which it is carried, is mainly amorphous. Crystallinity of the silicates would be revealed by narrow resonances in this wavelength regime, corresponding to placing the Si-O bonds in a regular grid of silica tetrahedras. Typically, these resonances are 0.5--1 micron wide. The crystalline silicates most commonly found in astrophysical environments are forsterite and enstatite. Forsterite has a strong resonances at ∼11.3 microns and enstatite at ∼9.2 and ∼10.4 microns. We searched for these resonances in the absorption spectrum of the line-of-sight toward Sgr A, and did not significantly detect them. We will set an upper limit to the degree of crystallinity in interstellar silicates. This result will be compared with the degree of crystallinity observed in various circumstellar environments, providing a measure of dust processing in the interstellar medium....

Topics

• impedance spectroscopy
• amorphous
• phase
• forming
• crystallinity
• infrared spectroscopy
• crystalline lattice