On the crystallinity of silicate dust in the interstellar medium

An accurate knowledge of the mineralogy (chemical composition and crystal structure) of the silicate dust in the interstellar medium (ISM) is crucial for understanding its origin in evolved stars, the physical and chemical processing in the ISM, and its subsequent incorporation into protostellar nebulae, protoplanetary discs and cometary nuclei where it is subjected to further processing. While an appreciable fraction of silicate dust in evolved stars, in protoplanetary discs around pre-main-sequence stars, in debris discs around main-sequence stars and in cometary nuclei is found to be in crystalline form, very recent infrared spectroscopic studies of the dust along the sightline toward the Galactic Centre source Sgr A* placed an upper limit of similar to 1.1 per cent on the silicate crystalline fraction, well below the previous estimates of similar to 5 or similar to 60 per cent derived from the observed 10-mu m absorption profile for the local ISM toward Cyg OB2 No. 12. Since the sightline toward Sgr A* contains molecular cloud materials as revealed by the detection of the 3.1- and 6.0-mu m water ice absorption features, we argue that by taking into account the presence of ice mantles on silicate cores, the upper limit on the degree of silicate crystallinity in the ISM is increased to mu 3-5 per cent.