A.P. Jones , V. Frey , L. Verstraete & P. Cox
Institut d'Astrophysique Spatiale, Universite Paris XI - Batiment 121, 91405 Orsay Cedex, FRANCE
We present the results of models for the dust emission seen in the M17-SW HII region by the ISO SW spectrometer. We interpret the observed spectra in terms of the emission from amorphous carbons, and amorphous and crystalline silicates. We will show evidence for crystalline silicates, of predominantly forsteritic composition, in emission in the H II region. However, the bulk of the silicate material within the M17-SW H II region is amorphous with only of the order of 10% of the silicate being crystalline.
This is particularly intriguing within the context of the evidence from ISO for partially crystalline silicates in both young and evolved stars (Waelkens et al. 1996, AA, 315, L245; Waters et al. 1996, AA, 315, L261) and also in comets (Crovisier et al. 1996, AA, 315, L385). It is therefore now apparent that crystalline silicates are only seen in regions close to stars where the dust is heated to temperatures of order several 100K for long periods.
In contrast, all evidence from the 10 and 18 micron silicate bands in the diffuse ISM (e.g., Mathis 1990, ARAA, 28, 37 and references therein) indicates that the silicates in the ISM are completely amorphous. This strongly suggests that the crystalline component of the silicates results from the annealing of amorphous silicates, i.e., their partial crystallisation, in regions where they are subjected to heating to approximately 'room temperature' for protracted periods.