The absence of crystalline silicates in the diffuse interstellar medium

We have studied the dust along the line-of-sight towards the Galactic Center using Short Wavelength Spectrometer (SWS) data obtained with the Infrared Space Observatory (ISO). We focussed on the wavelength region from 8-13 micron which is dominated by the strong silicate absorption feature. Using the absorption profiles observed towards Galactic Center Sources (GCS) 3 and 4, which are C-rich Wolf-Rayet Stars, as reference objects, we are able to disentangle the interstellar silicate absorption and the silicate emission intrinsic to the source, toward Sgr A* and derive a very accurate profile for the intrinsic 9.7 micron band. The interstellar absorption band is smooth and featureless and is well reproduced using a mixture of 15.1% amorphous pyroxene and 84.9% of amorphous olivine by mass, all in spherical sub-micron-sized grains. There is no direct evidence for substructure due to interstellar crystalline silicates. We are able to determine an upper limit to the degree of crystallinity of silicates in the diffuse interstellar medium (ISM), and conclude that the crystalline fraction of the interstellar silicates is 0.2% (+/- 0.2%) by mass. This is much lower than the degree of crystallinity observed in silicates in the circumstellar environment of evolved stars, the main contributors of dust to the ISM. There are two possible explanations for this discrepancy. First, an amorphization process occurs in the ISM on a time scale significantly shorter than the destruction time scale, possibly caused by particle bombardment by heavyweight ions. Second, we consider the possibility that the crystalline silicates in stellar ejecta are diluted by an additional source of amorphous silicates, in particular supernovae.