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The 9.7 and 18 um silicate absorption profiles towards diffuse and molecular cloud lines-of-sight

Studying the composition of dust in the interstellar medium (ISM) is crucial in understanding the cycle of dust in our galaxy. The mid-infrared spectral signature of amorphous silicates, the most abundant dust species in the ISM, is studied in different lines-of-sight through the Galactic plane, thus probing different conditions in the ISM. We have analysed 10 spectra from the Spitzer archive, of which 6 lines-of-sight probe diffuse interstellar medium material and 4 probe molecular cloud material. The 9.7 um silicate absorption features in 7 of these spectra were studied in terms of their shape and strength. In addition, the shape of the 18 um silicate absorption features in 4 of the diffuse sightline spectra were analysed. The 9.7 um silicate absorption bands in the diffuse sightlines show a strikingly similar band shape. This is also the case for all but one of the 18 um silicate absorption bands observed in diffuse lines-of-sight. The 9.7 um bands in the 4 molecular sightlines show small variations in shape. These modest variations in the band shape are inconsistent with the interpretation of the large variations in τ_9.7/E(J-K) between diffuse and molecular sightlines in terms of silicate grain growth. Instead, we suggest that the large changes in τ_9.7 / E(J-K) must be due to changes in E(J-K).