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

TL;DR

This study analyzes silicate absorption features at 9.7 and 18 micrometers in different interstellar environments to understand dust composition and variations in the Galactic plane.

Contribution

It provides detailed spectral analysis of silicate features across multiple lines-of-sight, revealing consistent band shapes in diffuse regions and small variations in molecular clouds.

Findings

Silicate absorption bands are remarkably similar in diffuse ISM.

Small variations in molecular cloud silicate features suggest limited grain growth.

Large variations in extinction ratios are due to changes in E(J-K), not silicate properties.

Abstract

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 {\tau}_9.7/E(J-K) between diffuse and molecular sightlines in terms of silicate grain growth. Instead, we suggest that the large changes in {\tau}_9.7 / E(J-K) must be due to changes in E(J-K).