Dr. Jean Chiar

December 20, 2006

"Unlocking the Mysteries of Interstellar Dust Composition and Icy Mantle Formation with Sensitive Infrared Spectroscopy"


Interstellar dust is ubiquitous, yet its precise composition is still widely debated. The Spitzer Space Telescope's (SST) Infrared Spectrometer (IRS) provides the sensitivity to study previously unattainable lines of sight throughout the plane of the Milky Way Galaxy. We seek to study the hydrocarbon (carbon and hydrogen-containing molecules) and silicate (mineral) dust components of the interstellar medium for 56 lines of sight with varying amounts of dust obscuring the starlight. In addition, we will be probing the dust over a range of Galactic longitudes and latitudes. Specifically, we will measure the absorption features in infrared part of the electromagnetic spectrum. The hydrocarbon absorption features occur at 6.9 and 7.3 micrometers, and the silicate absorption features at 9.7 and 18.5 micrometers. The ratio of the depths of the hydrocarbon and silicate absorption features provides a direct handle on the hydrocarbon to silicate dust volume. It has been previously been noted that the ratio of the depth of the absorption to the amount of dust obscuration is distinct for locations within the Solar neighborhood (about 1000 lightyears from the Solar System) compared to the Galactic Center (which is 30,000 lightyears from our Solar System). Thus, these absorption features and their relative depths will also be related to the amount of dust obscuration and Galactic location. Finally, the silicate mineralogy can be assessed by studying the ratio of the two silicate absorption features, whose relative strengths have been shown to be indicative of olivine-rich or pyroxene-rich silicates.