Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium

We have analyzed the 9.7 and ``18'' micron interstellar silicate absorptionfeatures along the line of sight toward four heavily extincted galactic WC-typeWolf-Rayet (WR) stars. We construct two interstellar extinction curves from1.25 to 25 micron using near-IR extinction measurements from the literaturealong with the silicate profiles of WR 98a (representing the local ISM) and GCS3 (representing the Galactic Center). We have investigated the mineralogy ofthe interstellar silicates by comparing extinction profiles for amorphoussilicates with olivine and pyroxene stochiometry to the 9.7 and ``18'' micronabsorption features in the WR 98a spectrum. In this analysis, we haveconsidered solid and porous spheres and a continuous distribution ofellipsoids. While it is not possible to simultaneously provide a perfect matchto both profiles, we find the best match requires a mixture of these two typesof compounds. We also consider iron oxides, aluminosilicates and silicatecarbide (SiC) as grain components. Iron oxides cannot be accommodated in theobserved spectrum, while the amount of Si in SiC is limited to <4%. Finally, wediscuss the cosmic elemental abundance constraints on the silicate mineralogy,grain shape and porosity.Comment: 34 pages, 8 figures, 7 table