Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium

Abstract

We have analyzed the 9.7 and ''18'' m interstellar silicate absorption features along the line of sight toward four heavily extincted galactic WC-type Wolf-Rayet ( WR) stars. We construct two interstellar extinction curves from 1.25 to 25 m using near-IR extinction measurements from the literature, along with the silicate profiles of WR 98a (representing the local ISM ) and GCS 3 (representing the Galactic center). We have investigated the mineralogy of the interstellar silicates by comparing extinction profiles for amorphous silicates with olivine and pyroxene stoichiometry to the 9.7 and ''18'' m absorption features in the WR 98a spectrum. In this analysis, we have considered solid and porous spheres and a continuous distribution of ellipsoids. While it is not possible to simultaneously provide a perfect match to both profiles, we find that the best match requires a mixture of these two types of compounds. We also consider iron oxides, aluminosilicates, and silicate carbide (SiC) as grain components. Iron oxides cannot be accommodated in the observed spectrum, while the amount of Si in SiC is limited to <4%. Finally, we discuss the cosmic elemental abundance constraints on the silicate mineralogy, grain shape, and porosity.

The shape and composition of interstellar silicate grains

“…It is clear from this figure that the spectra using (coated) porous GRF or DHS particles give a much better fit to the observed spectral profile than the spectrum using homogeneous spheres. This is consistent with the conclusion by Chiar & Tielens (2006) that perfect homogeneous spheres cannot account for the observed 20 µm feature.…”

“…From our analysis of the 10 µm spectral extinction feature we can compute the expected shape of the 20 µm feature. We compare this to the observed extinction spectrum obtained by Chiar & Tielens (2006).…”

“…8. The extinction spectra of the best fit models computed over a longer wavelength range (black curves) and compared to the feature obtained by Chiar & Tielens (2006) (gray curves). significant impact on our results.…”

Infrared spectroscopy of asymptotic giant branch stars in the Galactic bulge

“…These extinction laws are consistent with earlier work by Lutz et al (1996) and Indebetouw et al (2005). On the other hand, Román-Zúñiga et al (2007) found small differences with Chiar & Tielens (2006), indicating that the extinction law in the near-and mid-IR may vary slightly as a function of environment.…”

“…The spectral shape of the extinction curve in the IR is discussed in great detail by Chiar & Tielens (2006). These authors construct two slightly different extinction laws: one that is appropriate for the 'local' ISM and another for the material towards the GC.…”