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Paper: Mid-Infrared Spectropolarimetry as a Discriminator of Interstellar Grain Models
Volume: 343, Astronomical Polarimetry: Current Status and Future Directions
Page: 332
Authors: Li, A.
Abstract: Although the precise nature of interstellar dust still remains unknown after over 70 years' extensive studies since Trumpler (1930, PASP, 42, 214) first revealed its presence in interstellar space through the discovery of color excess, it is now generally accepted that interstellar grains consist of amorphous silicates and some form of carbonaceous materials; the former is inferred from the 9.7μm Si-O stretching mode and 18μm O-Si-O bending mode absorption features in interstellar regions as well as the fact that the cosmically abundant heavy elements such as Si, Fe, Mg are highly depleted; the latter is mainly inferred from the 2175Å extinction hump (and the ubiquitous 3.4μm C-H stretching vibrational band) and the fact that silicates alone are not able to provide enough extinction (see Li 2004a, in Penetrating Bars Through Masks of Cosmic Dust, in press).
It is now also generally accepted that, through the analysis of the wavelengthdependent interstellar extinction and polarization curves as well as the near and mid IR emission, interstellar grains span a wide range of sizes from a few angstroms to a few micrometers: there must exist a substantial amount of “large” grains (with radii a > 0.025μm) which are primarily responsible for the extinction, polarization and scattering at visible wavelengths and the IR emission at wavelength longward of 60μm; there must also exist a substantial amount of “very small grains” (with a < 0.025μm) which contribute importantly to the extinction in the vacuum-UV and emit strongly in the near and mid IR at wavelength shortward of 60μm when transiently heated to high temperatures during quantized absorption events (see Li 2004b, in Astrophysics of Dust, 417).
While the interstellar extinction curve and the near- and mid-IR emission respectively provide useful constraints on the sizes of the large and very small dust components, the absorption and emission spectroscopic features and the depletion studies provide useful constraints on the dust composition, it is the mid-IR spectropolarimetry at the wavelength ranges of the 3.4μm hydrocarbon and 9.7μm silicate absorption features that provides a powerful constraint on the dust morphology (e.g. see Adamson et al. 1999, ApJ, 512, 224; Li & Greenberg 2002, ApJ, 577, 789; also see Chiar et al. in this volume).
In this talk I start with a brief overview of our current knowledge of interstellar dust size and composition, I then focus on the mid-IR spectropolarimetric constraints on interstellar grain models. For details please see Li & Greenberg (2002, ApJ, 577, 789) and a paper in preparation by the same authors.
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