Mid-Infrared Spectropolarimetric Constraints on the Core-Mantle Interstellar Dust Model

In the framework of the silicate core-carbonaceous organic mantle interstellar dust model, the bulk of the visual/near-IR extinction and the entire polarization are from nonspherical and aligned core-mantle grains. The 3.4 micron C-H and 9.7 micron Si-O absorption features, respectively arising from the hydrocarbon mantle and the amorphous silicate core, are expected to be polarized to a modestly different degree. Spectropolarimetric observations toward the same lines of sight both in the 3.4 micron region and in the 9.7 micron region would be of great value to test the core-mantle dust model. The fact that the 3.4 micron feature is not polarized along the line of sight toward the Galactic center source IRS 7 is not yet sufficient to reject the core-mantle model due to the lack of spectropolarimetric observation of this region in the 9.7 micron region.