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Spectropolarimetric Constraints on the Nature of Interstellar Grains

While it is well recognized that interstellar grains are made of amorphous silicates and some form of carbonaceous materials, it remains debated regarding what exact chemical and physical form the carbonaceous component takes. Contemporary grain models assume that the silicate and carbon components are either physically separated, or they form a core-mantle structure, or they agglomerate to form porous composites. The core-mantle model posits that the mantle is made of some sort of aliphatic hydrocarbon materials and is responsible for the 3.4 micrometer absorption feature ubiquitously seen in the diffuse interstellar medium (ISM) of the Milky Way and external galaxies. This model is challenged by the nondetection of polarization in the 3.4 micrometer absorption feature as the 9.7 micrometer silicate feature is observed to be polarized. To alleviate this challenge, we calculate the degree of polarization of the 3.4 micrometer feature for spheroidal silicate dust coated by a layer of spherical aliphatic hydrocarbon. It is found that the 3.4 micrometer feature polarization still exceeds the observational upper limit, even though spherical aliphatic hydrocarbon mantles are expected to cause much less polarization than nonspherical (e.g., spheroidal) mantles. We have also shown that the composite grain model which consists of amorphous silicate, aliphatic hydrocarbon, and vacuum also predicts the 3.4 micrometer feature polarization to well exceed what is observed. These results support the earlier arguments that the aliphatic hydrocarbon component is physically separated from the silicate component unless the 3.4 micrometer absorption feature is just a minor carbon sink in the ISM.