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.
