The Dielectric Function of "Astrodust" and Predictions for Polarization
  in the 3.4$\mu$m and 10$\mu$m Features
  The dielectric function of interstellar dust material is modeled using
observations of extinction and polarization in the infrared, together with
estimates for the mass of interstellar dust. The "astrodust" material is
assumed to be a mix of amorphous silicates and other materials, including
hydrocarbons producing an absorption feature at 3.4$\mu$m. The detailed shape
of the 10$\mu$m polarization profile depends on the assumed porosity and grain
shape, but the 10$\mu$m spectropolarimetric data are not yet good enough to
clearly favor one shape over another, nor to constrain the porosity. The
expected 3.4$\mu$m feature polarization is consistent with existing upper
limits, provided the 3.4$\mu$m absorption is preferentially located in grain
surface layers; a separate population of non-aligned carbonaceous grains is not
required. We predict the 3.4$\mu$m polarization feature to be $(\Delta
p)_{3.4\mu{\rm m}}/p(10\mu{\rm m})\approx 0.016$, just below current upper
limits. Polarization by the same grains at submm wavelengths is also
calculated.
