The Dielectric Function of "Astrodust" and Predictions for Polarization in the 3.4$\mu$m and 10$\mu$m Features

TL;DR

This paper models the dielectric function of interstellar 'astrodust' to predict polarization features at 3.4μm and 10μm, providing insights into grain composition, shape, and alignment effects in the interstellar medium.

Contribution

It introduces a detailed model of interstellar dust dielectric properties and predicts polarization features, refining understanding of dust grain composition and alignment.

Findings

Predicted 3.4μm polarization feature is about 1.6% of the 10μm polarization.

10μm polarization profile depends on grain shape and porosity but current data are inconclusive.

The 3.4μm feature polarization is consistent with existing upper limits, assuming surface layer absorption.

Abstract

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.