Abstract
We propose polarization of scattered optical light from intermediate Galactic
latitude infrared cirrus as a new diagnostic to constrain models of
interstellar dust and the anisotropic interstellar radiation field (aISRF). For
single scattering by a sphere, with Mie scattering phase functions for
intensity and polarized intensity for a dust model at a given wavelength (Sloan
$r$ and $g$ bands), and with models of anisotropic illumination from the entire
sky (represented in HEALPix), we develop the formalism for calculating useful
summary parameters for an integrated flux nebula (IFN): average of the phase
function weighted by the illumination, polarization angle ($\psi$), and
polarization fraction ($p$). To demonstrate the diagnostic discrimination of
polarization from scattered light, we report on the effects of different
anisotropic illumination models and different dust models on the summary
parameters for the Spider IFN. The summary parameters are also sensitive to the
IFN location, as we illustrate using FRaNKIE illumination models. For assessing
the viability of dust and aISRF models, we find that observations of $\psi$ and
$p$ of scattered light are indeed powerful new diagnostics to complement joint
modeling of the intensity of scattered light (related to the average phase
function) and the intensity of thermal dust emission. However, optically thin
IFNs that can be modelled using single scattering are faint and $p$ is not
large, as it could be with Rayleigh scattering, and so these observations need
to be carried out with care and precision. Results for the Draco nebula
compared to the Spider illustrate the challenge.
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