  Winds of asymptotic giant branch (AGB) stars are commonly assumed to be
driven by radiative acceleration of dust grains. For M-type AGB stars, the
nature of the wind-driving dust species has been a matter of intense debate. A
proposed source of the radiation pressure triggering the outflows is photon
scattering on Fe-free silicate grains. This wind-driving mechanism requires
grain radii of about 0.1 - 1 micron in order to make the dust particles
efficient at scattering radiation around the stellar flux maximum. Grain size
is therefore an important parameter for understanding the physics behind the
winds of M-type AGB stars. We seek to investigate the diagnostic potential of
scattered polarized light for determining dust grain sizes. We have developed a
new tool for computing synthetic images of scattered light in dust and gas
shells around AGB stars, which can be applied to detailed models of dynamical
atmospheres and dust-driven winds. We present maps of polarized light using
dynamical models computed with the DARWIN code. The synthetic images clearly
show that the intensity of the polarized light, the position of the inner edge
of the dust shell, and the size of the dust grains near the inner edge are all
changing with the luminosity phase. Non-spherical structures in the dust shells
can also have an impact on the polarized light. We simulate this effect by
combining different pulsation phases into a single 3D structure before
computing synthetic images. An asymmetry of the circumstellar envelope can
create a net polarization, which can be used as diagnostics for the grain size.
The ratio between the size of the scattering particles and the observed
wavelength determines at what wavelengths net polarization switches direction.
If observed, this can be used to constrain average particle sizes.
