Modeling Porous Dust Grains with Ballistic Aggregates. II. Light
  Scattering Properties
  We study the light scattering properties of random ballistic aggregates
constructed in Shen et al. (Paper I). Using the discrete-dipole-approximation,
we compute the scattering phase function and linear polarization for random
aggregates with various sizes and porosities, and with two different
compositions: 100% silicate and 50% silicate-50% graphite. We investigate the
dependence of light scattering properties on wavelength, cluster size and
porosity using these aggregate models. We find that while the shape of the
phase function depends mainly on the size parameter of the aggregates, the
linear polarization depends on both the size parameter and the porosity of the
aggregates, with increasing degree of polarization as the porosity increases.
Contrary to previous studies, we argue that monomer size has negligible effects
on the light scattering properties of ballistic aggregates, as long as the
constituent monomer is smaller than the incident wavelength up to
2*pi*a_0/lambda\sim 1.6 where a_0 is the monomer radius. Previous claims for
such monomer size effects are in fact the combined effects of size parameter
and porosity. Finally, we present aggregate models that can reproduce the phase
function and polarization of scattered light from the AU Mic debris disk and
from cometary dust, including the negative polarization observed for comets at
scattering angles 160<theta<180 deg. These aggregates have moderate porosities,
P\sim 0.6, and are of sub-micron-size for the debris disk case, or micron-size
for the comet case.
