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 2πa0/λ ∼ 1.6 where a0 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◦ . θ < 180◦. These aggregates have moderate porosities, P ≈ 0.6, and are of sub-μm-size for the debris disk case, or μm-size for the comet case. Subject headings: dust, extinction – polarization – scattering – circumstellar matter – comets – interplanetary medium – stars: individual (AU Mic, GJ 803)