Modeling Porous Dust Grains with Ballistic Aggregates. II. Light Scattering Properties

TL;DR

This study models the light scattering properties of porous ballistic dust aggregates, revealing how size, porosity, and composition influence scattering and polarization, with applications to debris disks and cometary dust.

Contribution

It introduces detailed aggregate models that accurately reproduce observed scattering and polarization features of astrophysical dust, emphasizing the role of porosity over monomer size effects.

Findings

Polarization increases with aggregate porosity.

Monomer size has negligible effect if smaller than wavelength.

Models match observed scattering in debris disks and comets.

Abstract

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.