Sensitivity of Polarization to Grain Shape: II. Aggregates

A previous study (Paper I) investigated the polarization properties of avariety of simple convex grain shapes, some of which were found to beconsistent with the observed polarization properties of interstellar dust fromfar-ultraviolet to far-infrared. Here we study the optical properties of 45non-convex shapes, all aggregates of N equal-sized spheres. We considerN=2, N=3, and N=256 random aggregates obtained from 3 differentaggregation schemes. We also consider "trimmed" N=256 aggregates obtained bysystematically trimming initially random aggregates to increase eitherflattening or elongation. The "macroporosities" of the studied aggregates rangefrom P_ macro=0.18 (for the N=2 bisphere) to P_macro≈ 0.85 (for the N=256 "BA" aggregates). The only aggregatesconsistent with observations of starlight polarization and polarized thermalemission are shapes that have been trimmed to increase their asymmetry. Ifinterstellar grains are high-porosity aggregates, there must be processescausing extreme elongation or flattening; if not, interstellar grains must bedominated by fairly compact structures, with at most moderate porosities. Theratio of polarization in the 10μm silicate feature to starlightpolarization in the optical is shown to be insensitive to porosity and shape.X-ray scattering may be the best tool to determine the porosity of interstellargrains. We propose that modest porosities of interstellar grains could be theresult of "photolytic densification". High polarization fractions observed insome Class-0 cores require processes to reduce porosities and/or increaseasymmetries of aggregates in dense regions.