Modeling (Sub)millimeter Scattering Properties of Fractal and Consolidated Porous Particles: Applications to Protoplanetary Disks

Abstract The early evolution of dust in protoplanetary disks remains uncertain, as the maximum particle size inferred from (sub)millimeter polarization can differ by up to an order of magnitude from that inferred from spectral energy distribution modeling. To test whether porosity and morphology can reduce this tension, we perform light-scattering numerical simulations for two dust populations: (i) consolidated porous particles computed with the discrete dipole approximation ( ADDA ) and (ii) highly porous aggregate models, including fractal and hierarchical aggregates, computed with the multiple-sphere T -matrix method ( MSTM ). Using DSHARP optical constants, we compute scattering matrices, cross sections, and effective albedo ω ¯ eff for a size distribution n ( a ) ∝ a q with q = −3.5, a min = 0.1 μ m , and 10 wavelengths from 0.87 to 10 mm. We find that increasing porosity strengthens forward scattering and enhances polarization near θ ≈ 90 ∘ . For compact spheres, P ( 9 0 ∘ ) ω ¯ eff peaks near a max ∼ λ / 2 π and then declines, whereas porous particles show a broader peak extending to larger sizes, keeping polarization-based constraints compatible with a max ∼ 1 mm . Porosity also lowers κ abs at fixed dust mass relative to compact spheres, implying larger inferred dust masses for a given continuum flux.