The Umov effect in polydisperse dust aggregates

Abstract

This study explores the optical behavior of polydisperse dust aggregates, focusing on four structural types: ballistic cluster–cluster aggregates (BCCA), ballistic agglomeration (BA), and its variants with one (BAM1) and two migrations (BAM2). We analyze how the polarization maximum ($P_{max}$) and albedo ($A$) (both in logarithmic scale) vary with key scattering parameters such as the complex refractive index ($m=n+ik$), cluster radius ($R_c=0.8$–$1.2\mu m$), and porosity ($P\approx 0.64-0.99$), assuming 1024 monomers per aggregate. The analysis incorporates silicate, organic refractory, and Halley-like dust compositions. The Umov effect – an inverse correlation between $P_{max}$ and $A$ – is evident when $k$ is varied in the range $0.001<k<0.1$ (with $n=1.6$) across BA, BAM1, and BAM2 structures. In contrast, when porosity is varied across the range $P\approx 0.64\text{–}0.87$ for polydisperse aggregates of all three dust types, a linear inverse relationship between $P_{max}$ and $A$ is observed. BCCA aggregates generally do not follow the Umov effect due to small monomer size parameters, but for $x\ge 0.643$ ($\lambda =0.45\mu m$), the Umov effect is restored. When the size parameter ($X_c=2\pi R_c/\lambda$) is varied, the Umov effect holds for BA ($P\approx 0.87$), BAM1 ($P\approx 0.74$), and BAM2 ($P\approx 0.64$) with silicate compositions. For organic refractory and Halley-like dust compositions, BA and BAM1 continue to follow the Umov effect, but BAM2 deviates from it, likely due to its compact morphology and higher absorption. Additional BAM2 realizations with $P\approx 0.66$ and 0.67 consistently exhibit deviations, though a slight slope increase with porosity is noted. These findings underscore the importance of aggregate structure, porosity, and composition in determining light-scattering characteristics of cosmic dust.