Study of light scattering properties of dust aggregates with a wide variation of porosity

Abstract

We study the light scattering properties of moderately large dust aggregates ($0.8~\upmu \mbox{m} \lesssim R \lesssim 2.0~\upmu \mbox{m}$) with a wide variation of porosity ($\mathcal{P}$) from 0.57 to 0.98. The computations are performed using the Superposition T-matrix code with BAM2 cluster ($\mathcal{P} \sim 0.57\mbox{--}0.64$), BAM1 cluster ($\mathcal{P} \sim 0.74$), BA or BPCA cluster ($\mathcal{P} \sim 0.85\mbox{--}0.87$) and BCCA cluster ($\mathcal{P} \sim 0.98$). The simulations are executed at two wavelengths $0.45~\upmu \mbox{m}$ and $0.65~\upmu \mbox{m}$ with highly absorbing particles (organic refractory) as well as with low absorbing particles (amorphous silicates) to understand the photopolarimetric behavior (phase function, polarization, and color) of dust aggregates. The effect of aggregate size parameter ($X$) on the light scattering properties of aggregates (BA and BAM2) having different porosities is explored in this study. We find that the positive polarization maximum ($P_{\max}$), the amplitude of the negative polarization ($P_{\min}$) and phase function at the exact backscattering direction ($S_{11}(180^{\circ })$) are correlated with the porosity of aggregates. Compact aggregates show deeper negative polarization as compared to porous aggregates when the characteristic radius ($R$) of the aggregates are considered to be the same. Further lower porosity aggregates show higher $S_{11}(180^{\circ })$ and vice versa. When $\mathcal{P}$ is increased in a range from 0.64 to 0.98, both ${S}_{11}(180^{\circ })$ and $P_{\min}$ decrease linearly, whereas $P_{\max}$ increases linearly. We also find that the porosity of the aggregates plays a crucial role in determining the polarimetric color for high absorbing organic refractories. The compact clusters (BAM1 and BAM2) show the negative polarimetric color whereas BA clusters show almost positive polarimetric color at all values of scattering angle. We have also made some comparisons of our simulated results with PROGRA² experimental results.