Aggregate dust model to describe polarization properties of Comet Hale-Bopp

Abstract

In this work, the observed linear-polarization data of Comet Hale–Bopp are studied at wavelengths λ= 0.485 and 0.684 μm through simulations using Ballistic Particle–Cluster Aggregate and Ballistic Cluster–Cluster Aggregate. Using superposition t-matrix code, the best-fitting values of complex refractive indices are calculated which can well match the observed polarization data at the above two wavelengths. It is found from our analysis that there is not a single value of the monomer radius (am) that can simultaneously fit the observed polarization data at two wavelengths 0.485 and 0.684 μm. However, for different am in the two wavelengths, corresponding to similar size parameter of the monomer (x≈ 1.56), better fit can be achieved. The best-fitting complex refractive indices (n, k) coming out from this analysis are found to be (1.778, 0.059) at λ= 0.485 μm when am= 0.12 μm and (1.755, 0.080) at λ= 0.684 μm when am= 0.17 μm. The above values of refractive index show silicate behaviour as the values are close to amorphous olivine. Thus, it can be concluded that the silicate composition can best match the observed polarization data of Comet Hale–Bopp at λ= 0.485 and 0.684 μm. Finally, the polarization properties of Comet Hale–Bopp are compared with Comet Levy 1990XX.