Aggregate dust model to study the polarization properties of comet C/1996 B2 Hyakutake

The observed linear polarization data of comet Hyakutake are studied at wavelengths λ = 0.365 μm, λ = 0.485 μm and 0.684μm through simulations using Ballistic Particle-Cluster Aggregate and Ballistic Cluster-Cluster Aggregate aggregates of 128 spherical monomers. We first found that the size parameter of the monomer, x ∼ 1.56-1.70, turned out to be the most suitable which provides the best fits to the observed dust scattering properties at three wavelengths: λ = 0.365 μm, 0.485 μm and 0.684 μm. Thus, the effective radius of the aggregate (r) lies in the range 0.45 μm ≤ r ≤ 0.49 μm at λ = 0.365 μm; 0.60 μm ≤ r ≤ 0.66 μm at λ = 0.485 μm and 0.88 μm ≤ r ≤ 0.94 μm at λ = 0.684 μm. Now using superposition t-matrix code and the power-law size distribution, n(r) ∼ r-3, the best-fitting values of complex refractive indices are calculated for the observed polarization data at the above three wavelengths. The best-fitting complex refractive indices (n, k) are found to be (1.745, 0.095) at λ = 0.365 μm, (1.743, 0.100) at λ = 0.485 μm and (1.695, 0.100) at λ = 0.684 μm. The refractive indices derived from the present analysis correspond to a mixture of both silicates and organics, which are in good agreement with the in situ measurement of comets by different spacecraft.