Astrophysics Aggregate dust model to study the polarization properties of

Abstract The observed linear polarization data of comet Hyakutake are studied at wavelengths λ = 0.365μm, λ = 0.485μm and 0.684μm through simulations us-ing Ballistic Particle-Cluster Aggregate and Ballistic Cluster-Cluster Aggregate ag-gregates 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 = 0.365μm, 0.485μm and 0.684μm. Thus, the effective radius of the aggregate (r) lies in the range 0.45μm ≤ r ≤ 0.49μm at λ = 0.365μm; 0.60μm ≤ r ≤ 0.66μm at λ = 0.485μm and 0.88μm ≤ r ≤ 0.94μm at λ = 0.684μm. Now using superposition T-MATRIX code and the power-law size distribution, n(r) ∼ r−3, the best-fitting values of com-plex 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 = 0.365μm, (1.743, 0.100) at λ = 0.485μm and (1.695, 0.100) at λ = 0.684μm. 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. Key words: comets: general — dust, extinction — scattering — polarization