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

Abstract

In our present study, the observed linear polarization data of comet Hyakutake are studied at wavelengths $\lambda=0.365\mu m$, $\lambda=0.485\mu m$ and 0.684$\mu m$ through simulations using Ballistic Particle-Cluster Aggregate and Ballistic Cluster-Cluster Aggregate aggregates of 128 spherical monomers. We first investigated that the size parameter of the monomer, $x$ $\sim$ 1.56 -- 1.70, turned out to be most suitable which provides the best fits to the observed dust scattering properties at three wavelengths $\lambda = 0.365$$\mu m$, 0.485$\mu m$ and 0.684$\mu m$. Thus the effective radius of the aggregate (r) lies in the range $0.45 \mu m \le r \le 0.49 \mu m$ at $\lambda = 0.365$$\mu m$; $ 0.60 \mu m \le r \le 0.66 \mu m$ at $\lambda = 0.485$$\mu m$ and $0.88 \mu m \le r \le 0.94 \mu m$ at $\lambda = 0.684$$\mu m$. Now using superposition \textsc{t-matrix} code and the power-law size distribution, $n(r) \sim r^{-3}$, the best-fitting values of complex refractive indices are calculated which can best fit 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 $\lambda = 0.365 $ $\mu m$, (1.743, 0.100) at $\lambda = 0.485 $ $\mu m$ and (1.695, 0.100) at $\lambda = 0.684$ $\mu m$. The refractive indices coming out from the present analysis correspond to mixture of both silicates and organics, which are in good agreement with the \textit{in situ} measurement of comets by different spacecraft.