Polarimetric studies of comet Hale-Bopp

Abstract

In the present work, the non-spherical dust grain characteristics of comet Hale-Bopp are studied using the T-matrix method and the modified power law distribution function. It is found that the observed data fits very well with the power index ($\alpha) = - 3$. The best fit values of complex refractive index $(n, k)$ and the aspect ratio (E) at $\alpha = - 3$ are calculated to be $(1.382, 0.035, 0.936)$ and $(1.379, 0.041,0.936)$ at $\lambda = 0.485 \mu m$ and $0.684\mu m$ respectively. Kerola & Larson (K-L) analysed the same comet using the T-matrix method and the power law distribution function ($\alpha = -3$), and found that the prolate grains can explain the observed polarization in a more satisfactory manner as compared to the other shapes. But their analysis could not reproduce the negative polarization branch beyond scattering angle $157^0$. However, the results obtained from the present work successfully generate the expected negative polarization curve beyond $157^0$ and the fitting in this case is much better than K-L's work. So it is concluded from the present study that the use of modified power law distribution function (with $\alpha = - 3$) can fit the observed data in a better way, as compared to the power law distribution function used by previous authors.