date: 2012 March 14 (Wed) 17:00-17:30
room: CPS Conference Room
speaker: Asoke K. Sen (Assam University)
organizer: Yoichi Itoh
title: Model for cometary grains to explain polarization
abstract: Both the in situ measurement of Comet 1P/ Halley and the Stardust-returned samples of Comet Wild 2 showed the presence of a mixture of compact and aggregate particles, with both silicates and organic refractory being in the composition of the cometary dust. Results obtained recently from the Stardust mission suggest that the overall ratio of compact to aggregate particles is 65:35 (or 13:7) for Comet 81P/Wild 2.

A model has been proposed that considers cometary dust as a mixture of compact and aggregate particles, with a composition of silicate and organic. The compact particles are assumed to be spheroidal and aggregates as both ballistic cluster-cluster aggregate (BCCA) and ballistic agglomeration with two migrations (BAM2). The mixing ratio of compact to aggregate particles is taken to be 13:7. The power-law size distribution n(a) ~ a-2.6, obtained from a re-analysis of the Giotto spacecraft data, were used for both compact and aggregate particles. A mixture of prolate, spherical and oblate compact particles with an axial ratio (E) of 0.8-1.2 was considered. Using a T-matrix code for polydisperse spheroids (0.1 μm < a < 10 μm) and superposition T matrix code for aggregates (0.2 μm < av < 1 μm), the average simulated polarization curves are generated, which can be bestfit the observed polarization data at the four wavelengths: 0.365, 0.485, 0.670 and 0.684 μm. The mixing percentages of aggregates obtained from the present modeling are 50 per cent BAM2 and 50 per cent BCCA particles. The silicate-to-organic mixing percentages are 78 per cent silicate and 22 per cent organic, in terms of volume. The present model successfully reproduces the observed polarization data, especially the negative branch, for Comet 1P/Halley, more effectively as compared to other work done in the past. It is found that among the aggregates, the BAM2 aggregate plays a major role in deciding the cross-over angle and depth of the negative polarization branch.
keywords: polarization - scattering - comets: general - dust, extinction