Journal Article

Modelling the polarization properties of Comet 1P/Halley using a mixture of compact and aggregate particles

H. S. Das, D. Paul, A. Suklabaidya and A. K. Sen

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 1, pages 94-100
Published in print September 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19016.x
Modelling the polarization properties of Comet 1P/Halley using a mixture of compact and aggregate particles

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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. In the present work, we propose a model that considers cometary dust as a mixture of compact and aggregate particles, with a composition of silicate and organic. We consider compact particles as spheroidal particles and aggregates as both ballistic cluster–cluster aggregate (BCCA) and ballistic agglomeration with two migrations (BAM2) aggregate with a certain size distribution. The mixing ratio of compact to aggregate particles is taken to be 13:7. For modelling Comet 1P/Halley, the power-law size distribution n(a) ∼a−2.6, obtained from a re-analysis of the Giotto spacecraft data, for both compact and aggregate particles, is used. We consider a mixture of BAM2 and BCCA aggregates with a lower cut-off size of about 0.20 μ m and an upper cut-off of about 1 μ m. We also consider a mixture of prolate, spherical and oblate compact particles with an axial ratio (E) of 0.8–1.2 where a lower cut-off size of about 0.1 μ m and an upper cut-off of about 10 μ m are considered. Using a T-matrix code for polydisperse spheroids (0.1 μ m ≤a≤ 10 μ m) and superposition T-matrix code for aggregates (0.2 μ m ≤av≤ 1 μ m), the average simulated polarization curves are generated, which can best fit the observed polarization data at the four wavelengths: λ= 0.365, 0.485, 0.670 and 0.684 μ m. The suitable mixing percentages of aggregates obtained from the present modelling are 50 per cent BAM2 and 50 per cent BCCA particles, and 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 at the above four wavelengths, 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

Journal Article.  5398 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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