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L. Kolokolova (University of Florida)
Extensive optical observations of recent bright comets confirmed that light-scattering properties of cometary grains have common characteristics, regardless of differences in the properties of the comets, such as their nucleus size, activity, dust-to-gas ratio, etc. For example, the dependence of their brightness on phase angle \alpha has a weak enhancement at small \alpha, a strong, broad peak at large \alpha, and is relatively flat at intermediate phase angles. The geometric albedo of cometary grains is low (~0.05% at \alpha=5o). The cometary dust has red color that does not significantly change with phase angle. The dependence of linear polarization on phase angle is well represented by a bell-shaped curve with a maximum at \alpha=90-100o and a negative branch at \alpha < 20-30o. The spectral gradient of polarization (polarimetric color) is positive in the visual and increases as \alpha rises from 10o to 90o. There are two models that have been successful in replication of the dust properties listed above: (1) the model of aggregates of submicron particles and (2) the model of multishaped (e.g. ensembles of spheroids of a variety of axes ratios) polydisperse particles. In this paper results of a survey on light-scattering properties of the latter model are reported. It was found that polydisperse multishaped particles could reproduce correct shape of the intensity and polarization curves as well as the red color of cometary dust at realistic refractive indices of their materials. However, it is impossible to fit, even qualitatively, all the observational characteristics using the same set of parameters. Also, red polarimetric color within \alpha=10-90o cannot be obtained even for spectrally changing refractive indices. These results increase the chances that the model of aggregated submicron particles is more workable to describe properties of cometary dust.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.