Polarization properties of odd comet 17P/Holmes
ABSTRACT We present results of polarimetric observations of comet 17P/Holmes that in 2007 surprised the observers by its outburst that resulted in dramatically increased brightness and unusual shape of the coma. Polarimetric properties of the comet also appeared to be very peculiar. Even though the comet showed negative polarization typical for the small phase angles, its absolute values were much lower than usually observed at these phase angles and the spectral gradient of the polarization was much more pronounced than usually observed for comets (although negative that is typical for other comets too). Both these results are different from the regular trends and indicate some specifics of the comet Holmes dust during the outburst. We discuss how such unusual polarimetric properties of the comet can be explained and show how a peculiar combination of aggregated and compact particles or a composition with a unique ratio of silicates to organics can explain the observational results.
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ABSTRACT: The radiative-transfer coherent-backscattering (RT-CB) model is unique among light-scattering methodologies as it can be used to calculate accurate light-scattering properties of sparsely populated particle volumes with sizes ranging from subwavelength to infinity. We use the RT-CB model to examine the evolution of light-scattering properties as a volume of particles increases from wavelength-sized to several hundreds of wavelengths. We examine the evolution of light-scattering intensity phase function and polarization, as well as linear and circular polarization ratios. We confirm the expected trends for backscattering features to shift to smaller phase angles as the volume increases. In addition, we also see the amplitude of these features increases to some maximum for volumes having size parameters kR~100, before decaying to less than half this amplitude as their volumes approach infinity.Journal of Quantitative Spectroscopy and Radiative Transfer 05/2014; 150. DOI:10.1016/j.jqsrt.2014.05.019 · 2.29 Impact Factor
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ABSTRACT: At small phase angles, the whole coma aperture-averaged linear polarization of comets nearly coincides with that of C-type asteroids. However, the experimental study of light scattering by independent dust particles and regoliths, consisting of exactly the same particles, shows that their negative polarization branches are substantially different. Therefore, the similarity in the linear polarization of comets and C-type asteroids has to be interpreted in terms of a random coincidence rather than evidence for common properties of dust particles. Imaging polarimetry of comets shows two features with a distinctive behavior of the linear polarization: jets and circumnucleus haloes. Dust particles in jets produce only positive polarization through all the phase angles; whereas, dust particles in the halo reveal a significant negative polarization branch with Pmin = -6%. By comparison with the experimental study of light sc/SUBattering, such a difference in the negative polarization could indicate common properties of dust in the circumnucleus haloes and C-type asteroids. The high negative polarization can be confidently attributed to weakly absorbing particles. The real part of the refractive index Re(m) is 1.5-1.6, and the imaginary part is limited to Im(m) ≤ 0.02. The morphology of dust particles in the circumnucleus haloes can be rather fluffy with a material density of about 0.8 g/cm3. The power-law index for the size distribution is estimated to be about a = 1.5-2.10/2011; 63(10):1077-1085. DOI:10.5047/eps.2011.03.005
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ABSTRACT: We present multispectral photo-polarimetric observations of comet 17P/Holmes taken at three different dates. These observations show the evolution of the negative polarization branch (NPB) as a function of time and wavelength. We perform discrete-dipole approximation (DDA) simulations on agglomerated debris particles of various sizes and refractive indices. Our simulations show that the observations are consistent with the cloud being composed of agglomerated debris particles having refractive indices of approximately m=1.5–1.6+0.1i. Our results are also consistent with the particles obeying a power-law size distribution r–a and having a lower particle-radius cut-off of approximately 0.6μm, where the index a∼3.5 for the early observations and shrinks to a∼1.5 for the later observations. This is consistent with the smaller, more accelerated particles in the distribution being propelled out of the field of view.Journal of Quantitative Spectroscopy and Radiative Transfer 07/2011; 112(11):1848-1863. DOI:10.1016/j.jqsrt.2011.01.020 · 2.29 Impact Factor