Polarization properties of odd comet 17P/Holmes

Main Astronomical Observatory of National Academy of Sciences, Zabolotnoho Street 27, 03680 Kyiv, Ukraine; University of Maryland, College Park, MD 20740, USA; Institute of Astronomy of Karazin Kharkiv National University, Sums’ka Street 35, 61022 Kharkiv, Ukraine; International Center for Astronomical, Medical and Ecological Research, Zabolotnoho Street 27, Kyiv, Ukraine; Crimean Astrophysical Observatory, 98409 Nauchny, Crimea, Ukraine; Astronomical Observatory of Odesa National University, Shevchenko Park, 65014 Odesa, Ukraine
Journal of Quantitative Spectroscopy and Radiative Transfer (Impact Factor: 2.29). 09/2009; DOI: 10.1016/j.jqsrt.2009.03.031

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: 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 01/2011; 112(11):1848-1863. · 2.29 Impact Factor
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    ABSTRACT: We analyse the photopolarimetric response measured in Comet C/1975 V1 (West) using model agglomerated debris particles. Such particles have highly irregular morphology and volume density with properties that are consistent with the samples retrieved by Stardust from Comet 81P/Wild 2. We find that an entire set of observational data including phase function, phase dependence of the degree of linear polarization, photometric, and polarimetric colour in Comet C/1975 V1 (West) can be quantitatively reproduced with a two-component mixture of weakly and highly absorbing particles, having refractive indices m = 1.5+0i or 1.6+0.0005i and 2.43+0.59i, respectively. These refractive indices are consistent with Mg-rich silicates and amorphous carbon, two species considered to be abundant in comets. Our modelling predicts the volume ratio of these materials being about 1:3. We note that in situ mass spectrometry measurements of Comet 1P/Halley suggested a similar ratio of weakly to highly absorbing particles, with a large uncertainty. The best fit to observations is obtained for a polydisperse ensembles of particles that obey the power-law size distribution r-1.5 and r-2. Such a distribution also is in good agreement with in situ studies of Comet 1P/Halley. While our modelling can reproduce the photopolarimetric properties of Comet C/1975 V1 (West), it also is one of the simplest approach, since it has fewer free parameters than other approaches.
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