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: 2.38). 01/2009; 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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