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First Polarimetric Studies of the Distant Comets C/2010 S1 (LINEAR)
and C/2010 R1 (LINEAR) with the SAO RAS 6-m Telescope

Russian version

    Currently, there is very little information on comets which are active beyond the orbit of Jupiter. Dust tails and comas of such comets are observed at large distances from the Sun (greater than 5 au), where water ice sublimation is inconsiderable. In its turn, sublimation of impurities of more volatile components (CO, CO2, and so on) also can not provide the rate of gas outflow required for formation of extended tails and comas in distant comets.
    Previous observations conducted by our team with the SAO RAS 6-m telescope showed that some of these comets have extended tails dissimilar to the tails of comets forming at close heliocentric distances. Along with this, there are no molecular emissions in the optical spectra of the majority of these comets. Modeling of dynamical properties of the particles which form the tails of distant comets shows that these particles are quite sizable (up to 1 mm) and low density (~1 g/cm3). Supposedly, they can be ice particles with heat-resistant inclusions, which have not undergone heat treatment.
    We have been conducting polarimetric observations of distant comets with the SAO RAS 6-m telescope using the multi-mode SCORPIO-2 spectrograph since 2011. We for the first time measured the linear polarization of scattered emission for the two distant comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at heliocentric distances of 5.9-7.0 au. It is shown that the degree of linear polarization is significantly higher than the typical value (~1.5%) for comets observed at a heliocentric distance smaller than 3 au. Figure 1 presents the maps of the distribution of linear polarization (projected onto the scattering plane) for the comets studied with the SAO RAS 6-m telescope.
    The results of numeric modeling of linear polarization variations for Comet C/2010 S1 (LINEAR) with the phase angle allowed us to determine the size and refractive index of the particles on which supposedly the sunlight scatters in the cometary coma. For Comet C/2010 S1 (LINEAR), the dust can be presented as aggregates with a characteristic radius of about 1 μm consisting of ~1000 spherical monomers with a radius of 0.1 μm and a refractive index of 1.65 ± 0.05 (Fig.2).
Ivanova O.V., Dlugach J.M., Afanasiev V.L., Reshetnyk V.M., & Korsun P.P. (2015). CCD polarimetry of distant comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at the 6-m telescope of the SAO RAS. Planetary and Space Science. doi:10.1016/j.pss.2015.05.009

Contact - Ivanova O.V.
Fig.1. Maps of the distribution of linear polarization (projected onto the scattering plane) for the comets in the V (a), g-sdss (b), and r-sdss (c) filters, constructed using the observed data from the SAO RAS 6-m telescope. Left: Comet C/2010 S1 (LINEAR) on November 25, 2011 (a) and on November 12, 2012 (b). Right (c): Comet C/2010 R1 (LINEAR) on February 6, 2013. The arrows mark the direction to the Sun and the image orientation (North and East)
Fig.2. Model of cometary dust as aggregates consisting of spherical monomers