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Ground-Based Support of the "Rosetta" Mission Orbited
to Comet 67P/Churyumov-Gerasimenko
with the 6-m SAO RAS Telescope

Russian version

    Comet 67P/Churyumov-Gerasimenko (hereinafter 67P) was the research target of the Rosetta space mission. Before the Rosetta started, comet 67P had been poorly studied. Thus, its physical properties were obtained as a result of a detailed study within the international campaign of ground-based observations of the comet (2013-2016). Observations with largest ground-based and space telescopes including the 6-m SAO RAS telescope were used for comparison with the Rosetta mission measurements.
    Within the international campaign, observations of the comet 67P with the 6-m telescope BTA (Big Telescope Alt-azimuth) were carried out with the SCORPIO-2 multi-mode focal reducer of the Special Astrophysical Observatory (Russia) which was used in spectroscopic, photometric, and polarimetric modes. It should be noted that the obtained polarimetric data were more informative, than similar observation data obtained with other big telescopes for the comet.
    Our observations covered the post-perihelion period. The obtained photometric data allowed us not only to select long-term structures in the cometary coma but also to confirm that these structures (the tail and jets) were located at approximately the same position angles which were detected in previous apparitions.
    A special feature of this result is the fact that observations of activity of comet 67P from the Rosetta spacecraft did not reveal any long-term structures in the comet coma, and model analysis showed that structures should diffuse at small distances already from the nucleus and structures in the coma should not be observed from the Earth. Therefore, the nature of long-term structures in cometary comas remains unclear.
    Radial surface brightness profiles significantly differ for the coma, jets, and tail. These results allow us to assume that there is a non-isotropic ejection of dust particles from the whole surface of the nucleus as well as from local areas on the nucleus.
    The dust colour in the structures, as well as in the tail, is noticeably redder than that of the surrounding coma which is associated with large particles or/and particles of different composition, possibly, more rich in organics. A sharp decrease of brightness at large distances from the comet nucleus also confirms our results about the presence in the cometary coma of large slowly moving particles. Analysis of the colour map indicates the concentration of coarse dust near the nucleus as well as the majority of small fast-moving dust particles on the periphery of the comet coma.
    The analysis of linear polarization maps obtained for 67P, for the first time ever for this comet, allowed us not only to select regions of high (near the nucleus and periphery) and low polarization (in jets) in the cometary coma but also to find changes in the position of these regions with time. The model analysis of polarization maps made it possible to explain the resulting colour and polarization due to light scattering by large particles (of the order of hundreds of microns). At a distance from the nucleus, the particles become smaller and, finally, reach submicron sizes. The reasons for changing the particle size can be: fragmentation, sublimation, sorting, due to different particle velocities, etc. Circular polarization in was not detected in the comet.
    Spectroscopic observations allowed us to detect emission lines of CN, C2, C3 and C+ in the cometary coma. It was obtained that comet 67P correspond to the carbon-depleted comet type.
1) Snodgrass C., A'Hearn M.F., Aceituno F., Afanasiev V., Ivanova O.,..., Kiselev N., ..., Rosenbush V. et al. 2017. The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission. Philosophical Transactions of the Royal Society A, Volume 375, Issue 2097, id.20160249, 22 p. ( http://adsabs.harvard.edu/abs/2017RSPTA.37560249S)
2) Ivanova O.V., Rosenbush V.K., Kiselev N.N., Afanasiev V.L., Korsun P.P. 2017. Post-perihelion observations of comet 67P/Churyumov-Gerasimenko at the 6 m BTA telescope: optical spectroscopy. Monthly Notices of the Royal Astronomical Society, Volume 469, Issue Suppl_2, p. S386-S395. ( http://adsabs.harvard.edu/abs/2017MNRAS.469S.386I)
3) Rosenbush V.K., Ivanova O.V., Kiselev N.N., Kolokolova L.O., Afanasiev V.L. 2017. Spatial variations of brightness, colour and polarization of dust in comet 67P/Churyumov-Gerasimenko. Monthly Notices of the Royal Astronomical Society, Volume 469, Issue Suppl_2, 21 July 2017, p. S475-S491. ( http://adsabs.harvard.edu/abs/2017MNRAS.469S.475R)

Contact - Oleksandra Ivanova, Victor Afanasiev, Nikolay Kiselev, Vera Rosenbush.

Fig.1. (g-r) colour map of comet 67P/C-G derived on November 8, 2015 (left). On the right, the near-nucleus region of the coma is shown on a larger scale

Fig.2. Linear polarization maps of comet 67P/C-G. The upper row shows spatial distribution of polarization degree over the whole comet: the left-hand image was obtained on November 8, 2015 in the R filter; the central image - in the R filter also on December 9, 2015; the right-hand image - on April 5, 2016 in the r-sdss filter

Fig.3. Spectrum of 67P was obtained on November 8, 2015: a) the observed spectrum with the superimposed continuum; b) the "observed spectrum - fitted continuum"; c) the polynomial correction of the solar spectrum