Comet 67P/Churyumov-Gerasimenko (hereinafter 67P) was the research
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.
Contact - Oleksandra Ivanova,
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.
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,
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.
(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
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
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