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.
Contact - Ivanova O.V.
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.
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)
Model of cometary dust as aggregates consisting of spherical monomers