We present first measurements of the degree of linear polarization of distant comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at heliocentric distances r = 5.9 -7.0 AU. Observations were carried out with the SCORPIO-2 focal reducer at the 6-m telescope of the Special Astrophysical Observatory (Russia). Both comets showed considerable level of activity (significant dust comae and tails) beyond a zone where water ice sublimation is negligible (up to 5 AU). Significant spatial variations both in the intensity and polarization are found in both comets. The slope of radial profiles of intensity changes gradually with the distance from the photocenter: from  0.7 near the nucleus up to about  1.3 for larger distances (up to 100000 km). The variation in polarization profiles indicates the non uniformity in the polarization distribution over the coma. The polarization degree over the coma gradually increases (in absolute value) with increasing the photocentric distance from of about 1.9% up to 3% for comet C/2010 S1 (LINEAR), and from of about 2.5% up to 3.5% for comet C/2010 R1 (LINEAR). These polarization values are significantly higher than typical value of the whole coma polarization ( −1.5%) for comets at heliocentric distances less than 5 AU. The obtained photometric and polarimetric data are compared with those derived early for other comets at smaller heliocentric distances. Numerical modeling of light scattering characteristics was performed for media composed of particles with different refractive index, shape, and size. The computations were made by using the superposition T-matrix method. We obtained that for comet C/2010 S1 (LINEAR), the dust in the form of aggregates of overall radius R ~ 1.3 μm composed of N = 1000 spherical monomers with radius a = 0.1 μm, refractive index m = 1.65 + i 0.05, allows to obtain a satisfactory agreement between the results of polarimetric observations of comet C/2010 S1 and computations.

We present a model of cometary dust capable of simulating the dynamics within the first few tens of km of the comet surface. Recent measurements by the Grain Impact Analyser and Dust Accumulator and Cometary Secondary Ion Mass Analyser instruments on Rosetta show that the nucleus emits fluffy dust particles with porosities above 50 per cent and sizes up to at least mm (Fulle et al. 2015b;Rotundi et al. 2015;Schulz et al. 2015). Retrieval of the physical properties of these particles requires a model of the effective forces governing their dynamics. Here, we present a model capable of simulating realistic, large and porous particles using hierarchical aggregates, which shows previous extrapolations to be inadequate. The main strengths of our approach are that we can simulate very large (mm-scale) non-spherical agglomerates and can accurately determine their (1) effective cross-section and ratio of crosssection to mass, (2) gas drag coefficient, and (3) light scattering properties. In practical terms, we find that a more detailed treatment of the dust structure results in three to five times higher velocities for large dust particles in the inner coma than previously estimated using spherical particles of the same mass. We apply our model to the dynamics of dust in the vicinity of the nucleus of comet 67P and successfully reproduce the dust speeds reported early on when the comet was roughly 3.5 au from the Sun. At this stage, we employ a simple spherical comet nucleus, we model activity as constant velocity gas expansion from a uniformly active surface, and use Mie scattering. We discuss pathways to improve on these simplifications in the future.

Phone: +421527879126 E-Mail address: oivanova@ta3.sk Highlights• We present the results of photometric observations of comet C/2012 S1 (ISON) performed from September 29 to December 16, 2012.• We present the results of spectroscopic observations of comet C/2012 S1 (ISON) performed on February 9, 2013.• Photometric colours of the comet are redder than solar colours.• Analysis of spectral data shows the growth of the dust grain reflectivity with increasing wavelength.• Molecular emissions were not detected in the spectra of this comet.• The optical characteristics of the dust significantly depend on the chemical composition and structure. Effective medium approximations for light scattering by porous aggregates should be used for their evaluations. ABSTRACTWe present an analysis of the photometric and spectroscopic data of the comet C/2012 S1 (ISON) observed at the heliocentric distances of 6.21 -4.81 AU. The photometric observations were made with the 60-cm Zeiss-600 telescope (ICAMER, peak Terskol, Russia) and the spectroscopic observations were performed using the SCORPIO-2 focal reducer mounted in the prime focus of the 6-m BTA telescope (SAO RAS, Russia). We analyse the B, V and R-band images to describe the dusty cometary coma and to investigate its brightness, colours and dust production rate. The spectra cover the wavelength range of 3600-7070 Å. No emissions which are expected in this wavelength region were detected above the 3σ level. The continuum shows a reddening effect with the normalized gradient of reflectivity along dispersion of 9.3 ± 1.1% per 1000 Å. A dust-loss rate was derived using the obtained values and under the different model assumptions. Our simulations clearly indicate that to retrieve dust production from the observational Afρ parameter is an ambiguous task.The result of such a procedure is strongly dependent on dynamical (e.g. effective density and cross-section) as well as optical (e.g. scattering coefficient and phase function) characteristics of dust grains. A variation of the mentioned parameters can lead to dramatic changes in the evaluation of mass production. We demonstrate that the dynamic and optical properties are interconnected via the microscopic properties of dust grains (effective size and porosity).