CCD polarimetry of distant comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at the 6-m telescope of the SAO RAS

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 the 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 100,000 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 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 indexes, shapes, and sizes. 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 and refractive index m=1.65+i0.05, allows to obtain a satisfactory agreement between the results of polarimetric observations of comet C/2010 S1 and computations.