Speaker
Description
Cometary dust provides a unique record of the primitive material from which the Solar System formed. Observations of the brightness, color, and polarization of cometary comae can be used to constrain the physical and compositional properties of their dust particles. Comet C/2012 S1 (ISON) was classified as a dynamically new comet arriving from the Oort Cloud on a near-parabolic orbit and was expected to retain relatively unprocessed primordial material. Since the comet disintegrated during its perihelion passage, observations obtained before perihelion are especially important. Moreover, only a small amount of polarimetric information was available for this comet.
We report photometric and imaging polarimetric observations of C/2012 S1 (ISON) carried out with the 6-m BTA telescope at heliocentric distances of 4.81, 0.89, and 0.76 au, corresponding to phase angles of 7.3°, 57.6°, and 66.8°. The observations included broadband photometry and imaging measurements of both linear and circular polarization in several spectral bands. Long-slit spectroscopy obtained at 4.81 au revealed no detectable gaseous emission, indicating that the coma was dominated by dust at large heliocentric distances.
We investigate dust activity using the Afρ parameter, study the spectral color of the coma, and examine the spatial distribution of polarization. Afρ decreased from approximately 1000 cm⁻¹ at a distance of 4.8 AU to approximately 300 cm⁻¹ closer to the Sun. Spatially resolved color measurements revealed variations in dust properties throughout the coma, including an increase in the spectral gradient with increasing distance from the core. The coma was found to remain predominantly dusty at heliocentric distances of approximately 3.8–6.0 AU and exhibit a moderately red continuum with a normalized reflectivity gradient of approximately 6–10% per 1000 Å. At a geocentric distance of 0.76 AU, we detected a significant spatial variation in the degree of polarization from 7% to 14%. The spatial structure of these variations is similar to the spatial structures revealed by photometric observations.
Acknowledgements
The research of Leonid Shakun and Oleksandra Ivanova is supported by the Slovak Research and Development Agency under Contract No. APVV-24-0076 and the grant of Slovak Academy of Sciences (grant Vega No. 2/0067/26).
