Silicate composition of sungrazing comet C/1965 S1 (Ikeya-Seki)

The sungrazing comets, that pass extremely close to the Sun, are rare, but are of considerable scientific interest due to their unusual polarimetric properties. This paper is devoted to the interpretation of polarimetric observational data from comet Ikeya-Seki (C/1965 S1), also known as the "Big Comet of 1965." The data of polarimetric observations of this comet at a wavelength of 0.53 mu m are sharply out of the general range of other observed comets in the wavelength range of 0.52-0.55 mu m. The phase curve of comet Ikeya-Seki, unlike all other comets, has an incredibly steep slope, the exact explanation of which has not been given so far. In this work, we reproduce this slope using computer simulation. It was shown that irregularly shaped particles under no parameters can give such a sharp slope in the degree of linear polarization. However, the use of spherical particles is able to reproduce this slope quite accurately. The best fit is achieved using Fe-rich silicate (fayalite) particles with a radius of approximate to 0.633 mu m. Therefore, when studying near-solar comets, it is necessary to consider the effect on the scattering pattern of regular shape particles as well as the presence of Fe-rich silicate (fayalite) material.