Unusual polarimetric properties for interstellar comet 2I/Borisov

<p><strong>Introduction</strong></p> <p>Only two interstellar objects have been observed passing through our Solar System to date. The first one, 1I/&#8216;Oumuamua, discovered in 2017 in its post-perihelion phase, showed mostly asteroidal characteristics with some potential evidence of comet-like behaviour, but 2I/Borisov has showed clear evidence of comet activity since its discovery in 2019, prior to its perihelion passage in December of that year. This made it the first confirmed interstellar comet, so naturally many different observations were conducted by astronomers around the world to learn more about the peculiar object.</p> <p><strong>Methodology</strong></p> <p>In this work we present the analysis of polarimetric observations of the interstellar comet 2I/Borisov conducted with the FORS2 instrument located on the European Southern Observatory's Very Large Telescope Unit 1 (Antu). A series of polarimetric observations was performed between December 2019 and March 2020 using three different broadband filters in the visible regime in order to ascertain the physical characteristics of the coma dust particles.</p> <p><strong>Results & Conclusions</strong></p> <p>We find that polarisation of 2I/Borisov is higher than is usually measured for Solar System comets. This distinguishes the interstellar comet from all dynamically evolved comets in our Solar System. This evidence of 2I/Borisov being more pristine than the observed Solar System comets is corroborated by the fact that the comet with the closest polarimetric properties is C/1995 O1 (Hale&#8211;Bopp), which is believed to have approached the Sun only once before its apparition in 1997. Furthermore, and unlike both Hale&#8211;Bopp and many other comets, interstellar comet 2I/Borisov shows a polarimetrically homogeneous coma, unchanging with cometocentric distance, again suggesting it is even more pristine than the observed comets from our own Solar System. It is thus likely that it has never passed very close to its native star.</p>