Unusual polarimetric properties for interstellar comet 2I/Borisov

S Bagnulo,T Santana-Ros,M Devogèle,A Cellino,R Nežič,L Kolokolova,Ph Bendjoya,G Borisov,A A Christou

doi:10.1038/s41467-021-22000-x

Abstract

So far, only two interstellar objects have been observed within our Solar System. While the first one, 1I/‘Oumuamua, had asteroidal characteristics, the second one, 2I/Borisov, showed clear evidence of cometary activity. We performed polarimetric observations of comet 2I/Borisov using the European Southern Observatory Very Large Telescope to derive the physical characteristics of its coma dust particles. Here we show that the polarization of 2I/Borisov is higher than what is typically measured for Solar System comets. This feature distinguishes 2I/Borisov from dynamically evolved objects such as Jupiter-family and all short- and long-period comets in our Solar System. The only object with similar polarimetric properties as 2I/Borisov is comet C/1995 O1 (Hale-Bopp), an object that is believed to have approached the Sun only once before its apparition in 1997. Unlike Hale-Bopp and many other comets, though, comet 2I/Borisov shows a polarimetrically homogeneous coma, suggesting that it is an even more pristine object.

Highlights

Only two interstellar objects have been observed within our Solar System
Polarisation is measured as the flux perpendicular to the plane Sun-Object-Observer minus the flux parallel to that plane, divided by the sum of the two fluxes; this measurement is usually repeated in different viewing conditions described by the so-called phase angle
In particular it is found that at small phase angles (≤20∘), the linear polarisation is directed along the scattering plane; because the way the polarisation is measured, this situation is described as negative polarisation

Summary

Introduction

Only two interstellar objects have been observed within our Solar System. While the first one, 1I/‘Oumuamua, had asteroidal characteristics, the second one, 2I/Borisov, showed clear evidence of cometary activity. We performed polarimetric observations of comet 2I/Borisov using the European Southern Observatory Very Large Telescope to derive the physical characteristics of its coma dust particles. Its orbital eccentricity (3.356191 ± 0.000015) shows that the object is not gravitationally bound to the Solar System, making it the first unambiguous case of a comet arriving from interstellar space This is only the second recognised case of a small body entering the Solar System from interstellar space. The first such case, namely that of 1I/′Oumuamua, discovered on 18 October 2017 by the Pan-STARRS 1 telescope, was that of an apparently asteroidal body, lacking detectable signs of cometary activity[1]. Measurements of cometary linear polarisation provides information about physical characteristics of the coma dust particles that is difficult to obtain by other observing techniques. A remarkable result of polarimetric investigations was the prediction that cometary dust is made of aggregates of submicron grains[17], which was later confirmed by the in situ studies of the Rosetta mission[18] and of the samples returned by the Stardust mission[19]

Methods

Results

Conclusion