A colour portrait of the interstellar comet 2I/Borisov

Abstract The aim of this paper is to report on the results of a multicolour observing campaign performed at the LBT on the interstellar comet 2I/Borisov during three epochs while target was approaching its perihelion: UBVRI images have been observed in October and December 2019, when the comet was at ∼2.3 and 2.0 au, respectively. Comet Borisov presented a rather complex morphology, especially in the December observing epoch, when a bi-lobate structure on a global scale was visible, with a jet-like structure characterised by a disconnection event, probably due to the presence of an active area on the nucleus' surface combined with its rotational properties. U–B colour, measured for the first time for this comet, monotonically decreases as Borisov approaches perihelion, from 0.35 ± 0.06 in October 19th to −0.068 ± 0.034 in December 2nd; B–V colour shows a more “chaotic” behaviour, with values for October epochs quite similar to solar colours, and a higher value of 0.76 ± 0.02 in December 2nd, probably due to a combination of an increase of the B flux (CN emission) and of an increase in V flux (due to C2). V – R and R–I colours derived in both the October and December observing epochs depict a scenario of a coma slightly redder than the Sun, with a RI-reddening value of ∼11%/1000 A and ∼5%/1000 A, respectively. The R- Afρ at the reference aperture of r ∼104 km from the optocentre decreases from ∼100 cm to ∼70 cm among the two observing epochs, showing that the Borisov's dust environment is quite similar to those of Short Period Comets of our Solar System. The application of a first-order photometric model, with the plausible scenario of grains with radius a = 100 μm and bulk density ρd = 1000 kg/m3 moving at vd in the range of 3–30 m/s, allows to derive a dust production rate Qd decrease from a range of 3–25 kg/s in October to a range of 2–18 kg/s in December. All these results confirm that the interstellar comet 2I/Borisov has a coma environment quite similar to that of the Short Period Comets (mostly Jupiter Family targets) orbiting in our inner Solar System.