Interstellar comet 2I/Borisov: dust composition from multiband photometry and modelling

ABSTRACT We present results from multiband photometry of the interstellar comet 2I/Borisov (C/2019 Q4). The observations were carried out in the two months before its perihelion passage. The UBVri photometric data obtained for comet 2I were converted to its reflectance by means of observations of neighbouring solar analogues, which allowed the reflectance of the comet with wavelength to be calculated. The registered prominent changes in the reflectance spectra of 2I while it was approaching the Sun from 2.40 to 2.01 au, and numerical simulations of the light scattering by aggregate particles provide insight into the chemical-mineralogical and structural properties of the cometary dust particles. The close agreement between the trends observed in the spectral characteristics of 2I and some models suggests that, during the observations, conglomerates of magnesium-ferrous (and, probably, organic) submicron particles prevailed in the scattering by its matter, H2O ice particles induced no noticeable spectral signals, and the gas contribution to the scattering by the coma was relatively small.