Monitoring polarization in comet 46P/Wirtanen

We measure the degree of linear polarization of comet 46P/Wirtanen during two months, embracing the perihelion passage in 2018 December with phase angles ranging from α = 18.1° to 46.4°. The polarimetric response P_Q obtained resembles what was previously found in comet C/1975 V1 (West). This suggests 46P/Wirtanen belongs to a group of comets with high maximum positive polarization. In the first half of 2019 February, we conducted BVRI photometry of 46P/Wirtanen and found either neutral or blue colour of its dust, which is in good accordance with measurements of C/1975 V1 (West). While aperture-averaged polarimetry of 46P/Wirtanen reveals a nearly zero polarization P_Q at the lowest phase angle α = 18.1°, simultaneous imaging polarimetry suggests that the negative polarization (P_Q < 0) arises in a region of within 5000 km of the nucleus, where the negative polarization could be as strong as P_Q = -(1.44 ± 0.15) per cent. This observation suggests the existence of the circumnucleus halo and that the coma is populated by at least two types of dust particles. One of those reveals a low positive polarization at side scattering and high negative polarization near backscattering. Both polarimetric features are simultaneously produced by weakly absorbing Mg-rich silicate particles. Another type of dust produces solely positive polarization that could be attributed to carbonaceous particles. This composition of 46P/Wirtanen coma appears to be similar with what was previously found in comet C/1975 V1 (West).