Monitoring polarization in comet 46P/Wirtanen

Abstract

We measure the degree of linear polarization of comet 46P/Wirtanen during two months, embracing the perihelion passage in December, 2018 with phase angles ranging from α = 18.1° to 46.4°. The polarimetric response PQ 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 February of 2019, 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 PQ at the lowest phase angle α = 18.1°, simultaneous imaging polarimetry suggests that the negative polarization (PQ < 0) arises in a region of within 5,000 km of the nucleus, where the negative polarization could be as strong as PQ = –(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).

Radial Distribution of the Dust Comae of Comets 45P/Honda–Mrkos–Pajdus̆áková and 46P/Wirtanen

“…Specifically, for 46P/Wirtanen, we can also determine that these grains must have been moving at a minimum velocity of 15 m s −1 away from the nucleus (E. Howell 2021, personal communication). In accordance with the radar measurements, Zheltobryukhov et al (2020) also found evidence of a circumnucleus halo of dust, suggesting either diffuse outgassing or the merge of multiple weak jets. For the large grains to be important in our measurements of radial profile slopes, there would have to be a very massive, ongoing flux of large particles.…”

Radial Distribution of the Dust Comae of Comets 45P/Honda-Mrkos-Pajduusáková and 46P/Wirtanen

“…Specifically, for 46P/Wirtanen, we can also determine that these grains must have been moving at a minimum velocity of 15 ms −1 away from the nucleus (personal communication, Ellen Howell). In accordance with the radar measurements, Zheltobryukhov et al (2020) also found evidence of a circumnucleus halo of dust, suggesting either diffuse outgassing, or the merge of multiple weak jets. For the large grains to be important in our measurements of radial profile slopes, there would have to be a very massive, ongoing flux of large particles.…”

Six Outbursts of Comet 46P/Wirtanen

“…Lamy et al (1998) measured the coma to have S V,R = 8.3±7.7% per 100 nm in HST filter photometry. Zheltobryukhov et al (2020) measured neutral-to-blue colors in BV RI broadband photometry on 2019 February 8 and 10 UTC (T − T P =57-60 days) in a 5000 km aperture radius (compare with our 5 =1330 km radius). In terms of spectral slope, they report S V,R = −16.7 ± 7.1% per 100 nm and −7.5 ± 16.3, S R,I = −8.7 ± 4.8% per 100 nm and −8.1 ± 8.4.…”

“…Note that these colors are not necessarily that of the dust coma, as there are gas emission bands present at these wavelengths, especially C 2 in g, but also NH 2 in r, R, and F689M, and CN in i. For example, Zheltobryukhov et al (2020) estimate a gas contamination of 5% in the V -band in a 1 (100 km) radius aperture, a fraction that should grow with aperture size given the different radial profiles of C 2 and dust. See Fink et al (1998) and Fig.…”