Extremely low linear polarization of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto)
Abstract
We measured the degree of linear polarization P of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) with the broadband Johnson V filter in mid-November of 2018. Within a radius of ρ ≈ 17,000 km of the inner coma, we detected an extremely low linear polarization at phase angles α ≈ 83°-91.2° and constrained the polarization maximum to Pmax ≈ (6.8 ± 1.8)%. This is the lowest Pmax ever measured in a comet. Using model agglomerated debris particles, we reproduced the polarimetric response of comet C/2018 V1. Four retrieved refractive indices closely match what was experimentally found in Mg-rich silicates with little or no iron content. Moreover, the size distribution of the agglomerated debris particles appears in good quantitative agreement with the in situ findings of comet 1P/Halley. The dust model of polarization of comet C/2018 V1 suggests a strongly negative polarization with amplitude |Pmin| ≈ 5%-7%; whereas, an interpretation based on gaseous emission requires no negative polarization at small phase angles. This dramatic difference could be used to discriminate gaseous-emission and dust explanations in low-Pmax comets in future.
- Publication:
-
Icarus
- Pub Date:
- January 2020
- DOI:
- 10.1016/j.icarus.2019.113453
- arXiv:
- arXiv:2012.10724
- Bibcode:
- 2020Icar..33613453Z
- Keywords:
-
- Comet C/2018 V1 (Machholz-Fujikawa-Iwamoto);
- Polarization;
- Modeling;
- Agglomerated debris particles;
- Mg-rich silicates;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 9 pages, 5 figures