Extremely low linear polarization of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto)

TL;DR

This study reports the lowest polarization ever measured in a comet, using polarimetric observations and dust modeling to understand the comet's dust properties and polarization behavior.

Contribution

It provides the first measurement of extremely low linear polarization in comet C/2018 V1 and models its dust particles to explain the polarization characteristics.

Findings

Measured Pmax = (6.8 +/- 1.8)% at phase angles 83-91.2 degrees

Dust particles modeled as agglomerated debris with Mg-rich silicates

Polarization model suggests strong negative polarization, distinguishing dust from gaseous emission

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 \r{ho}=17,000 km of the inner coma, we detected an extremely low linear polarization at phase angles from 83 to 91.2 degree 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.