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We measured the degree of linear polarization P of comet C/2018 V1(Machholz-Fujikawa-Iwamoto) with the broadband Johnson V filter in mid-Novemberof 2018. Within a radius of \r{ho}=17,000 km of the inner coma, we detected anextremely low linear polarization at phase angles from 83 to 91.2 degree andconstrained the polarization maximum to Pmax = (6.8 +/- 1.8)%. This is thelowest Pmax ever measured in a comet. Using model agglomerated debrisparticles, we reproduced the polarimetric response of comet C/2018 V1. Fourretrieved refractive indices closely match what was experimentally found inMg-rich silicates with little or no iron content. Moreover, the sizedistribution of the agglomerated debris particles appears in good quantitativeagreement with the in situ findings of comet 1P/Halley. The dust model ofpolarization of comet C/2018 V1 suggests a strongly negative polarization withamplitude |Pmin| = 5%-7%; whereas, an interpretation based on gaseous emissionrequires no negative polarization at small phase angles. This dramaticdifference could be used to discriminate gaseous-emission and dust explanationsin low-Pmax comets in future.

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