2023MNRAS.518.1617C

We measured the degree of linear polarization of Comet C/2020 S3 (Erasmus) on 2020 November 13, 20, 22, and 23, while the comet was observed at large phase angles, α = 62.6°-66.6°. On the first two epochs, the polarization closely matched what was previously observed in Comet C/1989 X1 (Austin). On the third epoch, the polarization was found to rise slightly, and on the latest epoch, it rose significantly, exceeding that of Comet Austin. On the last observation, the polarization of Comet Erasmus appears to be more consistent with what was previously seen in Comet C/1996 B2 (Hyakutake) at a similar phase angle. While such short-term transient behaviour has been seen previously, Comets Austin and Hyakutake belong to two different classes in classifications based on the amplitude of their positive polarization P_max and, hence, Comet Erasmus revealed a transition from the class of low-P_max comets to that of high-P_max comets within only a few days. Polarization images and modelling suggest that the transition occurred due to a decrease in the relative abundance of Mg-rich silicate particles in the inner coma by 1/3, revealing a qualitative change in emanations of dust particles from the Erasmus nucleus.