Multi-band aperture polarimetry of Betelgeuse during the 2019-20 dimming

Betelegeuse (α Orion) is a red supergiant (M2 Iab) and a semi-regular long period variable (SRc) with periods prominent around 400 and 2100 days (Montarg`es et al. 2016). Its most recent dimming event was first noticed by Guinan et al. (2019), who subsequently determined the minimum magnitude to be the lowest on record (Guinan et al. 2020), and whose reporting catalysed other observations. New VLT-SPHERE images, when compared to those from a year earlier, reveal a darkened lower hemisphere (Montarg`es et al. 2020). Levesque & Massey (2020) analysed their own spectrophotometry in the context of those images, finding the photosphere temperature to be nearly unchanged and attributing Betelgeuse’s dimming to episodic mass-loss and a subsequent increase in large grain circumstellar dust. Aperture polarimetry was used extensively in the study of late-type supergiant variability until the 1980s (Clarke 2010). Betelgeuse, like similar stars, can be highly polarised at blue wavelengths reducing ∝ λ −4 to a minimum at red and reversing in the infrared; it undergoes slow changes in both polarisation magnitude (p) and position angle (ζ), on similar timescales to, but not necessarily correlated with its photometric periods. The most popular explanations relate to absorption and scattering from either convective cells (hotspots), circumstellar dust or both. Auri`ere et al. (2016) describe a scenario where near-surface features induce a net polarisation by asymmetrically depolarising the continuum. The polarisation behaviour then depends on feature geometry and composition. Recent polarimetric imaging with SPHERE-ZIMPOL provides justification for both mechanisms (Kervella et al. 2016); such measurements provide superior detail, but the resources required restrict their frequency. (Please note an erratum for this article is available via https://iopscience.iop.org/article/10.3847/2515-5172/ab83fc/meta.)