The Great Dimming of Betelgeuse seen by the Himawari-8 meteorological satellite

Betelgeuse, one of the most studied red supergiant stars, dimmed in the optical by ~1.2 mag between late 2019 and early 2020, reaching an historical minimum called "the Great Dimming." Thanks to enormous observational effort to date, two hypotheses remain that can explain the Dimming: a decrease in the effective temperature and an enhancement of the extinction caused by newly produced circumstellar dust. However, the lack of multi-wavelength monitoring observations, especially in the mid infrared where emission from circumstellar dust can be detected, has prevented us from closely examining these hypotheses. Here we present 4.5-year, 16-band photometry of Betelgeuse between 2017-2021 in the 0.45-13.5 micron wavelength range making use of images taken by the Himawari-8 geostationary meteorological satellite. By examining the optical and near-infrared light curves, we show that both a decreased effective temperature and increased dust extinction may have contributed by almost the same amount to the Great Dimming. Moreover, using the mid-infrared light curves, we find that the enhanced circumstellar extinction actually contributed to the Dimming. Thus, the Dimming event of Betelgeuse provides us an opportunity to examine the mechanism responsible for the mass loss of red supergiants, which affects the fate of massive stars as supernovae.Comment: 19 pages, 3+4+6 figures. Published open access in Nature Astronomy. The Himawari-8 light curves of Betelgeuse are available at https://d-taniguchi-astro.github.io/homepage/Data_Himawari_en.htm