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

Abstract

Betelgeuse, one of the most studied red supergiant stars 1,2 , dimmed in the optical by ∼1.2 mag between late 2019 and early 2020, reaching an historical minimum 3,4,5 called "the Great Dimming." Thanks to enormous observational effort to date, two hypotheses remain that can explain the Dimming 1 : a decrease in the effective temperature 6,7 and an enhancement of the extinction caused by newly produced circumstellar dust 8,9 . 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 µm wavelength range making use of images taken by the Himawari-8 10 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 11 .Himawari-8 is a Japanese geostationary meteorological satellite orbiting 35, 786 km above the equator at 140.7 • east 10 . Since 7 July 2015, Himawari-8 has taken images of the entire disk of the Earth once every ten minutes using its optical and infrared imager, the Advanced Himawari Imager (AHI; Extended Data Fig. 1). The Himawari-8 satellite also observes the region of outer space around the edge of the Earth's disk during every scan; this motivated us to develop a brand-new concept: using meteorological satellites as "space telescopes" for astronomy (Fig. 1).One of the unique aspects of this concept to use the Himawari-8 for astronomy is that it enables us to obtain high-cadence time series of mid-infrared images, which is hard to acquire with the usual astronomical instruments. Small ground-based telescopes can acquire time-series measurements 12 , but the observations only cover wavelengths within the atmospheric window, and they are interrupted by the Sun for several months in most cases 13,14 . In contrast, astronomical satellites and aircraft-carrying telescopes can observe stars in most wavelength ranges 15,16,17 , but the observations require higher costs than ground-based telescopes. Survey telescopes orbiting the Earth for non-astronomical purposes-like the Himawari-8 meteorological satellite-have the potential to overcome these problems. In this work, we demonstrate that the mystery of the Great Dimming of Betelgeuse can be solved with the aid of the time-series observations from the Himawari-8, especially that in the mid infrared, with which the amount of dust around Betelgeuse can be determined.