Weather satellite probes the great dimming of Betelgeuse

Between late 2019 and early 2020, Betelgeuse—the second-brightest star in the Orion constellation—dimmed to a historic low. Initial observations hinted that it may have been because of a change in the star’s effective temperature or an extinction of its light by surrounding dust. The mystery has remained, however, because of a lack of observations on multiple wavelengths, particularly in the mid-IR, that would identify dust emission.

Now researchers from the University of Tokyo led by Daisuke Taniguchi have appeared to solve the astronomical mystery with a weather satellite. In October 2014, the Japan Meteorological Agency launched Himawari-8 into geostationary orbit. Its mission has been to collect optical and IR images for environmental monitoring and to improve numerical weather models.

Although not by design, the satellite as it orbits around Earth routinely captures photographs of Betelgeuse. Taniguchi and his colleagues thought they could use those images to study the star’s dimming. By analyzing light-curve data, which show the star’s varying brightness over time, they found that temperature changes and dust can explain the great dimming.

The time series above shows the drop in Betelgeuse’s brightness at a wavelength of 640 nm. Those data are part of a 4.5-year catalog that spans the optical wavelength range and into the IR. The researchers compared the light-curve data with a model of Betelgeuse’s spectral-energy distribution to infer the star’s effective temperature and other variables. The comparison was revealing: The star’s brightness declined by an apparent magnitude of 1.2, half of which can be explained by a 140 K drop in effective temperature that was observed relative to the model.

So what could explain the rest of the dimming? Dust particles can block a star’s light . The mid-IR light curves of Betelgeuse provided Taniguchi and his colleagues with enough information to determine the surrounding dust’s optical depth, which provides a measure of the light’s extinction and is correlated with the amount of dust around the star. Sure enough, the dust optical depth increased during the dimming event.

Previous investigations around other red supergiant stars have identified two potential dust populations. One is a somewhat diffuse dust layer that’s located several tens of stellar radii from the photosphere—the visible surface of the star. The evidence so far shows that the amount of dust in the diffuse layer remains stable over time, so that’s probably not the culprit for Betelgeuse’s dimming. The other possibility is a dust cloud that’s just a few tenths of a stellar radius from the photosphere.

To learn more about the dust-cloud possibility, the researchers used another piece of data that was serendipitously measured by Himawari-8: water vapor. The wavelength ranges in which the satellite observes water in Earth’s upper atmosphere also capture information about the flux of water molecules around Betelgeuse. A time series of the abundance of gas containing water vapor around the star shows a possible transition period that’s roughly the same time as its dimming. The initial evidence supports the idea that a clump of cool gas containing water and dust may have condensed near Betelgeuse’s photosphere and reduced its brightness. (D. Taniguchi, K. Yamazaki, S. Uno, Nat. Astron., 2022, doi:10.1038/s41550-022-01680-5 .)