Spatially Resolved Observations of Betelgeuse at λ7 mm and λ1.3 cm Just prior to the Great Dimming

Abstract

We present spatially resolved observations of Betelgeuse (α Orionis) obtained with the Karl G. Jansky Very Large Array at λ ∼ 7 mm (44 GHz) and λ ∼ 1.3 cm (22 GHz) on 2019 August 2, just prior to the onset of the historical optical dimming that occurred between late 2019 and early 2020. Our measurements suggest recent changes in the temperature and density structure of the atmosphere between radii r ∼ 2R⋆ and 3R⋆. At λ7 mm the star is ∼20% dimmer than in previously published observing epochs between 1996 and 2004. We measure a mean gas temperature of T B = 2270 ± 260 K at r ∼ 2.1R⋆, where R ⋆ is the canonical photospheric radius. This is ∼2σ lower than previously reported temperatures at comparable radii and >1200 K lower than predicted by previous semiempirical models of the atmosphere. The measured brightness temperature at r ∼ 2.6R ⋆ (T B = 2580 ± 260 K) is also cooler than expected based on trends in past measurements. The stellar brightness profile in our current measurements appears relatively smooth and symmetric, with no obvious signatures of giant convective cells or other surface features. However, the azimuthally averaged brightness profile is found to be more complex than a uniform elliptical disk. Our observations were obtained approximately 6 weeks before spectroscopic measurements in the ultraviolet revealed evidence of increases in the chromospheric electron density in the southern hemisphere of Betelgeuse, coupled with a large-scale outflow. We discuss possible scenarios linking these events with the observed radio properties of the star, including the passage of a strong shock wave.