New Spatially Resolved Observations of Betelgeuse from the VLA and ALMA: Evidence for Atmospheric Perturbations from a Close Companion

Abstract We present new spatially resolved observations of the red supergiant Betelgeuse ( α Orionis) at seven frequencies: 22 GHz ( λ 1.4cm), 44 GHz ( λ 6.8 mm), 107 GHz ( λ 2.8 mm), 136 GHz ( λ 2.2 mm), 224 GHz ( λ 1.3 mm), 338 GHz ( λ 0.89 mm), and 485 GHz ( λ 0.62 mm), using the Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array. These observations (spanning 2021–2026) enable a characterization of the mean brightness temperature ( T B ), shape, and symmetry of the star’s extended atmosphere. At 22 GHz and 44 GHz T B has increased following the historic lows observed in 2019, just prior to the Great Dimming of late 2019/early 2020. We find that T B peaks at a larger projected radius (≳2.7 R ⋆ ) than in measurements prior to 2016, consistent with a possible evolution in the atmospheric density profile. In addition we find a systematic decrease in T B from ≈3500 K at ∼2.7 R ⋆ to T B ≈ 2220 K at ∼1.2 R ⋆ , confirming a previously reported temperature minimum between the photosphere and chromosphere. At all observed frequencies T B is lower than temperatures inferred from ultraviolet measurements at comparable radii, indicating the continued presence of cooler plasma ( T ≤ T eff ) mixed with the warmer chromospheric material. The new measurements yield a global spectral index α = 1.35 ± 0.03, with no evidence for a change in α over more than two decades in frequency. Recently, evidence of a close-in companion to Betelgeuse has been reported, with an orbital separation r ∼ 2.3 R ⋆ . We have identified a correlation between the ellipticity of Betelgeuse, as obtained from radio measurements, and the companion’s orbital phase. The results are consistent with periodic perturbations of the atmosphere by the companion.