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

We present new spatially resolved observations of the red supergiant Betelgeuse (alpha Orionis) at seven frequencies: 22 GHz (lambda 1.36cm); 44 GHz (lambda 6.8mm); 107 GHz (lambda 2.8mm); 136 GHz (lambda 2.2mm); 224 GHz (lambda 1.3mm); 338 GHz (lambda 0.89mm); and 485 GHz (lambda 0.62mm) 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 (r about2.7R) compared with 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 about3500 K at about2.7R to T B about 2220K at about1.2R, 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 alpha=1.35 plus or minus 0.03, with no evidence for a change in alpha over more than two decades in frequency. Recently, evidence of a close-in companion to Betelgeuse has been reported, with an orbital separation r about2.3R. 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.