E-MERLIN resolves Betelgeuse at λ 5 cm: hotspots at 5 R⋆

Abstract

Abstract Convection, pulsation and magnetic fields have all been suggested as mechanisms for the transport of mass and energy from the optical photosphere of red supergiants, out to the region where the stellar wind is launched. We imaged the red supergiant Betelgeuse at 0.06–0.18 arcsec resolution, using e-Multi-Element Radio-Linked Interferometer Network (e-MERLIN) at 5.5–6.0 GHz, with a sensitivity of ∼10 μJy beam−1. Most of the radio emission comes from within an ellipse (0.235 × 0.218) arcsec2 (∼5 times the optical radius), with a flux density of 1.62 mJy, giving an average brightness temperature ∼1250 K. This radio photosphere contains two hotspots of 0.53 and 0.79 mJy beam−1, separated by 90 mas, with brightness temperatures 5400 ± 600 K and 3800 ± 500 K. Similar hotspots, at more than double the distance from the photosphere of those seen in any other regime, were detected by the less-sensitive ‘old’ MERLIN in 1992, 1995 and 1996 and many exceed the photospheric temperature of 3600 K. Such brightness temperatures are high enough to emanate from pockets of chromospheric plasma. Other possibilities include local shock heating, the convective dredge-up of hot material or exceptionally cool, low-density regions, transparent down to the hottest layer at ∼40 mas radius. We also detect an arc 0.2–0.3 arcsec to the SW, brightness temperature ∼150 K, in a similar direction to extensions seen on both smaller and larger scales in the infrared and in CO at mm wavelengths. These preliminary results will be followed by further e-MERLIN, Very Large Array and Atacama Large Millimeter/sub-millimeter Array (ALMA) observations to help resolve the problem of mass elevation from 1 to 10 R⋆ in red supergiants.