ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse

¹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
e-mail: ogorman@cp.dias.ie
² Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309, USA
³ Instituto de Astronomá, Universidad Caólica del Norte, Avenida Angamos 0610, Antofagasta, Chile
⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Department of Astrophysics and Planetary Science, Villanova University, Villanova, PA 19085, USA
⁶ Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong
⁷ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁸ Lund University, Lund, Sweden
⁹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden

Received: 17 February 2020
Accepted: 5 May 2020

Abstract

We first present spatially resolved ALMA and VLA continuum observations of the early-M red supergiant Antares to search for the presence of a chromosphere at radio wavelengths. We resolve the free-free emission of the Antares atmosphere at 11 unique wavelengths between 0.7 mm (ALMA band 8) and 10 cm (VLA S band). The projected angular diameter is found to continually increase with increasing wavelength, from a low of 50.7 mas at 0.7 mm up to a diameter of 431 mas at 10 cm, which corresponds to 1.35 and 11.6 times the photospheric angular diameter, respectively. All four ALMA measurements show that the shape of the atmosphere is elongated, with a flattening of 15% at a similar position angle. The disk-averaged gas temperature of the atmosphere initially rises from a value of 2700 K at 1.35 R_⋆ (i.e., 0.35 R_⋆ above the photosphere) to a peak value of 3800 K at ∼2.5 R_⋆, after which it then more gradually decreases to 1650 K at 11.6 R_⋆. The rise in gas temperature between 1.35 R_⋆ and ∼2.5 R_⋆ is evidence for a chromospheric temperature rise above the photosphere of a red supergiant. We detect a clear change in the spectral index across the sampled wavelength range, with the flux density S_ν ∝ ν^1.42 between 0.7 mm and 1.4 cm, which we associate with chromosphere-dominated emission, while the flux density S_ν ∝ ν^0.8 between 4.3 cm and 10 cm, which we associate with wind-dominated emission. We show that the Antares MOLsphere is transparent at our observed wavelengths, and the lukewarm chromosphere that we detect is therefore real and not just an average of the cool MOLsphere and hot ultraviolet emitting gas. We then perform nonlocal thermal equilibrium modeling of the far-ultraviolet radiation field of another early-M red supergiant, Betelgeuse, and find that an additional hot (i.e., > 7000 K) chromospheric photoionization component with a much smaller filling factor must also exist throughout the chromospheres of these stars.