Betelgeuse, a nearby red supergiant, is a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings1,2,3,4. This picture has been challenged by the discovery of a dense and almost static shell5 that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse’s wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind6 and forms an almost static, photoionization-confined shell. Other red supergiants should have much more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova light curve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction.
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J.M. and S.M. are grateful to P. Kervella, T. Le Bertre and G. Perrin, the organisers of the Betelgeuse Workshop in Paris (November 2012), where the ideas for this work were first developed. J.M. acknowledges funding from a fellowship from the Alexander von Humboldt Foundation and from the Deutsche Forschungsgemeinschaft priority program 1573, ‘Physics of the Interstellar Medium’. S.M. acknowledges the receipt of research funding from the National Research Foundation (NRF) of South Africa. T.J.M. is supported by the Japan Society for the Promotion of Science Postdoctoral Fellowships for Research Abroad (26·51). H.R.N. acknowledges funding from a NSF grant (AST-0807664). R.K. acknowledges support from STFC (ST/L000709/1). The authors acknowledge the John von Neumann Institute for Computing for a grant of computing time on the JUROPA supercomputer at Jülich Supercomputing Centre.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Dependence of the photoionization-confined shell radius on the properties of the stellar wind and external ionizing radiation.
The panels plot RIF as a function of mass-loss rate, (a), external ionizing photon flux, Fγ (b), and wind velocity, vn (c). Data points are from spherically symmetric radiation hydrodynamics simulations and black lines are from equation (1). In a, the fixed parameters are vn = 15 km s−1 and Fγ = 1010 cm−2 s−1; in b they are vn = 15 km s−1 and either (blue points) or (red points); and c they are and Fγ = 1010 cm−2 s−1.
Extended Data Figure 2 Dependence of the photoionization-confined shell mass on the properties of the stellar wind and external ionizing radiation.
The panels plot Mshell as a function of (a), Fγ (b) and vn (c). Data points are steady-state masses from spherically symmetric radiation hydrodynamics simulations and black lines are from equation (3). Again, in a the fixed parameters are vn = 15 km s−1 and Fγ = 1010 cm−2 s−1; in b they are vn = 15 km s−1 and either (blue points) or (red points); and in c they are and Fγ = 1010 cm−2 s−1.
Extended Data Figure 3 Growth of shell mass, Mshell, as a function of time for two different photoionization-confined shell simulations.
The shell accumulates mass linearly with time until it begins to saturate at about 1/3 to 1/2 of its final mass. The solid line shows in a and in b. a, Photoionization-confined shell appropriate for Betelgeuse, with , vn = 14 km s−1 and Fγ = 2 × 107 cm−2 s−1. b, More extreme model with , vn = 15 km s−1 and Fγ = 1013 cm−2 s−1.
Extended Data Figure 4 Structure of the circumstellar medium around Betelgeuse from a spherically symmetric radiation hydrodynamics simulation.
Hydrogen number density, gas velocity, temperature and wind fraction are plotted as functions of distance from the star after 0.01 Myr of evolution. The wind fraction equals 1 in the wind and equals 0 in the ISM. The photoionization-confined shell is still very thin and has low mass at this early time, and the fully ionized ISM interface at r = 0.2 pc shows that the expanding wind drives a forward shock and a reverse shock. Supplementary Information contains a video showing an animation of the time evolution.
Evolution of the circumstellar structures around Betelgeuse from a spherically symmetric radiation-hydrodynamics simulation
Hydrogen number density, gas velocity, temperature, and wind fraction are plotted as a function of distance from the star. The wind fraction is = 1 in the wind and = 0 in the ISM. The photoionization-confined shell reaches its final position rapidly and accumulates mass, whereas the wind pushes the ISM to ever-larger radii over time. (MP4 303 kb)
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Mackey, J., Mohamed, S., Gvaramadze, V. et al. Interacting supernovae from photoionization-confined shells around red supergiant stars. Nature 512, 282–285 (2014). https://doi.org/10.1038/nature13522