Résumé
Aims. Understanding convection in red supergiants and the mechanisms that trigger the mass loss from these evolved stars are the general goals of most observations of Betelgeuse and its inner circumstellar environment.Methods. Linear spectropolarimetry of the atomic lines of the spectrum of Betelgeuse reveals information about the three-dimensional (3D) distribution of brightness in its atmosphere. We model the distribution of plasma and its velocities and use inversion algorithms to fit the observed linear polarization.Results. We obtain the first 3D images of the photosphere of Betelgeuse. Within the limits of the used approximations, we recover vertical convective flows and measure the velocity of the rising plasma at different heights in the photosphere. In several cases, we find this velocity to be constant with height, indicating the presence of forces other than gravity acting on the plasma and counteracting it. In some cases, these forces are sufficient to maintain plasma rising at 60 km s−1 to heights where this velocity is comparable to the escape velocity.Conclusions. Forces are present in the photosphere of Betelgeuse that allow plasma to reach velocities close to the escape velocity. These mechanisms may suffice to trigger mass loss and sustain the observed large stellar winds of these evolved stars.