Abstract
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.
Highlights
Betelgeuse is a very interesting target for the observation and understanding of red supergiants (RSG)
The study of the convective movements in these cold, low-gravity stars advances our understanding of turbulent fluid motions (Freytag et al 2002; Chiavassa et al 2011). This very same turbulence may give rise to the magnetic fields observed in Betelgeuse (Aurière et al 2010; Mathias et al 2018). These convective motions in the photosphere may largely contribute to the increased mass loss at these stages of the stellar evolution, forming a strong stellar wind which greatly contributes to the chemical enrichment of the circumstellar and interstellar medium
The 2D images produced by López Ariste et al (2018) are the result of fitting the observed linear polarization profiles with synthetic ones obtained from a model
Summary
Betelgeuse is a very interesting target for the observation and understanding of red supergiants (RSG). Its large angular diameter (42.11 mas in the K band, Montargès et al 2021), due to its relative proximity (about 200 pc) and to its actual size as an MIab supergiant, has made Betelgeuse a favorite target for interferometry, as these characteristics allow detailed images of its photosphere which have unveiled large convective structures (e.g., Haubois et al 2009; Montargès et al 2016) These structures were recently confirmed through an unexpected technique based on the discovery (Aurière et al 2016) and interpretation (López Ariste et al 2018) of linear polarization in the atomic lines of the spectrum of Betelgeuse. The analysis of these signals allowed López Ariste et al (2018) to produce images of the star. The large velocities measured, 40 and even 60 km s−1 in some cases, are kept constant up to heights where they are comparable to, but smaller than, the escape velocity
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