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Betelgeuse is an M supergiant that harbors spots and giant granules at itssurface and presents linear polarization of its continuum. We have previouslydiscovered linear polarization signatures associated with individual lines inthe spectra of cool and evolved stars. Here, we investigate whether a similarlinearly polarized spectrum exists for Betelgeuse. We used thespectropolarimeter Narval, combining multiple polarimetric sequences to obtainhigh signal-to-noise ratio spectra of individual lines, as well as theleast-squares deconvolution (LSD) approach. We have discovered the existence ofa linearly polarized spectrum for Betelgeuse, detecting a rather strong signal(at a few times 10$^{-4}$ of the continuum intensity level), both in individuallines and in the LSD profiles. Studying its properties and the signal observedfor the resonant \ion{Na}{i}\,D lines, we conclude that we are mainly observingdepolarization of the continuum by the absorption lines. The linearpolarization of the Betelgeuse continuum is due to the anisotropy of theradiation field induced by brightness spots at the surface and Rayleighscattering in the atmosphere. We have developed a geometrical model tointerpret the observed polarization, from which we infer the presence of twobrightness spots and their positions on the surface of Betelgeuse. We show thatapplying the model to each velocity bin along the Stokes Q and U profilesallows the derivation of a map of the bright spots. We use the Narval linearpolarization observations of Betelgeuse obtained over a period of 1.4 years tostudy the evolution of the spots. Our study of the linearly polarized spectrumof Betelgeuse provides a novel method for studying the evolution of brightnessspots at its surface and complements quasi-simultaneous observations obtainedwith PIONIER at the VLTI.

Discovery of a complex linearly polarized spectrum of Betelgeuse dominated by depolarization of the continuum