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

¹ Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, 31028 Toulouse Cedex 4, France
e-mail: michel.auriere@irap.omp.eu
² CNRS, UMR 5277, Institut de Recherche en Astrophysique et Planétologie, 57 avenue d’Azereix, 65000 Tarbes, France
³ CNRS, UMR 5277, Institut de Recherche en Astrophysique et Planétologie, 14 avenue Édouard Belin, 31400 Toulouse, France
⁴ LUPM, UMR 5299, CNRS and Université Montpellier, Place E. Bataillon, 34090 Montpellier, France
⁵ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France
⁶ LESIA, Observatoire de Paris, PSL Research University, CNRS UMR 8109, Sorbonne Universités, UPMC, Université Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁷ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bvd de L’Observatoire CS 34229, 06004 Nice Cedex 4, France
⁸ Kiepenheuer-Institut for Sonnenphysik, 79 104 Freiburg, Germany
⁹ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko shose, 1784 Sofia, Bulgaria
¹⁰ European Southern Observatories, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹¹ Department of Physics, Royal Military College of Canada, PO Box 17000, Station “Forces”, Kingston, Ontario, K7K 4B4, Canada
¹² Université de Bordeaux, LAB, UMR 5804, 33270 Floirac, France
¹³ CNRS, LAB, UMR 5804, 33270 Floirac, France
¹⁴ Unidad Mixta Internacional Franco-Chilena de Astronomía (UMI 3386), CNRS/INSU, France & Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile

Received: 5 January 2016
Accepted: 3 May 2016

Abstract

Context. Betelgeuse is an M supergiant that harbors spots and giant granules at its surface and presents linear polarization of its continuum.

Aims. We have previously discovered linear polarization signatures associated with individual lines in the spectra of cool and evolved stars. Here, we investigate whether a similar linearly polarized spectrum exists for Betelgeuse.

Methods. We used the spectropolarimeter Narval, combining multiple polarimetric sequences to obtain high signal-to-noise ratio spectra of individual lines, as well as the least-squares deconvolution (LSD) approach, to investigate the presence of an averaged linearly polarized profile for the photospheric lines.

Results. We have discovered the existence of a linearly polarized spectrum for Betelgeuse, detecting a rather strong signal (at a few times 10^-4 of the continuum intensity level), both in individual lines and in the LSD profiles. Studying its properties and the signal observed for the resonant Na i D lines, we conclude that we are mainly observing depolarization of the continuum by the absorption lines. The linear polarization of the Betelgeuse continuum is due to the anisotropy of the radiation field induced by brightness spots at the surface and Rayleigh scattering in the atmosphere. We have developed a geometrical model to interpret the observed polarization, from which we infer the presence of two brightness spots and their positions on the surface of Betelgeuse. We show that applying the model to each velocity bin along the Stokes Q and U profiles allows the derivation of a map of the bright spots. We use the Narval linear polarization observations of Betelgeuse obtained over a period of 1.4 yr to study the evolution of the spots and of the atmosphere.

Conclusions. Our study of the linearly polarized spectrum of Betelgeuse provides a novel method for studying the evolution of brightness spots at its surface and complements quasi-simultaneous observations obtained with PIONIER at the VLTI.