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

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.

The close circumstellar environment of Betelgeuse

“…The observed surface structures in this wavelength range are generally consistent with the results from radiative hydrodynamical simulations of convection (Chiavassa et al 2010;Montargès et al 2014). Aurière et al (2016) demonstrated that spectropolarimetry at visible wavelengths makes it possible to detect the presence of hot spots through their depolarization of the continuum spectrum of the star. Using this technique, hot spots have been reported by Tessore et al (2017b) on Betelgeuse at the epoch of the ALMA observations.…”

“…Several authors have reported the observation of bright spots on the disk of Betelgeuse (Young et al 2000;Haubois et al 2006;Ravi et al 2011;Aurière et al 2016), although their presence is not systematic (Burns et al 1997). Interferometric observations in the near-infrared domain by Montargès et al (2016) and Haubois et al (2009) have shown the presence of temperature inhomogeneities on the infrared photosphere of Betelgeuse.…”

The solar-like ‘Second Spectrum’ and polarized metal lines in the emission of the post-AGB binary 89 Herculis

“…Thomson) scattering and temporal evolution of hot spots. Extending Clarke and Schwarz's interpretation, Aurière et al (2016) suggested that the linear polarisation observed in the spectral line of Betelgeuse is indeed an effect of the depolarisation of the continuum set to zero. have computed the expected polarisation of the continuum in the 400 -700 nm range due to hot spots.…”

“…Harrington & Kuhn (2009b,a) and Lèbre et al (2011) have shown that such a property is often present in the Balmer lines of evolved stars. The existence of linearly polarisation across individual metal lines in absorption has been reported for the Mira star χCygni by Lèbre et al (2014) and for the red supergiant Betelgeuse by Aurière et al (2016). Using the Least Squares Deconvolution (LSD, Donati et al 1997), Tessore et al (2015) found evidence of linear polarisation in the spectral lines of the RV Tau variable R Scuti and Sabin et al (2015) in the lines of the RV Tau variable U Monoceroti as well in the post-AGB star 89 Herculis.…”

The first “second solar spectrum” ever observed in a star different than the sun: A high‐resolution spectropolarimetric Atlas of 89 Herculis

“…Harrington & Kuhn (, ) and Lèbre et al () have shown that linear polarization is often present in the Balmer lines of evolved stars. Using the least squares deconvolution method (LSD, Donati et al ), linear polarization across metal lines in absorption was detected for the Mira star χ Cygni (Lèbre et al ), for the red supergiant Betelgeuse (Aurière et al ), for the RV Tau variable R Scuti (Tessore et al ), and for the RV Tau variable U Monoceroti as well as for the post‐AGB binary star 89 Herculis (Sabin et al ).…”