Convective cells in Betelgeuse: imaging through spectropolarimetry

Abstract

We assess the ability to image the photosphere of red supergiants and, in particular Betelgeuse, through the modelling of the observed linear polarization in atomic spectral lines. We also aim to analyse the resulting images over time, to measure the size and dynamics of the convective structures in these stars. Rayleigh scattering polarizes the continuum and spectral lines depolarize it.This depolarization is seen as a linear polarization signal parallel to the radial direction on the stellar disk. Integrated over the disk, it would result in a null signal, except if brightness asymmetries/inhomogeneities are present. This is the basic concept behind our imaging technique. The several tests and comparisons performed prove that our technique reliably retrieves the salient brightness structures in the photosphere of Betelgeuse, and should be relevant to other red supergiants. We demonstrate that these structures are convective cells, with a characteristic size of more than 60% of the stellar radius. We also derive the characteristic upflow and downflow speeds, 22 and 10 km/s respectively. We find weak magnetic fields concentrated in the downflow lanes in between granules. We follow those convective structures in time. Changes happen on timescales of one week, but individual structures can be tracked over four years of observations.

arXiv commentAccepted for publication in A&A

References in corpus (11)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Atmospheric dynamics and the mass loss process in red supergiant stars
• Imaging the spotty surface of Betelgeuse in the H band
• Radiative hydrodynamics simulations of red supergiant stars. IV gray versus non-gray opacities
• The close circumstellar environment of Betelgeuse - V. Rotation velocity and molecular envelope properties from ALMA
• The close circumstellar environment of Betelgeuse. IV. VLTI/PIONIER interferometric monitoring of the photosphere
• Measuring surface magnetic fields of red supergiant stars
• Evolution of the magnetic field of Betelgeuse from 2009 - 2017
• The impact of quantum interferences between different J-levels on scattering polarization in spectral lines
• Discovery of a complex linearly polarized spectrum of Betelgeuse dominated by depolarization of the continuum
• The convective photosphere of the red supergiant CE Tau. I. VLTI/PIONIER H-band interferometric imaging

Cited by in corpus (24)

• A dusty veil shading Betelgeuse during its Great Dimming
• Standing on the shoulders of giants: New mass and distance estimates for Betelgeuse through combined evolutionary, asteroseismic, and hydrodynamical simulations with MESA
• Betelgeuse Just Isn't That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse
• The Photospheric Temperatures of Betelgeuse during the Great Dimming of 2019/2020: No New Dust Required
• On massive dust clumps in the envelope of the red supergiant VY Canis Majoris
• Is Betelgeuse Really Rotating? Synthetic ALMA Observations of Large-scale Convection in 3D Simulations of Red Supergiants
• Three-dimensional imaging of convective cells in the photosphere of Betelgeuse
• Red Supergiant Candidates for Multimessenger Monitoring of the Next Galactic Supernova
• Red Supergiants, Yellow Hypergiants, and Post-RSG Evolution
• Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg
• On Granulation and Irregular Variation of Red Supergiants
• The historical record of massive star formation in Cygnus
• Multiple components in the molecular outflow of the red supergiant NML Cyg
• Pollux: A weak dynamo-driven dipolar magnetic field and implications for its probable planet
• Asymmetric shocks in $\chi$ Cyg observed with linear spectropolarimetry
• Finding magnetic north: an extraordinary magnetic field detection in Polaris and first results of a magnetic survey of classical Cepheids
• The height of convective plumes in the red supergiant $\mu$ Cep
• The K supergiant runaway star HD 137071
• Making sense of the spectral line profiles of Betelgeuse and other Red SuperGiants
• The variability of Betelgeuse explained by surface convection
• Millimetre observations of the S-type AGB star $\chi$ Cygni: variability of the emission of the inner envelope
• Betelgeuse scope: Single-mode-fibers-assisted optical interferometer design for dedicated stellar activity monitoring
• First images of Antares photosphere from spectropolarimetry
• Precision Analysis of Evolved Stars