Measuring surface magnetic fields of red supergiant stars

Tessore, B.; Lèbre, A.; Morin, J.; Mathias, P.; Josselin, E.; Aurière, M.

doi:10.1051/0004-6361/201730473

Cited by 44 publications

(39 citation statements)

References 51 publications

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“…) appears typical of other RSGs(Tessore et al 2017). The time-scale for photospheric magnetic field variations(Mathias et al 2018) is consistent with the 5.8 yr time-scale observed in radial velocity and V magnitudes which has been attributed to giant convective cells.…”

supporting

confidence: 84%

SOFIA-EXES Observations of Betelgeuse during the Great Dimming of 2019/2020

Harper

DeWitt²,

Richter³

et al. 2020

ApJL

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In 2019 October Betelgeuse began a decline in V-band brightness that went beyond the minimum expected from its quasi-periodic ∼ 420 day cycle, becoming the faintest in recorded photometric history. Observations obtained in 2019 December with VLT/SPHERE (Montargès 2020) have shown that the southern half of the star has become markedly fainter than in 2019 January indicating that a major change has occurred in, or near, the photosphere. We present SOFIA-EXES high spectral-resolution observations of [Fe II] 25.99 µm and [S I] 25.25 µm emission lines from Betelgeuse obtained during the unprecedented 2020 February V-band brightness minimum to investigate potential changes in the circumstellar flow. These spectra are compared to observations obtained in 2015 and 2017 when the V magnitude was typical of brighter phases. We find only very small changes in the gas velocities reflected by either of the line profiles, no significant changes in the flux to continuum ratios, and hence no significant changes in the [Fe II]/[S I] flux ratios. There is evidence that absorption features have appeared in the 2020 continuum. The Alfvén wave-crossing time from the upper-photosphere is sufficiently long that one would not expect a change in the large scale magnetic field to reach the circumstellar [Fe II] and [S I] line forming regions, 3 < R(R * ) < 20. However, the light-crossing time is of order a few hours and a reduction in luminosity may reduce the dust-gas heating rate and [O I] 63.19 µm emission which has contributions from R > 20R * , where significant circumstellar oxygen-rich dust is observed.

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supporting

confidence: 84%

SOFIA-EXES Observations of Betelgeuse during the Great Dimming of 2019/2020

Harper

DeWitt²,

Richter³

et al. 2020

ApJL

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“…The magnetic field strength has some uncertainty but could be as high as 1-3 G. Aurière et al (2010) obtain a longitudinal (along the line of sight) magnetic field strength of the order of 1 G in α Ori (Betelgeuse), the most well-studied RSG star. Gausslevel field strength has been confirmed in two other RSGs by Tessore et al (2017). We can compare the above values to a magnetic field strength at the stellar surface obtained by a balance between magnetic field energy density and wind kinetic energy density (Fransson & Björnsson 1998;ud-Doula & Owocki 2002)…”

Section: Wind Magnetic Fieldsupporting

confidence: 55%

Core-collapse supernovae as cosmic ray sources

Marcowith

Dwarkadas

Renaud

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Core collapse supernovae (CCSNe) produce fast shocks which pervade the dense circum-stellar medium (CSM) of the stellar progenitor. Cosmic rays (CRs) if accelerated at these shocks can induce the growth of electromagnetic fluctuations in the foreshock medium. In this study, using a self-similar description of the shock evolution, we calculate the growth timescales of CR-driven instabilities. We select a sample of nearby core collapse radio supernova of type II and Ib/Ic. From radio data we infer the parameters which enter in the calculation of the instability growth times. We find that extended IIb SNe shocks can trigger fast intra day instabilities, strong magnetic field amplification and CR acceleration. In particular, the non-resonant streaming instability can contribute to about 50% of the magnetic field intensity deduced from radio data. This results in the acceleration of CRs in the range 1-10 PeV within a few days after the shock breakout. In order to produce strong magnetic field amplification and CR acceleration a fast shocks pervading a dense CSM is necessary. In that aspect IIn supernovae are also good candidates. But a detailed modeling of the blast wave dynamics coupled with particle acceleration is mandatory for this class of object before providing any firm conclusions. Finally, we find that the trans-relativistic object SN 2009bb even if it produces more modest magnetic field amplification can accelerate CRs up to 2-3 PeV within 20 days after the outburst.

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“…Recent Narval observations of the RSG µ Cep by Tessore et al (2017) show that this star presents, as does Betelgeuse, a much more important linear polarization signal than circular polarization. From a dedicated observational procedure performed with Narval, these authors are able to disentangle and model the crosstalk.…”

Section: Influence Of Crosstalk On Stokes V Measurementsmentioning

confidence: 94%

“…For all these detections, the polarization origin of the Stokes V signal has been evaluated, and a Zeeman origin is the most likely hypothesis. More recently, Tessore et al (2017) have detected magnetic fields at the surface of two RSGs (CE Tau and µ Cep), and P. Mathias et al: The magnetic field of Betelgeuse Table 2. Log of Narval (Stokes U and Q) observations of Betelgeuse and polarimetric measurements (see Sect.…”

Section: The Stellar Origin Of the Stokes V Signalmentioning

confidence: 99%

Evolution of the magnetic field of Betelgeuse from 2009–2017

Mathias

Aurière

Ariste

et al. 2018

A&A

Self Cite

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Context. Betelgeuse is an M-type supergiant that presents a circularly polarized (Stokes V) signal in its line profiles, interpreted in terms of a surface magnetic field. Aims. The weak circular polarization signal has been monitored over 7.5 years in order to follow its evolution on different timescales, and eventually to determine its physical origin. Linear polarization measurements have also been obtained regularly in the last few years. Methods. We used both the ESPaDOnS and Narval spectropolarimeters to obtain high signal-to-noise ratio (S/N) spectra, which were processed by means of the least-squares deconvolution (LSD) method. In order to ensure the reality of the very weak circular polarization, special care has been taken to limit instrumental effects. In addition, several tests were performed on the Stokes V signal to establish its stellar and Zeeman origin. Results. We confirm the magnetic nature of the circular polarization, pointing to a surface magnetic field of the order of 1 G. The Stokes V profiles present variations over different timescales, the most prominent one being close to the long secondary period (LSP; around 2000 d for Betelgeuse) often invoked in red evolved stars. This long period is also dominant for all the other Stokes parameters. The circular polarization is tentatively modeled by means of magnetic field concentrations mimicking spots, showing in particular that the velocity associated with each "spot" also follows the long timescale, and that this signal is nearly always slightly redshifted. Conclusions. From the coupled variations of both linear and circular polarization signatures in amplitude, velocity and timescale, we favour giant convection cells as the main engine at the origin of polarization signatures and variations in all the Stokes parameters. This strengthens support for the hypothesis that large convective cells are at the origin of the LSP.

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Measuring surface magnetic fields of red supergiant stars

Cited by 44 publications

References 51 publications

SOFIA-EXES Observations of Betelgeuse during the Great Dimming of 2019/2020

SOFIA-EXES Observations of Betelgeuse during the Great Dimming of 2019/2020

Core-collapse supernovae as cosmic ray sources

Evolution of the magnetic field of Betelgeuse from 2009–2017

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