Measuring surface magnetic fields of red supergiant stars

Abstract : RSG stars are very massive cool evolved stars. Recently, a weak magnetic field was measured at the surface of $\alpha$ Ori and this is so far the only M-type supergiant for which a direct detection of a surface magnetic field has been reported. By extending the search for surface magnetic field in a sample of late-type supergiants, we want to determine whether the surface magnetic field detected on $\alpha$ Ori is a common feature among the M-type supergiants. With the spectropolarimeter Narval at TBL we undertook a search for surface magnetic fields in a sample of cool supergiant stars, and we analysed circular polarisation spectra using the least-squares deconvolution technique. We detect weak Zeeman signatures of stellar origin in the targets CE Tau, $\alpha^1$ Her and $\mu$ Cep. For the latter star, we also show that cross-talk from the strong linear polarisation signals detected on this star must be taken into account. For CE Tau and $\mu$ Cep, the longitudinal component of the detected surface fields is at the Gauss-level, such as in $\alpha$~Ori. We measured a longitudinal field almost an order of magnitude stronger for $\alpha^1$ Her. We also report variability of the longitudinal magnetic field of CE Tau and $\alpha^1$ Her, with changes in good agreement with the typical atmospheric dynamics time-scales. We also report a non-detection of magnetic field at the surface of the yellow supergiant star $\rho$ Cas. The two RSG stars of our sample, CE Tau and $\mu$ Cep, display magnetic fields very similar to that of $\alpha$ Ori. The non-detection of a magnetic field on the post-RSG star $\rho$ Cas suggests that the magnetic field disappears, or at least becomes undetectable with present methods, at later evolutionary stages. Our analysis of $\alpha^1$ Her supports the proposed reclassification of the star as an AGB star.
Type de document :
Article dans une revue
Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 603, pp.A129. 〈10.1051/0004-6361/201730473〉
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Soumis le : jeudi 1 juin 2017 - 10:28:48
Dernière modification le : mercredi 17 octobre 2018 - 14:32:04

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Benjamin Tessore, Agnès Lebre, Julien Morin, Philippe Mathias, Eric Josselin, et al.. Measuring surface magnetic fields of red supergiant stars. Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 603, pp.A129. 〈10.1051/0004-6361/201730473〉. 〈hal-01531016〉



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