Measuring surface magnetic fields of red supergiant stars (original) (raw)

A&A 603, A129 (2017)

1 LUPM, Université de Montpellier, CNRS, Place Eugène Bataillon, 34095 Montpellier, France
e-mail: benjamin.tessore@umontpellier.fr
2 Université de Toulouse, UPS-OMP, Institut de recherche en Astrophysique et Planétologie, 31000 Toulouse, France
3 CNRS, UMR5277, Institut de recherche en Astrophysique et Planétologie, 14 avenue Édouard Belin, 31400 Toulouse, France

Received: 20 January 2017
Accepted: 19 April 2017

Abstract

Context. Red supergiant (RSG) stars are very massive cool evolved stars. Recently, a weak magnetic field was measured at the surface of α Ori and this is so far the only M-type supergiant for which a direct detection of a surface magnetic field has been reported.

Aims. 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 α Ori is a common feature among the M-type supergiants.

Methods. With the spectropolarimeter Narval at Télescope Bernard-Lyot 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.

Results. We detect weak Zeeman signatures of stellar origin in the targets CE Tau, _α_1 Her and μ 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 μ Cep, the longitudinal component of the detected surface fields is at the Gauss-level, such as in α Ori. We measured a longitudinal field almost an order of magnitude stronger for _α_1 Her. We also report variability of the longitudinal magnetic field of CE Tau and _α_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 ρ Cas.

Conclusions. The two RSG stars of our sample, CE Tau and μ Cep, display magnetic fields very similar to that of α Ori. The non-detection of a magnetic field on the post-RSG star ρ Cas suggests that the magnetic field disappears, or at least becomes undetectable with present methods, at later evolutionary stages. Our analysis of _α_1 Her supports the proposed reclassification of the star as an M-type asymptotic giant branch star.

Key words: supergiants / stars: late-type / stars: magnetic field / techniques: polarimetric

⋆

Based on observations obtained at the Télescope Bernard Lyot (TBL) at the Observatoire du Pic du Midi, operated by the Observatoire Midi-Pyrénées, Université de Toulouse (Paul Sabatier), Centre National de la Recherche Scientifique of France.