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Title:
The weak magnetic field of the O9.7 supergiant ζOrionisA
Authors:
Bouret, J.-C.; Donati, J.-F.; Martins, F.; Escolano, C.; Marcolino, W.; Lanz, T.; Howarth, I. D.
Affiliation:
AA(LAM-UMR 6110, CNRS & University de Provence, 38 rue Frédéric Joliot-Curie, F-13388 Marseille cedex 13, France), AB(LATT-UMR 5572, CNRS & University de Toulouse, 14 Av. E. Belin, F-31400 Toulouse, France), AC(GRAAL-UMR 5024, CNRS & University de Montpellier II, Place Bataillon, F-34095 Montpellier, France), AD(LAM-UMR 6110, CNRS & University de Provence, 38 rue Frédéric Joliot-Curie, F-13388 Marseille cedex 13, France), AE(LAM-UMR 6110, CNRS & University de Provence, 38 rue Frédéric Joliot-Curie, F-13388 Marseille cedex 13, France), AF(Department of Astronomy, University of Maryland, College Park, MD 20742, USA), AG(Department of Physics and Astronomy, University College London, Gower Street, London WC1E6BT)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 389, Issue 1, pp. 75-85. (MNRAS Homepage)
Publication Date:
09/2008
Origin:
MNRAS
MNRAS Keywords:
stars: early-type , stars: individual: ζOriA , stars: magnetic fields , stars: rotation , stars: winds, outflows
DOI:
10.1111/j.1365-2966.2008.13575.x
Bibliographic Code:
2008MNRAS.389...75B

Abstract

We report here the detection of a weak magnetic field of 50-100G on the O9.7 supergiant ζOrionisA (ζOriA), using spectropolarimetric observations obtained with NARVAL at the 2-m Télescope Bernard Lyot atop Pic du Midi (France). ζOriA is the third O star known to host a magnetic field (along with θ1OriC and HD191612), and the first detection on a `normal' rapidly rotating O star. The magnetic field of ζOriA is the weakest magnetic field ever detected on a massive star. The measured field is lower than the thermal equipartition limit (about 100G). By fitting non-local thermodynamic equilibrium (NLTE) model atmospheres to our spectra, we determined that ζOriA is a 40Msolar star with a radius of 25Rsolar and an age of about 5-6Myr, showing no surface nitrogen enhancement and losing mass at a rate of about 2 × 10-6Msolaryr-1.

The magnetic topology of ζOriA is apparently more complex than a dipole and involves two main magnetic polarities located on both sides of the same hemisphere; our data also suggest that ζOriA rotates in about 7.0d and is about 40° away from pole-on to an Earth-based observer. Despite its weakness, the detected magnetic field significantly affects the wind structure; the corresponding Alfvén radius is however very close to the surface, thus generating a different rotational modulation in wind lines than that reported on the two other known magnetic O stars.

The rapid rotation of ζOriA with respect to θ1OriC appears as a surprise, both stars having similar unsigned magnetic fluxes (once rescaled to the same radius); it may suggest that the subequipartition field detected on ζOriA is not a fossil remnant (as opposed to that of θ1 OriC and HD191612), but the result of an exotic dynamo action produced through magnetohydrodynamics (MHD) instabilities.

Based on observations obtained at the Télescope Bernard Lyot (TBL), operated by the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique of France.

E-mail: jean-claude.bouret@oamp.fr

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