Journal Article

The weak magnetic field of the O9.7 supergiant ζ Orionis A*

J.-C. Bouret, J.-F. Donati, F. Martins, C. Escolano, W. Marcolino, T. Lanz and I. D. Howarth

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 389, issue 1, pages 75-85
Published in print September 2008 | ISSN: 0035-8711
Published online August 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13575.x
The weak magnetic field of the O9.7 supergiant ζ Orionis A*

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We report here the detection of a weak magnetic field of 50–100 G on the O9.7 supergiant ζ Orionis A (ζ Ori A), using spectropolarimetric observations obtained with NARVAL at the 2-m Télescope Bernard Lyot atop Pic du Midi (France). ζ Ori A is the third O star known to host a magnetic field (along with θ1Ori C and HD 191612), and the first detection on a ‘normal’ rapidly rotating O star. The magnetic field of ζ Ori A is the weakest magnetic field ever detected on a massive star. The measured field is lower than the thermal equipartition limit (about 100 G). By fitting non-local thermodynamic equilibrium (NLTE) model atmospheres to our spectra, we determined that ζ Ori A is a 40 M star with a radius of 25 R and an age of about 5–6 Myr, showing no surface nitrogen enhancement and losing mass at a rate of about 2 × 10−6M yr−1.

The magnetic topology of ζ Ori A 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 ζ Ori A rotates in about 7.0 d 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 ζ Ori A with respect to θ1Ori C 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 ζ Ori A is not a fossil remnant (as opposed to that of θ1 Ori C and HD 191612), but the result of an exotic dynamo action produced through magnetohydrodynamics (MHD) instabilities.

Keywords: stars: early-type; stars: individual: ζ Ori A; stars: magnetic fields; stars: rotation; stars: winds, outflows

Journal Article.  7832 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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