The magnetic field of the supergiant star ζ Ori A

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Scientific paper

We present the results obtained on the O9.7 supergiant ζ Ori with the spectropolarimeter NARVAL at the 2M Telescope Bernard Lyot atop Pic du Midi (France). We detected the presence of a weak magnetic field of about 50-100G, making ζ Ori the third O star known to host a magnetic field and the first magnetic O star with a 'normal' rotationnal velocity. The magnetic field of Zeta Ori is the weakest magnetic field ever detected on a massive star and is lower than the thermal equipartition limit (about 100 G). By fitting synthetic spectra (obtained from NLTE stellar atmosphere models), we derived the physical properties of ζ Ori. This lattest is a 40 M_{&sun;} star, with a radius of 25 R_{&sun;} and appears quite evolved with an age of 5-6Myr. Despite its evolutionnary status, ζ Ori does not show signs of nitrogen surface enrichment. Concerning the wind of ζ Ori, we estimated a mass loss rate of about 2×10^{-6} M_{&sun;}.yr^{-1}. The magnetic topology of ζ Ori is apparently more complex than a simple dipole and involves two main magnetic polarities located on both sides of the northern hemisphere. Our data also suggest that ζ Ori rotates in about 7.0 days and is about 40 degrees away from pole-on to an Earth-based observer. Despite its weakness, the detected field appears sufficient to affect significantly the wind structure: the corresponding Alfvén radius is however very close to the surface of the star, thus generating a rotational modulation in wind lines different than that reported on the two other known magnetic O stars.
Finally, the rapid rotation of ζ Ori with respect to θ^{1} Ori C is surprising since both stars have similar unsigned magnetic fluxes (once rescaled to the same radius). This may indicate that the field of ζ Ori is not a fossil remnant (as opposed to that of θ^{1} Ori C and HD191612) but rather the result of exotic dynamo processes produced through MHD instabilities.

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