2008MNRAS.389...75B

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 40M_☉ star with a radius of 25R_☉and an age of about 5-6Myr, showing no surface nitrogen enhancement and losing mass at a rate of about 2x10^–6M_☉/yr.

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.