The weak magnetic field of the O9.7 supergiant {zeta} Orionis A

• Published in: Monthly Notices of the Royal Astronomical Society
• Main Authors: Bouret, J.-C., Donati, J.-F., Martins, F., Escolano, C., Marcolino, W., Lanz, T., Howarth, I. D.
• Format: Text
• Language: English
• Published: Oxford University Press 2008
• Subjects: Papers; narval
• Online Access: http://mnras.oxfordjournals.org/cgi/content/short/389/1/75; https://doi.org/10.1111/j.1365-2966.2008.13575.x

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.