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

• Main Authors: J. -c. Bouret, J. -f. Donati, F. Martins, C. Escolano, W. Marcolino, T. Lanz, I. D. Howarth
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2008
• Subjects: narval
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.311.5964; http://arxiv.org/pdf/0806.2162v1.pdf

We report here the detection of a weak magnetic field of 50–100 G on the O9.7 supergiant ζ Ori A, using spectropolarimetric observations obtained with NARVAL at the 2m Télescope Bernard Lyot atop Pic du Midi (France). ζ Ori A is the third O star known to host a magnetic field (along with θ 1 Ori 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 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 −6 M ⊙ 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