2008/06/12 by J. -C. Bouret, J.-C. Bouret, J.-F. Donati +7 · 69 citations
Physics and Astronomy · #Astro and Planetary Science #Astrophysics and Star Formation Studies #Equipartition theorem #Field (mathematics) #Magnetic field #RADIUS #Red supergiant #Stellar evolution #Stellar, planetary, and galactic studies #Supergiant #Thermal #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2008.13575.x
published in Monthly Notices of the Royal Astronomical Society 389(1), 75-85 (Oxford University Press) · 12 pages
arxiv created 2008/06/12 · openalex publication_date 2008/07/30 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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.