From observations made with the ESPaDOnS spectropolarimeter, recently installed on the 3.6‐m Canada–France–Hawaii Telescope, we report the discovery of a strong magnetic field in the Of?p spectrum variable HD 191612 – only the second known magnetic O star (following θ1 Ori C). The stability of the observed Zeeman signature over four nights of observation, together with the non‐rotational shape of line profiles, argues that the rotation period of HD 191612 is significantly longer than the 9‐d value previously proposed. We suggest that the recently identified 538‐d spectral variability period is the rotation period, in which case the observed line‐of‐sight magnetic field of −220 ± 38 G implies a large‐scale field (assumed dipolar) with a polar strength of about −1.5 kG. If confirmed, this scenario suggests that HD 191612 is, essentially, an evolved version of the near‐zero‐age main‐sequence magnetic O star θ1 Ori C, but with an even stronger field (about 15 kG at an age similar to that of θ1 Ori C). We suggest that the rotation rate of HD 191612, which is exceptionally slow by accepted O‐star standards, could be due to angular momentum dissipation through a magnetically confined wind.

We present the analysis performed on spectropolarimetric data of 97 O-type targets included in the framework of the MiMeS (Magnetism in Massive Stars) Survey. Mean Least-Squares Deconvolved Stokes I and V line profiles were extracted for each observation, from which we measured the radial velocity, rotational and nonrotational broadening velocities, and longitudinal magnetic field B ℓ . The investigation of the Stokes I profiles led to the discovery of 2 new multi-line spectroscopic systems (HD 46106, HD 204827) and confirmed the presence of a suspected companion in HD 37041. We present a modified strategy of the Least-Squares Deconvolution technique aimed at optimising the detection of magnetic signatures while minimising the detection of spurious signatures in Stokes V . Using this analysis, we confirm the detection of a magnetic field in 6 targets previously reported as magnetic by the MiMeS collaboration (HD 108, HD 47129A2, HD 57682, HD 148937, CPD-28 2561, and NGC 1624-2), as well as report the presence of signal in Stokes V in 3 new magnetic candidates (HD 36486, HD 162978, HD 199579). Overall, we find a magnetic incidence rate of 7 ± 3%, for 108 individual O stars (including all O-type components part of multi-line systems), with a median uncertainty of the B ℓ measurements of about 50 G. An inspection of the data reveals no obvious biases affecting the incidence rate or the preference for detecting magnetic signatures in the magnetic stars. Similar to A-and B-type stars, we find no link between the stars' physical properties (e.g. T eff , mass, age) and the presence of a magnetic field. However, the Of?p stars represent a distinct class of magnetic O-type stars.

We report the detection of a strong, organized magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6‐m Canada–France–Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Programme. From the fitting of our spectra using non‐local thermodynamic equilibrium model atmospheres, we determined that HD 57682 is a 17+19−9 M⊙ star with a radius of 7.0+2.4−1.8 R⊙ and a relatively low mass‐loss rate of 1.4+3.1−0.95× 10−9 M⊙ yr−1. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer v sin i= 15 ± 3 km s−1. This v sin i implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes V profiles and longitudinal field. Using a Bayesian analysis of the velocity‐resolved Stokes V profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of 1680+134−356 G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.

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 θ1 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 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 × 10−6 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 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 θ1 Ori 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.

We report the detection of a magnetic field on the Of?p star HD 108. Spectropolarimetric observations conducted in and Echelle SpectroPolarimetric Device for the Observation of Stars at Canada-France-Hawaii Telescope (ESPaDOnS@CFHT) reveal a clear Zeeman signature in the average Stokes V profile, stable on time-scales of days to months and slowly increasing in amplitude on time-scales of years. We speculate that this time-scale is the same as that on which Hα emission is varying and is equal to the rotation period of the star. The corresponding longitudinal magnetic field, measured during each of the three seasons, increases slowly from 100 to 150 G, implying that the polar strength of the putatively dipolar large-scale magnetic field of HD 108 is at least 0.5 kG and most likely of the order of 1-2 kG.The stellar and wind properties are derived through a quantitative spectroscopic analysis with the code CMFGEN. The effective temperature is difficult to constrain because of the unusually strong He I λλ4471, 5876 lines. Values in the range of 33 000-37 000 K are preferred. A mass-loss rate of about 10 −7 M yr −1 (with a clumping factor f = 0.01) and a wind terminal velocity of 2000 km s −1 are derived. The wind confinement parameter η is larger than 100, implying that the wind of HD 108 is magnetically confined.Stochastic short-term variability is observed in the wind-sensitive lines but not in the photospheric lines, excluding the presence of pulsations. Material infall in the confined wind is the most likely origin for lines formed in the inner wind. Wind clumping also probably causes part of the Hα variability. The projected rotational velocity of HD 108 is lower than 50 km s −1 , consistent with the spectroscopic and photometric variation time-scales of a few decades. Overall, HD 108 is very similar to the magnetic O star HD 191612 except for an even slower rotation.