Context. The evolution of massive stars is still partly unconstrained. Mass, metallicity, mass loss, and rotation are the main drivers of stellar evolution. Binarity and the magnetic field may also significantly affect the fate of massive stars. Aims. Our goal is to investigate the evolution of single O stars in the Galaxy. Methods. For that, we used a sample of 74 objects comprising all luminosity classes and spectral types from O4 to O9.7. We relied on optical spectroscopy obtained in the context of the MiMeS survey of massive stars. We performed spectral modelling with the code CMFGEN. We determined the surface properties of the sample stars, with special emphasis on abundances of carbon, nitrogen, and oxygen. Results. Most of our sample stars have initial masses in the range of 20 to 50 M . We show that nitrogen is more enriched and carbon and oxygen are more depleted in supergiants than in dwarfs, with giants showing intermediate degrees of mixing. CNO abundances are observed in the range of values predicted by nucleosynthesis through the CNO cycle. More massive stars, within a given luminosity class, appear to be more chemically enriched than lower mass stars. We compare our results with predictions of three types of evolutionary models and show that for two sets of models, 80% of our sample can be explained by stellar evolution including rotation. The effect of magnetism on surface abundances is unconstrained. Conclusions. Our study indicates that in the 20−50 M mass range, the surface chemical abundances of most single O stars in the Galaxy are fairly well accounted for by stellar evolution of rotating stars.

Aims. We investigate the stellar and wind properties of a sample of late-type O dwarfs. Previous analyses of such stars have found very low mass-loss rates; rates much lower than predicted by theory (the weak wind problem). Methods. Far-UV to optical spectra of five Galactic O stars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788 (O8V/O9V), ζ Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used a grid of TLUSTY models to obtain effective temperatures, gravities, rotational velocities, and to identify wind lines. Wind parameters for each object were obtained using expanding atmosphere models calculated with the CMFGEN code.Results. The spectra of our sample have primarily a photospheric origin. A weak wind signature is seen in C iv λλ1548, 1551, from which mass-loss rates consistent with previous CMFGEN results for O8-O9V stars were derived (∼10 −10 −10 −9 M yr −1 ). A discrepancy of roughly two orders of magnitude is found between these mass-loss rates and the values predicted by theory (Ṁ Vink ), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log L /L < ∼ 5.2. We have estimated the carbon abundance for the stars of our sample and concluded that its value cannot be reduced to sufficiently small values to solve the weak wind problem. Upper limits onṀ were established for all objects using lines of different ions: P v λλ1118, 1128, C iii λ1176, N v λλ1239, 1243, Si iv λλ1394, 1403, and N iv λ1718. All the values obtained are in disagreement with theoretical predictions, bringing support to the reality of weak winds. Together with C iv λλ1548, 1551, the use of N v λλ1239, 1243 results in the lowest mass-loss rates: the upper limits indicate thatṀ must be less than about −1.0 dexṀ Vink . Upper mass-loss rate limits obtained for other transitions are also low: they indicate thatṀ must be less than about (−0.5 ± 0.2) dexṀ Vink . We studied the behavior of the Hα line with different mass-loss rates. For two stars, only models with very lowṀ's provide the best fit to the UV and optical spectra. We also explored ways to fit the observed spectra with the theoretical mass-loss rates. By using large amounts of X-rays, we could reduce the predicted wind emission to the observed levels. However, unrealistic X-ray luminosities had to be used (log L X /L Bol > ∼ −3.5). The validity of the models used in our analyses is discussed.

Aims. We aim to study the properties of massive stars at low metallicity, with an emphasis on their evolution, rotation, and surface abundances. We focus on O-type dwarfs in the Small Magellanic Cloud. These stars are expected to have weak winds that do not remove significant amounts of their initial angular momentum. Methods. We analyzed the UV and optical spectra of twenty-three objects using the NLTE stellar atmosphere code  and derived photospheric and wind properties. Results. The observed binary fraction of the sample is ≈26%, which is consistent with more systematic studies if one considers that the actual binary fraction is potentially larger owing to low-luminosity companions and that the sample was biased because it excluded obvious spectroscopic binaries. The location of the fastest rotators in the Hertzsprung-Russell (H-R) diagram built with fast-rotating evolutionary models and isochrones indicates that these could be several Myr old. The offset in the position of these fast rotators compared with the other stars confirms the predictions of evolutionary models that fast-rotating stars tend to evolve more vertically in the H-R diagram. Only one star of luminosity class Vz, expected to best characterize extreme youth, is located on the zero-age main sequence, the other two stars are more evolved. We found that the distribution of O and B stars in the (N) -v sin i diagram is the same, which suggests that the mechanisms responsible for the chemical enrichment of slowly rotating massive stars depend only weakly on the star's mass. We furthermore confirm that the group of slowly rotating N-rich stars is not reproduced by the evolutionary tracks. Even for more massive stars and faster rotators, our results call for stronger mixing in the models to explain the range of observed N abundances. All stars have an N/C ratio as a function of stellar luminosity that match the predictions of the stellar evolution models well. More massive stars have a higher N/C ratio than the less massive stars. Faster rotators show on average a higher N/C ratio than slower rotators, again consistent with the expected trend of stronger mixing as rotation increases. When comparing the N/O versus N/C ratios with those of stellar evolution models, the same global qualitative agreement is reached. The only discrepant behavior is observed for the youngest two stars of the sample, which both show very strong signs of mixing, which is unexpected for their evolutionary status.

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.

This paper reports high‐precision Stokes V spectra of HD 191612 acquired using the ESPaDOnS spectropolarimeter at the Canada–France–Hawaii Telescope, in the context of the Magnetism in Massive Stars (MiMeS) Project. Using measurements of the equivalent width of the Hα line and radial velocities of various metallic lines, we have updated both the spectroscopic and orbital ephemerides of this star. We confirm the presence of a strong magnetic field in the photosphere of HD 191612, and detect its variability. We establish that the longitudinal field varies in a manner consistent with the spectroscopic period of 537.6 d, in an approximately sinusoidal fashion. The phases of minimum and maximum longitudinal field are, respectively, coincident with the phases of maximum and minimum Hα equivalent width and Hp magnitude. This demonstrates a firm connection between the magnetic field and the processes responsible for the line and continuum variability. Interpreting the variation of the longitudinal magnetic field within the context of the dipole oblique rotator model, and adopting an inclination i= 30° obtained assuming alignment of the orbital and rotational angular momenta, we obtain a best‐fitting surface magnetic field model with obliquity β= 67°± 5° and polar strength Bd= 2450 ± 400 G. The inferred magnetic field strength implies an equatorial wind magnetic confinement parameter η*≃ 50, supporting a picture in which the Hα emission and photometric variability have their origin in an oblique, rigidly rotating magnetospheric structure resulting from a magnetically channelled wind. This interpretation is supported by our successful Monte Carlo radiative transfer modelling of the photometric variation, which assumes the enhanced plasma densities in the magnetic equatorial plane above the star implied by such a picture, according to a geometry that is consistent with that derived from the magnetic field. Predictions of the continuum linear polarization resulting from Thompson scattering from the magnetospheric material indicate that the Stokes Q and U variations are highly sensitive to the magnetospheric geometry, and that expected amplitudes are in the range of current instrumentation.

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.

We report the discovery and analysis of a very strong magnetic field in the rapidly rotating early B‐type star HR 5907, based on observations obtained as part of the Magnetism in Massive Stars (MiMeS) project. We infer a rotation period of 0.508 276+0.000 015−0.000 012 d from photometric and Hα EW measurements, making this the shortest period, non‐degenerate, magnetic massive star known to date. From the comparison of IUE UV and optical spectroscopy with LTE bruce/kylie models we find a solid‐angle integrated, uniform black‐body temperature of 17 000 ± 1000 K, a projected rotational velocity of 290 ± 10 km s−1, an equatorial radius of 3.1 ± 0.2 R⊙, a stellar mass of 5.5 ± 0.5 M⊙, and an inclination angle of the rotation axis to our line‐of‐sight of 70 ± 10°. Our measurements of the longitudinal magnetic field, which vary between −500 and −2000 G, phase coherently with the rotation period and imply a surface dipole field strength of ∼15.7 kG. On the other hand, from fits to mean Least‐Squares Deconvolved Stokes V line profiles we infer a dipole field strength of ∼10.4 kG. This disagreement may result from a magnetic configuration more complex than our model, and/or from the non‐uniform helium surface abundance distribution. In either case we obtain a magnetic obliquity nearly aligned with the rotation axis (). Our optical spectroscopy also shows weak variability in carbon, silicon and nitrogen lines. The emission variability in hydrogen Balmer and Paschen lines indicates the presence of a dense, highly structured magnetosphere, interpreted as a centrifugally supported, magnetically confined circumstellar disc.