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.