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.

This is the first in a series of papers in which we describe and report the analysis of a large survey of Herbig Ae/Be stars in circular spectropolarimetry. Using the ESPaDOnS and Narval high-resolution spectropolarimeters at the Canada-France-Hawaii and Bernard Lyot Telescopes, respectively, we have acquired 132 circularly-polarised spectra of 70 Herbig Ae/Be stars and Herbig candidates. The large majority of these spectra are characterised by a resolving power of about 65,000, and a spectral coverage from about 3700 Å to 1 µm. The peak signal-to-noise ratio per CCD pixel ranges from below 100 (for the faintest targets) to over 1000 (for the brightest). The observations were acquired with the primary aim of searching for magnetic fields in these objects. However, our spectra are suitable for a variety of other important measurements, including rotational properties, variability, binarity, chemical abundances, circumstellar environment conditions and structure, etc. In this first paper, we describe the sample selection, the observations and their reduction, and the measurements that will comprise the basis of much of our following analysis. We describe the determination of fundamental parameters for each target. We detail the Least-Squares Deconvolution that we have applied to each of our spectra, including the selection, editing and tuning of the LSD line masks. We describe the fitting of the LSD Stokes I profiles using a multi-component model that yields the rotationally-broadened photospheric profile (providing the projected rotational velocity and radial velocity for each observation) as well as circumstellar emission and absorption components. Finally, we diagnose the longitudinal Zeeman effect via the measured circular polarisation, and report the longitudinal magnetic field and Stokes V Zeeman signature detection probability. As an appendix, we provide a detailed review of each star observed.

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.

Context. The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not previously known to host detectable magnetic fields. Aims. We aim to confirm the field detection reported by Lignières et al. (2009, A&A, 500, L41) and provide additional observational constraints about the field characteristics, by modelling the large-scale magnetic geometry of the star and by investigating a possible seasonal variability of the reconstructed field topology. Methods. We analyse a total of 799 high-resolution circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009. Using about 1100 spectral lines, we employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20 000. The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs. Results. We confirm the detection of circularly polarized signatures in the mean line profiles. The signal shows up in four independent data sets acquired with both NARVAL and ESPaDOnS. The amplitude of the polarized signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis, as expected for a signal of magnetic origin. The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 ± 0.008 d. The reconstruction of the magnetic topology unveils a magnetic region of radial field orientation, closely concentrated around the rotation pole. This polar feature is accompanied by a small number of magnetic patches at lower latitudes. No significant variability in the field structure is observed over a time span of one year. Conclusions. The repeated observational evidence that Vega possesses a weak photospheric magnetic field strongly suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain. Vega may well be the first confirmed member of a much larger, as yet unexplored, class of weakly-magnetic stars now investigatable with the current generation of stellar spectropolarimeters.

The origin of the magnetic fields observed in some intermediate‐mass and high‐mass main‐sequence stars is still a matter of vigorous debate. The favoured hypothesis is a fossil field origin, in which the observed fields are the condensed remnants of magnetic fields present in the original molecular cloud from which the stars formed. According to this theory a few per cent of the pre‐main‐sequence (PMS) Herbig Ae/Be star should be magnetic with a magnetic topology similar to that of main‐sequence intermediate‐mass stars. After our recent discovery of four magnetic Herbig stars, we have decided to study in detail one of them, HD 200775, to determine if its magnetic topology is similar to that of the main‐sequence magnetic stars. With this aim, we monitored this star in Stokes I and V over more than 2 yr, using the new spectropolarimeters ESPaDOnS at Canada–France–Hawaii Telescope (CFHT), and Narval at Bernard Lyot Telescope (TBL). By analysing the intensity spectrum we find that HD 200775 is a double‐lined spectroscopic binary system, whose secondary seems similar, in temperature, to the primary. We have carefully compared the observed spectrum to a synthetic one, and we found no evidence of abundance anomalies in its spectrum. We infer the luminosity ratio of the components from the Stokes I profiles. Then, using the temperature and luminosity of HD 200775 found in the literature, we estimate the age, the mass and the radius of both components from their HR diagram positions. From our measurements of the radial velocities of both stars we determine the ephemeris and the orbital parameters of the system. A Stokes V Zeeman signature is clearly visible in most of the least‐squares deconvolution profiles and varies on a time‐scale on the order of 1 d. We have fitted the 30 profiles simultaneously, using a χ2 minimization method, with a centred and a decentred‐dipole model. The best‐fitting model is obtained with a reduced χ2= 1.0 and provides a rotation period of 4.3281 ± 0.0010 d, an inclination angle of 60°± 11° and a magnetic obliquity angle β= 125°± 8°. The polar strength of the magnetic dipole field is 1000 ± 150 G, which is decentred by 0.05 ± 0.04 R* from the centre of the star. The derived magnetic field model is qualitatively identical to those commonly observed in the Ap/Bp stars. Our determination of the inclination of the rotation axis leads to a radius of the primary which is smaller than that derived from the HR diagram position. This can be explained by a larger intrinsic luminosity of the secondary relative to the primary, due to a larger circumstellar extinction of the secondary relative to the primary.