The MiMeS (Magnetism in Massive Stars) project is a large-scale, high-resolution, sensitive spectropolarimetric investigation of the magnetic properties of O- and early B-type stars. Initiated in ...2008 and completed in 2013, the project was supported by three Large Program allocations, as well as various programmes initiated by independent principal investigators, and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation of Stars) at the Canada–France–Hawaii Telescope, Narval at the Télescope Bernard Lyot and HARPSpol at the European Southern Observatory La Silla 3.6 m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications.
Abstract
The intriguing WN4b star WR 6 has been known to display epoch-dependent spectroscopic, photometric and polarimetric variability for several decades. In this paper, we set out to verify if a ...simplified analytical model in which corotating interaction regions (CIRs) threading an otherwise spherical wind is able to reproduce the many broad-band continuum light curves from the literature with a reasonable set of parameters. We modified the optically thin model developed by Ignace, St-Louis & Proulx-Giraldeau to approximately account for multiple scattering and used it to fit 13 separate data sets of this star. By including two CIRs in the wind, we obtained reasonable fits for all data sets with coherent values for the inclination of the rotation axis (i0 = 166°) and for its orientation in the plane of the sky, although in the latter case we obtained two equally acceptable values (ψ = 63° and 152°) from the polarimetry. Additional line profile variation simulations using the Sobolev approximation for the line transfer allowed us to eliminate the ψ = 152° solution. With the adopted configuration (i0 = 166° and ψ = 63°), we were able to reproduce all data sets relatively well with two CIRs located near the stellar equator and always separated by ∼90° in longitude. The epoch dependence comes from the fact that these CIRs migrate along the surface of the star. Density contrasts smaller than a factor of 2 and large opening angles for the CIR ($\beta \gtrapprox 35^{\circ}$) were found to best reproduce the type of spectroscopic variability reported in the literature.
The very massive binary NGC 3603-A1 Schnurr, O.; Casoli, J.; Chené, A.-N. ...
Monthly notices of the Royal Astronomical Society. Letters,
September 2008, Volume:
389, Issue:
1
Journal Article
Peer reviewed
Open access
Using Very Large Telescope/Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI), we have obtained repeated adaptive-optics assisted, near-infrared spectroscopy of the three ...central WN6ha stars in the core of the very young (∼1 Myr), massive and dense Galactic cluster NGC 3603. One of these stars, NGC 3603-A1, is a known 3.77 d, double-eclipsing binary, while another one, NGC 3603-C, is one of the brightest X-ray sources among all known Galactic WR stars, which usually is a strong indication for binarity. Our study reveals that star C is indeed an 8.9-d binary, although only the WN6ha component is visible in our spectra; therefore, we temporarily classify star C as an SB1 system. A1, on the other hand, is found to consist of two emission-line stars of similar, but not necessarily of identical spectral type, which can be followed over most the orbit. Using radial velocities for both components and the previously known inclination angle of the system, we are able to derive absolute masses for both stars in A1. We find M1 = (116 ± 31) M⊙ for the primary and M2 = (89 ± 16) M⊙ for the secondary component of A1. While uncertainties are large, A1 is intrinsically half a magnitude brighter than WR20a, the current record holder with 83 and 82 M⊙, respectively; therefore, it is likely that the primary in A1 is indeed the most massive star weighed so far.
In this paper, high quality spectropolarimetric observations of the Wolf-Rayet (WR) star WR1 (HD 4004) obtained with ESPaDOnS at the Canada-France-Hawaii Telescope are presented. All major emission ...lines present in the spectrum show depolarization in the relative Stokes parameters Q/I and U/I. From the behavior of the amount of line depolarization as a function of line strength, the intrinsic continuum light polarization of WR1 is estimated to be P/I = 0.443% + or - 0.028% with an angle of theta = -26degrees.2. Although such a level of polarization could in principle be caused by a wind flattened by fast rotation, the scenario in which it is a consequence of the presence of corotating interaction regions (CIRs) in the wind is preferred. This is supported by previous photometric and spectroscopic observations showing periodic variations with a period of 16.9 days. This is now the third WR star thought to exhibit CIRs in its wind that is found to have line depolarization. Previous authors have found a strong correlation between line depolarization and the presence of an ejected nebula, which they interpret as a sign that the star has relatively recently reached the WR phase since the nebula are thought to dissipate very fast. In cases where the presence of CIRs in the wind is favored to explain the depolarization across spectral lines, the above-mentioned correlation may indicate that those massive stars have only very recently transited from the previous evolutionary phase to the WR phase.
Context. The BRIght Target Explorer (BRITE) mission collects photometric time series in two passbands with the aim of investigating stellar structure and evolution. Since their launches in the years ...2013 and 2014, the constellation of five BRITE nano-satellites has observed a total of more than 700 individual bright stars in 64 fields. Some targets have been observed multiple times. Thus, the total time base of the datasets acquired for those stars can be as long as nine years. Aims. Our aim is to provide a complete description of ready-to-use BRITE data, to show the scientific potential of the BRITE-Constellation data by identifying the most interesting targets, and to demonstrate how scientists can use these data in their research, and encourage them to do so. Methods. We applied a decorrelation process to the automatically reduced BRITE-Constellation data to correct for instrumental effects. We performed a statistical analysis of the light curves obtained for the 300 stars observed in the first 14 fields during the first ~2.5 yr of the mission. We also performed cross-identification with the International Variable Star Index. Results. We present the data obtained by the BRITE-Constellation mission in the first 14 fields it observed, from November 2013 to April 2016. We also describe the properties of the data for these fields and the 300 stars observed in them. Using these data, we detect variability in 64% of the presented sample of stars. Sixty-four stars, or 21.3% of the sample, have not yet been identified as variable in the literature and their data have not been analysed in detail. They can therefore provide valuable scientific material for further research. All data are made publicly available through the BRITE Public Data Archive and the Canadian Astronomy Data Centre.
Context. The envelopes of stars near the Eddington limit are prone to various instabilities. A high Eddington factor in connection with the iron opacity peak leads to convective instability, and a ...corresponding envelope inflation may induce pulsational instability. Here, we investigate the occurrence and consequences of both instabilities in models of Wolf-Rayet stars. Aims. We determine the convective velocities in the sub-surface convective zones to estimate the amplitude of the turbulent velocity at the base of the wind that potentially leads to the formation of small-scale wind structures, as observed in several Wolf-Rayet stars. We also investigate the effect of stellar wind mass loss on the pulsations of our stellar models. Methods. We approximated solar metallicity Wolf-Rayet stars in the range 2−17 M⊙ by models of mass-losing helium stars, computed with the Bonn stellar evolution code. We characterized the properties of convection in the envelope of these stars adopting the standard mixing length theory. Results. Our results show the occurrence of sub-surface convective regions in all studied models. Small (≈1 km s-1) surface velocity amplitudes are predicted for models with masses below ≈10 M⊙. For models with M ≳ 10 M⊙, the surface velocity amplitudes are of the order of 10 km s-1. Moreover we find the occurrence of pulsations for stars in the mass range 9−14 M⊙, while mass loss appears to stabilize the more massive Wolf-Rayet stars. We confront our results with observationally derived line variabilities of 17 WN stars, of which we analysed eight here for the first time. The data suggest variability to occur for stars above 10 M⊙, which is increasing linearly with mass above this value, in agreement with our results. We further find our models in the mass range 9−14M⊙ to be unstable to radial pulsations, and predict local magnetic fields of the order of hundreds of gauss in Wolf-Rayet stars more massive than ≈10 M⊙. Conclusions. Our study relates the surface velocity fluctuations induced by sub-surface convection to the formation of clumping in the inner part of the wind. From this mechanism, we expect a stronger variability in more massive Wolf-Rayet stars, and a weaker variability in corresponding low metallicity Wolf-Rayet stars.
ABSTRACT
Massive star winds are structured both stochastically (‘clumps’) and often coherently (Co-rotation Interaction Regions, or CIRs). Evidence for CIRs threading the winds of Wolf–Rayet (WR) ...stars arises from multiple diagnostics including linear polarimetry. Some observations indicate changes in polarization position angle across optical recombination emission lines from a WR star wind but limited to blueshifted Doppler velocities. We explore a model involving a spherical wind with a single conical CIR stemming from a rotating star as qualitative proof-of-concept. To obtain a realistic distribution of limb polarization and limb darkening across the pseudo-photosphere formed in the optically thick wind of a WR star, we used Monte Carlo radiative transfer (MCRT). Results are shown for a parameter study. For line properties similar to WR 6 (EZ CMa; HD 50896), combining the MCRT results, a simple model for the CIR, and the Sobolev approximation for the line formation, we were able to reproduce variations in both polarization amplitude and position angle commensurate with observations. Characterizing CIRs in WR winds has added importance for providing stellar rotation periods since the vsin i values are unobtainable because the pseudo-photosphere forms in the wind itself.
ABSTRACT
Θ Mus = HD 113904 is a massive multiple system containing the WC5/6 + O6/7V binary WR48 in a $19.1\, \mathrm{ d}$ circular orbit. Previous attempts to constrain the variable photometric ...properties of this binary subsystem have been thwarted by the dominating stochastically variable light from a 10-times brighter blue supergiant (BSG), located only 46 mas away. Even now, with extensive optical space-based photometry from one of the BRITE-Constellation satellites, we were unable to beat down the intrinsic stochastic variability from the BSG enough to provide a convincing detection of a low-level atmospheric eclipse of the WC + O system, as often seen in other short-period WR + O systems. We explore the variability of the dominating BSG and find that its behaviour is similar to that of other BSGs, with a forest of low-frequency Fourier peaks likely from stochastic gravity waves reaching the stellar surface. Then, by adopting an orbital inclination from another more reliable source, we obtain a clumping-independent, linear-density-dependent upper limit of the mass-loss rate for the WR component of $(6.5 \pm 0.3) \times 10^{-5}\, \mathrm{ M}_{\odot }\,\mathrm{ yr}^{ -1}$, which is consistent with values of other WC5/6 subtypes. This corresponds to an upper limit of 5.0 ± 0.2 mmag for the depth of the atmospheric eclipse in the WR48 subsystem when observed together with the BSG.
ABSTRACT
WR22 = HD 92740 is a bright (V = 6.4 mag), intrinsically luminous, double-line WN7h + O9III-V binary exhibiting one sharp 8 per cent deep eclipse near periastron in its elliptical (e ... = 0.6) 80-day orbit, when the WR-star passes in front of the O star, with no secondary eclipse. We apply two models (L96, A13) to probe the optical space-based light curves from BRITE-Constellation, including three separate, complete eclipses, that show increased (o-c) scatter compared to the rest of the observations outside the eclipses, likely due to O-star light encountering WR wind-clumps. L96 is a simple atmospheric-eclipse model, often applied to close WR + O binaries, where the O-star is considered a point-source. A13 considers a finite-disk O-star and allows for atmospheric, photospheric and reflection components to the eclipse, permitting a better characterization of its shape through a more physically realistic description of the structures for both stars in WR22. Nevertheless, A13 is still susceptible to uncertainties in the luminosity of the O-star before unique values for the orbital inclination and WR mass-loss rate can be estimated. We present solutions for the two extremes of the O-star, O9V and O9III. As photometry alone cannot allow us to discriminate between these, we compared our results to the spectral models found in the literature and determined the correct solution to be O9V. Our best-fitting A13 Model 1 gives i = 83.5 ± 0.4°, with $\dot{M}_{\rm WR} = (1.86 \pm 0.2) \times 10^{-5} \dot{M}_{\odot }/yr$. The flux ratio in the red BRITE band in this model is FO/FWR = 0.064 ± 0.002.
ABSTRACT
The hot WN star WR 2 (HD 6327) has been claimed to have many singular characteristics. To explain its unusually rounded and relatively weak emission line profiles, it has been proposed that ...WR 2 is rotating close to break-up with a magnetically confined wind. Alternatively, the line profiles could be explained by the dilution of WR 2’s spectrum by that of a companion. In this paper, we present a study of WR 2 using near-infrared AO imaging and optical spectroscopy and polarimetry. Our spectra reveal the presence of weak photospheric absorption lines from a B 2.5–4V companion, which however contributes only 5–10 per cent to the total light, suggesting that the companion is a background object. Therefore, its flux cannot be causing any significant dilution of the WR star’s emission lines. The absence of intrinsic linear continuum polarization from WR 2 does not support the proposed fast rotation. Our Stokes V spectrum was not of sufficient quality to test the presence of a moderately strong organized magnetic field but our new modelling indicates that to confine the wind the putative magnetic field must be significantly stronger than was previously suggested sufficiently strong as to make its presence implausible.
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