We present the analysis performed on spectropolarimetric data of 97 O-type targets included in the framework of the Magnetism in Massive Stars (MiMeS) 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 non-rotational broadening velocities, and longitudinal magnetic field B sub( l). The investigation of the Stokes I profiles led to the discovery of two new multiline 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 optimizing the detection of magnetic signatures while minimizing the detection of spurious signatures in Stokes V. Using this analysis, we confirm the detection of a magnetic field in six 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 three new magnetic candidates (HD 36486, HD 162978, and HD 199579). Overall, we find a magnetic incidence rate of 7 plus or minus 3 per cent, for 108 individual O stars (including all O-type components part of multiline systems), with a median uncertainty of the B sub( 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 sub( eff), mass, and 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 magnetic field on the Of?p star HD 108. Spectropolarimetric observations conducted in 2007, 2008 and 2009, respectively, with NARVAL@Télescope Bernard Lyot (TBL) 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.
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.
We present a spectroscopic analysis of Hubble Space Telescope/Cosmic Origins Spectrograph observations of three massive stars in the low metallicity dwarf galaxies IC 1613 and WLM. These stars, were ...previously observed with Very Large Telescope (VLT)/X-shooter by Tramper et al., who claimed that their mass-loss rates are higher than expected from theoretical predictions for the underlying metallicity. A comparison of the far ultraviolet (FUV) spectra with those of stars of similar spectral types/luminosity classes in the Galaxy, and the Magellanic Clouds provides a direct, model-independent check of the mass-loss–metallicity relation. Then, a quantitative spectroscopic analysis is carried out using the non-LTE (NLTE) stellar atmosphere code cmfgen. We derive the photospheric and wind characteristics, benefiting from a much better sensitivity of the FUV lines to wind properties than Hα. Iron and CNO abundances are measured, providing an independent check of the stellar metallicity. The spectroscopic analysis indicates that Z/Z⊙ = 1/5, similar to a Small Magellanic Cloud-type environment, and higher than usually quoted for IC 1613 and WLM. The mass-loss rates are smaller than the empirical ones by Tramper et al., and those predicted by the widely used theoretical recipe by Vink et al. On the other hand, we show that the empirical, FUV-based, mass-loss rates are in good agreement with those derived from mass fluxes computed by Lucy. We do not concur with Tramper et al. that there is a breakdown in the mass-loss–metallicity relation.
Massive stars at low metallicity Bouret, J-C; Lanz, T; Martins, F ...
Astronomy and astrophysics (Berlin),
07/2013, Volume:
555
Journal Article
Peer reviewed
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. We analyzed the UV and optical spectra of twenty-three objects using the NLTE stellar atmosphere code CMFGEN and derived photospheric and wind properties.
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 $\lambda\lambda$1548, 1551, from which mass-loss rates consistent with previous CMFGEN results for O8-O9V stars were derived (${\sim}10^{-10}{-}10^{-9}~M_\odot$ yr-1). A discrepancy of roughly two orders of magnitude is found between these mass-loss rates and the values predicted by theory ($\dot{M}_{\rm Vink}$), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log $L_\star/L_\odot \la 5.2$. 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 on $\dot{M}$ were established for all objects using lines of different ions: P v $\lambda\lambda$1118, 1128, C iii $\lambda 1176$, N v $\lambda\lambda$1239, 1243, Si iv $\lambda\lambda$1394, 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 $\lambda\lambda$1548, 1551, the use of N v $\lambda\lambda$1239, 1243 results in the lowest mass-loss rates: the upper limits indicate that $\dot{M}$ must be less than about -1.0 dex $\dot{M}_{\rm Vink}$. Upper mass-loss rate limits obtained for other transitions are also low: they indicate that $\dot{M}$ must be less than about (-0.5 ± 0.2) dex $\dot{M}_{\rm Vink}$. We studied the behavior of the Hα line with different mass-loss rates. For two stars, only models with very low $\dot{M}$'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 $L_{\rm X}/L_{\rm Bol} \ga -3.5$). The validity of the models used in our analyses is discussed.
We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars, the O4If+ supergiant HD 190429A and the O4V((f)) dwarf HD 96715, using archival FUSE and IUE data. We have conducted a ...quantitative analysis using the two NLTE model atmosphere and wind codes, tlusty and cmfgen, which incorporate a detailed treatment of NLTE metal line blanketing. From the far-UV spectrum, we have derived the stellar and wind parameters and the surface composition of the two stars. The surface of HD 190429A has a composition typical of an evolved O supergiant (nitrogen-rich, carbon and oxygen-poor), while HD 96715 exhibits surface nitrogen enhancement similar to the enrichment found in SMC O dwarfs which has been attributed to rotationally-induced mixing. Following studies of Magellanic Cloud O stars, we find that homogeneous wind models could not match the observed profile of O vλ1371 and require very low phosphorus abundance to fit the P vλλ1118-1128 resonance lines. We show, on the other hand, that we are able to match the O v and P v lines using clumped wind models. In addition to these lines, we find that N ivλ1718 is also sensitive to wind clumping. For both stars, we have calculated clumped wind models that match well all these lines from different species and that remain consistent with Hα data. In particular, we have achieved an excellent match of the P v resonance doublet, indicating that our physical description of clumping is adequate. These fits therefore provide a coherent and thus much stronger evidence of wind clumping in O stars than earlier claims. We show that the success of the clumped wind models in matching these lines results from increased recombination in the clumps, hence from a better description of the wind ionization structure. We find that the wind of these two stars is highly clumped, as expressed by very small volume filling factors, namely $f_\infty = 0.04$ for HD 190429A and $f_\infty = 0.02$ for HD 96715. In agreement with our analysis of SMC stars, clumping starts deep in the wind, just above the sonic point. The most crucial consequence of our analysis is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more. These lower mass loss rates will affect substantially the evolution of massive stars. Accounting for wind clumping is essential when determining the wind properties of O stars. Our study therefore calls for a fundamental revision in our understanding of mass loss and of O-type star stellar winds.
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.
Context. Massive stars show a variety of spectral variabilities: discrete absorption components in UV P-Cygni profiles, optical line profile variability, X-ray variability, and radial velocity ...modulations. Aims. Our goal is to study the spectral variability of single OB stars to better understand the relation between photospheric and wind variability. For that, we rely on high spectral resolution and on high signal-to-noise ratio optical spectra collected with the spectrograph NARVAL on the Télescope Bernard Lyot at Pic du Midi. Methods. We investigated the variability of twelve spectral lines by means of the temporal variance spectrum. The selected lines probe the radial structure of the atmosphere from the photosphere to the outer wind. We also performed a spectroscopic analysis with atmosphere models to derive the stellar and wind properties and to constrain the formation region of the selected lines. Results. We show that variability is observed in the wind lines of all bright giants and supergiants on a daily timescale. Lines formed in the photosphere are sometimes variable, sometimes not. The dwarf stars do not show any sign of variability. If variability is observed on a daily timescale, it can also (but not always) be observed on hourly timescales, albeit with lower amplitude. There is a very clear correlation between amplitude of the variability and fraction of the line formed in the wind. Strong anti-correlations between the different parts of the temporal variance spectrum are observed. Conclusions. Our results indicate that variability is stronger in lines formed in the wind. A link between photospheric and wind variability is not obvious from our study, since wind variability is observed regardless of the level of photospheric variability. Different photospheric lines also show different degrees of variability.
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