An X‐Ray Survey of Wolf‐Rayet Stars in the Magellanic Clouds. I. TheChandraACIS Data Set

Wolf-Rayet stars (WR stars) were discovered by French astronomers Charles Wolf and Georges Rayet in 1867. The Wolf-Rayet (WR) stars are the evolved descents of the most massive, extremely hot (temperatures up to 200000 K) and very luminous (10<sup>5</sup><inline-formula><tex-math id="Z-20191028034712-1">\begin{document}$ L_{\odot} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034712-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034712-1.png"/></alternatives></inline-formula>－10<sup>6</sup><inline-formula><tex-math id="M1">\begin{document}$L_{\odot}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M1.png"/></alternatives></inline-formula>) O stars, with 25<inline-formula><tex-math id="Z-20191028034831-1">\begin{document}$ M_{\odot} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034831-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034831-1.png"/></alternatives></inline-formula>－30<inline-formula><tex-math id="M2">\begin{document}$M_{\odot}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M2.png"/></alternatives></inline-formula> solar mass for solar metallicity. The WR stars possess very strong stellar winds, which have velocities up to 3000 km/s and wind mass loss rate <inline-formula><tex-math id="M200">\begin{document}$10^{-5} M_{\odot}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M200.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M200.png"/></alternatives></inline-formula> a year. These winds are observed in the broad emission line profiles (sometimes, even P-Cygni profiles) of WR spectra in the optical and UV range. Actually, these winds are so strong that they can peel the star and convert it into a nude nucleus without envelope. It has been found that three bright galactic stars located at Cygnus region have broad strong emission bands, rather than absorptions lines, superposed on the typical continuum of hot stars. In 1930 Beals correctly identified these features as emission lines produced by high ionized elements such as helium, carbon, nitrogen and oxygen. The physical factors which can affect the evolution of WR stars are explored in this paper. These physical factors include stellar mass, initial velocities, orbital periods, metallicities, etc. According to the equations for angular momentum transfer and chemical element diffusion, we can ascertainhow these physical factors influence the evolution of WR stars and the mixing of chemical elements in WR stars.The result indicates that massive stars with high initial velocities and metallicities have strong stellar winds and be prone to producing WR stars. In contrast with the counterpart with high metallicities,it is hard for the single star with low metallicity to generate WR star due to weak wind. However, the star with very high initial velocity and low metallicity can form chemical homogenious evolution. Thestar has an enlarged convective core and a very thin hydrogen envelope and it can also generate WR star. The component in the binary system with short orbital period can transfer mass to the companion star through Roche lobe overflow, and this physical process can produce WR star under the condition of low metallicity. Furthermore, mass removal due to Roche lobe overflow reduces the temperature of stellar convective core and rate of nuclear reaction. It is shown that mass metallicities of chemical elements including <sup>4</sup>He, <sup>12</sup>C, <sup>19</sup>F, <sup>22</sup>Ne, <sup>23</sup>Na, <sup>25</sup>Mg in the primary star are higher than those in the single stars, whereas mass metallicities of chemical elements including <sup>1</sup>H, <sup>14</sup>N, <sup>16</sup>O, <sup>20</sup>Ne, and <sup>26</sup>Al are lower than those in the single counterparts. In a word, the conditions for massive stars with high initial velocities and metallicities in the binary system with short orbital period favor the formation of WR stars.

Wolf-Rayet (WR) stars in the Magellanic Clouds (MCs) are ideal for studying the production of X-ray emission by their strong fast stellar winds. We have started a systematic survey for X-ray emission from WR stars in the MCs using archival Chandra, ROSAT, and XMM-Newton observations. In Paper I, we reported the detection of X-ray emission from 29 WR stars using Chandra ACIS observations of 70 WR stars in the MCs. In this paper, we report the search and analysis of archival ROSAT PSPC and HRI observations of WR stars. While useful ROSAT observations are available for 117 WR stars in the MCs, X-ray emission is detected from only seven of them. The detection rate of X-ray emission from MCs WR stars in the ROSAT survey is much smaller than in the Chandra ACIS survey, illustrating the necessity of high angular resolution and sensitivity. LMC-WR 101-102 and 116 were detected by both ROSAT and Chandra, but no large long-term variations are evident.

In order to understand the evolution of Wolf-Rayet (WR) stars and their interaction with the surrounding circumstellar and interstellar gas we have undertaken an emission-line imaging survey of the (almost complete) WR star population in the Magellanic Clouds (Dopita. et al . 1994). Interference filter CCD images have been obtained in Hα and [O iii ] λ5007 for all WR stars in the LMC and the SMC. The survey was conducted using the 2.3 m telescope at the Siding Spring Observatory ANU. The field of view was 6′.7, and the pixel size was 0″.65/pix. A total of 115 WR. stars in the LMC (Breysacher 1981; Lortet, 1991) and 9 WR stars in the SMC (Azzopardi & Breysacher, 1979; Morgan et al . 1991) were observed in this survey. This survey is the first complete survey of the ionized material around WR stars in the Magellanic Clouds, and indeed is the first complete survey in any galaxy. We have almost doubled the number of ring nebula known in the MCs, and have revealed a number of cases in which stellar ejecta has almost certainly been identified. As a consequence, we find that the incidence of ring nebulae around WR stars in the LMC is very similar to that in the solar neighborhood. (According to Lozinskaya, 1982; 1983; 1992 only 30-40% of WR. and Of stars in the distance-complete sample in the Galaxy are associated with ring nebulae; the nebula types of stellar ejecta and wind-blown bubble are even more scarce, about. 10–15%.

Context. The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. Aims. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Methods. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-local thermodynamic equilibrium (LTE) stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. Results. AzV 14 is a close binary system with a period of P = 3.7058 ± 0.0013 d. The binary consists of two similar main sequence stars with masses of M1, 2 ≈ 32 M⊙. Both stars have weak stellar winds with mass-loss rates of log Ṁ/(M⊙ yr−1) = −7.7 ± 0.2. Binary evolutionary models can explain the empirically derived stellar and orbital parameters, including the position of the AzV 14 components on the Hertzsprung-Russell diagram, revealing its current age of 3.3 Myr. The model predicts that the primary will evolve into a WR star with Teff ≈ 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with Teff ≈ 50 kK. Furthermore, WR stars that descend from binary components that have accreted significant amount of mass are predicted to have increased oxygen abundances compared to other WR stars. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. Conclusions. Inspired by the binary evolutionary models, we hypothesize that the populations of WR stars in low-metallicity galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass. These results may have wide-ranging implications for our understanding of massive star feedback and binary evolution channels at low metallicity.

The spin of Wolf-Rayet (WR) stars at low metallicity (Z) is most relevant for our understanding of gravitational wave sources such as GW 150914, as well as the incidence of long-duration gamma-ray bursts (GRBs). Two scenarios have been suggested for both phenomena: one of them involves rapid rotation and quasi-chemical homogeneous evolution (CHE), the other invokes classical evolution through mass loss in single and binary systems. WR spin rates might enable us to test these two scenarios. In order to obtain empirical constraints on black hole progenitor spin, we infer wind asymmetries in all 12 known WR stars in the Small Magellanic Cloud (SMC) at Z = 1/5 Zsun, as well as within a significantly enlarged sample of single and binary WR stars in the Large Magellanic Cloud (LMC at Z = 1/2 Zsun), tripling the sample of Vink (2007). This brings the total LMC sample to 39, making it appropriate for comparison to the Galactic sample. We measure WR wind asymmetries with VLT-FORS linear spectropolarimetry. We report the detection of new line effects in the LMC WN star BAT99-43 and the WC star BAT99-70, as well as the famous WR/LBV HD 5980 in the SMC, which might be evolving chemically homogeneously. With the previous reported line effects in the late-type WNL (Ofpe/WN9) objects BAT99-22 and BAT99-33, this brings the total LMC WR sample to 4, i.e. a frequency of ~10%. Perhaps surprisingly, the incidence of line effects amongst low-Z WR stars is not found to be any higher than amongst the Galactic WR sample, challenging the rotationally-induced CHE model. As WR mass loss is likely Z-dependent, our Magellanic Cloud line-effect WR stars may maintain their surface rotation and fulfill the basic conditions for producing long GRBs, both via the classical post-red supergiant (RSG) or luminous blue variable (LBV) channel, as well as resulting from CHE due to physics specific to very massive stars (VMS).

Context. Classical Wolf-Rayet (WR) stars are massive, hydrogen-depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity Z, stars exceeding a certain initial mass MsingleWR(Z) can reach the WR phase through intrinsic mass-loss or eruptions (single-star channel). In principle, stars of lower masses can reach the WR phase via stripping through binary interactions (binary channel). Because winds become weaker at low Z, it is commonly assumed that the binary channel dominates the formation of WR stars in environments with low metallicity such as the Small and Large Magellanic Clouds (SMC, LMC). However, the reported WR binary fractions of 30−40% in the SMC (Z = 0.002) and LMC (Z = 0.006) are comparable to that of the Galaxy (Z = 0.014), and no evidence for the dominance of the binary channel at low Z could be identified observationally. Here, we explain this apparent contradiction by considering the minimum initial mass MspecWR(Z) needed for the stripped product to appear as a WR star. Aims. By constraining MspecWR(Z) and MsingleWR(Z), we estimate the importance of binaries in forming WR stars as a function of Z. Methods. We calibrated MspecWR using the lowest-luminosity WR stars in the Magellanic Clouds and the Galaxy. A range of MsingleWR values were explored using various evolution codes. We estimated the additional contribution of the binary channel by considering the interval [MspecWR(Z), MsingleWR(Z)], which characterizes the initial-mass range in which the binary channel can form additional WR stars. Results. The WR-phenomenon ceases below luminosities of log L ≈ 4.9, 5.25, and 5.6 [L⊙] in the Galaxy, the LMC, and the SMC, respectively, which translates to minimum He-star masses of 7.5, 11, 17 M⊙ and minimum initial masses of MspecWR = 18, 23, 37 M⊙. Stripped stars with lower initial masses in the respective galaxies would tend not to appear as WR stars. The minimum mass necessary for self-stripping, MsingleWR(Z), is strongly model-dependent, but it lies in the range 20−30, 30−60, and ≳40 M⊙ for the Galaxy, LMC, and SMC, respectively. We find that that the additional contribution of the binary channel is a non-trivial and model-dependent function of Z that cannot be conclusively claimed to be monotonically increasing with decreasing Z. Conclusions. The WR spectral appearance arises from the presence of strong winds. Therefore, both MspecWR and MsingleWR increase with decreasing metallicity. Considering this, we show that one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low Z, or that the WR binary fraction grows with decreasing Z.

Context. Oxygen sequence Wolf-Rayet (WO) stars are thought to represent the final evolutionary stage of the most massive stars. The characteristic strong O _(VI) emission possibly originates from an enhanced oxygen abundance in the stellar wind. Alternatively, the O _(VI) emission can be caused by the high temperature of these stars, in which case the WO stars are the high-temperature extension of the more common carbon sequence Wolf-Rayet (WC) stars.
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\nAims. By constraining the physical properties and evolutionary status of DR1, a WO star in the low-metallicity Local Group dwarf galaxy IC 1613 and one of only two objects of its class known in a SMC-like metallicity environment, we aim to investigate the nature of WO stars and their evolutionary connection with WC stars.
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\nMethods. We use the non-local thermodynamic equilibrium atmosphere code CMFGEN to model the observed spectrum of DR1 and to derive its stellar and wind parameters. We compare our values with other studies of WC and WO stars, as well as with the predictions of evolutionary models. We also model the surrounding nebula using the photo-ionization code CLOUDY.
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\nResults. The oxygen and carbon abundances that we obtain are comparable to values found for WC stars. The temperature and luminosity are, however, higher than those of WC stars. DR1 is embedded in the hottest known H _(II) region in the Local Group. The nebular properties can be consistently reproduced by CLOUDY models adopting DR1 as central ionizing source.
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\nConclusions. Comparison of the abundances and temperature of DR1 with core helium-burning models show that DR1 is currently well into the second half of helium burning. If the properties of DR1 are representative for the WO class, it would imply that WO stars are the high-temperature and high-luminosity extension of the WC stars, and do not necessarily represent a later evolutionary stage.

\n Context.The favoured progenitors of long-duration gamma-ray bursts (GRBs) are rapidly rotating \nWolf-Rayet (WR) stars. However, most Galactic WR stars are slow rotators, as stellar winds are thought to remove \nangular momentum. This poses a serious challenge to the collapsar model. Recent observations indicate that GRBs occur predominately in low \nmetallicity (Z) environments, which may resolve the problem: lower Z leads to less mass loss, which may inhibit \nangular momentum removal, allowing WR stars to remain rotating rapidly until collapse.\n Aims.We wish to determine whether low Z WR stars rotate on average more rapidly than Galactic WR stars.\n Methods.We perform a Very Large Telescope (VLT) linear spectropolarimetry survey of WR stars in \nthe low Z environment of the Large Magellanic Cloud (LMC) and compare our results with the Galactic \nsample of Harries et al. (1998, MNRAS, 296, 1072).\n Results.We find that only 2 out of 13 (i.e. 15%) of LMC WR stars show line polarization effects, compared to a similarly low fraction \nof ~15-20% for Galactic WR stars.\n Conclusions.The low incidence of line polarization effects in LMC WR stars suggests that the threshold metallicity where significant \ndifferences in WR rotational properties occur is below that of the LMC ($Z \\sim 0.5$ $Z_{\\odot}$), possibly constraining GRB progenitor channels \nto this upper metallicity.\n

Context. BAT99 126 is a multiple system in the Large Magellanic Cloud containing a Wolf-Rayet (WR) star, which has a reported spectroscopic (orbital) period of 25.5 days and a photometric (orbital) period of 1.55 days, and hence is potentially one of the shortest WR binaries known to date. Such short-period binary systems that contain a WR star in low-metallicity environments are prime candidate progenitors of black-hole (BH) mergers. Aims. By thoroughly analysing the spectroscopic and photometric data, we aim to establish the true multiplicity of BAT99 126, to characterise the orbit(s) of the system, to measure the physical properties of its individual components, and to determine the overall evolutionary status of the system. Methods. Using newly acquired high resolution spectra taken with the Ultra-violet and Visual Echelle Spectrograph mounted on the Very Large Telescope, we measured radial velocities via cross-correlation and line-profile fitting, and performed a spectral analysis of the individual components using model atmosphere codes. We estimated the age of the system and derived an evolutionary scenario for the 1.55-day system. Results. BAT99 126 comprises at least four components. The 1.55-day photometric signal originates in an eclipsing binary that consists of two O-type stars of spectral types O4 V and O6.5 V, which are both rapid rotators (300 km s−1 and 230 km s−1, respectively). From the broad emission lines of the WR star, we derived a spectral type WN2.5-3. We further reject the previously reported 25.5-d period for the WR star and find that there is no detectable orbital motion within our uncertainties. The presence of additional narrow Si III and O II lines in the composite spectrum corresponds to a fourth component, a B1 V star. There is clear evidence that the B-type star shows a radial velocity variation; however, the data do not allow for a determination of the orbital parameters. The configurations of the B-type star, the WR star, and possible additional undetected components remain unknown. We derived masses for the O-type components of 36 ± 5 M⊙ and 15 ± 2 M⊙, respectively, and estimated the age of the system to be 4.2 Myr. We find evidence of previous or ongoing mass-transfer between the two O-type components and infer initial masses of 23 M⊙ for the O4 V star and 29 M⊙ for the O6.5 V star. The O+O binary likely went through a phase of conservative mass transfer and is currently a near-contact system. Conclusion. We show that BAT99 126 is a multiple – quadruple or higher-order – system with a total initial mass of at least 160 M⊙. The 1.55-day O+O binary most likely will not evolve towards a BH+BH merger, but instead will merge before the collapse of the components to BHs.

We consider the evolutionary status of observed close binary systems containing black holes and Wolf-Rayet (WR) stars. When the component masses and the orbital period of a system are known, the reason for the formation of a WR star in an initial massive system of two main-sequence stars can be established. Such WR stars can form due to the action of the stellar wind from a massive OB star (MOB≥50M⊙), conservative mass transfer between components with close initial masses, or the loss of the common envelope in a system with a large (up to ∼25) initial component mass ratio. The strong impact of observational selection effects on the creation of samples of close binaries with black holes and WR stars is demonstrated. We estimate theoretical mass-loss rates for WR stars, which are essential for our understanding the observed ratio of the numbers of carbon and nitrogen WR stars in the Galaxy \(\dot M_{WR} (M_ \odot yr^{ - 1} ) = 5 \times 10^{ - 7} (M_{WR} /M_ \odot )^{1.3} \). We also estimate the minimum initial masses of the components in close binaries producing black holes and WR stars to be ∼25M⊙. The spatial velocities of systems with black holes indicate that, during the formation of a black hole from a WR star, the mass loss reaches at least several solar masses. The rate of formation of rapidly rotating Kerr black holes in close binaries in the Galaxy is ∼3×10−6 yr−1. Their formation may be accompanied by a burst of gamma radiation, possibly providing clues to the nature of gamma-ray bursts. The initial distribution of the component mass ratios for close binaries is dN∼dq=dM2/M1 in the interval 0.04≲q0≤1, suggesting a single mechanism for their formation.

The evolution of close binary systems containing Wolf-Rayet (WR) stars and black holes (BHs) is analyzed numerically. Both the stellar wind from the donor star itself and the induced stellar wind due to irradiation of the donor with hard radiation arising during accretion onto the relativistic component are considered. The mass and angular momentum losses due to the stellar wind are also taken into account at phases when the WR star fills its Roche lobe. It is shown that, if a WR star with a mass higher than ∼10M ⊙ fills its Roche lobe in an initial evolutionary phase, the donor star will eventually lose contact with the Roche lobe as the binary loses mass and angular momentum via the stellar wind, suggesting that the semi-detached binary will become detached. The star will remain a bright X-ray source, since the stellar wind that is captured by the black hole ensures a near-Eddington accretion rate. If the initial mass of the helium donor is below ∼5M ⊙, the donor may only temporarily detach from its Roche lobe. Induced stellar wind plays a significant role in the evolution of binaries containing helium donors with initial masses of ∼2M ⊙. We compute the evolution of three observed WR-BH binaries: Cyg X-3, IC 10 X-1, and NGC 300 X-1, as well as the evolution of the SS 433 binary system, which is a progenitor of such systems, under the assumption that this binary will avoid a common-envelope stage in its further evolution, as it does in its current evolutionary phase.

Context. Gaia DR3, released in June 2022, included low-resolution XP spectra that have been used for the classification of various types of emission-line objects through machine-learning techniques. The Gaia Extended Stellar Parametrizer for Emission-Line Stars (ESP-ELS) algorithm identified 565 sources as potential Wolf-Rayet (WR) stars. Over half of them were already known as WR stars in the Milky Way and Magellanic Clouds. Aims. We utilized Gaia DR3 data to identify new Galactic WR stars. Methods. We extracted all sources from the Gaia catalog classified as WC- or WN-type stars by the ESP-ELS algorithm. By applying judicious 2MASS color selection criteria, leveraging Gaia Hα measurements, and filtering out objects already cataloged in various databases, we selected 37 bright candidates (G ≤ 16 mag) and 22 faint candidates (G > 16 mag). Spectroscopic follow-up observations of these candidates were conducted using the 2SPOT facilities in Chile and France, as well as the 1 m C2PU’s Epsilon telescope at the Calern Observatory. Results. This paper focuses on the brighter sample. Among the 37 targets, we confirmed 17 and 16 new Galactic WC- and WN-type WR stars, respectively. Three of them were recently reported as new WR stars in an independent study. Conclusions. The Gaia mission provides a valuable resource for identifying WR stars missed in earlier surveys. While this work concentrated on a relatively small starting sample provided by the ESP-ELS algorithm, our findings highlight the potential for refining selection criteria to identify additional candidates not included in the outputs of the algorithm. Furthermore, the observation program underscores the utility of small telescopes in acquiring initial spectral data for sources with magnitudes up to G ~ 16 mag.

A nearly complete sample of 24 Magellanic Cloud WC/WO subclass Wolf–Rayet stars is studied spectroscopically and photometrically to determine its binary frequency. Theory predicts the Roche lobe overflow produced Wolf–Rayet binary frequency to be 52±14 per cent in the Large Magellanic Cloud and 100 per cent in the Small Magellanic Cloud, not counting non-Roche lobe overflow Wolf–Rayet binaries. Lower ambient metallicity (Z) leads to lower opacity, preventing all but the most massive (hence luminous) single stars from reaching the Wolf–Rayet stage. However, theory predicts that Roche lobe overflow even in binaries of modest mass will lead to Wolf–Rayet stars in binaries with periods below approximately 200 d, for initial periods below approximately 1000 d, independent of Z. By examining their absolute continuum magnitudes, radial velocity variations, emission-line equivalent widths and full widths at half-maximum, a WC/WO binary frequency of only 13 per cent, significantly lower than the prediction, is found in the Large Magellanic Cloud. In the unlikely event that all of the cases with a less certain binary status actually turn out to be binary, current theory and observation would agree. (The Small Magellanic Cloud contains only one WC/WO star, which happens to be a binary.) The three WC+O binaries in the Large Magellanic Cloud all have periods well below 1000 d. The large majority of WC/WO stars in such environments apparently can form without the aid of a binary companion. Current evolutionary scenarios appear to have difficulty explaining either the relatively large number of Wolf–Rayet stars in the Magellanic Clouds, or the formation of Wolf–Rayet stars in general.

WR 102-1 was detected by Suzaku as a conspicuous point source in the 6.7 keV intensity map of the central region of the Milky Way. The source was suggested as a possible Wolf–Rayet binary based on its X-ray and infrared spectral characteristics. The iron line emission is expected to originate in the Wolf–Rayet star’s dynamic stellar wind when colliding the companion’s mild stellar wind. Here, we report the result of a long-term X-ray monitoring of WR 102-1 since 1998 using archival data of ASCA, XMM–Newton, Chandra, Suzaku, and Swift to reveal variations of the iron K-emission line and the circumstellar absorption. Consequently, we have detected significant redshifts of the iron K-emission line from the XMM–Newton observation in 2003 March and the Suzaku observation in 2006 September. Furthermore, when the redshift was observed, which suggests that the Wolf–Rayet star was in front of the companion star, the circumstellar absorption values were smaller than other periods. These results appear contrary to the expectation if the Wolf–Rayet’s stellar wind is spherically symmetric, but may be understood if the Wolf–Rayet star’s stellar wind is significantly distorted due to the rapid orbital motion near the periastron.

Non-thermal radio emission in Wolf-Rayet (WR) stars is hypothesized to be synchrotron emission from shocks in the wind. For single star models, the shocks arise from instabilities in the WR wind itself. In binary models, the shocks form at the wind-wind interaction zone between a WR star and a massive companion. In Niemela et al. (1998), we used WFPC2 PC imaging to investigate the binary theory. For two WR stars we linked the non-thermal emission with the colliding wind region between binary companions. These observations support the conclusion that non-thermal radio emission arises in a wind collision zone between binary companions outside of the radio photosphere of the WR star.However, not all WR stars exhibiting non-thermal radio emission are known to have a binary companion. To conclusively link non-thermal emission to binarity, we must demonstrate that all non-thermal emitters are binary, and that all thermal emitters are either single stars or binary systems with separations either too wide or too close to result in a wind-wind interaction that produces shocks.In an effort to accomplish this goal we used the FGS1R interferometer on board the Hubble Space Telescope (HST) to observe 9 non-thermal emitting WR stars as well as a control sample of 9 thermal emitting WR stars. The FGS can resolve angular separations as small as 0”.007 within the currently accepted size of the radio photosphere for the relatively nearby stars in our sample. We found only one binary among the non-thermal emitting WR stars; WR 48 has a companion at a separation of 40 mas which is probably responsible for colliding winds non-thermal emission. However, the lack of evidence for companions among the other non-thermal emitters casts some doubt on the applicability of the colliding winds model for all non-thermal WR stars.KeywordsWolf-RayetStars-Binaries-Non-thermalRadioEmission