Modelling spectral line profiles of wind-wind shock emissions from massive binary systems

One of the most intriguing spectral features of WR binary stars is the presence of time-dependent line profiles. Long term observations of several systems revealed the periodicity of this variability, synchronized with the orbital movement. Several partially successful models have been proposed to reproduce the observed data. The most promising assume that the origin of the emission is the wind-wind interaction zone. In this scenario, two high velocity and dense winds produce a strong shock layer, responsible for most of the X-rays observed from these systems. As the secondary star moves along its orbital path, the shock region of conical shape, changes its position with relation to the line of sight. As a consequence, the stream measured Doppler shift presents time variations resulting in position changes of the spectral line. In our work, we present an alternative model, introducing turbulence in the shock layer to account for the line broadening and opacity effects for the asymmetry in the line profiles. We showed that the gas turbulence avoids the need of an unnaturally large contact layer thickness to reproduce line broadening. Also, we demonstrated that if the emission from the opposing cone surface is absorbed, the result is a single peaked profile. This result fully satisfies the recent data obtained from massive binary systems, and can help on the determination of both winds and orbital parameters. We successfully applied this model to the Br22 system and determined its orbital parameters.Comment: To appear in the MNRA

Mass-Loss Rate Determination for the Massive Binary V444 Cyg using 3-D Monte-Carlo Simulations of Line and Polarization Variability

A newly developed 3-D Monte Carlo model is used, in conjunction with a multi-line non-LTE radiative transfer model, to determine the mass-loss rate of the Wolf-Rayet (W-R) star in the massive binary \object{V444 Cyg} (WN5+O6). This independent estimate of mass-loss rate is attained by fitting the observed \HeI (5876) \AA and \HeII (5412) \AA line profiles, and the continuum light curves of three Stokes parameters ((I, Q, U)) in the (V) band simultaneously. The high accuracy of our determination arises from the use of many observational constraints, and the sensitivity of the continuum polarization to the mass-loss rate. Our best fit model suggests that the mass-loss rate of the system is (\dot{M}_{\WR}=0.6(\pm 0.2) \times 10^{-5} M_{\sun} \mathrm{yr}^{-1} ), and is independent of the assumed distance to \object{V444 Cyg}. The fits did not allow a unique value for the radius of the W-R star to be derived. The range of the volume filling factor for the W-R star atmosphere is estimated to be in the range of 0.050 (for R_{\WR}=5.0 R_{\sun}) to 0.075 (for R_{\WR}=2.5 R_{\sun}). We also found that the blue-side of \HeI (5876 ) \AA and \HeII (5412) \AA lines at phase 0.8 is relatively unaffected by the emission from the wind-wind interaction zone and the absorption by the O-star atmosphere; hence, the profiles at this phase are suitable for spectral line fittings using a spherical radiative transfer model.Comment: 18 pages, 17 figures: Accepeted for publication in A&

A Spectroscopic Survey of WNL Stars in the LMC: General Properties and Binary Status

We report the results of an intense, spectroscopic survey of all 41 late-type, nitrogen-rich Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) observable with ground-based telescopes. This survey concludes the decade-long effort of the Montr\'eal Massive Star Group to monitor every known WR star in the Magellanic Clouds except for the 6 crowded WNL stars in R136, which will be discussed elsewhere. The focus of our survey was to monitor the so-called WNL stars for radial-velocity (RV) variability in order to identify the short- to intermediate-period (P \la 200 days) binaries among them. Our results are in line with results of previous studies of other WR subtypes, and show that the binary frequency among LMC WNL stars is statistically consistent with that of WNL stars in the Milky Way. We have identified four previously unknown binaries, bringing the total number of known WNL binaries in the LMC to nine. Since it is very likely that none but one of the binaries are classical, helium-burning WNL stars, but rather superluminous, hence extremely massive, hydrogen-burning objects, our study has dramatically increased the number of known binaries harbouring such objects, and thus paved the way to determine their masses through model-independent, Keplerian orbits. It is expected that some of the stars in our binaries will be among the most massive known. With the binary status of each WR star now known, we also studied the photometric and X-ray properties of our program stars using archival MACHO photometry as well as Chandra and ROSAT data. We find that one of our presumably single WNL stars is among the X-ray brightest WR sources known. We also identify a binary candidate from its RV variability and X-ray luminosity which harbours the most luminous WR star known in the Local Group.Comment: 25 pages, 11 figures; accepted for MNRA

WR bubbles and HeII emission

We present the very first high quality images of the HeII 4686 emission in three high excitation nebulae of the Magellanic Clouds. A fourth high excitation nebula, situated around the WR star BAT99-2, was analysed in a previous letter. Using VLT FORS data, we investigate the morphology of the ring nebulae around the early-type WN stars BAT99-49 & AB7. We derive the total HeII fluxes for each object and compare them with the most recent theoretical WR models. Using Halpha, [OIII] and HeI 5876 images along with long-slit spectroscopy, we investigate the physical properties of these ring nebulae and find only moderate chemical enrichment. We also surveyed seven other LMC WR stars but we failed to detect any HeII emission but note that the nebula around BAT99-11 shows a N/O ratio and an oxygen abundance slightly lower than the LMC values, while the nebula around BAT99-134 presents moderate chemical enrichment similar to the one seen near BAT99-2, 49 and AB7. The third high excitation nebula presented in this paper, N44C, does not harbor stars hotter than mid-O main sequence stars. It was suggested to be a fossil X-ray nebula ionized but our observations of N44C reveal no substantial changes in the excitation compared to previous results reported in the literature.Comment: 13 pages, 8 figures (7 in jpg), accepted by A&A, also available from http://vela.astro.ulg.ac.be/Preprints/P81/index.htm

The massive eclipsing LMC Wolf-Rayet binary BAT99-129. 1 Orbital parameters, hydrogen content and spectroscopic characteristics

BAT99-129 in the LMC is one among a handful of extra-galactic eclipsing Wolf-Rayet binaries known. We present blue, medium-resolution, phase-dependent NTT-EMMI spectra of this system that allow us to separate the spectra of the two components of the binary and to obtain a reliable orbital solution for both stars. We assign an O5V spectral type to the companion, and WN3(h)a to the Wolf-Rayet component. We discuss the spectroscopic characteristics of the system: luminosity ratio, radii, rotation velocities. We find a possible oversynchronous rotation velocity for the O star. Surprisingly, the extracted Wolf-Rayet spectrum clearly shows the presence of blueshifted absorption lines, similar to what has been found in all single hot WN stars in the SMC and some in the LMC. We also discuss the presence of such intrinsic lines in the context of hydrogen in SMC and LMC Wolf-Rayet stars, WR+O binary evolution and GRB progenitors. Altogether, BAT99~129 is the extragalactic counterpart of the well-known Galactic WR binary V444 Cygni.Comment: 14 pages, 9 figures, accepted by A&A for publicatio

Reduced Wolf-Rayet Line Luminosities at Low Metallicity

New NTT/EMMI spectrophotometry of single WN2-5 stars in the Magellanic Clouds are presented, from which HeII 4686 line luminosities have been derived, and compared with observations of other Magellanic Cloud WR stars. SMC WN3-4 stars possess line luminosities which are a factor of 4 times lower than LMC counterparts, incorporating several binary SMC WN3-4 stars. Similar results are found for WN5-6 stars, despite reduced statistics, incorporating observations of single LMC WN5-9 stars. CIV 5808 line luminosities of carbon sequence WR stars in the SMC and IC1613 (both WO subtypes) are a factor of 3 lower than LMC WC stars from Mt Stromlo/DBS spectrophotometry, although similar results are also obtained for the sole LMC WO star. We demonstrate how reduced line luminosities at low metallicity follow naturally if WR winds are Z-dependent, as recent results suggest. We apply mass loss-Z scalings to atmospheric non-LTE models of Milky Way and LMC WR stars to predict the wind signatures of WR stars in the metal-poor star forming WR galaxy IZw18. WN HeII 4686 line luminosities are 7-20 times lower than in Z-rich counterparts of identical bolometric luminosity, whilst WC CIV 5808 line luminosities are 3-6 times lower. Significant He^+ Lyman continuum fluxes are predicted for Z-poor early-type WR stars. Consequently, our results suggest the need for larger population of WR stars in IZw18 than is presently assumed, particularly for WN stars, potentially posing a severe challenge to evolutionary models at very low Z. Finally, reduced wind strengths from WR stars at low Z impacts upon the immediate circumstellar environment of long duration GRB afterglows, particularly since the host galaxies of high-redshift GRBs tend to be Z-poor.Comment: 14 pages, 12 figures, accepted for A&A, revision fixes error with Eqn

Another single hydrogen-rich Wolf-Rayet star in the SMC?

A 12th Wolf-Rayet star in the SMC has recently been discovered by Massey et al. (2003). In order to determine its spectral type and a preliminary binary status, we obtained 3 high signal-to-noise spectra separated in time at the ESO-NTT. Compared to other WR stars in the SMC, SMC-WR12 appears to belong to the subgroup of faint, single and hydrogen-rich WN stars. We discuss the evolutionary status of WR12 and show that relatively low mass rotating progenitors can better account for the properties of single hydrogen-rich WN stars in the SMC.Comment: 5 pages, 2 figures, accepted by A&A, include latest correction

On the binary frequency of a complete sample of magellanic, WC Wolf-Rayet stars and a spectroscopic study of WC binary colliding winds

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal