The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions

Rauw, Grégor; Crowther, P. A.; Becker, M. De; Gosset, Éric; Nazé, Yaël; Sana, H.; Hucht, K. van der; Vreux, Jean-Marie; Williams, P. M.

doi:10.1051/0004-6361:20042136

Cited by 87 publications

(144 citation statements)

References 45 publications

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“…Moreover, we inferred a spectrophotometric distance of 8.0 ± 1.4 kpc. While this distance measurement is in excellent agreement with that of WR 20a (Rauw et al 2005), it is nevertheless problematic in several ways. Indeed, despite the much earlier spectral types inferred for the most massive cluster members, the current census of early-type stars in Westerlund 2 accounts for only 20% of the ionizing photons required to explain the observed radio emission of the RCW 49 complex.…”

Section: Introductionmentioning

confidence: 61%

“…These observations revealed strong evidence of ongoing star formation activity and underlined the need for a detailed study of the properties of the early-type stars in the cluster core and their impact on the surrounding nebula. On the other hand, WR 20a -one of the two Wolf-Rayet stars in RCW 49 -was found to be a very massive eclipsing binary consisting of two WN6ha stars with individual masses of about 80 M (Rauw et al 2004(Rauw et al , 2005Bonanos et al 2004). Since previous spectroscopic studies revealed only one O6: and six O7: stars, in the cluster core , the existence of a pair of stars that massive was somewhat unexpected.…”

Section: Introductionmentioning

confidence: 96%

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A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

Rauw

Sana

Nazé

2011

A&A

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Context. The distance of the very young open cluster Westerlund 2, which contains the very massive binary system WR 20a and is likely associated with a TeV source, has been the subject of much debate. Aims. We attempt a joint analysis of spectroscopic and photometric data of eclipsing binaries in the cluster to constrain its distance. Methods. A sample of 15 stars, including three eclipsing binaries (MSP 44, MSP 96, and MSP 223) was monitored with the FLAMES multi-object spectrograph. The spectroscopic data are analysed together with existing B V photometry.Results. The analysis of the three eclipsing binaries clearly supports the larger values of the distance, around 8 kpc, and rules out values of about 2.4−2.8 kpc that have been suggested in the literature. Furthermore, our spectroscopic monitoring reveals no clear signature of binarity with periods shorter than 50 days in either the WN6ha star WR 20b, the early O-type stars MSP 18, MSP 171, MSP 182, MSP 183, MSP 199, and MSP 203, or three previously unknown mid O-type stars. The only newly identified candidate binary system is MSP 167. The absence of a binary signature is especially surprising for WR 20b and MSP 18, which were previously found to be bright X-ray sources. Conclusions. The distance of Westerlund 2 is confirmed to be around 8 kpc as previously suggested based on the spectrophotometry of its population of O-type stars and the analysis of the light curve of WR 20a. Our results suggest that short-period binary systems are not likely to be common, at least not among the population of O-type stars in the cluster.

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Section: Introductionmentioning

confidence: 61%

Section: Introductionmentioning

confidence: 96%

A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

Rauw

Sana

Nazé

2011

A&A

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“…We separate the light contributions of individual stars to each system by considering mass ratios derived by spectroscopic monitoring of their orbits. The stars making up WR 20a are of identical spectral type and have very similar masses (Rauw et al 2005), hence we assume an equal light contribution from each star in the J, H and KS-bands, and alter the systemic photometry accordingly. Similarly, WR 43A in the NGC 3603 cluster is comprised of two stars with very high measured masses, 116± 31 M⊙ and 89± 16 M⊙ (q= 0.8± 0.2; Schnurr et al 2008).…”

Section: Correction For Binary Companionsmentioning

confidence: 99%

Spatial distribution of Galactic Wolf–Rayet stars and implications for the global population

Rosslowe

Crowther

2015

Monthly Notices of the Royal Astronomical Society

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We construct revised near-infrared absolute magnitude calibrations for 126 Galactic Wolf-Rayet (WR) stars at known distances, based in part upon recent large scale spectroscopic surveys. Application to 246 WR stars located in the field, permits us to map their Galactic distribution. As anticipated, WR stars generally lie in the thin disk (∼40 pc half width at half maximum) between Galactocentric radii 3.5-10 kpc, in accordance with other star formation tracers. We highlight 12 WR stars located at vertical distances of 300 pc from the midplane. Analysis of the radial variation in WR subtypes exposes a ubiquitously higher N W C /N W N ratio than predicted by stellar evolutionary models accounting for stellar rotation. Models for non-rotating stars or accounting for close binary evolution are more consistent with observations. We consolidate information acquired about the known WR content of the Milky Way to build a simple model of the complete population. We derive observable quantities over a range of wavelengths, allowing us to estimate a total number of 1200 +300 −100 Galactic WR stars, implying an average duration of ∼ 0.25 Myr for the WR phase at the current Milky Way star formation rate. Of relevance to future spectroscopic surveys, we use this model WR population to predict follow-up spectroscopy to K S ≃ 13 mag will be necessary to identify 95% of Galactic WR stars. We anticipate that ESA's Gaia mission will make few additional WR star discoveries via low-resolution spectroscopy, though will significantly refine existing distance determinations. Appendix A provides a complete inventory of 322 Galactic WR stars discovered since the VIIth catalogue (313 including Annex), including a revised nomenclature scheme.

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“…As these electrons spiral around in the magnetic field, they emit synchrotron radiation, which we detect as non-thermal radio emission. In addition, the hot compressed material in the colliding-wind region (CWR) also emits X-rays (Stevens et al 1992;Pittard & Parkin 2010) and influences the shape of the optical spectral lines (Rauw et al 2005). While the general outline of the explanation for non-thermal radio emission is clear, there are still problems when detailed modelling of specific systems is attempted.…”

Section: Introductionmentioning

confidence: 99%

The 2.35 year itch of Cygnus OB2 #9

Blomme

Nazé

Volpi

et al. 2013

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Context. Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star binaries. The non-thermal radiation is due to synchrotron emission in the colliding-wind region. Cyg OB2 #9 has only recently been discovered to be a binary system, and a multi-wavelength campaign was organized to study its 2011 periastron passage. Aims. We want to better determine the parameters of this system and model the wind-wind collision. This will lead to a better understanding of the Fermi mechanism that accelerates electrons up to relativistic speeds in shocks and its occurrence in collidingwind binaries. We report here on the results of the radio observations obtained in the monitoring campaign and present a simple model to interpret the data. Methods. We used the Expanded Very Large Array (EVLA) radio interferometer to obtain 6 cm and 20 cm continuum fluxes during the Cyg OB2 #9 periastron passage in 2011. We introduce a simple model to solve the radiative transfer in the stellar winds and the colliding-wind region, and thus determine the expected behaviour of the radio light curve. Results. The observed radio light curve shows a steep drop in flux sometime before periastron. The fluxes drop to a level that is comparable to the expected free-free emission from the stellar winds, suggesting that the non-thermal emitting region is completely hidden at that time. After periastron passage, the fluxes slowly increase. We use the asymmetry of the light curve to show that the primary has the stronger wind. This is somewhat unexpected if we use the astrophysical parameters based on theoretical calibrations. But it becomes entirely feasible if we take into account that a given spectral type-luminosity class combination covers a range of astrophysical parameters. The colliding-wind region also contributes to the free-free emission, which can help explain the high values of the spectral index seen after periastron passage. Combining our data with older Very Large Array (VLA) data allows us to derive a period P = 860.0 ± 3.7 days for this system. With this period, we update the orbital parameters that were derived in the first paper of this series. Conclusions. A simple model introduced to explain only the radio data already allows some constraints to be put on the parameters of this binary system. Future, more sophisticated, modelling that will also include optical, X-ray, and interferometric information will provide even better constraints.

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The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions

Cited by 87 publications

References 45 publications

A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

Spatial distribution of Galactic Wolf–Rayet stars and implications for the global population

The 2.35 year itch of Cygnus OB2 #9

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