M. De Becker

WR 20a: A massive cornerstone binary system comprising two extreme early-type stars

We analyse spectroscopic observations of WR 20a revealing that this star is a massive early-type binary system with a most probable orbital period of ∼3.675 days. Our spectra indicate that both components are most likely of WN6ha or O3If ∗ /WN6ha spectral type. The orbital solution for a period of 3.675 days yields extremely large minimum masses of 70.7 ± 4.0 and 68.8± 3.8 Mfor the two stars. These properties make WR 20a a cornerstone system for the study of massive star evolution.

The spectrum of the very massive binary system WR20a (WN6ha + WN6ha): Fundamental parameters and wind interactions

We analyse the optical spectrum of the very massive binary system WR 20a (WN6ha + WN6ha). The most prominent emission lines, Hα and He  λ 4686, display strong phase-locked profile variability. From the variations of their equivalent widths and from a tomographic analysis, we find that part of the line emission probably arises in a wind interaction region between the stars. Our analysis of the optical spectrum of WR 20a indicates a reddening of AV � 6.0 mag and a distance of ∼7.9 kpc, suggesting that the star actually belongs to the open cluster Westerlund 2. The location of the system at ∼1.1 pc from the cluster core could indicate that WR 20a was gently ejected from the core via dynamical interactions. Using a non-LTE model atmosphere code, we derive the fundamental parameters of each component: Teff = 43 000 ± 2000 K, log Lbol/L� � 6.0, u M = 8.5 × 10 −6 Myr −1 (assuming a clumped wind with a volume filling factor f = 0.1). Nitrogen is enhanced in the atmospheres of the components of WR 20a, while carbon is definitely depleted. Finally, the position of the binary components in the Hertzsprung-Russell diagram suggests that they are core hydrogen burning stars in a pre-LBV stage and their current atmospheric chemical composition probably results from rotational mixing that might be enhanced in a close binary compared to as ingle star of same age.

Early-type stars in the core of the young open cluster Westerlund 2 ⋆⋆⋆

Aims. The properties of the early-type stars in the core of the Westerlund 2 cluster are examined in order to establish a link bet ween the cluster and the very massive Wolf-Rayet binary WR 20a as well as the Hii complex RCW 49. Methods. Photometric monitoring as well as spectroscopic observations of Westerlund 2 are used to search for light variability and to establish the spectral types of the early-type stars in the c luster core. Results. The first light curves of the eclipsing binary WR 20a in B and V filters are analysed and a distance of 8 kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometri c variability above 1% at the 1-σ level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significant ly earlier than previously thought. Conclusions. The distance of the early-type stars in Westerlund 2 is established to be in excellent agreement with the distance of WR 20a, indicating that WR 20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0± 1.4 kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund 2 does not provide enough ionizing photons to account for the radio emission of the RCW 49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW 49.

New findings on the prototypical Of?p stars

Aims. In recent years several in-depth investigations of the three prototypical Of?p stars were undertaken. These multiwavelength studies revealed the peculiar properties of these objects (in the X-rays as well as in the optical): magnetic fields, periodic line profile variations, recurrent photometric changes. However, many questions remain unsolved. Methods. To clarify some of the properties of the Of?p stars, we have continued their monitoring. A new xmm-Newton observation and two new optical datasets were obtained. Results. Additional information about the prototypical Of?p trio has been found. HD 108 has now reached its quiescent, minimumemission state for the first time in 50–60 yr. The echelle spectra of HD 148937 confirm the presence of the 7d variations in the Balmer lines and reveal similar periodic variations (though of lower amplitudes) in the He i λ 5876 and He ii λ 4686 lines, underlining its similarities with the other two prototypical Of?p stars. The new xmm-Newton observation of HD 191612 was taken at the same phase in the line modulation cycle, but at a different orbital phase from previous data. It clearly shows that the X-ray emission of HD 191612 is modulated by the 538d period and not by the orbital period of 1542d – it is thus not of colliding-wind origin. The phenomenon responsible for the optical changes appears also at work in the high-energy domain. There are problems however: our MHD simulations of the wind magnetic confinement predict both a harder X-ray flux of a much larger strength than what is observed (the modelled differential emission measure peaks at 30–40 MK, whereas the observed one peaks at 2 MK) and narrow lines (hot gas moving with velocities of 100–200 km s −1 , whereas the observed full width at half maximum is ∼2000 km s −1 ).

A spectroscopic study of the non-thermal radio emitter Cyg OB2#8A: Discovery of a new binary system

We present the results of a spectroscopic campaign revealing that the non-thermal radio emitter Cyg OB2 #8A is an O6 + O5.5 binary system. We propose the very first orbital solution indicating a period of about 21.9 days. The system appears to be eccentric (0.24 ± 0.04) and is likely seen under a rather low inclination angle. The mass ratio of the components is close to unity. The impact of the binarity of this star in the framework of our understanding of non-thermal radio emission from early-type stars is briefly discussed.

High-resolution optical spectroscopy of Plaskett's star

Context. Plaskett’s star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 association. Previous work suggests that this system displays the Struve-Sahade effect although the measurements of the secondary radial velocities are very difficult and give controversial results. Both components have powerful stellar winds that collide and produce a strong X-ray emission. Aims. Our aim is to study the physical parameters of this system in detail and to investigate the relation between its spectral properties and its evolutionary status. Methods. We present here analysis of an extensive set of high-resolution optical spectra of HD 47 129. We used a disentangling method to separate the individual spectra of each star. We derived a new orbital solution and discuss the spectral classification of both components. A Doppler tomography technique applied to the emission lines Hα and He ii λ 4686 yields a Doppler map that illustrates the wind interactions in the system. Finally, an atmosphere code is used to determine the different chemical abundances of the system components and the wind parameters. Results. HD 47 129 appears to be an O8 III/I + O7.5 III binary system in a post RLOF evolutionary stage, where matter has been transferred from the primary to the secondary star. The He overabundance of the secondary supports this scenario. In addition, the N overabundance and C underabundance of the primary component confirm previous results based on X-ray spectroscopy and

HD 108: The mystery deepens with XMM-Newton observations

In 2001, using a large spectroscopic dataset from an extensive monitoring campaign, we discovered that the peculiar Of star HD 108 displayed extreme line variations. This strange behaviour could be attributed to a variety of models, and an investigation of the high energy properties of HD 108 was needed to test the predictions from these models. Our dedicated XMM-Newton observation of HD 108 shows that its spectrum is well represented by a two temperature thermal plasma model with kT1 ∼ 0.2 keV and kT2 ∼ 1.4 keV. In addition, we find that the star does not display any si gnificant short-term changes during the XMM-Newtonexposure. Compared to previous Einstein and ROSAT detections, it also appears that HD 108 does not present long-term flux variations either. While the line variations continue to modify HD 108’s spectrum in the optical domain, t he Xray emission of the star appears thus surprisingly stable: n o simple model is for the moment able to explain such an unexpected behaviour. Thanks to its high sensitivity, the XMM-Newton observatory has also enabled the serendipitous discovery of 57 new X-ray sources in the field of HD 108. Their properties are also discu ssed in this paper.

Catalogue of particle-accelerating colliding-wind binaries

Massive systems made of two or more stars are known to be the site for interesting physical processes – including at least in some cases – particle acceleration. Over the past decade, this topic motivated a particular effort to unveil the properties of these systems and characterize the circumstances responsible for the acceleration of particles and the potential role of pre-supernova massive stars in the production of high energy particles in our Galaxy. Although previous studies on this topic were mostly devoted to processes in general, or to a few individual objects in particular, a unified target-oriented census of particle-accelerating colliding-wind binaries (hereafter PACWBs) does not exist yet. This paper aims at making a general and unified census of these systems, emphasizing their main properties. A general discussion includes energetic considerations along with wind properties in relation with non-thermal emission processes that are likely at work in colliding-wind binaries. Finally, some guidelines for future observational and theoretical studies are drawn.

Early-type stars in the young open cluster IC 1805 - II. The probably single stars HD 15570 and HD 15629, and the massive binary/triple system HD 15558

Aims. We address the issue of the multiplicity of the three brightest early-type stars of the young open cluster IC 1805, namely HD 15570, HD 15629 and HD 15558. Methods. For the three stars, we measured the radial velocity by fitting Gaussian curves to line profiles in the optical domain. In the case of the massive binary HD 15558, we also used a spectral disentangling method to separate the spectra of the primary and of the secondary in order to derive the radial velocities of the two components. These measurements were used to compute orbital solutions for HD 15558. Results. For HD 15570 and HD 15629, the radial velocities do not present any significant trend attributable to a binary motion on time scales of a few days, nor from one year to the next. In the case of HD 15558 we obtained an improved SB1 orbital solution with a period of about 442 days, and we report for the first time on the detection of the spectral signature of its secondary star. We derive spectral types O5.5III(f) and O7V for the primary and the secondary of HD 15558. We tentatively compute a first SB2 orbital solution although the radial velocities from the secondary star should be considered with caution. The mass ratio is rather high, i.e. about 3, and leads to very extreme minimum masses, in particular for the primary object. Minimum masses of the order of 150 ± 50 – – – – – –

An XMM-Newton observation of the massive binary HD 159176

We report the analysis of an XMM-Newton observation of the close binary HD 159176 (O7 V + O7 V). The ob- served LX/Lbol ratio reveals an X-ray luminosity exceeding by a factor ∼7 the expected value for X-ray emission from single O-stars, therefore suggesting a wind-wind interaction scenario. EPIC and RGS spectra are fitted consistently with a two tem- perature mekal optically thin thermal plasma model, with temperatures ranging from ∼ 2t o 6× 10 6 K. At first sight, these rather low temperatures are consistent with the expectations for a close binary system where the winds collide well before reaching their terminal velocities. We also investigate the variability of the X-ray light curve of HD 159176 on various short time scales. No significant variability is found and we conclude that if hydrodynamical instabilities exist in the wind interaction region of HD 159176, they are not sufficient to produce an observable signature in the X-ray emission. Hydrodynamic simulations using wind parameters from the literature reveal some puzzling discrepancies. The most striking one concerns the predicted X-ray luminosity which is one or more orders of magnitude larger than the observed one. A significant reduction of the mass loss rate of the components compared to the values quoted in the literature alleviates the discrepancy but is not sufficient to fully ac- count for the observed luminosity. Because hydrodynamical models are best for the adiabatic case whereas the colliding winds in HD 159176 are most likely highly radiative, a totally new approach has been envisaged, using a geometrical steady-state colliding wind model suitable for the case of radiative winds. This model successfully reproduces the spectral shape of the EPIC spectrum, but further developments are still needed to alleviate the disagreement between theoretical and observed X-ray luminosities.