Colliding-wind Binary Systems Research Articles

Simulating Wind-Blown Nebulae from Single and Binary Massive Stars

Winds from massive stars expand supersonically into their surroundings, creating dynamic and fascinating nebulae that can give us insight into physical processes in interstellar plasma, and into the evolutionary history of the stars. Around single stars, parsec-scale bubbles such as bow shocks and ring nebulae are formed, whereas in colliding-wind binary (CWB) systems the high wind density produces intense time- and space-dependent emission across the electromagnetic spectrum from radio to gamma-rays. This contribution summarizes some recent results from 3D MHD modelling of bow shocks around runaway stars such as ζ Oph, and of the wind-collision zone of the CWB systems WR140 and WR21a. A resolution study of 3D simulations of bow shocks shows that X-ray emission from the shocked wind is time-variable and that converged results can be obtained once the Kelvin-Helmholtz instability at the contact discontinuity is resolved. Simulations of the CWB system WR140 show that inverse-Compton cooling of the shocked plasma can trigger runaway cooling when the orbit is near periastron, producing strong compression and dynamical instabilities. This sharply reduces the hard-X-ray emission around periastron, in agreement with observations. Scaling tests of the simulation software pion are also presented for a model of the CWB system WR21a run on up to 8192 cores using the HPC system Karolina.

Performance and Scaling of PION for Modelling Colliding-Wind Binary Systems

Colliding-wind binaries are star systems with strong wind-wind interaction producing X-ray-, radio- and sometimes gamma-ray-emitting plasma. Computer modelling of these systems including magnetic fields and all relevant radiative processes is still in its infancy, but is important for constraining shock physics (including particle acceleration) and interstellar dust formation processes. Here we summarise some recent scientific results on these systems and present results of EuroHPC benchmarking tests of the astropysical fluid-dynamics code PION on the Czech supercomputer Karolina. Excellent strong and weak scaling are demonstrated, showing that PION can be used efficiently on Tier-0 systems for 3D simulations of colliding winds in binary systems. Further improvements were obtained in hybrid OpenMP/MPI parallelisation on the basis of the benchmarking test results.

Exploring dust growth in the episodic WCd system WR140

ABSTRACT The wind collision region (WCR) in a colliding wind binary (CWB) is a particularly violent place, as such, it is surprising that it is also a region where significant quantities of interstellar dust can form. In extreme cases, approximately 30 per cent of the total mass-loss rate of a system can be converted into dust. These regions are poorly understood, as observation and simulation of these systems are difficult. In our previous paper, we simulated dust growth in CWB systems using an advected scalar model and found our model to be suitable for qualitative study. For this paper, we simulated the periodic dust forming CWB (WCd) system WR140 with our dust model, to determine how dust growth changes over the systems periastron passage. We found that dust production increases significantly at periastron passage, which is consistent with IR emission of the surrounding dusty shell. We also find that the dust production rate of the system decreases rapidly as the stars recede from each other, though the rate of decrease is significantly lower than the rate of increase during periastron passage. This was found to be due to strong cooling and its associated thermal instabilities, resulting in cool, high-density pockets of gas in the WCR where dust forms. The WCR also shows a degree of hysteresis, resulting in a radiative post-shock flow even when the stars are separated enough for the region to behave adiabatically.

The Expansion of the X-Ray Nebula Around η Car

The massive colliding wind binary system η Car is embedded in an X-ray emitting region having a characteristic temperature of a few million degrees, associated with ejecta produced during the 1840s, and in earlier outbursts. We use CHANDRA X-ray imaging observations obtained over the past two decades to directly measure the expansion of the X-ray nebula for the first time. A combined CHANDRA/ACIS image shows a faint, nearly uniform elliptic structure. This faint elliptical “shell” has a similar orientation and shape as the Homunculus nebula but is about 3 times larger. We measure proper motions of brighter regions associated with the X-ray emitting ring. We compare spectra of the soft X-ray emitting plasma in CHANDRA/ACIS and XMM-Newton PN observations and show that the PN observations indicate a decline in X-ray flux which is comparable to that derived from NICER observations. We associate the diffuse elliptical emission surrounding the bright X-ray “ring” with the blast wave produced during the Great Eruption. We suggest that the interaction of this blast wave with pre-existing clumps of ejecta produces the bright, broken X-ray emitting ring. We extrapolate the trend in X-ray energy back to the time of the Great Eruption using a simple model and show that the X-ray energy was comparable to the kinetic energy of the Homunculus, suggesting equipartition of energy between fast, low-density ejecta and slower, dense ejecta.

Accretion in massive colliding-wind binaries and the effect of the wind momentum ratio

ABSTRACTWe carry out a numerical experiment for ejecting winds in a massive colliding-wind binary system and quantify the accretion on to the secondary star under different primary mass-loss rates. We set a binary system comprising a luminous blue variable (LBV) as the primary and a Wolf–Rayet (WR) star as the secondary, and vary the mass-loss rate of the LBV to obtain different values of the wind momentum ratio η. Our simulations include two sets of cases: one in which the stars are stationary; and one that includes the orbital motion. As η decreases, the colliding-wind structure moves closer to the secondary. We find that for η ≲ 0.05, the accretion threshold is reached and clumps that originate from instabilities are accreted on to the secondary. For each value of η, we calculate the mass accretion rate and identify different regions in the $\dot{M} _{\rm acc}$ – η diagram. For 0.001 ≲ η ≲ 0.05, the accretion is sub-Bondi–Hoyle–Lyttleton (BHL), and the average accretion rate satisfies the power law $\dot{M}_{\rm acc} \propto \eta ^{-1.73}$ for static stars. The accretion is not continuous but rather changes from sporadic to a larger duty cycle as η decreases. For η ≲ 0.001, the accretion becomes continuous in time, and the accretion rate is BHL, up to a factor of 0.4–0.8. The simulations that include the orbital motion give qualitatively similar results, with the steeper power law $\dot{M}_{\rm acc} \propto \eta ^{-1.86}$ for the sub-BHL region and lower η as an accretion threshold.

CHANG-ES – XXVII. A radio/X-ray catalogue of compact sources in and around edge-on galaxies

ABSTRACT We present catalogues of discrete, compact radio sources in and around the discs of 35 edge-on galaxies in the Continuum Halos in Nearby Galaxies – an EVLA Survey. The sources were extracted using the pybdsf program at both 1.6 GHz (L band) and 6.0 GHz (C band) from matching resolution (≈3 arcsec) data. We also present catalogues of X-ray sources from Chandra data sets for 27 of the galaxies. The sources at the two radio frequency bands were positionally cross-correlated with each other, and the result cross-correlated with the X-ray sources. All catalogues are included for download with this paper. We detect a total of 2507 sources at L band and 1413 sources at C band. 75 sources have been successfully cross-correlated in both radio bands plus X-ray. Three new nuclear sources are candidates for low luminosity active galactic nuclei in NGC 3877, NGC 4192, and NGC 5792; the one in NGC 3877 also appears to be variable. We also find new nuclear sources in two companion galaxies: NGC 4435 (companion to NGC 4438) and NGC 4298 (companion to NGC 4302). We have also discovered what appears to be a foreground double star; each star has X-ray emission and there is radio emission at both L band and C band in between them. This could be a colliding wind binary system. Suggestions for follow-up studies are offered.

Melnick 33Na: a very massive colliding-wind binary system in 30 Doradus

ABSTRACT We present spectroscopic analysis of the luminous X-ray source Melnick 33Na (Mk 33Na, HSH95 16) in the Large Magellanic Cloud (LMC) 30 Doradus region (Tarantula Nebula), utilizing new time-series Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectroscopy. We confirm Mk 33Na as a double-lined O-type spectroscopic binary with a mass ratio q = 0.63 ± 0.02, e = 0.33 ± 0.01, and orbital period of 18.3 ± 0.1 d, supporting the favoured period from X-ray observations obtained via the Tarantula – Revealed by X-rays survey. Disentangled spectra of each component provide spectral types of OC2.5 If* and O4 V for the primary and secondary, respectively. Unusually for an O supergiant the primary exhibits strong C iv 4658 emission and weak N v 4603-20, justifying the OC classification. Spectroscopic analysis favours extreme physical properties for the primary (Teff = 50 kK, log L/L⊙ = 6.15) with system components of M1 = 83 ± 19 M⊙ and M2 = 48 ± 11 M⊙ obtained from evolutionary models, which can be reconciled with results from our orbital analysis (e.g. M1sin 3i = 20.0 ± 1.2 M⊙) if the system inclination is ∼38° and it has an age of 0.9–1.6 Myr. This establishes Mk 33Na as one of the highest mass binary systems in the LMC, alongside other X-ray luminous early-type binaries Mk34 (WN5h+WN5h), R144 (WN5/6h+WN6/7h), and especially R139 (O6.5 Iafc + O6 Iaf).

The Colliding Winds of WR 25 in High-resolution X-Rays

Abstract WR 25 is a colliding-wind binary star system comprising a very massive O2.5If*/WN6 primary and an O-star secondary in an eccentric orbit with a 208 day period. These hot stars have strong, highly supersonic winds that interact to form a bright X-ray source from wind-collision shocks whose conditions change with stellar separation. Different views through the winds of Wolf–Rayet and O stars are afforded with orbital phase as the stars move about their orbits, allowing for exploration of wind structure in ways not easy or even possible for single stars. We have analyzed an on-axis Chandra/HETGS spectrum of WR 25 obtained shortly before periastron when the X-rays emanating from the system are the brightest. From the on-axis observations, we constrain the line fluxes, centroids, and widths of various emission lines, including He triplets of Si xiii and Mg xi. We have also been able to include several serendipitous off-axis HETG spectra from the archive and study their flux variation with phase. This is the first report on high-resolution spectral studies of WR 25 in X-rays.

Particle acceleration and non-thermal emission in colliding-wind binary systems

ABSTRACT We present a model for the creation of non-thermal particles via diffusive shock acceleration in a colliding-wind binary. Our model accounts for the oblique nature of the global shocks bounding the wind–wind collision region and the finite velocity of the scattering centres to the gas. It also includes magnetic field amplification by the cosmic ray induced streaming instability and the dynamical back reaction of the amplified field. We assume that the injection of the ions and electrons is independent of the shock obliquity and that the scattering centres move relative to the fluid at the Alfvén velocity (resulting in steeper non-thermal particle distributions). We find that the Mach number, Alfvénic Mach number, and transverse field strength vary strongly along and between the shocks, resulting in significant and non-linear variations in the particle acceleration efficiency and shock nature (turbulent versus non-turbulent). We find much reduced compression ratios at the oblique shocks in most of our models compared to our earlier work, though total gas compression ratios that exceed 20 can still be obtained in certain situations. We also investigate the dependence of the non-thermal emission on the stellar separation and determine when emission from secondary electrons becomes important. We finish by applying our model to WR 146, one of the brightest colliding wind binaries in the radio band. We are able to match the observed radio emission and find that roughly 30 per cent of the wind power at the shocks is channelled into non-thermal particles.

Colliding-wind binary systems: diffusive shock acceleration and non-thermal emission

ABSTRACT We present a model for the non-thermal emission from a colliding-wind binary. Relativistic protons and electrons are assumed to be accelerated through diffusive shock acceleration (DSA) at the global shocks bounding the wind–wind collision region. The non-linear effects of the backreaction due to the cosmic ray pressure on the particle acceleration process and the cooling of the non-thermal particles as they flow downstream from the shocks are included. We explore how the non-thermal particle distribution and the keV−GeV emission changes with the stellar separation and the viewing angle of the system, and with the momentum ratio of the winds. We confirm earlier findings that DSA is very efficient when magnetic field amplification is not included, leading to significantly modified shocks. We also find that the non-thermal flux scales with the binary separation in a complicated way and that the anisotropic inverse Compton emission shows only a moderate variation with viewing angle due to the spatial extent of the wind–wind collision.

Detection of very-high-energy γ-ray emission from the colliding wind binary η Car with H.E.S.S.

Aims. Colliding wind binary systems have long been suspected to be high-energy (HE; 100 MeV < E < 100 GeV) γ-ray emitters. η Car is the most prominent member of this object class and is confirmed to emit phase-locked HE γ rays from hundreds of MeV to ~100 GeV energies. This work aims to search for and characterise the very-high-energy (VHE; E >100 GeV) γ-ray emission from η Car around the last periastron passage in 2014 with the ground-based High Energy Stereoscopic System (H.E.S.S.). Methods. The region around η Car was observed with H.E.S.S. between orbital phase p = 0.78−1.10, with a closer sampling at p ≈ 0.95 and p ≈ 1.10 (assuming a period of 2023 days). Optimised hardware settings as well as adjustments to the data reduction, reconstruction, and signal selection were needed to suppress and take into account the strong, extended, and inhomogeneous night sky background (NSB) in the η Car field of view. Tailored run-wise Monte-Carlo simulations (RWS) were required to accurately treat the additional noise from NSB photons in the instrument response functions. Results. H.E.S.S. detected VHE γ-ray emission from the direction of η Car shortly before and after the minimum in the X-ray light-curve close to periastron. Using the point spread function provided by RWS, the reconstructed signal is point-like and the spectrum is best described by a power law. The overall flux and spectral index in VHE γ rays agree within statistical and systematic errors before and after periastron. The γ-ray spectrum extends up to at least ~400 GeV. This implies a maximum magnetic field in a leptonic scenario in the emission region of 0.5 Gauss. No indication for phase-locked flux variations is detected in the H.E.S.S. data.

Megahertz emission of massive early-type stars in the Cygnus region

AbstractMassive, early-type stars have been detected as radio sources for many decades. Their thermal winds radiate free–free continuum and in binary systems hosting a colliding-wind region, non-thermal emission has also been detected. To date, the most abundant data have been collected from frequencies higher than 1 GHz. We present here the results obtained from observations at 325 and 610 MHz, carried out with the Giant Metrewave Radio Telescope, of all known Wolf-Rayet and O-type stars encompassed in area of$\sim$15 sq degrees centred on the Cygnus region. We report on the detection of 11 massive stars, including both Wolf-Rayet and O-type systems. The measured flux densities at decimeter wavelengths allowed us to study the radio spectrum of the binary systems and to propose a consistent interpretation in terms of physical processes affecting the wide-band radio emission from these objects. WR 140 was detected at 610 MHz, but not at 325 MHz, very likely because of the strong impact of free–free absorption (FFA). We also report—for the first time—on the detection of a colliding-wind binary system down to 150 MHz, pertaining to the system of WR 146, making use of complementary information extracted from the Tata Institute of Fundamental Research GMRT Sky Survey. Its spectral energy distribution clearly shows the turnover at a frequency of about 600 MHz, that we interpret to be due to FFA. Finally, we report on the identification of two additional particle-accelerating colliding-wind binaries, namely Cyg OB2 12 and ALS 15108 AB.

Proton Acceleration in Colliding Stellar Wind Binaries

Abstract The interaction between the strong winds in stellar colliding-wind binary (CWB) systems produces two shock fronts, delimiting the wind-collision region (WCR). There, particles are expected to be accelerated mainly via diffusive shock acceleration. We investigate the injection and acceleration of protons in typical CWB systems by means of Monte Carlo simulations, with both a test-particle approach and a nonlinear method modeling a shock locally modified by the backreaction of the accelerated protons. We use magnetohydrodynamic simulations to determine the background plasma in the WCR and its vicinity. This allows us to consider particle acceleration at both shocks, on either side of the WCR, with a realistic large-scale magnetic field. We highlight the possible effects of particle acceleration on the local shock profiles at the WCR. We include the effect of magnetic field amplification, due to resonant-streaming instability, and compare results without and with the backreaction of the accelerated protons. In the latter case, we find a lower flux of the nonthermal proton population and a considerable magnetic field amplification. This would significantly increase the synchrotron losses of relativistic electrons accelerated in CWB systems, lowering the maximal energy they can reach and strongly reducing the inverse Compton fluxes. As a result, γ-rays from CWBs would be predominantly due to the decay of neutral pions produced in nucleon–nucleon collisions. This might provide a way to explain why, in the vast majority of cases, CWB systems have not been identified as γ-ray sources, although they emit synchrotron radiation.

Anisotropic winds in a Wolf–Rayet binary identify a potential gamma-ray burst progenitor

The massive evolved Wolf-Rayet stars sometimes occur in colliding-wind binary systems in which dust plumes are formed as a result of the collision of stellar winds. These structures are known to encode the parameters of the binary orbit and winds. Here, we report observations of a previously undiscovered Wolf-Rayet system, 2XMM J160050.7-514245, with a spectroscopically determined wind speed of $\approx$3400 km s$^{-1}$. In the thermal infrared, the system is adorned with a prominent $\approx$12$''$ spiral dust plume, revealed by proper motion studies to be expanding at only $\approx$570 km s$^{-1}$. As the dust and gas appear coeval, these observations are inconsistent with existing models of the dynamics of such colliding wind systems. We propose that this contradiction can be resolved if the system is capable of launching extremely anisotropic winds. Near-critical stellar rotation is known to drive such winds, suggesting this Wolf-Rayet system as a potential Galactic progenitor system for long-duration gamma-ray bursts.

3D time-dependent hydrodynamical and radiative transfer modeling of Eta Carinae’s innermost fossil colliding wind structures

AbstractEta Carinae is the most massive active binary within 10,000 light-years. While famous for the largest non-terminal stellar explosion ever recorded, observations reveal a supermassive (∼120 M⊙) binary consisting of an LBV and either a WR or extreme O star in a very eccentric orbit (e=0.9) with a 5.54-year period. Dramatic changes across multiple wavelengths are routinely observed as the stars move about in their highly elliptical orbits, especially around periastron when the hot (∼40 kK) companion star delves deep into the denser and much cooler (∼15 kK) extended wind photosphere of the LBV primary. Many of these changes are due to a dynamic wind-wind collision region (WWCR) that forms between the stars, plus expanding radiation-illuminated fossil WWCRs formed one, two, and three 5.54-year orbital cycles ago. These fossil WWCRs have been spatially and spectrally resolved by the Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) at multiple epochs, resulting in data cubes that spatially map Eta Carinae’s innermost WWCRs and follow temporal changes in several forbidden emission lines (e.g. [Fe iii] 4659 Å, [Fe ii] 4815 Å) across the 5.54-year cycle. We present initial results of 3D time-dependent hydrodynamical and radiative-transfer simulations of the Eta Carinae binary and its WWCRs with the goal of producing synthetic data cubes of forbidden emission lines for comparison to the available HST/STIS observations. Comparison of the theoretical models to the observations reveals important details about the binary’s orbital motion, photoionization properties, and recent (5–15year) mass loss history. Such an analysis also provides a baseline for following future changes in Eta Carinae, essential for understanding the late-stage evolution of a nearby supernova progenitor. Our modeling methods can also be adapted to a number of other colliding wind binary systems (e.g. WR 140) that are scheduled to be studied with future observatories (e.g. the James Webb Space Telescope).

The 155-day X-ray cycle of the very massive Wolf–Rayet star Melnick 34 in the Large Magellanic Cloud

The Wolf–Rayet star Mk 34 was observed more than 50 times as part of the deep T-ReX Chandra ACIS-I X-ray imaging survey of the Tarantula Nebula in the Large Magellanic Cloud conducted between 2014 May and 2016 January. Its brightness showed one bright maximum and repeated faint minima which help define an X-ray recurrence time of 155.1 ± 0.1 d that is probably the orbital period of an eccentric binary system. The maximum immediately precedes the minimum in the folded X-ray light curve as confirmed by new Swift XRT observations. Notwithstanding its extreme median luminosity of 1.2 × 1035 erg s−1, which makes it over an order of magnitude brighter than comparable stars in the Milky Way, Mk 34 is almost certainly a colliding-wind binary system. Its spectrum shows phase-related changes of luminosity and absorption that are probably related to the orbital dynamics of two of the most massive stars known.

3D Magnetohydrodynamic Models of Nonthermal Photon Emission in the Binary System γ2 Velorum

Abstract Recent reports claiming an association of the massive star binary system Velorum (WR 11) with a high-energy γ-ray source observed by Fermi-LAT contrast the so far exclusive role of η Carinae as the hitherto only detected γ-ray emitter in the source class of particle-accelerating colliding-wind binary (CWB) systems. We offer support to this claim of association by providing dedicated model predictions for the nonthermal photon emission spectrum of Velorum. We use 3D magnetohydrodynamic modeling (MHD) to investigate the structure and conditions of the wind-collision region (WCR) of Velorum including the important effect of radiative braking in the stellar winds. A transport equation is then solved for the entire computational domain to study the propagation of relativistic electrons and protons. The resulting distributions of particles are subsequently used to compute nonthermal photon emission components. In agreement with observation in X-ray spectroscopy, our simulations yield a large shock-cone opening angle. We find the nonthermal γ-ray emission of Velorum to be of hadronic origin owing to the strong radiation fields in the binary system, which inhibit the acceleration of electrons to energies sufficiently high for efficient inverse-Compton radiation. We also discuss the strong dependence of a hadronic γ-ray component on the energy-dependent diffusion used in the simulations. Of two mass-loss rates for the WR star found in literature, only the higher rate is able to accommodate the observed γ-ray spectrum with reasonable values for important simulation parameters such as the injection ratio of high-energy particles within the WCR.

The shortest periodic and flaring flux variability of a methanol maser emission at 6.7 GHz in G 014.23−00.50

Abstract We detected flaring flux variability that regularly occurred with a period of 23.9 d on a 6.7 GHz methanol maser emission at Vlsr = 25.30 km s−1 in G 014.23−00.50 through highly frequent monitoring using the Hitachi 32 m radio telescope. Analyzing data from 2013 January 5 to 2016 January 21, the periodic variability has persisted in at least 47 cycles, corresponding to ∼1100 d. The period of 23.9 d is the shortest one observed in masers around high-mass young stellar objects so far. The flaring component normally falls below the detection limit (3 σ) of ∼0.9 Jy. In the flaring periods, the component rises above the detection limit with a ratio of the peak flux density more than 180 in comparison with the quiescent phase, showing intermittent periodic variability. The timescale of the flux rise was typically two days or shorter, and both symmetric and asymmetric profiles of flux variability were observed through intraday monitoring. These characteristics might be explained by a change in the flux of seed photons in a colliding-wind binary (CWB) system, or a variation of the dust temperature by the extra heating source of a shock formed by a CWB system within a gap region in a circumbinary disk, in which the orbital semi-major axes of the binary are 0.26–0.34 au.

COLLIDING-WIND BINARIES WITH STRONG MAGNETIC FIELDS

ABSTRACT The dynamics of colliding-wind binary (CWB) systems and conditions for efficient particle acceleration therein have attracted multiple numerical studies in recent years. These numerical models seek an explanation of the thermal and nonthermal emission of these systems as seen by observations. In the nonthermal regime, radio and X-ray emission is observed for several of these CWBs, while gamma-ray emission has so far only been found in η Carinae and possibly in WR 11. Energetic electrons are deemed responsible for a large fraction of the observed high-energy photons in these systems. Only in the gamma-ray regime might there be, depending on the properties of the stars, a significant contribution of emission from neutral pion decay. Thus, studying the emission from CWBs requires detailed models of the acceleration and propagation of energetic electrons. This in turn requires a detailed understanding of the magnetic field, which will affect not only the energy losses of the electrons but also, in the case of synchrotron emission, the directional dependence of the emissivity. In this study we investigate magnetohydrodynamic simulations of different CWB systems with magnetic fields that are strong enough to have a significant effect on the winds. Such strong fields require a detailed treatment of the near-star wind acceleration zone. We show the implementation of such simulations and discuss results that demonstrate the effect of the magnetic field on the structure of the wind collision region.

The first full orbit ofηCarinae seen byFermi

The binary system eta Carinae has completed its first 5.54y orbit since the beginning of science operation of the Fermi Large Area Telescope (LAT). We are now able to investigate the high-energy gamma-ray source at the position of eta Carinae over its full orbital period. By this, we can address and confirm earlier predictions for temporal and spectral variability. Newer versions of the LAT datasets, instrument response functions and background models allow for a more accurate analysis. Therefore it is important to re-evaluate the previously analyzed time period along with the new data to further constrain location, spectral shape, and flux time history of the gamma-ray source. We confirm earlier predictions of increasing flux values above 10 GeV toward the next periastron passage. For the most recent part of the data sample, flux values as high as those before the first periastron passage in 2008 are recorded. A comparison of spectral energy distributions around periastron and apastron passages reveals strong variation in the high-energy band. This is due to a second spectral component that is present only around periastron. Improved spatial consistency with the gamma-ray source at the position of eta Carinae along with the confirmation of temporal variability above 10 GeV in conjunction with the orbital period strengthens the argument for unambiguous source identification. Spectral variability provides additional constraints for future modeling of the particle acceleration and gamma-ray emission in colliding-wind binary systems.