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X-Ray Spectral Variation of η Carinae through the 2003 X-Ray Minimum
We report the results of an observing campaign on η Car around the2003 X-ray minimum, mainly using the XMM-Newton observatory. These arethe first spatially resolved X-ray monitoring observations of thestellar X-ray spectrum during the minimum. The hard X-ray emission,associated with the wind-wind collision (WWC) in the binary system,varied strongly in flux on timescales of days, but not significantly ontimescales of hours. The X-ray flux in the 2-10 keV band seen byXMM-Newton was only 0.7% of the flux maximum seen by RXTE. The slope ofthe X-ray continuum above 5 keV did not vary in any observation, whichsuggests that the electron temperature of the hottest plasma did notvary significantly at any phase. Through the minimum, the absorption tothe stellar source increased by a factor of 5-10 toNH~(3-4)×1023 cm-2. Thesevariations were qualitatively consistent with emission from the WWCplasma entering into the dense wind of the massive primary star. Duringthe minimum, X-ray spectra also showed significant excesses in thethermal Fe XXV emission line on the red side, while they showed only afactor of 2 increase in equivalent width of the Fe fluorescence line at6.4 keV. These features are not fully consistent with the eclipse of theX-ray plasma and may suggest an intrinsic fading of the X-rayemissivity. The drop in the WWC emission revealed the presence of anadditional X-ray component that exhibited no variation on timescales ofweeks to years. This component may be produced by the collision ofhigh-speed outflows at v~1000-2000 km s-1 from η Car withambient gas within a few thousand AU from the star.
| Wind-wind collision in the η Carinae binary system - III. The HeII λ4686 line profile
We modelled the HeII λ4686 line profiles observed in the ηCarinae binary system close to the 2003.5 spectroscopic event, assumingthat they were formed in the shocked gas that flows at both sides of thecontact surface formed by wind-wind collision. We used a constant flowvelocity and added turbulence in the form of a Gaussian velocitydistribution. We allowed emission from both the primary and secondaryshocks but introduced infinite opacity at the contact surface, implyingthat only the side of the contact cone visible to the observercontributed to the line profile. Using the orbital parameters of thebinary system derived from the 7-mm light curve during the lastspectroscopic event (Paper II) we were able to reproduce the lineprofiles obtained with the Hubble Space Telescope at different epochs,as well as the line mean velocities obtained with ground-basedtelescopes. A very important feature of our model is that the lineprofile depends on the inclination of the orbital plane; we found thatto explain the latitude-dependent mean velocity of the line, scatteredinto the line of sight by the Homunculus, the orbit cannot lie in theHomunculus equatorial plane, as usually assumed. This result, togetherwith the relative position of the stars during the spectroscopic events,allowed us to explain most of the observational features, like thevariation of the `Purple Haze' with the orbital phase, and to conciliatethe X-ray absorption with the postulated shell effect used to explainthe optical and ultraviolet light curves.
| A Clumping-independent Diagnostic of Stellar Mass-Loss Rates: Rapid Clump Destruction in Adiabatic Colliding Winds
Clumping in hot star winds can significantly affect estimates ofmass-loss rates, the inferred evolution of the star, and theenvironmental impact of the wind. A hydrodynamical simulation of acolliding winds binary (CWB) with clumpy winds reveals that the clumpsare rapidly destroyed after passing through the confining shocks of thewind-wind collision region (WCR) for reasonable parameters of the clumpsif the flow in the WCR is adiabatic. Despite large density andtemperature fluctuations in the postshock gas, the overall effect of theinteraction is to smooth the existing structure in the winds. Averagedover the entire interaction region, the resulting X-ray emission is verysimilar to that from the collision of smooth winds. The insensitivity ofthe X-ray emission to clumping suggests it is an excellent diagnostic ofthe stellar mass-loss rates (M˙) in wide CWBs and may prove to be auseful addition to existing techniques for deriving M˙, many ofwhich are extremely sensitive to clumping. Clumpy winds also haveimplications for a variety of phenomena at the WCR: particleacceleration may occur throughout the WCR due to supersonic MHDturbulence, reacceleration at multiple shocks, and reconnection; astatistical description of the properties of the WCR may be required forstudies of nonequilibrium ionization and the rate of electron heating;and the physical mixing of the two winds will be enhanced, as seemsnecessary to trigger dust formation.
| η Carinae across the 2003.5 Minimum: Spectroscopic Evidence for Massive Binary Interactions
We have analyzed high spatial, moderate spectral resolution observationsof η Carinae (η Car) obtained with the STIS from 1998.0 to2004.3. The data were obtained at discrete times covering an entire 2024day spectroscopic cycle, with focus on the X-ray/ionization low statethat began in 2003 June. The spectra show prominent P Cygni lines in HI, Fe II, and He I, which are complicated by blends and contamination bynebular emission and absorption. All lines show phase- andspecies-dependent variations in emission and absorption. For most of thecycle the He I emission is blueshifted relative to the H I and Fe II PCygni emission lines, which are centered at approximately systemvelocity. The blueshifted He I absorption components vary in intensityand velocity throughout the 2024 day period. We construct radialvelocity curves for the absorption component of the He I and H I lines.The He I absorption shows significant radial velocity variationsthroughout the cycle, with a rapid change of over 200 km s-1near the 2003.5 event. The H I velocity curve is similar to that of theHe I absorption, although offset in phase and reduced in amplitude. Weinterpret the complex line profile variations in He I, H I, and Fe II tobe a consequence of the dynamic interaction of the dense wind of ηCar A with the less dense, faster wind plus the radiation field of a hotcompanion star, η Car B. We use the variations seen in He I and theother P Cygni lines to constrain the geometry of the orbit and thecharacter of η Car B.
| Non-thermal radio emission from O-type stars. II. HD 167971
HD 167971 is a triple system consisting of a 3.3-day eclipsing binary(O5-8 V + O5-8 V) and an O8 supergiant. It is also a well knownnon-thermal radio emitter. We observed the radio emission of HD 167971with the Very Large Array (VLA) and the Australia Telescope CompactArray (ATCA). By combining these data with VLA archive observations weconstructed a radio lightcurve covering a 20-yr time-range. We searchedfor, but failed to find, the 3.3-day spectroscopic period of the binaryin the radio data.This could be due to the absence of intrinsic synchrotron radiationin the colliding-wind region between the two components of the eclipsingbinary,or due to the largeamount of free-free absorption that blocks the synchrotronradiation. We are able to explain many of the observed characteristicsof the radio dataif the non-thermal emission is produced in a colliding-wind regionbetween the supergiant and the combined winds of the binary.Furthermore, if the system is gravitationally bound, the orbital motionoccurs over a period of ~20 years or longer, as suggested by thelong-term variability in the radio data. We argue that the variabilityis due to the free-free absorption that changes with orbital phase ormay also in part be due to changes in separation, should the orbit beeccentric.Table A.1 is only available in electronic form at http://www.aanda.org
| A new paradigm for the X-ray emission of O stars from XMM-Newton observations of the O9.7 supergiant ζ Orionis
XMM-Newton observations of the O supergiant ζ Orionis (O9.7 Ib)extend knowledge of its high-resolution spectrum beyond the C VI line at33.7 Å and suggest a new framework for the interpretation of theX-ray spectra of single hot stars. All the lines are broad andasymmetric with similar velocity profiles. X-rays probably originate inthe wind's terminal velocity regime in collisionless shocks controlledby magnetic fields rather than in cooling shocks in the accelerationzone. During post-shock relaxation, exchange of energy between ions andelectrons is so slow that electron heating does not take place beforehot gas is quenched by the majority cool gas. The observed plasma is notin equilibrium and the electron bremsstrahlung continuum is weak. Chargeexchange, ionization and excitation are likely to be produced byprotons. Fully thermalized post-shock velocities ensure highcross-sections and account for the observed line widths, with someallowance probably necessary for non-thermal particle acceleration. Ingeneral, the form of X-ray spectra in both single and binary stars islikely to be determined principally by the amount of post-shock electronheating: magnetically confined X-ray plasma in binary systems can evolvefurther towards the higher electron temperatures of equilibrium while insingle stars this does not take place. The long mean-free path forCoulomb energy exchange between fast-moving ions may also inhibit thedevelopment of line-driven instabilities.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.
| Towards an understanding of the Of?p star HD191612: phase-resolved multiwavelength observations
We present the analysis of phase-resolved X-ray and optical observationsof the peculiar hot star HD191612 (Of?p). This star is known to displayline-profile variations that are recurrent with a period of 538d and itsspectrum was found to present the signature of a magnetic field. In theX-rays, it is slightly overluminous compared to the canonicalLX/LBOL) relation and appears brighter when theoptical lines are strongest. Our XMM-Newton observations further revealthat the X-ray spectrum of HD191612 exhibits rather broad lines and isdominated by a `cool' (0.2-0.6keV) thermal component, twocharacteristics at odds with the proposed magnetic rotator model. Wealso report for the first time the low-level variability of the metallic(absorption/emission) lines and HeII absorptions that appear to beassociated with radial-velocity shifts. Finally, we compare our resultswith observations of the early-type stars and discuss several possiblescenarios.Based on observations collected at the Haute-Provence Observatory(France) and with XMM-Newton, an ESA Science Mission with instrumentsand contributions directly funded by ESA Member States and the USA(NASA).E-mail: naze@astro.ulg.ac.be ‡Post-doctoral Researcher FNRS (Belgium). §Research Associate FNRS (Belgium). ¶Operated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555.
| The Keck Aperture-masking Experiment: Near-Infrared Sizes of Dusty Wolf-Rayet Stars
We report the results of a high angular resolution near-infrared surveyof dusty Wolf-Rayet stars using the Keck I Telescope, including newmultiwavelength images of the pinwheel nebulae WR 98a, WR 104, and WR112. Angular sizes were measured for an additional eight dustyWolf-Rayet stars using aperture-masking interferometry, allowing us toprobe characteristic sizes down to ~20 mas (~40 AU for typical sources).With angular sizes and specific fluxes, we can directly measure thewavelength-dependent surface brightness and size relations for oursample. We discovered tight correlations of these properties within oursample that could not be explained by simple spherically symmetric dustshells or even the more realistic ``pinwheel nebula''(three-dimensional) radiative transfer model, when using Zubko's opticalconstants. While the tightly correlated surface brightness relations weuncovered offer compelling indirect evidence of a shared and distinctivedust shell geometry among our sample, long-baseline interferometersshould target the marginally resolved objects in our sample in order toconclusively establish the presence or absence of the putativeunderlying colliding-wind binaries thought to produce the dust shellsaround WC Wolf-Rayet stars.
| Collision of Two Identical Hypersonic Stellar Winds in Binary Systems
We investigate the hydrodynamics of two identical hypersonic stellarwinds in a binary system. The interaction of these winds manifestsitself in the form of two shocks and a contact surface between them. Weneglect the binary rotation and assume that the gas flow ahead of theshocks is spherically symmetric. In this case the contact surface thatseparates the gas emanating from the different stars coincides with themidplane of the binary components. In the shock the gas is heated andflows away nearly along the contact surface. We find the shock shape andthe hot gas parameters in the shock layer between the shock and thecontact surface.
| X-Ray Variability in the Young Massive Triple θ2 Orionis A
Massive stars rarely show intrinsic X-ray variability. One exception isθ2 Ori A, which has shown strong variability over thelast 5 years. We observed a large outburst of the X-ray source with theHigh Energy Transmission Grating Spectrometer on board Chandra andcompare the emissivity and line properties in states of low and highflux. The low state indicates temperatures well above 25 MK. In the highstate we find high emissivities in the range from 3 to over 100 MK. Theoutburst event in stellar terms is one of the most powerful everobserved and the most energetic one in the ONC, with a lower totalenergy limit of 1.5×1037 ergs. The line diagnosticsindicate that the line-emitting regions in the low states are as closeas within 1-2 stellar radii from the O star's photosphere, whereas thehard states suggest a distance of 3-5 stellar radii. We discuss theresults in the context of stellar flares, magnetic confinement, andbinary interactions. By matching the dates of all observations with theorbital phases of the spectroscopy binary orbit, we find that outburstsoccur very close to the periastron passage of the stars. We argue thatthe high X-ray states are possibly the result of reconnection eventsfrom magnetic interactions of the primary and secondary stars of thespectroscopic binary. Effects from wind collisions seem unlikely forthis system. The line properties in the low state seem consistent withsome form of magnetic confinement. We also detect Fe fluorescenceindicative of the existence of substantial amounts of neutral Fe in thevicinity of the X-ray emission.
| Radio, X-ray, and γ-ray emission models of the colliding-wind binary WR140
We use hydrodynamical models of the wind-collision region in thearchetype colliding-wind system WR140 to determine the spatial andspectral distributions of the radio, X-ray, and γ-ray emissionfrom shock-accelerated electrons. Our calculations are for orbital phase0.837 when the observed radio emission is close to maximum. Using theobserved thermal X-ray emission at this phase in conjunction with theradio emission to constrain the mass-loss rates, we find that the O starmass-loss rate is consistent with the reduced estimates for O4-5supergiants by Fullerton, Massa & Prinja, and the wind-momentumratio, η = 0.02. This is independent of the opening angle deducedfrom radio very long baseline interferometry observations of the WCRthat we demonstrate fail to constrain the opening angle.We show that the turnover at ~3 GHz in the radio emission is due tofree-free absorption, since models based on the Razin effect have anunacceptably large fraction of energy in non-thermal electrons. We findthat the spectral index of the non-thermal electron energy distributionis flatter than the canonical value for diffusive shock acceleration,namely p < 2. Several mechanisms are discussed that could lead tosuch an index. Our inability to obtain fits to the radio data with p> 2 does not exclude the possibility of shock modification, butstronger evidence than that which currently exists is necessary for itssupport.Tighter constraints on p and the nature of the shocks in WR140 will beobtained from future observations at MeV and GeV energies, for which wegenerally predict lower fluxes than those in previous works. Since thehigh stellar photon fluxes prevent the acceleration of electrons beyondγ >~ 105-106, TeV emission fromcolliding-wind binary systems will provide unambiguous evidence ofpion-decay emission from accelerated ions. We finish by commenting onthe emission and physics of the multiple wind collisions in densestellar clusters, paying particular attention to the Galactic Centre.
| XMM-Newton observations of the massive colliding wind binary and non-thermal radio emitter CygOB2#8A [O6If + O5.5III(f)]
We report on the results of four XMM-Newton observations separated byabout ten days from each other of CygOB2#8A [O6If + O5.5III(f)]. Thismassive colliding wind binary is a very bright X-ray emitter - one ofthe first X-ray emitting O-stars discovered by the Einstein satellite -as well as a confirmed non-thermal radio emitter whose binarity wasdiscovered quite recently. The X-ray spectrum between 0.5 and 10.0keV isessentially thermal, and is best fitted with a three-component modelwith temperatures of about 3, 9 and 20MK. The X-ray luminosity correctedfor the interstellar absorption is rather large, i.e. about1034ergs-1. Compared to the `canonical'LX/Lbol ratio of O-type stars, CygOB2#8A was afactor of 19-28 overluminous in X-rays during our observations. The EPICspectra did not reveal any evidence for the presence of a non-thermalcontribution in X-rays. This is not unexpected considering that thesimultaneous detections of non-thermal radiation in the radio and softX-ray (below 10.0keV) domains is unlikely. Our data reveal a significantdecrease in the X-ray flux from apastron to periastron with an amplitudeof about 20 per cent. Combining our XMM-Newton results with those fromprevious ROSAT-PSPC and ASCA-SIS observations, we obtain a light curvesuggesting a phase-locked X-ray variability. The maximum emission leveloccurs around phase 0.75, and the minimum is probably seen shortly afterthe periastron passage. Using hydrodynamic simulations of the wind-windcollision, we find a high X-ray emission level close to phase 0.75, anda minimum at periastron as well. The high X-ray luminosity, the strongphase-locked variability and the spectral shape of the X-ray emission ofCygOB2#8A revealed by our investigation point undoubtedly to X-rayemission dominated by colliding winds.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA).E-mail: debecker@astro.ulg.ac.be ‡Research Associate FNRS (Belgium).
| The interplay between star formation and the nuclear environment of our Galaxy: deep X-ray observations of the Galactic centre Arches and Quintuplet clusters
The Galactic centre (GC) provides a unique laboratory for a detailedexamination of the interplay between massive star formation and thenuclear environment of our Galaxy. Here, we present a 100-ks ChandraAdvanced CCD Imaging Spectrometer (ACIS) observation of the Arches andQuintuplet star clusters. We also report on a complementary mapping ofthe dense molecular gas near the Arches cluster made with the OwensValley Millimeter Array. We present a catalogue of 244 point-like X-raysources detected in the observation. Their number-flux relationindicates an overpopulation of relatively bright X-ray sources, whichare apparently associated with the clusters. The sources in the core ofthe Arches and Quintuplet clusters are most likely extreme collidingwind massive star binaries. The diffuse X-ray emission from the core ofthe Arches cluster has a spectrum showing a 6.7-keV emission line and asurface intensity profile declining steeply with radius, indicating anorigin in a cluster wind. In the outer regions near the Arches cluster,the overall diffuse X-ray enhancement demonstrates a bow shockmorphology and is prominent in the Fe Kα 6.4-keV line emissionwith an equivalent width of ~1.4 keV. Much of this enhancement mayresult from an ongoing collision between the cluster and the adjacentmolecular cloud, which have a relative velocity >~120km-1.The older and less-compact Quintuplet cluster contains much weaker X-raysources and diffuse emission, probably originating from low-mass stellarobjects as well as a cluster wind. However, the overall population ofthese objects, constrained by the observed total diffuse X-rayluminosities, is substantially smaller than expected for both clusters,if they have normal Miller & Scalo initial mass functions. Thisdeficiency of low-mass objects may be a manifestation of the unique starformation environment of the GC, where high-velocity cloud-cloud andcloud-cluster collisions are frequent.
| The XMM-Newton view of Plaskett's star and its surroundings
XMM-Newton data of Plaskett's star (HD 47129) are used in order toanalyse its X-ray spectrum and variability and hence to derive furtherconstraints on the wind interaction in this early-type binary (O6 I +O7.5 I) system.Conventional models fail to provide a consistent fit of the EuropeanPhoton Imaging Camera (EPIC) and Reflexion Grating Spectrometer (RGS)spectra. The lines seen in the RGS spectrum have a temperature ofmaximum emissivity between 0.18 and 1.4 keV. The EPIC and RGS spectraare best fitted by a non-equilibrium model consisting of abremsstrahlung continuum at 2.2 +/- 0.1 keV and a number of independentemission lines. Our tests also suggest that an overabundance in nitrogenby a factor of ~6 might be indicated to best represent the RGS spectrum.On the other hand, a short-term variability study of the light curves ofthe system indicates that the X-ray flux of Plaskett's star did notdisplay any significant variability during our observation. This resultholds for all time-scales investigated here (from a few minutes to aboutone hour). Combining our XMM-Newton data with ROSAT archivalobservations, we find, however, a significant variability on the orbitaltime-scale. If this behaviour is indeed phase locked, it suggests aminimum in the X-ray flux when the primary star is in front. This mightbe attributed to an occultation of the colliding wind region by the bodyof the primary.Finally, 71 other X-ray sources have been detected in the field aroundPlaskett's star and most of them have a near-infrared (near-IR)counterpart with colours that are consistent with those of slightlyreddened main-sequence objects. Actually, a sizeable fraction of theX-ray sources in the EPIC images could be either foreground orbackground sources with no direct connection to HD 47129.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.E-mail: linder@astro.ulg.ac.be (NL), rauw@astro.ulg.ac.be (GR) ‡Research Associate FNRS, Belgium.
| Nonthermal High-Energy Emission from Colliding Winds of Massive Stars
Colliding winds of massive star binary systems are considered aspotential sites of nonthermal high-energy photon production. Motivatedby the detection of synchrotron radio emission from the colliding windlocation, we here investigate the properties of high-energy photonproduction in colliding winds of long-period WR+OB systems. Analyticalformulae for the steady state proton- and electron-particle spectra arederived assuming diffusive particle acceleration out of a pool ofthermal wind particles, taking into account adiabatic and all relevantradiative losses, and include advection/convection out of the windcollision zone. This includes analytical approximations for the electronenergy losses in the Klein-Nishina transition regime. For the first timein the context of CWB systems, our calculations use the fullKlein-Nishina cross section and account for the anisotropy of theinverse Compton scattering process. This leads to orbital fluxvariations by up to several orders of magnitude, which may, however, beblurred by the system's geometry. Both anisotropy and Klein-Nishinaeffects may yield characteristic spectral and variability signatures inthe γ-ray domain. Since propagation effects lead to a deficit oflow-energy particles in the convection-dominated zone, one expectsimprints in the radiation spectra. If protons are accelerated to atleast several GeV, π0-decay γ-rays might beobservable, depending on the injected electron-to-proton ratio. We showthat photon-photon pair production is generally not negligible,potentially affecting the emitted spectrum above ~50 GeV, depending onorbital phase and system inclination. The calculations are applied tothe archetypal WR+OB systems WR 140 and WR 147 to predict their expectedspectral and temporal characteristics and to assess their detectabilitywith current and upcoming γ-ray experiments.
| Accretion onto the Companion of η Carinae during the Spectroscopic Event. II. X-Ray Emission Cycle
We calculate the X-ray luminosity and light curve for the stellar binarysystem η Car for the entire orbital period of 5.54 yr. By using anew approach we find, as suggested in previous works, that the collisionof the winds blown by the two stars can explain the X-ray emission andtemporal behavior. Most X-ray emission in the 2-10 keV band results fromthe shocked secondary stellar wind. The observed rise in X-rayluminosity just before minimum is due to the increase in density andsubsequent decrease in radiative cooling time of the shocked fastsecondary wind. Absorption, particularly of the soft X-rays from theprimary wind, increases as the system approaches periastron and theshocks are produced deep inside the primary wind. However, absorptioncannot account for the drastic X-ray minimum. The 70 day minimum isassumed to result from the collapse of the collision region of the twowinds onto the secondary star. This process is assumed to shut down thesecondary wind, and hence the main X-ray source. We show that thisassumption provides a phenomenological description of the X-ray behavioraround the minimum.
| Radio Detection of Colliding Wind Binaries
Four massive, early-type stars, three of which are confirmed binaries,have been observed with the Australia Telescope Compact Array at 1.4,2.4, 4.8, and 8.6 GHz. The earliest star cataloged so far, HD 93129A,was also observed at 17.8 and 24.5 GHz. Here we present an analysis ofthe spectra as well as the structure of the stellar systems. All fourspectra show clear evidence of non-thermal emission, indicative of abinary system with a colliding wind region. We discuss the magneticfield of the emitting region of HD 93129A and make predictions on theradiation at high energies. Archive X-ray observations towards thetarget sources are also investigated and interpreted in the light of thenon-thermal emission detected.
| A Rich Population of X-Ray-emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1
Recent optical and infrared studies have revealed that the heavilyreddened starburst cluster Westerlund 1 (Wd 1) contains at least 22Wolf-Rayet (W-R) stars, constituting the richest W-R population of anyGalactic cluster. We present results of a sensitive Chandra X-rayobservation of Wd 1 that detected 12 of the 22 known W-R stars and themysterious emission-line star W9. The fraction of detected WN stars isnearly identical to that of WC stars. The WN stars WR-A and WR-B, aswell as W9, are exceptionally luminous in X-rays and have similar hard,heavily absorbed X-ray spectra with strong Si XIII and S XV emissionlines. The luminous high-temperature X-ray emission of these three starsis characteristic of colliding-wind binary systems, but their binarystatus remains to be determined. Spectral fits of the X-ray-brightsources WR-A and W9 with isothermal plane-parallel shock models requirehigh absorption column densities, log NH=22.56(cm-2), and yield characteristic shock temperatureskTs~3 keV (Ts~35 MK).
| Radio emission models of colliding-wind binary systems. Inclusion of IC cooling
Radio emission models of colliding wind binaries (CWBs) have beendiscussed by Dougherty et al. (2003). We extend these models byconsidering the temporal and spatial evolution of the energydistribution of relativistic electrons as they advect downstream fromtheir shock acceleration site. The energy spectrum evolves significantlydue to the strength of inverse-Compton (IC) cooling in these systems,and a full numerical evaluation of the synchrotron emission andabsorption coefficients is made. We have demonstrated that the geometryof the WCR and the streamlines of the flow within it lead to a spatiallydependent break frequency in the synchrotron emission. We therefore donot observe a single, sharp break in the synchrotron spectrum integratedover the WCR, but rather a steepening of the synchrotron spectrumtowards higher frequencies. We also observe that emission from thewind-collision region (WCR) may appear brightest near the shocks, sincethe impact of IC cooling on the non-thermal electron distribution isgreatest near the contact discontinuity (CD), and demonstrate that theimpact of IC cooling on the observed radio emission increasessignificantly with decreasing binary separation. We study how thesynchrotron emission changes in response to departures fromequipartition, and investigate how the thermal flux from the WCR varieswith binary separation. Since the emission from the WCR is opticallythin, we see a substantial fraction of this emission at certain viewingangles, and we show that the thermal emission from a CWB can mimic athermal plus non-thermal composite spectrum if the thermal emission fromthe WCR becomes comparable to that from the unshocked winds. Wedemonstrate that the observed synchrotron emission depends upon theviewing angle and the wind-momentum ratio, and find that the observedsynchrotron emission decreases as the viewing angle moves through theWCR from the WR shock to the O shock. We obtain a number of insightsrelevant to models of closer systems such as WR 140.Finally, we apply our new models to the very wide system WR147. The acceleration of non-thermal electrons appears to bevery efficient in our models of WR 147, and wesuggest that the shock structure may be modified by feedback from theaccelerated particles.
| Evidence for colliding winds in WR 25 from XMM-Newton observations of X-ray variability
The Wolf-Rayet star WR 25 in the Carina Nebula is a surprisingly brightX-ray source, and amongst the brightest WR stars in X-rays. It is asuspected binary star, though its binary nature has been a matter ofcontroversy. We report here observations of WR 25 from the XMM-Newtonarchive which showed an increase in X-ray luminosity of more than afactor of two. The X-ray absorption also increased. Such a large changein X-ray output is unknown in single massive stars, though large changesin X-ray brightness are seen in binaries. The most likely explanation ofthe observed X-ray variability is as a result of colliding-wind emissionin a moderately eccentric binary with a period of about 4 years. Thenext periastron passage is expected in 2007. The X-ray spectrum issimilar to that of the archetype colliding-wind binary WR 140.
| Accretion by the Secondary in η Carinae During the Spectroscopic Event. I. Flow Parameters
We examine the influence of the gravity of the companion (the secondary)to the massive primary star η Carinae on the winds blown by theprimary and the secondary. The two winds collide with each other afterpassing through two respective shock waves, and escape the system whilestrongly emitting in the X-ray band. While during most of the 5.5 yrorbital period the companion's gravity has a negligible effect on thewinds, we find that near periastron the companion's gravity maysignificantly influence the flow, and the companion might accrete fromthe primary's wind under certain circumstances. Near periastron passage,the collision region of the two winds may collapse onto the secondarystar, a process that could substantially reduce the X-ray luminosity. Wesuggest that such an accretion process produces the long, almost flat,X-ray minimum in η Car.
| Multiwavelength studies of WR 21a and its surroundings
We present results of high-resolution radio continuum observationstowards the binary star WR 21a (Wack 2134) obtained with the AustraliaTelescope Compact Array (ATCA) at 4.8 and 8.64 GHz. We detected thesystem at 4.8 GHz (6 cm) with a flux density of 0.25±0.06 mJy andset an upper limit of 0.3 mJy at 8.64 GHz (3 cm). The derived spectralindex of α < 0.3 (Sν ∝να) suggests the presence of non-thermal emission,probably originating in a colliding-wind region. A second, unrelatedradio source was detected ~10 arcsec north of WR 21a at (RA,Dec)J2000=(10h25m56.49s, -57°48arcmin34.4 arcsec), with flux densities of 0.36 and 0.55 mJy at 4.8 and8.64 GHz, respectively, resulting in α = 0.72. H i observations inthe area are dominated by absorption against the prominent H ii regionRCW 49. Analysis of a complete set of archived X-ray observations of WR21a confirms its strong variability but throws into doubt previoussuggestions by Reig (1999) of a period of years for the system. Finally,we comment on the association with the nearby EGRET source 3EGJ1027-5817.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| Bulk Velocities, Chemical Composition, and Ionization Structure of the X-Ray Shocks in WR 140 near Periastron as Revealed by the Chandra Gratings
The Wolf-Rayet WC7+O4-5 binary WR 140 went through the periastronpassage of its 8 yr eccentric binary orbit in early 2001 as the twostars made their closest approach. Both stars have powerful supersonicstellar winds that crash into each other between the stars to produceX-rays. Chandra grating observations were made when the X-rays were attheir peak, making WR 140 the brightest hot-star X-ray source in the skyand giving the opportunity to study the velocity profiles of lines, allof which were resolved and blueshifted before periastron. In the generalcontext of shock physics, the measurements constrain the flow of hot gasand where different ions were made. The brightness of lines relative tothe strong continuum in conjunction with plasma models gives interimabundance estimates for eight different elements in WC-type materialincluding an Ne/S ratio in good agreement with earlier long-wavelengthmeasurements. The lower velocity widths of cool ions imply a plasma thatwas not in equilibrium, probably due to the collisionless nature of theshock transitions and the slow character of both the postshock energyexchange between ions and electrons and subsequent ionization. Electronheat conduction into fast-moving preshock gas was absent, probablysuppressed by the magnetic field involved in WR 140's synchrotronemission. After periastron, the spectrum was weaker due mainly toabsorption by cool Wolf-Rayet star material.
| Chandra X-ray observations of the young stellar cluster NGC 6193 in the Ara OB1 association
A 90-ks Chandra High Energy Transmission Grating observation of theyoung stellar cluster NGC 6193 in the southern Ara OB1 associationdetected 43 X-ray sources in a 2 × 2 arcmin2 coreregion centred on the massive O stars HD 150135 (O6.5V) and HD 150136(O3 + O6V). The cluster is dominated by exceptionally bright X-rayemission from the two O stars, which are separated by only 10 arcsec.The X-ray luminosity of HD 150136 is logLX= 33.39 (ergs-1), making it one of the most luminous O-star X-ray sourcesknown. All of the fainter X-ray sources in the core region havenear-infrared (near-IR) counterparts, but existing JHK photometryprovides little evidence for near-IR excesses. These core sources havetypical mean photon energies ~ 2 keV and about one-third arevariable. It is likely that some are young low-mass stars in thecluster, but cluster membership remains to be determined. Gratingspectra show that the X-ray properties of HD 150135 and HD 150136 aresimilar, but not identical. Both have moderately broadened unshiftedemission lines and their emission is dominated by cool plasma at kT~ 0.3keV, pointing to a wind-shock origin. However, the emission of HD 150136is slightly hotter and four times more luminous than its optical twin HD150135. We discuss the possibility that a radiative colliding wind shockcontributes to the prodigious X-ray output of the short-period (2.66 d)spectroscopic binary HD 150136. A surprising result is that the X-rayemission of HD 150136 is slowly variable on a time-scale of <1 d. Theorigin of the variability is not yet known but the observed behavioursuggests that it is an occultation effect.
| The origin of massive O-type field stars: II. Field O stars as runaways
In two papers we try to confirm that all Galactic high-mass stars areformed in a cluster environment, by excluding that O-type stars found inthe Galactic field actually formed there. In de Wit et al. (2004) wepresented deep K-band imaging of 5 arcmin fields centred on 43 massiveO-type field stars that revealed that the large majority of theseobjects are single objects. In this contribution we explore thepossibility that the field O stars are dynamically ejected from youngclusters, by investigating their peculiar space velocity distribution,their distance from the Galactic plane, and their spatial vicinity toknown young stellar clusters. We (re-)identify 22 field O-type stars ascandidate runaway OB-stars. The statistics show that 4 ± 2% ofall O-type stars with V<8m can be considered as formedoutside a cluster environment. Most are spectroscopically singleobjects, some are visual binaries. The derived percentage for O-typestars that form isolated in the field based on our statistical analysesis in agreement with what is expected from calculations adopting auniversal cluster richness distribution with power index of β= 1.7,assuming that the cluster richness distribution is continuous down tothe smallest clusters containing one single star.
| Spectroscopic study of the long-period dust-producing WC7pd+O9 binary HD192641
We present the results of an optical spectroscopic study of the massiveWolf-Rayet (WR) binary HD192641 = WR137. These 1986-2000 data cover thedust-formation maximum in 1997. Combining all available measurements ofradial velocities, we derive, for the first time, a spectroscopic orbitwith period 4766 +/- 66 d (13.05 +/- 0.18 yr). The resulting masses,adopting i= 67 °, are MO= 20 +/- 2Msolar forthe O component and MWR= 4.4 +/- 1.5Msolar for theWR component. These appear, respectively, approximately normal and onthe low side for the given spectral types. Analysis of the intensemultisite spectroscopic monitoring in 1999 shows that theCIIIλ5696 and CIVλλ5802/12 lines have the highestintrinsic variability levels. The periodogram analysis yields asmall-amplitude modulation in the absorption troughs of theCIVλλ5802/12 and HeIλ5876 lines with a period of0.83 d, which could be related either to pulsations or large-scalerotating structures as seen in the WN4 star EZ Canis Majoris (WR6).Wavelet analysis of the strong emission lines of CIIIλ5696 andCIVλλ5802/12 enabled us to isolate and follow for severalhours small structures (emission subpeaks) associated with densityenhancements within the wind of the Wolf-Rayet star. Cross-correlatingthe variability patterns seen in different lines, we find a weak butsignificant correlation between the variability in emission lines withdifferent ionization potentials, i.e. in lines formed at differentdistances from the WR stellar core. Adopting a β wind-velocity law,from the motion of individual subpeaks we find β~ 5, which issignificantly larger than the canonical value β~= 1 found in O starwinds.
| Non-thermal radio emission from O-type stars. I. HD168112
We present a radio lightcurve of the O5.5 III(f^+) star HD 168112, basedon archive data from the Very Large Array (VLA) and the AustraliaTelescope Compact Array (ATCA). The fluxes show considerable variabilityand a negative spectral index, thereby confirming that HD 168112 is anon-thermal radio emitter. The non-thermal radio emission is believed tobe due to synchrotron radiation from relativistic electrons that havebeen Fermi accelerated in shocks. For HD 168112, it is not known whetherthese shocks are due to a wind-wind collision in a binary system or tothe intrinsic instability of the stellar wind driving mechanism.Assuming HD 168112 to be a single star, our synchrotron model shows thatthe velocity jump of the shocks should be very high, or there should bea very large number of shocks in the wind. Neither of these iscompatible with time-dependent hydrodynamical calculations of O starwinds. If, on the other hand, we assume that HD 168112 is a binary, thehigh velocity jump is easily explained by ascribing it to the wind-windcollision. By further assuming the star to be an eccentric binary, wecan explain the observed radio variability by the colliding-wind regionmoving in and out of the region where free-free absorption is important.The radio data presented here show that the binary has a period ofbetween one and two years. By combining the radio data with X-ray data,we find that the most likely period is ~1.4 yr.
| High-Resolution Radio Observations of the Colliding-Wind Binary WR 140
Milliarcsecond resolution Very Long Baseline Array (VLBA) observationsof the archetype W-R+O star colliding-wind binary (CWB) system WR 140are presented for 23 epochs between orbital phases 0.74 and 0.97. At 8.4GHz, the emission in the wind-collision region (WCR) is clearly resolvedas a bow-shaped arc that rotates as the orbit progresses. We interpretthis rotation as due to the O star moving from southeast toapproximately east of the W-R star, which leads to solutions for theorbital inclination of 122deg+/-5deg, longitude ofthe ascending node of 353deg+/-3deg, and an orbitsemimajor axis of 9.0+/-0.5 mas. The distance to WR 140 is determined tobe 1.85+/-0.16 kpc, which requires the O star to be a supergiant. Theinclination implies that the mass of the W-R and O star is 20+/-4 and54+/-10 Msolar, respectively. We determine a wind momentumratio of 0.22, with an expected half-opening angle for the WCR of63°, consistent with 65deg+/-10deg derivedfrom the VLBA observations. Total flux measurements from Very LargeArray (VLA) observations show that the radio emission from WR 140 isvery closely the same from one orbit to the next, pointing stronglytoward emission, absorption, and cooling mechanism(s) that arecontrolled largely by the orbital motion. The synchrotron spectra evolvedramatically through the orbital phases observed, exhibiting bothoptically thin and optically thick emission. We discuss a number ofabsorption and cooling mechanisms that may determine the evolution ofthe synchrotron spectrum with orbital phase.
| Wind-wind collision in the η Carinae binary system: a shell-like event near periastron
The exact nature of η Carinae is still an open issue. Strictperiodicity in the light curves at several wavelengths seem to point toa binary system, but the observed radial velocities, measured from spacewith high spatial resolution, are in conflict with the ground-basedobservations used to calculate the binary orbit. Also, the observed 2-10keV X-ray flux is much larger that what is expected from a single star,and favours the wind-wind collision hypothesis, characteristic ofhigh-mass binary systems. However, to explain the duration of the dip inthe light curve by wind collisions, it is necessary to postulate a verylarge increase in the η Carinae mass loss rate. Finally, the opticaland ultraviolet light curves are better explained by periodicshell-ejection events. In this paper we conciliate the two hypotheses.We still assume a binary system to explain the strong X-ray emission,but we also take into account that, near periastron and because of thehighly eccentric orbit, the wind emerging from η Carinae accumulatesbehind the shock and can mimic a shell-like ejection event. For thisprocess to be effective, at periastron the secondary star should belocated between η Carinae and the observer, solving also thediscrepancy between the orbital parameters derived from ground- andspace-based observations. We show that, as the secondary moves in itsorbit, the shell cools down and the number of available stellar ionizingphotons is not enough to maintain the shell temperature at itsequilibrium value of about 7500 K. The central part of the shell remainscold and under these conditions grain formation and growth can takeplace in time-scales of hours. We also calculated the neutral gas columndensity intercepting the line of sight at each point of the orbit nearperiastron, and were able to reproduce the form and duration of theX-ray light curve without any change in the η Carinae mass lossrate. This same column density can explain the observed Hα lightcurve observed during the 2003 event.
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Observation and Astrometry data
| Constellation: | Cygnus |
| Right ascension: | 20h20m27.98s |
| Declination: | +43°51'16.3" |
| Apparent magnitude: | 6.885 |
| Distance: | 1612.903 parsecs |
| Proper motion RA: | -5.3 |
| Proper motion Dec: | -2.2 |
| B-T magnitude: | 7.379 |
| V-T magnitude: | 6.926 |
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