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X-rays from colliding stellar winds: the case of close Wolf-Rayet+O binary systems
We have analysed the X-ray emission from a sample of close Wolf-Rayet+O(WR+O) binaries using data from the public Chandra and XMM-Newtonarchives. Global spectral fits show that two-temperature plasma isneeded to match the X-ray emission from these objects, because the hotcomponent (kT > 2 keV) is an important ingredient of the spectralmodels. In close WR+O binaries, X-rays likely originate in collidingstellar wind (CSW) shocks, driven by the massive winds of the binarycomponents. The CSW shocks in these objects are expected to be radiativebecause of the high density of the plasma in the interaction region. Incontrast, our analysis shows that, in our sample of close WR+O binaries,the CSW shocks are adiabatic. This is possible only if the mass-lossrates of the stellar components in the binary are at least one order ofmagnitude smaller than the values currently accepted. The most likelyexplanation for the X-ray properties of close WR+O binaries is thattheir winds are two-component flows. The more massive component (denseclumps) has a role in the optical/ultraviolet emission from theseobjects, while the smooth rarefied component is a key factor in theX-ray emission from these objects.
| A search for naphthalene in diffuse interstellar clouds
We have obtained high-resolution optical spectroscopy of 10 reddenedO-type stars with Ultraviolet and Visual Echelle Spectrograph at VeryLarge Telescope to search for interstellar bands of the naphthalenecation (C10H8+) in the interveningclouds. No absorption features were detected near the laboratorystrongest band of this cation at 6707 Å except for star HD 125241(O9 I). Additional bands in the optical spectrum of this star appear tobe consistent with other transitions of this cation. Under theassumption that the bands are caused by naphthalene cations, we derive acolumn density ?= (1.2 ± 0.3) × 10 13cm-2 similar to the column density claimed in the Perseuscomplex star Cernis 52. The strength ratio of the two prominent diffuseinterstellar bands at 5780 and 5797 Å suggests the presence of a?-type cloud in the line of sight of HD 125241.
| A Compilation of Interstellar Column Densities
We have collated absorption line data toward 3008 stars in order tocreate a unified database of interstellar column densities. These datahave been taken from a number of different published sources and includemany different species and ionizations. The preliminary results from ouranalysis show a tight relation [N(H)/E(B - V) = 6.12 ×1021] between N(H) and E(B - V). Similar plots havebeen obtained with many different species, and their correlations alongwith the correlation coefficients are presented.
| Spectral modelling of circular massive binary systems. Towards an understanding of the Struve-Sahade effect?
Context. Some secondary effects are known to introduce variations inspectra of massive binaries. These phenomena, such as the Struve-Sahadeeffect and the difficulties in determining properly the spectral typehave been reported and documented in the literature. Aims: Wesimulate the spectra of circular massive binaries at different phases ofthe orbital cycle and account for the gravitational influence of thecompanion star on the shape and physical properties of the stellarsurface. Methods: We use the Roche potential to compute thestellar surface, von Zeipel theorem, and reflection effects to computethe surface temperature. We then interpolate in a grid of non-localthermodynamic equilibrium (NLTE) plan-parallel atmosphere model spectrato obtain the local spectrum at each surface point. We finally sum allthe contributions, accounting for the reflection, Doppler shift, andlimb-darkening to obtain the total spectrum. The computation is done fordifferent orbital phases and for different sets of physical and orbitalparameters. Results: Our models reproduce the Struve-Sahadeeffect for several lines. Another effect on the surface temperaturedistribution is visible but the distribution predicted by our currentmodel is not yet consistent with observations. Conclusions: Insome cases, the Struve-Sahade effect as well as more complex lineintensity variations could be linked to blends of intrinsicallyasymmetric line profiles that are not appropriately treated by thedeblending routine. Systematic variations in the lines of (nearly)contact systems are also predicted by the model.
| Random forest automated supervised classification of Hipparcos periodic variable stars
We present an evaluation of the performance of an automatedclassification of the Hipparcos periodic variable stars into 26 types.The sub-sample with the most reliable variability types available in theliterature is used to train supervised algorithms to characterize thetype dependencies on a number of attributes. The most useful attributesevaluated with the random forest methodology include, in decreasingorder of importance, the period, the amplitude, the V-I colour index,the absolute magnitude, the residual around the folded light-curvemodel, the magnitude distribution skewness and the amplitude of thesecond harmonic of the Fourier series model relative to that of thefundamental frequency. Random forests and a multi-stage scheme involvingBayesian network and Gaussian mixture methods lead to statisticallyequivalent results. In standard 10-fold cross-validation (CV)experiments, the rate of correct classification is between 90 and 100per cent, depending on the variability type. The main mis-classificationcases, up to a rate of about 10 per cent, arise due to confusion betweenSPB and ACV blue variables and between eclipsing binaries, ellipsoidalvariables and other variability types. Our training set and thepredicted types for the other Hipparcos periodic stars are availableonline.
| The masses, and the mass discrepancy of O-type stars
Context. The “mass discrepancy” in massive O starsrepresents a long-standing problem in stellar astrophysics withfar-reaching implications for the chemical and dynamical feedback ingalaxies. Aims: Our goal is to investigate this mass discrepancyby comparing state-of-the-art model masses with model-independent massesdetermined from eclipsing binaries. Methods: Using stellarevolution models and a recent calibration of stellar parameters forO-star spectral sub-classes, we present a convenient way to convertobserved solar metallicity O star spectral types into model masses,which we subsequently compare to our dynamical mass compilation. We alsoderive similar conversions for Large and Small Magellanic Cloudmetallicities. Results: We obtain a good agreement between modeland dynamical masses, suggesting the long-standing problem of asystematic mass discrepancy problem may have been solved. We alsoprovide error ranges for the model masses, as well as minimal andmaximal age estimates for when the model stars are in a given spectraltype box.Appendices and Table 9 are only available in electronic form at http://www.aanda.org
| 3D models of radiatively driven colliding winds in massive O + O star binaries - III. Thermal X-ray emission
The X-ray emission from the wind-wind collision in short-period massiveO + O star binaries is investigated. The emission is calculated from 3Dhydrodynamical models which incorporate gravity, the driving of thewinds, orbital motion of the stars and radiative cooling of the shockedplasma. Changes in the amount of stellar occultation and circumstellarattenuation introduce phase-dependent X-ray variability in systems withcircular orbits, while strong variations in the intrinsic emission alsooccur in systems with eccentric orbits. The X-ray emission in eccentricsystems can display strong hysteresis, with the emission softer afterperiastron than at corresponding orbital phases prior to periastron,reflecting the physical state of the shocked plasma at these times.Our simulated X-ray light curves bear many similarities to observedlight curves. In systems with circular orbits the light curves show twominima per orbit, which are identical (although not symmetric) if thewinds are identical. The maxima in the light curves are produced nearquadrature, with a phase delay introduced due to the aberration andcurvature of the wind collision region. Circular systems with unequalwinds produce minima of different depths and duration. In systems witheccentric orbits the maxima in the light curves may show a very sharppeak (depending on the orientation of the observer), followed by aprecipitous drop due to absorption and/or cooling. We show that the riseto maximum does not necessarily follow a 1/dsep law. Ourmodels further demonstrate that the effective circumstellar column canbe highly energy dependent. Therefore, spectral fits which assumeenergy-independent column(s) are overly simplified and may compromisethe interpretation of observed data.To better understand observational analyses of such systems we applyChandra and Suzaku response files, plus Poisson noise, to the spectracalculated from our simulations and fit these using standard XSPECmodels. We find that the recovered temperatures from two- orthree-temperature mekal fits are comparable to those from fits to theemission from real systems with similar stellar and orbitalparameters/nature. We also find that when the global abundance is thawedin the spectral fits, subsolar values are exclusively returned, despitethe calculations using solar values as input. This highlights theproblem of fitting oversimplified models to data, and of course is ofwider significance than just the work presented here.Further insight into the nature of the stellar winds and the wind-windcollision region in particular systems will require dedicatedhydrodynamical modelling, the results of which will follow in duecourse.
| 3D models of radiatively driven colliding winds in massive O + O star binaries - II. Thermal radio to submillimetre emission
In this paper, the thermal emission over cm to submillimetre (submm)wavelengths from the winds in short-period O + O star binaries isinvestigated (potential non-thermal emission is presently ignored). Thecalculations are based on 3D hydrodynamical models which incorporategravity, the driving of the winds, orbital motion of the stars andradiative cooling of the shocked plasma. The thermal emission arisesfrom the stellar winds and the region where they collide. We investigatethe flux and spectrum from a variety of models as a function of orbitalphase and orientation of the observer, and compare to the single starcase. The emission from the wind-wind collision region (WCR) is stronglydependent on its density and temperature, being optically thick inradiative systems and optically thin in adiabatic systems. The flux fromsystems where the WCR is highly radiative, as investigated for the firsttime in this paper, can be over an order of magnitude greater than thecombined flux from identically typed single stars. This excess occursover a broad range of wavelengths from cm to submm. In contrast, whenthe WCR is largely adiabatic, a significant excess in the thermal fluxoccurs only below 100GHz.In circular systems with (near) identical stars, the observedvariability in synthetic light curves is typically less than a factor of2. Eccentric systems may show order of magnitude or greater fluxvariability, especially if the plasma in the WCR transitions from anadiabatic to a radiative state and vice-versa - in such cases, the fluxcan display significant hysteresis with stellar separation. We furtherdemonstrate that clumping can affect the variability of radio lightcurves.We investigate the spectral index of the emission, and often findindices steeper than +0.6. Synthetic images display a variety ofmorphologies, with the emission sometimes resembling an intertwined`double-helix'. We conclude by comparing our results to observations.The predictions made in this paper are of interest to futureobservations with the next generation of radio and submm telescopes,including the EVLA, e-MERLIN, ALMA and the Square Kilometre Array, andfuture upgrades to the VLBA.
| Masses and luminosities of O- and B-type stars and red supergiants
Massive stars are of interest as progenitors of supernovae, i.e.neutron stars and black holes, which can be sources of gravitationalwaves. Recent population synthesis models can predict neutron star andgravitational wave observations but deal with a fixed supernova rate oran assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernovaprogenitors, i.e. with O- and early B-type stars, and also allsupergiants within 3 kpc. We restrict our sample to those massive starsdetected both in 2MASS and observed by Hipparcos, i.e. only those starswith parallax and precise photometry. To determine the luminositieswe calculated the extinctions from published multi-colour photometry,spectral types, luminosity class, all corrected for multiplicity andrecently revised Hipparcos distances. We use luminosities andtemperatures to estimate the masses and ages of these stars usingdifferent models from different authors. Having estimated theluminosities of all our stars within 3 kpc, in particular for all O- andearly B-type stars, we have determined the median and mean luminositiesfor all spectral types for luminosity classes I, III, and V. Ourluminosity values for supergiants deviate from earlier results: Previouswork generally overestimates distances and luminosities compared to ourdata, this is likely due to Hipparcos parallaxes (generally moreaccurate and larger than previous ground-based data) and the fact thatmany massive stars have recently been resolved into multiples of lowermasses and luminosities. From luminosities and effective temperatureswe derived masses and ages using mass tracks and isochrones fromdifferent authors. From masses and ages we estimated lifetimes andderived a lower limit for the supernova rate of ?20 events/Myraveraged over the next 10 Myr within 600 pc from the sun. These data arethen used to search for areas in the sky with higher likelihood for asupernova or gravitational wave event (like OB associations).
| X-ray emission characteristics of two Wolf-Rayet binaries: V444 Cyg and CD Cru
Using data from observations made with XMM-Newton, we present an X-rayanalysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru. The X-raylight curves show the phase-locked variability in both binaries, wherethe flux increased by a factor of ~2 for V444 Cyg and ~1.5 for CD Cru,from minimum to maximum. The maximum luminosities in the 0.3-7.5 keVenergy band were found to be 5.8 × 1032 and 2.8 ×1032 erg s-1 for V444 Cyg and CD Cru,respectively. The X-ray spectra of these stars confirmed largeextinction and revealed hot plasma with prominent emission-line featuresof highly ionized Ne, Mg, Si, S, Ar, Ca and Fe; these are found to beconsistent with a two-temperature plasma model. At a temperature of ~0.6keV, the cooler plasma was found to be constant at all phases for bothbinaries, which could be the result of a distribution of small-scaleshocks in radiation-driven outflows. The hot components in thesebinaries were found to be phase-dependent. They varied from 1.85 to 9.61keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption ofthe hard component varied with the orbital phase and was found to bemaximum during the primary eclipse of V444 Cyg. The high plasmatemperature and variability with orbital phase suggest that thehard-component emission is caused by a colliding wind shock between thebinary components.
| A photometric and spectroscopic investigation of star formation in the very young open cluster NGC 6383
Context. The very young open cluster NGC 6383 centered on the O-starbinary HD 159176 is an interesting place for studying the impact ofearly-type stars with strong radiation fields and powerful winds on theformation processes of low-mass stars. Aims: To investigate thisprocess, it is necessary to determine the characteristics (age,presence, or absence of circumstellar material) of the population oflow-mass pre-main-sequence (PMS) stars in the cluster. Methods:We obtained deep UBV(RI)c H? photometric data of theentire cluster as well as medium-resolution optical spectroscopy of asubsample of X-ray selected objects. Results: Our spectroscopicdata reveal only very weak H? emission lines in a few X-rayselected PMS candidates. We photometrically identify a number ofH? emission candidates but their cluster membership is uncertain.We find that the fainter objects in the field of view have a wide rangeof extinction (up to AV = 20), one X-ray selected OB starhaving AV ? 8. Conclusions: Our investigationuncovers a population of PMS stars in NGC 6383 that are probably coevalwith HD 159176. In addition, we detect a population of reddened objectsthat are probably located at different depths within the natal molecularcloud of the cluster. Finally, we identify a rather complex spatialdistribution of H? emitters, which is probably indicative of asevere contamination by foreground and background stars.Based on observations collected at the European Southern Observatory(ESO, La Silla, Chile).Table 2 is only available in electronic form atthe CDS via anonymous ftp cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/A25
| The relation between the most-massive star and its parental star cluster mass
We present a thorough literature study of the most-massive star,mmax, in several young star clusters in order to assesswhether or not star clusters are populated from the stellar initial massfunction (IMF) by random sampling over the mass range 0.01 <= m <=150Msolar without being constrained by the cluster mass,Mecl. The data reveal a partition of the sample into lowestmass objects (Mecl <= 102Msolar),moderate mass clusters (102Msolar
| Phase-resolved XMM-Newton observations of the massive WR+O binary WR 22
Aims. To better understand the phenomenon of colliding winds in massivebinary stars, we study the X-ray lightcurve of a WR+O system of theCarina region, a system well known for the high mass of its primary. Methods: Phase-resolved X-ray observations of the massive WR+O binarysystem WR 22 were performed with the XMM-Newton facility. We observedthe object at seven different phases from near apastron to nearperiastron. Results: The X-ray spectrum can be represented by atwo-component, optically thin, thermal plasma model with a first one ata typical temperature of 0.6 keV and a second hotter one in the range2.0-4.5 keV. The hot component is indicative of a colliding windphenomenon, but its flux is remarkably constant with time despite thehigh eccentricity of the orbit. Although surprising at first, thisactually does not contradict the results of the hydrodynamicalsimulations of the wind collision that we performed. When the systemgoes from apastron to periastron, the soft part of the X-ray flux isprogressively lowered by an increasing intervening absorbing column.This behaviour can be interpreted in terms of an X-ray emitting plasmalocated near the O star, but not fully intrinsic to it, and accompanyingthe star when it dives into the wind of the WR component. A model ispresented that interprets most of the observational constraints. Thismodel suggests that the mass-loss rate of dot{M}WR 1.6× 10-5 {M}? yr-1 assumed forthe WR could still be slightly too high, whereas it is already lowerthan other published values. From the comparison of the observed and theexpected absorptions at phases near periastron, we deduce that the hardX-ray emitting collision zone should at least have a typical size of50-60 R?, but that the size for the soft X-ray emittingregion could reach 244 R? if the assumed mass-loss rateis correct. We also present an upper limit to the X-ray luminosity ofthe WR component that further questions the existence of intrinsic X-rayemission from single WN stars.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA). Research Associate FNRS (Belgium). PostdoctoralResearcher FNRS (Belgium).
| A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| The interstellar Ca II distance scale
Aims: We attempt to extend the relation between the strengths of theinterstellar Ca II lines and the distances to early-type stars toobjects beyond 1 kiloparsec, with the line saturation taken intoaccount. Methods: We measure the Ca II K and Ca II H equivalentwidths, and compute Ca II column densities for 262 lines of sighttowards early-type stars with available Hipparcos parallaxes (?). Thetargets are located within a few hundred parsecs of the Galactic plane,and span all the range of Galactic longitudes. We fit the NCaII - parallax relation with a function of the form ? = 1 / (a\cdot NCa II + b), using a maximum-likelihood approach totake account of errors in both variables. We use the resultant formulato estimate distances to stars in OB associations and clusters, andcompare them to those found in the literature, usually estimated byspectrophotometric methods. Results: For lines of sight withEW(K)/EW(H) > 1.3, we obtain the following approximate formula forthe distance: DCa II = 77 + (2.78 + {2.60}/{{EW(K)}/{EW(H)} -0.932}) EW(H), where the equivalent widths EW(K) and EW(H) are inmÅ, and the distance DCa II in parsecs. The errors inDCa II, resulting from the uncertainty in the fit parametersand errors in the equivalent widths, are typically about 15% of thedistance. We can also expect the equation not to hold for objectssituated farther than a few hundred parsecs from the Galactic plane. Wefind several cases of significant column density differences betweenassociation or cluster members, especially notable in theTrumpler 16 cluster, indicating either a localcontribution to the Ca II column density, or background/foreground starsbeing confused with members. The ratio DCa II/D_assoc appearsto depend on the Galactic longitude, being highest in the range 70°< l < 120° and lowest for 200° < l < 300°. Thiseffect may be due to large-scale structure being present in the Ca IIlayer, or to the nonmember confusion being enhanced in these directions.Table 1 is only available in electronic form at http://www.aanda.org
| Hot stars observed by XMM-Newton. I. The catalog and the properties of OB stars
Aims: Following the advent of increasingly sensitive X-rayobservatories, deep observations of early-type stars became possible.However, the results for only a few objects or clusters have until nowbeen reported and there has been no large survey comparable to thatbased upon the ROSAT All-Sky Survey (RASS). Methods: A limitedsurvey of X-ray sources, consisting of all public XMM-Newtonobservations (2XMMi) and slew survey data (XMMSL1), is now available.The X-ray counterparts to hot, massive stars have been searched for inthese catalogs. Results: About 300 OB stars were detected withXMM-Newton. Half of them were bright enough for a spectral analysis tobe possible, and we make available the detailed spectral properties thatwere derived. The X-ray spectra of O stars are represented well by low(<1 keV) temperature components and seem to indicate that anabsorption column is present in addition to the interstellarcontribution. The X-ray fluxes are well correlated with the bolometricfluxes, with a scatter comparable to that of the RASS studies and thuslarger than found previously with XMM-Newton for some individualclusters. These results contrast with those of B stars that exhibit alarge scatter in the LX - LBOL relation, noadditional absorption being found, and the fits indicate a plasma athigher temperatures. Variability (either within one exposure or betweenmultiple exposures) was also investigated whenever possible: short-termvariations are far more rare than long-term ones (the former affects afew percent of the sample, while the latter concerns between one thirdand two thirds of the sources). Conclusions: This paper presentsthe results of the first high-sensitivity investigation of the overallhigh-energy properties of a sizable sample of hot stars.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA), and accessed via the 2XMMi and XMM slew surveycatalogs. Tables 1 and 5 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/506/1055 PostdoctoralResearcher FNRS. Visiting astronomer, UNAM-Morelos (Mexico).
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| X-ray spectroscopy of stars
Non-degenerate stars of essentially all spectral classes are soft X-raysources. Their X-ray spectra have been important in constrainingphysical processes that heat plasma in stellar environments totemperatures exceeding one million degrees. Low-mass stars on the coolerpart of the main sequence and their pre-main sequence predecessorsdefine the dominant stellar population in the galaxy by number. TheirX-ray spectra are reminiscent, in the broadest sense, of X-ray spectrafrom the solar corona. The Sun itself as a typical example of amain-sequence cool star has been a pivotal testbed for physical modelsto be applied to cool stars. X-ray emission from cool stars is indeedascribed to magnetically trapped hot gas analogous to the solar coronalplasma, although plasma parameters such as temperature, density, andelement abundances vary widely. Coronal structure, its thermalstratification and geometric extent can also be interpreted based onvarious spectral diagnostics. New features have been identified inpre-main sequence stars; some of these may be related to accretionshocks on the stellar surface, fluorescence on circumstellar disks dueto X-ray irradiation, or shock heating in stellar outflows. Massive, hotstars clearly dominate the interaction with the galactic interstellarmedium: they are the main sources of ionizing radiation, mechanicalenergy and chemical enrichment in galaxies. High-energy emission permitsto probe some of the most important processes at work in these stars,and put constraints on their most peculiar feature: the stellar wind.Medium and high- resolution spectroscopy have shed new light on theseobjects as well. Here, we review recent advances in our understanding ofcool and hot stars through the study of X-ray spectra, in particularhigh-resolution spectra now available from XMM -Newton and Chandra. Weaddress issues related to coronal structure, flares, the composition ofcoronal plasma, X-ray production in accretion streams and outflows,X-rays from single OB-type stars, massive binaries, magnetic hot objectsand evolved WR stars.
| 3D models of radiatively driven colliding winds in massive O+O star binaries - I. Hydrodynamics
The dynamics of the wind-wind collision in massive stellar binaries areinvestigated using 3D hydrodynamical models which incorporate gravity,the driving of the winds, the orbital motion of the stars and radiativecooling of the shocked plasma. In this first paper, we restrict ourstudy to main-sequence O+O binaries. The nature of the wind-windcollision region is highly dependent on the degree of cooling of theshocked plasma, and the ratio of the flow time-scale of the shockedplasma to the orbital time-scale. The pre-shock wind speeds are lower inclose systems as the winds collide prior to their acceleration toterminal speeds. Radiative inhibition may also reduce the pre-shock windspeeds. Together, these effects can lead to rapid cooling of thepost-shock gas. Radiative inhibition is less important in wider systems,where the winds are accelerated to higher speeds before they collide,and the resulting collision region can be largely adiabatic. In systemswith eccentric orbits, cold gas formed during periastron passage canpersist even at apastron, before being ablated and mixed into itssurroundings and/or accelerated out of the system.
| The High Angular Resolution Multiplicity of Massive Stars
We present the results of a speckle interferometric survey of Galacticmassive stars that complements and expands upon a similar survey madeover a decade ago. The speckle observations were made with the Kitt PeakNational Observatory and Cerro Tololo Inter-American Observatory 4 mtelescopes and USNO speckle camera, and they are sensitive to thedetection of binaries in the angular separation regime between 0farcs03and 5'' with relatively bright companions (ΔV < 3). We reporton the discovery of companions to 14 OB stars. In total we resolvedcompanions of 41 of 385 O-stars (11%), 4 of 37 Wolf-Rayet stars (11%),and 89 of 139 B-stars (64%; an enriched visual binary sample that weselected for future orbital determinations). We made a statisticalanalysis of the binary frequency among the subsample that are listed inthe Galactic O Star Catalog by compiling published data on other visualcompanions detected through adaptive optics studies and/or noted in theWashington Double Star Catalog and by collecting published informationon radial velocities and spectroscopic binaries. We find that the binaryfrequency is much higher among O-stars in clusters and associationscompared to the numbers for field and runaway O-stars, consistent withpredictions for the ejection processes for runaway stars. We present afirst orbit for the O-star δ Orionis; a linear solution of theclose, apparently optical, companion of the O-star ι Orionis; andan improved orbit of the Be star δ Scorpii. Finally, we listastrometric data for another 249 resolved and 221 unresolved targetsthat are lower mass stars that we observed for various other scienceprograms.
| High-resolution optical spectroscopy of Plaskett's star
Context: Plaskett's star (HD 47 129) is a verymassive O + O binary that belongs to the Mon OB2 association. Previouswork suggests that this system displays the Struve-Sahade effectalthough the measurements of the secondary radial velocities are verydifficult and give controversial results. Both components have powerfulstellar winds that collide and produce a strong X-ray emission. Aims: Our aim is to study the physical parameters of this system indetail and to investigate the relation between its spectral propertiesand its evolutionary status. Methods: We present here analysis ofan extensive set of high-resolution optical spectra of HD 47129. We used a disentangling method to separate the individualspectra of each star. We derived a new orbital solution and discuss thespectral classification of both components. A Doppler tomographytechnique applied to the emission lines H? and He II ? 4686yields a Doppler map that illustrates the wind interactions in thesystem. Finally, an atmosphere code is used to determine the differentchemical 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 matterhas been transferred from the primary to the secondary star. The Heoverabundance of the secondary supports this scenario. In addition, theN overabundance and C underabundance of the primary component confirmprevious results based on X-ray spectroscopy and indicate that theprimary is an evolved massive star. We also determined a new orbitalsolution, with MP sin^3i = 45.4 ± 2.4M? and MS sin^3i = 47.3 ± 0.3M?. Furthermore, the secondary star has a highrotational velocity (v sin i ˜ 300 km s-1) that deformsits surface, leading to a non-uniform distribution in effectivetemperature. This could explain the variations in the equivalent widthsof the secondary lines with phase. We suggest that the wind of thesecondary star is confined near the equatorial plane because of its highrotational velocity, affecting the ram pressure equilibrium in the windinteraction zone.Based on observations made at the European Southern Observatory (LaSilla, Chile) and at the Observatoire de Haute Provence (France).
| Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries(hierarchy 2 + 2), is studied in comparison with triple stars, withrespective sample sizes of 81 and 724. Seven representative quadruplesystems are discussed in greater detail. The main conclusions are asfollows. (i) Quadruple systems of ? Lyr type with similar massesand inner periods are common, in 42 per cent of the sample the outermass ratio is above 0.5 and the inner periods differ by less than 10times. (ii) The distributions of the inner periods in triple andquadruple stars are similar and bimodal. The inner mass ratios do notcorrelate with the inner periods. (iii) The statistics of outer periodsand mass ratios in triples and quadruples are different. The medianouter mass ratio in triples is 0.39 independently of the outer period,which has a smooth distribution. In contrast, the outer periods of 25per cent quadruples concentrate in the narrow range from 10 to 100yr,the outer mass ratios of these tight quadruples are above 0.6 and theirtwo inner periods are similar to each other. (iv) The outer and innermass ratios in triple and quadruple stars are not mutually correlated.In 13 per cent of quadruples both inner mass ratios are above 0.85(double twins). (v) The inner and outer orbital angular momenta andperiods in triple and quadruple systems with inner periods above 30dshow some correlation, the ratio of outer-to-inner periods is mostlycomprised between 5 and 104. In the systems with small periodratios the directions of the orbital spins are correlated, while in thesystems with large ratios they are not. The properties of multiple starsdo not correspond to the products of dynamical decay of small clusters,hence the N-body dynamics is not the dominant process of theirformation. On the other hand, rotationally driven (cascade)fragmentation possibly followed by migration of inner and/or outerorbits to shorter periods is a promising scenario to explain the originof triple and quadruple stars.
| A catalogue of multiplicity among bright stellar systems
We consider the multiplicity of stellar systems with (combined)magnitude brighter than 6.00 in Hipparcos magnitudes. We identify 4559such bright systems (including the Sun), and the frequencies ofmultiplicities 1, 2,..., 7 are found to be 2718, 1437, 285, 86, 20, 11and 2. We discuss the uncertainties, which are substantial. We alsoconsider the distributions of periods of orbits and suborbits. We notethat for even more restricted set of 478 systems with VH<= 4.00, the proportions of higher multiples up to sextuple areprogressively larger (213, 179, 54, 19, 8, 5), suggesting substantialincompleteness in even the reasonably well studied larger sample.This sample can be seen as relatively thoroughly studied formultiplicity, and reasonably representative of stars more massive thanthe Sun. But the restriction to VH <= 6 means that oursample contains hardly any systems where all components are low-massmain-sequence stars (K or M).Data on multiplicity are important as a constraint on (i) the starformation problem, (ii) the problem of the evolution of the Galacticstellar population and (iii) the interaction of dynamics and evolutionthrough the effect of Kozai cycles. We discuss these topics briefly.
| Adaptive Optics Photometry and Astrometry of Binary Stars. III. a Faint Companion Search of O-Star Systems
We present the results of an adaptive optics survey for faint companionsamong Galactic O-type star systems (with V lsim 8) using the AdvancedElectro-Optical System (AEOS) 3.6 m telescope on Haleakala. We surveyedthese O-star systems in the I-band, typically being able to detect acompanion with a magnitude difference of utrimI lsim 6 in theprojected separation range 0farcs5 < ρ < 1farcs0, andutrimI lsim 9.5 in the range 1farcs0 < ρ < 5farcs0.In the course of the survey, we discovered 40 new companions among 31 ofthe 116 objects examined and made astrometric and differential magnitudemeasurements of 24 additional known pairs, several of them beingconfirmation detections. We present new astrometric orbits for twobinaries, BU 1032AB (WDS 05387–0236 σ Ori AB) and SEE 322(WDS 17158–3344 HD 155889AB). We lack magnitude differences forother filter bands, so it is difficult to determine physical fromline-of-sight companions, but we present empirical arguments for thelimiting magnitude difference where field contamination is significant.Based on observations made at the Maui Space Surveillance Systemoperated by Detachment 15 of the US Air Force Research Laboratory'sDirected Energy Directorate.
| Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysicalprocesses likely to play a role in the production of non-thermalemission in massive stars, with emphasis on massive binaries. Eventhough the discussion will start in the radio domain where thenon-thermal emission was first detected, the census of physicalprocesses involved in the non-thermal emission from massive stars showsthat many spectral domains are concerned, from the radio to the veryhigh energies. First, the theoretical aspects of the non-thermalemission from early-type stars will be addressed. The main topics thatwill be discussed are respectively the physics of individual stellarwinds and their interaction in binary systems, the acceleration ofrelativistic electrons, the magnetic field of massive stars, and finallythe non-thermal emission processes relevant to the case of massivestars. Second, this general qualitative discussion will be followed by amore quantitative one, devoted to the most probable scenario wherenon-thermal radio emitters are massive binaries. I will show how severalstellar, wind and orbital parameters can be combined in order to makesome semi-quantitative predictions on the high-energy counterpart to thenon-thermal emission detected in the radio domain. These theoreticalconsiderations will be followed by a census of results obtained so far,and related to this topic. These results concern the radio, the visible,the X-ray and the γ-ray domains. Prospects for the very highenergy γ-ray emission from massive stars will also be addressed.Two particularly interesting examples—one O-type and oneWolf-Rayet binary—will be considered in details. Finally,strategies for future developments in this field will be discussed.
| The Struve-Sahade effect in the optical spectra of O-type binaries. I. Main-sequence systems
Aims:We present a spectroscopic analysis of four massive binary systemsthat are known or are good candidates to display the Struve-Sahadeeffect (defined as the apparent strengthening of the secondary spectrumof the binary when the star is approaching, and the correspondingweakening of the lines when it is receding). Methods: We use highresolution optical spectra to determine new orbital solutions andspectral types of HD 165 052, HD 100 213, HD 159 176 and DH Cep. As goodknowledge of the fundamental parameters of the considered systems isnecessary to examine the Struve-Sahade effect. We then study equivalentwidth variations in the lines of both components of these binariesduring their orbital cycle. Results: In the case of these foursystems, variations appear in the equivalent widths of some lines duringthe orbital cycle, but the definition given above can any longer bevalid, since it is now clear that the effect modifies the primaryspectrum as much as the secondary spectrum. Furthermore, the linesaffected, and the way in which they are affected, depend on theconsidered system. For at least two of them (HD 100 213 and HD 159 176)these variations probably reflect the ellipsoidal variable nature of thesystem.Based on observations made at the European Southern Observatory (LaSilla, Chile) and at the Observatoire de Haute Provence (France).
| An XMM-Newton view of the young open cluster NGC 6231 - III. Optically faint X-ray sources
We discuss the properties of the X-ray sources with faint opticalcounterparts in the very young open cluster NGC 6231. From theirpositions in the Hertzsprung-Russell diagram, we find that the bulk ofthese objects probably consists of low-mass pre-main-sequence (PMS)stars with masses in the range 0.3-3.0 Msolar. The agedistribution of these objects indicates that low-mass star formation inNGC 6231 started more than 10Myr ago and culminated in a starburst-likeevent about 1-4Myr ago when the bulk of the low-mass PMS stars as wellas the massive cluster members formed. We find no evidence for a spatialage gradient that could point towards a sequential star formationprocess. Only a few X-ray sources have counterparts with a reddeningexceeding the average value of the cluster or with infrared coloursindicating the presence of a moderate near-IR excess. The X-ray spectraof the brightest PMS sources are best fitted by rather hard thermalplasma models and a significant fraction of these sources display flaresin their light curve. The X-ray brightest flaring sources have decaytimes between 2 and 16ks. The X-ray selected PMS stars in NGC 6231 havelogLX/Lbol values that increase strongly withdecreasing bolometric luminosity and can reach a saturation level(logLX/Lbol ~ -2.4) for non-flaring sources andeven more extreme values during flares.Based on observations with XMM-Newton, an ESA science mission withinstruments and contributions directly funded by ESA member states andthe USA (NASA).E-mail: hsana@eso.org ‡FNRS Research Associate (Belgium).
| 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
| Investigating star formation in the young open cluster NGC 6383
Context: By studying young open clusters, the mechanisms important forstar formation over several Myr can be examined. For example, accretionrate as a function of rotational velocity can be investigated.Similarly, sequential star formation triggered by massive stars withhigh mass-loss rates can be studied in detail. Aims: We identifiedand characterized probable members of NGC 6383, as well as determinedcluster parameters. Methods: New Strömgren uvby CCD photometry,obtained by us, is presented. This new data, together with Johnson UBVand 2MASS data in the NIR, was used to investigate characteristics ofpre- as well as zero age main sequence cluster members. Results: Wepresent Strömgren uvby CCD photometry for 272 stars in the field ofNGC 6383 and derive its reddening, E(b-y) = 0.21(4) mag, as well asdistance, d = 1.7(3) kpc from the Sun. Several stars with NIR excess andobjects in the domain of the classical Herbig Ae/Be and T Tauri starswere detected. Two previously known variables were identified asrapidly-rotating PMS stars. The field population is clearly separatedfrom the probable members in the color-magnitude diagram. Conclusions:. NGC 6383 is a young open cluster, with an age of less than 4 Myr,undergoing continuous star formation. True pre-main sequence membersmight be found down to absolute magnitudes of +6 mag, with a variety ofrotational velocities and stellar activities.Based on observations at ESO-La Silla (Proposal 073.C-0144). Data tableis only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/157
| 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.
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