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The OB binary HD152219: a detached, double-lined, eclipsing system
We present the results of an optical spectroscopic campaign on themassive binary HD152219 located near the core of the NGC6231 cluster.Though the primary to secondary optical brightness ratio is probablyabout 10, we clearly detect the secondary spectral signature and wederive the first reliable SB2 orbital solution for the system. Theorbital period is close to 4.2403d and the orbit is slightly eccentric(e = 0.08 +/- 0.01). The system is most probably formed by an O9.5 giantand a B1-2 V-III star. We derive minimal masses of 18.6 +/- 0.3 and 7.3+/- 0.1Msolar for the primary and secondary, respectively,and we constrain the stellar radius at values about 11 and5Rsolar. INTEGRAL-Optical Monitoring Camera (OMC) data revealthat HD152219 is the third O-type eclipsing binary known in NGC6231. Inthe Hertzsprung-Russell (HR) diagram, the primary component lies on theblue edge of the β Cep-type instability strip and its spectrallines display clear profile variations that are reminiscent of thoseexpected from non-radial pulsations. Finally, we report the analysis ofXMM-Newton observations of the system. The X-ray spectrum is relativelysoft and is well reproduced by a two-temperature mekal model withkT1 = 0.26keV and kT2 = 0.67keV. The X-ray flux ismost probably variable on a time-scale of days. The average X-rayluminosity during our campaign is log(LX) ~ 31.8(ergs-1), but shows fluctuations of about 10 per cent aroundthis value.Based on observations collected at the European Southern Observatory (LaSilla, Chile) and with the XMM-Newton satellite, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA); also based on data from the Optical MonitoringCamera (OMC) Archive at LAEFF, processed by ISDC.E-mail: hsana@eso.org ‡FNRS Research Associate (Belgium).
| 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.
| 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.
| Identification of the Infrared Counterpart to a Newly Discovered X-Ray Source in the Galactic Center
We present first results of a campaign to find and identify new compactobjects in the Galactic center. Selecting candidates from a combinationof Chandra and 2MASS survey data, we search for accretion disksignatures via infrared spectroscopy. We have found the infraredcounterpart to the Chandra source CXO J174536.1-285638, the spectrum ofwhich has strong Brγ and He I emission. The presence of C III, NIII, and He II indicate a binary system. We suspect that the system issome form of high-mass binary system, either a high-mass X-ray binary ora colliding-wind binary.
| The B Supergiant Components of the Double-lined Binary HD 1383
We present new results from a study of high-quality red spectra of themassive binary star system HD 1383 (B0.5 Ib + B0.5 Ib). We determinedradial velocities and revised orbital elements (P=20.28184+/-0.0002days) and made Doppler tomographic reconstructions of the componentspectra. A comparison of these with model spectra from non-LTE,line-blanketed atmospheres indicates that the two stars have almostidentical masses (M2/M1=1.020+/-0.014),temperatures (Teff=28,000+/-1000 K), gravities(logg=3.25+/-0.25), and projected rotational velocities (Vsini<~30 kms-1). We investigate a number of constraints on the radii andmasses of the stars based on the absence of eclipses, surface gravity,stellar wind terminal velocity, and probable location in the Perseusspiral arm of the Galaxy, and these indicate a range in probable radiusand mass of R/Rsolar=14-20 and M/Msolar=16-35,respectively. These values are consistent with model evolutionary massesfor single stars of this temperature and gravity. Both stars are muchsmaller than their respective Roche radii, so the system is probably ina precontact stage of evolution. A fit of the system's spectral energydistribution yields a reddening of E(B-V)=0.55+/-0.05 and a ratio oftotal-to-selective extinction of R=2.97+/-0.15. We find no evidence ofHα emission from colliding stellar winds, which is probably theconsequence of the low gas densities in the colliding winds zone.
| An XMM-Newton view of the young open cluster NGC 6231. I. The catalogue
This paper is the first of a series dedicated to the X-ray properties ofthe young open cluster NGC 6231. Our data set relies on an XMM-Newtoncampaign of a nominal duration of 180 ks and reveals that NGC 6231 isvery rich in the X-ray domain too. Indeed, 610 X-ray sources aredetected in the present field of view, centered on the cluster core. Thelimiting sensitivity of our survey is approximately 6 ×10-15 erg cm-2 s-1 but clearly dependson the location in the field of view and on the source spectrum. Usingdifferent existing catalogues, over 85% of the X-ray sources could beassociated with at least one optical and/or infrared counterpart withina limited cross-correlation radius of 3´´ at maximum. Thesurface density distribution of the X-ray sources presents a slight N-Selongation. Once corrected for the spatial sensitivity variation of theEPIC instruments, the radial profile of the source surface density iswell described by a King profile with a central density of about 8sources per arcmin2 and a core radius close to 3.1 arcmin.The distribution of the X-ray sources seems closely related to theoptical source distribution. The expected number of foreground andbackground sources should represent about 9% of the detected sources,thus strongly suggesting that most of the observed X-ray emitters arephysically belonging to NGC 6231. Finally, beside a few bright but softobjects - corresponding to the early-type stars of the cluster - most ofthe sources are relatively faint (~5 × 10-15 ergcm-2 s-1) with an energy distribution peakedaround 1.0-2.0 keV.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| 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.
| The massive binary CPD - 41° 7742. II. Optical light curve and X-ray observations
In the first paper of this series, we presented a detailedhigh-resolution spectroscopic study of CPD - 41°7742, deriving for the first time an orbital solution for bothcomponents of the system. In this second paper, we focus on the analysisof the optical light curve and on recent XMM-Newton X-ray observations.In the optical, the system presents two eclipses, yielding aninclination i77°. Combining the constraints from the photometrywith the results of our previous work, we derive the absolute parametersof the system. We confirm that the two components of CPD -41° 7742 are main sequence stars (O9 V + B1-1.5 V) withmasses (M_118 Mȯ and M_210 Mȯ)and respective radii (R_17.5 Rȯ and R_25.4Rȯ) close to the typical values expected for such stars.We also report an unprecedented set of X-ray observations that almostuniformly cover the 2.44-day orbital cycle. The X-ray emission fromCPD - 41° 7742 is well described by atwo-temperature thermal plasma model with energies close to 0.6 and 1.0keV, thus slightly harder than typical early-type emission. The X-raylight curve shows clear signs of variability. The emission level ishigher when the primary is in front of the secondary. During the highemission state, the system shows a drop of its X-ray emission thatalmost exactly matches the optical eclipse. We interpret the mainfeatures of the X-ray light curve as the signature of a wind-photosphereinteraction, in which the overwhelming primary O9 star wind crashes intothe secondary surface. Alternatively the light curve could result from awind-wind interaction zone located near the secondary star surface. As asupport to our interpretation, we provide a phenomenological geometricmodel that qualitatively reproduces the observed modulations of theX-ray emission.
| An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604
We discuss the results of two XMM-Newton observations of the opencluster NGC 6604 obtained in April and September 2002. We concentratemainly on the multiple system HD 167971 (O5-8V + O5-8V + (O8I)). Thesoft part of the EPIC spectrum of this system is thermal with typicaltemperatures of about 2 × 106 to 9 ×106 K. The nature (thermal vs. non-thermal) of the hard partof the spectrum is not unambiguously revealed by our data. If theemission is thermal, the high temperature of the plasma (~2.3 ×107 to 4.6 × 107 K) would be typical of whatshould be expected from a wind-wind interaction zone within a longperiod binary system. This emission could arise from an interactionbetween the combined winds of the O5-8V + O5-8V close binary system andthat of the more distant O8I companion. Assuming instead that the hardpart of the spectrum is non-thermal, the photon index would be rathersteep (~3). Moreover, a marginal variability between our two XMM-Newtonpointings could be attributed to an eclipse of the O5-8V + O5-8V system.The overall X-ray luminosity points to a significant X-ray luminosityexcess of about a factor 4 possibly due to colliding winds. ConsideringHD 167971 along with several recent X-ray and radio observations, wepropose that the simultaneous observation of non-thermal radiation inthe X-ray (below 10.0 keV) and radio domains appears rather unlikely.Our investigation of our XMM-Newton data of NGC 6604 reveals a rathersparse distribution of X-ray emitters. Including the two brightnon-thermal radio emitters HD 168112 and HD 167971, we present a list of31 X-ray sources along with the results of the cross-correlation withoptical and infrared catalogues. A more complete spectral analysis ispresented for the brightest X-ray sources. Some of the members of NGC6604 present some characteristics suggesting they may be pre-mainsequence star candidates.
| Photometric study of the eclipsing binary V1034 Sco
We present multi-colour Strömgren photometry of the eclipsingbinary V1034 Sco in the galactic open cluster NGC 6231, providing forthe first time a full coverage of the orbital period (P = 2.44 days) anda complete analysis of the orbit. This extensive new photometry wasanalyzed together with multiband spectroscopy and results in an improvedephemeris, photometric elements and absolute parameters of thecomponents. The system is close, but clearly detached. The masses of thecomponents M1 = 16.8 ±0.5 M_ȯ and M2 =9.4± 0.3 M_ȯ do not correspond to the published spectralclassification (O9III + B1III). The parameters of the components areconsistent with the main-sequence luminosity class. We independentlyderive a distance modulus V0 - MV = 10.73±0.02.
| A phase-resolved XMM-Newton campaign on the colliding-wind binary HD 152248
We report the first results of an XMM-Newton monitoring campaign of theopen cluster NGC 6231 in the Sco OB 1 association. This first paperfocuses on the massive colliding-wind binary HD 152248, which is thebrightest X-ray source of the cluster. The campaign, with a totalduration of 180 ks, was split into six separate observations, followingthe orbital motion of HD 152248. The X-ray flux from this systempresents a clear, asymmetric modulation with the phase and ranges from0.73 to 1.18 × 10-12 erg s-1 cm-2in the 0.5-10.0 keV energy band. The maximum of the emission is reachedslightly after apastron. The EPIC spectra are quite soft, and peakaround 0.8-0.9 keV. We characterize their shape using severalcombinations of MEKAL models and power-law spectra and we detectsignificant spectral variability in the 0.5-2.5 keV energy band.We also perform 2D hydrodynamical simulations using different sets ofparameters that closely reproduce the physical and orbital configurationof the HD 152248 system at the time of the six XMM-Newton pointings.This allows a direct confrontation of the model predictions with theconstraints deduced from the X-ray observations of the system. We showthat the observed variation of the flux can be explained by a variationof the X-ray emission from the colliding-wind zone, diluted by thesofter X-ray contribution of the two O-type stars of the system. Oursimulations also reveal that the interaction region of HD 152248 shouldbe highly unstable, giving rise to shells of dense gas that areseparated by low-density regions.Finally, we perform a search for short-term variability in the lightcurves of the system and we show that trends are present within severalof the 30-ks exposures of our campaign. Further, most of these trendsare in good agreement with the orbital motion and provide a directconstraint on the first-order derivative of the flux. In the samecontext, we also search for long-range correlations in the X-ray data ofthe system, but we only marginally detect them in the high-energy tailof the signal.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| Masses and other parameters of massive binaries
Binary stars provide us with the means to measure stellar mass. Here Ipresent several lists of known O-type stars with reliable mass estimatesthat are members of eclipsing, double-lined spectroscopic binaries. Themasses of young, unevolved stars in binaries are suitable for testingthe predictions of evolutionary codes, and there is good agreementbetween the observed and predicted masses (based upon temperature andluminosity) if the lower temperature scale from line-blanketed modelatmospheres is adopted. A final table lists masses for systems in awide variety of advanced evolutionary stages.
| Optical Polarization Observations of NGC 6231: Evidence for a Past Supernova Fingerprint
We present the first linear multicolor polarization observations for asample of 35 stars in the direction of the Galactic cluster NGC 6231. Wehave found a complex pattern in the angles of the polarimetric vectors.Near the core of this cluster the structure shows a semicircular patternthat we have interpreted as a reorientation of the dust particlesshowing the morphology of the magnetic field. We propose that asupernova event occurred some time ago and produced a shock on the localISM. We discuss in this paper independent confirmations of this event,both from the studies on the diffuse interstellar absorptions and theresults of the pre-main-sequence stars. We also show that a supernova issupported by the evolutionary status of the cluster.Based on observations obtained at Complejo Astronómico ElLeoncito (CASLEO), operated under agreement between the CONICET and theNational Universities of La Plata, Córdoba, and San Juan,Argentina.
| Improved Hipparcos Parallaxes of Coma Berenices and NGC 6231
A method to reprocess the Hipparcos Intermediate Astrometry Data thatreduces the propagation of the along-scan spacecraft attitude errors wasdeveloped and successfully used to obtain a more consistent parallax ofthe Pleiades (Makarov, published in 2002). The same technique is usednow to correct the Hipparcos parallaxes of the Coma Berenices and NGC6231 open clusters, which are also in error. The new mean parallax ofComa is 12.40+/-0.17 mas (against previously 11.43 mas) and of NGC 62311.7+/-0.4 mas (against previously -0.8 mas). The new data for Coma arein excellent agreement with the pre-Hipparcos main-sequence fittingestimates. All six members of NGC 6231 that have negative parallaxes inHipparcos obtain positive parallaxes. These results suggest that themain source of astrometric error has been correctly identified, and thata more accurate Hipparcos catalog can be computed.
| Ultra-high-resolution observations of interstellar Na I and K I towards the Scorpius OB1 association
I present ultra-high-resolution (R~9×105) observationsof interstellar NaI and KI absorption lines towards three members of theScorpius OB1 association (HD 152235, 152236 and 152249). Theseobservations have, for the first time, resolved the intrinsic linewidthsof most of the discrete absorption components present along thesecomplex sightlines. The aims of the project were twofold: (i) toconstrain the physical conditions prevailing in the highly blueshiftedSco OB1 shell components, and especially to search for evidence ofactive shocks within them; and (ii) to further constrain thelow-velocity structure, where the NaI spectra are fully saturated butthe unsaturated KI lines allow a clear resolution of the individualvelocity components. The results of these analyses are discussed.Perhaps the most surprising result is the lack of any obviouscorrelation between the velocity dispersion of a velocity component(b-value) and its velocity. Specifically, the high-velocity shellcomponents are generally found to be no broader than the low-velocitycomponents attributed to foreground (often molecular) clouds, and cannottherefore be any hotter or more turbulent. Thus, with the possibleexception of the most blueshifted component towards HD 152236, there isno evidence for active shocks in the shell components at present.However, consideration of the relative time-scales for post-shockcooling and grain surface adsorption indicates that shock processing inthe past may still account for the low NaI/CaII ratios of thesecomponents found in previous work.
| Spectroscopic Binaries in Young Open Clusters
We have analysed the binarity and multiplicity characteristics of 120O-type stars in 22 very young open clusters and found marked differencesbetween the "rich" (N >= 6 O-type stars and primaries) and "poor" (N= 1) clusters. In the rich clusters, the binary frequencies vary between14% (1 SB among 7 stars) and 80% (8 SBs among 10 stars). Multiplesystems seem not to be frequent and stars are spread all over thecluster area. In poor clusters, the binary frequency of the O-typeobjects is nearly 100%, with orbital periods around 3 days. Severalbinaries are also eclipsing. Additional companions are always present.They form either hierarchical multiple stars or trapezium systems. Thesemassive multiple systems are generally found close to the clustercenter, although there are exceptions.
| Tomographic Separation of Composite Spectra. VII. The Physical Properties of the Massive Triple System HD 135240 (δ Circini)
We present the results of a radial velocity study of the massive,double-lined, O binary HD 135240 based primarily on UV spectroscopy fromthe International Ultraviolet Explorer. Cross-correlation methodsindicate the presence of a third stationary spectral line componentwhich indicates that the system is a triple consisting of a central 3.9day close binary with a distant companion. We measured radial velocitiesfrom the cross-correlation functions after removal of the thirdcomponent, and we combined these with velocities obtained from Hαspectroscopy to reassess the orbital elements. We applied a Dopplertomography algorithm to reconstruct the individual UV spectra of allthree stars, and we determine spectral classifications of O7 III-V, O9.5V, and B0.5 V for the primary, secondary, and tertiary, respectively,using UV criteria defined by Penny, Gies, & Bagnuolo. We comparethese reconstructed spectra to standard single-star spectra to find theUV flux ratios of the components(F2/F1=0.239+/-0.022, andF3/F1=0.179+/-0.021). Hipparcos photometry revealsthat the central pair is an eclipsing binary, and we present the firstmodel fit of the light curve from which we derive an orbitalinclination, i=74deg+/-3deg. This analysisindicates that neither star is currently experiencing Roche lobeoverflow. We place the individual components in the theoretical H-Rdiagram, and we show that the masses derived from the combinedspectroscopic and photometric analysis(Mp/Msolar=21.6+/-2.0 andMs/Msolar=12.4+/-1.0) are significantly lower thanthose computed from evolutionary tracks for single stars.
| The origin of the runaway high-mass X-ray binary HD 153919/4U1700-37
Based on its Hipparcos proper motion, we propose that the high-massX-ray binary HD 153919/4U1700-37 originates in the OB association ScoOB1. At a distance of 1.9 kpc the space velocity of 4U1700-37 withrespect to Sco OB1 is 75 km s-1. This runaway velocityindicates that the progenitor of the compact X-ray source lost about 7Msun during the (assumed symmetric) supernova explosion. Thesystem's kinematical age is about 2 +/- 0.5 million years which marksthe date of the supernova explosion forming the compact object. Thepresent age of Sco OB1 is la 8 Myr; its suggested core, NGC 6231, seemsto be somewhat younger ( ~ 5 Myr). If HD 153919/4U1700-37 was born as amember of Sco OB1, this implies that the initially most massive star inthe system terminated its evolution within la 6 million years,corresponding to an initial mass ga 30 Msun. With theseparameters the evolution of the binary system can be constrained. Basedon data obtained with ESA's astrometric satellite Hipparcos.
| O Iiject{HD 152248}: Evidence for a colliding wind interaction
We present the results of a four year spectroscopic campaign on theearly-type binary system HD 152248. Our analysis yields a newclassification as O7.5III(f) + O7III(f), in contradiction with previousclassifications of the components as supergiants. We derive improvedorbital elements and the corresponding masses (M_1=29.6 Msunand M_2=29.9 Msun) are somewhat larger than previouslyreported in the literature, although they are still significantly lowerthan the ones expected from evolutionary tracks. Both components of thesystem are close to filling their Roche lobe at periastron passage. Wealso investigate the equivalent width variations of the lines of the twocomponents and we discuss their relation to the Struve-Sahade effect.Finally, we show that the line profile variability of the He Iil4686 andHα lines is consistent with a strong wind interaction between thetwo stars, quite as predicted by current colliding wind models. Based ondata collected at the European Southern Observatory (La Silla, Chile)and at the Cerro Tololo Inter-American Observatory (Cerro Tololo,Chile).
| High-mass binaries in the very young open cluster NGC 6231. Implication for cluster and star formation
New radial-velocity observations of 37 O- and B stars in the very youngopen cluster NGC 6231 confirm the high frequency of short-periodspectroscopic binaries on the upper main sequence. Among the 14 O-typestars, covering all luminosity classes from dwarfs to supergiants, 8 aredefinitively double-lined systems and all periods but one are shorterthan 7 days. Several additional binaries have been detected among theearly B-type stars. NGC 6231 is an exceptional cluster to constrain thescenarios of cluster- and binary-star formation over a large range ofstellar masses. We discuss the evidences, based on NGC 6231 and 21 otherclusters, with a total of 120 O-type stars, for a clear dichotomy in themultiplicity rate and structure of very young open clusters containingO-type stars in function of the number of massive stars. However, wecannot answer the question whether the observed characteristics resultfrom the formation processes or from the early dynamical evolution.
| Coordinated monitoring of the eccentric O-star binary Iota Orionis: the X-ray analysis
We analyse two ASCA observations of the highly eccentric O9 III+B1 IIIbinary Iota Orionis obtained at periastron and apastron. Based on theassumption of a strong colliding-wind shock between the stellarcomponents, we expected to see significant variation in the X-rayemission between these phases. The observations proved otherwise: theX-ray luminosities and spectral distributions were remarkably similar.The only noteworthy feature in the X-ray data was the hint of aproximity effect during periastron passage. Although this `flare' is ofrelatively low significance, it is supported by the notable proximityeffects seen in the optical, and the phasing of the X-ray and opticalevents is in very good agreement. However, other interpretations arealso possible. In view of the degradation of the SIS instrument andsource contamination in the GIS data we discuss the accuracy of theseresults, and also analyse archival ROSAT observations. We investigatewhy we do not see a clear colliding-wind signature. A simple model showsthat the wind attenuation to the expected position of the shock apex isnegligible throughout the orbit, which poses the puzzling question ofwhy the expected 1/D variation (i.e. a factor of 7.5) in the intrinsicluminosity is not seen in the data. Two scenarios are proposed: eitherthe colliding-wind emission is unexpectedly weak such that intrinsicshocks in the winds dominate the emission, or, alternatively, theemission observed is colliding-wind emission but in a more complex formthan we would naively expect. Complex hydrodynamical models are thenanalysed. Despite strongly phase-variable emission from the models, bothare consistent with the observations. We find that if the mass-lossrates of the stars are low then intrinsic wind shocks could dominate theemission. However, when we assume higher mass-loss rates of the stars,we find that the observed emission could also be consistent with apurely colliding-wind origin. A summary of the strengths and weaknessesof each interpretation is presented. To distinguish between thedifferent models X-ray observations with improved phase coverage will benecessary.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 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/Abstract.html
| Tomographic Separation of Composite Spectra. VI. The Physical Properties of the Massive Close Binary HD 152248
We present the results of a Doppler tomographic reconstruction of the UVspectra of the double-lined O binary HD 152248 based on observationsmade with IUE. We used cross-correlation methods to obtain radialvelocities, confirm the orbital elements, estimate the UV flux ratio,and determine projected rotational velocities. The individual componentspectra are classified as O7 I+O7 I using UV criteria defined by Penny,Gies, & Bagnuolo. We present a model fit of the eclipsing lightcurve from which we derive an orbital inclination, i=72^deg+/-3^deg.This analysis indicates that neither star is currently experiencingRoche lobe overflow. We place the individual components in thetheoretical H-R diagram and show that the individual masses(M_p/M_solar=24.2+/-2.0 M_s/M_solar=25.8+/-2.0) derived from thecombined spectroscopic and photometric analysis are significantly lowerthan those computed from evolutionary tracks for single stars.
| UBVI imaging photometry of NGC 6231
CCD UBVI photometry in the field of the open cluster NGC 6231 wasobtained for 1060 stars down to V~ 19 mag. Memberships, reddening,distance and age of this cluster were investigated. Its lower sequencedisplays a notorious bend at V~ 13.5 mag followed by a high number offaint stars showing a large magnitude spread at constant colour. Thisdistribution of stars does not seem to be produced by a mereaccumulation of field stars seen in the direction of the cluster but bya real star excess in the zone. The evidence suggests these stars areserious candidates to be cluster members caught in their way towards theZAMS. Fitting them with pre main sequence isochrones an age spread ofabout 10 Myr is obtained. The luminosity and mass functions were foundto be flat for -7.5 < MV < 1.5 and from 3 to 80 {calMsun} respectively. Based on observations collected at theUniversity of Toronto Southern Observatory, Las Campanas, Chile. Table 1is available only in electronic form at the CDS via anonymous ftp(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Colliding Winds in Massive Binary Systems
Not Available
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| Evolution of mass segregation in open clusters: some observational evidences
On the basis of the best available member list and duplicityinformation, we have studied the radial structure of Praesepe and of thevery young open cluster NGC 6231. We have found mass segregation amongthe cluster members and between binaries and single stars, which isexplained by the greater average mass of the multiple systems. However,the degree of mass segregation for stars between 1.5 and 2.3 M_sun isless pronounced in Praesepe than in the Pleiades. Furthermore, masssegregation is already present in the very young open cluster NGC 6231although this cluster is likely still not dynamically relaxed. Wediscuss the implications of these results and propose a qualitativescenario for the evolution of mass segregation in open clusters. InPraesepe the mass function of single stars and primaries appears to besignificantly different, like in the Pleiades. We observe an absence ofellipticity of the outer part of Praesepe. Tables 2 and 3 are availableonly in electronic form from the Strasbourg ftp server at 130.79.128.5
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Observation and Astrometry data
| Constellation: | Skorpion |
| Right ascension: | 16h54m10.06s |
| Declination: | -41°49'30.1" |
| Apparent magnitude: | 6.058 |
| Distance: | 10000000 parsecs |
| Proper motion RA: | 0 |
| Proper motion Dec: | -1 |
| B-T magnitude: | 6.186 |
| V-T magnitude: | 6.069 |
Catalogs and designations:
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