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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.
| Search for pulsating pre-main-sequence stars in NGC6383
A search for pulsating pre-main-sequence (PMS) stars was performed inthe young open cluster NGC 6383 using CCD time-series photometry inJohnson B and V filters. With an age of only ~1.7Myr all cluster memberslater than spectral type A0 have not reached the ZAMS yet, hence beingideal candidates for investigating PMS pulsation among A- and F-typestars. In total 286 stars have been analysed using classical Fouriertechniques. From about a dozen stars within the boundaries of theclassical instability strip, two stars were found to pulsate: NGC 6383#170, with five frequencies simultaneously, and NGC 6383 #198, with asingle frequency. In addition, NGC 6383 #152 is a suspected PMS variablestar, but our data remain inconclusive. Linear, non-adiabatic modelsassuming PMS evolutionary phase and purely radial pulsation werecalculated for the two new PMS pulsators. NGC 6383 #170 appears topulsate radially in third and fifth overtones, while the other threefrequencies seem to be of non-radial nature. NGC 6383 #198 pulsatesmonoperiodically, most probably in the third radial overtone. Magnitudesand B-V colours were available in the literature for only one third ofall stars and we used them for calibrating the remaining.
| 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.
| 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.
| 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.
| A Steady, Radiative-Shock Method for Computing X-Ray Emission from Colliding Stellar Winds in Close, Massive-Star Binaries
We present a practical, efficient, semianalytic formalism for computingsteady state X-ray emission from radiative shocks between collidingstellar winds in relatively close (orbital period up to order tens ofdays) massive-star, binary systems. Our simplified approach idealizesthe individual wind flows as smooth and steady, ignoring the intrinsicinstabilities and associated structure thought to occur in such flows.By also suppressing thin-shell instabilities for wind-collisionradiative shocks, our steady state approach avoids the extensivestructure and mixing that has thus far precluded reliable computation ofX-ray emission spectra from time-dependent hydrodynamical simulations ofclose-binary, wind-collision systems; but in ignoring the unknownphysical level of such mixing, the luminosity and hardness of X-rayspectra derived here represent upper limits to what is possible for agiven set of wind and binary parameters. A key feature of our approachis the separation of calculations for the small-scale shock-emissionfrom the ram-pressure-balance model for determining the large-scale,geometric form of the wind-wind interaction front. Integrating thelocalized shock emission over the full interaction surface and using awarm-absorber opacity to take account of attenuation by both the smoothwind and the compressed, cooled material in the interaction front, themethod can predict spectra for a distant observer at any arbitraryorbital inclination and phase. We illustrate results for a sampleselection of wind, stellar, and binary parameters, providing both fullX-ray light curves and detailed spectra at selected orbital phases. Thederived spectra typically have a broad characteristic form, and bysynthetic processing with the standard XSPEC package, we demonstratethat they simply cannot be satisfactorily fitted with the usualattenuated single- or two-temperature thermal-emission models. Weconclude with a summary of the advantages and limitations of ourapproach and outline its potential application for interpreting detailedX-ray observations from close, massive-star binary systems.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD 168112 (O5.5III(f+))
We report the results of a multiwavelength study of the non-thermalradio emitter HD 168112 (O5.5III(f+)). The detailed analysisof two quasi-simultaneous XMM-Newton and VLA observations reveals strongvariability of this star both in the X-ray and radio ranges. The X-rayobservations separated by five months reveal a decrease of the X-rayflux of 30%. The radio emission on the other hand increases by afactor 5-7 between the two observations obtained roughly simultaneouslywith the XMM-Newton pointings. The X-ray data reveal a hard emissionthat is most likely produced by a thermal plasma at kT 2-3 keVwhile the VLA data confirm the non-thermal status of this star in theradio waveband. Comparison with archive X-ray and radio data confirmsthe variability of this source in both wavelength ranges over a yet illdefined time scale. The properties of HD 168112 in the X-ray and radiodomain point towards a binary system with a significant eccentricity andan orbital period of a few years. However, our optical spectra reveal nosignificant changes of the star's radial velocity suggesting that if HD168112 is indeed a binary, it must be seen under a fairly lowinclination.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member states andthe USA (NASA). Also based on observations collected with the VLA, aninstrument of the National Radio Astronomy Observatory, which is afacility of the National Science Foundation operated by AssociatedUniversities, Inc. Optical data were collected at the European SouthernObservatory (La Silla, Chile), and at the ObservatorioAstronómico Nacional of San Pedro Mártir (Mexico).
| An XMM-Newton observation of the massive binary HD 159176
We report the analysis of an XMM-Newton observation of the close binaryHD 159176 (O7 V + O7 V). The observed LX/Lbolratio reveals an X-ray luminosity exceeding by a factor 7 theexpected value for X-ray emission from single O-stars, thereforesuggesting a wind-wind interaction scenario. EPIC and RGS spectra arefitted consistently with a two temperature mekal optically thin thermalplasma model, with temperatures ranging from 2 to6×106 K. At first sight, these rather low temperaturesare consistent with the expectations for a close binary system where thewinds collide well before reaching their terminal velocities. We alsoinvestigate the variability of the X-ray light curve of HD 159176 onvarious short time scales. No significant variability is found and weconclude that if hydrodynamical instabilities exist in the windinteraction region of HD 159176, they are not sufficient to produce anobservable signature in the X-ray emission. Hydrodynamic simulationsusing wind parameters from the literature reveal some puzzlingdiscrepancies. The most striking one concerns the predicted X-rayluminosity which is one or more orders of magnitude larger than theobserved one. A significant reduction of the mass loss rate of thecomponents compared to the values quoted in the literature alleviatesthe discrepancy but is not sufficient to fully account for the observedluminosity. Because hydrodynamical models are best for the adiabaticcase whereas the colliding winds in HD 159176 are most likely highlyradiative, a totally new approach has been envisaged, using ageometrical steady-state colliding wind model suitable for the case ofradiative winds. This model successfully reproduces the spectral shapeof the EPIC spectrum, but further developments are still needed toalleviate the disagreement between theoretical and observed X-rayluminosities.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| Supernova remnants and γ-ray sources
Electronic Article Available from Elsevier Science.
| 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.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| An XMM-Newton observation of the very young open cluster NGC 6383
We report the detection of a number of X-ray sources associated with thevery young open cluster NGC 6383. About two thirds of these objects arecorrelated with a rather faint optical source and all but one have atleast one infrared counterpart within a correlation radius of 8 arcsec.Although NGC 6383 is not associated with a prominent star formingregion, the overall properties of many of the X-ray sources suggest thatthey may be candidates for low-mass pre-main sequence stars. The numberof X-ray sources increases towards the cluster center suggesting thatthere exists a close relation between the massive O-star binary systemHD 159 176 in the cluster core and the population of X-ray brightlow-mass objects in NGC 6383.Based on observations with XMM-Newton, an ESA science mission withinstruments and contributions directly funded by ESA member states andthe USA (NASA).Table 1 is only available in electronic form athttp://www.edpsciences.org
| New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
We carried out a survey of infrared star clusters and stellar groups onthe 2MASS J, H and Ks all-sky release Atlas in the Northernand Equatorial Milky Way (350deg < l < 360deg, 0deg < l < 230 deg). Thesearch in this zone complements that in the Southern Milky Way (Dutra etal. \cite{Dutra03}a). The method concentrates efforts on the directionsof known optical and radio nebulae. The present study provides 167 newinfrared clusters, stellar groups and candidates. Combining the twostudies for the whole Milky Way, 346 infrared clusters, stellar groupsand candidates were discovered, whereas 315 objects were previouslyknown. They constitute an important new sample for future detailedstudies.
| XMM-Newton high-resolution X-ray spectroscopy of the Wolf-Rayet object WR 25 in the Carina OB1 association
We report the analysis of the first high-resolution X-ray spectra of theWolf-Rayet (WR) object WR 25 (HD 93162, WN6ha+O4f) obtained with theReflection Grating Spectrometers (RGS) and the European Photon ImagingCameras (EPIC-MOS and PN) CCD spectrometers on board the XMM-Newtonsatellite. The spectrum exhibits bright emission lines of the H- andHe-like ions of Ne, Mg, Si and S, as well as Fe XVII i to Fe XX and FeXXV lines. Line fluxes have been measured. The RGS and EPIC spectrahave been simultaneously fitted to obtain self-consistent temperatures,emission measures, and elemental abundances. Strong absorption by thedense WR stellar wind and the interstellar medium (ISM) is observedequivalent to NH = 7 x 1021 cm-2.Multi-temperature (DEM) fitting yields two dominant components aroundtemperatures of 7.0 and 32 MK, respectively. The XMM intrinsic (i.e.unabsorbed, corrected for the stellar wind absorption and the absorptionof ISM) X-ray luminosity of WR 25 is Lx(0.5-10 keV) = 1.3 x1034 erg s-1, and Lx(0.5-10 keV) = 0.85x 1034 erg s-1, (when correcting for the ISM only)assuming d=3.24 kpc. The obtained chemical abundances are subsolar,except for S. This may be real, but could equally well be due to a weakcoupling to the continuum, which is strongly influenced by theabsorption column density and the subtracted background. The expectedhigh N-abundance, as observed in the optical wavelength region, couldnot be confirmed due to the strong wind absorption, blocking out itsspectral signature. The presence of the Fe XXV emission-line complex at~ 6.7 keV is argued as being indicative for colliding winds inside aWR+O binary system.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).
| A Dozen Colliding-Wind X-Ray Binaries in the Star Cluster R136 in the 30 Doradus Region
We analyzed archival Chandra X-ray observations of the central portionof the 30 Doradus region in the Large Magellanic Cloud. The imagecontains 20 X-ray point sources with luminosities between5×1032 and 2×1035 ergs s-1(0.2-3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectraltype O stars as optical counterparts. Nine of these are within ~3.4 pcof R136, the central star cluster of NGC 2070. We derive an empiricalrelation between the X-ray luminosity and the parameters for the stellarwind of the optical counterpart. The relation gives good agreement forknown colliding-wind binaries in the Milky Way Galaxy and for theidentified X-ray sources in NGC 2070. We conclude that probably allidentified X-ray sources in NGC 2070 are colliding-wind binaries andthat they are not associated with compact objects. This conclusioncontradicts earlier studies where it was argued, using ROSAT data, thattwo earlier discovered X-ray sources are accreting black hole binaries.Five of the 18 brightest stars in R136 are not visible in our X-rayobservations. These stars either are single, have low-mass companions,or have very wide orbits. The resulting binary fraction among early-typestars is then unusually high (at least 70%).
| The X-ray Source SLX 1732-304 in the Globular Cluster Terzan 1: The Spectral States and an X-ray Burst
ART-P/Granat observations of the X-ray burster SLX 1732-304 in theglobular cluster Terzan 1 are presented. The X-ray (3-20 keV) fluxesfrom the source differed by more than a factor of 4 during the observingsessions on September 8 (F_X = 6.95 x 10^{-10} erg/cm^2/s) and October6, 1990 (F_X = 1.64 x 10^{-10} erg/cm^2/s). The intensity variations ofSLX 1732-304 were apparently accompanied by variations in its hardness:whereas the source in its high state had the spectrum with a distinctexponential cutoff typical of bright low-mass X-ray binaries, itslow-state spectrum could be satisfactorily described by a simple powerlaw with a photon index alpha = 1.7. During the ART-P observation onSeptember 8, a type I X-ray burst was detected from SLX 1732-304.
| Faint X-Ray Sources Resolved in the ASCA Galactic Plane Survey and Their Contribution to the Galactic Ridge X-Ray Emission
The X-ray emission from the central region of the Galactic plane,|l|<~45° and |b|<~0.4d, was studied in the 0.7-10 keV energyband with a spatial resolution of ~3' with the Advanced Satellite forCosmology and Astrophysics (ASCA) observatory. We developed a newanalysis method for the ASCA data to resolve discrete sources from theextended Galactic ridge X-ray emission (GRXE). We successfully resolved163 discrete sources with an X-ray flux down to 10-12.5 ergscm-2 s-1 and determined the intensity variationsof the GRXE as a function of the Galactic longitude with a spatialresolution of about 1°. The longitudinal intensity variation in theenergy band above 4 keV, for which there is little absorption in theGalactic plane, shows a large enhanced feature within |l|<~30°.This suggests a strong enhancement of X-ray emissivity of the GRXEinside the 4 kpc arm of the Galaxy. Searches for identifications of theresolved X-ray sources with cataloged X-ray sources and optical starsshow that the 66% are unidentified. Spectral analysis of each sourceshows that a large number of the unidentified sources have hard X-rayspectra obscured by the Galactic interstellar medium. We classified thesources into several groups by the flux, the hardness and the softnessof the spectra, and performed further detailed analysis for the spectrasummed within each group. Possible candidates of X-ray origins of theseunidentified sources are discussed based on the grouping spectralanalysis. Also, we derived the logN-logS relations of the resolvedsources in the energy bands below and above 2 keV separately. ThelogN-logS relation of the Galactic X-ray sources above 2 keV wasobtained for the first time with this study. It is represented by apower-law with an index of -0.79+/-0.07 after correction for thecontribution of extragalactic X-ray sources. This flat power-lawrelation suggests that the spatial distribution of the X-ray sourcesshould have an armlike structure in which the solar system is included.The integrated surface brightness of the resolved sources is about 10%of the total GRXE in both energy bands. The approximately 90% of theemission remaining is still unresolved.
| 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.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| 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
| The central part of the young open cluster NGC 6383
The spectral and extinction properties of 14 pre-main sequencecandidates in the central part of the very young open cluster NGC 6383were investigated. None of these stars shows evidence for anomalouscircumstellar extinction. However, six out of 14 programme stars do showan infrared excess, indicative of the presence of circumstellar dust,heated up by the central star. One of these stars (number 4), also showsHα in emission and shows some indications for the presence ofcircumstellar gas in its spectrum, and might therefore be a newlyidentified Herbig Ae star. Based on observations collected at theEuropean Southern Observatory, La Silla, Chile.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| 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
| The interstellar clouds of ADAMS and Blaauw revisited: an HI absorption study - II.
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| The interstellar clouds of ADAMS and Blaauw revisited: an HI absorption study - I.
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| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| 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.
| The winds of hot close binaries. Paper 3: HD 159176
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