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Chandra Detects the Rare Oxygen-type Wolf-Rayet Star WR 142 and OB Stars in Berkeley 87
We present first results of a Chandra X-ray observation of the rareoxygen-type Wolf-Rayet (WR) star WR 142 (= Sand 5 = St 3) harbored inthe young, heavily obscured cluster Berkeley 87. Oxygen-type WO starsare thought to be the most evolved of the WRs and progenitors ofsupernovae or gamma-ray bursts. As part of an X-ray survey of supposedlysingle WR stars, we observed WR 142 and the surrounding Berkeley 87region with Chandra ACIS-I. We detect WR 142 as a faint yet extremelyhard X-ray source. Due to weak emission, its nature as a thermal ornon-thermal emitter is unclear and thus we discuss several emissionmechanisms. Additionally, we report seven detections and eightnon-detections by Chandra of massive OB stars in Berkeley 87, two ofwhich are bright yet soft X-ray sources whose spectra provide a dramaticcontrast to the hard emission from WR 142.
| X-ray Emission from Nitrogen-Type Wolf-Rayet Stars
We summarize new X-ray detections of four nitrogen-type Wolf-Rayet (WR)stars obtained in a limited survey aimed at establishing the X-rayproperties of WN stars across their full range of spectral subtypes.None of the detected stars is so far known to be a close binary. Wereport Chandra detections of WR 2 (WN2), WR 18 (WN4), and WR 134 (WN6),and an XMM-Newton detection of WR79a (WN9ha). These observations clearlydemonstrate that both WNE and WNL stars are X-ray sources. We alsodiscuss Chandra archive detections of the WN6h stars WR 20b, WR 24, andWR 136 and ROSAT non-detections of WR 16 (WN8h) and WR 78 (WN7h). TheX-ray spectra of all WN detections show prominent emission lines and anadmixture of cool (kT < 1 keV) and hot (kT > 2 keV) plasma. Thehotter plasma is not predicted by radiative wind shock models and otheras yet unidentified mechanisms are at work. Most stars show X-rayabsorption in excess of that expected from visual extinction (AV), likely due to their strong winds or cold circumstellargas. Existing data suggest a falloff in X-ray luminosity toward laterWN7-9 subtypes, which have higher L bol but slower, denserwinds than WN2-6 stars. This provides a clue that wind properties may bea more crucial factor in determining emergent X-ray emission levels thanbolometric luminosity.
| Near-infrared Counterparts to Chandra X-ray Sources Toward the Galactic Center. II. Discovery of Wolf-Rayet Stars and O Supergiants
We present new identifications of infrared counterparts to thepopulation of hard X-ray sources near the Galactic center detected bythe Chandra X-ray Observatory. We have spectroscopically confirmed 16new massive stellar counterparts to the X-ray population, includingnitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars, and Osupergiants. These discoveries increase the total sample of massivestellar X-ray sources in the Galactic center region to 30 (possibly 31).For the majority of these sources, the X-ray photometry is consistentwith thermal emission from plasma having temperatures in the range of kT= 1-8 keV or non-thermal emission having power-law indices in the rangeof -1 <~ ? <~ 3, and X-ray luminosities in the rangeof L X ~ 1032-1034 ergs-1 (0.5-8.0 keV). Several sources have exhibited X-rayvariability of several factors between observations. These X-rayproperties are not a ubiquitous feature of single massive stars but aretypical of massive binaries, in which the high-energy emission isgenerated by the collision of supersonic winds, or by accretion onto acompact companion. However, without direct evidence for companions, thepossibility of intrinsic hard X-ray generation from single stars cannotbe completely ruled out. The spectral energy distributions of thesesources exhibit significant infrared excess, attributable to free-freeemission from ionized stellar winds, supplemented by hot dust emissionin the case of the WC stars. With the exception of one object locatednear the outer regions of the Quintuplet cluster, most of the new starsappear isolated or in loose associations. Seven hydrogen-rich WN and Ostars are concentrated near the Sagittarius B H II region, while othersimilar stars and more highly evolved hydrogen-poor WN and WC stars liescattered within ?50 pc, in projection, of Sagitarrius A West. Wediscuss various mechanisms capable of generating the observed X-rays andthe implications these stars have for massive star formation in theGalaxy's Central Molecular Zone.
| Chandra Observations of WR 147 Reveal a Double X-ray Source
We report the first results from deep X-ray observations of theWolf-Rayet (WR) binary system WR 147 with the Chandra High EnergyTransmission Gratings. Analysis of the zeroth-order data reveals that WR147 is a double X-ray source. The northern counterpart is likelyassociated with the colliding wind region, while the southern componentis certainly identified with the WN star in this massive binary. Thelatter is the source of high-energy X-rays (including the Fe K?complex at 6.67 keV) whose production mechanism is yet unclear. For thefirst time, X-rays are observed directly from a WR star in a binarysystem.
| 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).
| 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.
| A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. I. Characterizing the Variability
We present the results of a systematic search for large-scalespectroscopic variability in apparently single Wolf-Rayet (WR) starsbrighter than v ~ 12.5. In this first paper we characterize the variousforms of variability detected and distinguish several separate groups.For each star in our sample, we obtained 4-5 high-resolution spectrawith signal-to-noise ratio ~100. Our ultimate goal is to identify newcandidates presenting variability that potentially comes from corotatinginteraction regions (CIRs). Out of a sample of 25 stars, 10 were foundto display large-scale changes of which four are of CIR-type (WR 1,WR 115, WR 120, and WR 134). The star WR 134 wasalready known to show such changes from previous studies. Three WN8stars present a different type of large-scale variability and we believedeserve a group of their own. Also, all three WC9d stars in our samplepresent large-scale variability, but it remains to be checked if theseare binaries, as many dust-making WR stars are double. Finally, of theremaining stars, 10 were found to show small-amplitude spectral changes,which we attribute to normal line-profile variability due toinhomogeneities in the wind, and five were found to show no spectralvariability, as far as can be concluded from the data in hand. Follow-upstudies are required to identify potential periods for our candidatesshowing CIR-type changes and eventually estimate a rotation rate forthese WR stars.
| Discovery of X-Ray Emission from the Wolf-Rayet Star WR 142 of Oxygen Subtype
We report the discovery of weak yet hard X-ray emission from theWolf-Rayet (WR) star WR 142 with the XMM-Newton X-ray telescope.Being of spectral subtype WO2, WR 142 is a massive star in a veryadvanced evolutionary stage shortly before its explosion as a supernovaor ?-ray burst. This is the first detection of X-ray emission froma WO-type star. We rule out any serendipitous X-ray sources within?1'' of WR 142. WR 142 has an X-ray luminosity of LX ? 7 × 1030 erg s-1,which constitutes only lsim10-8 of its bolometricluminosity. The hard X-ray spectrum suggests a plasma temperature ofabout 100 MK. Commonly, X-ray emission from stellar winds is attributedto embedded shocks due to the intrinsic instability of the radiationdriving. From qualitative considerations we conclude that this mechanismcannot account for the hardness of the observed radiation. There are nohints for a binary companion. Therefore the only remaining, albeitspeculative explanation must refer to magnetic activity. Possiblyrelated, WR 142 seems to rotate extremely fast, as indicated by theunusually round profiles of its optical emission lines. Our detectionimplies that the wind of WR 142 must be relatively transparent toX-rays, which can be due to strong wind ionization, wind clumping, ornonspherical geometry from rapid rotation.
| CN column densities and excitation temperatures
We analyse abundances and rotational temperatures of the interstellar CNmolecule. We have calculated the column densities and excitationtemperatures of the molecule along 73 lines of sight basing on ouroriginal measurements of the B 2?+ -X2?+ (0,0) vibrational band recorded in highsignal-to-noise ratio spectra and also for 88 directions based onmeasurements already available in literature. We compare the columndensities obtained from different bands of CN molecule available toground-based instruments. The obtained excitation temperatures in theanalysed directions show always an excess over the cosmic microwavebackground radiation (CMBR) temperature.
| The relation between CH and CN molecules and carriers of 5780 and 5797 diffuse interstellar bands
Optical absorption bands of the interstellar CN (near 3875 Å) andCH molecules (the violet and blue ones near 4300 and 3886 Å,respectively) were applied to determine the column densities of thesetwo radicals in a statistically meaningful sample of 84 reddened OBstars. Equivalent widths of the major 5780 and 5797 diffuse bands (DIBs)were measured along the lines of sight toward the same stars in spectraacquired using four echelle spectrographs situated in both the northernand southern hemispheres. The mutual relation between abundances of CHand CN molecules shows a large scatter; and especially the CN moleculeabundance varies strongly from cloud to cloud. The carriers of the major5780 and 5797 DIBs seem to be spatially correlated with column densitiesof CH rather than of the CN molecule. This is most likely true in thecase of a narrower feature: the 5797 DIB correlates with CH columndensity better than 5780 does. The correlations do suggest that the DIBcarriers are likely hydrocarbons. They apparently occupy molecularclouds since the H{2} abundance is closely related to that ofmethylidyne (CH), as has already been demonstrated.
| Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund 2
Context: Westerlund 2 is a young and massive, obscured stellar clusterof our Galaxy. It harbors the most massive star with well-determinedparameters, WR20a (82 + 83 M?), a dozen very earlyO-type stars (O3-7), and a wealth of PMS stars. Although of clearsignificance, this star cluster has not been well-studied. Aims:The high-energy properties of this cluster, especially those of itsearly-type stars are examined in detail. The variability of the X-raysources is investigated. Methods: A monitoring of the field wasperformed using three Chandra observations. This dataset probes daily aswell as monthly to yearly timescales and provides the deepest X-ray viewof the cluster to date. Results: The two Wolf-Rayet stars WR20a(WN6ha+WN6ha) and WR20b (WN6ha) were analyzed in detail. They are bothluminous and display hard spectra, but WR20b does not appear to vary. Incontrast, WR20a, a known eclipsing, colliding-wind binary, brightens inthe X-ray domain during eclipses, i.e. when the collision is seenface-on. This can be explained by the properties of the wind-windcollision zone, whose high density leads to a large absorbing column (2× 1024 cm-2). All twelve O-type starspreviously classified spectroscopically, two eclipsing binariespreviously identified and nine newly-identified, O-type star candidatesare detected in the high-energy domain; ten of those could be analyzedspectroscopically. Four are overluminous, but the others have typicalL_X/L_BOL ratios, suggesting that several O-type objects are actuallybinaries. Variability at the ~2? level is detected for a majorityof the sources, of unknown origin for single objects. Faint, soft,diffuse emission pervades the entire field-of-view but no clearstructure can be identified, even at the position of a blister proposedto correspond to the TeV source HESS J1023-575. The X-ray properties ofPMS objects, in particular the brightest flaring ones, are alsoinvestigated. They provided an additional argument in favor of a largedistance (~8 kpc) for the cluster.
| Near Infrared Spectra of Galactic Wolf-Rayet Stars
Spectra of 37 Galactic WR stars were observed and reduced in thespectral range 790--895 nm. The main spectral features are identifiedand the equivalent widths and FWHMs of the strongest emission lines aremeasured. The equivalent width of the diffuse interstellar band at 862nm is also measured and the new estimates of color excessesE(B-V) are derived by using an empirical relationship betweenthe equivalent width and the color excess. The equivalent width ratiosfor the lines C III 850 nm, C IV 886 nm and C II 880 nm were found tocorrelate well with the WC subtype.
| The Wolf-Rayet Stars HD 4004 and HD 50896: Two of a Kind
We present the results of the analysis of 151 spectra of the Wolf-Rayetstar HD 4004 (WR1) obtained in 1999 and in 2005. The line-profilevariability is found to be periodic, with P=7.684 days, and to havecharacteristics that are very similar to those of another variable WRstar, HD 50896 (WR6). The similarities point toward a common physicalphenomenon in both systems. Of the scenarios that can explain theobservations, such as colliding winds in two stars with similar windmomenta and the ejection of streams or jets from two opposite locationson the stellar surface, the latter seems more likely due to the lack ofobservational evidence for a strong wind-bearing companion.
| The Young Stellar Population in M17 Revealed by Chandra
We report here results from a Chandra ACIS observation of the stellarpopulations in and around the M17 H II region. The field reveals 886sources with observed X-ray luminosities (uncorrected for absorption)between ˜29.3 ergs s-1< log LX<32.8ergs s-1, 771 of which have stellar counterparts in infraredimages. In addition to comprehensive tables of X-ray source properties,several results are presented:1. The X-ray luminosity function is calibrated to that of the OrionNebula Cluster population to infer a total population of roughly8000-10,000 stars in M17, one-third lying in the central NGC 6618cluster.2. About 40% of the ACIS sources are heavily obscured withAV>10 mag. Some are concentrated around well-studiedstar-forming regions -- IRS 5/UC1, the Kleinmann-Wright Object, andM17-North -- but most are distributed across the field. As previouslyshown, star formation appears to be widely distributed in the molecularclouds. X-ray emission is detected from 64 of the hundreds of Class Iprotostar candidates that can be identified by near- and mid-infraredcolors. These constitute the most likely protostar candidates known inM17.3. The spatial distribution of X-ray stars is complex: in addition tothe central NGC 6618 cluster and well-known embedded groups, we find anew embedded cluster (designated M17-X), a 2 pc long arc of young starsalong the southwest edge of the M17 H II region, and 0.1 pc substructurewithin various populations. These structures may indicate that thepopulations are dynamically young.4. All (14/14) of the known O stars but only about half (19/34) of theknown B0-B3 stars in the M17 field are detected. These stars exhibitthe long-reported correlation between X-ray and bolometric luminositiesof LX˜10-7Lbol. While many O andearly-B stars show the soft X-ray emission expected from microshocks intheir winds or moderately hard emission that could be caused bymagnetically channeled wind shocks, six of these stars exhibit very hardthermal plasma components (kT>4 keV) that may be due to collidingwind binaries. More than 100 candidate new OB stars are found, including28 X-ray detected intermediate- and high-mass protostar candidates withinfrared excesses.5. Only a small fraction (perhaps 10%) of X-ray selected high- andintermediate-mass stars exhibit K-band-emitting protoplanetary disks,providing further evidence that inner disks evolve very rapidly aroundmore massive stars.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| The Galactic WN stars. Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
Context: .Very massive stars pass through the Wolf-Rayet (WR) stagebefore they finally explode. Details of their evolution have not yetbeen safely established, and their physics are not well understood.Their spectral analysis requires adequate model atmospheres, which havebeen developed step by step during the past decades and account in theirrecent version for line blanketing by the millions of lines from ironand iron-group elements. However, only very few WN stars have beenre-analyzed by means of line-blanketed models yet. Aims: .Thequantitative spectral analysis of a large sample of Galactic WN starswith the most advanced generation of model atmospheres should provide anempirical basis for various studies about the origin, evolution, andphysics of the Wolf-Rayet stars and their powerful winds. Methods:.We analyze a large sample of Galactic WN stars by means of the PotsdamWolf-Rayet (PoWR) model atmospheres, which account for iron lineblanketing and clumping. The results are compared with a syntheticpopulation, generated from the Geneva tracks for massive starevolution. Results: .We obtain a homogeneous set of stellar andatmospheric parameters for the Galactic WN stars, partly revisingearlier results. Conclusions: .Comparing the results of ourspectral analyses of the Galactic WN stars with the predictions of theGeneva evolutionary calculations, we conclude that there is roughqualitative agreement. However, the quantitative discrepancies are stillsevere, and there is no preference for the tracks that account for theeffects of rotation. It seems that the evolution of massive stars isstill not satisfactorily understood.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Multiwavelength studies of WR 21a and its surroundings
We present results of high-resolution radio continuum observationstowards the binary star WR 21a (Wack 2134) obtained with the AustraliaTelescope Compact Array (ATCA) at 4.8 and 8.64 GHz. We detected thesystem at 4.8 GHz (6 cm) with a flux density of 0.25±0.06 mJy andset an upper limit of 0.3 mJy at 8.64 GHz (3 cm). The derived spectralindex of α < 0.3 (Sν ∝να) suggests the presence of non-thermal emission,probably originating in a colliding-wind region. A second, unrelatedradio source was detected ~10 arcsec north of WR 21a at (RA,Dec)J2000=(10h25m56.49s, -57°48arcmin34.4 arcsec), with flux densities of 0.36 and 0.55 mJy at 4.8 and8.64 GHz, respectively, resulting in α = 0.72. H i observations inthe area are dominated by absorption against the prominent H ii regionRCW 49. Analysis of a complete set of archived X-ray observations of WR21a confirms its strong variability but throws into doubt previoussuggestions by Reig (1999) of a period of years for the system. Finally,we comment on the association with the nearby EGRET source 3EGJ1027-5817.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| 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.
| An XMM-Newton look at the Wolf-Rayet star WR 40. The star itself, its nebula and its neighbours
We present the results of an XMM-Newton observation of the field of theWolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the highsensitivity of the satellite, the star itself is not detected: we thusderive an upper limit on its X-ray flux and luminosity. Joining thisresult to recent reports of a non-detection of some WC stars, we suggestthat the X-ray emission from single normal Wolf-Rayet stars could oftenbe insignificant despite remarkable instabilities in the wind. On thebasis of a simple modelling of the opacity of the Wolf-Rayet wind of WR40, we show that any X-ray emission generated in the particular zonewhere the shocks are supposed to be numerous will indeed have littlechance to emerge from the dense wind of the Wolf-Rayet star. We alsoreport the non-detection of the ejecta nebula RCW 58 surrounding WR 40.Concerning the field around these objects, we detected 33 X-ray sources,most of them previously unknown: we establish a catalog of these sourcesand cross-correlate it with catalogs of optical/infrared sources.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| Kinematical Structure of Wolf-Rayet Winds. II. Internal Velocity Scatter in WN Stars
The shortward edge of the absorption core velocities - v_black asdetermined from low resolution archived IUE spectra from the INESdatabase are presented for three P Cyg profiles of NV 1240, HeII 1640and NIV 1720 for 51 Galactic and 64 LMC Wolf-Rayet stars of the WNsubtype. These data, together with v_black of CIV 1550 line presented inNiedzielski and Skorzynski (2002) are discussed. Evidences are presentedthat v_black of CIV 1550 rarely displays the largest wind velocity amongthe four lines studied in detail and therefore its application as anestimator of the terminal wind velocity in WN stars is questioned. Anaverage v_black of several lines is suggested instead but it is pointedout that v_black of HeII 1640 usually reveals the highest observablewind velocity in Galactic and LMC WN stars. It is shown that thestratification strength decreases from WNL to WNE stars and that for WNLstars there exists a positive relation between v_black and theIonization Potential. The velocity scatter between v_black obtained fromdifferent UV lines is found to correlate well with the X-ray luminosityof single WN stars (correlation coefficient R=0.82 for the data obtainedfrom the high resolution IUE spectra) and therefore two clumpy windmodels of single WN stars are presented that allow the velocity scatterto persist up to very large distances from the stellar surface (r approx500-1000 R_*). These models are used to explain the specific features ofsingle WN stars like broad absorption troughs of strong lines havingdifferent v_black, X-ray fluxes, IR/radio continua and stratificationrelations.
| XMM-Newton observations of the nitrogen-rich Wolf-Rayet star WR 1
We present XMM-Newton results for the X-ray spectrum from the N-richWolf-Rayet (WR) star WR 1. The EPIC instrument was used to obtain amedium-resolution spectrum. The following features characterize thisspectrum: (a) significant emission ``bumps'' appear that are coincidentwith the wavelengths of typical strong lines, such as MgXI, SiXIII, andSXV; (b) little emission is detected above 4 keV, in contrast to recentreports of a hard component in the stars WR 6 and WR 110 which are ofsimilar subtype; and (c) evidence for sulfur K-edge absorption at about2.6 keV, which could only arise from absorption of X-rays by the ambientstellar wind. The lack of hard emission in our dataset is suggestivethat WR 1 may truly be a single star, thus representing the firstdetailed X-ray spectrum that isolates the WR wind alone (in contrast tocolliding wind zones). Although the properties of the S-edge are notwell-constrained by our data, it does appear to be real, and itsdetection indicates that at least some of the hot gas in WR 1 mustreside interior to the radius of optical depth unity for the totalabsorptive opacity at the energy of the edge.
| Chandra Observations of Associates of η Carinae. I. Luminosities
The region around the η Carinae Nebula has three OB associations,which contain a Wolf-Rayet star and several massive O3 stars. An earlyChandra ACIS-I image was centered on η Car and includes Trumpler 16and part of Trumpler 14. The Chandra image confirms the well-knownresult that O and very early B stars are X-ray sources withLX~=10-7Lbol over an X-ray luminosityrange of about 100. Two new, anomalously strong X-ray sources have beenfound among the hot star population: Tr 16-244, a heavily reddened O3 Istar, and Tr 16-22, a heavily reddened O8.5 V star. Two stars have anunusually large LX/Lbol: HD 93162, a Wolf-Rayetstar (and possible binary), and Tr 16-22, a possible colliding-windbinary. In addition, a population of sources associated with cool starsis detected. In the color-magnitude diagram, these X-ray sources sitabove the sequence of field stars in the Carina arm. The OB stars are onaverage more X-ray-luminous than the cool star X-ray sources. X-raysources among A stars have X-ray luminosities similar to those of coolerstars and may be due to cooler companions. Upper limits are presentedfor B stars that are not detected in X-rays. These upper limits are alsothe upper limits for any cool companions that the hot stars may have.Hardness ratios are presented for the most luminous sources in bands0.5-0.9, 0.9-1.5, and 1.5-2.04 keV. The available information on thebinary nature of the hot stars is discussed, but binarity does notcorrelate with X-ray strength in a simple way.Based on observations made with the Chandra X-Ray Observatory.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars
The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksecXMM-Newton, observation, implying an upper limit to the X-ray luminosityof LX < 2.5x 1030 erg s-1 andto the X-ray to bolometric luminosity ratio ofLX/Lbol < 4*E-9. This confirmsindications from earlier less sensitive measurements that there has beenno convincing X-ray detection of any single WC star. This lack ofdetections is reinforced by XMM-Newton, and CHANDRA observations of WCstars. Thus the conclusion has to be drawn that the stars withradiatively-driven stellar winds of this particular class areinsignificant X-ray sources. We attribute this to photoelectronicabsorption by the stellar wind. The high opacity of the metal-rich anddense winds from WC stars puts the radius of optical depth unity athundreds or thousands of stellar radii for much of the X-ray band. Webelieve that the essential absence of hot plasma so far out in the windexacerbated by the large distances and correspondingly high ISM columndensities makes the WC stars too faint to be detectable with currenttechnology. The result also applies to many WC stars in binary systems,of which only about 20% are identified X-ray sources, presumably due tocolliding winds.
| 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).
| High-resolution X-ray spectroscopy of tau Scorpii (B0.2V) with XMM-Newton
We report the analysis of the first high-resolution X-ray spectrum ofthe B0.2V star tau Scorpii obtained with the Reflection GratingSpectrometers (RGS) and the EPIC-MOS CCD spectrometers on boardXMM-Newton. The spectrum exhibits bright emission lines of the H- andHe-like ions of C, N, O, Ne, Mg, and Si, as well as Fe XVII & FeXVIII lines. Line fluxes have been determined. Simultaneous fits to theRGS and EPIC spectra were used to obtain four plasma temperatures,emission measures, and the overall elemental abundances. Thismulti-temperature fitting yielded temperatures of 1.6, 5.2, 8.2, and ga20 MK. These temperatures are confirmed by DEM modelling. The nitrogenlines are relatively strong: the N/O abundance ratio is about 3x solar.No indication of a solar-type ``FIP effect'' was found for the otherelements. According to the derived models the X-ray luminosity in theenergy range 0.3-10 keV is Lx = 3.2 x 1031 ergs-1 at a distance of 132 pc. The sensitivity of the He-likeforbidden and intercombination lines to a strong ultraviolet radiationfield is used to derive upper limits to the radial distances at whichlines of Mg XI, Ne IX, O VII, and N VI originate. The results suggestthat the soft X-rays (la 8 MK) originate from shocks low in the windthat are produced by the common mechanism of radiation line-driveninstabilities. This is consistent with the observed emission lineprofiles that are much narrower (la 500 km s-1) than thebroad lines (up to 1500 km s-1) observed high up in the windof zeta Puppis. The hot ( ~ 20-40 MK) component may be explained by amodel involving dense clumps embedded in a wind which sweeps past themat high relative velocity ( ~ 1400-1700 km s-1). Such aninteraction would produce the strong shocks required.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).
| Deep H? survey of XMM-Newton galactic fields
Not Available
| The source content of low galactic latitude XMM-Newton surveys
We present results from a project conducted by the Survey Science Centerof the XMM-Newton satellite and aiming at the identification andcharacterisation of serendipitous EPIC sources at low galacticlatitudes. Deep multi-colour optical imaging and spectroscopicobservations have been obtained in the framework of several observingcampaigns carried out at ING, CFHT and ESO. These observations have leadto a number of optical identifications, mostly with active stars. Wedescribe the identified source content at low galactic latitudes andcompare stellar populations properties at low and high galacticlatitudes with those expected from stellar X-ray count models.
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Osservazione e dati astrometrici
| Costellazione: | Sagittario |
| Ascensione retta: | 18h07m56.96s |
| Declinazione: | -19°23'56.9" |
| Magnitudine apparente: | 9.768 |
| Moto proprio RA: | 2.1 |
| Moto proprio Dec: | -2.8 |
| B-T magnitude: | 10.574 |
| V-T magnitude: | 9.835 |
Cataloghi e designazioni:
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