Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| A census of the Carina Nebula - I. Cumulative energy input from massive stars
The Carina Nebula (NGC 3372) is our richest nearby laboratory in whichto study feedback through ultraviolet radiation and stellar winds fromvery massive stars during the formation of an OB association, at anearly phase in the evolution of the surrounding proto-superbubble beforesupernova explosions have influenced the environment. This feedback istriggering successive generations of new star formation around theperiphery of the nebula, while simultaneously evaporating the gas anddust reservoirs out of which young stars are trying to accrete material.This paper takes inventory of the combined effect from all the knownmassive stars that power the Carina Nebula through their total ionizingflux and integrated mechanical energy from their stellar winds. Carinais close enough and accessible enough that spectral types for individualstars are available, and many close binary and multiple systems haverecently been spatially resolved, so that one can simply add them.Adopting values from the literature for corresponding spectral types,the present-day total ionizing photon luminosity produced by the 65 Ostars and three WNL stars in Carina is QH~=1051s-1, the total bolometric luminosity of allstars earlier than B2 is 2.5 × 107Lsolar,and the total mechanical luminosity of stellar winds is LSW~=105Lsolar. The total QH was about 25per cent higher when η Carinae was on the main sequence, before itand its companion were surrounded by its obscuring dust shell; for thefirst 3Myr, the net ionizing flux of the 70 O stars in Carina was about150 times greater than in the Orion Nebula. About400-500Msolar has been contributed to the HII region bystellar wind mass-loss during the past 3Myr. Values for QHand LSW are also given for the individual clusters Tr14, 15and 16, and Bo10 and 11, which are more relevant on smaller spatialscales than the total values for the whole nebula.
| Wolf-Rayet and O star runaway populations from supernovae
We present numerical simulations of the runaway fractions expectedamongst O and Wolf-Rayet star populations resulting from stars ejectedfrom binaries by the supernova of the companion. Observationally, therunaway fraction for both types of star is similar, prompting theexplanation that close dynamical interactions are the main cause ofthese high-velocity stars. We show that, provided that the initialbinary fraction is high, a scenario in which two-thirds of massiverunaways are from supernovae is consistent with these observations. Ourmodels also predict a low frequency of runaways with neutron starcompanions and a very low fraction of observable Wolf-Rayet-compactcompanion systems.
| 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 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).
| Anomalous dust-to-gas ratios in the Galaxy
Lines of sight with E(B-V)/N(HI) considerably smaller than the averagevalue for the solar neighbourhood have been selected from the catalogueof Diplas & Savage. In order to develop quantitative considerations,estimates of the molecular hydrogen column density were obtained usingthe relation of Savage et al. extended at E(B-V) > 0.4 with therecent data of Rachford et al. Contrary to the prevailing opinion in theliterature for sightlines with similar behaviour, we found that only 22per cent of our sample was characterized by both an average gas densitylarger than 1 cm-3 and a value of RV larger thanthat in the diffuse interstellar medium. By computing extinction models,we were able to fit the E(B-V)/N(HI) by changing the value ofRV only for some sightlines. For the remaining ones, aρd/ρH ratio different from the averageGalactic value must be invoked. The application of the Kramers-Kronigrelation to the observed extinction curves confirmed this possibility.Moreover, attempts to fit such curves with models having grain volumescorresponding to the standard ρd/ρH ratiofailed.We find a linear relation between ρd/ρHand E(B-V)/N(H) for our sightlines. The average Galactic value marks theseparation into two groups characterized by lower abundances of C and Sitrapped into the grains when E(B-V)/N(H) is smaller than the Galacticvalue, and by larger abundances when E(B-V)/N(H) is greater.
| An Atlas of Far-Ultraviolet Spectra of Wolf-Rayet Stars from the FUSE Satellite
We present an atlas of far-ultraviolet spectra of 21 Wolf-Rayet (WR)stars in the Galaxy and Large and Small Magellanic Clouds, secured withthe Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The datacover the wavelength range of 912-1190 Å at a spectral resolutionof 0.1 Å and span examples of most subtypes in the WN and WCsequences. We discuss the FUV spectral morphology of the different WRsequences, emphasizing the wide range of ions and chemical speciesexhibiting well-developed P Cygni profiles and emission lines in thiswavelength range. For WN stars the relative strengths of C IV/C III, NIII/N II, P V/P IV, and Si IV/Si III show a decrease in strength of thehigh ions from WN3 to WN11 complemented by an increase in the lower ionsat later types. The ``super ions'' of O VI and S VI are consideredphotoionized wind features for WN3-WN6 stars, probably the result ofAuger ionization in WN7-WN9 stars, and probably absent at WN10-WN11. TheWN5h star Sk 41 in the SMC shows relatively weaker features, which canbe ascribed to the effects of a global galaxy metal deficiency. For theWC stars, a similar pattern of wind ionization-linked strengths in theemissions and P Cygni profiles is present, particularly evident in therelative strengths of lines in P V, S IV, Si IV, and Si III. O VI, and SVI features are only seen in the earliest WC subtypes. The high carbonabundance in WC stars is reflected by the presence of strong C IV and CIII lines throughout the sequence. We present new estimates of the windterminal velocities from measurements of saturated absorption componentsobserved in a wide range of I.P. species. Considerable revisions tov&infy; for the WN3 and WN5 (SMC) stars in our sample and,in particular for the WN10 and WN11 stars are found. The latter make useof the unique availability of the N II resonance line in the FUSEwaveband.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| 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.
| WR 20a Is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System
We present a high-precision I-band light curve for the Wolf-Rayet binaryWR 20a, obtained as a subproject of the Optical Gravitational LensingExperiment. Rauw et al. have recently presented spectroscopy for thissystem, strongly suggesting extremely large minimum masses of 70.7+/-4.0and 68.8+/-3.8 Msolar for the component stars of the system,with the exact values depending strongly on the period of the system. Wedetect deep eclipses of about 0.4 mag in the light curve of WR 20a,confirming and refining the suspected period of P=3.686 days andderiving an inclination angle of i=74.5d+/-2.0d. Using these photometricdata and the radial velocity data of Rauw et al., we derive the massesfor the two components of WR 20a to be 83.0+/-5.0 and 82.0+/-5.0Msolar. Therefore, WR 20a is confirmed to consist of twoextremely massive stars and to be the most massive binary known with anaccurate mass determination.Based on observations obtained with the 1.3 m Warsaw telescope at LasCampanas Observatory, which is operated by the Carnegie Institute ofWashington.
| Hubble Space Telescope NICMOS Variability Study of Massive Stars in the Young Dense Galactic Starburst NGC 3603
We have used the relatively long data string of the 1997-1999 NICMOSfocus tests on NGC 3603 to extract ~J-band light curves for severalhundred stars in the cluster core. Given the relatively modestphotometric precision [σ(J)>=0.05 mag], we were able to isolateonly a half-dozen variable candidates with peak-to-valley amplitudesabove ~0.2 mag. One of the variables is one of the two outstandinglybrightest cluster members, A1, located in the very dense cluster center.A1 shows double eclipses on each orbital cycle, with the same period(P=3.7724 days) as found previously and independently in unresolvedground-based radial velocity variations of the Wolf-Rayet (WR) emissioncomponent in the central core of NGC 3603. Very rough best estimates forthe masses of the components of A1 are in the range 30-90Msolar for the brighter and more massive H-rich WR component(WN6ha) and 25-50 Msolar for its assumed O star companion. Amore detailed study is urgently needed, given the potential for thisextremely luminous system to harbor the most massive main-sequence starever ``weighed.'' Another variable, HST 12, escaped the original search,which was based on larger than average standard deviation. It is aprobable field-star eclipsing variable with a moderately long period.
| Resolving OB Systems in the Carina Nebula with the Hubble Space Telescope Fine Guidance Sensor
We observed 23 OB stars in the Carina Nebula (NGC 3372) with the HubbleSpace Telescope's Fine Guidance Sensor 1r (FGS1r) in its high angularresolution mode. Five of these OB stars are newly resolved binaries withprojected separations ranging from 0.015" to 0.352" (37 to 880 AU at adistance of 2.5 kpc), and V-band magnitude differences ranging from 0.9to 2.8. The most important astrophysical result is the unexpectedresolution of the prototype O2 If* star HD 93129A as a 55 milliarcsecond(mas) double with a ΔmV of 0.9. This object has servedas a spectroscopic benchmark for the analysis of the most massive hotstars and their winds on the prior assumption that it is a single star.This discovery supports the interpretation of recent radio and X-rayobservations as evidence of colliding-wind phenomena in HD 93129A.Another interesting result is the determination of an upper limit ofabout 35 AU for the projected separation of the binary pairs in thehierarchical double spectroscopic binary HD 93206. The high incidence ofresolved binaries provides motivation for a more thorough, statisticallymeaningful study of multiplicity among the most massive stars in theyoung ionizing clusters of the nebula to obtain a complete sample of thelong-period systems that have evaded spectroscopic detection. However,considering that the nine spectroscopic binaries with accurate orbits inthe Carina Nebula have orbital dimensions <~1 AU, which at a distanceof 2.5 kpc subtends an angle of only 0.4 mas, well below the ~=10 masangular resolution of FGS1r, there remains a significant range oforbital periods and separations over which it is very difficult todetect multiplicity in the nebula with currently available instruments.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| 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.
| WR 20a: A massive cornerstone binary system comprising two extreme early-type stars
We analyse spectroscopic observations of WR 20a revealing that this staris a massive early-type binary system with a most probable orbitalperiod of 3.675 days. Our spectra indicate that both components aremost likely of WN6ha or O3If*/WN6ha spectral type. Theorbital solution for a period of 3.675 days yields extremely largeminimum masses of 70.7 ± 4.0 and 68.8 ± 3.8Mȯ for the two stars. These properties make WR 20a acornerstone system for the study of massive star evolution.Based on observations collected at the European Southern Observatory (LaSilla, Chile).
| XMM-Newton X-ray observations of the Carina nebula
We use new XMM-Newton observations to perform a detailed X-ray analysisof the Carina nebula region in the 0.3-12 keV energy range. Our sourcedetection yields 80 discrete X-ray sources, from which about 20 per centseem not to have optical counterparts. To get an idea of the energyspectrum of these sources, we construct an X-ray colour-colour diagramusing the energy bands 0.3-2, 2-4.5 and 4.5-12 keV. We analyse thespectra of the most intense X-ray sources associated with early-typestars, including the luminous blue variable η Carina and WR25. Weshow that the X-ray emission from these sources is well fitted bymultitemperature model spectra. We detect surprisingly intense X-rayemission at energies above 4 keV for some of the observed early-typestars, especially from CPD-59 2629 (Tr16-22) which presents particularlyhard X-ray emission. We detect intense soft X-ray emission, below <2keV, in HDE 303311, which presents an X-ray excess of about 100 timeshigher than has been observed in other O5V stars. We use these data toconstruct the Lx/Lbol relation for the 0.3-12 and3.0-12 keV energy ranges, for all the observed O-type stars, plus ηCarina and WR25. Most of the bright stars seem to agree with lowmetallicity spectral models. The Lx/Lbol ratio forO-type stars in the 0.3-12 keV range is well fitted by a constant~6.07.54.8× 10-7, in fairagreement with the canonical expression Lx/Lbol~ 2× 10-7 formerly estimated for the 0.3-2.4 keV energyband. In contrast, the Lx/Lbol relation for the3.0-12 keV range presents a strong deviation from the canonicalrelation, with a high dispersion of about four orders of magnitude. Wealso detect intrinsic X-ray time variability in seven sources, over thetime-scale of about 50 h covered by the observations. This includes anX-ray flare of about 2-h duration detected in DETWC Tr16J104429.2-594143, a source probably not physically associated with theCarina nebula. We discuss the different underlying physical mechanismsthat can be responsible for the X-ray emission from early-type stars.
| Preliminary results from XMM-Newton observations of the massive Wolf-Rayet binary WR 22 (WN7h+O9III-V)
We present preliminary results from observations of the massive WR+Obinary WR 22 performed with the XMM observatory. The star is brighterin the X-ray domain near apastron and fainter at periastron. The changein brightness is very progressive and mainly corresponds to a loweringof the soft part of the spectrum.
| Wolf-Rayet Stars, Black Holes, and Gamma-Ray Bursters in Close Binaries
We consider the evolutionary status of observed close binary systemscontaining black holes and Wolf-Rayet (WR) stars. When the componentmasses and the orbital period of a system are known, the reason for theformation of a WR star in an initial massive system of two main-sequencestars can be established. Such WR stars can form due to the action ofthe stellar wind from a massive OB star (M OB≥50M ȯ),conservative mass transfer between components with close initial masses,or the loss of the common envelope in a system with a large (up to25) initial component mass ratio. The strong impact ofobservational selection effects on the creation of samples of closebinaries with black holes and WR stars is demonstrated. We estimatetheoretical mass-loss rates for WR stars, which are essential for ourunderstanding the observed ratio of the numbers of carbon and nitrogenWR stars in the Galaxy . We also estimate the minimum initial masses ofthe components in close binaries producing black holes and WR stars tobe 25M ȯ. The spatial velocities of systems with black holesindicate that, during the formation of a black hole from a WR star, themass loss reaches at least several solar masses. The rate of formationof rapidly rotating Kerr black holes in close binaries in the Galaxy is3×10-6 yr-1. Their formation may be accompanied by a burst ofgamma radiation, possibly providing clues to the nature of gamma-raybursts. The initial distribution of the component mass ratios for closebinaries is dNdq=dM 2/M 1 in the interval 0.04≲q 0≤1,suggesting a single mechanism for their formation.
| Evolution of Wolf-Rayet Stars in Binary Systems: An Analysis of the Mass and Orbital-Eccentricity Distributions
We have undertaken a statistical study of the component mass ratios andthe orbital eccentricities of WR + O close binary, detachedmain-sequence (DMS), contact early-type (CE), and semidetached (SD)systems. A comparison of the characteristics of WR + O systems and ofDMS, CE, and SD systems has enabled us to draw certain conclusions aboutthe evolutionary paths of WR + O binaries and to demonstrate that up to90% of all known WR + O binaries formed as a result of mass transfer inmassive close O + O binary systems. Since there is a clear correlationbetween the component masses in SD systems with subgiants, the absenceof an anticorrelation between the masses of the WR stars and O stars inWR + O binaries cannot be considered evidence against the formation ofWR + O binaries via mass transfer. The spectroscopic transitionalorbital period P tr sp corresponding to the transition from nearlycircular orbits (e sp<0.1) to elliptical orbits (e sp≥0.1) is14d for WR + O systems and 2d 3d for OB + OB systems. Theperiod range in which all WR + O orbits are circular &$(1mathop dlimits_. 6 ≤slant P ≤slant 14(d) ); is close to the range for SD systems with subgiants, &0mathop dlimits_. 7 ≤slant P ≤slant 15(d); . The large difference between the P tr sp values for WR + O and OB +OB systems suggests that a mechanism of orbit circularization additionalto that for OB + OB systems at the DMS stage (tidal dissipation of theorbital energy due to radiative damping of the dynamical tides) acts inWR + O binaries. It is natural to suggest mass transfer in the parent O+ O binaries as this supplementary orbit-circularization mechanism.Since the transitional period between circular and elliptical orbits forclose binaries with convective envelopes and ages of 5×109 yearsis &P_{tr} = 12mathop dlimits_. 4$; , the orbits of most known SD systems with subgiants had enough timeto circularize during the DMS stage, prior to the mass transfer. Thus,for most SD systems, mass transfer plays a secondary role incircularization of their orbits. In many cases, the initial orbitaleccentricities of the O + O binary progenitors of WR + O systems arepreserved, due to the low viscosity of the O-star envelopes and theshort timescale for their nuclear evolution until the primary O starfills its Roche lobe and the mass transfer begins. The mass transfer inthe parent O + O systems is short-lived, and the number of orbitalcycles during the early mass-transfer stage is relatively low (lowerthan for the progenitors of SD systems by three or four orders ofmagnitude). The continued transfer of mass from the less massive to themore massive star after the component masses have become equal leads tothe formation of a WR + O system, and the orbit's residual eccentricityincreases to the observed value. The increase of the orbitaleccentricity is also facilitated by variable radial mass loss via thewind from the WR star in the WR + O system during its motion in theelliptical orbit. The result is that WR + O binaries can haveconsiderable orbital eccentricities, despite their intense masstransfer. For this reason, the presence of appreciable eccentricitiesamong WR + O binaries with large orbital periods cannot be consideredfirm evidence against mass transfer in the parent O + O binary systems.Only for the WR + O binaries with the longest orbital periods (4 of 35known systems, or 11 %) can the evolution of the parent O + O binariesoccur without filling of the Roche lobe by the primary O star, beinggoverned by radial outflow in the form of the stellar wind and possiblyby the LBV phenomenon, as in the case of HD 5980.
| X-Ray Spectroscopy of η Carinae with XMM-Newton
We present XMM-Newton observations of the luminous star η Carinae,including a high-resolution soft X-ray spectrum of the surroundingnebula obtained with the Reflection Grating Spectrometer (RGS). TheEuropean Photon Imaging Camera (EPIC) image of the field around ηCar shows many early-type stars and diffuse emission from hot, shockedgas. The EPIC spectrum of the star is similar to that observed inprevious X-ray observations and requires two temperature components. TheRGS spectrum of the surrounding nebula shows K-shell emission lines fromhydrogen- and helium-like nitrogen and neon and L-shell lines from ironbut little or no emission from oxygen. The observed emission lines arenot consistent with a single temperature, but the range of temperaturesobserved is not large, spanning ~0.15-0.6 keV. We obtain upper limitsfor oxygen line emission and derive a lower limit of N/O>9. This isconsistent with previous abundance determinations for the ejecta ofη Car and with theoretical models for the evolution of massive,rotating stars.
| 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).
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| The behavior of the NIV 5203.2 Å line in stars with Wolf-Rayet and early O-type spectra
Not Available
| The massive Wolf-Rayet binary SMC WR7
We present a study of optical spectra of the Wolf-Rayet star AzV 336a(=SMC WR7) in the Small Magellanic Cloud. Our study is based on dataobtained at several Observatories between 1988 and 2001. We find SMC WR7to be a double-lined WN+O6 spectroscopic binary with an orbital periodof 19.56d. The radial velocities of the He absorption lines of the O6component and the strong HeII emission at λ4686Å of the WNcomponent describe anti-phased orbital motions. However, they show asmall phase shift of ~1d. We discuss possible explanations for thisphase shift. The amplitude of the radial velocity variations of HeIIemission is twice that of the absorption lines. The binary componentshave fairly high minimum masses, ~18 and 34Msolar for the WNand O6 components, respectively.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| A short observational history of X-ray studies of Wolf-Rayet stars.
Not Available
| Kinematical Structure of Wolf-Rayet Winds. I.Terminal Wind Velocity
New terminal wind velocities for 164 Wolf-Rayet stars (from the Galaxyand LMC) based on PCyg profiles of lambda1550 CIV resonance line werederived from the archive high and low resolution IUE spectra availableform the INES database. The high resolution data on 59 WR stars (39 fromthe Galaxy and 20 from LMC) were used to calibrate the empiricalrelation lambda_min^Abs- lambda_peak^Emis vs terminal wind velocity,which was then used for determinations of the terminal wind velocitiesfrom the low resolution IUE data. We almost doubled the previous mostextended sample of such measurements. Our new measurements, based onhigh resolution data, are precise within 5-7%. Measurements, based onthe low resolution spectra have the formal errors of approx 40-60%. Acomparison of the present results with other determinations suggestshigher precision of approx 20%. We found that the terminal windvelocities for the Galactic WC and WN stars correlate with the WRspectral subtype. We also found that the LMC WN stars have winds slowerthan their Galactic counterparts, up to two times in the case of the WNEstars. No influence of binarity on terminal wind velocities was found.Our extended set of measurements allowed us to test application of theradiation driven wind theory to the WR stars. We found that, contrary toOB stars, terminal wind velocities of the WR stars correlate only weaklywith stellar temperature. We also note that the terminal to escapevelocity ratio for the WR stars is relatively low: 2.55 pm 1.14 for theGalactic WN stars and 1.78 pm 0.70 for the Galactic WCs. This ratiodecreases with temperature of WR stars, contrary to what is observed inthe case of OB stars. The presented results show complex influence ofchemical composition on the WR winds driving mechanism efficiency. Ourkinematical data on WR winds suggest evolutionary sequence: WNL -->WNE --> WCE --> WCL.
| The evolved early-type binary HDE 228766 revisited
We use an extensive set of spectroscopic observations to reinvestigatethe properties of the massive binary HDE 228766. Conventionalclassification criteria suggest that HDE 228766 consists of an O7primary and an Of+ secondary. However, several spectralfeatures of the secondary, such as the simultaneous presence of N III, NIV and N V emissions, make it a rather unusual object. We find that theorbital motion of the secondary is probably best described by the radialvelocities of the narrow N III emission lines. Our orbital solutionyields m1 sin 3i = 31.7 and m2 sin3i = 25.5 Msun for the primary and secondaryrespectively. The He II lambda 4542 absorption in the secondary'sspectrum appears considerably blue-shifted with respect to the narrowemission lines, indicating that the absorption is probably formed in theaccelerating part of the secondary's wind. We use a tomographictechnique to investigate the profile variability of the broader emissionlines. In addition to a strong emission from the secondary, the Hαline displays a weak emission feature that is probably associated with awind interaction region located near the surface of the primary star.Finally, our analysis of the spectrum with a non-LTE code indicates thatthe secondary is an evolved object that exhibits some CNO processedmaterial in its atmosphere and has a large mass loss rate. Assuming adistance of 3.5 kpc (which follows from adopting MV, s = -6for the secondary) we infer dot {M} =~ 10-5 Msunyr-1 for the secondary. Our results suggest that HDE 228766could be in a post-Roche lobe overflow stage. The secondary may beclassified as WN8ha and is currently in a transition stage between a``normal'' Of star and a WNL-type Wolf-Rayet star. Based on observationscollected at the Observatoire de Haute Provence (France) and theObservatorio Astronómico Nacional of San Pedro Mártir(Mexico).
| The mass-loss rates of Wolf-Rayet stars explained by optically thick radiation driven wind models
Observed, clumping-corrected mass-loss rates of Galactic Wolf-Rayet (WR)stars are compared with predictions of the optically thick radiationdriven wind models. We did not develop models for the whole wind, but westudied the conditions at the sonic point that would explain theobserved high mass-loss rates of WR-stars. We find that optically thickwind models can explain the observed values of the mass-loss rates onlyif two conditions are satisfied: (a) The sonic point (wherevflow=vsound) lies deep in the wind where thetemperature is either near 160 000 K, or in the range of 40 000 to 70000 K. (b) The flux-mean opacity must increase outward from the sonicpoint. With these conditions a simple approximate formula for themass-loss rates of WR-stars can be derived. The first condition impliesthat the sonic point is at an optical depth between about 3 and 30. Suchlarge optical depths require a slowly increasing velocity law in thesupersonic region, with a velocity-law index of beta =~ 5 for WR-stars,compared to beta =~ 1 for O-stars. The OPAL-opacity tables for thechemical composition of the WR-stars show that the opacity indeedincreases outward at the temperature range near 1.6 x 105 K,and between about 4 x 107 and 7 x 104 K, asrequired for the optically thick wind models. The opacity at the sonicpoints of the models is very similar to the OPAL-opacity at the sonicpoint temperature and density. The radius of the sonic point is abouthalf as large as the inner boundaries of the ``standard'' models forearly type WR-winds. Observational evidence, derived from line profilevariations and from the light-curves of WR-stars in eclipsing binarysystems, support the derived large values of beta and the small valuesof the sonic point radius. The models presented here show that the highmass-loss rates of WR-stars might be the result of optically thickradiation driven winds. The presence of two very distinct temperatureregimes for the sonic point implies a bifurcation in the wind models ofWR-stars.
| A Short History and Other Stories of Binary Stars
A short history of binary stars from the first discoveries is presented.Examples of currently known binary systems with components of highesttemperature and mass are discussed.
| The VIIth catalogue of galactic Wolf-Rayet stars
The VIIth catalogue of galactic PopulationI Wolf-Rayet stars providesimproved coordinates, spectral types and /bv photometry of known WRstars and adds 71 new WR stars to the previous WR catalogue. This censusof galactic WR stars reaches 227 stars, comprising 127 WN stars, 87 WCstars, 10 WN/WC stars and 3 WO stars. This includes 15 WNL and 11 WCLstars within 30 pc of the Galactic Center. We compile and discuss WRspectral classification, variability, periodicity, binarity, terminalwind velocities, correlation with open clusters and OB associations, andcorrelation with Hi bubbles, Hii regions and ring nebulae. Intrinsiccolours and absolute visual magnitudes per subtype are re-assessed for are-determination of optical photometric distances and galacticdistribution of WR stars. In the solar neighbourhood we find projectedon the galactic plane a surface density of 3.3 WR stars perkpc2, with a WC/WN number ratio of 1.5, and a WR binaryfrequency (including probable binaries) of 39%. The galactocentricdistance (RWR) distribution per subtype shows RWRincreasing with decreasing WR subtype, both for the WN and WC subtypes.This RWR distribution allows for the possibility ofWNE-->WCE and WNL-->WCL subtype evolution.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Carena |
| Ascensione retta: | 10h41m17.60s |
| Declinazione: | -59°40'37.0" |
| Magnitudine apparente: | 6.42 |
| Distanza: | 1639.344 parsec |
| Moto proprio RA: | -6.5 |
| Moto proprio Dec: | 4 |
| B-T magnitude: | 6.472 |
| V-T magnitude: | 6.401 |
Cataloghi e designazioni:
|
INHABITED SKY
