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Resim Yükleyin
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Ýlgili Makaleler
The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| 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.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| ESO imaging survey: optical follow-up of 12 selected XMM-Newton fields
This paper presents the data recently released for the XMM-Newton/WFIsurvey carried out as part of the ESO Imaging Survey (EIS) project. Theaim of this survey is to provide optical imaging follow-up data in BVRIfor identification of serendipitously detected X-ray sources in selectedXMM-Newton fields. In this paper, fully calibrated individual andstacked images of 12 fields as well as science-grade catalogs for the 8fields located at high-galactic latitude are presented. These productswere created, calibrated and released using the infrastructure providedby the EIS Data Reduction system and its associated EIS/MVM imageprocessing engine, both of which are briefly described here. The datacovers an area of ~3 square degrees for each of the four passbands. Themedian seeing as measured in the final stacked images is 0.94 arcsec,ranging from 0.60 arcsec and 1.51 arcsec. The median limiting magnitudes(AB system, 2´´ aperture, 5σ detection limit) are25.20, 24.92, 24.66, and 24.39 mag for B-, V-, R-, and I-band,respectively. When only the 8 high-galactic latitude fields are includedthese become 25.33, 25.05, 25.36, and 24.58 mag, in good agreement withthe planned depth of the survey. Visual inspection of images andcatalogs, comparison of statistics derived from the present data withthose obtained by other authors and model predictions, as well as directcomparison of the results obtained from independent reductions of thesame data, demonstrate the science-grade quality of the automaticallyproduced final images and catalogs. These survey products, together withtheir logs, are available to the community for science exploitation inconjunction with their X-ray counterparts. Preliminary results from theX-ray/optical cross-correlation analysis show that about 61% of thedetected X-ray point sources in deep XMM-Newton exposures have at leastone optical counterpart within 2´´ radius down to R ≃25 mag, 50% of which are so faint as to require VLT observations therebymeeting one of the top requirements of the survey, namely to producelarge samples for spectroscopic follow-up with the VLT, whereas only 15%of the objects have counterparts down to the DSS limiting magnitude.
| The qWR star HD 45166 . I. Observations and system parameters
The binary star HD 45166 has been observed since 1922but its orbital period has not yet been found. It is considered apeculiar Wolf-Rayet star, and its assigned classification has varied.For this reason we included the object as a candidate V Sge star andperformed spectroscopy in order to search for its putative orbitalperiod. High-resolution spectroscopic observations show that thespectrum, in emission and in absorption, is quite rich. The emissionlines have great diversity of widths and profiles. The full widths athalf maximum vary from 70 km s-1 for the weakest lines up to370 km s-1 for the most intense ones. The hydrogen and heliumlines are systematically broader than the CNO lines. Assuming thatHD 45166 is a double-line spectroscopic binary, itpresents an orbital period of P = 1.596 ± 0.003 day, with aneccentricity of e = 0.18 ± 0.08. In addition, a search forperiodicity using standard techniques reveals that the emission linespresent at least two other periods, of 5 and 15 h. The secondary starhas a spectral type of B7 V and, therefore, should have a mass of aboutM2 = 4.8 ~Mȯ. Given the radial velocityamplitudes, we determined the mass of the hot (primary) star asM1 = 4.2 ± 0.7~Mȯ and the inclinationangle of the system, i = 0.77° ± 0.09°. As theeccentricity of the orbit is non zero, the Roche lobes increase anddecrease as a function of the orbital phase. At periastron, thesecondary star fills its Roche lobe. The distance to the star has beenre-determined as d = 1.3 ± 0.2 kpc and a color excess ofE(B-V)=0.155~±~0.007 has been derived. This implies an absolute Bmagnitude of -0.6 for the primary star and -0.7 for the B7 star. Wesuggest that the discrete absorption components (DACs) observed in theultraviolet with a periodicity similar to the orbital period may beinduced by periastron events.Based on observations made at the 1.5 m ESO telescope at La Silla, Chile, and at Laboratório Nacional de Astrofísica/CNPq, Brazil.Tables 2-5 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/444/895
| Cygnus X-3 and the problem of the missing Wolf-Rayet X-ray binaries
Cygnus X-3 is a strong X-ray source (LX ≈ 1038erg s-1) which is thought to consist of a compact objectaccreting matter from a helium star. We analytically find that theestimated ranges of mass-loss rate and orbital-period derivative for CygX-3 are consistent with two models: i) the system is detached and themass loss from the system comes from the stellar wind of a massivehelium star, of which only a fraction that allows for the observed X-rayluminosity is accreted, or ii) the system is semidetached and aRoche-lobe-overflowing low- or moderate-mass helium donor transfers massto the compact object, followed by ejection of its excess over theEddington rate from the system. These analytical results appear to beconsistent with evolutionary calculations. By means of populationsynthesis we find that currently in the Galaxy there may exist ~1 X-raybinary with a black hole that accretes from a ⪆7~M_ȯ Wolf-Rayetstar and ~1 X-ray binary in which a neutron star accretes matter from aRoche-lobe-overflowing helium star with mass ⪉1.5~M_ȯ. Cyg X-3is probably one of these 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.
| 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.
| Observations of the Diffuse Far-Ultraviolet Background with the Far Ultraviolet Spectroscopic Explorer
We have used observations taken under the Far Ultraviolet SpectroscopicExplorer (FUSE) S405/505 channel realignment program to explore thediffuse far-ultraviolet (FUV; 1000-1200 Å) radiation field. Of the71 independent locations in that program, we have observed a diffusesignal in 32, ranging in brightness from 1600 to a maximum of2.9×105 photons cm-2 sr-1s-1 Å-1 in Orion. The FUSE data confirm thatthe diffuse FUV sky is patchy with regions of intense emission, usuallynear bright stars, but also with dark regions, even at low Galacticlatitudes. We find a weak correlation between the FUV flux and the 100μm ratio but with wide variations, perhaps due to differences in thelocal radiation field.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.
| 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.
| Hydrogen in the atmosphere of the evolved WN3 Wolf-Rayet star WR 3: defying an evolutionary paradigm?
WR 3 is the brightest very early-type WN star in the sky. Based onseveral years of time-resolved spectroscopy and precision photometry onvarious time-scales, we deduce that WR 3 is most likely a single,weak-lined star of type WN3ha (contrary to its current catalogue-type ofWN3 + O4), with H lines occurring both in emission and absorption in itswind. This conclusion is confirmed and strengthened via detailedmodelling of the spectrum of WR 3. Given the similarity of WR 3 withnumerous H-rich WNE stars in the Large Magellanic Cloud and especiallythe Small Magellanic Cloud, and its location towards the metal-deficientexterior of the Galaxy, we conclude that rotationally induced meridionalcirculation probably led to the apparently unusual formation of this hotGalactic WN star with enhanced hydrogen. Although we cannot completelyrule out the possibility of a binary with a low orbital inclinationand/or long period, we regard this latter possibility as highlyunlikely.
| Magnetorotational Supernova Explosions and the Formation of Neutron Stars in Close Binary Systems
The formation of neutron stars in the closest binary systems (Porb<12 h) gives the young neutron star/pulsar a high rotationalvelocity and energy. The presence of a magnetic field of 3×10113×1013 G, as is observed for radio pulsars, enables the neutronstar to transfer 1051 erg of its rotational energy to the envelopeover a time scale of less than an hour, leading to a magnetorotationalsupernova explosion. Estimates indicate that about 30% of all type-Ib,csupernovae may be the products of magnetorotational explosions. Youngpulsars produced by such supernovae should exhibit comparatively slowrotation (P rot>0.01 s), since a large fraction of their rotationalangular momentum is lost during the explosion. The magnetorotationalmechanism for the ejection of the envelope is also reflected by theshape of the envelope. It is possible that the Crab radio pulsar is anexample of a product of a magnetorotational supernova. A possiblescenario for the formation of the close binary radio pulsar discoveredrecently by Lyne et al. is considered.
| WX Cen (≡ WR 48c) - a possible Type Ia supernova progenitor
We confirm the orbital period of WX Cen ≡ WR 48c determined byDiaz & Steiner and refined its value to Porb= 0.4169615(+/-22) d. The light curve of this object has a peak-to-peak variationof approximately 0.32 mag. It is non-sinusoidal in the sense that it hasa V-shaped narrow minimum, similar to the ones seen in V Sge, V617 Sgrand in compact binary supersoft sources (CBSS).Most of the emission lines in the optical spectrum are due to Balmer,HeII, CIV, NV, OV and OVI. An analysis of the HeII Pickering seriesdecrement shows that the system has significant amount of hydrogen. Theemission lines of HeII 4686 Å became weaker between the 1991 and2000/2002 observations, indicating distinct levels of activity. Thespectra of WX Cen show variable absorption features in the Balmer lineswith V=-2900 km s-1 and in emission with V=+/-3500 kms-1. These highly variable features remind us of thesatellite emission lines found in the spectra of CBSS.We estimate the colour excess as E(B-V) = 0.63 on the basis of theobserved diffuse interstellar band at 5780 Å. Given thedistance-colour excess relation in the direction of WX Cen, this impliesa distance of 2.8 +/- 0.3 kpc. Interstellar absorption of the NaI Dlines show components at -4.1 km s-1, which corresponds tothe velocity of the Coalsack, and three other components at -23.9, -32.0and -39.0 km s-1. These components are also seen with similarstrengths in field stars that have distances between 1.8 and 2.7 kpc.The intrinsic colour of WX Cen is (B-V)0=-0.2 and theabsolute magnitude is MV=-0.5.Extended red wings in the strong emission lines are seen. A possibleexplanation is that the system has a spill-over stream similar to whatis seen in V617 Sgr. We predict that when observed in opposite phase,blue wings would be observed. A puzzling feature that remains to beexplained is the highly variable red wing (V~ 700 km s-1) ofthe OVI emission lines as well as of the red wings of the H and Helines.The velocity of the satellite-like feature is consistent with the ideathat the central star is a white dwarf with a mass of M~ 0.9Msolar. With the high accretion rate under consideration, thestar may become a Type Ia supernova (SN Ia) in a time-scale of 5 ×106 yr.
| The Multiple Spectroscopic and Photometric Periods of DI Crucis (WR 46)
In an effort to determine the orbital period of the enigmatic star DICru (HD 104994, WR 46), we made photometric and spectroscopicobservations of this object between 1996 and 2002. Both photometric andspectroscopic characteristics are quite complex. The star is highlyvariable on short (few hour) as well as on long (few month) timescales.The optical spectrum is rich in strong emission lines of high-ionizationspecies such as He II, N IV, N V, and O VI. Weak emission of C III isalso present. Emission lines have been compiled and identified from theultraviolet to the infrared. In the UV, emission of O V and N IV is alsoobserved, together with very weak emission of C IV. The N Vλλ4603, 4619/He II λ4686 line ratios vary by asignificant amount from night to night. Temporal variance spectrumanalysis shows that the He II λ4686 line has P Cyg-like variableabsorption, while N V λλ4603, 4619 lines have a strong andbroad variable component due to the continuum fluorescence from a source(stellar atmosphere/optically thick wind) of variable temperature. Wealso show that the object has variable degree of ionization, probablycaused by wind density variation. The star presents multiple periods inradial velocity and photometry. From our data we derived a main radialvelocity period of 0.3319 days with an amplitude of K=58 kms-1. This period is similar to the value found by Marchenkoand coworkers. When at intermediate brightness, this period is also seenin the photometric measurements. When the star is at bright phase, thephotometric variations do not present the same period. Photometricperiods ranging from 0.154 to 0.378 days are present, consistent withobservations reported by other authors. Besides the 0.3319 day period,other spectroscopic periods are also seen. On distinct epochs, theperiods are different; Marchenko et al. interpreted the 0.3319 dayperiod as the orbital one. Although we do not discard this possibility,the true binary nature (e.g., long-term coherence or detection of asecondary star) has not yet been demonstrated. DI Cru is a Population IWR object. Given the similarities (e.g., multiple periods likely due tononradial oscillations), it could be interpreted as a luminouscounterpart of the qWR star HD 45166.Based on observations made at the Observatório do Pico dosDias/LNA, Brazil, and at the 1.5 m ESO telescope at La Silla, Chile.
| NGC 3125-1: The Most Extreme Wolf-Rayet Star Cluster Known in the Local Universe
We use Space Telescope Imaging Spectrograph long-slit ultravioletspectroscopy of local starburst galaxies to study the massive starcontent of a representative sample of super-star clusters, with aprimary focus on their Wolf-Rayet (W-R) content as measured from the HeII λ1640 emission feature. The goals of this work are threefold.First, we quantify the W-R and O-star content for selected massive youngstar clusters. These results are compared with similar estimates madefrom optical spectroscopy available in the literature. We conclude thatthe He II λ4686 equivalent width is a poor diagnostic measure ofthe true W-R content. Second, we present the strongest known He IIλ1640 emission feature in a local starburst galaxy. This featureis clearly of stellar origin in the massive cluster NGC 3125-1, as it isbroadened (~1000 km s-1). Strong N IV λ1488 and N Vλ1720 emission lines commonly found in the spectra of individualW-R stars of WN subtype are also observed in the spectrum of NGC 3125-1.Finally, we create empirical spectral templates to gain a basicunderstanding of the recently observed strong He II λ1640 featureseen in Lyman break galaxies (LBGs) at redshifts z~3. The UV fieldobserved in local starbursts provides a good overall match to thecontinuum and weak photospheric features in LBGs in the spectral rangeλλ1300-1700 but cannot reproduce the He II λ1640emission seen in the composite LBG sample of Shapley et al. Anadditional (ad hoc) 10%-15% contribution from ``extreme'' W-R clusterssimilar to NGC 3125-1 on top of the field provides a good match to thestrength of this feature.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, Inc., under NASAcontract NAS5-26555.
| WR 7a: a V Sagittae or a qWR star?
The star WR 7a, also known as SPH 2, has a spectrum that resembles thatof V Sagittae stars although no OVI emission has been reported. TheTemporal Variance Spectrum - TVS - analysis of our data shows weak butstrongly variable emission of OVI lines which is below the noise levelin the intensity spectrum.Contrary to what is seen in V Sagittae stars, optical photometricmonitoring shows very little, if any, flickering. We found evidence ofperiodic variability. The most likely photometric period isPphot= 0.227(+/-14) d, while radial velocities suggest aperiod of Pspec= 0.204(+/-13) d. One-day aliases of theseperiods can not be ruled out. We call attention to similarities with HD45166 and DI Cru (= WR 46), where multiple periods are present. They maybe associated to the binary motion or to non-radial oscillations.In contrast to a previous conclusion by Pereira et al., we show that WR7a contains hydrogen. The spectrum of the primary star seems to bedetectable as the NV 4604Åabsorption line is visible. If so, itmeans that the wind is optically thin in the continuum and that it islikely to be a helium main sequence star.Given the similarity to HD 45166, we suggests that WR 7a may be a qWR -quasi Wolf-Rayet - star. Its classification is WN4h/CE in the Smith,Shara & Moffat three-dimensional classification system.
| Discovery of Highly Dynamic Matter Enhancements along the Polar Axis and Equatorial Plane in the Luminous Blue Variable Binary HD 5980
HD 5980 is a W-R+O eclipsing binary in the Small Magellanic Cloud thathas attracted considerable attention since it underwent a luminous bluevariable (LBV)-type outburst in 1994. Since then, intense spectroscopicand photometric monitoring have revealed the presence of periodicvariability on a timescale of 6-7 hr that cannot be explained by ourcurrent understanding of the system. In this paper, we present the firstresults from our polarimetric observations around secondary eclipse whenthe assumed Wolf-Rayet (W-R) star passes in front of the assumed LBVcomponent (phase φ=0.36). These data confirm the presence ofstochastic polarimetric variability on a typical timescale of ~30minutes, reaching a very high amplitude of almost 1% on our last of fivenights. At that time, we also found that the mass loss, which isresponsible for the polarimetric variability, presented fluctuations inaxial symmetry ranging from very rapid density enhancements along theorbital plane to polar ejections. We propose that either a fast rotatormodel for the W-R star or the presence of a close orbiting neutron starcould qualitatively explain the observed polarimetric, spectroscopic,and photometric behavior. The evidence presented in this paper iscrucial for an understanding of the erratic behavior of this importantbinary and should be taken into account in any consistent model of thesystem.
| Observed instabilities in OB and Wolf-Rayet stars
A wealth of recent observations confirms that the fast, dense winds ofOB and Wolf-Rayet (WR) stars are highly structured, with the structuresresulting from the inherently unstable nature of a radiatively-drivenwind, as well as being triggered from the underlying photosphere. Thisreview mainly addresses variability patterns observed in the winds andphotospheric regions of presumably single stars. Schematically wedivide the detectable structures into two broad categories: small-scaleand large-scale inhomogeneities, with the former mainly of a stochasticbehavior/origin, and the latter frequently demonstrating a recurrent,even periodic, behavior. We then discuss the nature and nurture ofinstabilities, highlighting phenomenological similarities anddifferences in the variability of OB and WR stars.
| Realistic ionizing fluxes for young stellar populations from 0.05 to 2 Zsolar
We present a new grid of ionizing fluxes for O and Wolf-Rayet (W-R)stars for use with evolutionary synthesis codes and single-star HIIregion analyses. A total of 230 expanding, non-LTE, line-blanketed modelatmospheres have been calculated for five metallicities (0.05, 0.2, 0.4,1 and 2 Zsolar) using the WM-BASIC code of Pauldrach,Hoffmann & Lennon for O stars and the CMFGEN code of Hillier &Miller for W-R stars. The stellar wind parameters are scaled withmetallicity for both O and W-R stars. We compare the ionizing fluxes ofthe new models with the CoStar models of Schaerer & de Koter and thepure helium W-R models of Schmutz, Leitherer & Gruenwald. We findsignificant differences, particularly above 54 eV, where the emergentflux is determined by the wind density as a function of metallicity. Thenew models have lower ionizing fluxes in the HeI continuum withimportant implications for nebular line ratios. We incorporate the newmodels into the evolutionary synthesis code STARBURST99 and compare theionizing outputs for an instantaneous burst and continuous starformation with the work of Schaerer & Vacca (SV98), and Leitherer etal. The changes in the output ionizing fluxes as a function of age aredramatic. We find that, in contrast to previous studies, nebularHeIIλ4686 will be at, or just below, the detection limit in lowmetallicity starbursts during the W-R phase. The new models have lowerfluxes in the HeI continuum for Z>= 0.4 Zsolar and ages<=7 Myr because of the increased line blanketing. We test theaccuracy of the new model atmosphere grid by constructingphotoionization models for simple HII regions, and assessing the impactof the new ionizing fluxes on important nebular diagnostic line ratios.For the case of an HII region where the ionizing flux is given by theWM-basic dwarf O star grid, we show that HeIλ5786/Hβdecreases between 1 and 2 Zsolar in a similar manner toobservations. We find that this decline is caused by the increasedeffect of line blanketing above solar metallicity. We therefore suggestthat a lowering of the upper mass limit at high abundances is notrequired to explain the diminishing strength of HeIλ5786/Hβ,as has been suggested in the past. For an HII region where the ionizingflux is provided by an instantaneous burst of total mass 106Msolar, we plot the softness parameter η' against theabundance indicator R23 for ages of 1-5 Myr. The new modelsare coincident with the observational data of Bresolin et al.,particularly during the W-R phase, unlike the previous models of SV98which generally over-predict the hardness of the ionizing radiation. Thenew model grid and updated STARBURST99 code can be downloaded fromhttp://www.star.ucl.ac.uk/starburst.
| The enigmatic WR46: A binary or a pulsator in disguise. III. Interpretation
Photometric and spectroscopic monitoring campaigns of WR46 (WN3p), as presented in Veen et al. (2002a,b; hereafterPapers I and II, respectively), yield the following results. The light-and colour variations reveal a dominant single-wave period ofPsw89=0.1412 d in 1989, andPsw91 =0.1363 d in 1991. Because of a smalldifference in the minima, this periodicity may be a double-wavephenomenon (Pdw). The line fluxes vary in concert with themagnitudes. The significant difference of the periods can be either dueto the occurence of two distinct periods, or due to a gradual change ofthe periodicity. A gradual brightening of the system of 0.12m appearedto accompany the period change. In addition, the light variations in1989 show strong evidence for an additional period Px=0.2304d. Generally, the radial velocities show a cyclic variability on a timescale of the photometric double-wave. However, often they do not vary atall. The observed variability confirms the Population I WR nature of thelight source, as noted independently by Marchenko et al. (2000). In thepresent paper, we first show how the photometric double-wave variabilitycan be interpreted as a rotating ellipsoidal density distribution in thestellar wind. Subsequently, we discuss what mechanisms could drive sucha configuration. First, stellar rotation of a single star is discardedas a likely cause. Second, the obvious interpretation of the double-wavephotometry, i.e., a close binary system, is investigated. Howeverunlikely, we discuss how the observed period change might be reconciledwithin a model of a strongly interacting binary. Third, aninterpretation of a non-radial multi-mode pulsator is investigated. Theobserved period change and the multi-frequency behaviour do support thisinterpretation. We propose that the pulsational mode l=1 and |m|=1 maymimic a ``binary'' light- and radial-velocity curve. However, thephasing of the radial velocity and the light curve may be inconsistent.The possibility l=2 and |m|=0 is also discussed. Finally, we suggest howthe enigma of the variability of WR 46 may be solved.
| The enigmatic WR46: A binary or a pulsator in disguise. II. The spectroscopy
We present spectroscopic monitoring of the Wolf-Rayet (WR) starWR 46 between 1989 and 1998, which has been obtainedsimultaneously with multicolour photometry (Veen et al. \cite{Veen02a},Paper I). The spectroscopic monitoring data show that the radiativefluxes of the optical emission lines (O Vi 3811/34, O Vi 5290, N V 4944,N V 4604/20, He Ii 4686, He Ii 4859, He Ii 5411, He Ii 6560) vary inconcert with the photometric single-wave (sw) frequency f_sw (Paper I),and also the difference of that period between 1989 and 1991. Theline-flux variability does not provide obvious support for a shortsecond period (Paper I). The radial-velocity variations show aremarkable behaviour: usually, they display a coherent single-wave onthe time scale of the double-wave period, while during some nights theradial velocity appears surprisingly to stay constant (see alsoMarchenko et al. \cite{Marchenko00}). These so-called stand-stills maybe related to the observed time-delay effects. A time-delay effectmanifests itself in several phenomena. Firstly, the line flux showssmall, but persistent, time-delays for lines originating from loweroptical depths, the outer-wind lines (N V 4604/20 and He Ii). Secondly,the radial-velocity variations display much larger time-delays than theline fluxes and their behaviour appears less consistent. Assuming thatthe double-wave period controls the radial velocity, the stand-still isobserved to start when the radial motion is in anti-phase with thepresumed orbital motion. Thirdly, the outer-wind lines are observed toenter a stand-still much later than the inner-wind lines. Fourthly, theradial-velocity variations of the peaks of the emission lines precedethe radial-velocity variations of the wings of those lines. In additionto line-flux- and radial-velocity variability, the He Ii 4686 emissionline shows pronounced line-profile changes on a time scale of hours. Ourmonitoring is not sufficient to study this in detail. Furthermore, wediscern a flaring behaviour, i.e., an emission bump appeared on the bluewing of two He Ii-lines (around -1700 km s-1) lasting lessthan 5 min. Finally, the line fluxes follow the observed brightenings,also on a time scale of years. We conclude that the short-term cyclicvariability confirms the WR nature as established from the WR standardmodel analysis by Crowther et al. (\cite{Crowther95}; hereafter referredto as CSH). The various time-delay effects are consistent with theformation of the spectrum in a stratified stellar wind. The outer layerstrail the inner ones. The variability is inconsistent with the formationof the spectrum in a stellar disc as proposed by Niemela et al.(\cite{Niemela95}) and Steiner & Diaz (\cite{Steiner98}). Thelong-term cyclic variability of the brightness and line fluxes isrelated to an increase of the mass-loss-rate, and, possibly, to theperiod changes. The interpretation of the nature of the variability isdeferred to Veen et al. (\cite{Veen02b}, Paper III). Based onobservations collected at the European Southern Observatory (ESO), LaSilla, Chile.
| The enigmatic WR46: A binary or a pulsator in disguise. I. The photometry
We discuss the observational history of the Wolf-Rayet object WR 46(WN3p), including a re-investigation of the original discovery platesfrom early this century. We find that the reported presence of N Iiilines is a mis-interpretation of N V lines and conclude that the objectdid not change its spectral type since the first recording one centuryago. We performed photometric monitoring in the period 1986-1999, andconfirm that the object shows cyclical variability on a time scale ofhours. The shape of the light curves varies from purely sinusoidal toirregular, and from an amplitude of nearly 0\fm1 to constancy. Inaddition, night-to-night variability of the mean brightness causesfolded light curves to display a large scatter. We investigate thefrequency behaviour of the photometric data. From the periodograms ofour two large data sets, in 1989 and in 1991, we identify frequencies ofsignificantly different values 7.08 cd-1 and 7.34cd-1, respectively. Moreover, the 1989 data show strongevidence for an additional frequency fx= 4.34cd-1. The periodograms of our eight smaller data sets showmore ambiguous behaviour. We discuss whether these latter data showevidence for multi-frequency behaviour, or whether they can bereconciled with a single clock with a changing clock-rate. As pointedout by van Genderen et al. (1991), if the data are folded using twicethe single-wave period, the light curves appear ellipsoidal with unequalminima. Although the difference in depth of the minima is hardlysignificant, it does occur in both large data sets. Moreover, thesimultaneously obtained radial velocity measurements are in betteragreement with the double-wave than the single-wave period (Paper II).Finally, Marchenko et al. (2000) observed WR 46 to have a single-waveperiod of the same order as the double-wave period identified here. Theperiodograms of the (V-W) colour index show that the colour changes arecontrolled by single-wave frequencies, or their sub-harmonics(double-wave periods), but not by fx. The colour variation ofWR 46 is peculiar in the sense that the object is red when bright andblue when faint. Although the spectrum of WR 46 has been suggested tooriginate from a stellar disc, this peculiar colour behaviour is in linewith its WR nature, which is also confirmed by its spectral variability(Marchenko et al. 2000; Paper II). In addition, our seasonal photometricaverages of WR 46 show a rise from 1989 to 1991 of 0.12m, confirming thebrightening detected by the Hipparcos-satellite (Marchenko et al. 1998).Eventually, WR 46 brightened by about 0.25m and subsequently declined ona time scale of years. Such a rise is unique among the WR stars in theHipparcos-survey, and has not been found anywhere else. We investigatethe changes to the double-wave behaviour and mean colour-indexcoinciding with the period change and brightening. Interpretation of theobject as either a multi-frequency non-radial WR pulsator, or a WRbinary with possible large orbital decay is deferred to Paper III.Largely based on observations collected at the European SouthernObservatory (ESO), La Silla, Chile.
| 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.
| A Catalog and Atlas of Cataclysmic Variables: The Living Edition
The Catalog and Atlas of Cataclysmic Variables (Edition 1: Downes &Shara; Edition 2: Downes, Webbink, & Shara) has been a valuablesource of information for the cataclysmic variable community. However,the goal of having a central location for all objects is slowly beinglost as each new edition is generated. There can also be a long timedelay between new information becoming available on an object and itspublication in the catalog. To eliminate these concerns, as well as tomake the catalog more accessible, we have created a Web site which willcontain a ``living'' edition of the catalog. We have also added orbitalperiod information, as well as finding charts for novae, to thecatalog.
| New Wolf-Rayet central stars of planetary nebulae identified on the AAO/UKST Hα Survey
The central stars of two of the new planetary nebulae found during scansof the AAO/UKST Hα Survey of the Milky Way have been found toexhibit Wolf-Rayet (WR) emission features. One (PMR 1) is an early-typestar of class either [WO4] or [WC4]. The other (PMR 2) is a late [WC]star which, depending on the classification scheme used, is eitherintermediate in class between [WC9] and [WC10] or the sole member of the[WC10] class. Both stars exhibit unusual spectral features which may beattributed to enhanced nitrogen in their atmospheres and could beindicative of unusual stellar evolution.
| 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.
| Upper limits to the radio-fluxes of the Wolf-Rayet stars WR 46 (WN3p) and WR 50 (WC7+abs)
We have observed the Wolf-Rayet stars WR 46 (WN3p+c) and WR 50 (WC7+abs)at 3 and 6 cm using the Australia Telescope in search of non-thermalradio emission. However, the sources were not detected and we deriveupper limits to their radio fluxes of 0.18 mJy (0.15 mJy in the case ofWR 46 at 6 cm). These are not in conflict with expected thermalemission, because the wind densities have been found to be lower than anaverage WR wind. Inversely, assuming the mass-loss rate as determinedfrom optical spectral analyses, the inferred lower limits to thedistances are in agreement with previous determinations. Both objectsare reported as short-period photometric variables, but we note that thevariability of WR 50 is suspicious.
| Exospheric models for the X-ray emission from single Wolf-Rayet stars
We review existing ROSAT detections of single Galactic Wolf-Rayet (WR)stars and develop wind models to interpret the X-ray emission. The ROSATdata, consisting of bandpass detections from the ROSAT All-Sky Survey(RASS) and some pointed observations, exhibit no correlations of the WRX-ray luminosity (LX) with any star or wind parameters ofinterest (e.g. bolometric luminosity, mass-loss rate or wind kineticenergy), although the dispersion in the measurements is quite large. Thelack of correlation between X-ray luminosity and wind parameters amongthe WR stars is unlike that of their progenitors, the O stars, whichshow trends with such parameters. In this paper we seek to (i) test byhow much the X-ray properties of the WR stars differ from the O starsand (ii) place limits on the temperature TX and fillingfactor fX of the X-ray-emitting gas in the WR winds. Adoptingempirically derived relationships for TX and fXfrom O-star winds, the predicted X-ray emission from WR stars is muchsmaller than observed with ROSAT. Abandoning the TX relationfrom O stars, we maximize the cooling from a single-temperature hot gasto derive lower limits for the filling factors in WR winds. Althoughthese filling factors are consistently found to be an order of magnitudegreater than those for O stars, we find that the data are consistent(albeit the data are noisy) with a trend of fx ∝(Mν&infy;)-1 in WR stars, as is also the casefor O stars.
| The Puzzle of HD 104994 (WR 46)
Intense coordinated spectroscopic and photometric monitoring of thesuspected Wolf-Rayet binary WR 46 in 1999 reveals clear periodicvariations, P=0.329+/-0.013 days, in the radial velocities of theemission lines of highest ionization potential, O VI and N V, founddeepest in the Wolf-Rayet wind and thus least likely to be perturbed bya companion. These are accompanied by coherent variability in theprofiles of lines with lower ionization/excitation potential and in thecontinuum flux. Most probably originating from orbital motion of theWolf-Rayet component of the binary, this periodic radial velocity signaldisappears from time to time, thus creating a puzzle yet to be solved.We show that the entangled patterns of the line profile variability aremainly governed by transitions between high and low states of thesystem's continuum flux. Based in part on observations obtained at theEuropean Southern Observatory, La Silla, Chile (ESO program 62.H-0110).
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| Takýmyýldýz: | Guney Haci |
| Sað Açýklýk: | 12h05m18.73s |
| Yükselim: | -62°03'10.2" |
| Görünürdeki Parlaklýk: | 10.604 |
| özdevim Sað Açýklýk: | -3.1 |
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| B-T magnitude: | 10.782 |
| V-T magnitude: | 10.619 |
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