目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| A Clumping-independent Diagnostic of Stellar Mass-Loss Rates: Rapid Clump Destruction in Adiabatic Colliding Winds
Clumping in hot star winds can significantly affect estimates ofmass-loss rates, the inferred evolution of the star, and theenvironmental impact of the wind. A hydrodynamical simulation of acolliding winds binary (CWB) with clumpy winds reveals that the clumpsare rapidly destroyed after passing through the confining shocks of thewind-wind collision region (WCR) for reasonable parameters of the clumpsif the flow in the WCR is adiabatic. Despite large density andtemperature fluctuations in the postshock gas, the overall effect of theinteraction is to smooth the existing structure in the winds. Averagedover the entire interaction region, the resulting X-ray emission is verysimilar to that from the collision of smooth winds. The insensitivity ofthe X-ray emission to clumping suggests it is an excellent diagnostic ofthe stellar mass-loss rates (M˙) in wide CWBs and may prove to be auseful addition to existing techniques for deriving M˙, many ofwhich are extremely sensitive to clumping. Clumpy winds also haveimplications for a variety of phenomena at the WCR: particleacceleration may occur throughout the WCR due to supersonic MHDturbulence, reacceleration at multiple shocks, and reconnection; astatistical description of the properties of the WCR may be required forstudies of nonequilibrium ionization and the rate of electron heating;and the physical mixing of the two winds will be enhanced, as seemsnecessary to trigger dust formation.
| η Carinae across the 2003.5 Minimum: Spectroscopic Evidence for Massive Binary Interactions
We have analyzed high spatial, moderate spectral resolution observationsof η Carinae (η Car) obtained with the STIS from 1998.0 to2004.3. The data were obtained at discrete times covering an entire 2024day spectroscopic cycle, with focus on the X-ray/ionization low statethat began in 2003 June. The spectra show prominent P Cygni lines in HI, Fe II, and He I, which are complicated by blends and contamination bynebular emission and absorption. All lines show phase- andspecies-dependent variations in emission and absorption. For most of thecycle the He I emission is blueshifted relative to the H I and Fe II PCygni emission lines, which are centered at approximately systemvelocity. The blueshifted He I absorption components vary in intensityand velocity throughout the 2024 day period. We construct radialvelocity curves for the absorption component of the He I and H I lines.The He I absorption shows significant radial velocity variationsthroughout the cycle, with a rapid change of over 200 km s-1near the 2003.5 event. The H I velocity curve is similar to that of theHe I absorption, although offset in phase and reduced in amplitude. Weinterpret the complex line profile variations in He I, H I, and Fe II tobe a consequence of the dynamic interaction of the dense wind of ηCar A with the less dense, faster wind plus the radiation field of a hotcompanion star, η Car B. We use the variations seen in He I and theother P Cygni lines to constrain the geometry of the orbit and thecharacter of η Car B.
| 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 Extreme Case of a Misaligned Highly Flattened Wind in the Wolf-Rayet Binary CX Cephei
CX Cep (WR 151) is the WR+O binary (WN5+O5 V) with the second shortestperiod known in our Galaxy. To examine the circumstellar matterdistribution and to better constraint the orbital parameters andmass-loss rate of the W-R star, we obtained broadband and multiband(i.e., UBVRI) linear polarization observations of the system. Ouranalysis of the phase-locked polarimetric modulation confirms the highorbital inclination of the system (i.e., i=65deg). Using theorbital solution of Lewis et al. (1993), we obtain masses of 33.9 and23.9 Msolar for the O and W-R stars, respectively, whichagree with their spectral types. A simple polarimetric model accountingfor finite stellar size effects allowed us to derive a mass-loss ratefor the W-R star of (0.3-0.5)×10-5 Msolaryr-1. This result was remarkably independent of the model'sinput parameters and favors an earlier spectral type for the W-Rcomponent (i.e., WN4). Finally, using our multiband observations, wefitted and subtracted from our data the interstellar polarization. Theresulting constant intrinsic polarization of 3%-4% is misaligned inrelation to the orbital plane (i.e.,θCIP=26deg vs. Ω=75deg)and is the highest intrinsic polarization ever observed for a W-R star.This misalignment points toward a rotational (or magnetic) origin forthe asymmetry and contradicts the most recent evolutionary models formassive stars (Meynet & Maeder 2003) that predict sphericallysymmetric winds during the W-R phase (i.e., CIP=0%).
| The massive eclipsing LMC Wolf-Rayet binary BAT99-129. I. Orbital parameters, hydrogen content and spectroscopic characteristics
BAT99-129 in the LMC is one among a handful ofextra-galactic eclipsing Wolf-Rayet binaries known. We present blue,medium-resolution, phase-dependent NTT-EMMI spectra of this system thatallow us to separate the spectra of the two components of the binary andto obtain a reliable orbital solution for both stars. We assign an O5Vspectral type to the companion, and WN3(h)a to the Wolf-Rayet component.We discuss the spectroscopic characteristics of the system: luminosityratio, radii, rotation velocities. We find a possible oversynchronousrotation velocity for the O star. Surprisingly, the extracted Wolf-Rayetspectrum clearly shows the presence of blueshifted absorption lines,similar to what has been found in all single hot WN stars in the SMC andsome in the LMC. We also discuss the presence of such intrinsic lines inthe context of hydrogen in SMC and LMC Wolf-Rayet stars, WR+O binaryevolution and GRB progenitors. Altogether, BAT99 129 is theextragalactic counterpart of the well-known Galactic WR binaryV444 Cygni.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| CQ Cephei - The most mysterious of Wolf-Rayet type binary systems.
Not Available
| Orbital Elements and Mass-Loss Rate of V 444 Cyg
The binary system HD 193576 = V 444 Cyg was observed in 1999--2006 toobtain radial velocities of its components -- O-type star and WR-typestar. From the measured data of the N V 4604 emission line the orbitalperiod P=4.212450±0.000048 days was found. This result confirmsthe rate of the period change given by Underhill et al. (1990). Usingthe radial velocity curves of the N V 4604 emission line and the H I4861 absorption line we found the following masses and orbital elementsfor the system: M O=28.4±0.6 Mȯ , MWR=12.4±0.5 Mȯ , a O=11.5±0.3 Mȯ ,a WR=26.2±0.3 rsunm , γ O=-26±2 km/s, γ WR=45±1 km/s,K O=136±3 km/s and K WR=309±2 km/s.Using our estimates for the orbital period and masses, and the rate ofperiod change given by Underhill et al. (1990), we got the mass-lossrate of the WR component ˙ M=4.6×10-6 &msunm; /yr.We confirm that radial velocity values obtained by the bisector methodare of sufficient accuracy to be used in analysis of the mass-loss andwind-wind collision processes in V 444 Cyg.
| Polarimetric Study of the Massive Interacting Binary W Serpentis: Discovery of High-Latitude Scattering Spot/Jet
We present multicolor (UBVRI) polarimetry and additional B-band CCDpolarimetry of the peculiar, strongly interacting binary W Ser, withgood coverage over the 14.16 day orbital period in the three observingseasons 2001-2003. An interesting finding is that the first harmonicdominates in polarization variations over the binary cycle in eachseason, indicating that the main contribution to the polarized flux iscoming from a localized region away from the orbital plane. We apply ournew model codes for electron scattering in circumstellar matter tointerpret the data and find that a ``spot or jet'' like region atlatitude φ~65deg has clearly the best signal-to-noiseratio (>30) among the detected scattering components. Significantshell and stream components are also found, but no disk is seen in thepolarized light. The upper limits for the total number of free electronsin an optically thin scattering disk are an order of magnitude lowerthan in the spherical shell around the primary star. A possible reasonis that the accretion disk is optically thick and has a clearly definedand visible pseudophotosphere. Multiple scattering and absorptioneffects in an optically thick regime reduce the polarization of theobserved light to small levels. Scattering from optically thin parts ofthe circumstellar matter, the shell, the stream, and the high-latitudespot/jet, possibly associated with optically thin polar regions of theaccretion disk, dominates in the observed polarization of W Ser.
| 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.
| Bulk Velocities, Chemical Composition, and Ionization Structure of the X-Ray Shocks in WR 140 near Periastron as Revealed by the Chandra Gratings
The Wolf-Rayet WC7+O4-5 binary WR 140 went through the periastronpassage of its 8 yr eccentric binary orbit in early 2001 as the twostars made their closest approach. Both stars have powerful supersonicstellar winds that crash into each other between the stars to produceX-rays. Chandra grating observations were made when the X-rays were attheir peak, making WR 140 the brightest hot-star X-ray source in the skyand giving the opportunity to study the velocity profiles of lines, allof which were resolved and blueshifted before periastron. In the generalcontext of shock physics, the measurements constrain the flow of hot gasand where different ions were made. The brightness of lines relative tothe strong continuum in conjunction with plasma models gives interimabundance estimates for eight different elements in WC-type materialincluding an Ne/S ratio in good agreement with earlier long-wavelengthmeasurements. The lower velocity widths of cool ions imply a plasma thatwas not in equilibrium, probably due to the collisionless nature of theshock transitions and the slow character of both the postshock energyexchange between ions and electrons and subsequent ionization. Electronheat conduction into fast-moving preshock gas was absent, probablysuppressed by the magnetic field involved in WR 140's synchrotronemission. After periastron, the spectrum was weaker due mainly toabsorption by cool Wolf-Rayet star material.
| FUSE Observations of the SMC 16 day Wolf-Rayet Binary Sanduleak 1 (WO4+O4): Atmospheric Eclipses and Colliding Stellar Winds
In this paper we present the results of a FUSE monitoring campaign ofthe SMC WO4+O4 V Wolf-Rayet binary Sanduleak 1. Our 18 spectra obtainedduring a little more than one orbital cycle in 2000 October combinedwith four archival spectra show variability in the S VI, C III, C IV,and O VI P Cygni profiles, which we attribute to emission from the shockcone resulting from the collision between the two strong winds and toatmospheric eclipses of the O star continuum light by the W-R wind. Allthe lines vary in concert indicating that the cooling is such that evenlines such as the OVI λλ1032, 1038 doublet form in thelinear part of the cone. We have also applied both a simple geometricalmodel and profile fits, including emission from the normal wind, extraemission from the shock cone, and the atmospheric eclipse. Adopting anorbital inclination of ~40°, we deduce a total cone opening angle of~80° and a streaming velocity for the gas along the shock cone of~3000 km s-1. The luminosity ratio required to fit ourspectra is LO/LW-R=3.5, and the stellar radii are3.5 and 12 Rsolar, respectively, for the W-R and O stars. Wealso present radiative driving models for this binary system having twomassive stars with strong winds and discuss radiative inhibition andbraking effects. In particular, we address the coupling of the O starradiation with the W-R star wind. Finally, we present a PICAhydrodynamic colliding-wind model for Sand 1. We find an opening anglefor the shock cone similar to that deduced from the line-profilefitting, but significantly longer cooling lengths along the shock cone.However, the model reveals some cold gas that is stripped off the O4surface and mixed with the hotter WO4 material, thereby accelerating itscooling. This could very well explain why shorter cooling lengths areinferred from the profile fits.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS-32985.
| Lower mass loss rates in O-type stars: Spectral signatures of dense clumps in the wind of two Galactic O4 stars
We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars,the O4If+ supergiant HD 190429A and the O4V((f)) dwarf HD 96715, usingarchival FUSE and IUE data. We have conducted a quantitative analysisusing the two NLTE model atmosphere and wind codes, tlusty and cmfgen,which incorporate a detailed treatment of NLTE metal line blanketing.From the far-UV spectrum, we have derived the stellar and windparameters and the surface composition of the two stars. The surface ofHD 190429A has a composition typical of an evolved O supergiant(nitrogen-rich, carbon and oxygen-poor), while HD 96715 exhibits surfacenitrogen enhancement similar to the enrichment found in SMC O dwarfswhich has been attributed to rotationally-induced mixing. Followingstudies of Magellanic Cloud O stars, we find that homogeneous windmodels could not match the observed profile of O vλ1371 andrequire very low phosphorus abundance to fit the Pvλλ1118-1128 resonance lines. We show, on the other hand,that we are able to match the O v and P v lines using clumped windmodels. In addition to these lines, we find that N ivλ1718 isalso sensitive to wind clumping. For both stars, we have calculatedclumped wind models that match well all these lines from differentspecies and that remain consistent with Hα data. In particular, wehave achieved an excellent match of the P v resonance doublet,indicating that our physical description of clumping is adequate. Thesefits therefore provide a coherent and thus much stronger evidence ofwind clumping in O stars than earlier claims. We show that the successof the clumped wind models in matching these lines results fromincreased recombination in the clumps, hence from a better descriptionof the wind ionization structure. We find that the wind of these twostars is highly clumped, as expressed by very small volume fillingfactors, namely f_&infy; = 0.04 for HD 190429A and f_&infy; = 0.02 forHD 96715. In agreement with our analysis of SMC stars, clumping startsdeep in the wind, just above the sonic point. The most crucialconsequence of our analysis is that the mass loss rates of O stars needto be revised downward significantly, by a factor of 3 and more. Theselower mass loss rates will affect substantially the evolution of massivestars. Accounting for wind clumping is essential when determining thewind properties of O stars. Our study therefore calls for a fundamentalrevision in our understanding of mass loss and of O-type star stellarwinds.
| 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.
| Constraining the mass transfer in massive binaries through progenitor evolution models of Wolf-Rayet+O binaries
Since close WR+O binaries are the result of a strong interaction of bothstars in massive close binary systems, they can be used to constrain thehighly uncertain mass and angular momentum budget during the major masstransfer phase. We explore the progenitor evolution of the three bestsuited WR+O binaries HD 90657, HD 186943 and HD 211853, which arecharacterized by a WR/O mass ratio of ~0.5 and periods of 6...10 days.We are doing so at three different levels of approximation: predictingthe massive binary evolution through simple mass loss and angularmomentum loss estimates, through full binary evolution models withparametrized mass transfer efficiency, and through binary evolutionmodels including rotation of both components and a physical model whichallows to compute mass and angular momentum loss from the binary systemas function of time during the mass transfer process. All three methodsgive consistently the same answers. Our results show that, if thesesystems formed through stable mass transfer, their initial periods weresmaller than their current ones, which implies that mass transfer hasstarted during the core hydrogen burning phase of the initially moremassive star. Furthermore, the mass transfer in all three cases musthave been highly non-conservative, with on average only ~10% of thetransferred mass being retained by the mass receiving star. This resultgives support to our system mass and angular momentum loss model, whichpredicts that, in the considered systems, about 90% of the overflowingmatter is expelled by the rapid rotation of the mass receiver close tothe Ω-limit, which is reached through the accretion of theremaining 10%.
| First Ever Polarimetric Detection of a Wind-Wind Interaction Region and a Misaligned Flattening of the Wind in the Wolf-Rayet Binary CQ Cephei
In this paper we present unfiltered and multiband (i.e., UBVRI)polarimetric observations of the short-period Wolf-Rayet binary CQ Cep.Using the basic assumptions of an optically thin, corotating envelopeand pointlike sources (i.e., BME78 assumptions), we determined theorbital parameters of the system (i.e.,i=99deg+/-1deg andΩ=76deg+/-2deg at the 2 σ level) withan accuracy many times better than any previous work. Residual non-BME78variability around phase 0.0 was present in our data, which we associatewith the polarimetric eclipse of the dense central parts of theWolf-Rayet (W-R) wind by the orbiting O star. We attribute the observedphase lag of -0.15 between our residuals and those expected for astandard polarimetric eclipse to a wind-wind interaction (WWI) regiondistorted by Coriolis forces using the model presented by Marchenko etal. This model was also able to explain the strong wavelength dependenceof the polarimetric amplitudes in our multiband observations. Ouranalysis also reveals important epoch-dependent departures of the matterdistribution from spherical symmetry that were not related to theorbital plane and therefore cannot be the result of tidal interaction.We conclude that binarity is not playing an important role in drivingthe wind of the W-R star in CQ Cep and contributing to the observednonspherical matter distribution. On the other hand, this asymmetrycould be explained by a rotationally induced disk misaligned with theorbital plane.
| 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.
| 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.
| The influence of binaries on galactic chemical evolution
Understanding the galaxy in which we live is one of the greatintellectual challenges facing modern science. With the advent of highquality observational data, the chemical evolution modeling of ourgalaxy has been the subject of numerous studies in the last years.However, all these studies have one missing element which is theevolution of close binaries. Reason: their evolution is very complex andsingle stars only perhaps can do the job. (Un)Fortunately at present weknow that the majority of the observed stars are members of a binary ormultiple system and that certain objects can only be formed throughbinary evolution. Therefore galactic studies that do not account forclose binary evolution may be far from realistic.Because of the large expertise developed through the years in stellarevolution in general and binary evolution in particular at the BrusselsAstrophysical Institute, we found ourselves in a privileged position tobe the first to do chemical evolutionary simulations with the inclusionof detailed binary evolution. The complexity of close binary evolutionhas kept many astronomers from including binary stars into theirstudies. However, it is not always the easiest way of living that givesyou the most excitement and satisfaction.
| Close binary stars in ob-association regions i. preliminary investigation
We performed a sample of O- and B-eclipsing binary stars inOB-association regions and obtained the preliminary list of 147 binariesin 45 OB-association regions. We tried to elucidate the question whether(or not) the close binaries belong to corresponding OB-associations,from the commonness of their proper motions, radial velocities anddistances. Based on the completeness of the data,the binaries aredevided into three groups and the scheme for calculation of degree ofbelonging of stars to OB-associations is developed. Necessary data arenot available for nine systems and they are given in a specific table.For 12 cases, the binaries project onto the regions of two associations.We show that 33 (22.3%) close binary stars are members, 65 (43.9%) areprobable members and 39 (26.4%) are less probable members of theOB-associations. We find that 11 binaries belong to the Galaxybackground. The comparison of the distributions of orbital periods forthe binaries in OB-associations and for O-, B-binaries of the Galaxybackground shows their considerable differences in the vicinity of thetwo-day period.
| Observed Features of a Two-Phase Stellar Wind from WR 136 in the Vicinity of NGC 6888
A number of features are detected outside the nebula NGC 6888, within1.2° (30 pc) of the star WR 136, which can be explained in atwo-phase stellar-wind model. These include regions with finefilamentary gas structure that do not contain sources of stellar wind,extended radial “streams,” ultra-compact HII regions withhigh-velocity gas motions, and high-velocity gas motions outside theenvelope of NGC 6888. The two-phase wind consists of a rarefiedcomponent and dense compact condensations, or “bullets.” Thebullets generate cylindrical shocks in the interstellar gas, resultingin the presence of high-velocity gas up 20 30 pc from the star, outsidethe cavity formed by the rarified component of the wind.
| A Steady, Radiative-Shock Method for Computing X-Ray Emission from Colliding Stellar Winds in Close, Massive-Star Binaries
We present a practical, efficient, semianalytic formalism for computingsteady state X-ray emission from radiative shocks between collidingstellar winds in relatively close (orbital period up to order tens ofdays) massive-star, binary systems. Our simplified approach idealizesthe individual wind flows as smooth and steady, ignoring the intrinsicinstabilities and associated structure thought to occur in such flows.By also suppressing thin-shell instabilities for wind-collisionradiative shocks, our steady state approach avoids the extensivestructure and mixing that has thus far precluded reliable computation ofX-ray emission spectra from time-dependent hydrodynamical simulations ofclose-binary, wind-collision systems; but in ignoring the unknownphysical level of such mixing, the luminosity and hardness of X-rayspectra derived here represent upper limits to what is possible for agiven set of wind and binary parameters. A key feature of our approachis the separation of calculations for the small-scale shock-emissionfrom the ram-pressure-balance model for determining the large-scale,geometric form of the wind-wind interaction front. Integrating thelocalized shock emission over the full interaction surface and using awarm-absorber opacity to take account of attenuation by both the smoothwind and the compressed, cooled material in the interaction front, themethod can predict spectra for a distant observer at any arbitraryorbital inclination and phase. We illustrate results for a sampleselection of wind, stellar, and binary parameters, providing both fullX-ray light curves and detailed spectra at selected orbital phases. Thederived spectra typically have a broad characteristic form, and bysynthetic processing with the standard XSPEC package, we demonstratethat they simply cannot be satisfactorily fitted with the usualattenuated single- or two-temperature thermal-emission models. Weconclude with a summary of the advantages and limitations of ourapproach and outline its potential application for interpreting detailedX-ray observations from close, massive-star binary systems.
| Line profile variations in WR+O binary systems. I. The code and basic predictions
We compute the P Cyg line profiles formed by the stellar winds of binarysystems containing a Wolf-Rayet and an O-type star, incorporating theeffects due to wind eclipses and wind-wind collisions. The contributionfrom both stellar winds to the P Cygni emission lines is modeled fordifferent orbital phases. The opacity and the source function arecalculated assuming a simplified atom and the Sobolev approximation, andthe emission-line profile is calculated by exact radiative transferthrough the 3D geometry wind. We analyze the cases of a P Cygni linethat is formed only in the WR wind, and the case of a line formed inboth the WR and the O-star winds. The line-profile variations that arepredicted by this model are presented. When compared with observations,the synthetic profiles and their phase-dependent variability provide anestimate for the opening angle of the WWC shock cone and the velocitylaw of both stellar winds. Ultraviolet (UV) observations of the binarysystem γ2 Vel are used to illustrate how the modelpredictions can be applied to the observational data.
| Thermal Instability in the Envelopes of Wolf--Rayet Stars
We briefly consider thermal instabilities developing in the expandingenvelopes of Wolf-Rayet stars and their possible implications. Due tothe specific physical conditions in the gaseous outflow from the star,thermal instabilities are likely to be very important in determining theflow’s structure.
| The case for Case C mass transfer in the galactic evolution of black hole binaries
Earlier works, which we review, have shown that if the Fe core in apresupernova star is to be sufficiently massive to collapse into a blackhole, earlier in the evolution of the star the He core must be covered(clothed) by a hydrogen envelope during He core burning and removed onlyfollowing this, in, e.g., common envelope evolution. This is classifiedas Case C mass transfer. These previous arguments were based chiefly onstellar evolution, especially depending on the way in which12C burned.In this work we argue for Case C mass transfer on the basis of binaryevolution. The giant progenitor of the black hole will have a largeradius ~1000Rsolar at the end of its supergiant stage. Itslifetime at that point will be short, ~1000 years, so it will not expandmuch further. Thus, the initial giant radius for Case C mass transferwill be constrained to a narrow band about ~1000Rsolar. Thishas the consequence that the final separation af followingcommon envelope evolution will depend nearly linearly on the mass of thecompanion md which becomes the donor after the He core of thegiant has collapsed into the black hole. The separation at which thiscollapse takes place is essentially af, because of the rapidevolution of the giant. (In at least two binaries the black hole donorseparation has been substantially increased because of mass loss in theblack hole formation. These can be reconstructed from the amount of massdeposited on the donor in this mass loss.)We show that the reconstructed preexplosion separations of the blackhole binaries fit well the linear relationship.
| 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.
| The galactic evolution of the supernova rates
Supernova rates (hypernova, type II, type Ib/c and type Ia) in aparticular galaxy depend on the metallicity (i.e. on the galaxy age), onthe physics of star formation and on the binary population. In order tostudy the time evolution of the galactic supernova rates, we use ourchemical evolutionary model that accounts in detail for the evolution ofsingle stars and binaries. In particular, supernovae of type Ia areconsidered to arise from exploding white dwarfs in interacting binariesand we adopt the two most plausible physical models: the singledegenerate model and the double degenerate model. Comparison betweentheoretical prediction and observations of supernova rates in differenttypes of galaxies allows to put constraints on the population ofintermediate mass and massive close binaries.The temporal evolution of the absolute galactic rates of different typesof supernovae (including the type Ia rate) is presented in such a waythat the results can be directly implemented into a galactic chemicalevolutionary model. Particularly for type Ia's the inclusion of binaryevolution leads to results considerably different from those in earlierpopulation synthesis approaches, in which binary evolution was notincluded in detail.
| The chemical evolution of the Galaxy: the importance of stars with an initial mass larger than 40 Msolar
In the present paper we investigate in how far stars with an initialmass larger than 40 Msolar affect the chemical enrichment ofthe Galaxy. We illustrate the importance for chemical yields of a mostup-to-date treatment of the various stellar wind mass loss episodes instellar evolutionary codes and we discuss the effects of a possiblesupernova-like outburst prior to massive black hole formation.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
