Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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.
| 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.
| Massive Close Binary Stars and Gamma-ray Bursts
We analyze the observed parameters of massive extremely close binariescontaining Wolf-Rayet stars and black holes, and identify those systemswhose supernova outbursts lead to the formation of rapidly rotating Kerrblack holes. It is proposed that the formation of such a black hole isaccompanied by a strong gamma-ray burst. Several types of observedsystems satisfy the conditions necessary for the formation of a Kerrblack hole: BH+WR, BH+OB, WR+O, and BH+K,M.
| 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.
| 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 Mid-Infrared Spectral Survey of Galactic Wolf-Rayet Stars
We present 8-13 μm spectra at resolution R~600 of 29 northernGalactic Wolf-Rayet stars, including the first ever reportedmid-infrared (MIR) spectrum for many. Among the subtypes of the starsstudied were 14 WC, 13 WN, 1 WN/WC, and an additional reclassified WN.Lines of He I and He II, along with fine-structure lines of Ne II and SIV, are strongly present in 22 of the sources observed, while six of thesources exhibit the powerful emission of heated circumstellar carbondust. We point out similarities between our spectra and Infrared SpaceObservatory (ISO) observations of several of the same sources and notean unresolved discrepancy between the two data sets for the WC6 star WR146. We investigate the diagnostic power of MIR He I and He II lines forsubtype discrimination and find the line ratio Wλ(9.7μm He II)/Wλ(11.3 μm He I+He II) can providemoderate discrimination within the WN and WC types, though the smallnumber of stars with corresponding line pairs detected made suchassessment difficult.
| 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.
| Wolf-Rayet Stars and Relativistic Objects: Distinctions between the Mass Distributions in Close Binary Systems
The observed properties of Wolf-Rayet stars and relativistic objects inclose binary systems are analyzed. The final masses M CO f for thecarbon-oxygen cores of WR stars in WR + O binaries are calculated takinginto account the radial loss of matter via stellar wind, which dependson the mass of the star. The analysis includes new data on the clumpystructure of WR winds, which appreciably decreases the requiredmass-loss rates for the WR stars. The masses M CO f lie in the range (12)M ȯ (20 44)M ȯ and have a continuous distribution. Themasses of the relativistic objects M x are 1 20M ȯ and have abimodal distribution: the mean masses for neutron stars and black holesare 1.35 ± 0.15M ȯ and 8 10M ȯ, respectively, with agap from 2 4M ȯ in which no neutron stars or black holes areobserved in close binaries. The mean final CO-core mass is &$/line M _{CO}(f) = 7.4 - 10.3M_ ȯ$; , close to the mean mass for the black holes. This suggests that it isnot only the mass of the progenitor that determines the nature of therelativistic object, but other parameters as well-rotation, magneticfield, etc. One SB1R Wolf-Rayet binary and 11 suspected WR + C binariesthat may have low-mass companions (main-sequence or subgiant M-A stars)are identified; these could be the progenitors of low-mass X-raybinaries with neutron stars and black holes.
| The Interstellar Environment of the Wolf-Rayet Star WR 143
As part of a systematic study of the environment of Galactic WR stars, aregion along the line of sight to Cygnus (l=77.5°,b=0°) has beenstudied. The neutral hydrogen 21 cm line distribution shows theexistence of a cavity expanding from the Wolf-Rayet star WR 143. Thiscavity, created by the stellar wind of the star, has a mean radius of 7pc (assuming a distance of about 1 kpc), an expansion velocity of morethan 8 km s-1, and a missing mass of some 150Msolar and seems to be surrounded by an H I shell, likelymade up of the gas pushed by the star. Although this hole (also calledbubble) is quite conspicuous in H I, there seems to be no trace of it atthe other wavelengths studied (radio continuum and infrared). All radioobservations were obtained at the Dominion Radio AstrophysicalObservatory as part of the Canadian Galactic Plane Survey.
| 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.
| X-ray Nova Binary Systems
The physical properties of X-ray novae as close binary systems areanalysed. Observational data in X-ray, UV, optical, IR and radio rangesare summarized. Modern theoretical considerations of the problem ofX-ray novae, taking into account some new ideas and results, aredescribed. Properties of optical stars in X-ray novae are analysed. Dataabout the masses of black holes in X-ray binary systems are summarized.Possible evolutionary links between WR stars in close binary systems andX-ray novae are analysed.
| Mass-loss rates of Wolf-Rayet stars as a function of stellar parameters
Clumping-corrected mass-loss rates of 64 Galactic Wolf-Rayet (WR) starsare used to study the dependence of mass-loss rates, momentum transferefficiencies and terminal velocities on the basic stellar parameters andchemical composition. The luminosities of the WR stars have beendetermined either directly from the masses, using the dependence of L onmass predicted by stellar evolution theory, or they were determined fromthe absolute visual magnitudes and the bolometric corrections. For thispurpose we improved the relation between the bolometric correction andthe spectral subclass. (1) The momentum transfer efficiencies η(i.e. the ratio between the wind momentum loss and radiative momentumloss) of WR stars are found to lie in the range of 1.4 to 17.6, with themean value of 6.2 for the 64 program stars. Such values can probably beexplained by radiative driving due to multiple scattering of photons ina WR wind with an ionization stratification. However, there may be aproblem in explaining the driving at low velocities. (2) We derived thelinear regression relations for the dependence of the terminal velocity,the momentum transfer efficiency and the mass-loss rates on luminosityand chemical composition. We found a tight relation between the terminalvelocity of the wind and the parameters of the hydrostatic core. Thisrelation enables the determination of the mass of the WR stars fromtheir observed terminal velocities and chemical composition with anaccuracy of about 0.1 dex for WN and WC stars. Using evolutionary modelsof WR stars, the luminosity can then be determined with an accuracy of0.25 dex or better. (3) We found that the mass-loss rates(&mathaccent "705Frelax dot;) of WR stars depend strongly onluminosity and also quite strongly on chemical composition. For thecombined sample of WN and WC stars we found that &mathaccent"705Frelax dot; in Mȯyr-1 can be expressed as&mathaccent "705Frelax dot; ≃ 1.0 ×10-11(L/L ȯ)1.29Y1.7Z0.5 (1) with an uncertainty of σ = 0.19dex (4) The new mass-loss rates are significantly smaller than adoptedin evolutionary calculations, by about 0.2 to 0.6 dex, depending on thecomposition and on the evolutionary calculations. For H-rich WN starsthe new mass-loss rates are 0.3 dex smaller than adopted in theevolutionary calculations of Meynet et al. (1994). (5) The lowermass-loss rates, derived in this paper compared to previously adoptedvalues, facilitate the formation of black holes as end points of theevolution of massive stars. However they might create a problem inexplaining the observed WN/WC ratios, unless rotational mixing ormass-loss due to eruptions is important.
| The first COMPTEL source catalogue
The imaging Compton telescope COMPTEL aboard NASA's Compton Gamma-RayObservatory has opened the MeV gamma-ray band as a new window toastronomy. COMPTEL provided the first complete all-sky survey in theenergy range 0.75 to 30 MeV. The catalogue, presented here, is largelyrestricted to published results. It contains firm as well as marginaldetections of continuum and line emitting sources and presents upperlimits for various types of objects. The numbers of the most significantdetections are 32 for steady sources and 31 for gamma-ray bursters.Among the continuum sources, detected so far, are spin-down pulsars,stellar black-hole candidates, supernova remnants, interstellar clouds,nuclei of active galaxies, gamma-ray bursters, and the Sun during solarflares. Line detections have been made in the light of the 1.809 MeV26Al line, the 1.157 MeV 44Ti line, the 847 and1238 keV 56Co lines, and the neutron capture line at 2.223MeV. For the identification of galactic sources, a modelling of thediffuse galactic emission is essential. Such a modelling at this timedoes not yet exist at the required degree of accuracy. Therefore, asecond COMPTEL source catalogue will be produced after a detailed andaccurate modelling of the diffuse interstellar emission has becomepossible.
| Long-term photometry of the Wolf-Rayet stars WR 137, WR 140, WR 148, and WR 153
In 1991, a long term UBV-photometry campaign of four Wolf-Rayet starswas started using the 60 cm telescope of the National AstronomicalObservatory Rozhen, Bulgaria. Here we report on our observationalresults and discuss the light variations. The star WR 137 was observedduring 1991 - 1998. No indications of eclipses were found, though randomlight variations with small amplitudes exist, which are probably due todynamical wind instabilities. WR 140 was also monitored between 1991 and1998. In 1993, a dip in the light curve in all passbands was observedshortly after periastron passage, with amplitude of 0.03 mag in V. Thisis interpreted in terms of an ``eclipse'' by dust condensation in theWR-wind. The amplitude of the eclipse increases towards shorterwavelengths; thus, electron scattering alone is not sufficient toexplain the observations. An additional source of opacity is required,possibly Rayleigh scattering. After the eclipse, the light in allpassbands gradually increased to reach the ``pre-eclipse'' level in1998. The very broad shape of the light minimum suggests that a dustenvelope was built up around the WR-star at periastron passage bywind-wind interaction, and was gradually dispersed after 1993. Ourobservations of WR 148 (WR + c?) confirm the 4.3 d period; however, theyalso show additional significant scatter. Another interesting finding isa long-term variation of the mean light (and, possibly, of theamplitude) on a time scale of years. There is some indication of a 4year cycle of that long-term variation. We discuss the implications forthe binary model. Our photometry of WR 153 is consistent with thequadruple model of this star by showing that both orbital periods, 6.7 d(pair A) and 3.5 d (pair B), exist in the light variations. A search inthe HIPPARCOS photometric data also reveals both periods, which is anindependant confirmation. No other periods in the light variability ofthat star are found. The longer period light curve shows only oneminimum, which might be due to an atmospheric eclipse; the shorterperiod light curve shows two minima, indicating that both stars in pairB are eclipsing each other. Based on observations collected at theNational Astronomical Observatory Rozhen, Bulgaria
| The Structure of Wolf-Rayet Winds. II. Observations of Ionization Stratification in the WN Subtype
Motivated by the question of the importance of ionization stratificationin solving the ``momentum problem'' of Wolf-Rayet stellar winds, we havechosen a sample of 14 WN stars for a systematic study. We performedmeasurements of the emission line widths on ultraviolet, optical, andinfrared spectra to obtain data spanning a large range of ionizationpotentials. We provide extensive tables of these measurements as well asline profile classifications. The presence of ionization stratificationin the wind should result in a correlation between ionization potentialand line width. We find most of the winds to be stratified and discussthe level of stratification found in each star. To test the importanceof ionization stratification to efficient radiation-to-wind momentumtransfer, we compare our empirically measured stratification strengthswith two sets of theoretical performance numbers and give thecorrelation statistics in each case.
| A search for nonthermal radio emission from OB and WR stars with RATAN-600
We have searched for nonthermal radio emission from 40 OB and WR stars.Enhanced nonthermal radio emission from an early-type star could beevidence for the presence of a collapsed companion, and thus for itsorigin as the result of a supernova explosion in a massive binary. Asshown in the evolutionary calculations of joint evolution of a neutronstar and a normal star in binaries (Lipunov & Prokhorov \cite{lp}),a considerable fraction of neutron stars in binary systems having anoptical companion must be in the ejector state. A neutron star in thisstate generates a relativistic wind like an isolated radio pulsar. Mostejectors in binary systems can not be identified as radio pulsarsbecause of absorption of radiowaves in the stellar wind of the normalcompanion, but instead, they may appear as sources of high-energy quantadue to the synchrotron radiation of relativistic particles (ejected bythe radio pulsar) in the magnetic field of a normal star (Lipunov &Prokhorov \cite{lp2}; Lipunov & Nazin \cite{naz}). In this case asource of nonthermal radiation in a wide range from radio to hardgamma-ray may appear as a result of a specific reflection effect in themagnetic field of the optical companion. Cyg X-3 and the periodicradioburster LS I +610303 may be examples of just this kind.To test this idea, measurements of radio flux densities in the rangefrom 0.96 to 21.7 GHz from selected OB and WR stars were made with theRATAN-600. No nonthermal radio emission from the selected stars weredetected.
| Hot Expanding Shells in the Envelope of the Sagittarius B2 Molecular Cloud
We present high-resolution (3") maps of the (3, 3) and (4, 4) lines ofNH_3 toward the southern part of the molecular envelope of theSagittarius B2 star-forming region. These maps reveal, for the firsttime, that the morphology of the hot gas in the Sgr B2 envelope isdominated by at least six rings, two arcs, and a filament. The sizes ofthe rings are between 1 and 2.6 pc and their thicknesses between 0.2 and0.4 pc. Most of the gas in the rings is warm, with kinetic temperatures,T_k, of 40-70 K, although some parts of the low-velocity rings reachtemperatures larger than 100 K. These hot rings and arcs representregions of enhanced H_2 density and/or enhanced NH_3 abundance in theSgr B2 envelope. Some of the hot rings show radial velocity gradients,which suggests that the rings and arcs correspond to three-dimensionalshells expanding at velocities of ~6-10 km s^-1. The walls of the hotshells are highly clumped and contain unresolved condensations (<~1")in the line maps. There are two kinds of unresolved condensations: thoseappearing in the (3, 3) line maps with only weak emission in the (4, 4)line and those with rather strong emission in the (4, 4) line comparedwith that of the (3, 3) line. For the first kind, we identify sixcondensations in the NH_3 (3, 3) line maps that have brightnesstemperatures larger than the kinetic temperatures. It is likely thatthese are newly found NH_3 masers. We also find that other masers in theregion such as class II CH_3OH and H_2CO masers are very well correlatedin velocity and position with the hot shells. The large number of masersobserved in the hot shells can be explained as a result of thecombination of high abundance of volatile molecules like NH_3, CH_3OH,and H_2CO (produced by hot temperature and/or shock chemistry) andsufficient velocity coherence at the edges of the expanding shells. Forthe second kind of condensations, we have identified three compactsources in the walls of the shells that are hot (T_k>~300 K) and havehigh H_2 densities (>~10^6-10^7 cm^-3). These exhibit characteristicssimilar to those of the hot cores associated with newly formed massivestars. The inferred dust luminosity for the hot cores is at least 10^5L_solar, similar to the Orion A hot core. The high luminosity of the hotcores and the lack of associated radio continuum emission indicates thatthese are internally heated and very likely associated with the densecircumstellar material surrounding newly formed stars. The detection ofhot cores suggests that massive star formation is not restricted to onlySgr B2N and Sgr B2M but is also taking place in the envelope of Sgr B2.Very likely, this star formation has been triggered by the expandingbubbles that produce the shells seen in the NH_3 inversion lines. Wediscuss the possible origin of these hot shells. We find that awind-blown bubble driven by typical Galactic Wolf-Rayet stars couldaccount for the kinetic energies and the momenta observed in the hotNH_3 shells. A cluster containing massive evolved stars such as theQuintuplet could easily explain the large concentration of hot shells,the heating of the warm envelope, and the peculiar chemistry observed inthe envelope of Sgr B2. The implication of these findings for theheating of Galactic center molecular clouds is briefly discussed.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| Observational constraints on the efficiency of acceleration in the optically thin parts of Wolf-Rayet winds
Wolf-Rayet stars have such strong winds that their inner regions areoptically thick, preventing us from seeing the hydrostatic stellarcores. One might expect considerable acceleration of the wind to occurin the optically thick part. However, we show empirically that at least50%, and in some cases up to 100%, of the wind's acceleration occurs inthe optically thin part of the WR wind.
| Wolf-Rayet stars before and after Hipparcos.
Not Available
| The stellar-mass black-hole candidates.
Not Available
| The WR and O-type star population predicted by massive star evolutionary synthesis
Evolutionary calculations of massive single stars and of massive closebinaries that we use in the population number synthesis (PNS) code arepresented. Special attention is given to the assumptions/uncertaintiesinfluencing these stellar evolutionary computations (and thus the PNSresults). A description is given of the PNS model together with theinitial statistical distributions of stellar parameters needed toperform number synthesis.We focus on the population of O-type stars andWR stars in regions where star formation was continuous in time and instarburst regions. We discuss the observations that have to be explainedby the model. These observations are then compared to the PNSpredictions.We conclude that: . probably the majority of the massivestars are formed as binary components with orbital period between 1 dayand 10 yr; most of them interact. . at most 8% of the O-type stars arerunaways due to a previous supernova explosion in a binary; recentstudies of pulsar space velocities and linking the latter to the effectof asymmetrical supernova explosions, reveal that only a smallpercentage of these runaways will have a neutron star companion. . withpresent day stellar evolutionary computations, it is difficult toexplain the observed WR/O number ratio in the solar neighbourhood and inthe inner Milky Way by assuming a constant star formation rate, with orwithout binaries. The observed ratio for the Magellanic Clouds is betterreproduced. . the majority of the single WR stars may have had a binarypast. . probably merely 2-3% (and certainly less than 8%) of all WRstars have a neutron star companion. . a comparison between theoreticalprediction and observations of young starbursts is meaningful only ifbinaries and the effect of binary evolution are correctly included. Themost stringent feature is the rejuvenation caused by mass transfer.
| Colliding Winds in Binaries: Observations
Colliding winds in binaries are discussed mainly from an observationalpoint of view. Collisions are especially energetic in the case of hot,luminous stars, which drive strong, fast winds. Emphasis is thereforedevoted to binaries containing Wolf-Rayet stars. The subject is dividedup into (1) continuum radiation (X-ray and non-thermal radio from thehot bow shock head, IR from dust formed in some WC + O binaries fardownstream in the collision shock cone) and (2) line radiation (opticaland UV, both from various regions downstream from the bow shock head).The latter is particularly useful in providing constraints on thevelocity field and hence ultimately the geometry of the wind collisionand the binary system itself.
| A spectropolarimetric survey of northern hemisphere Wolf-Rayet stars
We present a homogeneous, high signal-to-noise spectropolarimetricsurvey of 16 northern hemisphere Wolf-Rayet (WR) stars. A reduction inpolarization at emission-line wavelengths - the `line effect' - isidentified in four stars: WRs 134, 137, 139, and 141. The magnitude ofthe effect in WR 139 (V444 Cyg) is variable, while WR 136, previouslyreported to show the line effect, does not show it in our data. Assumingthe line effect generally to arise from axisymmetric distortions ofstellar winds, we show that a model in which all WRs have the sameintrinsic (equator-on) polarization, with the observed variations solelya result of inclination effects, is inconsistent with the observations.A model in which the intrinsic polarizations are uniformly distributedis more plausible, but best-fitting results are obtained if thedistribution of polarizations is biased towards small values, with only~20 per cent of stars having intrinsic polarizations greater than ~0.3per cent. Radiative transfer calculations indicate that the observedcontinuum polarizations can be matched by models with equator:poledensity ratios of 2-3. The model spectra have electron-scattering wingsthat are significantly stronger than observed (in both intensity andpolarized flux), confirming that the winds of stars showing intrinsicpolarization must be clumped on small scales as well as being distortedon large scales. We combine the results of our survey with observationsfrom the literature to give a sample of 29 stars which have bothaccurate spectropolarimetric observations and physical parametersderived from standard-model analyses. We find that the line-effect starsare clustered at high M, L in the luminosity-mass-loss rate plane(although they are unexceptional in the terminal velocity-subtype andthe surface-mass-flux-temperature planes). The mass-loss rates derivedfrom radio-continuum observations for these stars are in good accordwith the results of optical emission-line analyses, suggesting that (i)the wind structure of line-effect stars has a density contrast which iseffectively constant with radius, and (ii) the high M values may beartefacts of large-scale wind structure. Assuming that observedspectroscopic and photometric variability of the line-effect stars isrelated to the WR rotation period, we compute equatorial rotationvelocities. These velocities correspond to ~10 per cent of the corebreakup rates, and may be large enough to produce significantwind-compression effects according to the models of Ignace, Cassinelli& Bjorkman.
| Clumping-corrected mass-loss rates of Wolf-Rayet stars
Mass-loss rates of Galactic Wolf-Rayet stars have been determined fromtheir radio emission power and spectral index (alpha = dln {f_ν} /dln ν), accounting for the clumped structure and (potential) variableionization in their outer winds. The average spectral index between mm-and cm- wavelengths is ~ 0.77 for WN stars and ~ 0.75 for WC stars, incontrast with ~ 0.58 expected for smooth winds. The observed wavelengthdependence of alpha can be explained using clumped wind models in somecases, with shocks (at 30-100 stellar radii) producing a higherionization zone in the outer wind. We obtain an empirical formularelating mass-loss with observed optical emission line equivalentwidths, with application to stars without measured radio fluxes.Clumping-corrected mass-loss rates are generally lower than thoseobtained by current smooth wind models. Specifically we find log\mdot(clumpy)-log \mdot(smooth)=-0.19 (sigma=0.28) for WN stars, and log\mdot(clumpy)-log \mdot(smooth)=-0.62 (sigma=0.19) for WC stars. Newmass-loss rate estimates agree very well with (clumping independent)determinations of WR components in binary systems.
| The nitrogen spectra of Wolf-Rayet stars. A grid of models and its application to the Galactic WN sample
Adopting the ``standard model'' for Wolf-Rayet atmospheres, non-LTEradiation transfer calculations are performed which account for heliumand nitrogen. Grids of theoretical models are presented for the wholerelevant parameter range. The WN classification criteria are employed inorder to identify the subtype domains, and inconsistencies arediscussed. The (almost complete) sample of known Galactic WN stars isanalyzed by comparing the observed spectra with the synthetic spectra ofthe grid models. This is the first time that nitrogen line analyses areperformed for the whole WN sample, while previous comprehensive studieswere restricted to helium models. The obtained parameters roughlyconfirm the results from the previous helium analyses, as far as latesubtypes (WNL) and early subtypes with strong lines (WNE-s) areconcerned. For early subtypes with weak lines (WNE-w), however, theparameters are substantially revised. The hottest WN star, with astellar (effective) temperature of 140kK, is WR2, which could not beanalyzed previously from its helium lines due to the lack of He I. Theother members of the WNE-w subgroup have stellar temperatures between 40and 90kK, thus populating the same temperature range as the strong-linedWNE-w, but with less dense winds. The luminosities are revised accordingto the new parameters. Moreover, reddening corrections are newlydetermined form comparing IUE data with the UV model fluxes. The averageluminosity is now log L/Lsun = 5.5 for WNE stars (both,strong and weak lined), and log L/Lsun = 5.9 for WNL (notsignificantly revised). The empirical minimum WN luminosity is10(5.0}L_{sun) , reducing former incompatibilities with predictions fromevolutionary calculations. The ratio between mechanical and radiativemomentum flow is slightly affected by the revisions, but remains muchhigher than unity: 9, 9 and 29 for the WNL, WNE-w and WNE-s subclass,respectively. Partly based on observations collected at the EuropeanSouthern Observatory (ESO), La Silla, Chile, and on archival data fromthe International Ultraviolet Explorer (IUE)
| CYG X-3: can the compact object be a black hole?
By means of population synthesis we find that the expected Galacticnumber of black holes with massive helium star companions is ~ 100 anddepends on an assumed threshold for M_pre-BH. The overwhelming majorityof these systems has orbital periods in excess of 10hr, with a maximumat ~ 100hr, while under the Illarionov & Sunyaev(1975) diskformation criteria for accretion from the strong stellar wind of aWolf-Rayet star, disk accretion is possible only for orbital periodsbelow ~ 10hr. However, the number of such short-period systems isvanishingly small. If the accretor in Cyg X-3 is a 10{Msun}black hole, then the accretion rate will be super-Eddington.Super-Eddington accretion may be responsible for the formation of jetsin Cyg X-3 and may also support an X-ray luminosity as high as ~ 10(39)erg s(-1). From the orbital period distribution for neutron stars withmassive helium companions we find that if during the common envelopephase a neutron star accretes at dot{M}_Edd and spins-up to theequilibrium period, then in most systems the spinning neutron star actsas a``propeller'' and accretion from the WR star wind is impossible. Forthe model with two massive helium stars as an immediate progenitor ofCyg X-3, the requirement of accomodation of two WR stars in thepost-common-envelope orbit combined with severe mass loss by themprevents formation of BH+WR systems with orbital periods less thanseveral days.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Cygnus |
| Ascensió Recta: | 20h41m21.55s |
| Declinació: | +52°35'15.1" |
| Magnitud Aparent: | 10.244 |
| Moviment propi RA: | -5.8 |
| Moviment propi Dec: | -5.3 |
| B-T magnitude: | 10.831 |
| V-T magnitude: | 10.293 |
Catàlegs i designacions:
|
