Contents
Images
Upload your image
DSS Images Other Images
Related articles
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.
| 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%.
| Wolf-Rayet Stars, Black Holes, and Gamma-Ray Bursters in Close Binaries
We consider the evolutionary status of observed close binary systemscontaining black holes and Wolf-Rayet (WR) stars. When the componentmasses and the orbital period of a system are known, the reason for theformation of a WR star in an initial massive system of two main-sequencestars can be established. Such WR stars can form due to the action ofthe stellar wind from a massive OB star (M OB≥50M ȯ),conservative mass transfer between components with close initial masses,or the loss of the common envelope in a system with a large (up to25) initial component mass ratio. The strong impact ofobservational selection effects on the creation of samples of closebinaries with black holes and WR stars is demonstrated. We estimatetheoretical mass-loss rates for WR stars, which are essential for ourunderstanding the observed ratio of the numbers of carbon and nitrogenWR stars in the Galaxy . We also estimate the minimum initial masses ofthe components in close binaries producing black holes and WR stars tobe 25M ȯ. The spatial velocities of systems with black holesindicate that, during the formation of a black hole from a WR star, themass loss reaches at least several solar masses. The rate of formationof rapidly rotating Kerr black holes in close binaries in the Galaxy is3×10-6 yr-1. Their formation may be accompanied by a burst ofgamma radiation, possibly providing clues to the nature of gamma-raybursts. The initial distribution of the component mass ratios for closebinaries is dNdq=dM 2/M 1 in the interval 0.04≲q 0≤1,suggesting a single mechanism for their formation.
| Evolution of Wolf-Rayet Stars in Binary Systems: An Analysis of the Mass and Orbital-Eccentricity Distributions
We have undertaken a statistical study of the component mass ratios andthe orbital eccentricities of WR + O close binary, detachedmain-sequence (DMS), contact early-type (CE), and semidetached (SD)systems. A comparison of the characteristics of WR + O systems and ofDMS, CE, and SD systems has enabled us to draw certain conclusions aboutthe evolutionary paths of WR + O binaries and to demonstrate that up to90% of all known WR + O binaries formed as a result of mass transfer inmassive close O + O binary systems. Since there is a clear correlationbetween the component masses in SD systems with subgiants, the absenceof an anticorrelation between the masses of the WR stars and O stars inWR + O binaries cannot be considered evidence against the formation ofWR + O binaries via mass transfer. The spectroscopic transitionalorbital period P tr sp corresponding to the transition from nearlycircular orbits (e sp<0.1) to elliptical orbits (e sp≥0.1) is14d for WR + O systems and 2d 3d for OB + OB systems. Theperiod range in which all WR + O orbits are circular &$(1mathop dlimits_. 6 ≤slant P ≤slant 14(d) ); is close to the range for SD systems with subgiants, &0mathop dlimits_. 7 ≤slant P ≤slant 15(d); . The large difference between the P tr sp values for WR + O and OB +OB systems suggests that a mechanism of orbit circularization additionalto that for OB + OB systems at the DMS stage (tidal dissipation of theorbital energy due to radiative damping of the dynamical tides) acts inWR + O binaries. It is natural to suggest mass transfer in the parent O+ O binaries as this supplementary orbit-circularization mechanism.Since the transitional period between circular and elliptical orbits forclose binaries with convective envelopes and ages of 5×109 yearsis &P_{tr} = 12mathop dlimits_. 4$; , the orbits of most known SD systems with subgiants had enough timeto circularize during the DMS stage, prior to the mass transfer. Thus,for most SD systems, mass transfer plays a secondary role incircularization of their orbits. In many cases, the initial orbitaleccentricities of the O + O binary progenitors of WR + O systems arepreserved, due to the low viscosity of the O-star envelopes and theshort timescale for their nuclear evolution until the primary O starfills its Roche lobe and the mass transfer begins. The mass transfer inthe parent O + O systems is short-lived, and the number of orbitalcycles during the early mass-transfer stage is relatively low (lowerthan for the progenitors of SD systems by three or four orders ofmagnitude). The continued transfer of mass from the less massive to themore massive star after the component masses have become equal leads tothe formation of a WR + O system, and the orbit's residual eccentricityincreases to the observed value. The increase of the orbitaleccentricity is also facilitated by variable radial mass loss via thewind from the WR star in the WR + O system during its motion in theelliptical orbit. The result is that WR + O binaries can haveconsiderable orbital eccentricities, despite their intense masstransfer. For this reason, the presence of appreciable eccentricitiesamong WR + O binaries with large orbital periods cannot be consideredfirm evidence against mass transfer in the parent O + O binary systems.Only for the WR + O binaries with the longest orbital periods (4 of 35known systems, or 11 %) can the evolution of the parent O + O binariesoccur without filling of the Roche lobe by the primary O star, beinggoverned by radial outflow in the form of the stellar wind and possiblyby the LBV phenomenon, as in the case of HD 5980.
| Metallicity and binarity in WC and WO stars
Not Available
| Physical parameters of the high-mass X-ray binary 4U1700-37
We present the results of a detailed non-LTE analysis of the ultravioletand optical spectrum of the O6.5 Iaf+ star HD153919 - the mass donor in the high-mass X-ray binary4U1700-37. We find that the star has a luminositylog(L*/Lsun)=5.82 +/- 0.07, T_eff=35 000 +/- 1000K, radius R*=21.9+1.3-0.5Rsun, mass-loss rate dot {M}=9.5x 10-6Msun yr-1, and a significant overabundance ofnitrogen (and possibly carbon) relative to solar values. Given theeclipsing nature of the system these results allow us to determine themost likely masses of both components of the binary via Monte Carlosimulations. These suggest a mass for HD 153919 ofM* = 58 +/- 11 Msun - implying the initial mass ofthe companion was rather high (ga 60 Msun). The most likelymass for the compact companion is found to be M_x =2.44+/- 0.27Msun, with only 3.5 per cent of the trials resulting in amass less than 2.0 Msun and none less than 1.65Msun. Such a value is significantly in excess of the upperobservational limit to the masses of neutron stars of 1.45Msun found by Thorsett & Chakrabarthy (\cite{thorsett}),although a mass of 1.86 Msun has recently been reported forthe Vela X-1 pulsar (Barziv et al. \cite{barziv}).Our observational data is inconsistent with the canonical neutron starmass and the lowest black hole mass observed (ga 4.4 Msun;Nova Vel). Significantly changing observationalparameters can force the compact object mass into either of theseregimes but, given the strong proportionality between M* andM_x, the O-star mass changes by factors of greater than 2, well beyondthe limits determined from its evolutionary state and surface gravity.The low mass of the compact object implies that it is difficult to formhigh mass black holes through both the Case A & B mass transferchannels and, if the compact object is a neutron star, wouldsignificantly constrain the high density nuclear equation of state.Based on observations collected at the European Southern Observatory, LaSilla, Chile (64.H-0224).
| The Effect of Binarity and Metallicity in the Spectra of WC and WO Stars
A statistical analysis of the main emission lines common to the WC andWO stars is made based on an extensive set of spectral data. To definethe trends in equivalent width ( Wλ), line ratios, andline widths, median values are derived for single-spectrum stars ofdifferent spectral class. We find that in Galactic WO and WC4 stars,Wλ (C IV 581 nm) is smaller compared to inextragalactic objects. In both Galactic and extragalactic stars,Wλ (O V 559 nm) smoothly increases towards early WCand WO stars. It is argued that differences in stellar wind structure,in combination with the ambient metallicity, may be the cause of theanomalies. Variation of the profile of the 465 nm blend indicates asubstantial contribution of He II 468 nm for the WCE and WO stars. Inaddition, we comment on the carbon abundances in relation to theevolutionary status of these objects. We also give an estimate of theOB/WR continuum flux ratio in composite-spectrum systems.
| 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.
| Wolf-Rayet Stars and Cosmic Gamma-ray Bursts
The observational properties of cosmic gamma-ray bursts and ofWolf-Rayet (WR) stars and their CO cores at the end of their evolutionare analyzed. WR stars do not have hydrogen envelopes, facilitating thetransformation of the energy of collapse into observable gamma rays. Ofthe ≈90 well-localized gamma-ray bursts, 21 have opticalidentifications, of which 16 have measured redshifts (z=0.4 4.5). Thedistribution of gamma-ray bursts in energy N(ΔE) has a largescatter, from 3×1051 to 2×1054 erg. There is some evidencethat the distribution N(ΔE) is bimodal if we include the gamma-rayburst GRB 980425, which is associated with the peculiar type Icsupernova SN 1998bw in the nearby elliptical galaxy ESO 184-G82, forwhich ΔE γ≈1048erg. These characteristics of gamma-raybursts are reminiscent of the distribution of final masses for the COcores of WR stars, which uniformly covers a broad range: M CO=(1 2)Mȯ-(20 44)M ȯ. The possible bimodality of the gamma-ray burstenergy distribution (E 1=1048 erg; ΔE2=3×1051-2×1054erg) could be associated with the bimodalmass distribution for stellar relativistic objects (MNS=(1.35±0.15)M ȯ; M BH=4 15M ȯ). The fact that SN1998bw is a “peculiar” type Ic supernova, not typical forthe collapses of WR stars (which usually give rise to type Ib/csupernovae), could be related to the rotation of the collapsing CO core.This “drags out/rd the time for the collapse, leading to theformation of a neutron star, a decrease in the gamma-ray burst energy,and an increase in the fraction of kinetic energy transferred to thesupernova envelope. The expected rate of collapse of the CO cores of WRstars in the Galaxy is ≈10-3/yr. This is at least three orders ofmagnitude higher than the mean frequency of gamma-ray bursts per galaxy(≈10-6 10-7/yr). Two models for gamma-ray bursts with WR stars asprogenitors are considered: the hypernova model of Paczynski (1998) andthe pulsation instability CO-core collapse model proposed by Gershte&$/set{lower0.5emhbox{smashriptscriptstylesmile}}{l} $; n (2000). In both models, the rate of CO-core collapses can be broughtinto agreement with the observed rate of gamma-ray bursts by taking intoaccount the anisotropy of the gamma radiation, associated with either arelativistic jet or the random character of the initial CO-core collapsedue to instabilities. It is concluded that WR stars could be theprogenitors of gamma-ray bursts. This hypothesis predicts the existenceof two types of gamma-ray bursts, corresponding to the bimodal massdistribution for stellar relativistic objects, and of three types ofoptical afterglow, associated with collapses of the CO cores of WR starsthat are single, in WR+O binaries, and in hypothetical WR+(A-M) systems.The paper also briefly examines a model of gamma-ray bursts as transientphenomena in the early stages of the evolution of galaxies (z>1),when very massive stars (M>100M ȯ) weak in heavy elements couldform. Such massive stars should also lose their hydrogen envelopes andbe transformed into massive WR stars, whose collapses could beaccompanied by gamma-ray bursts. It is suggested that WR galaxies arethe most probable candidates for the host galaxies of gamma-ray bursts.
| Magellanic Cloud WC/WO Wolf-Rayet stars - II. Colliding winds in binaries
A search for evidence of colliding winds is undertaken among the fourcertain Magellanic Cloud WC/WO spectroscopic binaries found in thecompanion Paper I, as well as among two Galactic WC/WO binaries of verysimilar subtype. Two methods of analysis, which allow the determinationof orbital inclination and parameters relating to the shock cone fromspectroscopic studies of colliding winds, are attempted. In the firstmethod, Lührs' spectroscopic model is fitted to the moderatelystrong Ciii 5696-Å excess line emission arising in the shock conefor the stars Br22 and WR 9. The four other systems show only very weakCiii 5696-Å emission. Lührs' model follows well the meandisplacement of the line in velocity space, but is unable to reproducedetails in the line profile and fails to give a reliable estimate of theorbital inclination. In the second method, an alternative attempt isalso made to fit the variation of more global quantities, full width athalf-maximum and radial velocity of the excess emission, with phase.This method also gives satisfactory results in a qualitative way, butshows numerical degeneracy with orbital inclination. Colliding windeffects on the very strong Civ 5808-Å Wolf-Rayet emission line,present in all six binaries, are also found to behave qualitatively asexpected. After allowing for line enhancement in colliding windbinaries, it now appears that all Magellanic Cloud WC/WO stars occupy avery narrow range in spectral subclass: WC4/WO3.
| Magellanic Cloud WC/WO Wolf-Rayet stars - I. Binary frequency and Roche lobe overflow formation
A nearly complete sample of 24 Magellanic Cloud WC/WO subclassWolf-Rayet stars is studied spectroscopically and photometrically todetermine its binary frequency. Theory predicts the Roche lobe overflowproduced Wolf-Rayet binary frequency to be 52+/-14 per cent in the LargeMagellanic Cloud and 100 per cent in the Small Magellanic Cloud, notcounting non-Roche lobe overflow Wolf-Rayet binaries. Lower ambientmetallicity (Z) leads to lower opacity, preventing all but the mostmassive (hence luminous) single stars from reaching the Wolf-Rayetstage. However, theory predicts that Roche lobe overflow even inbinaries of modest mass will lead to Wolf-Rayet stars in binaries withperiods below approximately 200d, for initial periods belowapproximately 1000d, independent of Z. By examining their absolutecontinuum magnitudes, radial velocity variations, emission-lineequivalent widths and full widths at half-maximum, a WC/WO binaryfrequency of only 13 per cent, significantly lower than the prediction,is found in the Large Magellanic Cloud. In the unlikely event that allof the cases with a less certain binary status actually turn out to bebinary, current theory and observation would agree. (The SmallMagellanic Cloud contains only one WC/WO star, which happens to be abinary.) The three WC+O binaries in the Large Magellanic Cloud all haveperiods well below 1000d. The large majority of WC/WO stars in suchenvironments apparently can form without the aid of a binary companion.Current evolutionary scenarios appear to have difficulty explainingeither the relatively large number of Wolf-Rayet stars in the MagellanicClouds, or the formation of Wolf-Rayet stars in general.
| 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.
| Non-thermal emission in Wolf-Rayet stars: are massive companions required?
We examine the radio spectral indices of 23 Wolf-Rayet (WR) stars toidentify the nature of their radio emission. We identify nine systems asnon-thermal emitters. In seven of these systems the non-thermal emissiondominates the radio spectrum, while in the remaining two it is ofcomparable strength to the thermal, stellar wind emission, giving`composite' spectra. Among these nine systems, seven have knownspectroscopic or visual binary companions. The companions are allmassive O or early B-type stars, strongly supporting a connectionbetween the appearance of non-thermal emission in WR stars and thepresence of a massive companion. In three of these binaries, the originof non-thermal emission in a wind-collision region between the stars hasbeen well established in earlier work. The binary systems that exhibitonly thermal emission are all short-period systems where awind-collision zone is deep within the opaque region of the stellar windof the WR star. To detect non-thermal emission in these systems requiresoptically thin lines of sight to the wind-collision region.
| Modelling the spectra of colliding winds in the Wolf-Rayet WC7+O binaries WR 42 and WR 79
We have obtained complete phase coverage of the WC7+O binaries WR 42=HD97152 and WR 79=HD 152270 with high signal-to-noise ratio (S/N),moderate-resolution spectra. Remarkable orbital phase-locked profilevariations of the Ciii λ5696 line are observed and interpreted asarising from colliding wind effects. Within this scenario, we havemodelled the spectra using a purely geometrical model that assumes acone-shaped wind-wind interaction region which partially wraps aroundthe O star. Such modelling holds the exciting promise of revealing anumber of interesting parameters for WR+O binaries, such as the orbitalinclination, the streaming velocity of material in the interactionregion and the ratio of wind momentum flux. Knowledge of theseparameters in turn leads to the possibility of a better understanding ofWR star masses, mass-loss rates and wind region characteristics.
| 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 75th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 916 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| Unidentified 3EG gamma-ray sources at low galactic latitudes
We present a study on the possible association of unidentified gamma-ray sources in the Third EGRET (3EG) catalog with different types ofgalactic objects such as Wolf-Rayet and Of stars, supernova remnants(SNRs), and OB associations (considered as pulsar tracers). We have madeuse of numerical simulations of galactic populations of gamma -ray pointsources in order to determine the statistical significance of thepositional coincidences. New constraints on pure chance association arepresented for SNRs and OB associations. There is overwhelmingstatistical support for the hypothesis that some SNRs are gamma -raysources (we obtain probabilities of pure chance association <10(-5)). OB star associations present higher, but still negligibleprobabilities (<10(-3) ). It is also shown that although massivestars present just marginally significant correlation with 3EG sources,there is a posteriori support for at least three candidates.
| Radio Continuum Measurements of Southern Early-Type Stars. III. Nonthermal Emission from Wolf-Rayet Stars
The Australia Telescope Compact Array (ATCA) has been used to search forradio continuum emission at 2.4 and 1.4 GHz from a sample of 36 southernWolf-Rayet stars. Seven Wolf-Rayet stars were detected at 2.4 GHz, ofwhich two were also detected at 1.4 GHz. We have identified sixWolf-Rayet stars, WR 14, 39, 48, 90, 105, and 112, that have nonthermalemission. The ATCA data confirm that at least 40% of Wolf-Rayet starswith measured spectral indices have nonthermal emission at centimeterwavelengths. Properties of each of the six sources are discussed. Themeasured spectral indices are between 0 and -1.0, and the radioluminosities are of order 10^29 ergs s^-1. So far 10 confirmed sourcesof nonthermal emission are known, including the six ATCA detections andfour previously known cases, WR 125, 140, 146, and 147. In all cases,the nonthermal radio emission almost certainly originates from aninteraction between the Wolf-Rayet stellar wind and the wind from amassive companion star. The radio observations agree well withtheoretical predictions for colliding winds. Synchrotron emission occursfrom relativistic electrons accelerated in strong shocks. The nonthermalspectral indices are likely to be close to -0.5. For WR 39, the detectedradio emission is offset by ~3" from the optical position of WR 39 andby ~2" from the optical position of WR 38B. We suggest that the radioemission may originate from a wind-wind interaction between WR 39 and WR38B, although this is not confirmed. For WR 11, the radio spectral indexincreases from +0.3 between 3 and 6 cm to +1.2 between 13 and 20 cm.This is interpreted as evidence for a highly attenuated nonthermalcomponent that originates well within the ionized wind of the W-R starfrom an interaction with the wind of the O9 companion star.
| 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.
| 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.
| 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.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. II. Photometry
Abundant {HIPPARCOS photometry over 3 years of 141 O and Wolf-Rayetstars, including 8 massive X-ray binaries, provides a magnificentvariety of light curves at the sigma ~ 1-5% level. Among the mostinteresting results, we mention: optical outbursts in HD 102567 (MXRB),coinciding with periastron passages; drastic changes in the light curveshape of HD 153919 (MXRB); previously unknown long-term variability ofHD 39680 (O6V:[n]pe var) and WR 46 (WN3p); unusual flaring of HDE 308399(O9V); ellipsoidal variations of HD 64315, HD 115071 and HD 160641;rotationally modulated variations in HD 66811=zeta Pup (O4Inf) and HD210839=lambda Cep (O6I(n)fp); dust formation episode in WR 121 (WC9). Ina statistical sense, the incidence of variability is slightly higheramong the WR stars, which might be explained by the higher percentage ofknown binary systems. Among the presumably single WR stars, thecandidate runaways appear to be more variable then the rest. Based ondata from the ESA Hipparcos astrometry satellite
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Radio Continuum Measurements of Southern Early-Type Stars. II. A Distance-limited Sample of Wolf-Rayet Stars
A distance-limited sample of southern Wolf-Rayet stars within 3 kpc ofthe Sun has been observed with the Australia Telescope Compact Array at8.64 and 4.80 GHz. Radio continuum flux densities at one or bothfrequencies were obtained for 10 sources and upper limits for 20; foursources are found to be thermal emitters on the basis of the observedspectral index. Five sources are classified as nonthermal. One sourcecould not be classified. We derive mass-loss rates for the thermalsources. After combining them with all existing radio mass-loss rates ofWolf-Rayet stars in the northern and southern hemisphere, we perform acomparison with mass-loss rates derived from optical emission lines. Thetwo methods lead to consistent results, which suggests either that theassumption of a spherically symmetric, stationary, homogeneous stellarwind is correct or that deviations from this assumption affect bothmethods in the same way. Wolf-Rayet mass-loss rates are surprisinglyuniform across spectral type. We find an average mass-loss rate of 4 x10-5 Mȯ yr-1 for all types observed, except for WC9 stars, whichhave rates that are lower by at least a factor of 2. An alternativeexplanation could be partial recombination of helium from He+ to He0 inthe radio region, which would lead to a reduced number of freeelectrons, and therefore reduced radio flux for WC9 stars. Mass-lossrates of 8 x 10-5 Mȯ yr-1 for late WN stars favored in recentstellar evolution models disagree with the observations of thesesubtypes. The results of this survey suggest that ~40% of all Wolf-Rayetstars with measured spectral index are nonthermal emitters at centimeterwavelengths. This percentage is nearly twice as high as that ofnonthermal emitters among OB stars and is higher than that previouslyestimated for WR stars. The nature of the nonthermal emission is stillnot fully understood. Possible causes of nonthermal emission arediscussed. In particular, we speculate that nonthermal emission mayarise from an interaction between a thermal WR wind and surroundingmaterial owing to a shell ejected during a previous evolutionary stageor owing the wind of a companion star.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| The number of O-type runaways, the number of O and Wolf-Rayet stars with a compact companion and the formation rate of double pulsars predicted by massive close binary evolution.
Using a detailed model of massive close binary evolution and accountingproperly for the effects of asymmetric supernova explosions (SN) wherewe use recent observations of pulsar runaway velocities, we determinethe theoretically expected number of post-SN O-type stars with andwithout a compact companion (CC), the number of O-type runaways, thenumber of WR+CC systems and the formation rate of binary pulsars in ourGalaxy. We conclude that o at least 50% of the O-type runaways areformed through the binary scenario, o less than 3% of all WR stars mayhide a CC, o the formation rate of binary pulsars in our Galaxy=~0.003-0.01 times the formation rate of massive stars; this correspondsroughly to a binary pulsar formation rate of the order of 10^-5^/year inagreement with the observations. Our results reveal a significantfraction of single WR stars but with a binary history. We also predictthe existence of 'weird' WR stars, i.e a WR star with a CC in its centre(descendants of Thorne-˙(Z)ytkow objects).
| Colliding Winds in Massive Binaries Involving Wolf-Rayet Stars (Invited Paper)
Not Available
| Photometric Determination of Orbital Inclinations and Mass Loss Rates for Wolf-Rayet Stars in WR+O Binaries
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....112.2227L
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
