Contents
Images
Upload your image
DSS Images Other Images
Related articles
A Multiwavelength Optical Emission Line Survey of Warm Ionized Gas in the Galaxy
We report on observations of several optical emission lines toward avariety of newly revealed faint, large-scale Hα-emitting regionsin the Galaxy. The lines include [N II] λ6583, [N II]λ5755, [S II] λ6716, [O III] λ5007, and He Iλ5876 obtained with the Wisconsin Hα Mapper (WHAM) towardsight lines that probe superbubbles, high-latitude filamentary features,and the more diffuse warm ionized medium (WIM). Our observations includemaps covering thousands of square degrees toward the well-knownOrion-Eridanus bubble, a recently discovered60deg×20deg bipolar superbubble centered inPerseus, plus several classical H II regions surrounding OB stars andhot evolved stellar cores. We use the emission-line data to explore thetemperature and ionization conditions within the emitting gas and theirvariations between the different emission regions. We find that in thediffuse WIM and in the faint high-latitude filamentary structures theline ratios of [N II]/Hα and [S II]/Hα are generally high,while [O III]/Hα and He I/Hα are generally low compared tothe bright classical H II regions. This suggests that the gas producingthis faint widespread emission is warmer, in a lower ionization state,and ionized by a softer spectrum than gas in classical H II regionssurrounding O stars, the presumed ionization source for the WIM. Inaddition, we find differences in physical conditions between the largebubble structures and the more diffuse WIM, suggesting that theionization of superbubble walls by radiation from interior Oassociations does not account entirely for the range of conditions foundwithin the WIM, particularly the highest values of [N II]/Hα and[S II]/Hα.
| 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%.
| 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.
| 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.
| A Very Large Array 3.6 Centimeter Continuum Survey of Galactic Wolf-Rayet Stars
We report the results of a survey of radio continuum emission ofGalactic Wolf-Rayet (WR) stars north of δ=-46°. Theobservations were obtained at 8.46 GHz (3.6 cm) using the Very LargeArray, with an angular resolution of ~6"×9" and typical rms noiseof ~0.04 mJy beam-1. Our survey of 34 WR stars resulted in 15definite and five probable detections, 13 of these for the first time atradio wavelengths. All detections are unresolved (θ<~5"). Timevariations in flux are confirmed in the cases of WR 98a, 104, 105, and125. WR 79a and WR 89 are also variable in flux, and we suspect they arealso nonthermal emitters. Thus, of our sample 20%-30% of the detectedstars are nonthermal emitters. Average mass-loss rate determinationsobtained excluding definite and suspected nonthermal cases give similarvalues for WN (all subtypes) and WC5-7 stars[M(WN)=(4+/-3)×10-5 Msolar yr-1and M(WC5-7)=(4+/-2)×10-5 Msolaryr-1], while a lower value was obtained for WC8-9 stars[M(WC8-9)=(2+/-1)×10-5 Msolaryr-1]. Uncertainties in stellar distances largely contributeto the observed scatter in mass-loss rates. Upper limits to themass-loss rates were obtained in cases of undetected sources and forsources that probably show additional nonthermal emission.
| Catalogue of Algol type binary stars
A catalogue of (411) Algol-type (semi-detached) binary stars ispresented in the form of five separate tables of information. Thecatalogue has developed from an earlier version by including more recentinformation and an improved layout. A sixth table lists (1872) candidateAlgols, about which fewer details are known at present. Some issuesrelating to the classification and interpretation of Algol-like binariesare also discussed.Catalogue 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/cgi-bin/qcat?J/A+A/417/263
| 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.
| An Effelsberg HI study of the ISM around WR 126, WR 154 and WR 155
The neutral hydrogen distribution has been studied in the direction ofthree Galactic Wolf-Rayet (WR) stars using the 100 m Effelsberg radiotelescope. Cavities in the HI distribution, regions of low HIemissivity, are observed over a 8-9 km s-1, velocity rangefor WR 126 (≡ST 2), WR 154 (≡HD 213049) and WR 155(≡HD 214419). These minima are interpreted as the observable 21-cmHI line counterpart of interstellar bubbles created by the winds of theWR stars and their progenitors. The HI cavities are elongated structuresdepicting an axial ratio ranging from 1.3 (WR 155) to 3 (WR 126). The WRstars are always eccentric with respect to either the geometric centreof the HI cavity or the absolute minimum inside it. This offset rangesfrom 50% to 80% of the HI hole's minor axis. The major axis of thesestructures range from 13 (WR 155) to 27 pc (WR 126), while the missingHI mass amounts to 45-50 Mȯ (WR 126), 60Mȯ (WR 155) and 85 Mȯ (WR 154).Figures 1, 3, 5 are only available in electronic form athttp://www.edpsciences.org
| 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.
| Progenitor models of Wolf-Rayet binaries: short-period WNE+O binaries with mass ratios q = 0.5
We identify two possible paths for the progenitor evolution of observedWNE+O binaries with WNE/O mass ratios close to 0.5 and periods between 7and 10 d. We show, through detailed binary evolution models, that withthe assumption that the O-type star expels most of the matter flowing atit during mass transfer, one possibility to obtain the observed systemsis through Case A mass transfer. We find a second solution usingstandard common envelope evolution. We conclude that in either case theO-type star in the three investigated systems did not accretesignificant amounts of mass. We discuss the intricate situation that inother cases massive close binaries may evolve conservatively.
| Sums of investigation of the linear polarization behaviour of binary systems with a Wolf-Rayet component
Analysis of the long-term (on a scale of years) behaviour of linearpolarization of four WR binary systems (CQ Cep, CX Cep, V444 Cyg and HD211853) is presented. Common features of the long-term polarizationvariations of CQ Cep, CX Cep and HD 211853 in combination with theresults of the harmonic analysis of their polarization curves allowed usto make assumptions on the causes of the found variability. The basicreason of the long-term polarization variability is likely to be thephysical activity of the WR components which manifests itself in theepisodic swelling of the WR envelopes and subsequent expulsion of theiroutermost layers. The involvement into the study of five wider "WR+O"pairs (HDE 311884, HD 90657, HD 97152, HD 152270 and HD 186943) allowedus to confirm these assumptions. The results of the analysis of thepolarization curves of nine WR binary systems are summed up. Threeconfirmations of high massiveness of the WR comnponent HDE 311884 havebeen derived.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| The Quadruple Wolf-Rayet System GP Cephei: Spectral Types, Masses, Mass-Loss Rate, and Colliding Winds
We have reevaluated the orbital elements for each pair of the quadruple(W-R+O) + (O+O) stellar system GP Cep and propose new spectral typesWN6o/WCE + O3-6, B0: I + B1: V-III. It is shown that there is only oneWolf-Rayet (W-R) star in GP Cep, contrary to a previous claim. A rate ofchange P=1.3+/-0.2 s yr-1 is determined for the W-R+O pair,which leads to a new period of 6.6887 days and to a W-R mass-loss rateof (0.8-3.0)×10-5 Msolar yr-1.Masses for this pair are estimated to be MW-R>~6Msolar and MO>~21 Msolar. Theeffects of wind-wind collision in the W-R+O pair are studied. It isshown that even after allowing for dilution by the OB components of thequadruple system, these effects are not as strong as in the binary V444Cygni (WN5+O6, P=4.212 days). In GP Cep, the phase-dependent, relativelyweak excess emission does not originate in the arms of the bow shockcone. Rather, it emerges from the extra heated portion of the W-R windfacing the hot O companion. The trailing bow shock arm is clearly seen,however, as an enhanced He I absorption component near quadrature atphase ~0.73. An anomalous blueshifted He I absorption is present atphase ~0.9, as is also seen in V444 Cyg, in the WC8+O9 I/O8 III binaryγ Velorum and in the LBV-cotype binary R81 (B2.5 Iab:e). A 3.5 dayorbit for the eclipsing B star pair is confirmed.
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| 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.
| On the polarimetric studies of the multiple system HD 211853 including a Wolf-Rayet star
The results of short-term observations of HD 211853 conducted at SAO RASin 1999 September are compared with those of earlier investigations ofthe system by St.-Louis et al. (1988) and Polyakova (1993). Theamplitude of linear polarizaton variation of the system which is causedby the scatter of light of the O component in the WR star's envelope inthe main (WN6+O) pair increased from 0.2-0.3% to 1% in our observations.Analysis of the results of the first two polarimetric studies of thesystem yielded different values of the orbit inclination of the (WN6+O)pair: ipolar = 78.2° in St.-Louis et al. (1988) andipolar = 65° in Polyakova (1993). The crude failure tocomply with the criterion ΔΩ =|&Omega1-Ω2| = 90° that we revealedin the two analysis causes doubt in either estimate ofipolar. A co-operative examination of three series ofpolarization observations of HD 211853 allows an assumption to be madethat the first two curves of linear polarization variations of thesystem with phase of the orbital period P = 6.d6884(observations of 1984-1986 and 1989) displayed an excited state of the(WN6+O) pair preceding the expulsion of the outermost parts of the WRenvelope. The latter is evidenced by a sharp drop in linear polarizationof the system (to the level of polarization of the interstellarconjunction in the (WN6+O) pair (the O star is at the front). Basing onthe fact that HD 211853 is a member of the association Cep OB1, anattempt is made to determine the most likely values of physicalparameters of the stars of the (WN6+O) pair. From our estimates thespectrum of O star is close to O7V, mO ≍25msolar, mWN = 8-12msolar,αWR+αO = 47-50Rsolar and i= 65°-75°.
| Multi-frequency variations of the Wolf-Rayet system HD193793 (WC7pd+O4-5) III. IUE observations
The colliding-wind binary system WR 140 (HD 193793, WC7pd+O4-5, P = 7.94yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35short-wavelength high-resolution spectra. An absorption-lineradial-velocity solution is obtained from the photospheric lines of theO component, by comparison with a single O star. The resulting orbitalparameters, e = 0.87 +/- 0.05, omega = 31degr +/- 9degr andKO star = 25 +/- 15 km s-1, confirm the largeeccentricity of the orbit, within the uncertainties of previous opticalstudies. This brings the weighted mean UV-optical eccentricity to e =0.85 +/- 0.04. Occultation of the O-star light by the WC wind and theWC+O colliding-wind region results into orbital modulation of theP-Cygni profiles of the C ii, C iv and Si iv resonance lines. Nearperiastron passage, the absorption troughs of those resonance-lineprofiles increase abruptly in strength and width, followed by a gradualdecrease. In particular, near periastron the blue black-edges of theP-Cygni absorption troughs shift to larger outflow velocities. Wediscuss that the apparently larger wind velocity and velocity dispersionobserved at periastron could be explained by four phenomena: (i)geometrical resonance-line eclipse effects being the main cause of theobserved UV spectral variability, enhanced by sightline crossing of theturbulent wind-wind collision zone; (ii) the possibility of anorbital-plane enhanced WC7 stellar wind; (iii) possible common-envelopeacceleration by the combined WC and O stellar radiation fields; and (iv)possible enhanced radiatively driven mass loss due to tidal stresses,focused along the orbiting line of centers.
| 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.
| 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.
| 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.
| 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
| Spectra of Wolf-Rayet stars and planetary nebulae
Some spectrograms of Wolf-Rayet (WR) stars and planetary nebulae (PNe)are captured with a home-made spectroscope attached to aSchmidt-Cassegrain telescope, and the spectra are briefly analysed. Theconstruction and potential of a low resolution stellar spectroscope isdiscussed together with techniques to capture images.
| An investigation of the large-scale variability of the apparently single Wolf-Rayet star WR 1
In recent years, much studies have focused on determining the origin ofthe large-scale line-profile and/or photometric patterns of variabilitydisplayed by some apparently single Wolf-Rayet stars, with the existenceof an unseen (collapsed?) companion or of spatially extended windstructures as potential candidates. We present observations ofWR 1 which highlight the unusual character of thevariations in this object. Our narrowband photometric observationsreveal a gradual increase of the stellar continuum flux amounting toDelta v ~ 0.09 mag followed by a decline on about the same timescale(3-4 days). Only marginal evidence for variability is found during the11 following nights. Strong, daily line-profile variations are alsoobserved but they cannot be easily linked to the photometric variations.Similarly to the continuum flux variations, coherent time-dependentchanges are observed in 1996 in the centroid, equivalent width, andskewness of He Ii lambda 4686. Despite the generally coherent nature ofthe variations, we do not find evidence in our data for the periodsclaimed in previous studies. While the issue of a cyclical pattern ofvariability in WR 1 is still controversial, it isclear that this object might constitute in the future a cornerstone forour understanding of the mechanisms leading to the formation of largelyanisotropic outflows in Wolf-Rayet stars.
| ICCD Speckle Observations of Binary Stars. XXII. A Survey of Wolf-Rayet Starsfor Close Visual Companions
We present the results of a speckle interferometric survey for closevisual companions, mainly among 29 of the apparently brightestWolf-Rayet (W-R) stars. Only one target, WR 48 = theta Mus, was resolvedas a close astrometric binary (with a separation of 46+/-9 mas). Thissystem is probably a triple comprising a short-period W-R binary plus adistant O supergiant companion. Although our binary detection fractionis low, it is not an unexpected result given the selection effects thatmilitate against easy detection of binaries. New, higher resolutionobservations will almost certainly increase the fraction of binaries.There are four known binaries among the six W-R stars in our sample thathave nonthermal radio emission, and this connection supports the ideathat the nonthermal emission originates in the wind-wind collisionbetween components.
| Refined Orbital Parameters of HD 193928
Not Available
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Cepheus |
| Right ascension: | 22h18m45.60s |
| Declination: | +56°07'33.9" |
| Apparent magnitude: | 9.029 |
| Distance: | 552.486 parsecs |
| Proper motion RA: | -4.7 |
| Proper motion Dec: | -2.8 |
| B-T magnitude: | 9.444 |
| V-T magnitude: | 9.064 |
Catalogs and designations:
|
