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Публикации по объекту
MOST Finds No Coherent Oscillations in the Hot Carbon-rich Wolf-Rayet Star HD 165763 (WR 111)
We have photometrically monitored the V = 8 mag Galactic Population IWC5 star WR 111 for 3 weeks nonstop using the MOST microsatellite. Eachof the ~27,000 data points has a precision of ~3 mmag. We find nocoherent Fourier components above the 50 part per million level over thewhole interval for frequencies f>10 cd-1 (periods P <2.4 hr). This limit is nearly 2 orders of magnitude below recentpredictions for early-type WR stars based on strange-mode pulsationsimulations, with expected periods in the range 10-30 minutes.Simultaneous spectroscopic observations of WR 111 reveal a normal levelof stochastic clumps propagating in the wind, which possibly manifestthemselves in the slow 1/f rise in the MOST power spectrum below f~10cd-1. Time-frequency analysis of the MOST data shows noobvious short-lived frequencies above the 1 mmag level, in starkcontrast to the highly variable cool WR stars WR 123 (WN8) and WR 103(WC9d), monitored previously by MOST. Radiation pressure thereforeappears to be the main, if not sole, driver of WR 111's strong wind.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon, Inc., the University of Toronto Instituteof Aerospace Studies, and the University of British Columbia with theassistance of the University of Vienna.
| Mass loss from late-type WN stars and its Z-dependence. Very massive stars approaching the Eddington limit
Context: The mass loss from Wolf-Rayet (WR) stars is of fundamentalimportance for the final fate of massive stars and their chemicalyields. Its Z-dependence is discussed in relation to the formation oflong-duration Gamma Ray Bursts (GRBs) and the yields from early stellargenerations. However, the mechanism of formation of WR-type stellarwinds is still under debate. Aims: We present the first fullyself-consistent atmosphere/wind models for late-type WN stars. Weinvestigate the mechanisms leading to their strong mass loss, andexamine the dependence on stellar parameters, in particular on themetallicity Z. Methods: We perform a systematic parameter study of themass loss from WNL stars, utilizing a new generation of hydrodynamicnon-LTE model atmospheres. The models include a self-consistenttreatment of the wind hydrodynamics, and take Fe-group line-blanketingand clumping into account. They thus allow a realistic modelling of theexpanding atmospheres of WR stars. The results are verified bycomparison with observed WNL spectra. Results: We identify WNL stars asvery massive stars close to the Eddington limit, potentially still inthe phase of central H-burning. Due to their high L/M ratios, thesestars develop optically thick, radiatively driven winds. These windsshow qualitatively different properties than the thin winds of OB stars.The resultant mass loss depends strongly on Z, but also on the Eddingtonfactor Γ_e, and the stellar temperature T_star. We combine ourresults in a parametrized mass loss recipe for WNL stars. Conclusions:According to our present model computations, stars close to theEddington limit tend to form strong WR-type winds, even at very low Z.Our models thus predict an efficient mass loss mechanism for lowmetallicity stars. For extremely metal-poor stars, we find that theself-enrichment with primary nitrogen can drive WR-type mass loss. Thesefirst WN stars might play an important role in the enrichment of theearly ISM with freshly produced nitrogen.
| Near Infrared Spectra of Galactic Wolf-Rayet Stars
Spectra of 37 Galactic WR stars were observed and reduced in thespectral range 790--895 nm. The main spectral features are identifiedand the equivalent widths and FWHMs of the strongest emission lines aremeasured. The equivalent width of the diffuse interstellar band at 862nm is also measured and the new estimates of color excessesE(B-V) are derived by using an empirical relationship betweenthe equivalent width and the color excess. The equivalent width ratiosfor the lines C III 850 nm, C IV 886 nm and C II 880 nm were found tocorrelate well with the WC subtype.
| Neon Abundances from a Spitzer/IRS Survey of Wolf-Rayet Stars
We report on neon abundances derived from Spitzer high resolutionspectral data of eight Wolf-Rayet (WR) stars using the forbidden line of[Ne III] 15.56 μm. Our targets include four WN stars of subtypes 4-7,and four WC stars of subtypes 4-7. We derive ion fraction abundancesγ of Ne2+ for the winds of each star. The ion fractionabundance is a product of the ionization fraction Qi in stagei and the abundance by number AE of element E relativeto all nuclei. Values generally consistent with solar are obtained forthe WN stars, and values in excess of solar are obtained for the WCstars.
| Discovery of the strongly eccentric, short-period binary nature of the B-type system HD 313926 by the MOST satellite
The MOST photometric space mission discovered an eclipsing binary amongits guide stars in 2006 June which combines a relatively largeeccentricity e = 0.20 with an orbital period of only 2.27 d. HD 313926appears to consist of two early-type stars of spectral type B3-B7. Ithas the largest eccentricity among known early-type binaries withperiods less than 3.5 d. Despite the large components indicated by itsspectral type and light curve model, and its short period, the orbit ofHD 313926 has not yet circularized so it is probably very young, evencompared with other young B stars.Based on data from MOST, a Canadian Space Agency mission jointlyoperated by Dynacon Inc., the University of Toronto Institute forAerospace Studies and the University of British Columbia, with theassistance of the University of Vienna, and on data from the DavidDunlap Observatory, University of Toronto.E-mail: rucinski@astro.utoronto.ca
| Physical Properties of Wolf-Rayet Stars
The striking broad emission line spectroscopic appearance of Wolf-Rayet(WR) stars has long defied analysis, owing to the extreme physicalconditions within their line- and continuum-forming regions. Recently,model atmosphere studies have advanced sufficiently to enable thedetermination of stellar temperatures, luminosities, abundances,ionizing fluxes, and wind properties. The observed distributions ofnitrogen- (WN) and carbon (WC)-sequence WR stars in the Milky Way and innearby star-forming galaxies are discussed; these imply lower limits toprogenitor masses of ?25, 40, and 75 Mȯ forhydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars,respectively. WR stars in massive star binaries permit studies ofwind-wind interactions and dust formation in WC systems. They also showthat WR stars have typical masses of 10 25 Mȯ, extendingup to 80 Mȯ for H-rich WN stars. Theoretical andobservational evidence that WR winds depend on metallicity is presented,with implications for evolutionary models, ionizing fluxes, and the roleof WR stars within the context of core-collapse supernovae andlong-duration gamma-ray bursts.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| Radio, X-ray, and γ-ray emission models of the colliding-wind binary WR140
We use hydrodynamical models of the wind-collision region in thearchetype colliding-wind system WR140 to determine the spatial andspectral distributions of the radio, X-ray, and γ-ray emissionfrom shock-accelerated electrons. Our calculations are for orbital phase0.837 when the observed radio emission is close to maximum. Using theobserved thermal X-ray emission at this phase in conjunction with theradio emission to constrain the mass-loss rates, we find that the O starmass-loss rate is consistent with the reduced estimates for O4-5supergiants by Fullerton, Massa & Prinja, and the wind-momentumratio, η = 0.02. This is independent of the opening angle deducedfrom radio very long baseline interferometry observations of the WCRthat we demonstrate fail to constrain the opening angle.We show that the turnover at ~3 GHz in the radio emission is due tofree-free absorption, since models based on the Razin effect have anunacceptably large fraction of energy in non-thermal electrons. We findthat the spectral index of the non-thermal electron energy distributionis flatter than the canonical value for diffusive shock acceleration,namely p < 2. Several mechanisms are discussed that could lead tosuch an index. Our inability to obtain fits to the radio data with p> 2 does not exclude the possibility of shock modification, butstronger evidence than that which currently exists is necessary for itssupport.Tighter constraints on p and the nature of the shocks in WR140 will beobtained from future observations at MeV and GeV energies, for which wegenerally predict lower fluxes than those in previous works. Since thehigh stellar photon fluxes prevent the acceleration of electrons beyondγ >~ 105-106, TeV emission fromcolliding-wind binary systems will provide unambiguous evidence ofpion-decay emission from accelerated ions. We finish by commenting onthe emission and physics of the multiple wind collisions in densestellar clusters, paying particular attention to the Galactic Centre.
| WR 143: a Wolf-Rayet binary
Near-infrared spectroscopy and photometry of the Wolf-Rayet star WR 143(HD 195177) were obtained in the JHK photometric bands. High-resolutionspectra observed in the J and H bands exhibit a narrow 1.083-μm HeIline and the HI Paβ and Brackett series lines in emissionsuperposed on the broad emission-line spectrum of the Wolf-Rayet star,giving strong indications of the presence of a companion. From thenarrow emission lines observed, the companion is identified to be anearly-type Be star. The photometric magnitudes exhibit variations in theJHK bands, which are probably due to the variability of the companionstar. The flux density distribution is too steep for a Wolf-Rayetatmosphere. This is identified to be mainly due to the increasingcontribution from the early-type companion star towards shorterwavelengths.
| Bulk Velocities, Chemical Composition, and Ionization Structure of the X-Ray Shocks in WR 140 near Periastron as Revealed by the Chandra Gratings
The Wolf-Rayet WC7+O4-5 binary WR 140 went through the periastronpassage of its 8 yr eccentric binary orbit in early 2001 as the twostars made their closest approach. Both stars have powerful supersonicstellar winds that crash into each other between the stars to produceX-rays. Chandra grating observations were made when the X-rays were attheir peak, making WR 140 the brightest hot-star X-ray source in the skyand giving the opportunity to study the velocity profiles of lines, allof which were resolved and blueshifted before periastron. In the generalcontext of shock physics, the measurements constrain the flow of hot gasand where different ions were made. The brightness of lines relative tothe strong continuum in conjunction with plasma models gives interimabundance estimates for eight different elements in WC-type materialincluding an Ne/S ratio in good agreement with earlier long-wavelengthmeasurements. The lower velocity widths of cool ions imply a plasma thatwas not in equilibrium, probably due to the collisionless nature of theshock transitions and the slow character of both the postshock energyexchange between ions and electrons and subsequent ionization. Electronheat conduction into fast-moving preshock gas was absent, probablysuppressed by the magnetic field involved in WR 140's synchrotronemission. After periastron, the spectrum was weaker due mainly toabsorption by cool Wolf-Rayet star material.
| A spectroscopic search for the non-nuclear Wolf-Rayet population of the metal-rich spiral galaxy M 83
We present a catalogue of non-nuclear regions containing Wolf-Rayetstars in the metal-rich spiral galaxy M 83 (NGC 5236). From a total of283 candidate regions identified using He ii λ4686 imaging withVLT-FORS2, Multi Object Spectroscopy of 198 regions was carried out,confirming 132 WR sources. From this sub-sample, an exceptional contentof ~1035 ± 300 WR stars is inferred, with N(WC)/N(WN) ~ 1.2,continuing the trend to larger values at higher metallicity amongstLocal Group galaxies, and greatly exceeding current evolutionarypredictions at high metallicity. Late-type stars dominate the WCpopulation of M 83, with N(WC8-9)/N(WC4-7) = 9 and WO subtypes absent,consistent with metallicity dependent WC winds. Equal numbers of late toearly WN stars are observed, again in contrast to current evolutionarypredictions. Several sources contain large numbers of WR stars. Inparticular, #74 (alias region 35 from de Vaucouleurs et al.) contains~230 WR stars, and is identified as a Super Star Cluster from inspectionof archival HST/ACS images. Omitting this starburst cluster would resultin revised statistics of N(WC)/N(WN) ~ 1 and N(WC8-9)/N(WC4-7) ~ 6 forthe "quiescent" disk population. Including recent results for thenucleus and accounting for incompleteness in our spectroscopic sample,we suspect the total WR population of M 83 may exceed 3000 stars.
| Hydrodynamic model atmospheres for WR stars. Self-consistent modeling of a WC star wind
We present the first non-LTE atmosphere models for WR stars thatincorporate a self-consistent solution of the hydrodynamic equations.The models take iron-group line-blanketing and clumping into account,and compute the hydrodynamic structure of a radiatively driven windconsistently with the non-LTE radiation transport in the co-movingframe. We construct a self-consistent wind model that reproduces allobserved properties of an early-type WC star (WC5). We find that theWR-type mass-loss is initiated at high optical depth by the so-called“Hot Iron Bump” opacities (Fe IX-XVI). The acceleration ofthe outer wind regions is due to iron-group ions of lower excitation incombination with C and O. Consequently, the wind structure shows twoacceleration regions, one close to the hydrostatic wind base in theoptically thick part of the atmosphere, and another farther out in thewind. In addition to the radiative acceleration, the “IronBump” opacities are responsible for an intense heating of deepatmospheric layers. We find that the observed narrow O VI emission linesin the optical spectra of WC stars originate from this region. Fromtheir dependence on the clumping factor we gain important informationabout the location where the density inhomogeneities in WR-winds startto develop.
| Inferring hot-star-wind acceleration from Line Profile Variability
The migration of profile sub-peaks identified in time-monitored opticalemission lines of Wolf-Rayet (WR) star spectra provides a directdiagnostic of the dynamics of their stellar winds via a measured ΔvLOS/Δ t, a line-of-sight velocity change per unittime. Inferring the associated wind acceleration scale from such anapparent acceleration then relies on the adopted intrinsic velocity ofthe wind material at the origin of this variable pattern. Such acharacterization of the Line Emission Region (LER) is in principlesubject to inaccuracies arising from line optical depth effects andturbulence broadening. In this paper, we develop tools to quantify sucheffects and then apply these to reanalyze the LER properties oftime-monitored WR stars. We find that most program lines can be fittedwell with a pure optically thin formation mechanism, that the observedline-broadening is dominated by the finite velocity extent of the LER,and that the level of turbulence inferred through Line ProfileVariability (lpv) has only a minor broadening effect in the overallprofile. Our new estimates of LER velocity centroids are systematicallyshifted outwards closer to terminal velocity compared to previousdeterminations, now suggesting WR-wind acceleration length scales βR* of the order of 10-20 Rȯ, a factor of afew smaller than previously inferred. Based on radiation-hydrodynamicssimulations of the line-driven-instability mechanism, we computesynthetic lpv for Ciii5696 Å for WR 111. The results match wellthe measured observed migration of 20-30 m s-2, equivalent toβ R* 20 Rȯ. However, our modelstellar radius of 19 Rȯ, typical of an O-typesupergiant, is a factor 2-10 larger than generally expected for WR coreradii. Such small radii leave inferred acceleration scales to be moreextended than expected from dynamical models of line driving, whichtypically match a “beta” velocity lawv(r)=v&infy; (1-R*/r)β, withβ ≈ 1-2; but the severity of the discrepancy is substantiallyreduced compared to previous analyses. We conclude with a discussion ofhow using lines formed deeper in the wind would provide a strongerconstraint on the key wind dynamics in the peak acceleration region,while also potentially providing a diagnostic on the radial variation ofwind clumping, an issue that remains crucial for reliable determinationof O-star mass loss rates.
| An Atlas of Far-Ultraviolet Spectra of Wolf-Rayet Stars from the FUSE Satellite
We present an atlas of far-ultraviolet spectra of 21 Wolf-Rayet (WR)stars in the Galaxy and Large and Small Magellanic Clouds, secured withthe Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The datacover the wavelength range of 912-1190 Å at a spectral resolutionof 0.1 Å and span examples of most subtypes in the WN and WCsequences. We discuss the FUV spectral morphology of the different WRsequences, emphasizing the wide range of ions and chemical speciesexhibiting well-developed P Cygni profiles and emission lines in thiswavelength range. For WN stars the relative strengths of C IV/C III, NIII/N II, P V/P IV, and Si IV/Si III show a decrease in strength of thehigh ions from WN3 to WN11 complemented by an increase in the lower ionsat later types. The ``super ions'' of O VI and S VI are consideredphotoionized wind features for WN3-WN6 stars, probably the result ofAuger ionization in WN7-WN9 stars, and probably absent at WN10-WN11. TheWN5h star Sk 41 in the SMC shows relatively weaker features, which canbe ascribed to the effects of a global galaxy metal deficiency. For theWC stars, a similar pattern of wind ionization-linked strengths in theemissions and P Cygni profiles is present, particularly evident in therelative strengths of lines in P V, S IV, Si IV, and Si III. O VI, and SVI features are only seen in the earliest WC subtypes. The high carbonabundance in WC stars is reflected by the presence of strong C IV and CIII lines throughout the sequence. We present new estimates of the windterminal velocities from measurements of saturated absorption componentsobserved in a wide range of I.P. species. Considerable revisions tov&infy; for the WN3 and WN5 (SMC) stars in our sample and,in particular for the WN10 and WN11 stars are found. The latter make useof the unique availability of the N II resonance line in the FUSEwaveband.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| The influence of binaries on galactic chemical evolution
Understanding the galaxy in which we live is one of the greatintellectual challenges facing modern science. With the advent of highquality observational data, the chemical evolution modeling of ourgalaxy has been the subject of numerous studies in the last years.However, all these studies have one missing element which is theevolution of close binaries. Reason: their evolution is very complex andsingle stars only perhaps can do the job. (Un)Fortunately at present weknow that the majority of the observed stars are members of a binary ormultiple system and that certain objects can only be formed throughbinary evolution. Therefore galactic studies that do not account forclose binary evolution may be far from realistic.Because of the large expertise developed through the years in stellarevolution in general and binary evolution in particular at the BrusselsAstrophysical Institute, we found ourselves in a privileged position tobe the first to do chemical evolutionary simulations with the inclusionof detailed binary evolution. The complexity of close binary evolutionhas kept many astronomers from including binary stars into theirstudies. However, it is not always the easiest way of living that givesyou the most excitement and satisfaction.
| A Newly Discovered Open Cluster Surrounding the Wolf-Rayet Stars WR 38 and WR 38a
Photoelectric and CCD UBV survey data are presented for stars brighterthan about V~16.5 in the field of the faint Carina Wolf-Rayet (WR) starsWR 38 and WR 38a. Both WR stars appear to belong to an associatedcompact cluster (nuclear diameter ~0.3′) of at least six faint OBstars reddened by EB-V=1.60+/-0.02 s.e. and lying at adistance of ~14.5+/-1.6 kpc (V0-MV=15.80+/-0.25s.d.). As cluster members, the two WR stars have estimated luminositiesof Mv=-5.8 (WR 38, type WC4) and Mv=-5.0 (WR 38a,type WN5). The former value is slightly more luminous than expected forGalactic WR stars.
| Comprehensive modelling of the planetary nebula LMC-SMP 61 and its [WC]-type central star
We present a comprehensive study of the Magellanic Cloud planetarynebula SMP 61 and of its nucleus, a Wolf-Rayet type star classified [WC5-6]. The observational material consists of HST STIS spectroscopy andimaging, together with optical and UV spectroscopic data collected fromthe literature and infrared fluxes measured by IRAS. We have performed adetailed spectral analysis of the central star, using the Potsdam codefor expanding atmospheres in non-LTE. For the central star we determinethe following parameters: Lstar = 103.96Lȯ, Rstar = 0.42 Rȯ,Tstar = 87.5 kK, Mȯ = 10-6.12Mȯ yr-1 d, v∞ = 1400 kms-1, and a clumping factor of D = 4. The elemental abundancesby mass are XHe = 0.45, XC = 0.52, XN< 5 × 10-5, XO = 0.03, and XFe< 1 × 10-4. The fluxes from the model stellaratmosphere were used to compute photoionization models of the nebula.All the available observations, within their error bars, were used toconstrain these models. We find that the ionizing fluxes predicted bythe stellar model are consistent with the fluxes needed by thephotoionization model to reproduce the nebular emission, within theerror margins. However, there are indications that the stellar modeloverestimates the number and hardness of Lyman continuum photons. Thephotoionization models imply a clumped density structure of the nebularmaterial. The observed C II] λ 2326i/C II] λ 2326r lineratio implies the existence of carbon-rich clumps in the nebula. Suchclumps are likely produced by stellar wind ejecta, possibly mixed withthe nebular material. We discuss our results with regard to the stellarand nebular post-AGB evolution. The observed Fe-deficiency for thecentral star indicates that the material which is now visible on thestellar surface has been exposed to s-process nucleosynthesis duringprevious thermal pulses. The absence of nitrogen allows us to set anupper limit to the remaining H-envelope mass after a possible AGB finalthermal pulse. Finally, we infer from the total amount of carbondetected in the nebula that the strong [WC] mass-loss may have beenactive only for a limited period during the post-AGB evolution.Table \ref{neb:fluxes} is only available in electronic form athttp://www.edpsciences.org
| Cosmic Rays Acceleration in Wolf-Rayet Stellar Winds
Popescu et al (2004) gave a model for the observed cosmic rays between5×1015 and 3×1018 eV. Their source ispresumed to be the supernova of stars that explode in their winds. Theobserved cosmic rays abundance at the source are affected by spallationin the supernova shell, by the difference in ionization degree (beingone or two times ionized) at the injection in the supernova shock, thestars with initial masses 15MSun≤M≤30MSunhaving a different contribution to them than the stars with30MSun≤M≤50MSun, this being 2:1 for theelements with Z≥6. Still, the abundances after these corrections aredifferent by a factor Zi/ZHe, where Ziis the atomic number for the element i. This paper is dedicated to theexplanation of this factor and its physical meanings by consideringthat, prior to the shock injection, the wind particles are radiativeaccelerated.
| The chemical evolution of the Galaxy: the importance of stars with an initial mass larger than 40 Msolar
In the present paper we investigate in how far stars with an initialmass larger than 40 Msolar affect the chemical enrichment ofthe Galaxy. We illustrate the importance for chemical yields of a mostup-to-date treatment of the various stellar wind mass loss episodes instellar evolutionary codes and we discuss the effects of a possiblesupernova-like outburst prior to massive black hole formation.
| Stellar evolution with rotation. X. Wolf-Rayet star populations at solar metallicity
We examine the properties of Wolf-Rayet (WR) stars predicted by modelsof rotating stars taking account of the new mass loss rates for O-typestars and WR stars (Vink et al. \cite{Vink00}, \cite{Vink01}; Nugis& Lamers \cite{NuLa00}) and of the wind anisotropies induced byrotation. We find that the rotation velocities v of WR stars are modest,i.e. about 50 km s-1, not very dependent on the initial v andmasses. For the most massive stars, the evolution of v is very stronglyinfluenced by the values of the mass loss rates; below ~ 12 M_sun theevolution of rotation during the MS phase and later phases is dominatedby the internal coupling. Massive stars with extreme rotation may skipthe LBV phase.Models having a typical v for the O-type stars have WR lifetimes on theaverage two times longer than for non-rotating models. The increase ofthe WR lifetimes is mainly due to that of the H-rich eWNL phase.Rotation allows a transition WN/WC phase to be present for initialmasses lower than 60 M_sun. The durations of the other WR subphases areless affected by rotation. The mass threshold for forming WR stars islowered from 37 to 22 M_sun for typical rotation. The comparisons of thepredicted number ratios WR/O, WN/WC and of the number of transitionWN/WC stars show very good agreement with models with rotation, whilethis is not the case for models with the present-day mass loss rates andno rotation. As to the chemical abundances in WR stars, rotation bringsonly very small changes for WN stars, since they have equilibrium CNOvalues. However, WC stars with rotation have on average lower C/He andO/He ratios. The luminosity distribution of WC stars is also influencedby rotation.
| The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars
The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksecXMM-Newton, observation, implying an upper limit to the X-ray luminosityof LX < 2.5x 1030 erg s-1 andto the X-ray to bolometric luminosity ratio ofLX/Lbol < 4*E-9. This confirmsindications from earlier less sensitive measurements that there has beenno convincing X-ray detection of any single WC star. This lack ofdetections is reinforced by XMM-Newton, and CHANDRA observations of WCstars. Thus the conclusion has to be drawn that the stars withradiatively-driven stellar winds of this particular class areinsignificant X-ray sources. We attribute this to photoelectronicabsorption by the stellar wind. The high opacity of the metal-rich anddense winds from WC stars puts the radius of optical depth unity athundreds or thousands of stellar radii for much of the X-ray band. Webelieve that the essential absence of hot plasma so far out in the windexacerbated by the large distances and correspondingly high ISM columndensities makes the WC stars too faint to be detectable with currenttechnology. The result also applies to many WC stars in binary systems,of which only about 20% are identified X-ray sources, presumably due tocolliding winds.
| The Asiago Database on Photometric Systems (ADPS). II. Band and reddening parameters
The Asiago Database on Photometric Systems (ADPS) is a compilation ofbasic information and reference data on 201 photometric systems (bothground-based and space-born), available in printed form (Moro &Munari \cite{Moro00}, hereafter Paper I) and electronically(http://ulisse.pd.astro.it/ADPS). Seventeen new systems have been addedto ADPS since its publication, bringing the total to 218. In this PaperII, band and reddening parameters are homogeneously computed viasynthetic photometry for the censed photometric systems with known bandtransmission profiles (179 systems). Band parameters include varioustypes of wavelengths (mean, peak, Gaussian, and effective according to aseries of representative spectral types), widths (width at half maximum,at 80% and 10% of transmission's peak, FWHM of the fitting Gaussian,equivalent, and effective for representative spectral types), moment ofthe 2nd order, skewness and kurtosis indices, and polynomial expressionsfor the behavior of effective wavelength and effective width as functionof black-body temperature. Reddening parameters include A(lambda )/A(V)for three reddening laws (characterized by RV=5.0, 3.1 and2.1) and its range of variability over the HR diagram, the Cardelli etal. (\cite{Cardelli89}) a(x) and b(x) coefficients, second order fits toA(lambda )/ EB-V for three representative spectral types, andpolynomial expressions for the behavior of effective wavelength andeffective width as function of reddening (for the RV=3.1law).Figures 9-187 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/401/781}\fnmsep\thanks{Thesource spectra used in the computations are available via the webinterface to ADPS: http://ulisse.pd.astro.it/ADPS/}
| The effects of binaries on the evolution of Wolf-Rayet type spectral features in starbursts
In the present paper we investigate in detail the effects of binarieswith initial period between 1 day and 10 years on theoreticalsimulations of Wolf-Rayet (WR) type spectral features in starbursts. Wefocus on the evolution of the nebular Hbeta line instarburst in general, on the intensity ratios I(nebular He II;lambda4686)/I(Hbeta ), I(blue bump)/I(Hbeta ), and I(redbump)/I(Hbeta ) as a function of the equivalent width ofHbeta of WR galaxies in particular. The binary evolutionaryprocesses that dominate the evolution of the considered spectralfeatures are the Roche lobe overflow in case Br systems, the masstransfer efficiency, and the merger rate. We show that the predictionson nebular He II depend critically on the uncertainties in the theory ofWR atmospheres and particularly on uncertainties in the treatment of thesubsonic velocity region of the WR wind. The observations of lowmetallicity starbursts are best reproduced by a theoretical model with asignificant number of binaries and with a metallicity-dependent WR wind.
| Wolf-Rayet star parameters from spectral analyses
The Potsdam non-LTE code for expanding atmospheres, which accounts forclumping and iron-line blanketing, has been used to establish a grid ofmodel atmospheres for WC stars. A parameter degeneracy is discovered forearly-type WC models which do not depend on the `stellar temperature'.15 Galactic WC4-7 stars are analyzed, showing a very uniform carbonabundance (He:C = 55:40) with only few exceptions.
| Hydrodynamic model atmospheres for hot stars
Recent non-LTE models for expanding atmospheres, accounting for irongroup line-blanketing and clumping, show a radiative acceleration whichsupplies a large part of the driving force of WR and O star winds.Aiming at the calculation of fully consistent wind models, we developeda method to include the solution of the hydrodynamic equations into ouratmosphere code, taking into account the radiation pressure from the CMFradiation transport. In the present work we discuss the resulting windacceleration for `standard' WR and O-type star model atmospheres, andpresent a hydrodynamically consistent non-LTE model for the O4I(n)f starζ Pup. In addition we demonstrate the effect of clumping on theradiative acceleration.
| Advances in modeling of Wolf-Rayet stars
Due to advances in computer power and numerical techniques non-LTEline-blanketing calculations for Wolf-Rayet (WR) stars are now routine.The incorporation of blanketing has led to significant improvements inspectral analyses, and to a systematic increase in the derived WRluminosities. To make further progress we need to understand thedistribution, structure, and strength of inhomogeneities in the stellarwind, and in turn, how these influence diagnostics of WR stellarparameters, and radiative driving. Further, we need to understand thephysical process that initiates mass loss in WR stars. Problems withexisting wind calculations are examined, and the difficulty ofobservationally determining the shape of the velocity law around thesonic point is discussed. To determine the wind dynamics around thesonic point, it is essential to include ions with ionization potentialsin excess of 300 eV. A recent study of the O7 Iaf+ star AV 83is discussed. The analysis indicates the existence of a clumped wind,and a relatively slow wind acceleration with the velocity lawcharacterized by β = 2. The importance of extreme Of stars, forunderstanding both WR and O-type star winds, is stressed. Spectra ofsuch stars show numerous photospheric and wind features, allowing theentire wind to be probed observationally.
| Stellar parameters of Wolf-Rayet stars from far-UV to mid-IR observations
Recent results for Galactic and Magellanic Cloud Wolf-Rayet stars aresummarised based on line-blanketed, clumped model atmospheres togetherwith UV, optical and IR spectroscopy. The trend towards earlier WN andWC spectral types with decreasing metallicity is explained via thesensitivity of classification diagnostics to abundance/wind density,such that WR mass-loss rates are metallicity dependent. Pre-supernovaemasses for WC stars are determined, in reasonable agreement withCO-cores of recent Type Ic SN.
| Spectral Analysis of the LMC [WC] Star SMP 61
Not Available
| MASSIVE STARS IN THE LOCAL GROUP: Implications for Stellar Evolution and Star Formation
The galaxies of the Local Group serve as important laboratories forunderstanding the physics of massive stars. Here I discuss what isinvolved in identifying various kinds of massive stars in nearbygalaxies: the hydrogen-burning O-type stars and their evolved He-burningevolutionary descendants, the luminous blue variables, red supergiants,and Wolf-Rayet stars. Primarily I review what our knowledge of themassive star population in nearby galaxies has taught us about stellarevolution and star formation. I show that the current generation ofstellar evolutionary models do well at matching some of the observedfeatures and provide a look at the sort of new observational data thatwill provide a benchmark against which new models can be evaluated.
| An explanation for the curious mass loss history of massive stars: From OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss ratesand terminal velocities during their evolution from O-star through theLuminous Blue Variable phase to the Wolf-Rayet phase. The luminosityremains approximately unchanged during these phases. These large changesin wind properties are explained in the context of the radiation drivenwind theory, of which we consider four different models. They are due tothe evolutionary changes in radius, gravity and surface composition andto the change from optically thin (in continuum) line driven winds tooptically thick radiation driven winds.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Стрелец |
| Прямое восхождение: | 18h08m28.47s |
| Склонение: | -21°15'11.2" |
| Видимая звёздная величина: | 7.678 |
| Расстояние: | 1666.667 парсек |
| Собственное движение RA: | 1.3 |
| Собственное движение Dec: | -1.6 |
| B-T magnitude: | 7.855 |
| V-T magnitude: | 7.693 |
Каталоги и обозначения:
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