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The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| Forming a constant density medium close to long gamma-ray bursts
Aims.The progenitor stars of long Gamma-Ray Bursts (GRBs) are thought tobe Wolf-Rayet stars, which generate a massive and energetic wind.Nevertheless, about 25 percent of all GRB afterglows light curvesindicate a constant density medium close to the exploding star. Weexplore various ways to produce this, by creating situations where thewind termination shock arrives very close to the star, as the shockedwind material has a nearly constant density. Methods: .Typically,the distance between a Wolf-Rayet star and the wind termination shock istoo large to allow afterglow formation in the shocked wind material.Here, we investigate possible causes allowing for a smaller distance: Ahigh density or a high pressure in the surrounding interstellar medium(ISM), a weak Wolf-Rayet star wind, the presence of a binary companion,and fast motion of the Wolf-Rayet star relative to the ISM.Results: .We find that all four scenarios are possible in a limitedparameter space, but that none of them is by itself likely to explainthe large fraction of constant density afterglows. Conclusions:.A low GRB progenitor metallicity, and a high GRB energy make theoccurrence of a GRB afterglow in a constant density medium more likely.This may be consistent with constant densities being preferentiallyfound for energetic, high redshift GRBs.
| A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging andspectroscopy of the Wolf-Rayet population in the massive clusterWesterlund 1 are presented. Narrow-band near-infrared (IR) imagingtogether with follow up spectroscopy reveals four new Wolf-Rayet stars,of which three were independently identified recently by Groh et al.,bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S)- representing 8 per cent of the known Galactic Wolf-Rayet population -comprising eight WC stars and 16 (15) WN stars. Revised coordinates andnear-IR photometry are presented, whilst a quantitative near-IR spectralclassification scheme for Wolf-Rayet stars is presented and applied tomembers of Westerlund 1. Late subtypes are dominant, with no subtypesearlier than WN5 or WC8 for the nitrogen and carbon sequences,respectively. A qualitative inspection of the WN stars suggests thatmost (~75 per cent) are highly H deficient. The Wolf-Rayet binaryfraction is high (>=62 per cent), on the basis of dust emission fromWC stars, in addition to a significant WN binary fraction from hardX-ray detections according to Clark et al. We exploit the large WNpopulation of Westerlund 1 to reassess its distance (~5.0kpc) andextinction (AKS ~ 0.96mag), such that it islocated at the edge of the Galactic bar, with an oxygen metallicity ~60per cent higher than Orion. The observed ratio of WR stars to red andyellow hypergiants, N(WR)/N(RSG + YHG) ~3, favours an age of~4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitorsof initial mass ~40-55Msolar. Qualitative estimates ofcurrent masses for non-dusty, H-free WR stars are presented, revealing10-18Msolar, such that ~75 per cent of the initial stellarmass has been removed via stellar winds or close binary evolution. Wepresent a revision to the cluster turn-off mass for other Milky Wayclusters in which Wolf-Rayet stars are known, based upon the latesttemperature calibration for OB stars. Finally, comparisons between theobserved WR population and subtype distribution in Westerlund 1 andinstantaneous burst evolutionary synthesis models are presented.Based on observations made with ESO telescopes at the La SillaObservatory under programme IDs 073.D-0321 and 075.D-0469.E-mail: Paul.crowther@sheffield.ac.uk
| The Galactic WN stars. Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
Context: .Very massive stars pass through the Wolf-Rayet (WR) stagebefore they finally explode. Details of their evolution have not yetbeen safely established, and their physics are not well understood.Their spectral analysis requires adequate model atmospheres, which havebeen developed step by step during the past decades and account in theirrecent version for line blanketing by the millions of lines from ironand iron-group elements. However, only very few WN stars have beenre-analyzed by means of line-blanketed models yet. Aims: .Thequantitative spectral analysis of a large sample of Galactic WN starswith the most advanced generation of model atmospheres should provide anempirical basis for various studies about the origin, evolution, andphysics of the Wolf-Rayet stars and their powerful winds. Methods:.We analyze a large sample of Galactic WN stars by means of the PotsdamWolf-Rayet (PoWR) model atmospheres, which account for iron lineblanketing and clumping. The results are compared with a syntheticpopulation, generated from the Geneva tracks for massive starevolution. Results: .We obtain a homogeneous set of stellar andatmospheric parameters for the Galactic WN stars, partly revisingearlier results. Conclusions: .Comparing the results of ourspectral analyses of the Galactic WN stars with the predictions of theGeneva evolutionary calculations, we conclude that there is roughqualitative agreement. However, the quantitative discrepancies are stillsevere, and there is no preference for the tracks that account for theeffects of rotation. It seems that the evolution of massive stars isstill not satisfactorily understood.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Stratification of optical emission from NGC 6888 as a trace of the interaction between Wolf-Rayet stellar wind and the shell of a red supergiant
We suggest a model that explains the stratification peculiarities of the[O III] and Hα line emission from some of the ring nebulae aroundWolf-Rayet stars. These peculiarities lie in the fact that the [O III]line emission regions are farther from the central star than theHα regions, with the distance between them reaching several tenthsof a parsec. We show that the radiative shock produced by a Wolf-Rayetstellar wind and propagating with a velocity of 100 km s-1 cannotexplain such large distances between these regions due to the lowvelocity of the gas outflow from the shock front. The suggested modeltakes into account the fact that the shock produced by a Wolf-Rayetstellar wind propagates in a two-phase medium: a rarefied medium anddense compact clouds. The gas downstream of a fast shock traveling in ararefied gas compresses the clouds. Slow radiative shocks are generatedin the clouds; these shocks heat the latter to temperatures at whichions of doubly ionized oxygen are formed. The clouds cool down,radiating in the lines of this ion, to temperatures at which Balmer lineemission begins. The distance between the [O III] and Hα lineemission regions is determined by the cooling time of the cloudsdownstream of the slow shock and by the velocity of the fast shock.Using the ring nebula NGC 6888 as an example, we show that the gasdownstream of the fast shock must be at the phase of adiabatic expansionrather than deceleration with radiative cooling, as assumed previously.
| Reduced Wolf-Rayet line luminosities at low metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Small andLarge Magellanic Clouds are presented, from which He ii λ4686line luminosities have been derived, and compared with observations ofother Magellanic Cloud Wolf-Rayet stars. SMC WN3-4 stars possess lineluminosities which are a factor of 4 times lower than LMC counterparts,incorporating several binary SMC WN3-4 stars from the literature.Similar results are found for WN5-6 stars, despite reduced statistics,incorporating observations of single LMC WN5-9 stars from theliterature. C iv λ5808 line luminosities of carbon sequence WRstars in the SMC and IC 1613 (both WO subtypes) from the recentliterature are a factor of 3 lower than LMC WC stars from Mt Stromlo/DBSspectrophotometry, although similar results are also obtained for thesole LMC WO star. We demonstrate how reduced line luminosities at lowmetallicity follow naturally if WR winds are metallicity-dependent, asrecent empirical and theoretical results suggest. We apply massloss-metallicity scalings to atmospheric non-LTE models of Milky Way andLMC WR stars to predict the wind signatures of WR stars in themetal-poor star forming WR galaxy I Zw 18. WN He ii λ4686 lineluminosities are 7-20 times lower than in metal-rich counterparts ofidentical bolometric luminosity, whilst WC C iv λ5808 lineluminosities are 3-6 times lower. Significant He+ Lymancontinuum fluxes are predicted for metal-poor early-type WR stars.Consequently, our results suggest a larger population of WR stars in IZw 18 than is presently assumed, particularly for WN stars, potentiallyposing a severe challenge to evolutionary models at very lowmetallicity. Finally, reduced wind strengths from WR stars at lowmetallicities impacts upon the immediate circumstellar environment oflong duration GRB afterglows, particularly since the host galaxies ofhigh-redshift GRBs tend to be metal-poor.
| Oscillations in the Massive Wolf-Rayet Star WR 123 with the MOST Satellite
We present the results of intensive visual-broadband photometricmonitoring of the highly variable WN8 Wolf-Rayet star WR 123, obtainedby the MOST (Microvariability and Oscillations of STars) satellite. Thisfirst Canadian astronomical space telescope observed WR 123 for 38 daysnonstop during 2004 June and July. Fourier analysis shows that noperiodic signal is stable for more than several days in thelow-frequency domain (f< 1 day-1), where most of thestochastic power is contained. Also, no significant variability is seenin the high-frequency domain (10 day-1 < f < 1400day-1) down to the level of 0.2 mmag, an order of magnitudelower than theoretical predictions for strange-mode pulsations. On theother hand, there seems to be a relatively stable 9.8 hr periodic signalpresent throughout the whole run. This period is probably too short torepresent the axial rotation of the star, unless it is related tomultiple substructures equidistantly spread along the stellar equator.It is also too short to be orbital in nature; it is more likely to berelated to pulsational instablilities (although with a much longerperiod than expected), thus finally revealing a possible fundamentaldriver behind the highly variable wind of this object, and others ofsimilar type.
| On the metallicity dependence of Wolf-Rayet winds
We have performed a pilot study of mass loss predictions for late-typeWolf-Rayet (WR) stars as a function of metal abundance, over a rangebetween 10-5 ≤ (Z/Zȯ) ≤ 10. We findthat the winds of nitrogen-rich Wolf-Rayet stars are dominated by ironlines, with a dependence of mass loss on Z similar to that of massive OBstars. For more evolved, carbon-rich, WR stars the wind strength is alsofound to be dependent on the Fe abundance, so that they depend on thechemical environment of the host galaxy, but with a mass lossmetallicity dependence that is less steep than for OB stars. Our findingthat WR mass loss is Z-dependent is a new one, with importantconsequences for black hole formation and X-ray population studies inexternal galaxies. A further finding of our study is that the Zdependence of C-rich WR stars becomes weaker at metallicities belowZ/Zȯ ⪉ 1/10, and mass loss no longer declines oncethe metal abundance drops below (Z/Zȯ) ≃10-3. This is the result of an increased importance ofradiative driving by intermediate mass elements, such as carbon. Incombination with rapid rotation and/or proximity to the Eddington limit- likely to be relevant for massive Population iii stars - this effectmay indicate a role for mass loss in the appearance and evolution ofthese objects, as well as a potential role for stellar winds inenriching the intergalactic medium of the early Universe.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| 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.
| Spectroscopic study of the long-period dust-producing WC7pd+O9 binary HD192641
We present the results of an optical spectroscopic study of the massiveWolf-Rayet (WR) binary HD192641 = WR137. These 1986-2000 data cover thedust-formation maximum in 1997. Combining all available measurements ofradial velocities, we derive, for the first time, a spectroscopic orbitwith period 4766 +/- 66 d (13.05 +/- 0.18 yr). The resulting masses,adopting i= 67 °, are MO= 20 +/- 2Msolar forthe O component and MWR= 4.4 +/- 1.5Msolar for theWR component. These appear, respectively, approximately normal and onthe low side for the given spectral types. Analysis of the intensemultisite spectroscopic monitoring in 1999 shows that theCIIIλ5696 and CIVλλ5802/12 lines have the highestintrinsic variability levels. The periodogram analysis yields asmall-amplitude modulation in the absorption troughs of theCIVλλ5802/12 and HeIλ5876 lines with a period of0.83 d, which could be related either to pulsations or large-scalerotating structures as seen in the WN4 star EZ Canis Majoris (WR6).Wavelet analysis of the strong emission lines of CIIIλ5696 andCIVλλ5802/12 enabled us to isolate and follow for severalhours small structures (emission subpeaks) associated with densityenhancements within the wind of the Wolf-Rayet star. Cross-correlatingthe variability patterns seen in different lines, we find a weak butsignificant correlation between the variability in emission lines withdifferent ionization potentials, i.e. in lines formed at differentdistances from the WR stellar core. Adopting a β wind-velocity law,from the motion of individual subpeaks we find β~ 5, which issignificantly larger than the canonical value β~= 1 found in O starwinds.
| Polarization Effects in the Radiation of Magnetized Envelopes and Extended Accretion Structures
We have calculated the degree and position angle of the polarization ofradiation scattered in a magnetized, optically thin or optically thickenvelope around a central source, taking into account Faraday rotationof the plane of polarization during the propagation of the scatteredradiation and the finite size of the radiation source. The wavelengthdependence of the degree of polarization can be used to estimate themagnetic field of the source (a star, the region around a neutron star,or a black hole), and we have used our calculations to estimate themagnetic fields in a number of individual objects: several hot O andWolf-Rayet stars, compact objects in X-ray close binaries with blackholes (SS 433, Cyg X-1), and supernovae. The spectrum of the linearpolarization can be used to determine the magnetic field in the vicinityof a central supermassive black hole, where the polarized opticalradiation is generated. In a real physical model, this value can beextrapolated to the region of the last stable orbit. In the future, theproposed technique will make it possible to directly estimate themagnetic field in the region of the last stable orbit of a supermassiveblack hole using X-ray polarimetry.
| The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions
We analyse the optical spectrum of the very massive binary system WR 20a(WN6ha + WN6ha). The most prominent emission lines, Hα and He iiλ 4686, display strong phase-locked profile variability. From thevariations of their equivalent widths and from a tomographic analysis,we find that part of the line emission probably arises in a windinteraction region between the stars. Our analysis of the opticalspectrum of WR 20a indicates a reddening of AV ≃ 6.0mag and a distance of 7.9 kpc, suggesting that the star actuallybelongs to the open cluster Westerlund 2. The location of the system at 1.1 pc from the cluster core could indicate that WR 20a was gentlyejected from the core via dynamical interactions. Using a non-LTE modelatmosphere code, we derive the fundamental parameters of each component:Teff = 43 000 ± 2000 K,log{Lbol/Lȯ} ≃ 6.0, dot{M} = 8.5× 10-6 Mȯ yr-1 (assuming aclumped wind with a volume filling factor f = 0.1). Nitrogen is enhancedin the atmospheres of the components of WR 20a, while carbon isdefinitely depleted. Finally, the position of the binary components inthe Hertzsprung-Russell diagram suggests that they are core hydrogenburning stars in a pre-LBV stage and their current atmospheric chemicalcomposition probably results from rotational mixing that might beenhanced in a close binary compared to a single star of same age.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| 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 XMM-Newton look at the Wolf-Rayet star WR 40. The star itself, its nebula and its neighbours
We present the results of an XMM-Newton observation of the field of theWolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the highsensitivity of the satellite, the star itself is not detected: we thusderive an upper limit on its X-ray flux and luminosity. Joining thisresult to recent reports of a non-detection of some WC stars, we suggestthat the X-ray emission from single normal Wolf-Rayet stars could oftenbe insignificant despite remarkable instabilities in the wind. On thebasis of a simple modelling of the opacity of the Wolf-Rayet wind of WR40, we show that any X-ray emission generated in the particular zonewhere the shocks are supposed to be numerous will indeed have littlechance to emerge from the dense wind of the Wolf-Rayet star. We alsoreport the non-detection of the ejecta nebula RCW 58 surrounding WR 40.Concerning the field around these objects, we detected 33 X-ray sources,most of them previously unknown: we establish a catalog of these sourcesand cross-correlate it with catalogs of optical/infrared sources.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).
| 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.
| Observations of the Diffuse Far-Ultraviolet Background with the Far Ultraviolet Spectroscopic Explorer
We have used observations taken under the Far Ultraviolet SpectroscopicExplorer (FUSE) S405/505 channel realignment program to explore thediffuse far-ultraviolet (FUV; 1000-1200 Å) radiation field. Of the71 independent locations in that program, we have observed a diffusesignal in 32, ranging in brightness from 1600 to a maximum of2.9×105 photons cm-2 sr-1s-1 Å-1 in Orion. The FUSE data confirm thatthe diffuse FUV sky is patchy with regions of intense emission, usuallynear bright stars, but also with dark regions, even at low Galacticlatitudes. We find a weak correlation between the FUV flux and the 100μm ratio but with wide variations, perhaps due to differences in thelocal radiation field.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.
| 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.
| 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.
| WR 7a: a V Sagittae or a qWR star?
The star WR 7a, also known as SPH 2, has a spectrum that resembles thatof V Sagittae stars although no OVI emission has been reported. TheTemporal Variance Spectrum - TVS - analysis of our data shows weak butstrongly variable emission of OVI lines which is below the noise levelin the intensity spectrum.Contrary to what is seen in V Sagittae stars, optical photometricmonitoring shows very little, if any, flickering. We found evidence ofperiodic variability. The most likely photometric period isPphot= 0.227(+/-14) d, while radial velocities suggest aperiod of Pspec= 0.204(+/-13) d. One-day aliases of theseperiods can not be ruled out. We call attention to similarities with HD45166 and DI Cru (= WR 46), where multiple periods are present. They maybe associated to the binary motion or to non-radial oscillations.In contrast to a previous conclusion by Pereira et al., we show that WR7a contains hydrogen. The spectrum of the primary star seems to bedetectable as the NV 4604Åabsorption line is visible. If so, itmeans that the wind is optically thin in the continuum and that it islikely to be a helium main sequence star.Given the similarity to HD 45166, we suggests that WR 7a may be a qWR -quasi Wolf-Rayet - star. Its classification is WN4h/CE in the Smith,Shara & Moffat three-dimensional classification system.
| A unique Galactic planetary nebula with a [WN] central star
We report the discovery of the first probable Galactic [WN] central starof a planetary nebula (CSPN). The planetary nebula candidate was foundduring our systematic scans of the AAO/UKST Hα Survey of the MilkyWay. Subsequent confirmatory spectroscopy of the nebula and central starreveals the remarkable nature of this object. The nebular spectrum showsemission lines with large expansion velocities exceeding 150 kms-1, suggesting that perhaps the object is not a conventionalplanetary nebula. The central star itself is very red and is identifiedas being of the [WN] class, which makes it unique in the Galaxy. A largebody of supplementary observational data supports the hypothesis thatthis object is indeed a planetary nebula and not a Population IWolf-Rayet star with a ring nebula.
| Massive Stars and the Energy Balance of the Interstellar Medium. I. The Impact of an Isolated 60 Msolar Star
We present results of numerical simulations carried out with atwo-dimensional radiation hydrodynamics code in order to study theimpact of massive stars on their surrounding interstellar medium. Thisfirst paper deals with the evolution of the circumstellar gas around anisolated 60 Msolar star. The interaction of the photoionizedH II region with the stellar wind bubble forms a variety of interestingstructures like shells, clouds, fingers, and spokes. These resultsdemonstrate that complex structures found in H II regions are notnecessarily relics from the time before the gas became ionized but mayresult from dynamical processes during the course of the H II regionevolution. We have also analyzed the transfer and deposit of the stellarwind and radiation energy into the circumstellar medium until the starexplodes as a supernova. Although the total mechanical wind energysupplied by the star is negligible compared to the accumulated energy ofthe Lyman continuum photons, the kinetic energy imparted to thecircumstellar gas over the star's lifetime is 4 times higher than for acomparable windless simulation. Furthermore, the thermal energy of warmphotoionized gas is lower by some 55%. Our results document thenecessity to consider both ionizing radiation and stellar winds for anappropriate description of the interaction of OB stars with theircircumstellar environment.
| 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/}
| Candidates for Ablated Flows
There is observational evidence of the mass loading of flows by ablatedmaterial in a variety of circumstellar and interstellar phenomena. Herethe strength of this evidence will be evaluated for the flows observedin the hydrogen deficient planetary nebulae A 30 and 78, the cometaryknots in the Helix nebula, the hypersonic ``strings'' in the Eta Carinaenebula, the inner shell of the prototypical luminous blue variable starP Cygni, the Honeycomb nebula in the halo of 30 Doradus, and the weakmonopolar jets from the M 42 proplyds.
| 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.
| Hydrodynamic Interaction of Strong Shocks with Inhomogeneous Media. I. Adiabatic Case
Many astrophysical flows occur in inhomogeneous (clumpy) media. Wepresent results of a numerical study of steady, planar shocksinteracting with a system of embedded cylindrical clouds. Our study usesa two-dimensional geometry. Our numerical code uses an adaptive meshrefinement, allowing us to achieve sufficiently high resolution both atthe largest and the smallest scales. We neglect any radiative losses,heat conduction, and gravitational forces. Detailed analysis of thesimulations shows that interaction of embedded inhomogeneities with theshock/postshock wind depends primarily on the thickness of the cloudlayer and arrangement of the clouds in the layer. The total cloud massand the total number of individual clouds is not a significant factor.We define two classes of cloud distributions: thin and thick layers. Wedefine the critical cloud separation along the direction of the flow andperpendicular to it, distinguishing between the interacting andnoninteracting regimes of cloud evolution. Finally, we discuss massloading and mixing in such systems.
| VLA Radio Continuum and IRAS Observations of the Ring Nebulae around WR 101 and WR 113
We report radio continuum observations at 1465 MHz obtained with theVery Large Array (VLA) in the DnC configuration toward the ring nebulaeassociated with the stars WR 101 and WR 113, with resolutions of ~38"and 30", respectively. IRAS images of the nebulae with resolutions ofabout 2' (90 Msolar, 40 cm-3) are also analyzed. Aremarkable resemblance among the optical, infrared, and radio images ofthese ring nebulae is observed. The VLA data indicate that Anon. WR 101is thermal in nature. An ionized mass of ~230+/-40 Msolar andelectron densities in the range ~40-55 cm-3 were estimatedfor Anon. WR 101. The derived ionized masses and electron densities inthe inner and outer shells of the nebula related to WR 113 are ~20+/-10Msolar, 180-500 cm-3 and ~90 Msolar 40cm-3, respectively. Based on infrared data at 60 and 100μm, the derived masses and temperatures for the dust component in thering nebula around WR 101 are 0.3-1 Msolar and ~40 K. Theassociated masses suggest that the ring nebula related to WR 101 and theouter arc associated with WR 113 consist of swept-up interstellarmatter, while the relatively low ionized mass associated with the innershell of the nebula around WR 113 may contain a nonnegligiblecontribution of expelled ejecta material. The derived electron densitiesfor the nebula around WR 101 and the inner shell around WR 113 arecomparable to electron densities for other W-R ring nebulae. Low fillingfactors are inferred for both nebulae. The nebulae probably originatedduring the current W-R phase of the stars.
| 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.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Киль |
| Прямое восхождение: | 11h06m17.20s |
| Склонение: | -65°30'35.3" |
| Видимая звёздная величина: | 7.696 |
| Расстояние: | 5000 парсек |
| Собственное движение RA: | -1.2 |
| Собственное движение Dec: | -2.4 |
| B-T magnitude: | 7.856 |
| V-T magnitude: | 7.71 |
Каталоги и обозначения:
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