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Kinematics of stars in the line of sight to the open cluster Collinder21
We present a radial velocity study and revised spectral classificationfor 41 stars in the line of sight to the open cluster Collinder 121,whose existence has been often questionned, as well as its relation to ayoung stellar association containing the WR star HD 50896 (WR6). Fromour spectroscopic data and photometry available in the literature we areable to identify a real clustering at a distance of 1.1 +/- 0.2 kpc fromthe Sun (derived through the spectroscopic parallax of its members) andsharing an average heliocentric radial velocity of 33.9 +/- 2.5 kms^{-1}. This group is coincident in distance with that found byKaltcheva (2000) based on Strömgren and Hβ photometry. FromHIPPARCOS proper motions, available for 19 stars among the clusterprobable members included in our study, we also confirm a common spatialkinematics for the group. The radial velocity determined for Collinder121 is in agreement with that obtained for the ring nebula S 308(associated to WR 6) and its surrounding HI structure, thus a physicalvinculation with the cluster is inferred.
| 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: [email protected]
| Analysis of δ Librae including Hipparcos astrometry
New spectroscopy of the classical Algol system δ Lib, combinedwith high-quality optical and infrared photometry, provides the basisfor a good understanding of the close binary system's main parameters.Detailed analysis of the photometry reveals the significant role of athird light source, pointing to the existence of a companion to theeclipsing system of mass ~1Msolar.We review the methodology of applying high-accuracy positionalinformation, available from the Hipparcos Intermediate Astrometric Dataarchive, to stars that may have such companions. Analysis of theastrometry of δ Lib also points to a third star similar to the onealready identified by Worek from radial-velocity data, although withslightly revised parameters. O-C data do not contradict this, but theirgeneral precision (while confirming the close pair's Algol status) failsto allow a decision on the third orbit parameters: Worek's or revisedones. Taking the photometry, spectroscopy and astrometry together,however, the existence of a third star of comparable mass to the Sun, asa relatively close companion to the eclipsing binary (~4 au), isconfirmed.
| Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| 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.
| Hard X-Rays from Ultracompact H II Regions in W49A
We report the Chandra detection of hard X-ray emission from the Welchring in W49A, an organized structure of ultracompact (UC) H II regionscontaining a dozen nascent early-type stars. Two UC H II regions areassociated with hard X-ray emission in a deep Advanced CCD ImagingSpectrometer (ACIS) image exposed for ~96.7 ks. One of the two X-raysources has no near-infrared counterpart and is extended by ~5", or ~0.3pc, at a distance of ~11.4 kpc, which is spatially aligned with thecometary radio continuum emission associated with the UC H II region.The X-ray spectrum of the emission, when fit with a thermal model,indicates a heavily absorbed plasma with extinction of~5×1023 cm-2, temperature ~7 keV, and X-rayluminosity in the 3.0-8.0 keV band of ~3×1033 ergss-1. Both the luminosity and the size of the emissionresemble the extended hard emission found in UC H II regions inSagittarius B2, yet they are smaller by an order of magnitude than theemission found in massive star clusters such as NGC 3603. Threepossibilities are discussed for the cause of the hard extended emissionin the Welch ring: an ensemble of unresolved point sources, shockedinteracting winds of the young O stars, and a wind-blown bubbleinteracting with ambient cold matter.
| The Origin of Structures in Wolf-Rayet Winds: FUSE Observations of WR 135
We report the detection with FUSE of strong, highly blueshiftedabsorption features appearing in the absorption troughs of practicallyall major P Cygni profiles in the presumably single Wolf-Rayet star WR135. These features also appear in the shock-sensitive O VIλλ1032, 1038 doublet, coincident both in time and invelocity space with the rest of the lower ionization species. Choosingbetween two alternative interpretations (large-scale, coherentstructures vs. localized, random shocks), we favor the latter. Theabsolute value of the velocity as well as the velocity dispersion in theshocked region, the density of the shocked gas, and the timescales ofthe observed variability allow us to relate the observed shocks to theincidence of numerous overdense clumps (blobs) in the wind of a hot,massive star.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.
| Massive Stars and the Energy Balance of the Interstellar Medium. II. The 35 Msolar Star and a Solution to the ``Missing Wind Problem''
We continue our numerical analysis of the morphological and energeticinfluence of massive stars on their ambient interstellar medium for a 35Msolar star that evolves from the main-sequence through redsupergiant and Wolf-Rayet phases, until it ultimately explodes as asupernova. We find that structure formation in the circumstellar gasduring the early main-sequence evolution occurs as in the 60Msolar case but is much less pronounced because of the lowermechanical wind luminosity of the star. On the other hand, since theshell-like structure of the H II region is largely preserved, effectsthat rely on this symmetry become more important. At the end of thestellar lifetime 1% of the energy released as Lyman continuum radiationand stellar wind has been transferred to the circumstellar gas. Fromthis fraction 10% is kinetic energy of bulk motion, 36% is thermalenergy, and the remaining 54% is ionization energy of hydrogen. Thesweeping up of the slow red supergiant wind by the fast Wolf-Rayet windproduces remarkable morphological structures and emission signatures,which are compared with existing observations of the Wolf-Rayet bubbleS308, whose central star has probably evolved in a manner very similarto our model star. Our model reproduces the correct order of magnitudeof observed X-ray luminosity, the temperature of the emitting plasma,and the limb brightening of the intensity profile. This is remarkable,because current analytical and numerical models of Wolf-Rayet bubblesfail to consistently explain these features. A key result is that almostthe entire X-ray emission in this stage comes from the shell of redsupergiant wind swept up by the shocked Wolf-Rayet wind rather than fromthe shocked Wolf-Rayet wind itself as hitherto assumed and modeled. Thisoffers a possible solution to what is called the ``missing windproblem'' of Wolf-Rayet bubbles.
| 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.
| Supernova remnant S 147 and its associated neutron star(s)
The supernova remnant S 147 harbors the pulsar PSRJ 0538+2817 whose characteristic age is more than an order ofmagnitude greater than the kinematic age of the system (inferred fromthe angular offset of the pulsar from the geometric center of thesupernova remnant and the pulsar proper motion). To reconcile thisdiscrepancy we propose that PSR J 0538+2817 could be the stellarremnant of the first supernova explosion in a massive binary system andtherefore could be as old as its characteristic age. Our proposalimplies that S 147 is the diffuse remnant of the second supernovaexplosion (that disrupted the binary system) and that a much youngersecond neutron star (not necessarily manifesting itself as a radiopulsar) should be associated with S 147. We use the existingobservational data on the system to suggest that the progenitor of thesupernova that formed S 147 was a Wolf-Rayet star (so that thesupernova explosion occurred within a wind bubble surrounded by amassive shell) and to constrain the parameters of the binary system. Wealso restrict the magnitude and direction of the kick velocity receivedby the young neutron star at birth and find that the kick vector shouldnot strongly deviate from the orbital plane of the binary system.
| 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.
| 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.
| 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.
| X-Ray Emission from Wind-blown Bubbles. III. ASCA SIS Observations of NGC 6888
We present ASCA SIS observations of the wind-blown bubble NGC 6888.Owing to the higher sensitivity of the SIS for higher energy photonscompared to the ROSAT PSPC, we are able to detect aT~8×106 K plasma component in addition to theT~1.3×106 K component previously detected in PSPCobservations. No significant temperature variations are detected withinNGC 6888. García-Segura & Mac Low's analytical models of WRbubbles constrained by the observed size, expansion velocity, and massof the nebular shell underpredict the stellar wind luminosity and cannotreproduce simultaneously the observed X-ray luminosity, spectrum,surface brightness profile, and SIS count rate of NGC 6888's bubbleinterior. The agreement between observations and expectations frommodels may be improved if one or more of the following ad hocassumptions are made: (1) the stellar wind luminosity was weaker in thepast, (2) the bubble is at a special evolutionary stage and the nebularshell has recently been decelerated to 1/2 of its previous expansionvelocity, and (3) the heat conduction between the hot interior and thecool nebular shell is suppressed. Chandra and XMM-Newton observationswith high spatial resolution and high sensitivity are needed toaccurately determine the physical conditions of NGC 6888's interior hotgas for critical comparisons with bubble models.
| The Neon Abundance of Galactic Wolf-Rayet Stars
The fast, dense winds that characterize Wolf-Rayet (W-R) stars obscuretheir underlying cores and complicate the verification of evolving coreand nucleosynthesis models. Core evolution can be probed by measuringabundances of wind-borne nuclear-processed elements, partiallyovercoming this limitation. Using ground-based mid-infrared spectroscopyand the 12.81 μm [Ne II] emission line measured in four Galactic W-Rstars, we estimate neon abundances and compare them to long-standingpredictions from evolved-core models. For the WC star WR 121, thisabundance is found to be >~11 times the cosmic value, in goodagreement with predictions. For the three less-evolved WN stars, littleneon enhancement above cosmic values is measured, as expected. Wediscuss the impact of clumping in W-R winds on this measurement and thepromise of using metal abundance ratios to eliminate sensitivity to winddensity and ionization structure.
| Multiwavelength studies of WR 21a and its surroundings
We present results of high-resolution radio continuum observationstowards the binary star WR 21a (Wack 2134) obtained with the AustraliaTelescope Compact Array (ATCA) at 4.8 and 8.64 GHz. We detected thesystem at 4.8 GHz (6 cm) with a flux density of 0.25±0.06 mJy andset an upper limit of 0.3 mJy at 8.64 GHz (3 cm). The derived spectralindex of α < 0.3 (Sν ∝να) suggests the presence of non-thermal emission,probably originating in a colliding-wind region. A second, unrelatedradio source was detected ~10 arcsec north of WR 21a at (RA,Dec)J2000=(10h25m56.49s, -57°48arcmin34.4 arcsec), with flux densities of 0.36 and 0.55 mJy at 4.8 and8.64 GHz, respectively, resulting in α = 0.72. H i observations inthe area are dominated by absorption against the prominent H ii regionRCW 49. Analysis of a complete set of archived X-ray observations of WR21a confirms its strong variability but throws into doubt previoussuggestions by Reig (1999) of a period of years for the system. Finally,we comment on the association with the nearby EGRET source 3EGJ1027-5817.
| 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.
| 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).
| 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.
| 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.
| Spitzer Space Telescope Infrared Spectrograph (IRS) Spectroscopy of the Prototype Wolf-Rayet Star EZ Canis Majoris (HD 50896)
We present mid-infrared Spitzer Space Telescope Infrared Spectrograph(IRS) spectroscopy of the prototype WN star EZ Canis Majoris (HD 50896,WN4b). Numerous stellar wind lines of He II are revealed, plusfine-structure lines of [Ne III] 15.5 μm and [O IV] 25.9 μm. Wecarry out a spectroscopic analysis of HD 50896 allowing for lineblanketing and clumping, which is compared to the mid-IR observations.We make use of these stellar properties to accurately deriveNe/He=(1.2-1.8)×10-4 andO/He=(4.2-4.8)×10-5 by number, for the first time in anearly WN star. In addition, we obtain N/C~40 and N/O~50 by number, inperfect agreement with current predictions for rotating massive stars atthe end of interior hydrogen burning.
| The Diversity of Gamma-Ray Burst Afterglows and the Surroundings of Massive Stars
The finding of a Type Ic supernova connected with GRB 030329 showed amassive star origin for this burst, supporting evidence for thisassociation in previous bursts with light-curve bumps at the appropriatetime for a supernova. Here we explore the possibility that all longbursts have massive star progenitors, interacting with either the freelyexpanding wind of the progenitor or the shocked wind. We present modelsfor the afterglows of GRB 020405 and GRB 021211, which are a challengeto wind interaction models. Considering sources for which windinteraction models are acceptable, a range of wind densities isrequired, from values typical of Galactic Wolf-Rayet stars to values~102 times smaller. The reason for the low densities isunclear, but it may involve low progenitor masses and/or lowmetallicities. If mass is a factor, a low-density event should beassociated with a low-mass supernova. The interpretation of burstsapparently interacting with constant density media as interaction with ashocked wind requires both a range of mass-loss densities and a range ofexternal pressures. The highest pressures, p/k>~108cm-3 K, may be due to an extreme starburst environment, whichwould imply that the burst is superposed on an active star-formingregion. Although the range of observed events can be accommodated by theshocked-wind theory, special circumstances are necessary to bring thisabout. Finally, we consider the high-velocity, high-ionizationabsorption features observed in some afterglow spectra. If the featuresare circumstellar, the presence of the burst in a starburst region maybe important for the formation of clumps near the burst.
| 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.
| 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
| Planetary Nebulae, Bubbles, and Superbubbles: What Can We Learn From Their Kinematics?
We present three examples on how the confrontation between theinformation we can derive from the kinematics of planetary nebulae,bubbles, and superbubbles and the theoretical models of evolution ofthese objects allows us to have a better understanding of the physicalmechanisms involved in the creation and evolution of these objects andto conceive better ways to observe them.
| An Inside-Out View of Bubbles
Fast stellar winds can sweep up ambient media and form bubbles. Theevolution of a bubble is largely controlled by the content and physicalconditions of the shocked fast wind in its interior. This hot gas wasnot clearly observed until the recent advent of Chandra and XMM-NewtonX-ray observatories. To date, diffuse X-ray emission has beenunambiguously detected from two circumstellar bubbles blown by WR stars,four planetary nebulae, and two superbubbles blown by young clusters.Model fits to the X-ray spectra show that the circumstellar bubbles aredominated by hot gas with low temperatures ( < 3×10^6 K), whilethe interstellar bubbles contain significant fractions of hotter gas (< 5×10^6 K) . In all cases, large discrepancies in the X-rayluminosity are found between observations and conventional models ofbubbles. Future theoretical models of bubbles need to re-examine thevalidity of heat conduction and take into account realistic microscopicprocesses such as mass loading from dense clumps/knots and turbulentmixing. Chandra ACIS-S observation of NGC 6888 will shed light on theseastrophysical processes.
| The galactic evolution of the supernova rates
Supernova rates (hypernova, type II, type Ib/c and type Ia) in aparticular galaxy depend on the metallicity (i.e. on the galaxy age), onthe physics of star formation and on the binary population. In order tostudy the time evolution of the galactic supernova rates, we use ourchemical evolutionary model that accounts in detail for the evolution ofsingle stars and binaries. In particular, supernovae of type Ia areconsidered to arise from exploding white dwarfs in interacting binariesand we adopt the two most plausible physical models: the singledegenerate model and the double degenerate model. Comparison betweentheoretical prediction and observations of supernova rates in differenttypes of galaxies allows to put constraints on the population ofintermediate mass and massive close binaries.The temporal evolution of the absolute galactic rates of different typesof supernovae (including the type Ia rate) is presented in such a waythat the results can be directly implemented into a galactic chemicalevolutionary model. Particularly for type Ia's the inclusion of binaryevolution leads to results considerably different from those in earlierpopulation synthesis approaches, in which binary evolution was notincluded in detail.
| 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.
| On the nature of Collinder 121: insights from the low-mass pre-main sequence
We present a VI photometric catalogue towards the open cluster Cr 121.XMM-Newton and ROSAT data are used to discover a low-mass pre-mainsequence (PMS) along this sightline. de Zeeuw et al. have identified Cr121 as a moving group, using Hipparcos data, at a distance of 592 pc; wereject the scenario that these low-mass PMS stars are associated withthat association. By considering the higher mass main sequence stellarmembership of the groups along this sightline, the density of low-massPMS stars and their age spread we argue that the low-mass PMS stars areassociated with a young, compact cluster at a distance of 1050 pc. Thisis consistent with Collinder's original description of Cr 121, and weargue that this distant compact cluster should retain its originaldesignation. The moving group detected by de Zeeuw et al. resembles aforeground association and we agree with Eggen that this should becalled CMa OB2.This study demonstrates that although the de Zeeuw et al. census of OBassociations is an invaluable resource for studying local starformation, it must be interpreted in the context of other data whenconsidering structure over distances of the same order as the limits ofthe Hipparcos parallaxes.
| 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.
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Osservazione e dati astrometrici
| Costellazione: | Cane Maggiore |
| Ascensione retta: | 06h54m13.00s |
| Declinazione: | -23°55'42.0" |
| Magnitudine apparente: | 6.91 |
| Distanza: | 574.713 parsec |
| Moto proprio RA: | -1 |
| Moto proprio Dec: | 2.7 |
| B-T magnitude: | 6.662 |
| V-T magnitude: | 6.721 |
Cataloghi e designazioni:
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