Contents
Images
Upload your image
DSS Images Other Images
Related articles
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.
| 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
| An Extreme Case of a Misaligned Highly Flattened Wind in the Wolf-Rayet Binary CX Cephei
CX Cep (WR 151) is the WR+O binary (WN5+O5 V) with the second shortestperiod known in our Galaxy. To examine the circumstellar matterdistribution and to better constraint the orbital parameters andmass-loss rate of the W-R star, we obtained broadband and multiband(i.e., UBVRI) linear polarization observations of the system. Ouranalysis of the phase-locked polarimetric modulation confirms the highorbital inclination of the system (i.e., i=65deg). Using theorbital solution of Lewis et al. (1993), we obtain masses of 33.9 and23.9 Msolar for the O and W-R stars, respectively, whichagree with their spectral types. A simple polarimetric model accountingfor finite stellar size effects allowed us to derive a mass-loss ratefor the W-R star of (0.3-0.5)×10-5 Msolaryr-1. This result was remarkably independent of the model'sinput parameters and favors an earlier spectral type for the W-Rcomponent (i.e., WN4). Finally, using our multiband observations, wefitted and subtracted from our data the interstellar polarization. Theresulting constant intrinsic polarization of 3%-4% is misaligned inrelation to the orbital plane (i.e.,θCIP=26deg vs. Ω=75deg)and is the highest intrinsic polarization ever observed for a W-R star.This misalignment points toward a rotational (or magnetic) origin forthe asymmetry and contradicts the most recent evolutionary models formassive stars (Meynet & Maeder 2003) that predict sphericallysymmetric winds during the W-R phase (i.e., CIP=0%).
| 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.
| Gamma-Ray Burst Dust Echoes Revisited: Expectations at Early Times
Gamma-ray burst (GRB) dust echoes were first proposed as an alternativeexplanation for the supernova-like (SN-like) components to theafterglows of GRB 980326 and GRB 970228. However, the spectroscopicidentification of Type Ic SN 2003dh associated with GRB 030329, as wellas the identification of SN-like components of the afterglows of otherGRBs, appears to have confirmed the GRB/SN paradigm. However, the likelyprogenitors of Type Ic SNe are Wolf-Rayet WC stars, and late-type WCstars have been observed to be surrounded by dust at a distance of1014-1015 cm from the star. Consequently, werevisit the possibility of GRB dust echoes, not on a timescale of weeksafter the burst but on a timescale of minutes to hours. We find that ifthe optical flash is sufficiently bright and the jet sufficiently wide,GRB afterglows may be accompanied by chromatic variations on thistimescale. From these signatures, model parameters such as the innerradius of the dust distribution and the initial opening angle of the jetmay be deduced. With rapid and regular localizations of GRBs by HETE-2,INTEGRAL, and now Swift, and new and improved robotic telescope systems,these early-time GRB dust echoes may soon be detected. We describe ingreater detail one such robotic telescope system, called PROMPT, whichthe University of North Carolina at Chapel Hill is building at the CerroTololo Inter-American Observatory.
| 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.
| A Survey of Unidentified EGRET Sources at Very High Energies
The Whipple Observatory 10 m γ-ray telescope has been used tosurvey the error boxes of EGRET unidentified sources in an attempt tofind counterparts at energies of 350 GeV and above. Twenty-oneunidentified sources detected by EGRET (more than 10% of the totalnumber) have been included in this survey. In no case is a statisticallysignificant signal found in the EGRET error box, which implies that, atleast for this sample, the γ-ray spectra of these sources steepenbetween 100 MeV and 350 GeV. For each EGRET source location, we listcandidate associations and derive upper limits on the integralγ-ray flux above 350 GeV.
| Wind-wind collision in the η Carinae binary system: a shell-like event near periastron
The exact nature of η Carinae is still an open issue. Strictperiodicity in the light curves at several wavelengths seem to point toa binary system, but the observed radial velocities, measured from spacewith high spatial resolution, are in conflict with the ground-basedobservations used to calculate the binary orbit. Also, the observed 2-10keV X-ray flux is much larger that what is expected from a single star,and favours the wind-wind collision hypothesis, characteristic ofhigh-mass binary systems. However, to explain the duration of the dip inthe light curve by wind collisions, it is necessary to postulate a verylarge increase in the η Carinae mass loss rate. Finally, the opticaland ultraviolet light curves are better explained by periodicshell-ejection events. In this paper we conciliate the two hypotheses.We still assume a binary system to explain the strong X-ray emission,but we also take into account that, near periastron and because of thehighly eccentric orbit, the wind emerging from η Carinae accumulatesbehind the shock and can mimic a shell-like ejection event. For thisprocess to be effective, at periastron the secondary star should belocated between η Carinae and the observer, solving also thediscrepancy between the orbital parameters derived from ground- andspace-based observations. We show that, as the secondary moves in itsorbit, the shell cools down and the number of available stellar ionizingphotons is not enough to maintain the shell temperature at itsequilibrium value of about 7500 K. The central part of the shell remainscold and under these conditions grain formation and growth can takeplace in time-scales of hours. We also calculated the neutral gas columndensity intercepting the line of sight at each point of the orbit nearperiastron, and were able to reproduce the form and duration of theX-ray light curve without any change in the η Carinae mass lossrate. This same column density can explain the observed Hα lightcurve observed during the 2003 event.
| 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.
| Observed Features of a Two-Phase Stellar Wind from WR 136 in the Vicinity of NGC 6888
A number of features are detected outside the nebula NGC 6888, within1.2° (30 pc) of the star WR 136, which can be explained in atwo-phase stellar-wind model. These include regions with finefilamentary gas structure that do not contain sources of stellar wind,extended radial “streams,” ultra-compact HII regions withhigh-velocity gas motions, and high-velocity gas motions outside theenvelope of NGC 6888. The two-phase wind consists of a rarefiedcomponent and dense compact condensations, or “bullets.” Thebullets generate cylindrical shocks in the interstellar gas, resultingin the presence of high-velocity gas up 20 30 pc from the star, outsidethe cavity formed by the rarified component of the wind.
| Three-dimensional dust radiative-transfer models: the Pinwheel Nebula of WR 104
We present radiative-transfer modelling of the dusty spiral PinwheelNebula observed around the Wolf-Rayet/OB-star binary WR 104. The modelsare based on the three-dimensional radiative-transfer code TORUS,modified to include an adaptive mesh that allows us to adequatelyresolve both the inner spiral turns (subau scales) and the outer regionsof the nebula (distances of 104 au from the central source).The spiral model provides a good fit to both the spectral energydistribution and Keck aperture masking interferometry, reproducing boththe maximum entropy recovered images and the visibility curves. Wededuce a dust creation rate of 8 +/- 1 × 10-7Msolar yr-1, corresponding to approximately 2 percent by mass of the carbon produced by the Wolf-Rayet star. Simultaneousmodelling of the imaging and spectral data enables us to constrain boththe opening angle of the wind-wind collision interface and the dustgrain size. We conclude that the dust grains in the inner part of thePinwheel Nebula are small (~100 Å), in agreement with theoreticalpredictions, although we cannot rule out the presence of larger grains(~1 μm) further from the central binary. The opening angle of thewind-wind collision interface appears to be about 40°, in broadagreement with the wind parameters estimated for the central binary. Wediscuss the success and deficiencies of the model, and the likelybenefits of applying similar techniques to the more complex nebulaeobserved around other WR/O star binaries.
| 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.
| 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.
| Wolf-Rayet Stars, Black Holes, and Gamma-Ray Bursters in Close Binaries
We consider the evolutionary status of observed close binary systemscontaining black holes and Wolf-Rayet (WR) stars. When the componentmasses and the orbital period of a system are known, the reason for theformation of a WR star in an initial massive system of two main-sequencestars can be established. Such WR stars can form due to the action ofthe stellar wind from a massive OB star (M OB≥50M ȯ),conservative mass transfer between components with close initial masses,or the loss of the common envelope in a system with a large (up to25) initial component mass ratio. The strong impact ofobservational selection effects on the creation of samples of closebinaries with black holes and WR stars is demonstrated. We estimatetheoretical mass-loss rates for WR stars, which are essential for ourunderstanding the observed ratio of the numbers of carbon and nitrogenWR stars in the Galaxy . We also estimate the minimum initial masses ofthe components in close binaries producing black holes and WR stars tobe 25M ȯ. The spatial velocities of systems with black holesindicate that, during the formation of a black hole from a WR star, themass loss reaches at least several solar masses. The rate of formationof rapidly rotating Kerr black holes in close binaries in the Galaxy is3×10-6 yr-1. Their formation may be accompanied by a burst ofgamma radiation, possibly providing clues to the nature of gamma-raybursts. The initial distribution of the component mass ratios for closebinaries is dNdq=dM 2/M 1 in the interval 0.04≲q 0≤1,suggesting a single mechanism for their formation.
| 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.
| Gamma-ray emission from Wolf-Rayet binaries
In the colliding wind region of early-type binaries, electrons can beaccelerated up to relativistic energies displaying power-law spectra, asdemonstrated by the detection of non-thermal radio emission from severalWR+OB systems. The particle acceleration region, located between thestars, is exposed to strong photon fields in such a way that inverseCompton cooling of the electrons could result in a substantialhigh-energy non-thermal flux. In particular cases, the ratio of theenergy densities of magnetic to photon fields in the colliding windregion will determine whether a given source can produce or notsignificant gamma-ray emission. We present here a study of the binariesWR 140, WR 146, and WR 147 in the light of recent radio and gamma-rayobservations. We show that with reasonable assumptions for the magneticfield strength WR 140 can produce the gamma-ray flux from the EGRETsource 3EG J2022+4317. WR 146 and WR 147 are below the detectionthreshold, but new and forthcoming instruments like INTEGRAL and GLASTmight detect non-thermal emission from them.
| Dust Formation Events in Colliding Winds: An Application to Eta Car
Recent IR observations indicate that many massive binary systems presentdust formation episodes in regions close to the stars during theperiastron passage. These systems are known to be high-energy sources,and it is believed that wind collisions are the origin of the emission.In this work we show that wind collisions not only increase the X-rayemission but also allow dust formation. As an application we study etaCar, which presents, near periastron, an increase in the X-ray emissionfollowed by a sudden decrease that lasts for about a month. We reproducethis feature calculating the optical depth due to dust formation alongthe orbital period.
| Evolution of Wolf-Rayet Stars in Binary Systems: An Analysis of the Mass and Orbital-Eccentricity Distributions
We have undertaken a statistical study of the component mass ratios andthe orbital eccentricities of WR + O close binary, detachedmain-sequence (DMS), contact early-type (CE), and semidetached (SD)systems. A comparison of the characteristics of WR + O systems and ofDMS, CE, and SD systems has enabled us to draw certain conclusions aboutthe evolutionary paths of WR + O binaries and to demonstrate that up to90% of all known WR + O binaries formed as a result of mass transfer inmassive close O + O binary systems. Since there is a clear correlationbetween the component masses in SD systems with subgiants, the absenceof an anticorrelation between the masses of the WR stars and O stars inWR + O binaries cannot be considered evidence against the formation ofWR + O binaries via mass transfer. The spectroscopic transitionalorbital period P tr sp corresponding to the transition from nearlycircular orbits (e sp<0.1) to elliptical orbits (e sp≥0.1) is14d for WR + O systems and 2d 3d for OB + OB systems. Theperiod range in which all WR + O orbits are circular &$(1mathop dlimits_. 6 ≤slant P ≤slant 14(d) ); is close to the range for SD systems with subgiants, &0mathop dlimits_. 7 ≤slant P ≤slant 15(d); . The large difference between the P tr sp values for WR + O and OB +OB systems suggests that a mechanism of orbit circularization additionalto that for OB + OB systems at the DMS stage (tidal dissipation of theorbital energy due to radiative damping of the dynamical tides) acts inWR + O binaries. It is natural to suggest mass transfer in the parent O+ O binaries as this supplementary orbit-circularization mechanism.Since the transitional period between circular and elliptical orbits forclose binaries with convective envelopes and ages of 5×109 yearsis &P_{tr} = 12mathop dlimits_. 4$; , the orbits of most known SD systems with subgiants had enough timeto circularize during the DMS stage, prior to the mass transfer. Thus,for most SD systems, mass transfer plays a secondary role incircularization of their orbits. In many cases, the initial orbitaleccentricities of the O + O binary progenitors of WR + O systems arepreserved, due to the low viscosity of the O-star envelopes and theshort timescale for their nuclear evolution until the primary O starfills its Roche lobe and the mass transfer begins. The mass transfer inthe parent O + O systems is short-lived, and the number of orbitalcycles during the early mass-transfer stage is relatively low (lowerthan for the progenitors of SD systems by three or four orders ofmagnitude). The continued transfer of mass from the less massive to themore massive star after the component masses have become equal leads tothe formation of a WR + O system, and the orbit's residual eccentricityincreases to the observed value. The increase of the orbitaleccentricity is also facilitated by variable radial mass loss via thewind from the WR star in the WR + O system during its motion in theelliptical orbit. The result is that WR + O binaries can haveconsiderable orbital eccentricities, despite their intense masstransfer. For this reason, the presence of appreciable eccentricitiesamong WR + O binaries with large orbital periods cannot be consideredfirm evidence against mass transfer in the parent O + O binary systems.Only for the WR + O binaries with the longest orbital periods (4 of 35known systems, or 11 %) can the evolution of the parent O + O binariesoccur without filling of the Roche lobe by the primary O star, beinggoverned by radial outflow in the form of the stellar wind and possiblyby the LBV phenomenon, as in the case of HD 5980.
| Optical line emission from the supernova remnant G 73.9+0.9
Flux calibrated images of the field around the known supernova remnant G73.9+0.9 in the Hiα+[N Ii], [S Ii], [O Ii], and [O Iii] emissionlines are presented. The low ionization images are characterized bydiffuse emission both within and outside the extent of the remnant. Theflux calibrated images revealed a few small scale structures in the eastareas of G 73.9+0.9. The long-slit spectra identify the emission fromone of them as emission from shock heated gas. This patchy structure is 3 arcmin long and emits Hα flux at a level of 33× 10-17 erg s-1 cm-2arcsec-2. The bright diffuse arc-like structure in the centerof the field seems to be associated to G 73.9+0.9 given its spectralsignature and positional relation with the non-thermal radio emission. A8 arcmin long filamentary structure with an absolute Hα fluxof 9 × 10-17 erg s-1 cm-2arcsec-2 is detected in the [O Iii] emission line to thesouth of G 73.9+0.9 but is probably unrelated. The deep long-slitspectra suggest complete recombination zones, shock velocities below 90km s-1, low electron densities (<50 cm-3) andnon-negligible magnetic field strengths.
| Metallicity and binarity in WC and WO stars
Not Available
| Dust formation as a diagnostic of Wolf-Rayet winds
Dust formation by Wolf-Rayet stars is a powerful diagnostic ofexceptional activity in their winds. Observed behaviour ranges fromstars which make dust continually (e.g. WR 104, WC9d+B0.5V) to thosewhich make dust for short episodes at periodic intervals (e.g. WR 140,WC7pd+O4-5). In both examples, dust formation is related to collidingWR and OB stellar winds in binary orbits. We have been using IRphotometry primarily from the SAAO and ESO and spectroscopy with ISO-SWS to investigate systems having intermediate properties (WR 98a,WC8-9vd+?) or more complex behaviour (WR 48a, WC8ed+?).
| Deep transient optical fading in the WC9 Star WR 106
We discovered that the WR9-type star WR 106 (HDE 313643) underwent adeep episodic fading in 2000. The depth of the fading (Delta V ~ 2.9mag) surpassed those of all known similar ``eclipse-like" fadings in WRstars. This fading episode was likely to be produced by a line-of-sightepisodic dust formation rather than a periodic enhancement of dustproduction in the WR-star wind during the passage of the companion starthough an elliptical orbit. The overall 2000 episode was composed of atleast two distinct fadings. These individual fadings seem to moresupport that the initial dust formation triggered a second dustformation, or that the two independent dust formations occurred by thesame triggering mechanism rather than a stepwise dust formation. We alsodiscuss on phenomenological similarity of the present fading with thedouble fading of R CrB observed in 1999-2000.
| Discovery of Extremely Large-Amplitude Quasi-Periodic Photometric Variability in a WC9-Type Wolf-Rayet Binary, WR 104
A spectroscopic observation of comet 19P/Borrelly was carried out on2001 September 23, using a low-dispersion spectrograph in the wavelengthrange 3800-7600Å. It was performed about 20hours after anencounter of the spacecraft Deep Space 1 (DS1) with the comet. Wedetected species such as CN, C2,[OI],andNH2, and estimated thegas-production rates of CN, C2,andNH2 relative to that ofH2Obased on a Haser model. Theproduction-rate ratios and the gas-to-dust ratiowere:Q(CN)/Q(H2O) =0.094%,Q(C2)/Q(H2O) =0.096%,Q(NH2)/Q(H2O) = 0.13%, andlog[Q(H2O)/Afρ] = 26.0.Ourcalculated relative producti! ! on rates ofC2 andNH2 to CN were similar to to CNwere similar to those of previous apparitions. Therefore, thecircumstances of the comet when DS1 encountered may be the same as thoseduring the last several apparitions, and many ground-based observationsperformed during previous apparitions will be useful to reveal thenature of the comet during the in situ observations by DS1.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| Gamma-ray line emission from OB associations and young open clusters. II. The Cygnus region
Gamma-ray and microwave observations of the Cygnus region reveal anintense signal of 1.809 Me line emission, attributed to radioactivedecay of 26, that is closely correlated with 53 GHz free-freeemission, originating from the ionised interstellar medium. We modelledboth emissions using a multi-wavelength evolutionary synthesis code formassive star associations that we applied to the known massive starpopulations in Cygnus. For all OB associations and young open clustersin the field, we determined the population age, distance, and richnessas well as the uncertainties in all these quantities from publishedphotometric and spectroscopic data. We propagate the populationuncertainties in model uncertainties by means of a Bayesian method. Theyoung globular cluster Cyg OB2 turns out to be the dominant26 nucleosynthesis and ionisation source in Cygnus. Our modelreproduces the ionising luminosity of the Cygnus region very well, yetit underestimates 26 production by about a factor of 2. Weattribute this underestimation to shortcomings of currentnucleosynthesis models, and suggest the inclusion of stellar rotationas possible mechanism to enhance 26 production. We alsomodelled 60Fe nucleosynthesis in the Cygnus region, yet thesmall number of recent supernova events suggests only little60Fe production. Consequently, a detection of the 1.137 Meand 1.332 Me decay lines of 60Fe from Cygnus by the upcomingINTEGRAL observatory is not expected. Appendices A and B, and Tables 1,2, and 5 are only available in electronic form athttp://www.edpsciences.org
| The Effect of Binarity and Metallicity in the Spectra of WC and WO Stars
A statistical analysis of the main emission lines common to the WC andWO stars is made based on an extensive set of spectral data. To definethe trends in equivalent width ( Wλ), line ratios, andline widths, median values are derived for single-spectrum stars ofdifferent spectral class. We find that in Galactic WO and WC4 stars,Wλ (C IV 581 nm) is smaller compared to inextragalactic objects. In both Galactic and extragalactic stars,Wλ (O V 559 nm) smoothly increases towards early WCand WO stars. It is argued that differences in stellar wind structure,in combination with the ambient metallicity, may be the cause of theanomalies. Variation of the profile of the 465 nm blend indicates asubstantial contribution of He II 468 nm for the WCE and WO stars. Inaddition, we comment on the carbon abundances in relation to theevolutionary status of these objects. We also give an estimate of theOB/WR continuum flux ratio in composite-spectrum systems.
| Proper Motions of New Dust in the Colliding Wind Binary WR 140
The eccentric W-R + O binary system WR 140 produces dust for a fewmonths at intervals of 7.94 yr coincident with periastron passage. Wepresent the first resolved images of this dust shell, at binary phasesφ~0.039 and ~0.055, using aperture masking techniques on the Keck Itelescope to achieve diffraction-limited resolution. Proper motions ofapproximately 1.1 mas per day were detected, implying a distance<~1.5 kpc from the known wind speed. The dust plume observed is notas simple as the ``pinwheel'' nebulae seen around other W-R collidingwind binaries, indicating the orbital plane is highly inclined to ourline of sight and/or the dust formation is very clumpy. Follow-upimaging in the mid-infrared and with adaptive optics is urgentlyrequired to track the dust motion further, necessary for unambiguouslydetermining the orbital geometry, which we only partially constrainhere. With full knowledge of the orbital elements, these infrared imagescan be used to reconstruct the dust distribution along the collidingwind interface, providing a unique tool for probing the postshockphysical conditions of violent astrophysical flows.
| 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.
| Massive Binary WR 112 and Properties of Wolf-Rayet Dust
Some hot, massive, Population I Wolf-Rayet (W-R) stars of the carbonsubclass are known to be prolific dust producers. How dust can form insuch a hostile environment remains a mystery. Here we report thediscovery of a relatively cool, extended, multiarc dust envelope aroundthe star WR 112, most likely formed by wind-wind collision in along-period binary system. We derive the binary orbital parameters, thedust temperature, and the dust mass distributions in the envelope. Wefind that amorphous carbon is a main constituent of the dust, inagreement with earlier estimates and theoretical predictions. However,the characteristic size of the dust grains is estimated to be ~1 μm,significantly larger than theoretical limits. The dust production rateis 6.1×10-7 Msolar yr-1, and thetotal detectable dust mass is found to be about2.8×10-5 Msolar (for d=4.15 kpc). We alsoshow that, despite the hostile environment, at least ~20% of theinitially formed dust may reach the interstellar medium.
| Multi-frequency variations of the Wolf-Rayet system HD193793 (WC7pd+O4-5) III. IUE observations
The colliding-wind binary system WR 140 (HD 193793, WC7pd+O4-5, P = 7.94yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35short-wavelength high-resolution spectra. An absorption-lineradial-velocity solution is obtained from the photospheric lines of theO component, by comparison with a single O star. The resulting orbitalparameters, e = 0.87 +/- 0.05, omega = 31degr +/- 9degr andKO star = 25 +/- 15 km s-1, confirm the largeeccentricity of the orbit, within the uncertainties of previous opticalstudies. This brings the weighted mean UV-optical eccentricity to e =0.85 +/- 0.04. Occultation of the O-star light by the WC wind and theWC+O colliding-wind region results into orbital modulation of theP-Cygni profiles of the C ii, C iv and Si iv resonance lines. Nearperiastron passage, the absorption troughs of those resonance-lineprofiles increase abruptly in strength and width, followed by a gradualdecrease. In particular, near periastron the blue black-edges of theP-Cygni absorption troughs shift to larger outflow velocities. Wediscuss that the apparently larger wind velocity and velocity dispersionobserved at periastron could be explained by four phenomena: (i)geometrical resonance-line eclipse effects being the main cause of theobserved UV spectral variability, enhanced by sightline crossing of theturbulent wind-wind collision zone; (ii) the possibility of anorbital-plane enhanced WC7 stellar wind; (iii) possible common-envelopeacceleration by the combined WC and O stellar radiation fields; and (iv)possible enhanced radiatively driven mass loss due to tidal stresses,focused along the orbiting line of centers.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | はくちょう座 |
| Right ascension: | 20h14m31.77s |
| Declination: | +36°39'39.6" |
| Apparent magnitude: | 7.937 |
| Distance: | 10000000 parsecs |
| Proper motion RA: | -2.9 |
| Proper motion Dec: | -6.8 |
| B-T magnitude: | 8.293 |
| V-T magnitude: | 7.967 |
Catalogs and designations:
|
