Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Spectral atlas of massive stars around He I 10 830 Å
We present a digital atlas of peculiar, high-luminosity massive stars inthe near-infrared region (10 470-11 000 Å) at medium resolution(R≃7000). The spectra are centered around He I 10 830 Å,which is formed in the wind of those stars, and is a crucial line toobtain their physical parameters. The instrumental configuration alsosampled a rich variety of emission lines of Fe II, Mg II, C I, N I, andPa γ. Secure identifications for most spectral lines are given,based on synthetic atmosphere models calculated by our group. We alsopropose that two unidentified absorption features have interstellarand/or circumstellar origin. For the strongest one (10 780 Å) anempirical calibration between E(B-V) and equivalent width is provided.The atlas displays the spectra of massive stars organized in fourcategories, namely Be stars, OBA Iape (or luminous blue variables, LBVcandidates and ex/dormant LBVs), OB supergiants and Wolf-Rayet stars.For comparison, the photospheric spectra of non emission-line stars arepresented. Selected LBVs were observed in different epochs from 2001 to2004, and their spectral variability reveals that some stars, such asη Car, AG Car and HR Car, suffered dramatic spectroscopic changesduring this time interval.Based on observations made at Observatório do Pico dos Dias/LNA(Brazil). Figures 5 to 18 are only available in electronic form athttp://www.aanda.org Electronic version of the spectra (fichiers FITS)is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/993
| 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.
| Detailed Far-Ultraviolet to Optical Analysis of Four [WR] Stars
We present far-UV to optical analyses of four hydrogen-deficient centralstars of planetary nebulae: BD +30 3639, NGC 40, NGC 5315, and NGC 6905.Using the radiative transfer code CMFGEN, we determined new physicalparameters and chemical abundances for these stars. The results wereanalyzed in the context of the [WR]-->PG 1159 evolution via thetransformed radius-temperature (RT×T*) andH-R diagrams. NGC 5315 showed itself as an odd object among thepreviously analyzed central stars. Its temperature (~76 kK) isconsiderably lower than other early-type [WR] stars (~120-150 kK). Fromour models for NGC 5315 and NGC 6905, it is unclear if early-type [WR]stars have smaller C/He mass ratios than other spectral classes, asclaimed in the literature. A ratio of ~0.8 is found for NGC 6905. Weanalyzed FUSE spectra of these stars for the first time and identifiedphosphorus in the spectra of BD +30 3639, NGC 40, and NGC 5315 throughthe doublet transition P V λλ1118, 1128 (3p2Po-3s 2S). The Fe, Si, P, S, and Neabundances were analyzed in the context of the nucleosynthesis occurringin previous evolutionary phases. We found evidence for Fe deficiency inBD +30 3639 and NGC 5315, and we determined a solar Si abundance for BD+30 3639 and NGC 40. Regarding P, an oversolar abundance in the NGC 5315model was preferred. Upper limits for the S abundance were estimated. Wefound that Ne is overabundant in BD +30 3639. In the other stars, Ne isweak or undetectable. Our results are in agreement with theoreticalpredictions and show the usefulness of [WR] stars as test beds fornucleosynthesis calculations in the AGB and post-AGB phases.
| MOST Detects g- and p-Modes in the B Supergiant HD 163899 (B2 Ib/II)
The Microvariability and Oscillations of Stars (MOST) satellite observedthe B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star anddetected 48 frequencies <~2.8 cycles day-1 with amplitudesof a few millimagnitudes (mmag) and less. The frequency range embracesg- and p-mode pulsations. It was generally thought that no g-modes areexcited in less luminous B supergiants because strong radiative dampingis expected in the core. Our theoretical models, however, show that suchg-modes are excited in massive post-main-sequence stars, in accordancewith these observations. The nonradial pulsations excited in modelsbetween 20 Msolar at logTeff~4.41 and 15Msolar at logTeff~4.36 are roughly consistent withthe observed frequency range. Excitation by the Fe bump in opacity ispossible because g-modes can be partially reflected at a convective zoneassociated with the hydrogen-burning shell, which significantly reducesradiative damping in the core. The MOST light curve of HD 163899 showsthat such a reflection of g-modes actually occurs and reveals theexistence of a previously unrecognized type of variable, slowlypulsating B supergiants (SPBsg) distinct from α Cyg variables.Such g-modes have great potential for asteroseismology.Based on data from the MOST satellite, a Canadian Space Agency mission,operated jointly by Dynacon, Inc., the University of Toronto Instituteof Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| Discovery of the New Slowly Pulsating B Star HD 163830 (B5 II/III) from MOST Space-based Photometry
We report the discovery of a new slowly pulsating B star, with thelargest number of detected frequencies to date by more than a factor of3, based on 37 days of MOST (Microvariability and Oscillations of STars)satellite guide star photometry. The star HD 163830 (V=9.3, B5 II/III)varies in 20 detected frequencies in the range 0.035-1.06day-1 (0.4-12.3 μHz) with amplitudes from 0.7 to 7.6 mmag(with a signal-to-noise ratio from 4 to 41). Eighteen of thesefrequencies are consistent with low-degree, high-order nonradial g-modesof seismic models of an evolved 4.5 Msolar star. We areunable to identify one unique model due to lack of mode identifications.The lowest two frequencies may be associated with the rotation of HD163830, but firm proof of this must await future spectroscopic data.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon Inc., the University of Toronto Institutefor Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| 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.
| An Ultraviolet to Mid-Infrared Study of the Physical and Wind Properties of HD 164270 (WC9) and Comparison to BD +30 3639 ([WC9])
We present new Spitzer IRS observations of HD 164270 (WC9, WR103). Aquantitative analysis of the UV, optical, near-, and mid-IR spectra ofHD 164270 is presented, allowing for line blanketing and wind clumping,revealing T*~48 kK, logL/Lsolar~4.9, andM˙~10-5 Msolar yr-1 for a volumefilling factor of f~0.1. Our models predict that He is partiallyrecombined in the outer stellar wind, such that recent radio-derivedmass-loss rates of WC9 stars have been underestimated. We obtainC/He~0.2 and O/He~0.01 by number from optical diagnostics. Mid-IRfine-structure lines of [Ne II] 12.8 μm and [S III] 18.7 μm areobserved, with [Ne III] 15.5 μm and [S IV] 10.5 μm absent. Fromthese we obtain Ne/He~Ne+/He=2.2×10-3 bynumber, 7 times higher than the solar value (as recently derived byAsplund et al.), and S/He~S2+/He=5.1×10-5 bynumber. From a comparison with similar results for other WC subtypes weconclude that WC9 stars are as chemically advanced as earlier subtypes.We consider why late WC stars are exclusively observed inhigh-metallicity environments. In addition, we compare theUV/optical/mid-IR spectroscopic morphology of HD 164270 with theplanetary nebula central star BD +30 3639 ([WC9]). Their UV and opticalsignatures are remarkably similar, such that our quantitativecomparisons confirm similarities in stellar temperature, wind densities,and chemistry first proposed by Smith & Aller, in spite ofcompletely different evolutionary histories, with HD 164270 presently afactor of 10 more massive than BD +30 3639. At mid-IR wavelengths, thedust from the dense young nebula of BD +30 3639 completely dominates itsappearance, in contrast with HD 164270.
| MOST Detects g-Modes in the Be Star HD 163868
We have extracted a 37 day light curve with a precision of 0.0012 magper point for the Microvariability and Oscillations of Stars (MOST)guide star, HD 163868 (B5 Ve). Its rich frequency spectrum resemblesthat of a slowly pulsating B (SPB) star but, being a rapid rotator, wedesignate it SPBe. The 60 most significant periods lie in three distinctgroups centered on 8 days and 14 and 7 hr. We demonstrate that the 14and 7 hr periods can be modeled by two swarms of high-order, progradesectorial g-modes (m=-1, -2), which are destabilized by the iron opacitybump. Our model also predicts a group of r-modes with periods near 2.3days, which correspond to frequencies observed in the tail of the 8 daygroup. The remaining periodicities, between 7 and 11 days, cannot beexplained by unstable modes in our model.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon, Inc., the University of Toronto Instituteof Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| Five WC9 stars discovered in the AAO/UKST Hα survey
We report the discovery of five massive Wolf-Rayet (WR) stars resultingfrom a programme of follow-up spectroscopy of candidate emission-linestars in the Anglo-Australian Observatory United Kingdom SchmidtTelescope (AAO/UKST) Southern Galactic Plane Hα survey. The6195-6775 Åspectra of the stars are presented and discussed. A WC9class is assigned to all five stars through comparison of their spectrawith those of known late-type WC stars, bringing the known total numberof Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibitnear-infrared (NIR) excesses characteristic of hot dust emission (asseen in the great majority of known WC9 stars), we find that two of thestars show no discernible evidence of such excesses. This increases thenumber of known WC9 stars without NIR excesses to seven. Reddenings anddistances for all five stars are estimated.
| 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.
| Inferring hot-star-wind acceleration from Line Profile Variability
The migration of profile sub-peaks identified in time-monitored opticalemission lines of Wolf-Rayet (WR) star spectra provides a directdiagnostic of the dynamics of their stellar winds via a measured ΔvLOS/Δ t, a line-of-sight velocity change per unittime. Inferring the associated wind acceleration scale from such anapparent acceleration then relies on the adopted intrinsic velocity ofthe wind material at the origin of this variable pattern. Such acharacterization of the Line Emission Region (LER) is in principlesubject to inaccuracies arising from line optical depth effects andturbulence broadening. In this paper, we develop tools to quantify sucheffects and then apply these to reanalyze the LER properties oftime-monitored WR stars. We find that most program lines can be fittedwell with a pure optically thin formation mechanism, that the observedline-broadening is dominated by the finite velocity extent of the LER,and that the level of turbulence inferred through Line ProfileVariability (lpv) has only a minor broadening effect in the overallprofile. Our new estimates of LER velocity centroids are systematicallyshifted outwards closer to terminal velocity compared to previousdeterminations, now suggesting WR-wind acceleration length scales βR* of the order of 10-20 Rȯ, a factor of afew smaller than previously inferred. Based on radiation-hydrodynamicssimulations of the line-driven-instability mechanism, we computesynthetic lpv for Ciii5696 Å for WR 111. The results match wellthe measured observed migration of 20-30 m s-2, equivalent toβ R* 20 Rȯ. However, our modelstellar radius of 19 Rȯ, typical of an O-typesupergiant, is a factor 2-10 larger than generally expected for WR coreradii. Such small radii leave inferred acceleration scales to be moreextended than expected from dynamical models of line driving, whichtypically match a “beta” velocity lawv(r)=v&infy; (1-R*/r)β, withβ ≈ 1-2; but the severity of the discrepancy is substantiallyreduced compared to previous analyses. We conclude with a discussion ofhow using lines formed deeper in the wind would provide a strongerconstraint on the key wind dynamics in the peak acceleration region,while also potentially providing a diagnostic on the radial variation ofwind clumping, an issue that remains crucial for reliable determinationof O-star mass loss rates.
| An Atlas of Far-Ultraviolet Spectra of Wolf-Rayet Stars from the FUSE Satellite
We present an atlas of far-ultraviolet spectra of 21 Wolf-Rayet (WR)stars in the Galaxy and Large and Small Magellanic Clouds, secured withthe Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The datacover the wavelength range of 912-1190 Å at a spectral resolutionof 0.1 Å and span examples of most subtypes in the WN and WCsequences. We discuss the FUV spectral morphology of the different WRsequences, emphasizing the wide range of ions and chemical speciesexhibiting well-developed P Cygni profiles and emission lines in thiswavelength range. For WN stars the relative strengths of C IV/C III, NIII/N II, P V/P IV, and Si IV/Si III show a decrease in strength of thehigh ions from WN3 to WN11 complemented by an increase in the lower ionsat later types. The ``super ions'' of O VI and S VI are consideredphotoionized wind features for WN3-WN6 stars, probably the result ofAuger ionization in WN7-WN9 stars, and probably absent at WN10-WN11. TheWN5h star Sk 41 in the SMC shows relatively weaker features, which canbe ascribed to the effects of a global galaxy metal deficiency. For theWC stars, a similar pattern of wind ionization-linked strengths in theemissions and P Cygni profiles is present, particularly evident in therelative strengths of lines in P V, S IV, Si IV, and Si III. O VI, and SVI features are only seen in the earliest WC subtypes. The high carbonabundance in WC stars is reflected by the presence of strong C IV and CIII lines throughout the sequence. We present new estimates of the windterminal velocities from measurements of saturated absorption componentsobserved in a wide range of I.P. species. Considerable revisions tov&infy; for the WN3 and WN5 (SMC) stars in our sample and,in particular for the WN10 and WN11 stars are found. The latter make useof the unique availability of the N II resonance line in the FUSEwaveband.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| A Very Large Array 3.6 Centimeter Continuum Survey of Galactic Wolf-Rayet Stars
We report the results of a survey of radio continuum emission ofGalactic Wolf-Rayet (WR) stars north of δ=-46°. Theobservations were obtained at 8.46 GHz (3.6 cm) using the Very LargeArray, with an angular resolution of ~6"×9" and typical rms noiseof ~0.04 mJy beam-1. Our survey of 34 WR stars resulted in 15definite and five probable detections, 13 of these for the first time atradio wavelengths. All detections are unresolved (θ<~5"). Timevariations in flux are confirmed in the cases of WR 98a, 104, 105, and125. WR 79a and WR 89 are also variable in flux, and we suspect they arealso nonthermal emitters. Thus, of our sample 20%-30% of the detectedstars are nonthermal emitters. Average mass-loss rate determinationsobtained excluding definite and suspected nonthermal cases give similarvalues for WN (all subtypes) and WC5-7 stars[M(WN)=(4+/-3)×10-5 Msolar yr-1and M(WC5-7)=(4+/-2)×10-5 Msolaryr-1], while a lower value was obtained for WC8-9 stars[M(WC8-9)=(2+/-1)×10-5 Msolaryr-1]. Uncertainties in stellar distances largely contributeto the observed scatter in mass-loss rates. Upper limits to themass-loss rates were obtained in cases of undetected sources and forsources that probably show additional nonthermal emission.
| 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.
| Near-IR spectroscopic search for binary companions to dusty WC stars
Many late WC type Wolf-Rayet (WR) stars are known to form dust in theirwinds. While, in the case of episodic dust makers, it is now clear thatdust formation takes place due to a colliding wind phenomenon, we stilldo not know for sure if binarity is the main reason for dust formationin persistent dust makers. This work is to understand, if persistentdust formation is also due to colliding winds in close binary systems. Asystematic search for colliding winds is taken up via high resolutionspectroscopy of the HeI λ1.083μm emission line. Preliminaryindications of binarity in many persistent dust makers are seen.
| Wind inhomogeneities in [WC] central stars of planetary nebulae
Not Available
| Insights into Wolf-Rayet WC9 stars from spectroscopy}
We present new spectroscopic observations of WC9 stars. As predicted byWilliams & van der Hucht (2000), O II lines are weak in thedust-free star WR 88, which also shows a high ionization. Spectralvariability is detected in WR 106 and WR 121. These findings arebriefly discussed in terms of the conditions for dust formation inWolf-Rayet winds.
| The mass-loss rates of Wolf-Rayet stars explained by optically thick radiation driven wind models
Observed, clumping-corrected mass-loss rates of Galactic Wolf-Rayet (WR)stars are compared with predictions of the optically thick radiationdriven wind models. We did not develop models for the whole wind, but westudied the conditions at the sonic point that would explain theobserved high mass-loss rates of WR-stars. We find that optically thickwind models can explain the observed values of the mass-loss rates onlyif two conditions are satisfied: (a) The sonic point (wherevflow=vsound) lies deep in the wind where thetemperature is either near 160 000 K, or in the range of 40 000 to 70000 K. (b) The flux-mean opacity must increase outward from the sonicpoint. With these conditions a simple approximate formula for themass-loss rates of WR-stars can be derived. The first condition impliesthat the sonic point is at an optical depth between about 3 and 30. Suchlarge optical depths require a slowly increasing velocity law in thesupersonic region, with a velocity-law index of beta =~ 5 for WR-stars,compared to beta =~ 1 for O-stars. The OPAL-opacity tables for thechemical composition of the WR-stars show that the opacity indeedincreases outward at the temperature range near 1.6 x 105 K,and between about 4 x 107 and 7 x 104 K, asrequired for the optically thick wind models. The opacity at the sonicpoints of the models is very similar to the OPAL-opacity at the sonicpoint temperature and density. The radius of the sonic point is abouthalf as large as the inner boundaries of the ``standard'' models forearly type WR-winds. Observational evidence, derived from line profilevariations and from the light-curves of WR-stars in eclipsing binarysystems, support the derived large values of beta and the small valuesof the sonic point radius. The models presented here show that the highmass-loss rates of WR-stars might be the result of optically thickradiation driven winds. The presence of two very distinct temperatureregimes for the sonic point implies a bifurcation in the wind models ofWR-stars.
| The enigmatic WR46: A binary or a pulsator in disguise. III. Interpretation
Photometric and spectroscopic monitoring campaigns of WR46 (WN3p), as presented in Veen et al. (2002a,b; hereafterPapers I and II, respectively), yield the following results. The light-and colour variations reveal a dominant single-wave period ofPsw89=0.1412 d in 1989, andPsw91 =0.1363 d in 1991. Because of a smalldifference in the minima, this periodicity may be a double-wavephenomenon (Pdw). The line fluxes vary in concert with themagnitudes. The significant difference of the periods can be either dueto the occurence of two distinct periods, or due to a gradual change ofthe periodicity. A gradual brightening of the system of 0.12m appearedto accompany the period change. In addition, the light variations in1989 show strong evidence for an additional period Px=0.2304d. Generally, the radial velocities show a cyclic variability on a timescale of the photometric double-wave. However, often they do not vary atall. The observed variability confirms the Population I WR nature of thelight source, as noted independently by Marchenko et al. (2000). In thepresent paper, we first show how the photometric double-wave variabilitycan be interpreted as a rotating ellipsoidal density distribution in thestellar wind. Subsequently, we discuss what mechanisms could drive sucha configuration. First, stellar rotation of a single star is discardedas a likely cause. Second, the obvious interpretation of the double-wavephotometry, i.e., a close binary system, is investigated. Howeverunlikely, we discuss how the observed period change might be reconciledwithin a model of a strongly interacting binary. Third, aninterpretation of a non-radial multi-mode pulsator is investigated. Theobserved period change and the multi-frequency behaviour do support thisinterpretation. We propose that the pulsational mode l=1 and |m|=1 maymimic a ``binary'' light- and radial-velocity curve. However, thephasing of the radial velocity and the light curve may be inconsistent.The possibility l=2 and |m|=0 is also discussed. Finally, we suggest howthe enigma of the variability of WR 46 may be solved.
| 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.
| [WC] Stellar Wind Turbulent Outflows Feeding the ISM
Wind fluctuations in [WC 8-9]-type central stars of planetary nebulaehave been detected. Moving features seen on the top of the C III lambda5696 and C IV lambda lambda 5801/12 emission lines of HD 826 (NGC 40)and BD +30^o3639 are interpreted as outflowing clumps which are radiallyaccelerated in the Wolf-Rayet winds. Kinematic parameters of the blobswere derived and compared to those observed for massive Wolf-Rayetstars.
| [WC] central stars of planetary nebula and the born-again phenomenon
[WC] central stars of planetary nebulae are members of the larger classof hydrogen-deficient central stars. The whole class constitutes about20% of all spectroscopically-known central stars. Observationalconnections between [WC] central stars and the born-again phenomenonshow that at least a fraction of the [WC] stars can be created throughthis scenario. However, it is unlikely that the class as a whole evolvedthrough this channel. In this paper the arguments against a born-againorigin for the whole class of [WC] central stars of planetary nebula areoutlined. It is suggested that the role of the H-deficient weak emissionlines stars might be crucial in explaining the origin of [WC] stars. Itis also demonstrated how difficult it is to pin down the exact stellarparameters of these objects (which help to position them on the HRdiagram). This is due to the largely unknown distances and to the factthat small changes in the model assumptions can have large repercussionson the derived parameters. This difficulty hampers our efforts todetermine the true evolutionary position of individual [WC] centralstars, as well as their relationship to one another, and therefore pindown their origin.
| Speckle interferometry and radiative transfer modelling of the Wolf-Rayet star WR 118
WR 118 is a highly evolved Wolf-Rayet star of theWC10 subtype surrounded by a permanent dust shell absorbing andre-emitting in the infrared a considerable fraction of the stellarluminosity. We present the first diffraction-limited 2.13 mu m speckleinterferometric observations of WR 118 with 73 masresolution. The speckle interferograms were obtained with the 6 mtelescope at the Special Astrophysical Observatory. The two-dimensionalvisibility function of the object does not show any significantdeviation from circular symmetry. The visibility curve declines towardsthe diffraction cut-off frequency to ~ 0.66 and can be approximated by alinear function. Radiative transfer calculations have been carried outto model the spectral energy distribution, given in the range of 0.5-25mu m, and our 2.13 mu m visibility function, assuming spherical symmetryof the dust shell. Both can be fitted with a model containingdouble-sized grains (``small'' and ``large'') with the radii of a = 0.05mu m and 0.38 mu m, and a mass fraction of the large grains greater than65%. Alternatively, a good match can be obtained with the grain sizedistribution function n(a) ~ a-3, with a ranging between0.005 mu m and 0.6 mu m. At the inner boundary of the modelled dustshell (angular diameter Thetain = (17 +/- 1) mas), thetemperature of the smallest grains and the dust shell density are 1750 K+/- 100 K and (1 +/- 0.2) x 10-19 g/cm3,respectively. The dust formation rate is found to be (1.3 +/- 0.5) x10-7 Msun/yr, assuming Vwind = 1200 kms-1.
| Turbulent outflows from [WC]-type nuclei of planetary nebulae. II. The [WC 8] central star of \object{NGC 40}.
Using spectroscopic observations taken at the Observatoire deHaute-Provence (France) and the Observatoire du mont Mégantic(Canada), we describe wind fluctuations in the [WC 8]-type central starof the planetary nebula NGC 40, HD826, which was observed intensively during 22 nights. Movingfeatures seen on the top of the C Iiilambda 5696 and C Ivlambda lambda5801/12 (+C Iiilambda 5826) emission lines are interpreted as outflowing``blobs'' which are radially accelerated outwards in the Wolf-Rayetwind. The amplitudes of the variations range up to 25-30% of theadjacent continuum flux, over timescales of hours. The variabilities ofboth lines are quite well correlated, although they are somewhat weakerfor the C Iv complex. Subpeaks (or gaps) on the top of the C Iii linegenerally move towards the nearest line edge in a symmetric fashion inthe blue and the red. Kinematic parameters of the blobs were derived andcompared to those observed for massive and other low-mass Wolf-Rayetstars. Especially impressive are the significantly larger observedmaximum radial acceleration values of the blobs, compared to thosealready reported for massive WC 5-9, or low-mass [WC 9] stars. This isattributed to the very small stellar radius of HD826. In addition the beta velocity field is found to possiblyunderestimate the true gradient within the stellar wind flow. On thewhole, the wind of HD 826 is highly stochasticallyvariable on a very short time-scale. This supports a turbulent origin.Based on observations taken at the Observatoire de Haute-Provence,France, and at the Observatoire du mont Mégantic, Canada.
| Modeling of Rapid Variability in the Spectral Line Profiles of Wolf-Rayet Stars
Profiles of variable emission lines in the spectra of Wolf-Rayet starsare calculated using a stochastic cloud model for the inhomogeneousatmospheres of early-type stars. The model assumes that most of the lineflux is formed in cold, dense condensations (clouds) that move through ararified inter-cloud medium whose density monotonically decreasesoutwards. The formation of clouds is taken to be stochastic. Waveletanalysis is used to estimate the parameters of cloud ensembles. Themodel can reproduce the general pattern of line-profile variabilityobserved in the spectra of Wolf-Rayet stars.
| The VIIth catalogue of galactic Wolf-Rayet stars
The VIIth catalogue of galactic PopulationI Wolf-Rayet stars providesimproved coordinates, spectral types and /bv photometry of known WRstars and adds 71 new WR stars to the previous WR catalogue. This censusof galactic WR stars reaches 227 stars, comprising 127 WN stars, 87 WCstars, 10 WN/WC stars and 3 WO stars. This includes 15 WNL and 11 WCLstars within 30 pc of the Galactic Center. We compile and discuss WRspectral classification, variability, periodicity, binarity, terminalwind velocities, correlation with open clusters and OB associations, andcorrelation with Hi bubbles, Hii regions and ring nebulae. Intrinsiccolours and absolute visual magnitudes per subtype are re-assessed for are-determination of optical photometric distances and galacticdistribution of WR stars. In the solar neighbourhood we find projectedon the galactic plane a surface density of 3.3 WR stars perkpc2, with a WC/WN number ratio of 1.5, and a WR binaryfrequency (including probable binaries) of 39%. The galactocentricdistance (RWR) distribution per subtype shows RWRincreasing with decreasing WR subtype, both for the WN and WC subtypes.This RWR distribution allows for the possibility ofWNE-->WCE and WNL-->WCL subtype evolution.
| Wolf-Rayet Stars and Relativistic Objects: Distinctions between the Mass Distributions in Close Binary Systems
The observed properties of Wolf-Rayet stars and relativistic objects inclose binary systems are analyzed. The final masses M CO f for thecarbon-oxygen cores of WR stars in WR + O binaries are calculated takinginto account the radial loss of matter via stellar wind, which dependson the mass of the star. The analysis includes new data on the clumpystructure of WR winds, which appreciably decreases the requiredmass-loss rates for the WR stars. The masses M CO f lie in the range (12)M ȯ (20 44)M ȯ and have a continuous distribution. Themasses of the relativistic objects M x are 1 20M ȯ and have abimodal distribution: the mean masses for neutron stars and black holesare 1.35 ± 0.15M ȯ and 8 10M ȯ, respectively, with agap from 2 4M ȯ in which no neutron stars or black holes areobserved in close binaries. The mean final CO-core mass is &$/line M _{CO}(f) = 7.4 - 10.3M_ ȯ$; , close to the mean mass for the black holes. This suggests that it isnot only the mass of the progenitor that determines the nature of therelativistic object, but other parameters as well-rotation, magneticfield, etc. One SB1R Wolf-Rayet binary and 11 suspected WR + C binariesthat may have low-mass companions (main-sequence or subgiant M-A stars)are identified; these could be the progenitors of low-mass X-raybinaries with neutron stars and black holes.
| Turbulent outflows from [WC]-type nuclei of planetary nebulae. I. BD +30 3639 and other [WC 9-10] stars
Using spectroscopic observations taken at the Observatoire deHaute-Provence (OHP, France) and the European Southern Observatory (ESO,Chile), we describe wind fluctuations in four [WC 9-10]-type centralstars of planetary nebulae, especially BD +30°3639, which was observed intensively during 15 nights. Movingfeatures seen on the top of the C Iiilambda 5696 emission line areinterpreted as outflowing ``blobs'' which are radially acceleratedoutwards, as seen in the winds of massive Wolf-Rayet stars. We find lineprofile variations occurring on a time scale of hours. Kinematicparameters of the blobs are derived and compared to those of massiveWolf-Rayet stars. The wind fragmentation process appears independent ofthe strong differences between both types of hot stars.
| X-ray Nova Binary Systems
The physical properties of X-ray novae as close binary systems areanalysed. Observational data in X-ray, UV, optical, IR and radio rangesare summarized. Modern theoretical considerations of the problem ofX-ray novae, taking into account some new ideas and results, aredescribed. Properties of optical stars in X-ray novae are analysed. Dataabout the masses of black holes in X-ray binary systems are summarized.Possible evolutionary links between WR stars in close binary systems andX-ray novae are analysed.
| Mass-loss rates of Wolf-Rayet stars as a function of stellar parameters
Clumping-corrected mass-loss rates of 64 Galactic Wolf-Rayet (WR) starsare used to study the dependence of mass-loss rates, momentum transferefficiencies and terminal velocities on the basic stellar parameters andchemical composition. The luminosities of the WR stars have beendetermined either directly from the masses, using the dependence of L onmass predicted by stellar evolution theory, or they were determined fromthe absolute visual magnitudes and the bolometric corrections. For thispurpose we improved the relation between the bolometric correction andthe spectral subclass. (1) The momentum transfer efficiencies η(i.e. the ratio between the wind momentum loss and radiative momentumloss) of WR stars are found to lie in the range of 1.4 to 17.6, with themean value of 6.2 for the 64 program stars. Such values can probably beexplained by radiative driving due to multiple scattering of photons ina WR wind with an ionization stratification. However, there may be aproblem in explaining the driving at low velocities. (2) We derived thelinear regression relations for the dependence of the terminal velocity,the momentum transfer efficiency and the mass-loss rates on luminosityand chemical composition. We found a tight relation between the terminalvelocity of the wind and the parameters of the hydrostatic core. Thisrelation enables the determination of the mass of the WR stars fromtheir observed terminal velocities and chemical composition with anaccuracy of about 0.1 dex for WN and WC stars. Using evolutionary modelsof WR stars, the luminosity can then be determined with an accuracy of0.25 dex or better. (3) We found that the mass-loss rates(&mathaccent "705Frelax dot;) of WR stars depend strongly onluminosity and also quite strongly on chemical composition. For thecombined sample of WN and WC stars we found that &mathaccent"705Frelax dot; in Mȯyr-1 can be expressed as&mathaccent "705Frelax dot; ≃ 1.0 ×10-11(L/L ȯ)1.29Y1.7Z0.5 (1) with an uncertainty of σ = 0.19dex (4) The new mass-loss rates are significantly smaller than adoptedin evolutionary calculations, by about 0.2 to 0.6 dex, depending on thecomposition and on the evolutionary calculations. For H-rich WN starsthe new mass-loss rates are 0.3 dex smaller than adopted in theevolutionary calculations of Meynet et al. (1994). (5) The lowermass-loss rates, derived in this paper compared to previously adoptedvalues, facilitate the formation of black holes as end points of theevolution of massive stars. However they might create a problem inexplaining the observed WN/WC ratios, unless rotational mixing ormass-loss due to eruptions is important.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Yay |
| Sað Açýklýk: | 18h01m43.15s |
| Yükselim: | -32°42'55.2" |
| Görünürdeki Parlaklýk: | 8.825 |
| Uzaklýk: | 10000000 parsek |
| özdevim Sað Açýklýk: | 3.4 |
| özdevim Yükselim: | -3.2 |
| B-T magnitude: | 9.066 |
| V-T magnitude: | 8.845 |
Kataloglar ve belirtme:
|
