Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
New X-Ray Detections of WNL Stars
Previous studies have demonstrated that putatively single nitrogen-typeWolf-Rayet stars (WN stars) without known companions are X-ray sources.However, almost all WN star X-ray detections so far have been of earlierWN2-WN6 spectral subtypes. Later WN7-WN9 subtypes (also known as WNLstars) have proved more difficult to detect, an important exceptionbeing WR 79a (WN9ha). We present here new X-ray detections of the WNLstars WR 16 (WN8h) and WR 78 (WN7h). These new results, when combinedwith previous detections, demonstrate that X-ray emission is present inWN stars across the full range of spectral types, including later WNLstars. The two WN8 stars observed to date (WR 16 and WR 40) showunusually low X-ray luminosities (Lx ) compared to other WNstars, and it is noteworthy that they also have the lowest terminal windspeeds (v ?). Existing X-ray detections of about adozen WN stars reveal a trend of increasing Lx with windluminosity Lwind =(1/2)?v2?, suggesting that wind kineticenergy may play a key role in establishing X-ray luminosity levels in WNstars.
| The Galactic WC stars. Stellar parameters from spectral analyses indicate a new evolutionary sequence
Context. The life cycles of massive stars from the main sequence totheir explosion as supernovae or gamma ray bursts are not yet fullyclear, and the empirical results from spectral analyses are partly inconflict with current evolutionary models. The spectral analysis ofWolf-Rayet stars requires the detailed modeling of expanding stellaratmospheres in non-LTE. The Galactic WN stars have been comprehensivelyanalyzed with such models of the latest stage of sophistication, while asimilarly comprehensive study of the Galactic WC sample remains undone. Aims: We aim to establish the stellar parameters and mass-lossrates of the Galactic WC stars. These data provide the empirical basisof studies of (i) the role of WC stars in the evolution of massivestars, (ii) the wind-driving mechanisms, and (iii) the feedback of WCstars as input to models of the chemical and dynamical evolution ofgalaxies. Methods: We analyze the nearly complete sample ofun-obscured Galactic WC stars, using optical spectra as well asultraviolet spectra when available. The observations are fitted withtheoretical spectra, using the Potsdam Wolf-Rayet (PoWR) modelatmosphere code. A large grid of line-blanked models has beenestablished for the range of WC subtypes WC4 - WC8, and smaller gridsfor the WC9 parameter domain. Both WO stars and WN/WC transit types arealso analyzed using special models. Results: Stellar andatmospheric parameters are derived for more than 50 Galactic WC and twoWO stars, covering almost the whole Galactic WC population as far as thestars are single, and un-obscured in the visual. In theHertzsprung-Russell diagram, the WC stars reside between the hydrogenand the helium zero-age main sequences, having luminosities L from104.9 to 105.6 L&sun;. The mass-lossrates scale very tightly with L0.8. The two WO stars in oursample turn out to be outstandingly hot (?200 kK) and do not fit intothe WC scheme. Conclusions: By comparing the empirical WCpositions in the Hertzsprung-Russell diagram with evolutionary models,and from recent supernova statistics, we conclude that WC stars haveevolved from initial masses between 20 solar masses and 45M&sun;. In contrast to previous assumptions, it seems that WCstars in general do not descend from the most massive stars. Only the WOstars might stem from progenitors that have been initially more massivethan 45 M&sun;.Appendices A and B are available in electronic form at http://www.aanda.org
| A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. II. A Global View of the Wind Variability
This study is the second part of a survey searching for large-scalespectroscopic variability in apparently single Wolf-Rayet (WR) stars. Ina previous paper (Paper I), we described and characterized thespectroscopic variability level of 25 WR stars observable from thenorthern hemisphere and found 3 new candidates presenting large-scalewind variability, potentially originating from large-scale structuresnamed corotating interaction regions (CIRs). In this second paper, wediscuss an additional 39 stars observable from the southern hemisphere.For each star in our sample, we obtained 4-5 high-resolution spectrawith a signal-to-noise ratio of ~100 and determined its variabilitylevel using the approach described in Paper I. In total, 10 new starsare found to show large-scale spectral variability of which 7 presentCIR-type changes (WR 8, WR 44, WR55, WR 58, WR 61, WR 63, WR 100). Ofthe remaining stars, 20 were found to show small-amplitude changes and 9were found to show no spectral variability as far as can be concludedfrom the data on hand. Also, we discuss the spectroscopic variabilitylevel of all single galactic WR stars that are brighter than v ~ 12.5,and some WR stars with 12.5 < v <= 13.5, i.e., all the starspresented in our two papers and four more stars for which spectra havealready been published in the literature. We find that 23/68 stars(33.8%) present large-scale variability, but only 12/54 stars (~22.1%)are potentially of CIR type. Also, we find that 31/68 stars (45.6%) onlyshow small-scale variability, most likely due to clumping in the wind.Finally, no spectral variability is detected based on the data on handfor 14/68 (20.6%) stars. Interestingly, the variability with the highestamplitude also has the widest mean velocity dispersion.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| The Spitzer Atlas of Stellar Spectra (SASS)
We present the Spitzer Atlas of Stellar Spectra, which includes 159stellar spectra (5-32 ?m R ~ 100) taken with the InfraredSpectrograph on the Spitzer Space Telescope. This Atlas gathersrepresentative spectra of a broad section of the Hertzsprung-Russelldiagram, intended to serve as a general stellar spectral reference inthe mid-infrared. It includes stars from all luminosity classes, as wellas Wolf-Rayet (WR) objects. Furthermore, it includes some objects ofintrinsic interest, such as blue stragglers and certain pulsatingvariables. All of the spectra have been uniformly reduced, and all areavailable online. For dwarfs and giants, the spectra of early-typeobjects are relatively featureless, characterized by the presence ofhydrogen lines in A spectral types. Besides these, the most noticeablephotospheric features correspond to water vapor and silicon monoxide inlate-type objects and methane and ammonia features at the latestspectral types. Most supergiant spectra in the Atlas present evidence ofcircumstellar gas and/or dust. The sample includes five M supergiantspectra, which show strong dust excesses and in some cases polycyclicaromatic hydrocarbon features. Sequences of WR stars present thewell-known pattern of lines of He I and He II, as well as forbiddenlines of ionized metals. The characteristic flat-top shape of the [NeIII] line is evident even at these low spectral resolutions. SeveralLuminous Blue Variables and other transition stars are present in theAtlas and show very diverse spectra, dominated by circumstellar gas anddust features. We show that the [8]-[24] Spitzer colors (IRAC and MIPS)are poor predictors of spectral type for most luminosity classes.
| Chandra Detects the Rare Oxygen-type Wolf-Rayet Star WR 142 and OB Stars in Berkeley 87
We present first results of a Chandra X-ray observation of the rareoxygen-type Wolf-Rayet (WR) star WR 142 (= Sand 5 = St 3) harbored inthe young, heavily obscured cluster Berkeley 87. Oxygen-type WO starsare thought to be the most evolved of the WRs and progenitors ofsupernovae or gamma-ray bursts. As part of an X-ray survey of supposedlysingle WR stars, we observed WR 142 and the surrounding Berkeley 87region with Chandra ACIS-I. We detect WR 142 as a faint yet extremelyhard X-ray source. Due to weak emission, its nature as a thermal ornon-thermal emitter is unclear and thus we discuss several emissionmechanisms. Additionally, we report seven detections and eightnon-detections by Chandra of massive OB stars in Berkeley 87, two ofwhich are bright yet soft X-ray sources whose spectra provide a dramaticcontrast to the hard emission from WR 142.
| X-ray Emission from Nitrogen-Type Wolf-Rayet Stars
We summarize new X-ray detections of four nitrogen-type Wolf-Rayet (WR)stars obtained in a limited survey aimed at establishing the X-rayproperties of WN stars across their full range of spectral subtypes.None of the detected stars is so far known to be a close binary. Wereport Chandra detections of WR 2 (WN2), WR 18 (WN4), and WR 134 (WN6),and an XMM-Newton detection of WR79a (WN9ha). These observations clearlydemonstrate that both WNE and WNL stars are X-ray sources. We alsodiscuss Chandra archive detections of the WN6h stars WR 20b, WR 24, andWR 136 and ROSAT non-detections of WR 16 (WN8h) and WR 78 (WN7h). TheX-ray spectra of all WN detections show prominent emission lines and anadmixture of cool (kT < 1 keV) and hot (kT > 2 keV) plasma. Thehotter plasma is not predicted by radiative wind shock models and otheras yet unidentified mechanisms are at work. Most stars show X-rayabsorption in excess of that expected from visual extinction (AV), likely due to their strong winds or cold circumstellargas. Existing data suggest a falloff in X-ray luminosity toward laterWN7-9 subtypes, which have higher L bol but slower, denserwinds than WN2-6 stars. This provides a clue that wind properties may bea more crucial factor in determining emergent X-ray emission levels thanbolometric luminosity.
| X-ray spectroscopy of stars
Non-degenerate stars of essentially all spectral classes are soft X-raysources. Their X-ray spectra have been important in constrainingphysical processes that heat plasma in stellar environments totemperatures exceeding one million degrees. Low-mass stars on the coolerpart of the main sequence and their pre-main sequence predecessorsdefine the dominant stellar population in the galaxy by number. TheirX-ray spectra are reminiscent, in the broadest sense, of X-ray spectrafrom the solar corona. The Sun itself as a typical example of amain-sequence cool star has been a pivotal testbed for physical modelsto be applied to cool stars. X-ray emission from cool stars is indeedascribed to magnetically trapped hot gas analogous to the solar coronalplasma, although plasma parameters such as temperature, density, andelement abundances vary widely. Coronal structure, its thermalstratification and geometric extent can also be interpreted based onvarious spectral diagnostics. New features have been identified inpre-main sequence stars; some of these may be related to accretionshocks on the stellar surface, fluorescence on circumstellar disks dueto X-ray irradiation, or shock heating in stellar outflows. Massive, hotstars clearly dominate the interaction with the galactic interstellarmedium: they are the main sources of ionizing radiation, mechanicalenergy and chemical enrichment in galaxies. High-energy emission permitsto probe some of the most important processes at work in these stars,and put constraints on their most peculiar feature: the stellar wind.Medium and high- resolution spectroscopy have shed new light on theseobjects as well. Here, we review recent advances in our understanding ofcool and hot stars through the study of X-ray spectra, in particularhigh-resolution spectra now available from XMM -Newton and Chandra. Weaddress issues related to coronal structure, flares, the composition ofcoronal plasma, X-ray production in accretion streams and outflows,X-rays from single OB-type stars, massive binaries, magnetic hot objectsand evolved WR stars.
| A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. I. Characterizing the Variability
We present the results of a systematic search for large-scalespectroscopic variability in apparently single Wolf-Rayet (WR) starsbrighter than v ~ 12.5. In this first paper we characterize the variousforms of variability detected and distinguish several separate groups.For each star in our sample, we obtained 4-5 high-resolution spectrawith signal-to-noise ratio ~100. Our ultimate goal is to identify newcandidates presenting variability that potentially comes from corotatinginteraction regions (CIRs). Out of a sample of 25 stars, 10 were foundto display large-scale changes of which four are of CIR-type (WR 1,WR 115, WR 120, and WR 134). The star WR 134 wasalready known to show such changes from previous studies. Three WN8stars present a different type of large-scale variability and we believedeserve a group of their own. Also, all three WC9d stars in our samplepresent large-scale variability, but it remains to be checked if theseare binaries, as many dust-making WR stars are double. Finally, of theremaining stars, 10 were found to show small-amplitude spectral changes,which we attribute to normal line-profile variability due toinhomogeneities in the wind, and five were found to show no spectralvariability, as far as can be concluded from the data in hand. Follow-upstudies are required to identify potential periods for our candidatesshowing CIR-type changes and eventually estimate a rotation rate forthese WR stars.
| Discovery of X-Ray Emission from the Wolf-Rayet Star WR 142 of Oxygen Subtype
We report the discovery of weak yet hard X-ray emission from theWolf-Rayet (WR) star WR 142 with the XMM-Newton X-ray telescope.Being of spectral subtype WO2, WR 142 is a massive star in a veryadvanced evolutionary stage shortly before its explosion as a supernovaor ?-ray burst. This is the first detection of X-ray emission froma WO-type star. We rule out any serendipitous X-ray sources within?1'' of WR 142. WR 142 has an X-ray luminosity of LX ? 7 × 1030 erg s-1,which constitutes only lsim10-8 of its bolometricluminosity. The hard X-ray spectrum suggests a plasma temperature ofabout 100 MK. Commonly, X-ray emission from stellar winds is attributedto embedded shocks due to the intrinsic instability of the radiationdriving. From qualitative considerations we conclude that this mechanismcannot account for the hardness of the observed radiation. There are nohints for a binary companion. Therefore the only remaining, albeitspeculative explanation must refer to magnetic activity. Possiblyrelated, WR 142 seems to rotate extremely fast, as indicated by theunusually round profiles of its optical emission lines. Our detectionimplies that the wind of WR 142 must be relatively transparent toX-rays, which can be due to strong wind ionization, wind clumping, ornonspherical geometry from rapid rotation.
| Automatic Detection of Expanding H I Shells in the Canadian Galactic Plane Survey Data
The identification of expanding H I shells is difficult because of theirvariable morphology. In this paper we present an automatic detector forH I shells, based on the more stable dynamical characteristics ofexpanding bubbles with radii <40 pc. The detection is performed intwo stages. First, artificial neural networks are trained to recognizethe dynamical signature of an expanding bubble in the velocity spectraof 21 cm data. The second stage consists of subsequent validations basedon the potential bubble's morphology. The technique is tested on 11known bubbles, and 10 of them are successfully detected. Conducting asystematic detection on a 48deg×9deg regionin the Perseus arm, we obtain 7100 detections with spatial distributionfollowing the stellar distribution of the Galactic disk. The estimatedradius and expansion velocity distributions for objects with R<=10 pcagree with the distributions predicted by models of adiabaticallyexpanding bubble populations. The fraction of the Perseus arm volumeoccupied by the detected objects, which can be interpreted as the smallbubbles' contribution to the Galactic porosity Q, is calculated toQR<40pc=0.007+0.025-0.003. Four newbubble cases and eight serious candidates, related to known progenitors,are proposed.
| 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.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| 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.
| 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.
| Ionized gas and stars in the central kiloparsec of the type 2 Seyfert galaxy NGC 2110 - I. The data
In this paper, we present new results from an extensive set of HubbleSpace Telescope (HST) and ground-based observations of the Seyfertgalaxy NGC 2110. The HST data sets include Wide Field Planetary Camera 2(WFPC2) observations as well as the first high-spatial resolutionspectroscopy ([OI], [NII], Hα and [SII] lines) of this galaxyobtained using the Space Telescope Imaging Spectrograph (STIS). Theground-based data are three-dimensional (x, y, λ) spectrographicobservations obtained using the integral field spectrograph OASIS on theCanada-France-Hawaii Telescope, complemented by near-infrared long-slitspectroscopy obtained using NIRSPEC on the Keck-II telescope. The OASISobservations cover regions containing both stellar absorption lines andmajor optical emission lines. The NIRSPEC observations cover the H and Kbands.Combined with archival HST observations, the WFPC2 data provide us witha high-spatial resolution extinction map. The OASIS data allowedbidimensional mapping of the stellar and gaseous kinematics, as well asof the spectral properties of the ionized gas. These results arecompared to those obtained in the near-infrared with NIRSPEC/Keck. Last,we used the STIS data to probe the ionized gas kinematics and propertiesin the inner 4arcsec along PA = 156° at unprecedented spatialresolution.Our two-dimensional (2D) map of the stellar velocity field and thenear-infrared stellar velocity profile are symmetric about the nucleus,confirming the results of previous long-slit observations. The asymmetryof the velocity field of the ionized gas is present at the same levelfor visible and near-infrared lines, indicating this is not a reddeningeffect. Multiple Gaussian fitting of the emission-line profile allowedthe contributions of the broad and a narrow components to bedisentangled. The intensity peak of the [OIII] narrow component islocated north of the nucleus, indicating that the bulk of the narrow[OIII] emission comes from the jet-like structure (Mulchaey et al.) andnot from the nucleus itself. We suggest that the northern arm is theanomalous one, contrary to what has been claimed earlier. Last, we alsoshow that the elongated region of high gas velocity dispersion locatedclose to the nucleus and discovered by Gonzàlez Delgado et al. isintrinsic to the narrow component.
| The central star of the planetary nebula N 66 in the Large Magellanic Cloud: A detailed analysis of its dramatic evolution 1983-2000
The central star of the planetary nebula N 66 (alias WS 35, SMP 83 andHV 5967) in the Large Magellanic Cloud enhanced its brightnessdramatically in 1993 and 1994. Within the subsequent four years itreturned to the previous level. Its spectrum resembles that of aWolf-Rayet star of the nitrogen sequence (WN4.5). We monitored theobject intensively from ground and with the Hubble Space Telescope. Nowwe present the complete set of spectroscopic observations from thedifferent epochs before, during and after the brightness outburst of N66. The stellar spectra from the different epochs are analyzed in detailby means of most advanced non-LTE models for expanding stellaratmospheres. The main results are: the luminosity, log L/Lsun= 4.6, before and after the outburst is exceptionally high for a centralstar of a planetary nebula. During the outburst in 1994, it even climbedup to log L/L_sun = 5.4 for about one year. The effective temperatureof about 112 kK remained roughly constant, i.e. the luminosity mainlyincreased because of a larger effective stellar radius. The mass lossrate increased from 10-5.7 Msun yr-1 inthe quiet state to 10-5.0 Msun yr-1during the outburst. The chemical composition of the stellar atmosphereis that of incompletely CNO-processed matter: it is dominated by heliumwith a rest of hydrogen, nitrogen being slightly enhanced and carbonstrongly depleted. We extensively discuss possible scenarios for thenature and evolutionary origin of N 66, which should explain theexceptional stellar parameters, the atmospheric composition, theoutburst mechanism, and the existence of the bipolar nebula which wasejected only a few thousand years ago and contains about 0.6 solarmasses of hydrogen-rich matter. If being a single star, N 66 might be(i) a low-mass star after the Asymptotic Giant Branch, as usuallyadopted for central stars of planetary nebulae, (ii) a massive, i.e.non-degenerate star, or (iii) a merger produced from two white dwarfs.Although there are no direct indications for binarity, we alternativelydiscuss whether N 66 might be (iv) a massive star which lost itshydrogen envelope in a recent common-envelope phase with a less massivecompanion, or (v) a white dwarf accreting mass from a companion with ahigh rate. None of the scenarios is free of any contradiction to atleast one of the observational facts. However, the binary scenarios poseless severe problems. If N 66 is a white dwarf accreting matter in aclose-binary system, its present accretion rate would bring it to theChandrasekhar limit within a few hundred thousand years. Thus N 66 mightbe a candidate for a future type Ia supernova explosion in our cosmicneighborhood.Partly based on observations with the NASA/ESA Hubble Space Telescopeperformed at the STScI which is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contract NAS5-26555.Partly based on observations with the International Ultraviolet Explorer(IUE).
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Radio observations of interstellar bubbles surrounding massive stars}
We show radio continuum observations of the WR ring nebulae around WR101 and WR 113 obtained using the VLA and HI 21 cm line data of theinterstellar bubble around the O type stars BD +24 deg 3866 and BD+25deg 3952 obtained with the DRAO Synthesis Telescope. We review previousradio continuum and HI line results toward WR and O-type stars.
| 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 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.
| Exospheric models for the X-ray emission from single Wolf-Rayet stars
We review existing ROSAT detections of single Galactic Wolf-Rayet (WR)stars and develop wind models to interpret the X-ray emission. The ROSATdata, consisting of bandpass detections from the ROSAT All-Sky Survey(RASS) and some pointed observations, exhibit no correlations of the WRX-ray luminosity (LX) with any star or wind parameters ofinterest (e.g. bolometric luminosity, mass-loss rate or wind kineticenergy), although the dispersion in the measurements is quite large. Thelack of correlation between X-ray luminosity and wind parameters amongthe WR stars is unlike that of their progenitors, the O stars, whichshow trends with such parameters. In this paper we seek to (i) test byhow much the X-ray properties of the WR stars differ from the O starsand (ii) place limits on the temperature TX and fillingfactor fX of the X-ray-emitting gas in the WR winds. Adoptingempirically derived relationships for TX and fXfrom O-star winds, the predicted X-ray emission from WR stars is muchsmaller than observed with ROSAT. Abandoning the TX relationfrom O stars, we maximize the cooling from a single-temperature hot gasto derive lower limits for the filling factors in WR winds. Althoughthese filling factors are consistently found to be an order of magnitudegreater than those for O stars, we find that the data are consistent(albeit the data are noisy) with a trend of fx ∝(Mν&infy;)-1 in WR stars, as is also the casefor O stars.
| 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.
| The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. I. Results from 19 OB Associations in the Magellanic Clouds
We combine new CCD UBV photometry and spectroscopy with those from theliterature to investigate 19 Magellanic Cloud OB associations thatcontain Wolf-Rayet (W-R) and other types of evolved, massive stars. Ourspectroscopy reveals a wealth of newly identified interesting objects,including early O-type supergiants, a high-mass, double-lined binary inthe SMC, and, in the LMC, a newly confirmed luminous blue variable (LBV;R85), a newly discovered W-R star (Sk -69°194), and a newly foundluminous B[e] star (LH 85-10). We use these data to provide precisereddening determinations and construct physical H-R diagrams for theassociations. We find that about half of the associations may be highlycoeval, with the massive stars having formed over a short period(Δτ<1 Myr). The (initial) masses of the highest massunevolved stars in the coeval clusters may be used to estimate themasses of the progenitors of W-R and other evolved stars found in theseclusters. Similarly, the bolometric luminosities of the highest massunevolved stars can be used to determine the bolometric corrections(BCs) for the evolved stars, providing a valuable observational basisfor evaluating recent models of these complicated atmospheres. What wefind is the following: (1) Although their numbers is small, it appearsthat the W-R stars in the SMC come from only the highest mass (greaterthan 70 Msolar) stars. This is in accord with ourexpectations that at low metallicities only the most massive andluminous stars will have sufficient mass loss to become W-R stars. (2)In the LMC, the early-type WN (WNE) stars occur in clusters whoseturnoff masses range from 30 to 100 Msolar or more. Thissuggests that possibly all stars with mass greater than 30Msolar pass through a WNE stage at LMC metallicities. (3) Theone WC star in the SMC is found in a cluster with a turnoff mass of 70Msolar, the same as that for the SMC WN stars. In the LMC,the WC stars are found in clusters with turnoff masses of 45Msolar or higher, similar to what is found for the LMC WNstars. Thus we conclude that WC stars come from essentially the samemass range as do WN stars and indeed are often found in the sameclusters. This has important implications for interpreting therelationship between metallicity and the WC/WN ratio found in LocalGroup galaxies, which we discuss. (4) The LBVs in our sample come fromvery high mass stars (greater than 85 Msolar), similar towhat is known for the Galactic LBV η Car, suggesting that only themost massive stars go through an LBV phase. Recently, Ofpe/WN9 starshave been implicated as LBVs after one such star underwent an LBV-likeoutburst. However, our study includes two Ofpe/WN9 stars, BE 381 and Br18, which we find in clusters with much lower turnoff masses (25-35Msolar). We suggest that Ofpe/WN9 stars are unrelated to``true'' LBVs: not all ``LBV-like outbursts'' may have the same cause.Similarly, the B[e] stars have sometimes been described as LBV-like.Yet, the two stars in our sample appear to come from a large mass range(30-60 Msolar). This is consistent with other studies,suggesting that B[e] stars cover a large range in bolometricluminosities. (5) The bolometric corrections of early WN and WC starsare found to be extreme, with an average BC(WNE) of -6.0 mag and anaverage BC(WC4) of -5.5 mag. These values are considerably more negativethan those of even the hottest O-type stars. However, similar valueshave been found for WNE stars by applying Hillier's ``standard model''for W-R atmospheres. We find more modest BCs for the Ofpe/WN9 stars(BC=-2 to -4 mag), also consistent with recent analysis done with thestandard model. Extension of these studies to the Galactic clusters willprovide insight into how massive stars evolve at differentmetallicities.
| The Interstellar Medium around Galactic WN Stars: WR 2, WR 128, and WR 151
The neutral hydrogen distribution has been studied in the direction ofthree Galactic Wolf-Rayet (W-R) stars, using the 100 m Effelsberg radiotelescope. Neutral hydrogen voids, regions of low H I emissivity, areobserved over a 10-16 km s^-1 velocity range for the stars WR 2 (=HD6327), WR 128 (=HD 187282), and WR 151 (=CX Cep). These minima areinterpreted as the observable 21 cm counterpart of the interstellarbubble created by the powerful winds of W-R stars and their progenitors.The H I cavities are elongated structures having an axial ratio rangingfrom 1.3 (WR 151) to 2.0 (WR 128). As pointed out in previous studies,the W-R stars are always off-center with respect to either the geometriccenter of the H I void or the absolute minimum inside the cavity. Thisoffset ranges from 40% to 70% of the H I hole's minor axis. The majoraxes of these cavities are ~30 pc for both WR 2 and WR 128, and ~50 pcfor WR 151. The missing H I mass amounts to 96 M_solar (WR 128), 200M_solar (WR 2), and 480 M_solar (WR 151). Some of the physicalparameters of the ring nebula associated with WR 128, such as itsexpansion velocity, 80+/-22 km s^-1, radius, 6.3+/-0.2 pc, andcharacteristic age, (7.4+/-2.4)x10^4 yr, are derived from a simplegeometric model that accounts for the discrepancy between the radialvelocity of the H I hole and the ring nebula related to WR 128.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| The relationship between the WR classification and stellar models. II. The WN stars without hydrogen
We consider the relationships between the classification parameters ofWN stars in the new 3-dimensional classification of Smith et al. (1996)and the corresponding and related parameters that define stellaratmosphere models. Specifically, we consider: FWHM of HeII 4686 vs.v_infty ; hydrogen content by direct inspection vs. hydrogen content bymodelling and vs. colour (b-v)_0; ionisation subclass and M_v; vs.effective temperature. From these data we argue that the WN b and onlythe WN b stars (i.e. stars with EW 5411 > 40 Angstroms \ or FWHM 4686> 30 Angstroms) are entirely free of hydrogen. For the WN b stars, weconsider the relationships of EW 5411 and FWHM 4686 to the derivedtemperature T_*; the mass loss rate; and the surface mass flux. Itappears that, to first approximation, the stars are a one-parameterfamily and the spectral classification criteria are sufficient to givean indication of the intrinsic colour, absolute magnitude (not veryaccurately), effective temperature T_* and terminal velocity.Theoretical models suggest that the critical parameter defining most ofthe properties of a WN b star is its present mass. However, thebehaviour of FWHM 4686 suggests the presence of a second parameter thataffects the mass loss rate and terminal velocity of the wind. We suggestthat the second parameter may be either (or a combination of) theinternal mean molecular weight or the rotation rate of the star. Wefurther compare the relationships predicted by evolutionary models withthose found for observed stars (using atmosphere models), highlightingthe present difficulties in these comparisons.
| The nitrogen spectra of Wolf-Rayet stars. A grid of models and its application to the Galactic WN sample
Adopting the ``standard model'' for Wolf-Rayet atmospheres, non-LTEradiation transfer calculations are performed which account for heliumand nitrogen. Grids of theoretical models are presented for the wholerelevant parameter range. The WN classification criteria are employed inorder to identify the subtype domains, and inconsistencies arediscussed. The (almost complete) sample of known Galactic WN stars isanalyzed by comparing the observed spectra with the synthetic spectra ofthe grid models. This is the first time that nitrogen line analyses areperformed for the whole WN sample, while previous comprehensive studieswere restricted to helium models. The obtained parameters roughlyconfirm the results from the previous helium analyses, as far as latesubtypes (WNL) and early subtypes with strong lines (WNE-s) areconcerned. For early subtypes with weak lines (WNE-w), however, theparameters are substantially revised. The hottest WN star, with astellar (effective) temperature of 140kK, is WR2, which could not beanalyzed previously from its helium lines due to the lack of He I. Theother members of the WNE-w subgroup have stellar temperatures between 40and 90kK, thus populating the same temperature range as the strong-linedWNE-w, but with less dense winds. The luminosities are revised accordingto the new parameters. Moreover, reddening corrections are newlydetermined form comparing IUE data with the UV model fluxes. The averageluminosity is now log L/Lsun = 5.5 for WNE stars (both,strong and weak lined), and log L/Lsun = 5.9 for WNL (notsignificantly revised). The empirical minimum WN luminosity is10(5.0}L_{sun) , reducing former incompatibilities with predictions fromevolutionary calculations. The ratio between mechanical and radiativemomentum flow is slightly affected by the revisions, but remains muchhigher than unity: 9, 9 and 29 for the WNL, WNE-w and WNE-s subclass,respectively. Partly based on observations collected at the EuropeanSouthern Observatory (ESO), La Silla, Chile, and on archival data fromthe International Ultraviolet Explorer (IUE)
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. II. Photometry
Abundant {HIPPARCOS photometry over 3 years of 141 O and Wolf-Rayetstars, including 8 massive X-ray binaries, provides a magnificentvariety of light curves at the sigma ~ 1-5% level. Among the mostinteresting results, we mention: optical outbursts in HD 102567 (MXRB),coinciding with periastron passages; drastic changes in the light curveshape of HD 153919 (MXRB); previously unknown long-term variability ofHD 39680 (O6V:[n]pe var) and WR 46 (WN3p); unusual flaring of HDE 308399(O9V); ellipsoidal variations of HD 64315, HD 115071 and HD 160641;rotationally modulated variations in HD 66811=zeta Pup (O4Inf) and HD210839=lambda Cep (O6I(n)fp); dust formation episode in WR 121 (WC9). Ina statistical sense, the incidence of variability is slightly higheramong the WR stars, which might be explained by the higher percentage ofknown binary systems. Among the presumably single WR stars, thecandidate runaways appear to be more variable then the rest. Based ondata from the ESA Hipparcos astrometry satellite
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cassiopea |
| Ascensione retta: | 01h05m23.02s |
| Declinazione: | +60°25'18.9" |
| Magnitudine apparente: | 11.332 |
| Moto proprio RA: | -3.8 |
| Moto proprio Dec: | -3.3 |
| B-T magnitude: | 11.53 |
| V-T magnitude: | 11.349 |
Cataloghi e designazioni:
|
INHABITED SKY
