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Radio emission from the massive stars in the galactic super star cluster Westerlund 1
Aims: Current mass-loss rate estimates imply that main sequenceline-driven winds are not sufficient to strip away the H-rich envelopeto yield Wolf-Rayet (WR) stars. The rich transitional population of theyoung massive cluster Westerlund 1 (Wd 1) provides an ideal laboratoryto observe and constrain mass-loss processes throughout the transitionalphase of stellar evolution. Methods: We present an analysis ofdeep radio continuum observations of Wd 1 obtained with the AustraliaTelescope Compact Array at four frequency bands that permitinvestigation of the intrinsic characteristics of the radio emission. Results: We detect 18 cluster members, a sample dominated by thecool hypergiants, with additional detections amongst the hotter OBsupergiants and WR stars. The radio properties of the sample arediverse, with thermal, non-thermal and composite thermal/non-thermalsources present. Mass-loss rates determined for stars with partiallyoptically thick stellar winds are ~10-5 M?yr-1 across all spectral types, insufficient to enable theformation of WRs during a massive star lifetime, and the stars mustundergo a period of greatly enhanced mass loss. The sgB[e] star W9, thebrightest radio source in Wd 1, may provide an example, with a currentmass-loss rate an order of magnitude higher than the other clustermembers, and an extended nebula interpreted as a wind from an earlierepoch with a density ~3× the current wind. Such an envelopestructure in W9 is reminiscent of luminous blue variables, and one thatshows evidence of two eras of high, possibly eruptive mass loss.Surprisingly, three of the OB supergiants are detected, implyingunusually dense winds, though they are embedded in more extendedemission regions that may influence the derived parameters. They alsomay have composite spectra, suggesting binarity, which can lead to ahigher flux than expected from a stellar wind. Spatially resolvednebulae are associated with three of the four RSGs and three of the sixYHGs in the cluster, which are due to quiescent mass loss rather thanoutbursts. The extended nebulae of W20 and W26 have a cometarymorphology, implying significant interaction with either theintracluster medium or cluster wind. For some of the cool star winds,the ionizing source may be a companion star though the cluster radiationdensity is sufficiently high to provide the necessary ionizingradiation. Five WR stars are detected with composite spectra,interpreted as arising in colliding-wind binaries.
| On the evolution and fate of super-massive stars
Context: We study the evolution and fate of solar compositionsuper-massive stars in the mass range 60-1000 Mȯ. Ourstudy is relevant for very massive objects observed in young stellarcomplexes as well as for super-massive stars that could potentially formthrough runaway stellar collisions. Aims: We predict the outcomes ofstellar evolution by employing a mass-loss prescription that isconsistent with the observed Hertzsprung-Russell Diagram location of themost massive stars. Methods: We compute a series of stellar models withan appropriately modified version of the Eggleton evolutionary code. Results: We find that super-massive stars with initial masses up to 1000Mȯ end their lives as objects less massive than≃150 Mȯ. These objects are expected to collapseinto black holes (with M ⪉ 70 ~Mȯ) or explode aspair-instability supernovae. Conclusions: We argue that ifultraluminous X-ray sources (ULXs) contain intermediate-mass blackholes, these are unlikely to be the result of runaway stellar collisionsin the cores of young clusters.
| Galactic distributions and statistics of the HD stars in the michigan spectral catalogue.
Not Available
| Detection of the radial velocity curve of the B5-A0 supergiant companion star of Cir X-1?
In this paper, we report on phase-resolved I-band optical spectroscopicand photometric observations of Cir X-1 obtained with the Very LargeTelescope. The spectra are dominated by Paschen absorption lines atnearly all orbital phases except near phase zero (coinciding with theX-ray dip) when the absorption lines are filled in by broad Paschenemission lines. The radial velocity curve of the absorption linescorresponds to an eccentric orbit (e = 0.45) whose period and time ofperiastron passage are consistent with the period and phase predicted bythe most recent X-ray dip ephemeris. We found that the I-band magnitudedecreases from 17.6 to ~16.8 near phase 0.9-1.0 this brighteningcoincides in phase with the X-ray dip. Even though it is likely that theabsorption-line spectrum is associated with the companion star of CirX-1, we cannot exclude the possibility that the spectrum originates inthe accretion disc. However, if the spectrum belongs to the companionstar, it must be a supergiant of spectral type B5-A0. If we assume thatthe compact object does not move through the companion star atperiastron, the companion star mass is constrained to <~10Msolar for a 1.4-Msolar neutron star, whereas theinclination has to be . Alternatively, the measured absorption lines andtheir radial velocity curve can be associated with the accretion discsurrounding a 1.4-Msolar neutron star and its motion aroundthe centre of mass. An absorption-line spectrum from an accretion discis typically found when our line of sight passes through the accretiondisc rim implying a high inclination. In this scenario, the companionstar mass is found to be ~0.4 Msolar. However, from radioobservations it was found that the angle between the line of sight andthe jet axis is smaller than 5°. This would mean that the jetploughs through the accretion disc in this scenario, making thissolution less probable.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| HD 331319: A Post-AGB F Supergiant with He I Lines
Not Available
| Instability regions in the upper HR diagram
The following instability regions for blueward evolving-supergiants areoutlined and compared. (1) Areas in the Hertzsprung-Russell (HR) diagramwhere stars are dynamically unstable. (2) Areas where the effectiveacceleration in the upper part of the photospheres is negative, hencedirected outward. (3) Areas where the sonic points of the stellar winds(where vwind=vsound) are situated inside thephotospheres, at a level deeper than τRoss=0.01. Wecompare the results with the positions of actual stars in the HR diagramand we find evidence that the recent strong contraction of the yellowhypergiant HR 8752 was initiated in a period during which<0, whereupon the star became dynamicallyunstable. The instability and extreme shells around IRC+10420 aresuggested to be related to three factors: <0the sonic point is situated inside the photosphere; and the star isdynamically unstable.
| Yellow Hypergiants as Dynamically Unstable Post-Red Supergiant Stars
According to recent theoretical studies, the majority of single starsmore massive than 30 Msolar successfully evolve into redsupergiants but then lose most of their hydrogen envelopes andmetamorphose into hot blue remnants. While they are cool, they becomedynamically unstable as a result of high radiation pressure and partialionization of the gases in their outer layers. It is shown here thatthese unstable red supergiant models repeatedly shrink and reexpand on athermal timescale when perturbed by heavy bursts of mass loss.Consequently, they fill up the domain of yellow hypergiants on theHertzsprung-Russell diagram and display very fast rates of evolutionthere, as observed.
| Checking the yellow evolutionary void. Three evolutionary critical Hypergiants: HD 33579, HR 8752 & IRC +10420
We have checked the reality of the yellow evolutionary void (which is anarea in the Hertzsprung-Russell diagram where atmospheres of bluewardevolving super- and hypergiants are moderately unstable), by comparingone star inside the void: HD 33579 (= R76), and two at thelow-temperature border of it: HR 8752 (= HD 217476, V509 Cas) andIRC+10420. We found that the first star has a large mass and a fairlystable behaviour over time. These aspects suggest, together withabundance determinations by others, that it is a fairly young, stillredward-evolving supergiant. For such a star the void is not forbidden.The two other stars, HR 8752 resp. IRC+10420, have low masses whichplaces them in the post-red blueward loop. They show indications of theexpected bouncing effect for blueward returning red supergiants: whenapproaching the void they eject mass, resulting in a sudden reduction ofT_eff and a decrease of the atmospheric acceleration to g_eff =~ zero.Thereafter T_eff and g_eff increase again. For HR 8752 two such recent`bounces' have been identified. The photometric variations of HR 8752and of HD 33579 are due to high-l gravity-wave pulsations. Based onobservations at the La Palma Observatory and the ESO Observatory inChili.
| The Yellow Hypergiants HR 8752 and rho Cassiopeiae near the Evolutionary Border of Instability
High-resolution near-ultraviolet spectra of the yellow hypergiants HR8752 and rho Cassiopeiae indicate high effective temperatures, placingboth stars near the low-T_eff border of the ``yellow evolutionaryvoid.'' At present, the temperature of HR 8752 is higher than ever. Forthis star we found T_eff=7900+/-200 K, whereas rho Cassiopeiae hasT_eff=7300+/-200 K. Both HR 8752 and rho Cassiopeiae have developedstrong stellar winds with V_&infy;~=120 and V_&infy;~=100 km s^-1,respectively. For HR 8752 we estimate an upper limit for the sphericallysymmetric mass loss of 6.7x10^-6 M_solar yr^-1. Over the past decadestwo yellow hypergiants appear to have approached an evolutionary phase,which has never been observed before. We present the first spectroscopicevidence of the blueward motion of a cool super/hypergiant on the H-Rdiagram.
| Light variations of massive stars (alpha Cyg variables). XVI. The LMC supergiants R 85 (LBV) and R 110 (LBV) and the SMC supergiants R 42 and R 45
Multi-colour photometry of four variable supergiants in the LMC and SMC,viz. R 85, R 110, R 42 and R 45, is searched for periods, studied anddiscussed. The suspected LBV R 85 is undoubtedly an active LBV, thoughnot as spectacular as R 110. Their microvariations superimposed on the SDor-activity are analyzed as well as those exhibited by R 42 and R 45.Often, a period search is difficult because of the very complicatedmicro-variability. We suggest that this is caused by an intricatemulti-cyclic behaviour combined with stochastic processes. The length ofthe strongest cyclicity in the power spectrum of R 42 (128 d) is of theorder of the rotation periods of BA-type supergiants. In connection withour findings described in the present paper and the previous ones, wediscuss various competing theoretical models on the instability of alphaCyg variables, including the LBVs. Based on observations obtained at theEuropean Southern Observatory at La Silla, Chile (observing proposals55D-0317, 56D-0249, 57D-0133 and 58D-0118)
| HIPPARCOS photometry of 24 variable massive stars (alpha Cygni variables)
The photometric variability of 24 alpha Cyg variables, i.e. variablesuper-and hypergiants, observed by the Hipparcos satellite is described.Three of the selected stars are situated in the SMC, 12 in the LMC andthe remaining 9 in the Galactic plane. Four of them are hot S Dor-typevariables, or LBVs (luminous blue variables) and two are possiblemembers of this class. Light curves are presented for all stars. Forfive stars, among which one LBV, the variability was discovered from theHipparcos photometric data. Results of period searches are presentedand, when relevant, folded light curves are shown. The linear ephemerisof two variables was revised. For most of the program stars theHipparcos magnitude scale (Hp) differs from the V of the UBV system byla 0fm1 . For all variables temperatures and M_bol are given. Galacticforeground reddening for the objects in the Magellanic Clouds are givenbased on IRAS maps.
| The yellow hypergiants
We list the main stellar data of known hypergiants and similarlyluminous stars, and then concentrate on a review of the yellowhypergiants. These stars are post-red supergiants evolving alongblueward loops in the Hertzsprung-Russell diagram. Their properties,their location in the Hertzsprung-Russell diagram and their occasionalmass ejections are related to a region of atmospheric instability in theH-R diagram, the Yellow Void. The `bouncing against the border of theVoid' of three objects: ρ Cas, HR8752 and IRC+10420, is described.The apparent atmospheric instability of yellow hypergiants is related tothe atmospheric pulsations. There are indications that the approach tothe Void is associated with an increased amplitude of the pulsation andwith enhanced mass loss. The observed small-scale motion field is onlyapparently strongly supersonic; the observed large stochastic velocitiesare the quasi-stochastically varying thermal motions in the many hotsheets that occur in the wakes of many small shocks, while the realhydrodynamic velocity component is small and subsonic. This shock-wavefield is also responsible for the observed rate of mass loss and foremission in the wings of Hα . Most yellow hypergiants haveenvelopes containing gas and dust, but a thick extended envelope,presumably dissipating and showing bipolar outflow, is only known aroundIRC+10420. At the interface of the bipolar wind and the interstellarmedium one or more stationary shocks may develop as is observed in thecase of IRC+10420 and suspected with ρ Cas.
| An obstacle to the late evolution of massive stars
During their evolution, massive stars first swell and thus become redsupergiants, after which they shrink again to evolve to the bluesupergiant phase. When, during this blueward loop, the effectivephotospheric temperature T_eff has increased to values of about 7000 K,the main part of the photosphere becomes moderately unstable.Observations of the yellow hypergiant star HD 217476=HR 8752 show thatthe approach to this instability region may lead to phases of enhancedmass loss and consequent reduced surface temperature, after which T_effslowly increases until it again `bounces' against the atmosphericinstability region. Two such bounces have been observed in the last 30yr and a third seems imminent; frequent observations of this star aretherefore useful. Other `yellow hypergiants', like rhoCas and IRC+10420,seem to show the same behaviour. Repeated observations of these starsmay help us to understand how massive stars pass the instability regionduring their evolution towards the blue supergiant phase.
| New perceptions on the S Doradus phenomenon and the micro variations of five Luminous Blue Variables (LBVs)
We investigated the photometric histories of the LBVs HR Car in theGalaxy, R127, R110 and R71 in the LMC, and R40 in the SMC by collectingall available photometry. All these objects have been reasonably wellobserved over the last few decades and a number of "S Dor (SD) phases"(episodes of enhanced light) is well documented. Time-scales lie between1.4 and 25y. During the SD cycles two different types ofmicro-variations (amplitudes ~0.2') are present: one near the minima(time-scale from 2 to 6 weeks; colours generally blue in the maxima andred in the minima) and the other one near the maxima (time-scale ~100d;colours generally red in the maxima and blue in the minima). Halfway theascending and descending branches of the SD cycles the stars switchrather abruptly from one type to the other. We argue that the two typesof SD phases are probably caused by different instability mechanisms.Based, in part, on observations obtained at the European SouthernObservatory at La Silla, Chile.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The HIPPARCOS proper motion of the Magellanic Clouds
The proper motion of the Large (LMC) and Small (SMC) Magellanic Cloudusing data acquired with the Hipparcos satellite is presented. Hipparcosmeasured 36 stars in the LMC and 11 stars in the SMC. A correctlyweighted mean of the data yields the presently available most accuratevalues, mu_alpha cos(delta) = 1.94 +/- 0.29 mas/yr, mu_delta = - 0.14+/- 0.36 mas/yr for the LMC. For the SMC, mu_alpha cos(delta) = 1.23 +/-0.84 mas/yr, mu_delta = - 1.21 +/- 0.75 mas/yr is obtained, whereby careis taken to exclude likely tidal motions induced by the LMC. Bothgalaxies are moving approximately parallel to each other on the sky,with the Magellanic Stream trailing behind. The Hipparcos proper motionsare in agreement with previous measurements using PPM catalogue data byKroupa et al. (1994), and by Jones et al. (1994) using backgroundgalaxies in a far-outlying field of the LMC. For the LMC the Hipparcosdata suggest a weak rotation signal in a clockwise direction on the sky.Comparison of the Hipparcos proper motion with the proper motion of thefield used by Jones et al. (1994), which is about 7.3 kpc distant fromthe center of the LMC, also suggests clockwise rotation. Combining thethree independent measurements of the proper motion of the LMC and thetwo independent measurements of the proper motion of the SMC improvesthe estimate of the proper motion of the LMC and SMC. The correspondinggalactocentric space motion vectors are computed. Within theuncertainties, the LMC and SMC are found to be on parallel trajectories.Recent theoretical work concerning the origin of the Magellanic Systemis briefly reviewed, but a unique model of the Magellanic Stream, forthe origin of the Magellanic Clouds, and for the mass distribution inthe Galaxy cannot yet be decided upon. Future astrometric space missionsare necessary to significantly improve our present knowledge of thespace motion of the two most conspicuous galactic neighbours of theMilky Way.
| New discoveries on the S DOR phenomenon based on an investigation of the photometric history of the variables AG Car, S DOR and Eta Car.
A century of photometric observations of AG Car and S Dor isinvestigated for general characteristics and possible periodicities inthe occurrence of the S Dor (SD) phases, defined as episodes of a radiusand an apparent brightness variation at a more or less constantluminosity. We identified two types of such phases: the``very-long-term" (VLT) and the ``normal" SD phases. The latter aresuperimposed on the first. The time scales for the VLT-SD phases are inthe order of decades for both variables. The normal SD phases of AG Carand presumably also of S Dor obey stable periods of 371.4d +/-0.6d and6.8+/-0.1yr, respectively. We suspect that the SD-, or LBV phenomenon isprovoked by two types of pulsational modes. The oscillating O-C valuesfor both variables indicate the possible presence of beat cycles. Theirtime scales are of the order of years to decades. We found, at least forthe SD activity of AG Car, further support for a radius change of thestar and a more or less horizontal displacement in the theoreticalHR-diagram. However, S Dor has a higher luminosity in minimum than inmaximum, amounting to 0.5-1mag depending on the range in the visualmagnitude (1-2.4mag). Inconsistencies between existing temperaturescales during the light variations of AG Car were noticed. We found nocyclic pattern whatsoever in the SD phases of Eta Car. The time scalefor such events within the last 20yr lies between 1 and 3yr. There isevidence that over the last 20 yrs the central LBV in the Eta Car systemexperienced a rising branch of a VLT-SD phase, which appears to furnish,according to the secular colour change, indirect support for an ongoingdecrease of the circumstellar dust density.
| On the spectrum of Theta Tucanae
We present 9700 visual-magnitude estimates of key objects among southernmassive stars, viz. the LBVs/hypergiants HD6884 (R40), eta Car, AG Car,HDE326823, HDE269006 (R71), HD33579, HDE269128 and WR stars HD5980 andWR40. The visual estimates are complemented with photoelectric lightcurves obtained over the last two decades in the framework of theLong-Term Photometry of Variables project. Several of our data sets havebeen searched for periodicities, and this aspect of the data iscommented in graphical form (by means of amplitude spectra) and, wherenecessary, with additional notes. The visual and photoelectric datademonstrate that the sytematic monitoring of luminous massive stars isscientifically very rewarding.
| Long-term visual monitoring of southern luminous variables
On the basis of high-resolution spectra and Stroemgren y photometry, wediscuss the nature of theta Tucanae as a (binary) delta Scuti star, andpresent a set of high-resolution digital spectra of theta Tuc togetherwith comparison spectra of the delta Scuti star UV Arietis. The spectrumof theta Tuc shows an amazing similarity to the spectrum of Procyon. Thepulsational radial velocity amplitude of theta Tuc (for the principaloscillation frequency) is of the order of 6-8 kms. The photometricindices point to Teff=7575 +/- 130 K, [Fe/H] = 0.02, L/Ldot =1.4 +/- 0.1and log g ~ 3.8. The estimated mass is not higher than 2.0 (+/- 0.1)Mdot, and the pulsation constant Q is about 30% smaller as waspreviously thought.
| Spectroscopic study of the outflowing disk winds of B[e] supergiants in the Magellanic Clouds.
We report on UV high resolution spectroscopic observations of R50 in theSmall Magellanic Cloud, and R82 and HenS22 in the Large Magellanic Cloudobtained with the International Ultraviolet Explorer. The observed starsare supposed to represent edge-on cases of B[e] supergiants for which atwo-component stellar wind model has previously been suggested. Thespectra are characterized by P Cygni-type lines of FeII. Theobservations show that the three stars have very slowly expanding windswith terminal velocities derived from the blue absorption edges of 75,100, and 120km/s, respectively. Fits of the FeII lines of Hen S22 andR82 using the SEI method lead to even slower velocities of about 60 to80km/s, respectively. This is about a factor of ten slower than theterminal velocity of normal B-type supergiants. The results areconsistent with the assumption that the observed stars are viewededge-on. We derived optical depths of the absorption components of theFeII resonance lines of Hen S22 and R82 of larger than about 5, yieldinglower limits for the disk mass-loss rates of the order of 6x10^-7^ and5x10^-7^Msun_/yr, respectively. The very low terminalvelocity of the disk can be explained by the fact that the disks of theB[e] supergiants are on the low-velocity side of the bi-stability jumpof radiation driven winds (which reduces vinfinity_/v_esc_)and a rotational velocity of about 0.75 of the critical rotationvelocity (which reduces the effective v_esc_). The effective gravityderived from vinfinity_ and vinfinity_/v_esc_=1.3is very low. It is on the order of logg_eff_=0.2 to 0.7.
| A pulsating star inside η Carinae. I. Light variations, 1992-1994.
We present and analyze two seasons of intense photometric monitoring inthe Stroemgren uvbyHβ system of η Carinae (October 1992-August1994). The luminous blue variable (LBV) in the core did not show much SDor activity, i.e. it was in a relatively quiescent stage. Thissituation was very favourable for studying its optical micro variations.It appears that the central LBV pulsates (presumably in a non-radialmode) like other massive evolved stars, the α Cyg variables. Thequasi-period is 58.56d. The linear ephemeris is: JD_max_ = 2448875.0+58.56 E. Support was found for the existence of the presumedperiodicity of 52.4d of the so-called "dimples", shallow dips in thelight curve which only last for a few days. The cause may be the eclipseof a small companion or of a hot spot in an accretion disk. The Hβindex became bluer by ~0.07mag during the last 11/2 years and shows anoscillation in anti-phase with the 58.56d pulsation, suggesting that theHII region(s) responsible for the hydrogen line emission has arelatively high luminosity. This is another reason to suppose that asecond luminous source, perhaps a luminous disk (with a hot spot), maybe present in the η Car system.
| On the motion of the Magellanic Clouds
We have measured the proper motion of the Large and Small MagellanicClouds using Magellanic Cloud stars in the PPM Catalogue, and obtainμ = 1.7+/-0.9 mas yr^-1^ for the LMC. Systematic uncertainties arediscussed. Bound and unbound orbits of the Magellanic Clouds around theGalaxy are consistent with our result. The various models of theMagellanic Stream and their predictions for the motion of the MagellanicClouds are discussed. The predictions by several authors for the sametype of model differ by up to 0.3 mas yr^-1^. All models proposed todate that assume the Magellanic Clouds to lead the Magellanic Streampredict a proper motion for the LMC of between 1.5 and 2.0 mas yr^-1^,the smallest value being a prediction for a Galaxy with no halo. Otherindependent measurements of the proper motion of the LMC are discussed.These lie between 1 and 1.5 mas yr^-1^. Future astrometry will have toallow measurement of the proper motion of the LMC with an uncertainty nolarger than one-tenth of a milliarcsecond per year in order to help todistinguish significantly between models of the halo of the Galaxy. Theproper motion of the LMC cannot by itself distinguish between models ofthe Magellanic Stream.
| R 40: The first luminous blue variable in the Small Magellanic Cloud
During the past decade R 40 in the Small Magellanic Cloud (SMC) hasbrightened in the visual range by about 0.5 magnitudes. Correlated withthe brightening the spectral type has changed from B8Ia to A3Ia-O. Thisbehavior shows R 40 to be a luminous blue variable (LBV) the first onediscovered in the SMC. During the past eight years we obtained extensiveStroemgren photometry. R 40 was also observed in the IR range in theJHKL filter system. High resolution spectra were observed in Dec. 1991and Jan. 1993 in the optical range at European Southern Observatory(ESO) with CASPEC at the 3.6 m telescope and with our fiber-linkedechelle spectrograph at the 2.2 m telescope. In addition one highresolution LWP-IUE spectrum and one SWP and LWP low resolution spectrumwere secured during the present outburst phase of R 40. For comparisonIUE archive data were consulted; R 40 had been exposed in the lowresolution SWP and LWP mode in 1987. Apart from the secular brighteningat a timescale of years R 40 exhibits photometric pulsation-likemicrovariations with a quasi-period of 120 days which is also typicalfor LBVs. From the continuum energy distribution (from the satellite UVto the IR range) and from the CASPEC spectrum we derived stellarparameters of R 40 during the outburst phase in 1991 by making use ofKurucz model atmospheres: Teff = 8700 K, log g = 0.75 andwith a distance module of 18.85 for the SMC Mbol = -9.4 and R= 280 solar radii and M = 16 solar mass. The mass-loss rate was derivedfrom the P Cygni type H-alpha profile to 8 x 10-6 solarmass/yr which is small but not exceptional for an LBV (cf. e.g. R110 ofthe Large Magellanic Cloud (LMC)). Comparison of the present spectralenergy distribution with the one of 1987 shows that the brightening inthe visual is accompanied by a corresponding decrease of the satelliteUV continuum; i.e. as expected for LBVs the outburst occurs under thecondition of Mbol = const. Since the metallicity is down by0.6 dex in the SMC, R 40 demonstrates that low metallicity does notpreclude LBV outbursts.
| Luminosities of yellow supergiants from near-infrared spectra - Calibration through Magellanic Cloud stars
The possibility of using medium resolution spectrograms in the nearinfrared region to determine luminosities of A-G supergiants has beenexplored. A sample of 49 of these stars has been observed in the twoMagellanic Clouds, and using the intensities of the O I 7774 triplet andan index (CP), which is a combination of the Ca II triplet and Paschenlines intensities, a preliminary luminosity calibration, based on LMCstars, has been obtained. Such a calibration predicts reliableluminosities for Galactic supergiants, and offers the advantage of beingcompletely reddening independent. The reddening free CP index combinedwith BVRI color indices has also been used to estimate the interstellarreddenings of Magellanic Cloud stars.
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Stellar wind properties of A-type hypergiants
The visually bright objects from the Magellanic Clouds are examined interms of observed properties that indicate properties relevant to thestellar wind. IUE observations are accomplished in the long-wavelengthprimary echelle mode in the optical wavelength near H-alpha of theobjects HD 33579 and HD 7583. The Fe-II lines are found to have widthsthat are dependent upon log gf and the excitation potential; thesedependencies relate to an accelerated wind that is directed outwards.The terminal velocity is found to be approximately equal to 200 km/s forboth stars as evinced by the strong Fe-II lines. A flat velocity law isdetermined and a mass-loss rate of 2 x 10 exp 6 solar mass/yr is givenfor both of the stars. The wind properties of the A-types are concludedto relate to a uniform acceleration mechanism.
| Positional reference stars in the Magellanic Clouds
The equatorial coordinates are determined of 926 stars (mainly ofgalactic origin) in the direction of the Magellanic Clouds at the meanepoch T = 1978.4 with an overall accuracy characterized by the meanvalues of the O-C coordinates, Sa = 0.35 arcsec and Sd = 0.38 arcsec,calculated from the coordinates of the Perth reference stars. Thesevalues are larger than the accuracy expected for primary standard stars.They allow the new positions to be considered as those of reliablesecondary standard stars. The published positions correspond to anunquestionable improvement of the quality of the coordinates provided inthe current catalogs. This study represents an 'astrometric step' in thestarting of a 'Durchmusterung' of the Magellanic Clouds organized by deBoer (1988, 1989).
| The anomalous A-type supergiants in the Magellanic Clouds - Evidence for post-red supergiant evolution
Non-LTE model atmospheres are used to show that the strength of thehydrogen lines and the Balmer jump in a group of A-type supergiants inthe Large and Small Magellanic Clouds can best be explained by enhancedhelium in the atmospheres of these supergiants. On the HR diagram thesestars are found between M(Bol) is approximately equal to -6 to -8 mag,corresponding to an initial mass of 10 to 20 solar masses. Given theirlocation on the HR diagram, their spectral and color anomalies which canbe explained by enhanced helium, and the existence of spectroscopicallynormal A supergiants in the same luminosity range, it is suggested thatthese anomalous A-type supergiants are He-burning postred supergiantsand that they are excellent candidates for the predecessor of the kindof star that became SN 1987a.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | דג זהב |
| התרוממות ימנית: | 05h05m55.51s |
| סירוב: | -67°53'10.9" |
| גודל גלוי: | 9.102 |
| תנועה נכונה: | 2.7 |
| תנועה נכונה: | 1.7 |
| B-T magnitude: | 9.377 |
| V-T magnitude: | 9.125 |
קטלוגים וכינוים:
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INHABITED SKY
