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A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud
We present a study of the magnetic field of the Small Magellanic Cloud(SMC), carried out using radio Faraday rotation and optical starlightpolarization data. Consistent negative rotation measures (RMs) acrossthe SMC indicate that the line-of-sight magnetic field is directeduniformly away from us with a strength 0.19+/-0.06 μG. Applying theChandrasekhar-Fermi method to starlight polarization data yields anordered magnetic field in the plane of the sky of strength 1.6+/-0.4μG oriented at a position angle 4deg+/-12deg,measured counterclockwise from the great circle on the sky joining theSMC to the Large Magellanic Cloud (LMC). We construct athree-dimensional magnetic field model of the SMC, under the assumptionthat the RMs and starlight polarization probe the same underlyinglarge-scale field. The vector defining the overall orientation of theSMC magnetic field shows a potential alignment with the vector joiningthe center of the SMC to the center of the LMC, suggesting thepossibility of a ``pan-Magellanic'' magnetic field. A cosmic-ray-drivendynamo is the most viable explanation of the observed field geometry,but has difficulties accounting for the observed unidirectional fieldlines. A study of Faraday rotation through the Magellanic Bridge isneeded to further test the pan-Magellanic field hypothesis.
| Mass loss and the evolution of massive stars
We present the results of mass-loss predictions for hot stars. Mass lossis an important aspect for the evolution of massive stars, therotational properties of the progenitors of gamma-ray bursts, and isessential in assessing whether the most massive stars explode aspair-instability supernovae, or avoid them due to mass loss. As aresult, the rate of mass loss is critical for our understanding of thechemical enrichment of the interstellar medium. Of topical interest isthe question whether luminous blue variables are the progenitors ofsupernovae. Although there is a growing body of evidence to suggestthis, it remains as yet unexplained by modern stellar evolution models.
| Wra 751, a luminous blue variable developing an S Doradus cycle
Aims. The object Wra 751 is a luminous blue variable that latelyexhibits strong changes in light and color. We summarise the availablephotometry of Wra 751, present new photometric observations, and discussthese data with special attention on the systematic differences betweenthe various data sources. In addition, we establish an empiricalrelationship between b-y and B-V for this class of stars. Methods: Wra751 is a strong-active member of the S Dor class, and it exhibitsvery-long term S Doradus phases with an amplitude of about twomagnitudes in V and a cycle length of several decades. The associatedB-V colour-index amplitude is about 0.4 mag. At this moment this LBV,which is the reddest member of the class, is going through the bright(and red) stage of a long-term S Dor cycle. Results: The S Dorbehaviour of this system shows some resemblence to the temporalcharacteristics of the Galactic LBV AG Car, because the time scales andamplitudes of light and colour variability are very similar.Individual photometric measurements (Tables 2-4) are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/484/463
| Estimating the Structure and Geometry of Winds from Luminous Blue Variables via Fitting the Continuum Energy Distributions
By combining the UV spectra from IUE with photometric data in theoptical band, we present a quantitative study on the continuum energydistributions of LBVs to determine the structure and geometry of LBVwinds. It is shown that the shape of continuum energy distributionsaround the Balmer jump is sensitive to the velocity law of LBV winds. Asimple, spherically symmetric wind model including free-bound andfree-free radiation is constructed to compute the continuum energydistributions of LBVs. By matching theoretical ones to the observedcontinuum energy distributions around the Balmer jump, we have obtainedvalue of the exponent of the velocity law β in both minimum andmaximum state for five LBVs, i.e., AG Car, HR Car, R40, S Dor, and R127.We have found that β is about 0.5-0.7 in the minimum state andlarger than 1.5 in the maximum state. Transitions in the ionizationstates of metals between the minimum and maximum state of LBVs, whichlead to changes in the radiative acceleration due to spectral lines, aremost likely responsible for such effect on the velocity law. We havealso determined the geometry of the wind and found that a sphericallysymmetric wind model can well reproduce the observed continuum energydistributions of the five LBVs. Based on these results we suggest thatthe wind of LBVs be basically quasi-spherical, maybe with some clumpystructure in the spherical wind to produce some observed asphericalfeatures.
| New Insights Into the S Doradus Phenomenon and the Micro-Variations of Eight Luminous Blue Variables
Here we investigate the photometric histories of the luminous bluevariables (LBVs) eta, AG and HR Car within our Galaxy, S Dor, R127,R110, and R71 in the LMC, and R40 in the SMC by collecting all availablephotometry.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of LuminousBlue Variables (LBVs) in the Galaxy and the Magellanic Clouds, in orderto investigate the geometries of their winds. We find that at least halfof our sample show changes in polarization across the strong Hαemission line, indicating that the light from the stars is intrinsicallypolarized and therefore that asphericity already exists at the base ofthe wind. Multi-epoch spectropolarimetry on four targets revealsvariability in their intrinsic polarization. Three of these, AG Car, HRCar and P Cyg, show a position angle (PA) of polarization which appearsrandom with time. Such behaviour can be explained by the presence ofstrong wind-inhomogeneities, or “clumps” within the wind.Only one star, R 127, shows variability at a constant PA, and henceevidence for axi-symmetry as well as clumpiness. However, if viewed atlow inclination, and at limited temporal sampling, such a wind wouldproduce a seemingly random polarization of the type observed in theother three stars. Time-resolved spectropolarimetric monitoring of LBVsis therefore required to determine if LBV winds are axi-symmetric ingeneral. The high fraction of LBVs (>50%) showing intrinsicpolarization is to be compared with the lower ~20-25% for similarstudies of their evolutionary neighbours, O supergiants and Wolf-Rayetstars. We anticipate that this higher incidence is due to the lowereffective gravities of the LBVs, coupled with their variabletemperatures within the bi-stability jump regime. This is alsoconsistent with the higher incidence of wind asphericity that we find inLBVs with strong Hα emission and recent (last ~10 years) strongvariability.
| A Model of the Circumstellar Envelope of Luminous Blue Variables
The continuum energy distributions of the luminous blue variables R127and R110 in the outburst phase are fitted with a circumstellar envelopemodel. Both stars show two peaks in their continuum, one near1250Å and the other in the optical band. We suggest that their UVand optical fluxes may have different origins: the UV flux comes fromthe central star while the optical flux comes from an expandingcircumstellar envelope. We construct a model for LBVs consisting of twoLTE atmosphere models with different temperatures, and find it to be inagreement with the observed spectral energy distributions of R127 andR110. According to our numerical experiments, R127's continuum iscomposed of fluxes from a circumstellar envelope of Teff =8000 K, R = 485Rodot, and log g = 1, and from a central starof Teff = 17 000 K, R = 135Rodot, and log g = 2.5with a permeating factor f = 0.5; while R110's continuum can be fittedby a circumstellar envelope of Teff = 7000 K, R =350Rodot, and log g = 0.5, and a central star ofTeff = 25 000 K, R = 27Rodot, and log g = 3.0 witha permeating factor f = 0.65. Both models show that the non-sphericallysymmetric, optically thick regions are formed surrounding the centralstar in the outburst phase. The light of the central star is shielded bythe circumstellar envelope so that the visual brightness increases withthe decrease/increase of the temperature/radius of the optically thickregions.
| Long-term spectroscopic monitoring of the Luminous Blue Variable HD 160529
We have spectroscopically monitored the galactic Luminous Blue VariableHD 160529 and obtained an extensive high-resolution data set that coversthe years 1991 to 2002. During this period, the star evolved from anextended photometric minimum phase towards a new visual maximum. Inseveral observing seasons, we covered up to four months with almostdaily spectra. Our spectra typically cover most of the visual spectralrange with a high spectral resolution (lambda /Delta lambda ~ 20 000 ormore). This allows us to investigate the variability in many lines andon many time scales from days to years. We find a correlation betweenthe photospheric HeI lines and the brightness of the star, both on atime scale of months and on a time scale of years. The short-termvariations are smaller and do not follow the long-term trend, stronglysuggesting different physical mechanisms. Metal lines also show bothshort-term and long-term variations in strength and also a long-termtrend in radial velocity. Most of the line-profile variations can beattributed to changing strengths of lines. Propagating features in theline profiles are rarely observed. We find that the mass-loss rate of HD160529 is almost independent of temperature, i.e. visual brightness.Based on observations collected at the European Southern Observatory atLa Silla, Chile (proposals 69.D-0378, 269.D-5038).
| Chemical abundances of massive stars in Local Group galaxies
The relative abundances of elements in galaxies can provide valuableinformation on the stellar and chemical evolution of a galaxy. Whilenebulae can provide abundances for a variety of light elements, starsare the only way to directly determine the abundances of iron-group ands-process and r-process elements in a galaxy. The new 8m and 10m classtelescopes and high-efficiency spectrographs now make high-qualityspectral observations of bright supergiants possible in dwarf galaxiesin the Local Group. We have been concentrating on elemental abundancesin the metal-poor dwarf irregular galaxies, NGC 6822, WLM, Sextants A,and GR 8. Comparing abundance ratios to those predicted from their starformation histories, determined from color-magnitude diagrams, andcomparing those ratios between these galaxies can give us new insightsinto the evolution of these dwarf irregular galaxies. Iron-groupabundances also allow us to examine the metallicities of the stars inthese galaxies directly, which affects their inferred mass loss ratesand predicted stellar evolution properties.
| Outflow from and asymmetries in the nebula around the LBV candidate Sk-69o279
We present and discuss new long-slit Echelle spectra of the LMC LBVcandidate Sk-69o279 and put them in context with previousimages and spectra. While at first glance it resembles a simplespherically expanding symmetric shell, we find a considerably morecomplex morphology and kinematics. The spectra indicate thatmorphologically identified deviations from sphericity are outflows offaster material out of the main body of Sk-69o279. Themorphological as well as the kinematic similarity with other LBV nebulaemakes it likely that Sk-69o279 is an LBV candidate and posesthe question: in how far are outflows out of expanding LBV nebulae ageneral property of such nebulae - at least during some phases of theirevolutions.
| Cyclicities in the light variations of S Doradus stars III. P Cygni
On the basis of new photometric observations and archived data publishedsince 1907, we discuss the light variations of P Cygni. We conclude thatthere are alpha Cygni-type microvariations with a stable (pulsation)quasi-period of 17.3 days. There are also longer cycles of variationwith P ~ 100 d, so-called 100 d-type micro-variations, and with P ~1500-1600 d, a short S Dor-type phase.
| S Doradus variables in the Galaxy and the Magellanic Clouds
The goal in writing this paper is five fold: (1) to summarize thescientific achievements in the 20th century on S Dor variables (orLBVs); (2) to present an inventory of these variables in the Galaxy andthe Magellanic Clouds with a description of their physical state andinstability properties; (3) to emphasize the photometric achievements ofthe various types of instabilities. Generally this seems to be aneglected item resulting in a number of misunderstandings continuouslywandering through literature; (4) to investigate the structure of the SDor-area on the HR-diagram; (5) to estimate the total numbers of S Dorvariables in the three stellar systems. The position of the strongactive S Dor variables in minimum brightness obey the following linearrelation on the HR-diagram:log L/Lsun = 1.37 log T_eff -0.03. The relatively small dispersion of less active and supposed ex-and dormant S Dor variables with respect to this relation is twice aslarge at the blue side than at the red side. This might be caused byevolution to the WR stage and/or to high rotation. S Dor variables canbe subject to five types of instabilities: the very rare genuineeruptive episodes (the ``SD-eruptions''), two different brighteningphases caused by slow pulsations (the ``SD-phases''): one on a timescale of years, the other on a time scale of decades at a more or lessconstant luminosity and two types of microvariations: one on a timescale of weeks, the other on a time scale of about 100 d. So far, noperiodicities of light curve characteristics of any of theseinstabilities have ever been found. The durations of active andnon-active stages are estimated for about half of the sample based onscattered magnitude estimations such as from historical records, and onmodern monitoring campaigns. It would be a misunderstanding to believethat all S Dor variables should be always spectacular. It is estimatedthat most of them will not be spectacular at all for at least 70% oftheir lifetime as an S Dor variable. Tables 1 to 6 and 8 to 17 are onlyavailable in electronic form at http://www.edpsciences.org, Table 7 isonly available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/366/508. Figures 2--10,12, 14, 15, 17--19 are only available in electronic form athttp://www.edpsciences.org, see Note added in proof
| 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.
| An exploratory non-LTE analysis of B-type supergiants in the Small Magellanic Cloud
A preliminary differential non-LTE model atmosphere analysis of moderateresolution (R ~ 5 000) and signal-to-noise ratio spectra of 48 SmallMagellanic Cloud B-type supergiants is presented. Standard techniquesare adopted, viz. plane-parallel geometry and radiative and hydrostaticequilibrium. Spectroscopic atmospheric parameters (T_eff, log g andv_turb), luminosities and chemical abundances (He, C, N, O, Mg and Si)are estimated. These are compared with those deduced for a comparablesample of Galactic supergiants. The SMC targets appear to have similaratmospheric parameters, luminosities and helium abundances to theGalactic sample. Their magnesium and silicon underabundances arecompatible with those found for main sequence SMC objects and there isno evidence for any large variation in their oxygen abundances. Bycontrast both their carbon and nitrogen lines strengths are inconsistentwith single abundances, while their nitrogen to carbon abundance ratiosappear to vary by at least as much and probably more than that found inthe Galactic sample.
| P Cygni: An Extraordinary Luminous Blue Variable
P Cygni is a prototype for understanding mass loss from massive stars.This textbook star is known first of all because of two great eruptionsin the 17th century. In the first half of this century it has given itsname to a class of stars which are characterized by spectral linesconsisting of nearly undisplaced emissions accompanied by ablue-displaced absorption component. This characteristic P Cygni-typeprofile betrays the presence of a stellar wind, but P Cygni's wind isquite unlike that of other hot supergiants. P Cygni was the first starthat showed the effects of stellar evoluton from a study of itsphotometric history. It shares some common properties with the so-calledLuminous Blue Variables. However, P Cygni is a unique object. Thisreview deals with P Cygni's photometric properties, its circumstellarenvironment - including infrared and radio observations - and itsoptical and ultraviolet spectrum. Smaller sections deal with P Cygni'swind structure and evolution.
| Light variations of the blue hypergiants HD 168607 and HD 168625 (1973-1999)
Strömgren differential photometry of HD 168607 and HD 168625collected during the previous 27 years is analysed. We bring to an endour quarter of a century monitoring of these objects and conclude thatHD 168607 shows typical variability of an uneventful LBV, whereas HD168625 remains to be classified as a variable hypergiant, or amarginally-dormant LBV. The long-term light- and colour behaviour of HD168607 indicates the possible presence of a very slowly-developing S Dorphase - that is, a bright phase that coincides with redder colours.Besides the alpha Cygni-type microvariability, no other signs ofultrashort-periodic variability has been observed. Based on observationsobtained at the European Southern Observatory at La Silla, Chile(applications ESO 56D-0249, 58D-0118, 60D-0148, 61D-0128 and 62H-0110)
| A Semiempirical Test for Dynamical Instability in Luminous Blue Variables
By employing as input parameters only observationally determined stellarsurface quantities, detailed envelope models for six well-observedluminous blue variables at quiescence are constructed and tested formarginal dynamical instability. The best-observed of these objects, PCyg, proves to be exactly in a state of marginal dynamical instability,just as predicted. Within the larger observational errors estimated forthe five other objects, they too are probably in the same marginallyunstable state as P Cyg.
| VBLUW photometry of fifteen alpha CYG variables (including LBVs) in the Galaxy, the LMC and the SMC
VBLUW photometry (Walraven system) of 15 alpha Cyg-type variables,(luminous massive stars), including 10 LBVs and one suspected LBV arepresented. Formulae are given to transform the V and V-B of the Walravensystem to V and B-V of the Johnson UBV system.
| Discovery of Candidate Luminous Blue Variables in M31
Luminous blue variables (LBVs) constitute a short-lived, eruptive phasein the evolution of some of the most massive stars. Only a handful haveyet been identified in the Galaxy and in each of the nearby galaxies;there are four known LBVs in M31. We have found an efficient method toidentify candidate LBV stars in nearby galaxies. The candidates areidentified in a pair of deep, continuum-subtracted narrowband Hαand [S II] images as objects with extremely low [S II] to Hαratios, and with coincident stellar objects in continuum images. Five ofthe most promising new candidates identified by these criteria in thenortheastern half of M31 were subsequently confirmed by opticalspectroscopy to show spectra similar to the previously identified M31LBV, HS var 15. These five candidates also have much in common with B[e]stars, of which none were known to exist in M31. They are brightHα sources, (120 L_ȯ < L_Hα < 1300 L_ȯ) withno detectable [S II] emission, large Hα equivalent widths (-60 to-400 Å), and broad wings on the Hα profiles (FWZI =1200-2000 km s^-1). Most candidates have Fe II emission. We discuss theenvironments of the candidates and find that many objects are likelyolder than several million years because they tend not to be locatedinside bright H II regions. We predict, based on the current results,that at least 20-25 LBV/B[e] candidates may be present in M31.
| The Pistol Star
We present new near-infrared data and analysis, which indicate that thePistol Star is one of the most luminous stars known, adding another testpoint for massive star formation and stellar evolution theories. Weestimate an extinction of A_K = 3.2 +/- 0.5 using the near-infraredcolors of the star and of surrounding stars in the young Quintupletcluster. Using our wind/atmosphere code, we find two families of modelsthat fit the spectral energy distribution and detailed line profiles.The lower luminosity models give L = 10^6.6+/-0.2 L_ȯ and T_eff =10^4.15+/-0.01 K, while the higher luminosity models give L =10^7.2+/-0.2 L_ȯ and T_eff = 10^4.33+/-0.01 K; the error inluminosity assumes an uncertainty of +/-0.5 in A_K, while the error inT_eff is constrained by detailed line modeling. The models also reveal ahelium enriched surface. As previously existing stellar evolution modelsdo not extend to such high luminosities, we employ new evolutionarytracks for very massive stars to determine the initial mass and age ofthe Pistol Star, and estimate M_initial = 200-250 M_ȯ and an age of1.7-2.1 Myr. The inferred luminosity and temperature place the star in asparsely populated zone in the H-R diagram where luminous blue variables(LBVs) are often found. This is consistent with our evolutionary models,which predict that the star is in an unstable evolutionary stage. Weinterpret the star and its surrounding nebula as an LBV that hasrecently ejected large amounts of material. Our K-band speckle-imagingdata reveal the star to be single down to a projected separation of 110AU.
| 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)
| Ejected Nebulae as Probes of the Evolution of Massive Stars in the Large Magellanic Cloud
We present new Hubble Space Telescope (HST) Faint Object Spectrograph(FOS) and ground-based long-slit spectroscopic observations of thenebulae around the two LMC luminous blue variables (LBVs) R127 (=HDE269858) and R143 (=HDE 269929) and the Ofpe/WN9 star S119 (=HDE 269687).We have used the ground-based long-slit spectra to investigate thekinematics of the nebulae around R127 and R143. We find that thecircumstellar environment of R127 is complex and suggestive of twodiscrete ejections in the mass loss history of the central star. Thereis an inner expanding shell, with a radius of 0.6 pc, an expansionvelocity of 29 km s-1, and a dynamical age of 2 x 104 yr. There is alsomaterial beyond the bright inner nebula that may represent an outerexpanding shell with a radius of 1.9 pc and a dynamical age of 7 x 104yr. For R143, we find that the velocity field in the northern andsouthern circumstellar regions (1"-20") is predominantly constant.Therefore, the nebular emission previously believed to be associatedwith R143 (the fingers) is most likely part of the 30 Doradus complex,with the exception of a small emission-line region located ~=2" north ofthe star. This compact region displays a blueshifted motion with adifferential velocity of ~=130 km s-1 with respect to the central star.The spectrum of this clump shows a high [N II]/H alpha ratio, suggestiveof nitrogen-enriched material that has been ejected from the star some3.7 x 103 yr ago. We have used the optical HST/FOS spectra to determinereddenings, electron temperatures and densities, and N and O abundancesfor the three nebulae. For R143 our derived abundances indicate that theregion we observed to the south of the star is 30 Doradus H II gas,since it shows a typical LMC H II region N/O ratio. For R127 we findthat N is enriched by a factor of 10.7 +/- 2.2, and O is depleted by afactor of 2.0 +/- 1.0, with N/O = 0.89 +/- 0.40. For the S119 nebula wedo not have a secure electron temperature, but find N/O = 1.41-2.45,similar to the value found for R127. We compare the nebular abundancesto the expected surface abundances of LBVs and discuss the likelystellar evolutionary state at the time of nebular ejection. If theatmospheres of LBVs consist of CNO-processed material, this implies thatthe event that formed the nebula took place before or at the very startof the LBV phase. Comparison of the observed nebular abundances withother objects shows that the abundance pattern for the R127 nebula isremarkably similar to that determined for the inner ring of SN 1987A,which is thought to be composed of red supergiant (RSG) wind material.This coincidence, together with the observed low expansion velocity,suggests that the nebula was once the CN-processed convective envelopeof an RSG. We consider whether the R127 nebular parameters can bereproduced with the evolutionary models of Meynet et al., and we findthat the pre-LBV mass loss has to be low enough for redward evolutionsuch that a surface N/O ~ 1 is achieved while the star is a coolsupergiant inside the Humphreys-Davidson limit. Our findings are broadlyin accord with the model of Stothers & Chin that incorporates abrief RSG phase. We also consider the possibility of a pseudo-RSG phaseby discussing the suggestion that a star in a super-Eddington conditionmay develop a very extended outer convective envelope that may becomedetached. We also present a coarse analysis of the FOS stellarspectrophotometry using the Q-method to estimate reddenings and spectraltypes, and we compare our results to more sophisticated methods.
| Cyclicities in the light variations of Luminous Blue Variables. II. R40 developing an S Doradus phase
Stroemgren differential photometry of R40 collected during the timeinterval 1986-1996 is analysed together with Walraven photometry. Thegradual brightening of the star over the last 10 years can be describedby a linear trend with superimposed oscillations (in v, b and y) withfrequency 0.0008cd(-1) ( ~ 1300 d cycle). We interpret theseoscillations as ``normal S Dor'' phases, and suggest that thequasi-linear brightening of the star is the ascending branch of agrowing very-long-term S Dor phase (VLT-SD), as found by van Genderen etal. (1997a) in AG Car and S Dor itself. As R40 is now becoming fainterand bluer, the length of the VLT-SD cycle is about 20 years. Based onobservations obtained at the European Southern Observatory at La Silla,Chile (applications ESO 56D-0249 and 58D-0118)
| The Luminous Eclipsing SMC OB + WN Binary HD 5980 before and during the Recent LBV-like Outburst: an Extreme Case of Colliding Winds
The 1994 LBV-(luminous blue variable)like outburst of one of theluminous, hot components of the binary HD 5980 made it the brighteststar in the Small Magellanic Cloud for an interval of 5 months. The mostintriguing question to arise from this event is the following: Why didthe HD 5980 spectrum change from an H-poor WN3 with veiled OB absorptionlines about 20 yr ago to an H-rich WN11 without central absorption linesduring the outburst? In an attempt to answer this apparent enigma, wepresent and analyze new phase-dependent spectroscopic, polarimetric, andlight-curve observations. Together with other published data, these newobservations allow us to improve the orbital parameters considerably,except for the radial velocity amplitudes and hence the masses, whichare only roughly constrained. Especially important in HD 5980 is thestrong collision of the two nearly equal pre-outburst winds. Theemission-line spectrum generated by the collision tends to mask theunderlying line spectra of both components when the system is relativelyquiescent. We argue that the pre-erupting system consists of a veryluminous but moderately massive H-rich O type supergiant, possibly withemission lines, and a low-mass, H-poor, relatively faint WN companion,whose lines are mostly drowned out by wind collision emission, thespectrum of which largely imitates that of a WNE star. It was the Osupergiant that erupted in a normal way as an H-rich, visually brightWN11 star. In this way, the need for peculiar evolutionary scenarios(e.g., rapid evolution from a faint, low-mass, H-poor WNE star to aluminous, H-rich WNL star) is avoided.
| Light variations of massive stars (alpha Cyg variables). XV. The LMC supergiants R99 (LBV), R103, R123 (LBV) and R128
VBLUW photometry (Walraven system) of the four variable LMC supergiantsR99, R103, R123 and R128 is analysed, searched for periods anddiscussed. Based on former and present photometry we conclude that twoof the three emission-line objects are undoubtedly active LBVs (R99 andR123), although not so spectacular. R123, like AG Car near minimumbrightness, shows a low amplitude S Dor activity with superimposed alphaCyg-type variations. Based on observations obtained at the EuropeanSouthern Observatory at La Silla, Chile
| The long-term behaviour of the Be star HD163868
In this paper we discuss the light variations of HD163868 on the basisof all available photometric data originating from photometricmonitoring during the last two decades. We suggest that one explanationof the on-and-off type variability can be a binary configuration with acompact companion and a period of the order of 850 days, though theobserved effects could also be seen in the context of positiveinterference of NRPs. The bright stages are associated with strongreddening consistent with an increase in circumstellar material. We drawthe attention to the similarity with cyclically-recurrent mild S Dorphases of LBVs. Based on observations obtained at the European SouthernObservatory at La Silla, Chile (observing proposals 55D-0317, 56D-0249and 57D-0133) and on data obtained with the Observatoire deGenève Swiss Telescope at La Silla
| 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.
| UV Spectral Classification of O and B Stars in the Small Magellanic =
We present an ultraviolet classification system for 0 and B stars of theSmall Magellanic Cloud (SMC). This system is defined by a set ofstandard, low resolution, International Ultraviolet Explorer (JUE)spectra. This UV classification system results from the development ofspectral sequences demonstrating systematic patterus of UV spectralfeatures. These spectral sequences yield Uv classifications for 133 0and B stars of the SMC, which we also present here. Although independentof the MK System, our UV classifications show general agreement withthose deduced from visual data. This UV classification system isdesigned to be applicable to 0 and B stars in other nearby galaxies ofmetallicity comparable to the SMC. The classification technique itselfis applicable to any UV dataset of sufficient size and quality, and canbe used to extend our UV classification system to other galactic metalabundances. These UV classification systems will be essential foranalysis of datasets from new spaceborne instrumentation such as theSpace Telescope Imaging Spectrograph (STIS), which will be capable ofobserving stars in external galaxies for which no opticalclassifications exist. 0 1997 American Astronomical Society.[S0004-6256(97)0231 1-X]
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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: | טוקאן |
| התרוממות ימנית: | 01h07m18.22s |
| סירוב: | -72°28'03.7" |
| גודל גלוי: | 10.204 |
| תנועה נכונה: | 1.5 |
| תנועה נכונה: | -6.1 |
| B-T magnitude: | 10.323 |
| V-T magnitude: | 10.214 |
קטלוגים וכינוים:
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