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The lights of other days
The concept of first lights can be thought of as the most recentiteration in an extended interplay between the idea that the worldaround us can be explained in terms of processes that are still going onand amenable to examination (“uniformitarianism”) and theidea that many important things happened only long ago when the universewas very different (“catastrophism”). As is usually the casein such matters, there is a fair amount of truth on each side, but, justat the moment, the catastrophists are slightly in the lead.
| The birth-cluster of the galactic luminous blue variable WRA 751
We present the results of NTT/VLT UBV imaging of a 260arcmin2 region containing the Galactic Luminous Blue VariableWRA 751, in search for its birth-cluster, i.e. a cluster of young andmassive stars spatially and physically associated with it. On the basisof the classical reddening-free parameter Q, we have identified a sampleof 24 early-type stars with colours typical of spectral types earlierthan B3. Interestingly, these stars are clustered within a radius of 1'from WRA 751, corresponding to about 1% of the imaged field. These starstightly distribute around (B-V) ≃ 1.67, which in turn defines amean extinction AV ≃ 6.1 mag. The 5 brighter (V ≤16.2) and bluer (Q ≤ -0.9) stars of the sample have beensubsequently observed with FORS1 and classified as 3 late O- and 2 earlyB-stars. The absence of stars earlier than O8 indicates an age of thecluster older than 4 Myr, although it could be due to an incompletesampling of the upper end of the main sequence. Nevertheless, thedetection of OB stars of class I certainly indicates an age of a fewmillion years. At an assumed distance of 6 kpc, we estimate a clusterradius of 3.4 pc and a total mass of 2.2 × 103Mȯ. Our discovery is only the second known instance of aGalactic Luminous Blue Variable associated with its birth-cluster.
| Tycho-2 stars with infrared excess in the MSX Point Source Catalogue
Stars of all evolutionary phases have been found to have excess infraredemission due to the presence of circumstellar material. To identify suchstars, we have positionally correlated the infrared Mid-Course SpaceExperiment (MSX) Point Source Catalogue and the Tycho-2 opticalcatalogue. Near-mid-infrared colour criteria have been developed toselect infrared excess stars. The search yielded 1938 excess stars; overhalf (979) have never previously been detected by IRAS. The excess starswere found to be young objects such as Herbig Ae/Be and Be stars, andevolved objects such as OH/IR (infrared) and carbon stars. A number ofB-type excess stars were also discovered whose infrared colours couldnot be readily explained by known catalogued objects.
| 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.
| Infrared Observations of the Candidate LBV 1806-20 and Nearby Cluster Stars1,
We report near-infrared photometry, spectroscopy, and speckle imaging ofthe hot, luminous star we identify as candidate LBV 1806-20. We alsopresent photometry and spectroscopy of three nearby stars, which aremembers of the same star cluster containing LBV 1806-20 and SGR 1806-20.The spectroscopy and photometry show that LBV 1806-20 is similar in manyrespects to the luminous ``Pistol star,'' albeit with some importantdifferences. They also provide estimates of the effective temperatureand reddening of LBV 1806-20 and confirm distance estimates, leading toa best estimate for the luminosity of this star of greater than5×106Lsolar. The nearby cluster stars havespectral types and inferred absolute magnitudes that confirm thedistance (and thus luminosity) estimate for LBV 1806-20. If we dropkinematic measurements of the distance(15.1+1.8-1.3 kpc), we have a lower limit on thedistance of greater than 9.5 kpc and on the luminosity of greater than2×106Lsolar, based on the cluster stars. Ifwe drop both the kinematic and cluster star indicators for distance, anammonia absorption feature sets yet another lower limit to the distanceof greater than 5.7 kpc, with a corresponding luminosity estimate ofgreater than 7×105 Lsolar for the candidateLBV 1806-20. Furthermore, on the absis of very high angular resolutionspeckle images, we determine that LBV 1806-20 is not a cluster of starsbut is rather a single star or binary system. Simple arguments based onthe Eddington luminosity lead to an estimate of the total mass of LBV1806-20 (single or binary) exceeding 190Msolar. We discussthe possible uncertainties in these results and their implications forthe star formation history of this cluster.Based on data obtained at the Palomar Observatory 200 inch telescope,which is operated by the California Institute of Technology, the JetPropulsion Laboratory, and Cornell University.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| The Missing Luminous Blue Variables and the Bistability Jump
We discuss an interesting feature of the distribution of luminous bluevariables (LBVs) on the H-R diagram, and we propose a connection withthe bistability jump seen in the winds of early-type supergiants. Thereappears to be a deficiency of quiescent LBVs on the S Doradusinstability strip at luminosities between log(L/Lsolar)~=5.6and 5.8. The upper boundary, interestingly, is also where thetemperature-dependent S Doradus instability strip intersects thebistability jump at about Teff~=21,000 K. Because ofincreased opacity, winds of early-type supergiants are slower and denseron the cool side of the bistability jump, and we postulate that this maytrigger optically thick winds that inhibit quiescent LBVs from residingthere. We conduct numerical simulations of radiation-driven winds for arange of temperatures, masses, and velocity laws atlog(L/Lsolar)=5.7 to see what effect the bistability jumpshould have. We find that for relatively low stellar masses, theorder-of-magnitude increase in the wind density at the bistability jumpleads to the formation of a modest to strong pseudophotosphere thatmight alter a star's apparent position on the H-R diagram. The effect isstrongest for LBVs approaching 10 Msolar, where thepseudophotospheres are sufficiently extended to make an early B-typestar appear as a yellow hypergiant. Thus, the proposed mechanism will bemost relevant for LBVs that are post-red supergiants [curiously, theupper boundary at log(L/Lsolar)~=5.8 coincides with the upperluminosity limit for red supergiants]. Further work is obviously needed,especially with regard to a possible evolutionary connection between the``missing'' LBVs and the most luminous red supergiants and yellowhypergiants. Specifically, yellow hypergiants such as IRC +10420 andρ Cas occupy the same luminosity range as the missing LBVs and showapparent temperature variations at constant luminosity. If these yellowhypergiants do eventually become Wolf-Rayet stars, we speculate thatthey may skip the normal LBV phase, at least as far as their apparentpositions on the H-R diagram are concerned.
| Discovery and Evolution of an Unusual Luminous Variable Star in NGC 3432 (Supernova 2000ch)
We present photometric and spectroscopic observations of SN 2000ch, anunusual and extremely luminous variable star located in the galaxy NGC3432. The object was discovered on 2000 May 3.2 during the course of theLick Observatory Supernova Search, at an unfiltered magnitude of about17.4. Prediscovery images obtained in 1997, 1998, and 2000 April showthe object with R=19.2-19.5 mag. Optical spectra obtained beginning on2000 May 6 show a smooth, flat continuum and strong, broad hydrogenBalmer emission lines at wavelengths consistent with the catalogedredshift of NGC 3432, strengthening the association of the variable withthe galaxy. Photometric monitoring reveals a complex and erratic lightcurve over a time span of ~10 days. Subsequent optical spectra over thenext ~3 months continued to show strong Balmer emission lines with amean full width at half-maximum intensity ~1550 km s-1 and adistinct red asymmetry. A spectrum obtained 9 months after the outburstis similar to the previous spectra, but the integrated flux in Hαis nearly half that observed during the outburst. The object'sphotometric behavior, spectrum, and luminosity suggest that it is a verymassive and luminous variable star and might be related to some luminousblue variable stars such as η Carinae and SN 1997bs in NGC 3627. Thebrightest apparent magnitude implies an absolute magnitude ofMV~-12.7 at the distance of NGC 3432, a value that iscomparable to η Car during its outburst in the mid-19th century.
| On the structure and kinematics of nebulae around LBVs and LBV candidates in the LMC
We present a detailed analysis of the morphology and kinematics ofnebulae around LBVs and LBV candidates in the Large Magellanic Cloud.HST images and high-resolution Echelle Spectra were used to determinethe size, shape, brightness, and expansion velocities of the LBV nebulaearound R 127, R 143, and S 61. For S Dor, R 71, R 99, and R 84 wediscuss the possible presence of nebular emission, and derive upperlimits for the size and lower limits on the expansion velocities ofpossible nebulae. Including earlier results for the LBV candidates S 119and Sk-69o279 we find that in general the nebulae around LBVsin the LMC are comparable in size to those found in the Milky Way. Theexpansion velocities of the LMC nebulae, however, are significantlylower - by about a factor of 3 to 4 - than those of galactic nebulae ofcomparable size. Galactic and LMC nebulae show about the same diversityof morphologies, but only in the LMC do we find nebulae with outflow.Bipolarity - at least to some degree - is found in nebulae in the LMC aswell as in the Milky Way, and manifests a much more general featureamong LBV nebulae than previously known.
| The calibrating stars of the Mira P-L relation
Improved light curves extracted from the MACHO data base are used todiscuss the periods and amplitudes of the calibrating stars of the MiraP-L relations. Previously unpublished spectral types and positions aregiven for several of those that lie in the S Dor variable star field. Itis found that the periods derived from the discovery observations weregenerally sufficiently accurate that the K- and mbol-logPrelations are not significantly changed by the MACHO results.Furthermore, the periods have remained essentially constant over two tothree decades. The MACHO r-band amplitudes of the oxygen-rich LargeMagellanic Cloud (LMC) Miras are similar to those in galactic (Baade'swindow) fields at a given period but there appear to be moreshorter-period stars as a proportion of the whole than in the galaxy. InO-rich stars, the distinction in amplitude between the Miras and thesemiregulars is not as conspicuous at shorter periods (<200 d) as atlonger ones. The LMC carbon Miras have smaller amplitudes than theirO-rich counterparts. Of the six stars with P > 420 d that were foundto be too luminous to fit the P-L relation, two are now known to beLi-rich. This tends to confirm the suggestion that they are hot-bottomburners.
| Massive Stars in Transition: Spectroscopic Analysis of HR Carinae
We analysed the high-resolution data of the LBV (or S Dor variable) HRCarinae, taken with the FEROS spectrograph in the wavelength range 3900to 8200 Å . We performed a spectral analysis of the Balmer lineswith a non-LTE expanding atmosphere code in order to derive its stellarparameters and to infer its evolutive status.
| Luminous Blue Variables, cool hypergiants and some impostors in the H-R diagram
Current observations of the S Dor/LBVs and candidates and theimplications for their important role in massive star evolution arereviewed. Recent observations of the cool hypergiants are altering ourideas about their evolutionary state, their atmospheres and winds, andthe possible mechanisms for their asymmetric high mass loss episodeswhich may involve surface activity and magnetic fields. Recent resultsfor IRC+10 420, ρ Cas and VY CMa are highlighted. S Dor/LBVs ineruption, and the cool hypergiants in their high mass loss phases withtheir optically thick winds are not what their apparent spectra andtemperatures imply; they are then `impostors' on the H-R diagram. Theimportance of the very most massive stars, like η Carinae and the`supernovae impostors' are also discussed.
| Infrared [FeII] emission in the circumstellar nebulae of luminous blue variables
After a serendipitous discovery of bright [FeII]λ16435 emissionin nebulae around η Carinae and P Cygni, infrared spectra of otherluminous blue variables (LBV) and LBV candidates were obtained. Brightinfrared [FeII] emission appears to be a common property among LBVs withprominent nebulae; this is an interesting discovery because strong[FeII]λ16435 is typically seen in shock-excited objects likesupernova remnants and outflows from newly formed massive stars, as wellas in active galactic nuclei (AGN), where the excitation mechanism isuncertain. This paper presents spectra in the H-band (1.5 to 1.75 μm)for the central stars and nebulae of η Car, AG Car, P Cyg, Wra 751,HR Car, HD 168625, HD 160529, R 127 and S Doradus. Seven of nine targetsshow bright [FeII]λ16435 in their nebulae, while it is absent inall central stars except the LBV candidate Wra 751. The two objects (SDor and HD 160529) without prominent [FeII]λ16435 are not yetknown to have nebulae detected in optical images, and both lack brightthermal infrared emission from dust. The possible excitation mechanismsfor this line and the implications of its discovery in LBV nebulae arediscussed; there are good reasons to expect shock excitation in someobjects, but other mechanisms cannot be ruled out.
| 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.
| Early-type variables in the Magellanic Clouds. I. beta Cephei stars in the LMC bar
A thorough analysis of the OGLE-II time-series photometry of the LargeMagellanic Cloud bar supplemented by similar data from the MACHOdatabase led us to the discovery of three beta Cephei-type stars. Theseare the first known extragalactic beta Cephei-type stars. Two of thethree stars are multiperiodic. Two stars have inferred masses of about10 M_sun while the third is about 2 mag brighter and at least twice asmassive. All three variables are located in or very close to the massiveand young LMC associations (LH 41, 59 and 81). It is therefore veryprobable that the variables have higher than average metallicities. Thiswould reconcile our finding with theoretical predictions of the shapeand location of the beta Cephei instability strip in the H-R diagram.The low number of beta Cephei stars found in the LMC is anotherobservational confirmation of strong dependence of the mechanism drivingpulsations in these variables on metallicity. Follow-up spectroscopicdetermination of the metallicities in the discovered variables willprovide a good test for the theory of pulsational stability in massivemain-sequence stars.
| Light variations of massive stars (alpha Cyg variables). XVIII. The B[e] supergiants S 18 in the SMC and R 66 = HDE 268835 and R 126 = HD 37974 in the LMC
We discuss photometric monitoring (VBLUW system) of three B[e]supergiants. All three objects appear to be variable. They are subjectto two (R 66 and R 126 in the LMC) and three (S 18 in the SMC) types oflight oscillations which range from a few days to years, and areprobably due to pulsations. We argue that a classification as alpha Cygvariables is justified. Their classification as mixed B[e]/S Dorvariables is less certain, though not impossible. Also based on othercases, a strong B[e]-S Dor variable connection seems to be present.Based on observations obtained at the European Southern Observatory atLa Silla, Chile.
| Outflow-induced Dynamical and Radiative Instability in Stellar Envelopes with an Application to Luminous Blue Variables and Wolf-Rayet Stars
Theoretical models of the remnants of massive stars in a very hot,post-red-supergiant phase display no obvious instability if standardassumptions are made. However, the brightest observed classical luminousblue variables (LBVs) may well belong to such a phase. A simpletime-dependent theory of moving stellar envelopes is developed in orderto treat deep hydrodynamical disturbances caused by surface mass lossand to test the moving envelopes for dynamical instability. In the caseof steady state outflow, the theory reduces to the equivalent of theCastor, Abbott, & Klein formulation for optically thick winds atdistances well above the sonic point. The time-dependent versionindicates that the brightest and hottest LBVs are both dynamically andradiatively unstable, as a result of the substantial lowering of thegeneralized Eddington luminosity limit by the mass-loss acceleration. Itis suggested that dynamical instability, by triggering secular cycles ofmass loss, is primarily what differentiates LBVs from the purelyradiatively unstable Wolf-Rayet stars. Furthermore, when accuratemain-sequence mass-loss rates are used to calculate the evolutionarytracks, the predicted surface hydrogen and nitrogen abundances of theblue remnants agree much better with observations of the brightest LBVsthan before.
| P Cygni in a short S Doradus phase. Spectroscopic and photometric evidences
We studied the long-term spectral and photometric behaviour of P Cygniover the 13.8 year interval from March 1985 to January 1999. The UBVphotometry reveals a slow ( ~ 7.4-year), low-amplitude ( ~ 0.1 mag)variation in V in which the star becomes redder when it brightens andvice versa. It underwent a possible maximum in the winter of 1985(between JD 2 446 000 and JD 2 446 200), a minimum in the winter of 1989(between JD 2 447 500 and JD 2 447 700), a maximum in the fall of 1992(between JD 2 448 800 and JD 2 449 000), and a minimum in the spring of1996 (between JD 2 450 100 and JD 2 450 300) The properties of thisvariation are typical for weak-active S Dor variables in ``short S Dorphase'' \citep{vanGen01}. Simultaneous spectroscopic observations showchanges in the Hα equivalent width (corrected for the effect ofthe changing continuum) in positive correlation with the 7.4-yearphotometric oscillation in the V-band brightness. This result isinterpreted as an indication that in P Cygni an increase in the stellarbrightness, during the 7.4-year SD phase, is likely accompanied by anincrease in the mass-loss rate. In this behaviour P Cygni is similar toS Dor and R71. From simple considerations, it is concluded that the7.4-year SD cycle is probably a combination of an increasingradius/decreasing effective temperature and an expandingpseudo-photosphere.
| A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We report on a comprehensive search for Wolf-Rayet (W-R) stars in theSMC using interference filter imaging. Photometry of over 1.6 millionstellar images on multiple, overlapping fields covering 9.6deg2 found the previously known W-R stars at very highsignificance levels, two known Of-type stars, plus additionalcandidates, which we examined with slit spectroscopy. We discovered twonew Wolf-Rayet stars, both of type ``WN3+abs,'' bringing the totalnumber in the SMC to 11. We discuss their spectra, as well asreclassifying the previously known ones with our new data. Our surveyalso revealed four newly found Of-type stars, including one of the O5f?ptype, which is one of the earliest type stars known in the SMC. Anothernewly identified Of star is AV 398 (O8.5 If), a star often used inextinction studies under the assumption that it is of early B type. Werecover S18 (AV 154), a B[e] star whose spectrum currently lacks He IIλ4686 emission but which must have had strong emission a yearearlier; we compare this star to S Dor, suggesting that it is indeed aluminous blue variable. We also find a previously unknown symbiotic starwhose spectrum is nearly identical to the Galactic symbiotic AG Dra.More important, perhaps, than any of these discoveries is thedemonstration that there is not a significant number of W-R starswaiting to be discovered in the SMC. The number of W-R stars is a factorof 3 times lower in the SMC (per unit luminosity) than in the LMC. Thisstrongly suggests that at the low metallicity that characterizes the SMConly the most massive stars can evolve to W-R type.
| Simultaneous Hα and photometric observations of P Cygni
For the first time an extensive set of (quasi-) simultaneous photometric(UBV) and spectroscopic (Hα line profiles) observations of PCygni, covering a period from May, 1990 to June, 1994 was analyzed interms of time variability. It is found that the Hα equivalentwidth (EW) exhibits two different patterns of variability: a slower one,called Long-Term (LT) variability, with an amplitude of about 30Åand a characteristic duration of about 600 days and a faster one,called Short-Term (ST) variability, with an amplitude up to 10Åand duration of 40 to 60 days. Suggestive evidence for EWvariation on a longer time scale (about few years) also exists. Thevariations in the Hα luminosity are not solely due to changes inthe underlying continuum but also reflect variations in the physicalproperties of the wind. We find, in terms of a simplifiedspherically-symmetric wind model, that the LT variation of the line canbe successfully explained in terms of a 26% alteration of the mass-lossrate, possibly accompanied by variations in the velocity field. From theanalysis of the photometric behaviour of the star we find evidence for avery slow variation in the stellar brightness with an amplitude of about0.13 mag and a duration of about 2600 days, i.e. about 7 years. Duringthis variation, i.e. when the star brightens, the effective temperaturedecreases (by about 10%) and the radius increases (by about 7%). Theproperties of this Very Long Term (VLT) variation suggest that P Cygnihas probably experienced a normal S Dor-type variation with a minimumphase around 1988 and a maximum phase in 1992. Some hints for a positivecorrelation between mass loss variations and changes in the stellarradius, due to the normal SD variability, do exist implying that thebehaviour of P Cygni is more likely similar to that of R71 and S Dor butdifferent from e.g. AG Car, R127 and HD 160529. Superimposed on the VLTcomponent in the photometric variability of P Cygni, we observe STbrightness variations with an amplitude between 0.1 and 0.2 mag whichappears to recur on a time scale of three to four months. The colourbehaviour of these microvariations, at least of those which appear nearthe maximum phase of the VLT variation, is redder in B-V and bluer inU-B when the star brightens in V. The properties of this ST photometricvariability are similar to the properties of the so-called ``100 d-typemicro-variations'', recognized in other LBVs by van Genderen et al.(\cite{van97a},b). Based on time-scale evidences we suggest that themicrovariabilities observed are rather due to ``relaxationoscillations'' (Stothers & Chin \cite{Stothers95}) than tostrange-mode oscillations in the stellar interior. Evidence for a closerelationship between ST variations in Hα and changes in thestellar brightness and temperature is found. From other results about PCygni's spectral variations (Markova \cite{Markova2000a}), we concludethat the ST variability of the wind is most likely connected withprocesses in the stellar photosphere.
| 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
| Neue beobachterische Themen und Ziele als Erganzung zu den bisherigen Arbeiten innerhalb der BAV.
Not Available
| Far-Ultraviolet Stellar Emission Measurements from UVSTAR
We present the relevant results from three UVSTAR missions on STS69,STS85 and STS95. In particular we discuss three hot sub-dwarf star andɛ CMa (500-1250 Å) observations.
| The Milton Bureau Revisited
Under the direction of Cecilia Payne-Gaposchkin and Sergei Gaposchkin, aprogram was subsidized by the Milton Fund of Harvard Observatory in 1937for the study of all variable stars then known to be brighter than tenthphotographic magnitude at maximum. This included some 1512 stars forwhich a grand total of 1,263,562 estimates of magnitude were made,ranging from a low of 16 (except for a few novae) to 4084 observationsper star. The sky had been divided into 54 fields, and the results ofthe measurements presented field by field in two volumes of the Annalsof Harvard Observatory. Then, in another volume, the results werediscussed in four sections, each dealing with a particular class ofvariable: 1, those of RV Tauri type; 2, the eclipsing variables; 3,Cepheids and RR Lyrae variables, and 4, the red variables, especiallyMira-type and semiregular variables.For the present paper, many of these results have been compared withmodern determinations in the 1985-87 version of the "General Catalogueof Variable Stars (GCVS)". In particular, there are numerous instancesof disagreement as to whether a star should be classified RV or SR.Although there are many instances where the Milton Bureau determinationsof types of variability differ from the types given in moderncatalogues, the reasons for the differences are generallyunderstandable.For 17 RV Tauri type stars in this survey multiple periods have now beendetermined. Many of these still deserve continued observations in orderto ascertain the constance of the periods and improve the accuracy oftheir longest reported periods.
| 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.
| Infrared imaging and spectroscopy of the Luminous Blue Variables Wra 751 and AG Car
We present ground-based infrared imaging and ISO spectroscopy of theluminous blue variables Wra 751 and AG Car. The images show in bothcases a detached shell with a roughly circular distribution of emission.The infrared images of AG Car coincide very well with the opticalimages. The optical (Hα ) image of Wra 751 is different from theinfrared image; the Hα nebula is suggested to be a scatteringnebula containing cold dust particles. Fitting both the images and thespectra consistently with a 1-D radiative transfer model, we deriveproperties of their dust shells. Wra 751 is surrounded by a dust shellwith inner and outer radii of 0.17 and 0.34 pc respectively and a dustmass of 0.017 Msun . The dust shell of AG Car has inner andouter radii of 0.37 and 0.81 pc respectively and a total dust mass of0.25 Msun . Dust mass-loss rates during the formation of theshells are 2.7x 10-6 and 3.4x 10-5 Msun yr-1, respectively. The total dust mass and hence thederived dust mass-loss rates are uncertain by at least a factor of two.For AG Car, the derived dust mass and mass-loss rate are higher thanprevious estimates. This is mainly caused by the fact that acontribution of very large grains (> 10 mu m) is needed to explainthe flux levels at longer wavelengths. Dust models for both objects failto explain the flux shortward of 15 to 20 mu m: a population of smallwarm grains, not in thermal equilibrium with the central star isnecessary to explain this excess. Similarities between dust shellsaround Wolf-Rayet stars and Wra 751 and AG Car (mass, grain sizepopulation, morphology) suggest a similar formation history and imply anevolutionary connection. A similar connection with red supergiants issuggested on the basis of the dust composition and derived time-averagedmass-loss rates. based on observations obtained with ISO, an ESA projectwith instruments funded by ESA Member states (especially the PIcountries: France, Germany, the Netherlands and the United Kingdom) withthe participation of ISAS and NASA. Based on observations obtained atESO, La Silla, Chile
| An Unprecedented Change in the Spectrum of S Doradus: As Cool as It Gets
Recent observations of S Doradus, the archetype of luminous bluevariables (LBVs), reveal that the star's optical spectrum now resemblesan F-type supergiant, with a rich complex of absorption lines. Despitenearly 50 years of spectroscopic monitoring, such a spectrum has neverbefore been seen for S Dor despite numerous occasions when the star wasequally bright. However, such F-type spectra have been seen in otherLBVs, including Var B in M33 during a recent outburst, and in ηCarina during an outburst in 1893. The singly ionized metal lines arisein a layer moving away from the star (toward us) at 50 kms-1, consistent with the lines forming in a``pseudophotosphere'' originating in the stellar wind. The temperaturesuggested by the F-type spectrum is as cool as an LBV can get. Our CCDUBV photometry shows that the star has brightened only 0.3 mag in Vsince 1996, at which time it showed an emission-line spectrumcharacteristic of LBVs at minimum. The dereddened B-V color isconsistent with an effective temperature suggested by the spectrum (7500K), but there is a pronounced UV excess as evidenced by U-B. Why thebehavior of S Dor has suddenly changed remains a mystery.
| The OB Zoo: A Digital Atlas of Peculiar Spectra
A digital atlas of 20 high-luminosity, peculiar OB spectra in the3800-4900 Å range is presented. The atlas is organized anddiscussed in terms of the following four categories: WN-A or WNL stars,OB Iape or very late WN (WNVL) stars, iron stars, and B-supergiantluminous blue variables (LBVs). Several objects in the earliercategories are also active or quiescent LBVs. Some (but not all) ofthese objects have been well studied, and extensive references areprovided, as are comprehensive spectral-line identifications. Severalnew morphological relationships among the objects have been recognizedthrough this presentation. In particular, attention is drawn to theoccurrence of spatial pairing between nearly identical, unusual spectra,which may have implications for a particular mode of massive-starformation. This small sample includes one or both members of at leastfive such pairs. Physical explanations of these peculiar, likelytransitional spectra and the relationships among them are essential fora complete understanding of massive stellar evolution.
| 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.
| S Doradus
IAUC 7290 available at Central Bureau for Astronomical Telegrams.
| Light variations of massive stars (alpha Cygni variables). XVII. The LMC supergiants R 74 (LBV), R 78, HD 34664 = S 22 (B[e]/LBV), R 84 and R 116 (LBV?)
Multi-colour photometry (Walraven system) of five super- and hypergiantsin the LMC, viz. R 74, R 78, HD 34664, R 84 and R 116, is searched forvariability and periods, and discussed. Apart from R 84, of which theclaimed variability in the past must be due to a number of faint fieldstars at the edge of the apertures, all are variable. R 74 and HD 34664are weak-active LBVs with superimposed microvariations. HD 34664 is thesecond known B[e] star which is also an LBV. The first reported one is R4 in the SMC. This could alter some views on the evolutionary history ofB[e] stars and LBVs. R 78 is an alpha Cyg variable, but presumably noLBV. R 116 appears to be a close counterpart of the galactic ex-/dormantLBV zeta (1) Sco, also showing an intricate alpha Cyg-type multi-periodmicrovariability. Based on observations obtained at the EuropeanSouthern Observatory at La Silla, Chile}
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Observation and Astrometry data
| Constellation: | Schwertfisch |
| Right ascension: | 05h18m14.35s |
| Declination: | -69°15'01.1" |
| Apparent magnitude: | 9.567 |
| Proper motion RA: | 0.7 |
| Proper motion Dec: | 4.9 |
| B-T magnitude: | 9.66 |
| V-T magnitude: | 9.575 |
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