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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.
| The relation between the most-massive star and its parental star cluster mass
We present a thorough literature study of the most-massive star,mmax, in several young star clusters in order to assesswhether or not star clusters are populated from the stellar initial massfunction (IMF) by random sampling over the mass range 0.01 <= m <=150Msolar without being constrained by the cluster mass,Mecl. The data reveal a partition of the sample into lowestmass objects (Mecl <= 102Msolar),moderate mass clusters (102Msolar
| A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| The open cluster Pismis 11 and the very luminous blue supergiant HD 80077. I. Physical parameters of the cluster
Context: The very luminous blue supergiant HD 80077 has been claimed tobe a member of the young open cluster Pismis 11, and hence a hypergiant.Membership of the cluster would mean that it is one of the brighteststars in the Galaxy, and one of the few evolved very massive stars whosedistance can be accurately determined. Aims: We carry out acomprehensive study of the open cluster Pismis 11, which allows us toderive with accuracy its distance and reddening. Methods: Weobtained UBVRI photometry of the cluster field and low-resolutionspectroscopy of a number of putative members. We derive spectral typesfrom the spectra and determine that the reddening in this direction isstandard. We then carry out a careful photometric analysis that allowsus to determine individual reddening values, deriving unreddenedparameters that are used for the main sequence fit. Results: Weidentify 43 likely members of Pismis 11 and determine individualreddenings. We study the variation of extinction across the face of thecluster and find some dispersion, with a trend to higher values in theimmediate neighbourhood of HD 80077. We estimate a distance of 3.6 kpcfor the cluster. If HD 80077 is a member, it has M_bol < -10.5 and itis one of the three visually brightest stars in the Galaxy. Severalearly type stars in the vicinity of Pismis 11 fit well the clustersequence and are likely to represent an extended population at the samedistance. About 18 arcmin to the North of Pismis 11, we find a smallconcentration of stars, which form a clear sequence. We identify thisgroup as a previously uncatalogued open cluster, which we provisionallycall Alicante 5. The distance to Alicante 5 is also 3.6 kpc, suggestingthat these two clusters and neighbouring early-type stars form a smallassociation. Conclusions: We have identified a small associationaround Pismis 11, located at a distance of 3.6 kpc. Based on its propermotion, HD 80077 is not a runaway star and may be a member of thecluster. If this is the case, it would be one of the brightest stars inthe Galaxy.Partially based on observations collected at the European SouthernObservatory, La Silla, Chile (ESO 0.76D-0037, 0.78D-0176 and 71.D-0151).Tables 2, 3, 5, 6, 8, 9 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/493/79
| CN column densities and excitation temperatures
We analyse abundances and rotational temperatures of the interstellar CNmolecule. We have calculated the column densities and excitationtemperatures of the molecule along 73 lines of sight basing on ouroriginal measurements of the B 2?+ -X2?+ (0,0) vibrational band recorded in highsignal-to-noise ratio spectra and also for 88 directions based onmeasurements already available in literature. We compare the columndensities obtained from different bands of CN molecule available toground-based instruments. The obtained excitation temperatures in theanalysed directions show always an excess over the cosmic microwavebackground radiation (CMBR) temperature.
| Radio study on the mass loss bi-stability jump
By means of radio continuum observations of 29 stars with spectral typesO8-B3 we searched for evidence of the existence of a bi-stability jumpin stellar mass loss around T_{eff} ≈ 21 kK. Although there arequantitative discrepancies, the qualitative behaviour of the windefficiency with effective temperature appears to be in line withtheoretical predictions. Observations of new cases are fundamental toconfirm these results.
| 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.
| A Preliminary Investigation of the Diffuse Interstellar Line at 8621 Å
We have obtained high-resolution spectra at the Dominion AstrophysicalObservatory, the Calar Alto Observatory, and the European SouthernObservatory of hot stars in the near-infrared region to study thediffuse interstellar band (DIB) at 8621 Å. Field stars as well asselected members of the Perseus spiral arm, the ρ Ophiuchi complex,and the Cygnus OB2 association were observed and the equivalent widthsof the band measured, as well as interstellar K I absorption at 7699Å. In total we measure the equivalent width of the 8621 band in 64stars. In a series of figures we show the correlations of the DIB withreddening, polarization, K I, and strengths of other DIBs at 5780, 5797,and 6613 Å. The quality of the correlations are discussed by meansof the Spearman rank correlation test. Good correlations are found withreddening and, among the other DIBs, 8621 correlates best with 5780. Byexamining the three special regions mentioned above we confirm theweakness of the 8621 DIB in the hot stars of the ρ Oph star-formingregion. In the Perseus spiral arm region we find that the DIBs areformed largely in foreground clouds but not in the gas that appears tohave been blown away from the young massive stars of the Perseus arm. InCyg OB2 we find that the equivalent widths of the 8621 line lie abovethe linear correlation of equivalent width with E(B-V) and theirstrength does not vary as strongly with reddening. The heavily reddenedCyg OB2 star 12 is the most extreme example of the relativeinsensitivity of the 8621 equivalent width to reddening in the CygnusOB2 region.
| Galactic distributions and statistics of the HD stars in the michigan spectral catalogue.
Not Available
| Testing the predicted mass-loss bi-stability jump at radio wavelengths
Context: Massive stars play a dominant role in the Universe, but one ofthe main drivers for their evolution, their mass loss, remains poorlyunderstood. Aims: In this study, we test the theoretically predictedmass-loss behaviour as a function of stellar effective temperatureacross the so-called “bi-stability” jump. Methods: Weobserve OB supergiants in the spectral range O8-B3 at radio wavelengthsto measure their thermal radio flux densities, and complement thesemeasurements with data from the literature. We derive the radiomass-loss rates and wind efficiencies, and compare our results withHα mass-loss rates and predictions based on radiation-driven windmodels. Results: The wind efficiency shows the possible presence of alocal maximum around an effective temperature of 21 000 K - inqualitative agreement with predictions. Furthermore, we find that theabsolute values of the radio mass-loss rates show good agreement withempirical Hα rates derived assuming homogeneous winds - for thespectral range under consideration. However, the empirical mass-lossrates are larger (by a factor of a few) than the predicted rates fromradiation-driven wind theory for objects above the bi-stability jump(BSJ) temperature, whilst they are smaller (by a factor of a few) forstars below the BSJ temperature. The reason for these discrepanciesremains as yet unresolved. A new wind momenta-luminosity relation (WLR)for O8-B0 stars has been derived using the radio observations. Thevalidity of the WLR as a function of the fitting parameter related tothe force multiplier α_eff (Kudritzki & Puls, 2000, ARA&A,629) is discussed. Conclusions: Our most interesting finding is thatthe qualitative behaviour of the empirical wind efficiencies witheffective temperature is in line with the predicted behaviour, and thispresents the first hint of empirical evidence for the predictedmass-loss bi-stability jump. However, a larger sample of stars aroundthe BSJ needs to be observed to confirm this finding.
| Spectral atlas of massive stars around He I 10 830 Å
We present a digital atlas of peculiar, high-luminosity massive stars inthe near-infrared region (10 470-11 000 Å) at medium resolution(R≃7000). The spectra are centered around He I 10 830 Å,which is formed in the wind of those stars, and is a crucial line toobtain their physical parameters. The instrumental configuration alsosampled a rich variety of emission lines of Fe II, Mg II, C I, N I, andPa γ. Secure identifications for most spectral lines are given,based on synthetic atmosphere models calculated by our group. We alsopropose that two unidentified absorption features have interstellarand/or circumstellar origin. For the strongest one (10 780 Å) anempirical calibration between E(B-V) and equivalent width is provided.The atlas displays the spectra of massive stars organized in fourcategories, namely Be stars, OBA Iape (or luminous blue variables, LBVcandidates and ex/dormant LBVs), OB supergiants and Wolf-Rayet stars.For comparison, the photospheric spectra of non emission-line stars arepresented. Selected LBVs were observed in different epochs from 2001 to2004, and their spectral variability reveals that some stars, such asη Car, AG Car and HR Car, suffered dramatic spectroscopic changesduring this time interval.Based on observations made at Observatório do Pico dos Dias/LNA(Brazil). Figures 5 to 18 are only available in electronic form athttp://www.aanda.org Electronic version of the spectra (fichiers FITS)is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/993
| Arcsecond-resolution 12CO mapping of the yellow hypergiants IRC +10420 and AFGL 2343
Context: IRC +10420 and AFGL 2343 are the unique, known yellowhypergiants (YHGs) presenting a heavy circumstellar envelope (CSE). Aims: We aim to study the morphology, exceptional kinematics, andexcitation conditions of their CSEs, and the implications for mass-lossprocesses. Methods: We have mapped the 12CO J=2-1 and 1-0emission in these YHGs with the IRAM Plateau de Bure interferometer andthe 30 m telescope. We developed LVG models in order to analyze theircircumstellar characteristics. Results: The maps show that the overallshape of both CSEs is approximately spherical, although they also revealseveral aspherical features. The CSE around IRC +10420 shows a roundedextended halo surrounding a bright inner region, with both componentspresenting aspherical characteristics. It presents a brightness minimumat the center. The envelope around AFGL 2343 is a detached shell,showing spherical symmetry and clumpiness at a level of ~15% of themaximum brightness. The envelopes expand isotropically at ~35 kms-1, about two or three times faster than typical CSEs aroundAGB stars. High temperatures (~200 K) are derived for the innermostregions in IRC +10420, while denser and cooler (~30 K) gas is found inAFGL 2343. Conclusions: .The mass-loss processes in these YHGs havebeen found to be similar. The deduced mass-loss rates(~10-4-10-3 Mȯ yr-1)are much higher than those obtained in AGB stars, and they presentsignificant variations on time scales of ~1000 yr.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany),and IGN (Spain).
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| On the population of galactic Luminous Blue Variables
We report the first results of a long term infrared monitoring campaignof known and candidate galactic Luminous Blue Variables (LBVs). Inparticular, we are able to confirm the LBV nature ofG24.73+0.69, a luminous mid-B supergiant associatedwith a dusty ejection nebula. We find that prior to 2003 SeptemberG24.73+0.69 exhibited low amplitude (Δ JHK 0.4 mag) variability, but in the ~200 day period between 2003September-2004 April it abruptly brightened by ~0.7 mag in the broadbandJ filter. Subsequently, a further ~0.4 mag increase was observed between2004 April-October, resulting in an overall difference of ~1.1 magbetween (current) photometric mimimum and maximum; similar variabilityalso being observed in the H and K bands. In light of the numerousrecent IR studies of the galactic hot star population we also compile anupdated census of confirmed and candidate galactic LBVs, reporting 12and 23 members respectively for each class. Finally, we utilise this newcensus to construct an H-R diagram for the galactic LBV population,resulting in a striking confirmation of the LBV-minimum light strip.
| On the massive stellar population of the super star cluster Westerlund 1
We present new spectroscopic and photometric observations of the youngGalactic open cluster Westerlund 1 (Wd 1) that reveala unique population of massive evolved stars. We identify ~200 clustermembers and present spectroscopic classifications for ~25% of these. Wefind that all stars so classified are unambiguously post-Main Sequenceobjects, consistent with an apparent lack of an identifiable MainSequence in our photometric data to V 20. We are able to identifyrich populations of Wolf Rayet stars, OB supergiants and short livedtransitional objects. Of these, the latter group consists of both hot(Luminous Blue Variable and extreme B supergiant) and cool (YellowHypergiant and Red Supergiant) objects - we find that half the knownGalactic population of YHGs resides within Wd 1. We obtain a meanV-MV ~ 25 mag from the cluster Yellow Hypergiants, implying aMain Sequence turnoff at or below MV =-5 (O7 V or later).Based solely on the masses inferred for the 53 spectroscopicallyclassified stars, we determine an absolute minimum mass of ~1.5 ×10^3~Mȯ for Wd 1. However, considering the completephotometrically and spectroscopically selected cluster population andadopting a Kroupa IMF we infer a likely mass for Wd 1 of~10^5~Mȯ, noting that inevitable source confusion andincompleteness are likely to render this an underestimate. As such, Wd 1is the most massive compact young cluster yet identified in the LocalGroup, with a mass exceeding that of Galactic Centre clusters such asthe Arches and Quintuplet. Indeed, the luminosity, inferred mass andcompact nature of Wd 1 are comparable with those of Super Star Clusters- previously identified only in external galaxies - and is consistentwith expectations for a Globular Cluster progenitor.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| 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.
| 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.
| 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
| On the identification of the C60+ interstellar features
The identity of the carriers of the diffuse interstellar bands (DIBs) isone of the most fascinating puzzles of modern spectroscopy. Over thelast few years the number of known DIBs has grown substantially. In thispaper we discuss the two recently discovered near-infrared weakinterstellar features which have already been proposed as fingerprintsof the buckminsterfullerene [formmu2] We present and discussmeasurements of the two related DIBs within a larger sample of reddenedtargets, observed with different spectrometers, telescopes and siteconditions. We provide additional arguments in favour of theinterstellar origin of the two bands. We find evidence around the9577-Å DIB of far-wing structures, which may affect broad-bandmeasurements. We estimate corrections and errors for telluric andstellar blends, and show that the cores of the two DIBs are wellcorrelated with a ratio near unity within 20per cent. Finally, wediscuss their relation to the laboratory spectra of [formmu3] and thesearch for two expected weaker [formmu4] transitions.
| The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way
The picture of the young stellar groups in the Canis Major-Puppis-Vela(215 deg
| Strömgren and Hβ photometry of O and B type stars in star-forming regions. I. Canis Major - Puppis - Vela
Strömgren and Hβ photometry of OB-stars generally brighterthan 9.5 mag in the Canis Major - Puppis - Vela region of Milky Way isreported. The observations are based on the Milky Way luminous-star (LS)identifications and are designed to create a complete, magnitude-limitedsample of LS for this field. We present new uvby photometry for 127 LSand Hβ photometry for 25 of them. These observations are part of anongoing effort to improve the completeness of the existing uvbybetadata-base for the bright OB-type stars in the Milky Way, with the aim toinvestigate the structure of selected star-forming regions. Based ondata from the Strömgren Automatic Telescope of the CopenhagenAstronomical Observatory, La Silla. Tables 3 and 4 are only available inelectronic form at the CDS via anonymous ftp to 130.79.128.5 or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Interstellar C_2 absorption lines towards CH+ forming regions
Interstellar C_2 absorption line observations towards the southern OBassociations NGC 2439, Vela OB1, and Cen OB1 are used to infergaskinetic temperatures Tkin and densities nctowards lines of sight with previously determined large CH andCH+ column densities. Towards NGC 2439, the material ischaracterised by temperatures of Tkin = 75-85 K and densitiesexceeding nc > 1000 C_2m, and a fractional C_2 abundanceof about x(C_2) = 1.5 10-8. Temperatures and densitiesinferred towards two stars in Vela OB1 are Tkin = 65-85 K andnc >= 600 C_2m. C_2 fractional abundances in the cloudcomplex obscuring Vela OB1 are about half the value found in NGC 2439.For the lines of sight towards Cen OB1, values of Tkin =70-95 K and fractional abundances of x(C_2) ~ 5 10-9 arefound, and towards HD 114213, Tkin = 25 K and nc =400+/-100 C_2m. The observations demonstrate that C_2 resides in coolgas at temperature of Tkin < 100 K. The C_2 columndensities are correlated with those of CH, which is expected fromtheoretical models. This suggests that CH is efficiently formed in thecool material as well, together with C_2. This finding does not supportexpectations from recent MHD and vortex models of CH+formation, which stipulate that neutrals such as CH are abundantlyproduced in hot regions of many 100 K temperature.
| The Brightest Supergiants Predicted by Theory
The distribution of the brightest Galactic supergiants on theHertzsprung-Russell diagram is studied here with the help of newtheoretical evolutionary tracks for very massive stars. The shapes ofthe new tracks are determined largely by stellar wind mass loss duringthe main-sequence phase. In contrast to earlier theoretical work, thetracks for stars with the highest initial masses point down the mainsequence and converge to essentially the same unique end product, whichthen evolves toward the region of red supergiants. Enhanced mass lossdue to ionization-induced dynamical instability in the star's outerenvelope eventually terminates the redward movement. In these two ways,mass loss imposes upper limits on the predicted luminosities for extremeOB supergiants, yellow hypergiants, and red supergiants. By adoptingstandard mass-loss rates for main-sequence stars, the new evolutionarytracks are found to account satisfactorily for the empirical supergiantluminosity limits determined by Humphreys & Davidson and by deJager. Success is somewhat more mixed when main-sequence mass-loss rates3 times larger (as suggested by some recent observational studies) areadopted.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| 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.
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