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Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope
We have analyzed Hubble Space Telescope ACS and WFPC2 new and archivalimages of eight H II regions to look for new protoplanetary disks(proplyds) similar to those found in the Orion Nebula. We find a wealthof features similar in size (although many are larger) to the brightcusps around the Orion Nebula proplyds. None of them, however, containsa definitive central star. From this, we deduce that the new cusps maynot be proplyds but instead fragments of molecular cloud material. Outof all the features found in the eight H II regions examined, only one,an apparent edge-on silhouette in M17, may have a central star. Thisfeature might join the small number of bona fide proplyds found outsidethe Orion Nebula, in M8, M20, and possibly M16. In line with the resultsfound recently by Smith et al., the paucity of proplyds outside theOrion Nebula can be explained by their transient nature, as well as bythe specific environmental conditions under which they can be observed.Several fragments are seen as dark silhouettes against a brightbackground. We have reanalyzed those found in IC 2944 by Reipurth et al.and found new, similar ones in M16. None of these fragments contains acentral star, and we exclude the possibility that they are disks.Reipurth et al. concluded that the IC 2944 silhouettes are not starforming. We argue here that their assumption of a constant optical depthfor these fragments is not physical and that it is more likely thatthese fragments are star forming, a condition that is supported,although not proved, by their shapes and distributions. The process ofcloud fragmentation and photoevaporation produces a large number ofsmall fragments, while the size hierarchy expected in a photoevaporativeenvironment would not favor small fragments. The size distributionsobserved will constrain any future theories of cloud fragmentation. Onebright microjet candidate is found in M17, protruding from a large,limb-brightened fragment. A second, larger, jetlike feature, similar inshape and size to a Herbig-Haro jet, is found in Pismis 24. No centralstar appears to be associated with either of these jet candidates.Based on observations made with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| Quantitative spectroscopy of BA-type supergiants
Luminous BA-type supergiants have enormous potential for modernastrophysics. They allow topics ranging from non-LTE physics and theevolution of massive stars to the chemical evolution of galaxies andcosmology to be addressed. A hybrid non-LTE technique for thequantitative spectroscopy of these stars is discussed. Thorough testsand first applications of the spectrum synthesis method are presentedfor the bright Galactic objects η Leo (A0 Ib), HD 111613 (A2 Iabe),HD 92207 (A0 Iae) and β Ori (B8 Iae), based on high-resolution andhigh-S/N Echelle spectra. Stellar parameters are derived fromspectroscopic indicators, consistently from multiple non-LTE ionizationequilibria and Stark-broadened hydrogen line profiles, and they areverified by spectrophotometry. The internal accuracy of the methodallows the 1σ-uncertainties to be reduced to 1-2% in T_effand to 0.05-0.10 dex in log g. Elemental abundances are determined forover 20 chemical species, with many of the astrophysically mostinteresting in non-LTE (H, He, C, N, O, Mg, S, Ti, Fe). The non-LTEcomputations reduce random errors and remove systematic trends in theanalysis. Inappropriate LTE analyses tend to systematicallyunderestimate iron group abundances and overestimate the light andα-process element abundances by up to factors of two to three onthe mean. This is because of the different responses of these species toradiative and collisional processes in the microscopic picture, which isexplained by fundamental differences of their detailed atomic structure,and not taken into account in LTE. Contrary to common assumptions,significant non-LTE abundance corrections of ~0.3 dex can be found evenfor the weakest lines (Wλ 10 mÅ). Non-LTEabundance uncertainties amount to typically 0.05-0.10 dex (random) and~0.10 dex (systematic 1σ-errors). Near-solar abundances arederived for the heavier elements in the sample stars, and patternsindicative of mixing with nuclear-processed matter for the lightelements. These imply a blue-loop scenario for η Leo because offirst dredge-up abundance ratios, while the other three objects appearto have evolved directly from the main sequence. In the most ambitiouscomputations several ten-thousand spectral lines are accounted for inthe spectrum synthesis, permitting the accurate reproduction of theentire observed spectra from the visual to near-IR. This prerequisitefor the quantitative interpretation of intermediate-resolution spectraopens up BA-type supergiants as versatile tools for extragalacticstellar astronomy beyond the Local Group. The technique presented hereis also well suited to improve quantitative analyses of less extremestars of similar spectral types.
| Spectral Analysis of 4 Lacertae and ν Cephei
Elemental abundance analysis based on high S/N and high resolutionDominion Astrophysical Observatory spectrograms have been performed fortwo early type supergiants: 4 Lac (B9 Iab) and {\msf ν } Cep (A2 Ia).Lines as weak as of order 5 mÅ are employed in this study. Theprojected rotational velocities of these stars are 14 and 26 kms-1, respectively. Both stars show similar radial velocityamplitudes, macroturbulent velocities and the same general elementalabundance trends. Their He, CNO and light element abundances are solaror overabundant while the iron peak and heavy element abundances aresolar or underabundant. Detailed LTE model atmosphere abundance analysisshows that 4 Lac has nuclearly processed matter in its photosphere while{\msf ν } Cep does not.
| Bright OB stars in the Galaxy. II. Wind variability in O supergiants as traced by Hα
We investigate the line-profile variability (lpv) of Hα for alarge sample of O-type supergiants (15 objects between O4 and O9.7), inan objective, statistically rigorous manner. We employed the TemporalVariance Spectrum (TVS) analysis, developed for the case of photosphericabsorption lines and modified by us to take into account the effects ofwind emission. By means of a comparative analysis we place constraintson the properties of this variability - quantified in terms of a meanand a newly defined fractional amplitude of deviations - as a functionof stellar and wind parameters. The results of our analysis show thatall the stars in the sample show evidence of significant lpv inHα, mostly dominated by processes in the wind. The variationsoccur between zero and 0.3 v_&infy; (i.e., below 1.5 R_star ), in goodagreement with results from similar studies. A comparison between theobservations and corresponding line-profile simulations indicates thatfor stars with intermediate wind densities the properties of theHα variability can be explained by simple models consisting ofcoherent or broken shells (blobs) uniformly distributed over the windvolume, with an intrinsic scatter in the maximum density contrast ofabout a factor of two. For stars at lower and higher wind densities, onthe other hand, we found certain inconsistencies between theobservations and our predictions, most importantly concerning the meanamplitude and the symmetry properties of the TVS. This disagreementmight be explained by the presence of coherent large-scale structures,partly confined in a volume close to the star. Interpreted in terms of avariable mass-loss rate, the observed variations of Hα indicatechanges of ±4% with respect to the mean value of dot M for starswith stronger winds and of ± 16% for stars with weaker winds. Theeffect of these variations on the corresponding wind momenta is ratherinsignificant (less than 0.16 dex), increasing only the local scatterwithout affecting the Wind Momentum Luminosity Relationship.
| 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.
| HD 183143: A Hypergiant
We present spectroscopic evidence that the luminosity of HD 183143 ishigher by one magnitude than thought previously. The star is yet anotherB6-8 Ia-0 white hypergiant of the Galaxy. Its absolute visual magnitudeis close to -8 mag, and its distance is close to 2 kpc. We describespectroscopic manifestations of the nonstationary behavior of itsatmosphere and wind.
| Quantitative Spectroscopy of Supergiants
Blue supergiants of spectral types B and A are the visually brighteststars in spiral and irregular galaxies, with their most luminous members(at M_V=-10) outshining entire dwarf galaxies. This characteristicallows us to use them as probes to study the Local Universe in greatdetail. In principle, already the existing large telescopes andinstrumentation facilitate quantitative spectroscopy of these objects asfar as the Virgo and Fornax clusters of galaxies. Beyond theirchallenging stellar atmospheres and opportunities for testingsophisticated non-LTE physics they offer numerous applications to modernastrophysics. Quantitative spectroscopy of supergiants can contribute toimprove our understanding of massive star evolution. Galactic abundancegradients and abundance patterns, as can be obtained from studies oflarge ensembles of supergiants in our own and other galaxies, willfoster the understanding of galactochemical evolution. Finally, they arepromising independent indicators for calibrating the extragalacticdistance scale, by application of the wind momentum-luminosity and theflux-weighted gravity-luminosity relationships. In view of this largepotential, the objective of this thesis is to improve the status ofquantitative spectroscopy of BA-type supergiants and to provide firstapplications on a sample of Galactic and extragalactic targets, withinthe Local Group and beyond.
| 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).
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Non-LTE line formation for N: Abundances and stellar parameters. Model atom and first results on BA-type stars
A comprehensive model atom for non-LTE line formation calculations forneutral and singly-ionized nitrogen is presented. Highly accurateradiative and collisional atomic data are incorporated, recentlydetermined for astrophysical and fusion research using the R-matrixmethod in the close-coupling approximation. As a test and firstapplication of the model, nitrogen abundances are determined on thebasis of line-blanketed LTE model atmospheres for five stars, the mainsequence object Vega (A0 V) and the supergiantseta Leo (A0 Ib), HD 111613 (A2Iabe), HD 92207 (A0 Iae) and betaOri (B8 Iae), using high S/N and high-resolution spectra atvisual and near-IR wavelengths. The computed non-LTE line profiles fitthe observations excellently for a given nitrogen abundance in eachobject. Moreover, the ionization equilibrium of \ion{N}{i/ii} proves tobe a sensitive temperature indicator for late B-type and early A-typesupergiants - even at low metallicities - due to the apparent nitrogenoverabundance in these objects. All supergiants within our sample showan enrichment of nitrogen on the order of ~ 0.3-0.6 dex, indicating themixing of CN-cycled material into atmospheric layers, with the sum ofthe CNO abundances staying close to solar. This finding is in accordancewith recent stellar evolution models accounting for mass-loss androtation. For Vega, an underabundance of nitrogen by 0.25 dex is found,in good agreement with the similar underabundance of other lightelements. The dependence of the non-LTE effects on the atmosphericparameters is discussed with special emphasis on the supergiants where astrong radiation field at low particle densities favours deviations fromLTE. Non-LTE effects systematically strengthen the \ion{N}{i/ii} lines.For some N I lines in supergiants non-LTE abundance corrections inexcess of 1 dex are found and they react sensitively to modifications ofthe collisional excitation data. The influence of microturbulence on thestatistical-equilibrium calculations is also investigated: theline-strengths of the strong N I features show some sensitivity due tomodifications of the line-formation depths and the departurecoefficients, while the - in this parameter range - weak N Ii linesremain unaffected. Based on observations collected at the EuropeanSouthern Observatory, Chile (ESO Ndeg 62.H-0176).
| Non-LTE line-formation for neutral and singly-ionized carbon. Model atom and first results on BA-type stars
A comprehensive model atom for non-LTE line-formation calculations forneutral and singly-ionized carbon is presented. Highly accurateradiative and collisional atomic data are incorporated, recentlydetermined for astrophysical and fusion research using the R-matrixmethod in the close-coupling approximation. As a test and firstapplication of the model, carbon abundances are determined on the basisof line-blanketed LTE model atmospheres for five stars, the mainsequence object Vega (A0 V) and the supergiantseta Leo (A0 Ib), HD 111613 (A2Iabe), HD 92207 (A0 Iae) and betaOri (B8 Iae), using high S/N and high-resolution spectra atvisual and near-IR wavelengths. The computed non-LTE line profiles fitthe observations well for a single carbon abundance in each object. Fortwo supergiants, eta Leo and HD 111613, lines of both species aresimultaneously present in the spectra, giving consistent C I and C Iiabundances (within the error bars). However, the uncertainties of theabundances are large, on the order of ~ 0.3 dex(statistical+systematical error), thus the ionization equilibrium of\ion{C}{i/ii} is of restricted use for the determination of stellarparameters. All supergiants within our sample show a depletion of carbonon the order of 0.2-0.5 dex, indicating the mixing of CN-cycled materialinto the atmospheric layers, with the sum of the CNO abundancesremaining close to solar. This finding is in accordance with recentstellar evolution models accounting for mass-loss and rotation. ForVega, an underabundance of carbon by 0.3 dex is found, in excellentagreement with the similar underabundance of other light elements. Thedependence of the non-LTE effects on the atmospheric parameters isdiscussed and the influence of systematic errors is estimated. Specialemphasis is given to the supergiants where a strong radiation field atlow particle densities favours deviations from LTE. Non-LTE effectssystematically strengthen the \ion{C}{i/ii} lines. For the C I lines inthe infrared, a strong sensitivity to modifications in thephotoionization and collisional excitation data is found. An increasingdiscrepancy between our model predictions and the observations for the CIi doublet lambda lambda 6578-82 is perceived with rising luminosity,while the other C Ii doublet and quartet lines remain consistent.Furthermore, the influence of microturbulence on thestatistical-equilibrium calculations is investigated. Based onobservations collected at the European Southern Observatory, Chile (ESONdeg 62.H-0176).
| Star clusterings in the Carina complex: vec {UBVRI} photometry of NGC 3324 and Loden 165
We report on UBVRI photometry of two 5' x 5'fields in the region of the young open cluster NGC 3324. One of ourfields covers the core region, while the other is closer to the tidalradius of the cluster. Our study provides the first CCD photometry ofNGC 3324. We find that the cluster is very young and probably containsseveral pre Main Sequence (MS) stars. 25 members are identified on thebasis of their position in the (U-B) vs. (B-V) diagram. We investigatethe relation of the red super-giant HD 92207 with NGC 3324, suggestingthat it probably does not belong to the cluster. Our second field isclose to Loden 165, a possible cluster of stars that has never beenstudied so far. We show that this object is a probable open cluster,much older than NGC 3324 and much closer to the Sun. Based onobservations carried out at ESO La Silla. All the data are available atWEBDA http://obswww.unige.ch/webda/navigation.html.
| Non-LTE line formation for \ion{Mg}{I/II}: Abundances and stellar parameters. Model atom and first results on A-type stars
An extensive model atom for non-LTE line-formation calculations forneutral and singly-ionized magnesium is presented, taking into accountrecent improvements in the atomic data. As a test and first applicationof the model, stellar parameters and magnesium abundances are determinedon the basis of line-blanketed LTE model atmospheres for three stars:Vega (A0 V), eta Leo (A0 Ib) and HD 92207 (A0 Iae) using high S/N andhigh resolution spectra at visual and near-IR wavelengths. Theionization equilibrium of \ion{Mg}{i/ii} proves to be a sensibletemperature indicator for early A-type stars at all luminosities.Evidence is given that in late A and early F-type supergiants (T_effla \8000 K) the determination of accurate stellar parameters is hampered bythe presence of a pressure inversion region in the model atmospheres atline-formation depths. The \ion{Mg}{i/ii} lines in the observations arereproduced simultaneously by the calculated line profiles with highccuracy. For Vega spectral synthesis in the UV region of the\ion{Mg}{i/ii} resonance lines also proves excellent consistency withthe results from the visual. The dependence of the non-LTE effects onthe atmospheric parameters is discussed with special emphasis on thesupergiants where a strong radiation field at low particle densitiesfavours deviations from LTE, especially in the minor ionic species ofneutral magnesium. Non-LTE corrections turn out to be small in Mg I -typically la 0.3 dex - even in supergiants, but are essential for anaccurate effective temperature determination. From the Mg Ii spectrum,only the features at lambda lambda 4481 and 7877-96 Å reactsensitively to non-LTE effects. Furthermore, the influence ofmicroturbulence on the statistical-equilibrium calculations isinvestigated. The line strengths are found to be systematicallyaffected. Based on observations collected at the European SouthernObservatory, La Silla, Chile (ESO No 62.H-0176).
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| Winds from Hot Stars
This review deals with the winds from "normal" hot stars such asO-stars, B- and A-supergiants, and Central Stars of Planetary Nebulaewith O-type spectra. The advanced diagnostic methods of stellar winds,including an assessment of the accuracy of the determinations of globalstellar wind parameters (terminal velocities, mass-loss rates, windmomenta, and energies), are introduced and scaling relations as afunction of stellar parameters are provided. Observational results areinterpreted in the framework of the stationary, one-dimensional (1-D)theory of line-driven winds. Systematic effects caused by nonhomogeneousstructures, time dependence, and deviations from spherical symmetry arediscussed. The review finishes with a brief description of the role ofstellar winds as extragalactic distance indicators and as tracers of thechemical composition of galaxies at high redshift.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Non-LTE line formation for neutral oxygen. Model atom and first results on A-type stars
An extensive model atom for non-LTE line formation calculations for O Iis presented, taking into account recent improvements in the atomicdata. Based on line-blanketed LTE model atmospheres equivalent widthsare computed in LTE and non-LTE for the diagnostic O I lines of A- andlate B-type stars in the range Teff = 7500 K to 15000 K andluminosity classes V to Ia. Non-LTE abundance corrections are provided:they span a wide range in magnitude, from less than 0.1 dex for the weaklines in main sequence stars to more than 1.5 dex for the near-infraredlines in some supergiants. The dependence of the non-LTE effects on theatmospheric parameters is discussed with special emphasis onsupergiants. In particular, the near-infrared transitions are found toreact sensitively to the collisional excitation cross sections used inthe calculations. Further investigations concentrate on the influence ofmicroturbulence and on the rôle of wind outflow velocity fields onthe line formation. As a test and first application of the model, oxygenabundances for Vega (A0 V), etaLeo (A0 Ib) and HD 92207 (A0 Iae) arederived. The analysis of Vega confirms a slight oxygen underabundance (~ 0.3 dex) in this star while eta Leo and HD 92207 show a nearly solarvalue. For Vega the observed spectrum can be reproduced accurately bythe calculated line profiles. In supergiants consistent abundances canbe derived from the weak lines in the visible. At high luminosities theprominent near-infrared features - among other strong lines fromdifferent elemental species - are subject to additional broadening by anunidentified process which prevents an equally accurate theoreticalinterpretation. Based on observations collected at the European SouthernObservatory, Chile (ESO No 62.H-0176)
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| The wind momentum-luminosity relationship of galactic A- and B-supergiants
The Balmer lines of four A Ia-supergiants (spectral type A0 to A3) andfourteen B Ia and Ib-supergiants (spectral type B0 to B3) in the solarneighbourhood are analyzed by means of NLTE unified model atmospheres todetermine the properties of their stellar winds, in particular theirwind momenta. As in previous work for O-stars (Puls et al. \cite{pul96})a tight relationship between stellar wind momentum and luminosity(``WLR'') is found. However, the WLR varies as function of spectraltype. Wind momenta are strongest for O-supergiants, then decrease fromearly B (B0 and B1) to mid B (B1.5 to B3) spectral types and becomestronger again for A-supergiants. The slope of the WLR appears to besteeper for A- and mid B-supergiants than for O-supergiants. Thespectral type dependence is interpreted as an effect of ionizationchanging the effective number and the line strength distributionfunction of spectral lines absorbing photon momentum around the stellarflux maximum. This interpretation needs to be confirmed by theoreticalcalculations for radiation driven winds. The ``Pistol-Star'' in theGalactic Centre, an extreme mid B-hypergiant recently identified as oneof the most luminous stars (Figer et al. \cite{fig99}) is found tocoincide with the extrapolation of the mid B-supergiant WLR towardshigher luminosities. However, the wind momentum of the Luminous BlueVariable P Cygni, a mid B-supergiant with extremely strong mass-loss, is1.2 dex higher than the WLR of the ``normal'' supergiants. Thissignificant difference is explained in terms of the well-known stellarwind bi-stability of supergiants very close to the Eddinton-limit inthis particular range of effective temperatures. A-supergiants in M31observed with HIRES at the Keck telescope have wind momenta compatiblewith their galactic counterparts. The potential of the WLR as a new,independent extragalactic distance indicator is discussed. It isconcluded that with ten to twenty objects, photometry with HST andmedium resolution spectroscopy with 8m-telescopes from the grounddistance moduli can be obtained with an accuracy of about 0fm1 out tothe Virgo and Fornax clusters of galaxies.
| Stromgren and Hβ photometry of OB stars in the region of the Carina Spiral Feature
Stromgren and Hβ photometry of 130 stars in the Carina section ofthe Milky Way is presented. The color excesses and the distances of allstars are obtained. A good agreement is found between the CM and HRdiagrams and the spatial distribution of these stars. Tables 1, 2, 4 and5 are only available in electronic form at the CDS via anonymous ftp to130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
| 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 inhomogeneous circumstellar envelope of Rigel (beta Orionis A)
We report on time series Hα profiles of the late B-type supergiantRigel (beta Orionis A). Our observations indicate the presence ofvariable and asymmetric outflows and infalls of matter. We have recordeda blueshifted high-velocity absorption component in Hα almostsimultaneously with Kaufer et al. The final part of this extraordinaryevent has been observed. We have found that blueshifted high-velocityabsorption completely disappeared in one month, and, during another,redshifted strong absorption appeared at 50kms^-1. Based on ourobservations and those reported in the literature, we propose extendedrotating magnetic structures (closed loops) in the mantle of Rigel. Thefundamental parameters of the star are discussed.
| On the Variability of Early A-Type Supergiants
An examination of the Hipparcos photometry of 26 bright early A-typesupergiants shows that they are all variable.
| Long-term spectroscopic monitoring of BA-type supergiants. III. Variability of photospheric lines.
We obtained time series of spectra with high S/N and high resolution inwavelength and time of early-type A and late-type B supergiants (cf.Kaufer et al. 1996A&A...305..887K, Paper I, and Kaufer et al.1996A&A...314..599K, Paper II for the analysis of the variability ofthe stellar envelopes). In this work we inspect the time variations ofthe numerous photospheric line profiles in the optical spectrum. We findcomplex cyclical variations of the radial velocities with a typicalvelocity dispersion of σ=~3km/s. The correspondingequivalent-width variations are less than 1% of their mean if we assumea common modulation mechanism for both radial velocities and equivalentwidth. We do not find any depth dependence of the velocity fields in themetallic lines. For αCyg the Balmer lines show an increase of theradial velocity from H27 to H8 by 3km/s, which is identified with theonset of the radially accelerating velocity field of the stellar wind.The Cleaned periodograms of the radial-velocity curves show thesimultaneous excitation of multiple pulsation modes with periods longerand shorter than the estimated radial fundamental periods of theobjects, which might indicate the excitation of non-radial and radialovertones, respectively. The analysis of the line-profile variations(LPV) of the photospheric line spectrum reveals prograde travellingfeatures in the dynamical spectra. The travelling times of thesefeatures are in contradiction to the possible rotation periods of theseextended, slowly rotating objects. Therefore, we suggest that thesefeatures should be identified with non-radial pulsation modes, possiblyg-modes, of low order (l=|m|<~5).
| Radiation driven wind models for A, F and G supergiants.
We investigate the effects of radiation pressure on the atmospheres ofA, F and G-supergiants by calculating hydrodynamical model atmospheresfor stars with 5500<=T_eff_<=9500K. In the subsonic part of thewind, the radiation pressure by continuum and lines from Kurucz (1992,ATLAS 6 program) is taken into account. In the supersonic part of thewind, the radiation pressure is expressed in terms of the forcemultiplier formalism (Castor et al. 1975ApJ...195..157C) with thecorrection for the finite disk taken into account. The temperaturestructure is from the T(τ) relation of blanketed model atmospheres.The predicted mass loss rates of the A-supergiants agrees excellentlywith the observed values. However the predicted terminal velocities areabout a factor 3 higher than observed. We discuss several possiblecauses for this discrepancy. The most likely one is a change in theforce multiplier parameter α of the line radiation force fromabout 0.5 in the lower parts of the wind to a much smaller value ofabout 0.1 throughout most of the wind. This might be the result of achange in the ionization of the wind with distance, or a decoupling ofthe line driven ions in the wind from the ambient gas. The predictedmass loss rate of the G-type supergiant 22Vul, which is the onlyG-supergiant with a reliable mass loss rate, is a factor 10^5^ smallerthan observed. This is probably due to the fact that G-supergiants havechromospheres, which were not taken into account in our model. Ourmodels for F-supergiants could not be compared with observations becausethere are no reliable empirical mass loss rates or terminal windvelocities for normal F-supergiants. The F-supergiants ρCas andHR8752 have highly variable mass loss rates which obviously cannot beexplained by our models. We conclude that mass loss from A-typesupergiants is most likely due to a line driven wind but that the massloss from G-supergiants is not. It is interesting to find the spectraltype between F0 and G3 where the radiation driven wind models break downand to compare that with the type where the chromospheres becomenoticeable. The high opacity in the hydrogen ionization zone produces anet outward force in those layers. This gives rise to a pressureinversion in the subsonic part of the atmosphere, but does not lead tohigh mass loss rates.
| Peculiar motions of OB associations and their associated molecular clouds.
Not Available
| Astrometric positions of stars associated with nebulosities in the southern hemisphere.
Several stars associated with nebulosities and cited by S. van den Berghand W. Herbst are included in the preliminary programme for theHIPPARCOS mission. When performing preparatory measurements of plates,we encountered difficulties in identifying certain of these objects whenrelying only on coordinates, which led us to take advantage of thiswork, which relates to the southern sky, in order to determine theastrometric position of all the objects in the list, to within 0.35''.This catalogue is available from the Centre for Astronomic Data atStrasbourg.
| Long-term spectroscopic monitoring of BA-type supergiants. II. High-velocity absorptions in βOri and HD96919.
During our extended monitoring campaigns on late B and early A-typesupergiants (Kaufer et al. 1996A&A...305..887K, Paper I) we haveobserved extraordinarily deep and highly blue-shifted absorption eventsin the Hα line. In this work, new time-series observations showingthe most extreme cases of such events observed so far are presented fortwo objects, βOri (B8Ia) and HD96919 (B9Ia). The development ofthese high-velocity absorption (HVA) events in velocity and time arediscussed: the HVAs show no signs of spherically symmetric mass-lossevents with subsequent accelerated propagation into the wind. Theabsence of unshifted line emission in connection with the HVAs isespecially indicative of the non-sphericity of the active circumstellarregions. Simultaneously with the blue-shifted absorption, red-shiftedabsorption is found in Balmer and metallic lines, primarily during theonset of the developing event, which clearly reveals the complexstructure of the involved velocity fields. Mass outflow and mass infallare present in the envelope. As a picture for the circumstellarstructures that cause the sudden appearance of the HVAs over a largevelocity range, localized regions of enhanced mass loss on the stellarsurface, which build up extended, rotating streak lines in theequatorial plane are suggested. Finally, the role of a criticalionization structure in the condensed structures is discussed.
| Derivation of the Galactic rotation curve using space velocities
We present rotation curves of the Galaxy based on the space-velocitiesof 197 OB stars and 144 classical cepheids, respectively, which rangeover a galactocentric distance interval of about 6 to 12kpc. Nosignificant differences between these rotation curves and rotationcurves based solely on radial velocities assuming circular rotation arefound. We derive an angular velocity of the LSR of{OMEGA}_0_=5.5+/-0.4mas/a (OB stars) and {OMEGA}_0_=5.4+/-0.5mas/a(cepheids), which is in agreement with the IAU 1985 value of{OMEGA}_0_=5.5mas/a. If we correct for probable rotations of the FK5system, the corresponding angular velocities are {OMEGA}_0_=6.0mas/a (OBstars) and {OMEGA}_0_=6.2mas/a (cepheids). These values agree betterwith the value of {OMEGA}_0_=6.4mas/a derived from the VLA measurementof the proper motion of SgrA^*^.
| Long-term spectroscopic monitoring of BA-type supergiants. I. Halpha_ line-profile variability.
We have obtained time series of spectra in the wavelength range4000-6800 A over several months with high S/N and high resolution inwavelength (λ/{DELTA}λ=~20000) and time ({DELTA}t=~1d) ofthe late-type B and early-type A supergiants HD91619 (B7Ia), βOri(B8Ia), HD96919 (B9Ia), HD92207 (A0Ia), HD100262 (A2Ia) and αCyg(A2Ia). Halpha_ is found to show broad emission extended toabout +/-1200km/s for all objects except αCyg. Due to the lack ofstrong line-emission in Halpha_ the electron-scatteredphotons are expected to originate in deep atmospheric layers. In all ofthe objects the Halpha_-line profiles are found to be highlyvariable on different time scales reaching from days to months. Patternsof variation in Halpha_ are found to be quite symmetric aboutthe systemic velocity and are mainly due to variable blue andred-shifted emission superimposed on almost constant photospheric and/orwind profiles. These V/R variations are interpreted in terms of axialsymmetry of the envelopes of these objects. Time-series analyses of thevariations reveal Halpha_ time scales up to a factor of 6longer than expected radial fundamental pulsation periods but consistentwith rotational periods. Therefore, rotational modulation as a possiblesource of variability is concluded. Corotating weak magnetic surfacestructures are suggested as the source for a rotationally modulatedlower wind region. Suddenly appearing deep and highly blue-shiftedabsorptions in Halpha_ are ascribed to instabilities of theionization structure of the wind. Outwards propagating discreteabsorption components have been observed only once in HD92207.
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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: | שדרית |
| התרוממות ימנית: | 10h37m26.80s |
| סירוב: | -58°44'00.0" |
| גודל גלוי: | 5.45 |
| מרחק: | 2500 פארסק |
| תנועה נכונה: | -5.8 |
| תנועה נכונה: | 4.4 |
| B-T magnitude: | 6.024 |
| V-T magnitude: | 5.536 |
קטלוגים וכינוים:
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