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Towards an understanding of the Of?p star HD191612: phase-resolved multiwavelength observations
We present the analysis of phase-resolved X-ray and optical observationsof the peculiar hot star HD191612 (Of?p). This star is known to displayline-profile variations that are recurrent with a period of 538d and itsspectrum was found to present the signature of a magnetic field. In theX-rays, it is slightly overluminous compared to the canonicalLX/LBOL) relation and appears brighter when theoptical lines are strongest. Our XMM-Newton observations further revealthat the X-ray spectrum of HD191612 exhibits rather broad lines and isdominated by a `cool' (0.2-0.6keV) thermal component, twocharacteristics at odds with the proposed magnetic rotator model. Wealso report for the first time the low-level variability of the metallic(absorption/emission) lines and HeII absorptions that appear to beassociated with radial-velocity shifts. Finally, we compare our resultswith observations of the early-type stars and discuss several possiblescenarios.Based on observations collected at the Haute-Provence Observatory(France) and with XMM-Newton, an ESA Science Mission with instrumentsand contributions directly funded by ESA Member States and the USA(NASA).E-mail: naze@astro.ulg.ac.be ‡Post-doctoral Researcher FNRS (Belgium). §Research Associate FNRS (Belgium). ¶Operated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555.
| X-Ray Variability in the Young Massive Triple θ2 Orionis A
Massive stars rarely show intrinsic X-ray variability. One exception isθ2 Ori A, which has shown strong variability over thelast 5 years. We observed a large outburst of the X-ray source with theHigh Energy Transmission Grating Spectrometer on board Chandra andcompare the emissivity and line properties in states of low and highflux. The low state indicates temperatures well above 25 MK. In the highstate we find high emissivities in the range from 3 to over 100 MK. Theoutburst event in stellar terms is one of the most powerful everobserved and the most energetic one in the ONC, with a lower totalenergy limit of 1.5×1037 ergs. The line diagnosticsindicate that the line-emitting regions in the low states are as closeas within 1-2 stellar radii from the O star's photosphere, whereas thehard states suggest a distance of 3-5 stellar radii. We discuss theresults in the context of stellar flares, magnetic confinement, andbinary interactions. By matching the dates of all observations with theorbital phases of the spectroscopy binary orbit, we find that outburstsoccur very close to the periastron passage of the stars. We argue thatthe high X-ray states are possibly the result of reconnection eventsfrom magnetic interactions of the primary and secondary stars of thespectroscopic binary. Effects from wind collisions seem unlikely forthis system. The line properties in the low state seem consistent withsome form of magnetic confinement. We also detect Fe fluorescenceindicative of the existence of substantial amounts of neutral Fe in thevicinity of the X-ray emission.
| The surprising magnetic topology of τ Sco: fossil remnant or dynamo output?
We report the discovery of a medium-strength (~0.5 kG) magnetic field onthe young, massive star τ Sco (B0.2V), which becomes thethird-hottest magnetic star known. Circularly polarized Zeemansignatures are clearly detected in observations collected mostly withthe ESPaDOnS spectropolarimeter, recently installed on the 3.6-mCanada-France-Hawaii Telescope; temporal variability is also clearlyestablished in the polarimetry, and can be unambiguously attributed torotational modulation with a period close to 41 d. Archival ultraviolet(UV) spectra confirm that this modulation repeats over time-scales ofdecades, and refine the rotation period to 41.033 +/- 0.002 d.Despite the slow rotation rate of τ Sco, we none the less succeed inreconstructing the large-scale structure of its magnetic topology. Wefind that the magnetic structure is unusually complex for a hot star,with significant power in spherical-harmonic modes of degree up to 5.The surface topology is dominated by a potential field, although amoderate toroidal component is probably present. We fail to detectintrinsic temporal variability of the magnetic structure over the 1.5-yrperiod of our spectropolarimetric observations (in agreement with thestable temporal variations of the UV spectra), and infer that anydifferential surface rotation must be very small.The topology of the extended magnetic field that we derive from thephotospheric magnetic maps is also more complex than a global dipole,and features in particular a significantly warped torus of closedmagnetic loops encircling the star (tilted at about 90° to therotation axis), with additional, smaller, networks of closed-fieldlines. This topology appears to be consistent with the exceptional X-rayproperties of τ Sco and also provides a natural explanation of thevariability observed in wind-formed UV lines. Although we cannotcompletely rule out the possibility that the field is produced throughdynamo processes of an exotic kind, we conclude that its magnetic fieldis most probably a fossil remnant from the star formation stage.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Science de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.E-mail: donati@ast.obs-mip.fr (J-FD); idh@star.ucl.ac.uk (IDH);mmj@st-andrews.ac.uk (MMJ); petit@ast.obs-mip.fr (PP);claude.catala@obspm.fr (CC); jlandstr@uwo.ca (JDL);jean-claude.bouret@oamp.fr (J-CB); evelyne.alecian@obspm.fr (EA);jrb3@st-andrews.ac.uk (JRB); forveill@cfht.hawaii.edu (TF);fpaletou@ast.obs-mip.fr (FP); manset@cfht.hawaii.edu (NM)
| The magnetic field and confined wind of the O star θ1 Orionis C
Aims.In this paper we confirm the presence of a globally-ordered,kG-strength magnetic field in the photosphere of the young O starθ1 Orionis C, and examine the properties of itsoptical line profile variations. Methods: .A new series ofhigh-resolution MuSiCoS Stokes V and I spectra has been acquired whichsamples approximately uniformly the rotational cycle ofθ1 Orionis C. Using the Least-Squares Deconvolution(LSD) multiline technique, we have succeeded in detecting variableStokes V Zeeman signatures associated with the LSD mean line profile.These signatures have been modeled to determine the magnetic fieldgeometry. We have furthermore examined the profile variations of linesformed in both the wind and photosphere using dynamic spectra.Results: .Based on spectrum synthesis fitting of the LSD profiles, wedetermine that the polar strength of the magnetic dipole component is1150 B_d 1800 G and that the magnetic obliquity is 27° β 68°, assuming i=45± 20°. The best-fitvalues for i=45° are Bd = 1300 ± 150~G~ (1σ)and β = 50° ± 6° (1σ). Our data confirm theprevious detection of a magnetic field in this star, and furthermoredemonstrate the sinusoidal variability of the longitudinal field andaccurately determine the phases and intensities of the magnetic extrema.The analysis of "photospheric" and "wind" line profile variationssupports previous reports of the optical spectroscopic characteristics,and provides evidence for infall of material within the magneticequatorial plane.
| Modelling of isolated radio pulsars and magnetars on the fossil field hypothesis
We explore the hypothesis that the magnetic fields of neutron stars areof fossil origin. For parametrized models of the distribution ofmagnetic flux on the main sequence and of the birth spin period of theneutron stars, we calculate the expected properties of isolated radiopulsars in the Galaxy using as our starting point the initial massfunction and star formation rate as a function of Galactocentric radius.We then use the 1374-MHz Parkes Multi-Beam Survey of isolated radiopulsars to constrain the parameters in our model and to deduce therequired distribution of magnetic fields on the main sequence. We findagreement with observations for a model with a star formation rate thatcorresponds to a supernova rate of 2 per century in the Galaxy fromstars with masses in the range 8-45Msolar and predict 447000active pulsars in the Galaxy with luminosities greater than 0.19 mJykpc2. The progenitor OB stars have a field distribution whichpeaks at ~46 G with ~8 per cent of stars having fields in excess of 1000G. The higher-field progenitors yield a population of 24 neutron starswith fields in excess of 1014 G, periods ranging from 5 to 12s, and ages of up to 100000 yr, which we identify as the dominantcomponent of the magnetars. We also predict that high-field neutronstars (logB > 13.5) originate preferentially from higher-massprogenitors and have a mean mass of 1.6Msolar, which issignificantly above the mean mass of 1.4Msolar calculated forthe overall population of radio pulsars.
| Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit
Massive X-ray binaries are usually classified by the properties of thedonor star in classical, supergiant and Be X-ray binaries, the maindifference being the mass transfer mechanism between the two components.The massive X-ray binary 4U 2206+54 does not fit inany of these groups, and deserves a detailed study to understand how thetransfer of matter and the accretion on to the compact object takeplace. To this end we study an IUE spectrum of the donor and obtain awind terminal velocity (v_&infy;) of ~350 km s-1, which isabnormally slow for its spectral type. We also analyse here more than 9years of available RXTE/ASM data. We study the long-term X-rayvariability of the source and find it to be similar to that observed inthe wind-fed supergiant system Vela X-1, reinforcingthe idea that 4U 2206+54 is also a wind-fed system.We find a quasi-period decreasing from ~270 to ~130 d, noticed inprevious works but never studied in detail. We discuss possiblescenarios for its origin and conclude that long-term quasi-periodicvariations in the mass-loss rate of the primary are probably drivingsuch variability in the measured X-ray flux. We obtain an improvedorbital period of P_orb=9.5591±0.0007 d with maximum X-ray fluxat MJD 51856.6±0.1. Our study of the orbital X-ray variability inthe context of wind accretion suggests a moderate eccentricity around0.15 for this binary system. Moreover, the low value of v_&infy; solvesthe long-standing problem of the relatively high X-ray luminosity forthe unevolved nature of the donor, BD +53°2790,which is probably an O9.5 V star. We note that changes in v_&infy;and/or the mass-loss rate of the primary alone cannot explain thedifferent patterns displayed by the orbital X-ray variability. Wefinally emphasize that 4U 2206+54, together withLS 5039, could be part of a new population ofwind-fed HMXBs with main sequence donors, the natural progenitors ofsupergiant X-ray binaries.
| Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of θ1 Orionis C*
From observations made with the ESPaDOnS spectropolarimeter, recentlyinstalled on the 3.6-m Canada-France-Hawaii Telescope, we report thediscovery of a strong magnetic field in the Of?p spectrum variable HD191612 - only the second known magnetic O star (followingθ1 Ori C). The stability of the observed Zeemansignature over four nights of observation, together with thenon-rotational shape of line profiles, argues that the rotation periodof HD 191612 is significantly longer than the 9-d value previouslyproposed. We suggest that the recently identified 538-d spectralvariability period is the rotation period, in which case the observedline-of-sight magnetic field of -220 +/- 38 G implies a large-scalefield (assumed dipolar) with a polar strength of about -1.5 kG. Ifconfirmed, this scenario suggests that HD 191612 is, essentially, anevolved version of the near-zero-age main-sequence magnetic O starθ1 Ori C, but with an even stronger field (about 15 kGat an age similar to that of θ1 Ori C). We suggestthat the rotation rate of HD 191612, which is exceptionally slow byaccepted O-star standards, could be due to angular momentum dissipationthrough a magnetically confined wind.
| Le champ magnetique des etoiles.
Not Available
| A Period and a Prediction for the Of?p Spectrum Alternator HD 191612
The observational picture of the enigmatic O-type spectrum variable HD191612 has been sharpened substantially. A symmetrical, low-amplitudelight curve with a period near 540 days has recently been reported fromHipparcos photometry. This period satisfies all of the spectroscopysince at least 1982, including extensive new observations during 2003and 2004, and it has predicted the next transition during 2004September-October. Measurements of the Hα equivalent width reveala sharp emission peak in the phase diagram, in contrast to theapparently sinusoidal light curve. The He II absorption-line strength isessentially constant, while He I varies strongly, possibly filled in byemission in the O6 state, thus producing the apparent spectral typevariations. The O8 state appears to be the ``normal'' one. Twointermediate O7 observations have been obtained, which fall at theexpected phases, but these are the only modern observations of thetransitions so far. The period is too long for rotation or pulsation;although there is no direct evidence as yet for a companion, a model inwhich tidally induced oscillations drive an enhanced wind nearperiastron of an eccentric orbit appears promising. Further observationsduring the now predictable transitions may provide a critical test.Ultraviolet and X-ray observations during both states will likely alsoprove illuminating.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.Tables 1 and 2 are 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/410/905
| The Remarkable Alternating Spectra of the Of?p Star HD 191612
The spectrum of HD 191612 has been found to display large, recurrentvariations between two highly reproducible, peculiar states; at leastfour transformations have occurred since 1950. In one state, thespectral type is O6-O7, with C III λ4650 emission comparable to NIII λ4640 (the definition of the Of?p category) and P Cygniprofiles at He II λ4686 and Hα. In the other state, thespectral type is O8, with the C III emission absent, very strong N IIIλ4097 absorption, broad He II λ4686 absorption with narrowcentral emission (a profile that may be unprecedented in this line amongknown O-type spectra), and a broad asymmetrical absorption at Hα.One observing sequence over several consecutive nights shows no spectralvariations, practically ruling out a short-period, interacting binary asthe origin of the phenomenon; moreover, no significant radial velocityvariations have been found. Although the sporadic observational recordprior to the discovery of the variations in early 2001 precludesdefinite conclusions, it is possible that a given state is maintainedfor a decade or longer, but one transformation occurred within 13months, and the data obtained during 2002 suggest an event with ashorter timescale.The Of?p category currently contains only five members: three in theGalaxy and two in the Small Magellanic Cloud. The other two Galacticmembers also display bizarre and unexplained phenomena; in the case ofHD 108, they are strikingly similar to those described here. Because oftheir relatively high X-ray luminosities, all three Galactic objectshave been suggested to have collapsed companions. If the spectralvariations of HD 108 and HD 191612 are due to binary interactions, theyare likely multiyear, eccentric systems like WR 140 and η Carinae.The axisymmetric shell ejections of HD 148937 could have a similarorigin. Alternatively, these stars may be rapid rotators or in anunstable evolutionary transitional stage. Further intensivespectroscopic monitoring is required to reveal their nature.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| 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.
| 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
| 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
| 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.
| On the normal energy distribution in stellar spectra: O5-O8 stars
The spectral energy curves for a number of O4-O8 stars are analyzedusing measurements taken from six sources of spectrophotometric data.Based on the normal UBV color indices for O stars, these stars aredivided into three groups (O5, O6-O7, and O8), for each of which thenormal energy distribution is derived and presented. Comparison of thesedistributions indicates clear differences between the O5 and O8subclasses at UV wavelengths. A single normal distribution curve is,therefore, insufficient to characterize the O spectral class as a whole.
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| Flux Distributions for 59 Stars in Cygnus
Absolute flux distributions are given for 59 Cygnus stars in thespectral range of 320--720 nm with a step of 5 nm. Their rms errors areon the average 2--4%. The synthetic color indices calculated for theflux distributions are compared with the observed color indices in threephotometric systems.
| Metal line strengths of blue stragglers towards the young galactic association Perseus OB1.
We present equivalent width measurements of lines of HeI, CIII, NIII,OII, MgII and SiIV from high resolution optical spectra of eight bluestragglers towards the young galactic association Perseus OB1, togetherwith similar data from ultraviolet spectra. We have carried outintercomparisons of the observed line strengths between target stars ofsimilar effective temperatures, and comparisons between the target starsand data for other O-type stars, in order to search for possibleabundance anomalies in our target stars. We find firm evidence that twostars, HD 12323 and HD 13268, exhibit CNO bicycle processed material attheir surfaces, and we outline evolutionary scenarios to explain thisphenomenon. One further star, HD 12993, also shows marginal evidence forprocessed material at its surface. There is no evidence for abundanceanomalies in the five remaining stars, and we consider that they may notbe association members. Hence their identification as blue stragglers isin doubt.
| The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...460..914V&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Evolution of initially mixed massive main sequence stars
The main sequence evolution of massive stars is calculated under theassumption that an unspecified mixing process were able to keep themodels homogeneous during some fraction of the core hydrogen burningphase. The mixing could be due to rotation. The results are used todiscuss discrepancies in stellar parameters between recent spectralanalyses of OB stars and standard stellar evolution theory. The observedsurface helium abundances serve as input to determine the duration ofthe mixing. It is shown that mixing on the main sequence alone cannotexpain the derived values. although the agreement becomes much betterwhen using the tracks presented here.
| Anomalous Proper-Motions in the Cygnus Super Bubble Region
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..101...37C&db_key=AST
| Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg
The literature on all OB associations was reviewed, and their IRAS pointsource content was studied, between galactic longitude 55 and 150 deg.Only one third of the 24 associations listed by Ruprecht et al. (1981)have been the subject of individual studies designed to identify thebrightest stars. Distances to all of these were recomputed using themethod of cluster fitting of the B main sequence stars, which makes itpoossible to reexamine the absolute magnitude calibration of the Ostars, as well as for the red supergiant candidate stars. Also examinedwas the composite HR diagram for these associations. Associations withthe best defined main sequences, which also tend to contain very youngclusters, referred to here as OB clusters, have extremely few evolved Band A or red supergiants. Associations with poorly defined mainsequences and few OB clusters have many more evolved stars. They alsoshow an effect in the upper HR diagram referred to as a ledge byFitzpatrick and Garmany (1990) in similar data for the Large MagellanicCloud. It is suggested that the differences in the associations are notjust observational selection effects but represent real differences inage and formation history.
| Intrinsic parameters of galactic luminous OB stars
We present the results of a spectroscopic analysis of 25 galacticluminous OB stars. The gravities are in general lower than previouslyassumed, especially for giants. The helium abundances are also higherthan normal in many cases. Using data from the spectroscopic analysis,radii, luminosities and masses are derived. The observed wind terminalvelocities are used with the analytic approximation for the theory ofradiatively driven winds (Kudritzki et al., 1989) to obtain masses andmass loss rates. The masses derived from the spectroscopic analysis andthose derived independently from the wind theory agree, except for asmall difference attributed to deficiencies in the physics of the modelatmospheres used. A comparison with evolutionary tracks indicates that(1) evolution theory cannot explain the observed helium abundancepattern and that (2) the masses derived from the spectroscopic analysis(or the wind theory) are systematically lower than those derived fromevolutionary calculations.
| Close binary systems in regions of open clusters. I - V448 CYG in NGC 6871
New photoelectric UBVRI' observations of the massive early-typeeclipsing system V448 Cyg in the open cluster NGC 6871 were obtained.The light curve is found to be variable in all colors. The period of thesystem is modified. New photometric elements of V448 Cyg are determinedby Lavrov's method, and absolute parameters of the system are estimated.Difficulties in interpreting the larger-size low-mass component arediscussed. The distance to V448 Cyg is estimated at 11.2 m, and its ageis 8,000,000 yr.
| Discovery of a peculiar Of?-like star in the LMC
We present new, high dispersion spectroscopy of the LMC N82 indicating astar with a very peculiar spectrum which does not resemble exactly toany so far seen. Hence, it cannot be forced into any known categories.The closest comparisons may be the three known Galactic peculiar Of andOf?p objects found by Walborn (1973). This star may be a transitionobject between Of supergiants and W-R stars.
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משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | ברבור |
| התרוממות ימנית: | 20h09m28.61s |
| סירוב: | +35°44'01.3" |
| גודל גלוי: | 7.832 |
| מרחק: | 9090.909 פארסק |
| תנועה נכונה: | -4.9 |
| תנועה נכונה: | -4.5 |
| B-T magnitude: | 8.059 |
| V-T magnitude: | 7.851 |
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