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|A high-resolution spectroscopy survey of β Cephei pulsations in bright stars|
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
|Statistics of the Instability Strip of β Cephei Stars|
We present a study of the β Cephei instability strip based on asample of 49 stars of this type. After deriving their effectivetemperatures and luminosities from their observed (B-V), (U-B) colorsand parallaxes we find their positions in the HR diagram to be mostlyconfined to the main sequence, and their masses to lie between 7Mȯ and 30 Mȯ. Their distribution on theHR diagram matches well with our previous theoretical instability stripwhich has an upper bound in the luminosity and rather tight boundariesin the effective temperature.
|Spectroscopy and Time Variability of Absorption Lines in the Direction of the Vela Supernova Remnant|
We present high-resolution (R~75,000), high signal-to-noise (S/N~100) CaII λ3933.663 and Na I λλ5889.951, 5895.924 spectraof 68 stars in the direction of the Vela supernova remnant. The spectracomprise the most complete high-resolution, high S/N optical survey ofearly-type stars in this region of the sky. A subset of the sight lineshas been observed at multiple epochs, 1993/1994 and 1996. Of the 13stars observed twice, seven have spectra revealing changes in theequivalent width and/or velocity structure of lines, most of which arisefrom remnant gas. Such time variability has been reported previously forthe sight lines toward HD 72089 and HD 72997 by Danks & Sembach andfor HD 72127 by Hobbs and coworkers. We have confirmed the ongoing timevariability of these spectra and present new evidence of variability inthe spectra of HD 73658, HD 74455, HD 75309, and HD 75821. We havetabulated Na I and Ca II absorption-line information for the sight linesin our sample to serve as a benchmark for further investigations of thedynamics and evolution of the Vela SNR. Based on observations obtainedat the European Southern Observatory, La Silla, Chile.
|Resolved young open star clusters: keys to understanding massive hot stars.|
|A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars|
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
|A HIPPARCOS Census of the Nearby OB Associations|
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
|The ROSAT all-sky survey catalogue of optically bright OB-type stars.|
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
|Observational status and excitation mechanisms of beta-Cephei variables.|
|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.
|Hα emission and infrared excess in Be stars: probing the circumstellar disc.|
We present near-simultaneous (within one week) high-resolution Hαspectra and near-infrared photometry of 69 B and Be stars. We confirmthat, for the Be stars, the infrared excess and Hα emission arecorrelated, albeit with a rather large scatter. Given thenear-simultaneity of our observations, this scatter probably is notrelated to intrinsic variability, but rather reflects a genuinestar-to-star variation in physical conditions. In order to deriveinformation about the density and velocity structure of Be star discs,we calculate theoretical infrared excesses, Hα equivalent widthsand Hα line profiles for two different models. We find that themodel used by Poeckert & Marlborough (1978a) to fit the spectrum ofγ Cas, is not capable of reproducing the observed correlationbetween the infrared excess and the excess Hα equivalent width,producing - for given infrared excess - Hα emission lines that areweaker than observed. The model of Waters (1986), used successfully tofit the infrared excess continuum emission of Be stars, also fails,producing Hα emission lines that are too strong. We discuss theseresults in terms of the density structure of the wind.
|A photometric study of β Cephei stars. II. Determination of the degrees L of pulsation modes|
The wavelength dependence of photometric amplitudes is used as a meansof identifying the degrees l of pulsation modes of β Cephei starsstudied in an earlier paper. To this end, an expression for thephotometric amplitude of a non-rotating pulsating star is derived interms of the wavelength of the radiation received and the degree of thepulsation mode involved. The derivation differs from earlier derivationsin that the specific radiation intensity is considered instead of theoutward normal radiation flux. Furthermore, the effects of thenon-radial components of the Lagrangian displacement on the localsurface element of the star are taken into account. The angulardependence of the specific radiation intensity is determined by alimb-darkening function. The relative Lagrangian perturbation of thespecific radiation intensity is set equal to the relative Lagrangianperturbation of the outward normal radiation flux. The physicalparameters of the β Cephei stars are estimated by means ofcalibrations of photometric systems. From a comparison of thecalibrations of the Walraven, the Geneva, and the Stroemgren system forearly-type stars, it appears that the most reliable values are obtainedby means of the Walraven system. The influence of the uncertainties onthe physical parameters on the determination of the degrees l ofpulsation modes in β Cephei stars is examined. The expression forthe photometric amplitude of a pulsating star is used for thedetermination of the degree l of a pulsation mode by fitting curves ofthe wavelength dependences of theoretical photometric amplitudes forvarious degrees l to the wavelength dependence of observationallydetermined photometric amplitudes. In this way, values for the degrees lof most of the pulsation modes of the β Cephei stars considered arefound. It appears that not all β Cephei stars pulsate in at leastone radial mode and that multiperiodic β Cephei stars pulsate in avariety of combinations of pulsation modes.
|Beta Cephei stars from a photometric point of view|
This is an observational review, with an emphasis on photometric dataand their interpretation. Two lists are presented, one containing BetaCephei stars and the other Beta Cephei suspects. These lists then serveas a basis for discussing such topics as the location of Beta Cepheistars in the observational and theoretical H-R diagrams, theevolutionary state of these stars, the period-luminosity andperiod-luminosity-color relations, and observational identification ofpulsation modes. The paper also includes references to recent workconnected with the theoretical discovery that an opacity mechanism isresponsible for the excitation of Beta Cephei-star pulsations. Finally,observational programs for verifying the consequences of this discoveryare suggested.
|A photometric study of Beta Cephei stars. I - Frequency analyses|
A frequency analysis is presented for both new photometric data andolder radial-velocity measurements for 33 Beta Cephei stars. While thepresent results are largely in agreement with those of Cuypers (1985)and Engelbrecht (1986), marked differences are noted. A tabulation isgiven of the pulsation frequencies of the stars that are generallyaccepted to be Beta Cephei variables.
|Broad-band photometry of selected southern ultraviolet-bright stars.|
|Spectroscopy of southern Be stars 1984-1987|
The 93-cm Manuel Foster Observatory telescope in Chile has been used toobtain 919 spectrograms of 85 southern Be III-V stars. Balmeremission-lines (mainly H-beta) were noted in 74 percent of the stars,and Fe-II emission in 48 percent of them. Variations noted include rapidV/R variations of HR 1956, a primary shell phase of HR 2142, a shellphase of HR 2356, a transitory emission state of HR 2745, and thedisappearance of the H-beta emission of HR 5193.
|The VELA star cloud. III - The B8 to A0 stars and interstellar reddening|
The present discussion of intermediate band and H-beta observations of360 HD stars B8-A0 in Vela, and all of the CPD stars in a small adjacentregion, notes that the interstellar absorption in both cases can beunderstood as a combination of three absorbing clouds of 1 kpc-A(V) =1.5, 0.5, and 0.17. The major difference in the two environments is thepresence of numerous T Tauri stars in Taurus. Several clusters in theVela region are discussed.
|New radial pulsation constants for the Beta Cephei variables|
Recent new calibrations of luminosities, temperatures and bolometriccorrections for B stars in terms of the Beta index and the Stroemgrenparameter have necessitated the recalculation of the radial pulsationconstants, Q, for the Beta Cephei (or Beta Canis Majoris) variablestars. Corrections for the effect of binaries on the absolutemagnitudes, derived both from the luminosity calibration and from themean distance moduli of those variables in clusters, are calculated inan Appendix. The mean value of Q, although determined from absolutemagnitudes which are about 0.4 mag fainter than those from previouscalibrations of the Beta index, still suggests that the majority of thevariables are pulsating in the first overtone radial mode, as have mostinvestigations in recent years.
|A catalog of ultraviolet interstellar extinction excesses for 1415 stars|
Ultraviolet interstellar extinction excesses are presented for 1415stars with spectral types B7 and earlier. The excesses with respect to Vare derived from Astronomical Netherlands Satellite (ANS) 5-channel UVphotometry at central wavelengths of approximately 1550, 1800, 2500, and3300 A. A measure of the excess extinction in the 2200-A extinction bumpis also given. The data are valuable for investigating the systematicsof peculiar interstellar extinction and for studying the character of UVinterstellar extinction in the general direction of stars for which theextinction-curve shape is unknown.
|The local system of early type stars - Spatial extent and kinematics|
Published uvby and H-beta photometric data and proper motions arecompiled and analyzed to characterize the structure and kinematics ofthe bright early-type O-A0 stars in the solar vicinity, with a focus onthe Gould belt. The selection and calibration techniques are explained,and the data are presented in extensive tables and graphs and discussedin detail. The Gould belt stars of age less than 20 Myr are shown togive belt inclination 19 deg to the Galactic plane and node-lineorientation in the direction of Galactic rotation, while the symmetricaldistribution about the Galactic plane and kinematic properties (purecircular differential rotation) of the belt stars over 60 Myr oldresemble those of fainter nonbelt stars of all ages. The unresolveddiscrepancy between the expansion observed in the youngest nearby starsand the predictions of simple models of expansion from a point isattributed to the inhomogeneous distribution of interstellar matter.
|The Beta CEP variables - Fundamental radial pulsators|
New uvby-beta photometry for 61 B stars from B0-B9 in the young clusterNGC 3293 has shown that the commonly used calibration of the beta indexin terms of absolute magnitude, M(V), gives M(V) values about 0.6 magtoo bright for the early B stars in the cluster. Nine of the early Bstars in NGC 3293 are Beta Cep variables with similar properties(periods, beta and c0 indices) to the majority of the BetaCep stars. Therefore, the M(V) scale for all the variables is deduced tobe in error by 0.6 mag, since their absolute magnitudes are commonlyderived from beta. The application of this correction suggests thatthese stars pulsate in the fundamental radial mode instead of the firstovertone as has been recently supposed.
|A survey of ultraviolet objects|
An all-sky survey of ultraviolet objects is presented together with astatistical analysis that leads to the conclusion that there is asignificantly higher population of hot subdwarfs lying below themain-sequence than hitherto thought. The distribution of all ultravioletobjects, main sequence ultraviolet objects, and MK unclassifiedultraviolet objects are shown in galactic coordinates, and the absolutemagnitudes and color-color diagrams for these groups are presented.Scale heights are derived, giving values similar to planetary nebulaefor the hottest groups.
|66th Name-List of Variable Stars|
|A classification system for O-B2 stars based on the SI IV and C IV resonance lines|
Low-dispersion ultraviolet spectra from Skylab Experiment S-019 are usedto explore the variations of Si IV and C IV line strengths withtemperature and luminosity. These considerations lead to aclassification system in which the Si/C ratio is used to discriminateluminosity among the O stars and temperature among the O9-B2 stars oflower luminosity. Stars falling in these two regimes may bedistinguished either by the presence of C IV emission or on the basis ofC IV absorption strength. The log(Si IV/C IV) vs C IV diagram isproposed as a primary tool in such a classification system. The rapidvariation in the Si IV/C IV ratio from less than 1/10 at O9 to greaterthan 10 at B1.5 for luminosity class III-V stars appears to be anespecially useful criterion for the temperature classification of starsin this spectral range.
|The VELA star cloud. I - NGC 2547, TR 10, the Gamma Velorum system, and bright stars|
The first results of an intermediate-band, large-scale photometricsurvey of the Vela star cloud are discussed. Attention is given to theluminosity and reddening, as well as the apparent or proper motions ofall the CPD stars near NGC 2547, the brightest stars in Trumpler 10, arandom selection of stars in the region of Gamma Velorum, and the HRstars. The similarity of the Alpha Persei cluster with the clusters inthe Vela sheet is shown, and it is found that the Vela sheet, at leastover the region surveyed, is nearly perpendicular to the line of sight.The probability is demonstrated that a thin sheet of coeval (2.5 x 10 tothe 7th yr) stars, some 425 pc distant and with a similar metalabundance, lies in front of a dense dark cloud.
|Absorption spectra of interstellar grains. I - Reference stars|
In order to find the proper standards for the precise extraction ofinterstellar spectral features, an analysis was made of low-resolutionUV spectra of unreddened early-type stars. Stars having a E(B-V) colorexcess below 0.05m contained in the catalog of TD-1 satelliteobservations made during the S2/68 experiment are examined.
|Search for Beta Cephei stars south of declination -20 deg. I - Incidence of light variability among early B giants and subgiants: Summer objects|
|Visual multiples.2 The GAM Vel group.|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..541A&db_key=AST
|Structure and age of the local association /Pleiades group/|
Intermediate-band indices are used to derive luminosities for some 500early-type stars with well-determined proper motions and radialvelocities. Space motion vectors and galactic coordinates are computedfor the stars considered. It is found that the local association membersare mainly concentrated in the Sco-Cen region in the Southern Hemisphereand the Cas-Tau region in the north.
|Photoelectric spectrophotometry of Wolf-Rayet stars|
Photoelectric spectrum scans of five southern Wolf-Rayet stars in thespectral range 4600-4720 A were analyzed to study the variability ofbrightness and of emission line strengths. No variations of any kind inshort time scale were found. However, in WC stars night-to-nightvariations of 3 to 4 per cent were detected in the emission linestrengths.
|Short-term variability of gamma-2 Velorum|
Photoelectric spectrum scans of gamma-2 Velorum in the spectral range4600-4720 A were analyzed to study the short-term variability ofbrightness and of emission line strengths. The emission strengths of HeII (4686 A) and C III-IV (4650 A) show rms variations of 3 and 2 percent, respectively. These variations have time scales of 1 minute andlonger. Night-to-night variations were also found. The continuumradiation also shows small-amplitude variations. No periodic variationsattributable to this star were detected on any of the nights on whichthese observations were made.
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|Proper motion RA:||-4.4|
|Proper motion Dec:||10.2|
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