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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.
|Catalog of Galactic β Cephei Stars|
We present an extensive and up-to-date catalog of Galactic β Cepheistars. This catalog is intended to give a comprehensive overview ofobservational characteristics of all known β Cephei stars, coveringinformation until 2004 June. Ninety-three stars could be confirmed to beβ Cephei stars. We use data from more than 250 papers publishedover the last nearly 100 years, and we provide over 45 notes onindividual stars. For some stars we reanalyzed published data orconducted our own analyses. Sixty-one stars were rejected from the finalβ Cephei list, and 77 stars are suspected to be β Cepheistars. A list of critically selected pulsation frequencies for confirmedβ Cephei stars is also presented.We analyze the β Cephei stars as a group, such as the distributionsof their spectral types, projected rotational velocities, radialvelocities, pulsation periods, and Galactic coordinates. We confirm thatthe majority of the β Cephei stars are multiperiodic pulsators. Weshow that, besides two exceptions, the β Cephei stars with highpulsation amplitudes are slow rotators. Those higher amplitude starshave angular rotational velocities in the same range as thehigh-amplitude δ Scuti stars (Prot>~3 days).We construct a theoretical HR diagram that suggests that almost all 93β Cephei stars are main-sequence objects. We discuss theobservational boundaries of β Cephei pulsation and the physicalparameters of the stars. We corroborate that the excited pulsation modesare near to the radial fundamental mode in frequency and we show thatthe mass distribution of the stars peaks at 12 Msolar. Wepoint out that the theoretical instability strip of the β Cepheistars is filled neither at the cool nor at the hot end and attempt toexplain this observation.
|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.
|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 (18.104.22.168) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
|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.
|An extensive Delta a-photometric survey of southern B and A type bright stars|
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 22.214.171.124 orhttp://cdsweb.u-strasbg.fr/Abstract.html
|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.
|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.
|Nonadiabatic observables in beta Cephei models|
Using results of linear nonadiabatic calculations for oscillations ofbeta Cephei star models, we calculate amplitudes and phases for light,color and radial velocity for the unstable modes of low harmonicdegrees, l. The nonadiabatic observables are the amplitude ratios andthe phase differences for various oscillating parameters. We constructtheoretical diagrams involving these observables as well as pulsationperiods and compare them with the stellar data. Balona & Stobie(1979) showed that the diagrams based on two-color photometric data maybe used to determine the l-value of an observed mode. Our use of resultsof nonadiabatic calculations improves their method. We show, inparticular, that the diagrams employing the satellite ultravioletmeasurements are the best for discrimination between the l = 0, 1 and 2cases. The clearest separation of the domains for the three l-valuesoccurs in the diagram making use of both photometric and the radialvelocity data. The observational points fall into three theoretricaldomains and an assignment of the l-value is unambiguous. A comparison ofthe theory with the observations is also made using the Period versusAmplitude ratios diagrams in various photometric systems. The agreementis very satisfactory and, in most cases, the l-value can be determined.The nonadiabtic observables are useful not only to determine l but alsothe radial order of the observed modes as well as for constrain meanstellar parameters. As an example we consider the case of delta Ceti-asingle mode beta Cephei star. Our results point to the significantmulticolor photometric and spectroscopic data for asteroseismology.
|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.
|The distribution of interstellar dust in the solar neighborhood|
We surveyed the IRAS data base at the positions of the 1808 O6-B9.5stars in The Bright Star Catalog for extended objects with excessemission at 60 microns, indicating the presence of interstellar dust atthe location of the star. Within 400 pc the filling factor of theinterstellar medium, for dust clouds with a density greater than 0.5/cucm is 14.6 + or - 2.4%. Above a density of 1.0/cu cm, the densitydistribution function appears to follow a power law index - 1.25. Whenthe dust clouds are mapped onto the galactic plane, the sun appears tobe located in a low-density region of the interstellar medium of widthabout 60 pc extending at least 500 pc in the direction of longitudes 80deg - 260 deg, a feature we call the 'local trough'.
|Dense clumps of ionized gas near Pi Scorpii, as revealed by the fine-structure excitation of N II|
The column density and the emission of the ionized gas along the line ofsight toward the B1 V + B2 V binary star Pi Sco are measured on thebasis of the fine-structure absorption lines of the ground state N II.It is found that the bulk of this ionized gas must be clumped on alength scale of 0.025 pc, which is far smaller than the observed size ofthe diffuse H II region surrounding Pi Sco of about 6 pc. The observedcolumn density of S III toward Pi Sco yields an upper limit on thedistance of the absorbing, clumped gas from the star of less than about0.02 pc, assuming that both the N II and S III absorption arise from thesame gas. The possibility that the ionized gas originates from aphotoevaporating circumstellar disk directly surrounding Pi Sco isexcluded, since such a disk would have an unusual size of order 0.025 pcand would have had to survive for the estimated age of Pi Sco of 5-8Myr. The derived mean density of the clumped gas is of order 40/cu cm,so that the gas is at a pressure that far exceeds the mean pressure inthe H II region. It is concluded that the ionized gas could originatefrom evaporation flows off a cluster of compact neutral objects thatevaporate due to the ionizing radiation of Pi Sco.
|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.|
|Empirical temperature calibrations for early-type stars|
Three temperature calibrations of suitable photometric quantities havebeen derived for O and B stars. A sample of 120 stars with reliableT(eff.) determinations has been used for establishing each calibration.The different calibrations have been critically discussed and compared.Temperature determinations for 1009 program stars have been obtainedwith an accuracy of the order of 10 percent.
|An optimal extraction algorithm for CCD spectroscopy|
An optimal spectrum extraction procedure is described, and examples ofits performance with CCD data are presented. The algorithm delivers themaximum possible signal-to-noise ratio while preservingspectrophotometric accuracy. The effects of moderate geometricdistortion and of cosmic-ray hits on the spectrum are automaticallyaccounted for. In tests with background-noise limited CCD spectra,optimal extraction offers a 70-percent gain in effective exposure timein comparison with conventional extraction procedures.
|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.
|Two new multiperiodic Beta Cephei candidates in the Scorpius complex|
The discovery of two hitherto unknown Beta Cephei candidates isreported. The photometric variations of both stars indicate the presenceof at least three frequencies. The periods of HD 147985 are 0.13231 d,0.14493 d, and 0.15666 d; the periods of HD 15662 are 0.16890 d, 0.18861d, and 0.16978 d. Both variables belong to the Scorpius complex. Unlikethe Beta Cephei candidates discovered in NGC 6231 (Balona and Shobbrook,1983), they fit into the classical instability strip. It thus seemsunlikely that the peculiar position in the HR diagram of the variablesin NGC 6231 is related to the galactic location of the cluster. Thepossibility of a more extended instability strip is discussed in view ofthe results of recent surveys in the solar neighborhood. It is foundthat the actual instability strip closely parallels the main sequence.
|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.
|The interstellar 2200 A band - A catalogue of equivalent widths|
|65th Name-List of Variable Stars|
|Starlight polarization in the direction of two H I complexes|
Upper limits to the gas/dust ratio in the direction of two complexes arederived based on 21-cm line observations and starlight polarizationmeasurements. Elongated H I structures parallel to the galactic magneticfield suggest that the gas and dust comprise a common cloud or cloudsystem. Dust producing the starlight polarization is mixed with the H Igas, aiding in estimating the distance for the two H I complexes.Distances of 100 pc for the filament at low velocities and of 200 pc foran intermediate-velocity cloud complex (IVC) are established frompolarimetric data, and possible origins for the filaments and IVC aresuggested.
|The period-luminosity relation for Beta Cephei stars in the Geneva Photometric System|
The luminosities of Beta Cephei stars as obtained by means of thecalibration of the Geneva Photometric System lead to a better definedperiod-luminosity relation than do the luminosities obtained from thecalibration of the Beta-index. This is mainly because the spectraldomain covered by the filters of the Geneva System is nearer to theultraviolet than to the H-beta line and because the present luminositycalibration is not affected by emission in the H-alpha or H-beta linesand only marginally by emissions in the H-gamma line. However, somestars do not obey the period-luminosity relation. The period of HR 3088is shorter and the periods of Alpha Lupi and Tau(1) Lupi are longer thanthe periods predicted by the period-luminosity relation.
|On the period-luminosity relation of Beta Cephei stars|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1980A&A....84..374J&db_key=AST