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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.
| Radial velocity measurements of B stars in the Scorpius-Centaurus association
We derive single-epoch radial velocities for a sample of 56 B-type starsmembers of the subgroups Upper Scorpius, Upper Centaurus Lupus and LowerCentaurus Crux of the nearby Sco-Cen OB association. The radial velocitymeasurements were obtained by means of high-resolution echelle spectravia analysis of individual lines. The internal accuracy obtained in themeasurements is estimated to be typically 2-3 km s-1, butdepends on the projected rotational velocity of the target. Radialvelocity measurements taken for 2-3 epochs for the targets HD 120307, HD142990 and HD 139365 are variable and confirm that they arespectroscopic binaries, as previously identified in the literature.Spectral lines from two stellar components are resolved in the observedspectra of target stars HD 133242, HD 133955 and HD 143018, identifyingthem as spectroscopic binaries.
| Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.
| 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.
| Metallicity of mono- and multiperiodic β Cephei stars
Analyzing IUE ultraviolet spectra of β Cep pulsating stars wenoticed that multiperiodic variables have a larger mean metal abundancein the photosphere, [ m/H] , than monoperiodic ones. We applystatistical tests to verify this dichotomy. We obtain that, with a largeprobability, the multiperiodic β Cep stars have greater values of [m/H] . This result is consistent with the linear non-adiabatic theory ofpulsation of early B-type stars.
| Metallicities of the β Cephei stars from low-resolution ultraviolet spectra
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
| Disentangling component spectra of κ Scorpii, a spectroscopic binary with a pulsating primary. II. Interpretation of the line-profile variability
We analyse the complex short-term SiIII line-profile variability of the spectroscopic binary β Cep star κ Scorpii after orbit subtraction, before and after spectral disentangling. We refine the known oscillation frequency of the star: f1=4.99922 c d-1 and detect2f1. Variability is also found at frequencies nearf2≃ 4.85 c d-1 and f3≃ 5.69c d-1 or their aliases. These frequencies are not significantif we consider the spectra alone, but they survive our selection afterthe consideration that they were derived previously from independentground-based and space photometry by different teams. Moreover, we finddominant variability in the equivalent width with a frequency in theinterval [0.22,0.30] c d-1 which we interpret as therotational frequency frot of the star. The complex windowfunction does not allow us to determine definite values forf2, f3, frot. The variability withf1 is interpreted as a prograde non-radial oscillation modewith spherical wavenumbers (ℓ,m)=(2,-1) or (1,-1). The additionalfrequencies are explained in terms of rotational modulation superposedto the main oscillation. We also point out that we cannot disprove thevariability in κ Scorpii to originate from co-rotating structures. KOREL disentangling preserves the large-amplitude line-profile variability but its performance for complex low-amplitude variability remains to bestudied in detail.Based on observations obtained with the Coudé ÉchelleSpectrograph on the ESO CAT telescope and with the CORALIEéchelle spectrograph on the 1.2-m Euler Swiss telescope, bothsituated at La Silla, Chile.
| Interpretation of the variability of the β Cephei star λ Scorpii. I. The multiple character
We derive accurate values of the orbital parameters of the close binaryβ Cephei star λ Scorpii. Moreover, we present the firstdetermination of the properties of the triple system to which λScorpii belongs. Our analysis is based on a time series of 815high-resolution spectra, covering a timespan of 14 years. We find aclose orbit of 5.9525 d days (e=0.26) and a wide orbit of approximately1082d days (e=0.23). The orbital parameters of the triplestar and a spectrum synthesis lead us to conclude that the system iscomposed of two early-type B stars and a low-mass pre-main-sequence starrather than containing an ultra-massive white dwarf as claimed before.Our proposed configuration is compatible with population synthesis. Theradial velocity variations of the primary allow us to confirm thepresence of at least one pulsation mode with frequency 4.679410 cd-1 which is subject to the light-time effect in the triplesystem. A detailed analysis of the complex line-profile variations isdescribed in a subsequent paper.Based on observations obtained with the Coudé EchelleSpectrograph on the ESO CAT telescope and with the CORALIE EchelleSpectrograph on the 1.2-m Euler Swiss telescope, both situated at LaSilla, ChileTable \ref{observations} is only available in electronic form athttp://www.edpsciences.org
| β Cep stars from a spectroscopic point of view
In this review we present the current status of line-profile-variationstudies of β Cep stars. Such studies have been performed for 26bright members of this class of pulsating stars in the past 25 years. Wedescribe all these currently available data and summarize theinterpretations based on them in terms of the excited pulsation modes.We emphasize that line-profile variations offer a much more detailedpicture of the pulsational behaviour of pulsating stars compared toground-based photometric data. The latter, however, remain necessary tounravel the often complex frequency pattern and to achieve unambiguousmode identification for multiperiodic β Cep stars and also toderive the pulsational properties of the faint members of the class. Wehighlight the statistical properties of the sample of 26 stars for whichaccurate spectroscopic studies are available and point out some futureprospects.
| Interstellar Silicon Abundance
We present 34 measurements of silicon gas phase column densities in theinterstellar medium. We have used spectra containing the SiII 1808 Angline which were obtained with the Goddard High Resolution Spectrograph(GHRS) aboard the Hubble Space Telescope (HST). Extinction curveparameters are determined for analyzed lines of sight and relationbetween Si/H ratio and extinction parameters is discussed. We find theabundance of gas phase silicon in diffuse clouds to be lower than thesolar value by a factor of four.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 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/404/913
| Spectroscopic mode identification for the beta Cephei star EN (16) Lacertae
We perform for the first time spectroscopic mode identification in theeclipsing binary beta Cephei star EN (16) Lac. This mode identificationis based upon a time series of 942 line profiles of the He I lambdalambda 6678 Åline in its spectrum. All three known frequenciesf1, f,SUB>2,/SUB>, f3 of the star arepresent in the line-profile variations, but we failed to find additionalmodes. Using different identification methods we find conclusiveevidence for the radial nature of the main mode and for the l=2, m=0identification of the mode with frequency f2. A uniqueidentification of the third mode is not possible from the spectra, butwe do derive that l3<3. Fits to the amplitude and phasevariability of the modes imply a rotation frequency between 0.1 and 0.4c d-1. The star's rotation axis is not aligned with theorbital axis.Based on observations gathered with the coudé spectrographattached to the 2.0 m reflector telescope at Tautenburg Observatory.
| Identification of non-radial pulsation modes in the close-binary beta Cephei star nu Centauri
We present time series of high-resolution spectra of the beta Cepheistar nu Cen, showing multi-periodic variations in the SiIII lambda 4552and lambda 4567 line profiles. After correction for the binary motion,we analyse the time series of spectra by Fourier transforms for eachposition in the line profile. For selected frequencies we perform adirect fit of multiple sinusoids. Four frequencies are needed todescribe the pattern of moving bumps in the line profiles. We presentthe distributions of variational amplitude and phase across the lineprofile and use these diagnostics to show that most of the variabilitycan be explained by multiple non-radial pulsations. From the phasedistributions across the line profiles we derive estimates for thedegree l of the individual pulsation modes in nu Cen. We find l valuesranging from 6 to 10. We find no trace of a previously reportedlow-degree l <~ 2 pulsation with a period of 0.17 day. We brieflydiscuss these observations, and those of the other close-binary betaCephei stars alpha Vir and psi 2 Ori, in terms of tidaleffects on pulsation-mode stability. Based on data taken at ESO LaSilla.
| The mass ratio distribution of B-type visual binaries in the Sco OB2 association
A sample of 115 B-type stars in the Sco OB2 association is examined forexistence of visual companions in the J and K_s bands, using the ADONISnear-infrared adaptive optics system and coronograph. Practically allthe components in the separation range 0farcs3 -6farcs4 (45-900 AU) andmagnitudes down to K = 16 were detected. The K and J - K photometry ofthe primaries and differential photometry and astrometry of the 96secondaries are presented. Ten secondaries are new physical components,as inferred from the photometric and statistical criteria, while therest of the newly detected objects are faint background stars. After asmall correction for detection incompleteness and a conversion of thefluxes into masses, an unbiased distribution of the components massratio q was derived. The power law f(q)~ q-0.5 fits theobservations well, whereas a q-1.8 distribution, whichcorresponds to a random pairing of stars, is rejected. The companionstar fraction is 0.20+/-0.04 per decade of separation which iscomparable to the highest measured binary fraction among low-mass PMSstars and ~ 1.6 times higher than the binary fraction of low-mass dwarfsin the solar neighborhood and in open clusters in the same separationrange. Based on observations collected at the European SouthernObservatory, La Silla, Chile (ESO programme 65.H-0179). Tables 1, 3 andthe full version of Table 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/cgi-bin/qcat?J/A+A/382/92
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| Apsidal motion and non-radial pulsations in psi 2 Ori
We present a time series of high-resolution echelle spectra of thedouble-lined close binary psi 2 Ori. The spectra sample thewavelength region of 3800-6800 Å. In the absorption lines of theearly-B type primary we find clear evidence for non-radial pulsationswith intermediate values of the modal degree l. Using across-correlation technique we derive the radial velocity of bothcomponents. We compare our orbital solution with those reported in theliterature to derive the apsidal motion period in this system: 47.5 +/-0.7 year. We analyse the absorption line profiles of the primary usingFourier techniques to derive apparent pulsation periods and l values oftwo detected modes with apparent frequencies f_1=10.48 c/d and f_2=10.73c/d. We discuss whether the non-radial pulsations in this star areinternally excited or due to tidal forcing. Comparing the pulsationfrequencies with those expected for tidal forcing and for internallyexcited modes, we tentatively conclude that these modes are probably dueto internally excited beta Cephei pulsations. Based on observations madewith the Isaac Newton Telescope operated on the island of La Palma bythe Isaac Newton Group in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.
| Line-profile variations of the double-lined spectroscopic binary kappa Scorpii
We present a total time series of high signal-to-noise, high-resolutionspectra of the SiIII lambda 4552.6, lambda 4567.8 and lambda 4574.8Å lines of the fast-rotating, spectroscopic binary and beta Cepheistar kappa Scorpii. Among this data set is a sample of 422 spectra ofintensive monitoring during eight subsequent nights in July 1997. Wefind variability of the line profiles on two time-scales: a variation ofseveral months as a result of the orbital motion and rapid variations ofa few hours, which are explained in terms of non-radial pulsation modes.From the total dataset, covering a time span of 9 years, we derive forthe first time the orbital parameters of kappa Scorpii and find anorbital period of 195 days. The complex patterns on the grayscalerepresentations of the residual SiIII lambda 4552.6 Å profileswith respect to the average profile point towards more than one(non-axisymmetric) pulsation mode. A frequency analysis of the threenormalised velocity moments confirms the main period of 4.80 hours andthe second period of 4.93 hours, which were previously derived fromphotometric data. We attempt an identification of the pulsation modes bymeans of the moment method and line-profile fitting and find that kappaScorpii pulsates in a main prograde sectoral mode of degree 1 or 2 andin an additional tesseral mode for which l = 5, m = 1 are the mostlikely wavenumbers. In the variations of the line profiles we findindications for the presence of additional pulsation modes. Based onobservations obtained at ESO with the CAT/CES, La Silla, Chile.
| 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
| OB association members in the ACT and TRC catalogues
The Hipparcos Catalogue contains members of nearby OB associationsbrighter than 12th magnitude in V. However, membership lists arecomplete only to magnitude V=7.3. In this paper we discuss whetherproper motions listed in the `Astrographic Catalogue+Tycho' referencecatalogue (ACT) and the Tycho Reference Catalogue (TRC), which arecomplete to V~10.5mag, can be used to find additional associationmembers. Proper motions in the ACT/TRC have an average accuracy of~3masyr-1. We search for ACT/TRC stars which have propermotions consistent with the spatial velocity of the Hipparcos members ofthe nearby OB associations already identified by de Zeeuw et al. Thesestars are first selected using a convergent-point method, and thensubjected to further constraints on the proper-motion distribution,magnitude and colour to narrow down the final number of candidatemembers. Monte Carlo simulations show that the proper-motiondistribution, magnitude, and colour constraints remove ~97per cent ofthe field stars, while at the same time retain more than 90per cent ofthe cluster stars. The procedure has been applied to five nearbyassociations: the three subgroups of Sco OB2, plus Per OB3 and Cep OB6.In all cases except Cep OB6, we find evidence for new associationmembers fainter than the completeness limit of the Hipparcos Catalogue.However, narrow-band photometry and/or radial velocities are needed topinpoint the cluster members, and to study their physicalcharacteristics.
| 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.
| 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
| Beryllium abundance in lithium-rich giants
About 2% of the Population I red giants show lithium abundancessignificantly larger than expected by dilution due to mixing byclassical convection and, in some of them, the lithium abundance reachesvalues similar to (and even larger than) the Pop I value (meteoritic,open clusters etc.) around logN(Li) = 3.3. The classical convectionpredicts also a (smaller) dilution of beryllium and (an even smallerone) of boron. Two main interpretations of the Li-rich giants arepossible: the initial Li has been somehow preserved (perhaps by theinhibition of the classical mixing) on the contrary, a mixing, deeperthan the classical one, took place in some (or all?) giants, leading toa dilution of the superficial lithium but sometimes overcompensating forit by an internal production of lithium, with transport to the surface(by the Cameron-Fowler mechanism). In the first interpretation, Be (morerobust than Li) is a fortiori preserved, in the second one, a mixingdeeper than the classical one should dilute Be more than the classicalBe dilution. We have observed the Be II lambda 3130.420 and 3131.066Angstroms lines in two Li-rich giants and three reference stars: oneLi-poor giant and alpha Cen A and B. The observations were carried outat the ESO 3.6m telescope using the CASPEC spectrograph. By comparingthe observed spectra with spectrum synthesis calculations we show thatfor the three giant stars the Be abundance is low, suggesting that, inthese giants, Be is very depleted (>90%) from the initial Pop I value(logN(Be) ~ 1.4). This result implies that the original Li in thesestars must have been almost completely destroyed, and that the high Liabundances in the Li-rich red giants is most probably due to Liproduction in these stars. Observations collected at the EuropeanSouthern Observatory -- ESO, Chile
| Towards understanding rapid line-profile and light variations of early-type stars. 3. Some thoughts and reflections
The current situation in the research of rapid line-profile and lightvariations of early-type stars is critically reviewed. It is suggestedthat the ultimate understanding of the physical processes causing thesevariations can only come from an open-minded and complex approach to theproblem and from systematic observational effort. It is argued that theresults of the search for periodicities in the complicated variations ofthese objects depend critically on whether the method used isappropriate to the real physical situation. The danger of detection of afalse multiperiodicity is pointed out for two particular situations: (i)a single-periodic signal which undergoes slow periodic change, e.g., dueto the light time effect in a binary system, and (ii) a single-periodicsignal with a complicated phase curve (a model of not exactlyequidistant corotating spokes). It is argued that the observed rapidvariations need not be due to classical non-radial pulsations but mayarise from more complicated velocity fields in the stellar atmospheresand/or mantles. Two early-type stars, ěp and zeta Oph, arediscussed in detail. It is argued that both may be the cases where thevariations are caused by corotating structures slightly above thestellar photosphere. For ěn, the pattern of the variations canalso be affected by the motion of the star in a binary orbit. For zetaOph, a double-wave light curve with the corotation period of 0. (v {r) md}64 (suggested by the author earlier for the line-profile variations)was found from Hipparcos V photometry and its presence can also besuspected in other existing photometric data and in the recurrence timesof the narrow features seen in the UV spectra.
| On the normal spectral energy distribution of stars: Spectral types O9-B5
The normal energy distributions for fifteen spectral subtypes from O9 toB5 for luminosity classes V, IV, and III are derived. Threephotometrically uniform catalogs served as the source of thespectrophotometric data used. Synthetic color indices for all spectraltypes are calculated using the energy distribution curves obtained.Comparison of these indices with the expected normal color indicessuggests that the energy distributions derived are reliable.
| High S/N Echelle spectroscopy in young stellar groups. II. Rotational velocities of early-type stars in SCO OB2.
We investigate the rotational velocities of early-type stars in the ScoOB2 association. We measure v.sin(i) for 156 established and probablemembers of the association. The measurements are performed with threedifferent techniques, which are in increasing order of expectedv.sin(i): 1) converting the widths of spectral lines directly tov.sin(i), 2) comparing artificially broadened spectra of low v.sin(i)stars to the target spectrum, 3) comparing the HeI λ4026 lineprofile to theoretical models. The sample is extended with literaturedata for 47 established members of Sco OB2. Analysis of the v.sin(i)distributions shows that there are no significant differences betweenthe subgroups of Sco OB2. We find that members of the binary populationof Sco OB2 on the whole rotate more slowly than the single stars. Inaddition, we find that the B7-B9 single star members rotatesignificantly faster than their B0-B6 counterparts. We test varioushypotheses for the distribution of v.sin(i) in the association. Theresults show that we cannot clearly exclude any form of randomdistribution of the direction and/or magnitude of the intrinsicrotational velocity vector. We also investigate the effects of rotationon colours in the Walraven photometric system. We show that positions ofB7-B9 single dwarfs above the main sequence are a consequence ofrotation. This establishes the influence of rotation on the Walravencolours, due primarily to surface gravity effects.
| The Pulkovo Spectrophotometric Catalog of Bright Stars in the Range from 320 TO 1080 NM
A spectrophotometric catalog is presented, combining results of numerousobservations made by Pulkovo astronomers at different observing sites.The catalog consists of three parts: the first contains the data for 602stars in the spectral range of 320--735 nm with a resolution of 5 nm,the second one contains 285 stars in the spectral range of 500--1080 nmwith a resolution of 10 nm and the third one contains 278 stars combinedfrom the preceding catalogs in the spectral range of 320--1080 nm with aresolution of 10 nm. The data are presented in absolute energy unitsW/m(2) m, with a step of 2.5 nm and with an accuracy not lower than1.5--2.0%.
| The Distribution of Dust Clouds in the Interstellar Medium
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...457..764D&db_key=AST
| 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.
| The threshold for stellar winds in hot main-sequence stars
The profiles of ultraviolet resonance lines of C IV were surveyed in asample of 29 cluster and association members in the spectral type rangeO9-B2 III-V, together with a few field stars of interest. Thetemperatures and gravities of the stars were taken from the modelatmosphere analysis by Grigsby, Morrison, & Anderson (1992), and theluminosities were estimated on the basis of cluster and associationdistances from the recent literature. A parameter Pw wasdefined in order to describe the degree and assymetry of the C IVprofile. This parameter, together with total C IV equivalent width, wasfound to be well correlated with stellar luminosity and temperature. Afew anomalous stars were noted: tau Sco, HD 66665, HD 13621, and the ONstars HD12323 and HD 201345. The results suggest a sudden onset ofobservable mass loss at T(effective) = 27,500 +/- 500 K, log (L/solarluminosity) = 4.4 +/- 0.12, in agreement with the previous study byPrinja (1989). At T(effective) = 28,000 K and log g = 4, our non-LTEmodel atmospheres show an enhancement in the ground-state population ofC(+3) in their topmost layer, which could be responsible for initiationof the winds via radiation pressure on the C(+3) ions, or for the onsetof visibility of C(+3) ions in the wind because of an increase in theoptical depth in the C IV lines in the outermost layers.
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Membro dei seguenti gruppi:
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Osservazione e dati astrometrici
| Costellazione: | Centauro |
| Ascensione retta: | 13h49m30.30s |
| Declinazione: | -41°41'16.0" |
| Magnitudine apparente: | 3.41 |
| Distanza: | 145.56 parsec |
| Moto proprio RA: | -24.6 |
| Moto proprio Dec: | -20.5 |
| B-T magnitude: | 3.128 |
| V-T magnitude: | 3.352 |
Cataloghi e designazioni:
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INHABITED SKY
