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# μβ Sco (Pipirima)

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 A high-resolution spectroscopy survey of β Cephei pulsations in bright starsWe 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 associationWe 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. Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degrWe 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 The mass ratio distribution of B-type visual binaries in the Sco OB2 associationA 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 associationsRadial 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 Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statisticsThe 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 A HIPPARCOS Census of the Nearby OB AssociationsA 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 unboundmoving 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 HemisphereObservations 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 A Search for Helium Spectrum VariablesNot Available On the normal spectral energy distribution of stars: Spectral types O9-B5The 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. The Scorpius OB2 ComplexThe Sco OB2 complex is a member of the Local Association. Theassociation contains pre-main-sequence stars in addition to objects some3 x 10^7 yr old. If it is assumed that stars in wide binaries arecoeval, then the He-weak variables in the supercluster arepre-main-sequence, elevated above that sequence by an amount similar tothat of the 3 x 10^7 yr old, normal stars but contracting toward thesequence, not evolving from it. The apparent great depth of theassociation is probably a result of some foreground superclustermembers, superposed on Sco OB2, and a rift in the absorption cloudbetween Scorpius and Ophiuchus. The adjoining Chamaeleon complex alsoappears to be a member of the supercluster. Re-analysis of scintillation effects from gravitational waves.In this article, we continue A. Labeyrie's work concerning the detectionof Gravitational Waves through the deformations of optical wavefronts.We give the analytical expressions of the wavefront distortions. Specialattention is paid to the discussion of this approach with respect toother works. The wavefront distortions are expected to cause ascintillation effect for a distant observer. The observability of thiseffect is discussed in the cases of binary stars and millisecondpulsars. It seems unobservable for a single monochromatic source ofgravitational waves. 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 NMA 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 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 starsThe 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. 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. The determination of T_eff_ of B, A and F main sequence stars from the continuum between 3200 A and 3600 A.A method of determination of the effective temperature of B, A and Fmain sequence stars is proposed, using the slope of the continuumbetween 3200A and 3600A. The effective temperature calibration is basedon a sample of stars with energy distributions known from UV to the red.We have determined the Balmer jump and the effective temperatures for235 main sequence stars. The temperatures found have been compared withthose derived by Underhill et al. (1979), Kontizas & Theodossiou(1980), Theodossiou (1985), Morossi & Malagnini (1985). Thecomparison showed good agreement for most of the stars in common. On theother hand, the temperatures derived from the reddening-free colourfactor QUV, from the colour index (m1965-V) and from (B-V), given inGulati et al. (1989), are systematically lower than our temperatures,however the differences are within one-sigma error. The local distribution of NA I interstellar gasWe present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures. Non-LTE, line-blanketed model atmospheres for late O- and early B-type starsThe use of non-LTE line-blanketed model atmospheres to analyze thespectra of hot stars is reported. The stars analyzed are members ofclusters and associations, have spectral types in the range O9-B2 andluminosity classes in the range III-IV, have slow to moderate rotation,and are photometrically constant. Sampled line opacities of iron-groupelements were incorporated in the radiative transfer solution; solarabundances were assumed. Good to excellent agreement is obtained betweenthe computed profiles and essentially all the line profiles used to fixthe model, and reliable stellar parameters are derived. The synthetic MII 5581 equivalent widths agree well with the observed ones at the lowend of the temperature range studied, but, above 25,000 K, the syntheticline is generally stronger than the observed line. The behavior of theobserved equivalent widths of N II, N III, C II and C III lines as afunction of Teff is studied. Most of the lines show much scatter, withno consistent trend that could indicate abundance differences from starto star. Night-sky brightness from the visibility of stars near the horizonHundreds of naked-eye observations of bright stars near the southernhorizon have been made on clear moonless nights over the interval1977-91. They reveal a consistency in their visibility from one seasonor year to another, but a deterioration over the entire time intervalreveals a possible increase in light pollution and possibly airpollution as well. The method is constrained to the sky near thenorthern and southern horizons where stars remain at near-constantaltitude over a suitable time interval, but the repeatability assuresthat it is sound. The limiting magnitude is given as a function ofzenith distance, and color and seasonal effects do not appear toinfluence the results. The stellar temperature scale for stars of spectral types from O8 to F6 and the standard deviation of the MK spectral classificationEmpirical effective temperature of 211 early-type stars found in aprevious investigation (Kontizas and Theodossiou, 1980; Theodossiou,1985) are combined with the effective temperatures of 313 early-typestars from the literature. From these effective temperatures of a totalnumber of 524 early-type stars of spectral types from O8 to F6 a newstellar temperature scale is developed along with the standard deviationof the MK spectral classification. Effects of rotation on colours and line indices of stars. III - Binary and peculiar starsThe broad-band UBV colors and the intermediate band uvby colors of AlphaPersei, Pleiades, and the Scorpio-Centaurus association are analyzed forrotation effects. An attempt was made to discriminate normal singlestars from that of binary and peculiar stars after taking the observedrotation effects into account. It is found that the spread in theobserved colors does not in general allow such a discrimination, exceptthat the objects with large reddening are double-lined binaries,peculiar stars or emission-lined objects. The few normal stars in thesethree clusters with such large reddening are listed as they are likelyto belong to one of the above classes. Luminosities, masses, and ages of B-type starsMeasurements of H-gamma lines and photometry of the Balmer discontinuityare combined with models of stellar atmospheres and interiors to derivevalues of T(eff) log g, mass, and M(bol) for main-sequence B-type starsin the Alpha Per and the Pleiades clusters and in the Orion, Sco-Gen,and I Lac associations. The masses as a function of T(eff) agree withthe analyses of eclipsing binary orbits. The luminosities yielddistances for the observed clusters and associations that are in goodagreement with independent determinations. The observed main sequence isbroader than predicted by standard models and suggests that mixing mayplay a role in extending the main-sequence lifetime. Stars within eachcluster and association show a range of ages, a result that suggeststhat star formation occurred over a period of at least 1-3 x 10 to the7th yr. Walraven photometry of nearby southern OB associationsHomogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included. Catalogue of i and w/w crit values for rotating early type starsNot Available Stellar integrated fluxes in the wavelength range 380 NM - 900 NM derived from Johnson 13-colour photometryPetford et al. (1988) have reported measured integrated fluxes for 216stars with a wide spread of spectral type and luminosity, and mentionedthat a cubic-spline integration over the relevant Johnson 13-colormagnitudes, converted to fluxes using Johnson's calibration, is inexcellent agreement with those measurements. In this paper a list of thefluxes derived in this way, corrected for a small dependence on B-V, isgiven for all the 1215 stars in Johnson's 1975 catalog with completeentries. Physical parameters of stars in the Scorpio-Centaurus OB associationWalraven photometry is presented of established and probable members ofthe Scorpio-Centaurus OB association. For each star, effectivetemperature and surface gravity are derived using Kurucz (1979)atmosphere models. From the Straizys and Kuriliene (1981) tables,absolute magnitudes are calculated. Distance moduli and visualextinctions are determined for all stars. From a comparison of theHR-diagrams of the stars in each subgroup with theoretical isochrones,the ages of the three subgroups are derived. The distances to the threesubgroups are shown to be different; there is a general trend (alsowithin each subgroup) for the distances to be larger at higher galacticlongitudes. The visual extinction in the youngest subgroupUpper-Scorpius, is well correlated with the IRAS 100-micron map. Thedistance toward the Ophiuchus dark clouds is found to be 125 pc, basedon the photometric distances to the stars. Most of the early-type starsin Upper-Scorpius are located at the far side of the dark clouds. Effects of stellar rotation on the Geneva photometric systemThe effects of stellar rotation on colors and parameters of the Genevaphotometric system are considered, using homogeneous material. Attentionis focused on these parameters useful for deriving physical propertiesof B- and A-type stars. Two major photometric planes in this respect,the (X, Y) plane and the (d, Delta) plane, are not discernibly affectedby rotation. The temperature parameter, B(2) - V(1) is reddened byrotation to an extent that is in agreement with model calculations foruniformly rotating stars. The nitrogen abundance in the atmospheres of single and binary starsThe nitrogen abundances for 47 single, 11 visual double, and 41 closebinary stars are obtained from high-dispersion Wlambdadeterminations of nitrogen lines using atmospheric models andtheoretical calculations of N II lines, taking LTE errors into account.The mean value of lg N(N) is found to be 8.05 for single stars, 8.50 forclose binaries, and 8.15 for visual double stars. A correlation is foundbetween the nitrogen abundance and the helium and carbon abundances. Itis suggested that considerable mixing between the central part of starsand the atmosphere occurs in binary stars.
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