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Statistical properties of a sample of periodically variable B-type supergiants. Evidence for opacity-driven gravity-mode oscillations
Aims.We have studied a sample of 28 periodically variable B-typesupergiants selected from the HIPPARCOS mission and 12 comparison starscovering the whole B-type spectral range. Our goal is to test if theirvariability is compatible with opacity-driven non-radialoscillations. Methods: .We have used the NLTE atmosphere codeFASTWIND to derive the atmospheric and wind parameters of the completesample through line profile fitting. We applied the method to selectedH, He, and Si line profiles, measured with the high resolution CESspectrograph attached to the ESO CAT telescope in La Silla, Chile.Results: .From the location of the stars in the (log T_eff, log g)diagram, we suggest that variability of our sample supergiants is indeeddue to the gravity modes resulting from the opacity mechanism. We findnine of the comparison stars to be periodically variable as well, andsuggest them to be new α Cyg variables. We find marginal evidenceof a correlation between the amplitude of the photometric variabilityand the wind density. We investigate the wind momentum-luminosityrelation for the whole range of B spectral type supergiants, and findthat the later types (>B5) perfectly follow the relation for Asupergiants. Additionally, we provide a new spectral type - T_effcalibration for B supergiants. Conclusions: .Our results imply thepossibility of probing internal structure models of massive stars ofspectral type B through seismic tuning of gravity modes.Figures of the spectral line fits and discussion of the individualobjects, Appendices A, B and Table 6 are only available in electronicform at http://www.aanda.org
| The VLT-FLAMES survey of massive stars: stellar parameters and rotational velocities in NGC 3293, NGC 4755 and NGC 6611
An analysis is presented of VLT-FLAMES spectroscopy for three Galacticclusters, NGC 3293, NGC 4755 and NGC 6611. Non-LTE model atmospherecalculations have been used to estimate effective temperatures (fromeither the helium spectrum or the silicon ionization equilibrium) andgravities (from the hydrogen spectrum). Projected rotational velocitieshave been deduced from the helium spectrum (for fast and moderaterotators) or the metal line spectrum (for slow rotators). The origin ofthe low gravity estimates for apparently near main sequence objects isdiscussed and is related to the stellar rotational velocity. Theatmospheric parameters have been used to estimate cluster distances(which are generally in good agreement with previous determinations) andthese have been used to estimate stellar luminosities and evolutionarymasses. The observed Hertzsprung-Russell diagrams are compared withtheoretical predictions and some discrepancies including differences inthe main sequence luminosities are discussed. Cluster ages have beendeduced and evidence for non-coeval star formation is found for allthree of the clusters. Projected rotational velocities for targets inthe older clusters, NGC 3293 and NGC 4755, have been found to besystematically larger than those for the field, confirming recentresults in other similar age clusters. The distribution of projectedrotational velocities are consistent with a Gaussian distribution ofintrinsic rotational velocities. For the relatively unevolved targets inthe older clusters, NGC 3293 and NGC 4755, the peak of the velocitydistribution would be 250 km s-1 with afull-width-half-maximum of approximately 180 km s-1. For NGC6611, the sample size is relatively small but implies a lower meanrotational velocity. This may be evidence for the spin-down effect dueto angular momentum loss through stellar winds, although our results areconsistent with those found for very young high mass stars. For allthree clusters we deduce present day mass functions with Γ-valuesin the range of -1.5 to -1.8, which are similar to other young stellarclusters in the Milky Way.
| The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611
We introduce a new survey of massive stars in the Galaxy and theMagellanic Clouds using the Fibre Large Array Multi-Element Spectrograph(FLAMES) instrument at the Very Large Telescope (VLT). Here we presentobservations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph(R ≃ 25 000), in fields centered on the open clusters NGC 3293,NGC 4755 and NGC 6611. These data are supplemented by a further 50targets observed with the Fibre-Fed Extended Range Optical Spectrograph(FEROS, R = 48 000). Following a description of our scientificmotivations and target selection criteria, the data reduction methodsare described; of critical importance the FLAMES reduction pipeline isfound to yield spectra that are in excellent agreement with lessautomated methods. Spectral classifications and radial velocitymeasurements are presented for each star, with particular attention paidto morphological peculiarities and evidence of binarity. Theseobservations represent a significant increase in the known spectralcontent of NGC 3293 and NGC 4755, and will serve as standards againstwhich our subsequent FLAMES observations in the Magellanic Clouds willbe compared.
| Interstellar CN toward CH+-forming regions
Measurements on interstellar CN absorption are presented for stars inthree southern OB associations, NGC 2439, Vela OB1, and Cen OB1. CN isdetected in 21 out of 31 stars observed. The doublet ratio for the R(1)and P(1) lines of the (0, 0) band of the B 2Sigma+-X 2Sigma + violet system and acomparison of violet system data with measurements of the (1, 0) and (2,0) bands of the A 2Pi -X 2Sigma + redsystem are used to derive Doppler parameters and total column densities.Inferred CN column densities vary by more than an order of magnitude forlines of sight with similar CH column densities. Observations of the (0,0) band of the CH B 2Sigma --X 2Pisystem are used to revise previously published CH column densitiestoward the lines of sight studied in CN. Together with earlier resultson CH, CH+, and C2, the CN data presented hereprovide a homogeneous set of column densities and radial velocities ofdiatomic molecules in three individual translucent clouds. We use thesedata to study CN production via chemical models. Gas densities areinferred from models based on production via CH and C2 incool gas. Most sightlines in our sample test densities typical fordiffuse molecular gas (a few hundred cm-3 ) when theultraviolet flux permeating the gas is between 1 and 5 times the averageinterstellar flux. A few lines of sight indicate that CN is producedunder dark cloud conditions because relatively large densities areobtained or because this simple chemical scheme is unable to reproducethe observed CN columns. Low densities are indicated for directions withupper limits on CN. We add an ad hoc component of a number oflow-velocity (<10 km s-1) criss-crossing MHD shocks toexplain observed column densities of interstellar CH+. Theseshocks also produce about 10 to 30% of the total CH column along theline of sight.
| Light element abundances in the young open clusters NGC 3293, NGC 4755 and NGC 6231: Tracers of stellar evolution
The abundances of He (LTE), C, N, and O (NLTE) were derived for 21 Bstars in three young open clusters. Almost all the stars show subsolarCNO abundances. However, the mean oxygen abundance for each programmecluster appears to be in marginal agreement with the most recentrevisions of the solar value. After consideration of the CN abundancesin this sample, there is no clear evidence of internal mixing. Onlythree stars among the non-supergiants seem to show a nitrogenenhancement. Two of them have a fairly low projected equatorial velocity(admittedly, they may be rapid rotators seen pole-on); the third one isa definite fast rotator. In the lower gravity stars (three stars in thissample with log g < 3.0) some kind of mixing has apparently occurred.The supergiants do not differ significantly from the other programmestars in their respective helium contents. The mean helium abundance foreach cluster is close to the standard value, (He/H) ~ 11.0. Based onspectra collected at the European Southern Observatory, La Silla, Chile,as part of programme 47.7-045.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Misidentifications of Stars in NGC 4755 (κ Crucis)
Erroneous cross-identifications appearing in the literature and in theSIMBAD database for stars in the Jewel Box cluster, NGC 4755, arecorrected. These errors concern mistaken cross-references for CPD,Bright Star Catalog, and Luminous Stars in the Southern Milky Waynumbers to stars identified in the analysis of this cluster carried outby Arp & van Sant.
| H I Shells behind the Coalsack
We report the discovery of two new large H I shells in the direction ofthe Coalsack Nebula. Both shells were observed with the Parkes RadioTelescope as part of the Southern Galactic Plane Survey. The largestshell, GSH 304-00-12, is at a distance of ~1.2 kpc and has derivedphysical dimensions of 280×200 pc. The second shell, GSH305+01-24, is at a distance of ~2.2 kpc and has derived dimensions of280×440 pc. We present a simple numerical model to show that GSH305+01-24 most likely formed from stellar winds in the Centaurus OB1stellar association. There is associated radio, infrared, and Hαcontinuum emission. Both shells are situated in the Sagittarius-Carinaarm, with GSH 305+01-24 more distant. The far edge of GSH 304-00-12 isat the near side of the arm and opens into the interarm region. We findno evidence of closure at the near side of the shell and thereforedescribe the geometry as conical. Emission from the near side of theshell may be lost in absorption by the Coalsack Nebula.
| Photometric and kinematic studies of open star clusters. III. NGC 4103, NGC 5281, and NGC 4755
We present CCD photometry and proper motion studies of the three openstar clusters NGC 4103, NGC 5281, and NGC 4755 (kappa Cru). By fittingisochrones to the colour magnitude diagrams, we found that all threeobjects are young open star clusters with ages of at most t=45 Myr. Theyare located at distances from approx. 1600 pc to 2200 pc, derived fromdistance moduli (m-M)_0 ranging from 11 mag to 12 mag. We combinedmembership determinations based on proper motions and statistical fieldstar subtraction to derive the initial mass function (IMF) of theclusters. The shape of the IMFs could be represented by power laws withexponents of Gamma =-1.46 +/- 0.22 for NGC 4103, Gamma =-1.60 +/- 0.50for NGC 5281, and Gamma =-1.68 +/- 0.14 for NGC 4755, when - as areference - Salpeter's (\cite{salpeter}) value would be Gamma =-1.35.These results agree well with other IMF studies of open star clusters.Partly based on observations from ESO, La Silla.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Interstellar CH^+^ in southern OB associations.
Optical absorption line observations of interstellar CH^+^ and CH arepresented towards the southern OB associations CMa OB1, NGC 2439, VelaOB1, NGC 4755, and Cen OB1. A total of 5-11 stars per association wereobserved, with visual extinctions ranging from A_V_=0.5-4.5mag. Thederived CH^+^ and CH velocities agree within the measurement errors.Towards a particular association, the CH^+^ column density N(CH^+^) iscorrelated to the visual extinction of the background star. Thesefindings weaken the possibility that magnetic shocks are the generalmechanism that is required to produce interstellar CH^+^. It is foundthat N(CH^+^) is correlated to N(CH), which indicates that N(CH^+^) iscorrelated to the optical depth of a cloud. The correlations aredifficult to reconcile with scenarios where the sites of CH^+^ formationare constrained to the surface of molecular clouds. The observationssupport ideas which involve turbulence as a major CH^+^ productionmechanism. In particular, the results are in agreement with expectationsfrom a scenario where the CH^+^ formation proceeds in cool gas via afraction of fast, non-Maxwellian H_2_ or C^+^, created by thedissipation of interstellar turbulence.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
| 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 spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| Stromgren and H-Beta Photometry of Associations and Open Clusters - Part Three - CENTAURUS-OB1 and CRUX-OB1
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.269..289K&db_key=AST
| Observational tests of convective core overshooting in stars of intermediate to high mass in the Galaxy
This study presents the results of 14 tests for the presence ofconvective overshooting in large convecting stellar cores for stars withmasses of 4-17 solar masses which are members of detached close binarysystems and of open clusters in the Galaxy. A large body of theoreticaland observational data is scrutinized and subjected to averaging inorder to minimize accidental and systematic errors. A conservative upperlimit of d/HP less than 0.4 is found from at least four tests, as wellas a tighter upper limit of d/HP less than 0.2 from one good test thatis subject to only mild restrictions and is based on the maximumobserved effective temperature of evolved blue supergiants. It isconcluded that any current uncertainty about the distance scale forthese stars is unimportant in conducting the present tests forconvective core overshooting. The correct effective temperature scalefor the B0.5-B2 stars is almost certainly close to one of the proposedhot scales.
| The volume filling factor of the infrared cirrus IS 0.2
The filling factor of the infrared cirrus in which luminous stars areembedded is estimated. The stars are assumed to be randomly situatedwith respect to the cirrus. A direct ratio of the number of stars withassociated cirrus emission to those which in principle could be detectedindicates that the cirrus filling factor is roughly 0.2.
| Stromgren and H-beta photometry of stars earlier than G0 in the Southern Coalsack direction
Four-color and H-beta photometry have been obtained for 236 starsearlier than G0 in order to study the color excess distribution in theSouthern Coalsack direction. The beta histogram of the stars reveals arelative absence of stars in the range of A4 to A9. The (b-y)distribution for B-type stars suggests the existence of three differenttypes of stars located at different distances.
| Evolution of massive stars - Comparison of cluster sequences and models with mass loss
A comparison between theoretical isochrones of models with mass loss andobserved sequences of 25 clusters younger than 25 million years, ismade. The number counts are compared with the theoretically expectedstar numbers in various parts of the isochrones, and the effects ofdifferences in mass loss rates and stellar opacities are considered. Thestudy indicates that the theoretical main sequence band with mass lossis wider than the observed main sequence band for very young clusterswith turnoff at type earlier or equal to B0, and is smaller for olderclusters. A discrepancy of 1.0-1.5 mag is found for clusters with an ageof 10-21 million years. It is suggested that some extra-mixing,overshooting, or turbulent diffusion is at the origin of the observeddifferences, with an overshooting distance to pressure scale height atthe edge of the classical convective core ratio of about 0.3 for massivestars in the range of 9-15 solar masses.
| UBV photometry of the southern galactic cluster NGC 4755 = Kappa Crucis
UBV photoelectric magnitudes and colors are presented for 86 starstogether with photographic magnitudes and colors for 553 stars in thevicinity of the southern young open cluster NGC 4755. A true distancemodulus of 11.82 m is derived corresponding to a linear distance of 2.3kpc. The evolutionary age is determined to be between 8 and 11.5 x 10 tothe 6th years, depending on the assumed chemical composition of thecluster stars. The mean interstellar reddening E(B-V) across the clusterincreases with galactic latitude and angular distance from the Coalsackfrom about 0.40 m to 0.48 m with an average value of 0.44 m. The totalnumber of probable cluster members detected in the field down to alimiting visual magnitude of V = 16.0 m is 203. The linear gravitationalradius of the cluster obtained from star strip counts down to visualmagnitude 14.5 m is found to be 3.4 pc. The luminosity function derivedfor NGC 4755 is in good agreement with the initial luminosity functionas given by Sandage (1957). The cluster contains six supergiants andthree variable Be stars.
| A study of B-type supergiants with the uvby,beta photometric system
The applicability of the uvby,beta photometric system to theclassification and study of B-type supergiants (BTS) is investigatedusing published data on 157 BTS and observations of 17 BTS made with the36-in. reflector at McDonald Observatory. The results are presented intabular form and analyzed to produce preliminary calibrations ofluminosity class vs. beta index and of absolute magnitude (Mv) vs. beta(or delta Mv vs. delta beta) for four associations of stars. Theeffectiveness of various color indices as temperature indicators isdiscussed. It is shown that there is good correspondence between MK anduvby,beta classifications of B-type main-sequence stars, giants, andBTS, confirming the usefulness of the uvby,beta system in furtherresearch on BTS.
| New UBVRI photometry for 900 supergiants
A description is presented of the results obtained in connection with asystematic program of supergiant photometry on the Johnson UBVRI system.During the eight years after the start of the program, almost 1000 starshave been observed, about 400 three or more times each. The originalselection of stars used the spectral type catalog of Jaschek et al.(1964) to choose supergiants. Since observations were possible from bothChile and Canada, no declination limits were imposed, and no particularselection criteria were imposed other than to eliminate carbon stars.These are so red as to require enormous extrapolations of thetransformation equations.
| Catalog of Luminous Stars in the Southern Coalsack Zone
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1982RMxAA...5..183M&db_key=AST
| Balmer-line photometry of NGC 4755
To discriminate between emission and nonemission objects in the analysisof a number of polarization observations of stars in the southern opencluster NGC 4755, a comparison of the H alpha and H beta line strengthsis made. H alpha and H beta photometric observations are presented for28 and 33 stars, respectively, in NGC 4755; eight emission line starsare identified. V-magnitude, spectral type, and alpha and beta indicestogether with standard deviations are included. In general, nosignificant deviations of the beta indices from those measured by Perryet al. (1976) are found.
| Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978ApJS...38..309H&db_key=AST
| A search for Beta Cephei stars. II - NGC 4755
This paper reports the result of a search for Beta Cephei stars in thecluster NGC 4755. Three were found. New evidence about the evolutionarystage of Beta Cephei stars is put forward, which supports the hypothesisthat they are going through the core-hydrogen burning phase.
| Multicolor photometry of the open cluster NGC 4755.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..632P&db_key=AST
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Observation and Astrometry data
| Constellation: | Kreuz des Südens |
| Right ascension: | 12h53m59.80s |
| Declination: | -60°20'07.5" |
| Apparent magnitude: | 6.779 |
| Distance: | 877.193 parsecs |
| Proper motion RA: | -1.5 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 7 |
| V-T magnitude: | 6.798 |
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INHABITED SKY
