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Membership, rotation, and lithium abundances in the open clusters NGC 2451 A and B
High-resolution spectra of 30 late-type and 9 early-type candidatemembers of the young (50-80 Myr) open clusters NGC 2451 A and Bhave been analyzed in order to complement our previous photometric andX-ray study. Cluster membership of these X-ray selected stars has beenconfirmed or rejected on the basis of radial velocity and Hαchromospheric emission. The metallicity of both clusters seems to beabout solar - contrary to previous investigations. Lithium abundanceshave been determined by two different methods, namely curve-of-growthtechniques and spectrum synthesis, yielding quite consistent results.The pattern of Li abundances versus effective temperature resembles thatof the equally-old Alpha Per cluster, i.e., little Li depletion is seenfor solar-type and earlier-type stars, while towards cooler stars Li ismore and more depleted, possibly showing a star-to-star scatter below 5200 K. The hottest star in our sample shows a Li abundance0.5 dex higher than the meteoritic value. Rotational velocitieshave been determined in order to investigate the supposed dependence ofactivity and Li depletion on rotation.Based on observations performed at the European Southern Observatory, LaSilla/Chile.
| An X-ray study of the open clusters NGC 2451 A and B
We have conducted a detailed study of the object NGC 2451, whichactually consists of two different open clusters A and B along the sameline of sight at 206 pc and 370 pc distance, respectively. Althoughbelonging to the nearest clusters, they have not been much investigateduntil present due to strong contamination by field stars. ROSAT X-rayobservations and optical UBVR photometry are used to identify clustermembers by means of X-ray emission and colour-magnitude diagrams. Theidentified stars concentrate nicely around the expected main sequencesin the colour-magnitude diagram at the distances derived fromastrometric investigations. Altogether, 39 stars are identified asmember candidates of the nearer cluster A, 49 stars as member candidatesof the more distant cluster B, and 22 faint stars are probably membersof either of the two clusters, but due to large errors it is not clearto which one they belong. Further 40 stars identified with X-ray sourcesare probably non-members. For the first time, the range of knownprobable cluster members of NGC 2451 A and B has been extended downwardsthe main sequence to stars of spectral class M. Isochrone fitting yieldsan age of 50 to 80 Myrs for NGC 2451 A and ~50 Myrs for NGC 2451 B,consistent with the X-ray luminosity distribution functions, which arecomparable to other clusters in the same age range. Except from theoccurence of four flares, the stars of both clusters do not show stronglong-term X-ray variability exceeding a factor 5 over a time span of 1to 3 years.Based on observations performed by the ROSAT X-ray observatory and theEuropean Southern Observatory.Tables 3-6 are only available in electronic form athttp://www.edpsciences.org
| WIYN Open Cluster Study. VII. NGC 2451A and the Hipparcos Distance Scale
We provide new evidence that NGC 2451A is undoubtedly a young opencluster, although sparsely populated. New cluster membership has beenderived from relative proper motions of 5868 stars. In total, 136 starsdown to V~15 have membership probability Pμ>=2%. NewCCD BV photometry indicates that about 70 stars are indeed main-sequencestars of NGC 2451A. This is also supported by our measurements of radialvelocities. A total of 34 very likely cluster members yield a meanheliocentric radial velocity for NGC 2451A equal to +22.9 kms-1. The high quality of our BV photometry, a confirmation ofcluster membership from proper motions and radial velocities, and arecently obtained metallicity estimate for several cluster stars allowus to perform a precise isochrone fit. The Yale isochrones, updated bythe latest available input physics, have been fitted to the cluster'scolor-magnitude diagram, yielding a distance modulusV0-MV in the range 6.35 to 6.38, which is inexcellent agreement with the distance modulus (m-M)0=6.38derived from the Hipparcos data recently by van Leeuwen and Robichon etal. For NGC 2451A the isochrone fit yields an age of 60+/-20 Myr; hence,the cluster appears to be somewhat younger than the Pleiades. We alsopresent alternative evidence suggesting that the cluster could beslightly older than the Pleiades. The most important result of thisstudy is an excellent match between the main-sequence fitting andHipparcos distances to NGC 2451A. If we consider noticeable similaritiesbetween NGC 2451A and Pleiades, then the Hipparcos distance anomaly forPleiades may not require an explanation of astrophysical nature.
| 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
| 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
| Fractionated stellar wind and the H/He abundance anomalies in BP stars
Radiatively driven winds occur in all main sequence stars (Babel\cite{Bab1}, \cite{Bab2}). However, due to the weak coupling between thephoton absorbing metals and the inert elements H and He, the wind in thelow temperature domain is fractionated: He decouples from the wind atT_eff < 25 000 K, and below 17 000 K even H. The decoupled elementsfall back to the surface of the star thus creating overabundances andabundance stratifications. These anomalies, however, become manifestonly if atmospheric turbulence is suppressed (say by magnetic fields).In order to prove the validity of the described scenario, all B_p starsfor which reliable fundamental parameters exist, are discussed on thebasis of the (augmented) (g, T_eff)-diagram of Babel (\cite{Bab2}). Itis shown that the fractionation process is able to explain the observedtemperature sequence of He-rich and He-poor stars, additionally toclassical diffusion processes. A necessary condition is that a magneticfield is present. This explains why only a fraction of B stars exhibitsH/He anomalies. While classical diffusion operates in the quiet zones(no wind) of a star, fractionation takes place above the wind bases.
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Geneva photometry of the open cluster NGC 2451 and its exceptional Be star HR 2968 satellite.}
During many years, several authors discussed whether NGC2451 was a real open star cluster or not. By using parallaxesand proper motions from Hipparcos satellite, and Geneva multicolourphotometric measurements of 64 stars, the existence of two clusters Aand B is confirmed. Distances and Geneva colour excesses E[B-V] arerespectively 197 pc, 0.01 and 358 pc, 0.12 for NGC2451 A and B. The two clusters have the same age (log t =7.7). A unique Be star, HR 2968, which belongs tocluster B, has been monitored from 1978 to 1998 in Geneva photometry andfrom November 1989 to March 1993 by the Hipparcos satellite. This starshows exceptional luminosity variations: i) The mean luminosity, whichhas been stable since 1978 (normal B-star phase), increased from 1990 to1995 (Be phase), and then decreased until 1998; ii) Also, in 1990started a periodic light variation with a period of 371 d. Five periodsof this mid-term light variation were observed. Hipparcos and Genevaphotometries are in perfect agreement. A model is proposed to explainthis periodic variability: the Be star is the main component of a binarysystem having an eccentric orbit of period 371 d; from 1990, the Be starwas surrounded by matter expelled in its equatorial plane and, at eachperiastron passage, the companion star interacts gravitationally and/orradiatively with the disk. The physical parameters of the Be star andits companion (most probably a main sequence star) are determined byusing their belonging to NGC 2451 B and bycalculating an approximate orbit from published radial velocities. Themean values of the photometric data are only available in electronicform at the CDS via anonymous ftp to cdsarc.u.strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/Abstract.html
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| The nature of some doubtful open clusters as revealed by HIPPARCOS
We discuss the nature of some galactic open clusters by using propermotions and parallaxes from the Hipparcos and ACT catalogues. We showthat the clusters Collinder 399, Upgren 1, NGC 1252 and Melotte 227 donot exist. Collinder 132 is found to be mainly composed out of membersof an OB association, but there may be a star cluster present in thisarea too. Roeser & Bastian (1994) proposed that NGC 2451 consists oftwo star clusters. We show that NGC 2451 A definitively does exist, NGC2451 B may exist. A star cluster may also be present in the area ofRoslund 5. The Hipparcos data finally confirm the reality of Collinder135.
| 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.
| 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 new list of effective temperatures of chemically peculiar stars. II.
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| Effective temperature of AP and AM stars from Geneva photometry
The determination of the effective temperature of different kinds ofchemically peculiar stars from Geneva photometry is rediscussed. Wepropose an improved method of determining T(eff) of Ap stars fromGeneva's reddening-free X and Y parameters or from the B2-G index. Theeffect of duplicity on the determination of T(eff) of Am stars isdiscussed.
| On the determination of effective temperature and surface gravity of B, A, and F stars using Stromgren UVBY beta photometry
The determination of stellar temperatures and surface gravities by meansof the Stromgren uvby beta photometric system is examined in the regionof main sequence stars. We used a sample of stars with independentlyderived temperatures and gravities. In particular, the calibrations ofMoon & Dworetsky (1985), Lester et al. (1986), and Balona (1984) arediscussed. We compare our results with Castelli (1991) and present newtemperature calibrations for normal, Johnson, and Ap stars. The use ofintegrated fluxes and especially the infrared flux method fordetermining stellar temperatures is discussed. Surface gravities of Band A stars, derived by fitting theoretical profiles to the Balmerlines, are used to check the calibrations.
| Mid-ultraviolet and optical photometry of helium stars
Mid-ultraviolet and optical photometric analysis of helium stars arepresented. A linear relation exists between the effective temperaturederived from model atmospheres and (1965-V)0 index. The effectivetemperatures derived from (1965-V)0 index are somewhat higher than thatof MK spectral type estimates especially for late B-type helium objects.
| Liste des étoiles Ap et Am dans les amas ouverts (édition révisée)
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| Liste des étoiles Ap et Am dans les amas ouverts (Edition révisée)
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| The properties of OB supergiants in the Large Magellanic Cloud. II - Spectral types and intrinsic colors
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1988ApJ...335..703F&db_key=AST
| Chemically peculiar stars in open clusters. I - The catalog
The largest existing compilation is presented of Ap and Am open clusterstars. The catalog contains information on 381 chemically peculiar (CP)stars of the upper main sequence in 79 open clusters. The catalog iscomposed of the following tables: (1) the main body, which lists CP (orsuspected CP) stars which are kinematical (or suspected kinematical)members of open clusters; (2) the list of CP (or suspected CP) starssometimes numbered among cluster members but which are actuallykinematical nonmembers; (3) the list of stars sometimes designated as'peculiar' but, in fact, probably not CP; (4) references for numberingsystems of cluster stars; (5) references for membership; and (6)references for spectral and/or peculiarity types.
| Catalog of AP and AM stars in open clusters
The previous results of Raab (1922), Markarian (1951), and Collinder(1931) have been used to catalog Ap and Am stars that are in the fieldof open clusters. Tabular data are presented for the clusterdesignation, the HD or HDE number, the right ascension (1900), thedeclination (1900), and the magnitude. Also listed are the spectraltypes and, for certain stars, the probability of cluster membership.
| Effective temperatures and angular diameters of Bp-Ap SI stars and B and A normal stars - (u-b) and Delta(m2100)0 calibrations
The JHKLM photometric calibrations of Johnson (1966), Hayes (1979) andWamsteker (1981) are compared in computing empirical T(eff) and angulardiamters for nine standard stars and 12 Bp-Si stars from totalintegrated fluxes by the method of Blackwell et al. (1980). The resultsare presented in extensive tables and discussed in detail. It is foundthat the Hayes calibration gives the most consistent T(eff) values fromJ and K, and that T(eff) is overestimated by 700-1100 K if thenormal-star calibration is applied to peculiar stars. T(eff) is thendetermined from the (u-b) and Delta(m2100)0 calibrations for 10 standardand 39 peculiar stars, and it is confirmed that the Balmer jump ofpeculiar stars decreases with Si abundance and is smaller than that ofnormal stars of the same T(eff).
| The VELA star cloud. III - The B8 to A0 stars and interstellar reddening
The present discussion of intermediate band and H-beta observations of360 HD stars B8-A0 in Vela, and all of the CPD stars in a small adjacentregion, notes that the interstellar absorption in both cases can beunderstood as a combination of three absorbing clouds of 1 kpc-A(V) =1.5, 0.5, and 0.17. The major difference in the two environments is thepresence of numerous T Tauri stars in Taurus. Several clusters in theVela region are discussed.
| Statistical Investigation of Chemically Peculiar Stars - Part Four - Luminosity of Different Type Stars
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| On the Effective Temperatures of Chemically Peculiar Stars
Not Available
| Observational problems of investigation of stars with anomalous helium lines.
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| The local system of early type stars - Spatial extent and kinematics
Published uvby and H-beta photometric data and proper motions arecompiled and analyzed to characterize the structure and kinematics ofthe bright early-type O-A0 stars in the solar vicinity, with a focus onthe Gould belt. The selection and calibration techniques are explained,and the data are presented in extensive tables and graphs and discussedin detail. The Gould belt stars of age less than 20 Myr are shown togive belt inclination 19 deg to the Galactic plane and node-lineorientation in the direction of Galactic rotation, while the symmetricaldistribution about the Galactic plane and kinematic properties (purecircular differential rotation) of the belt stars over 60 Myr oldresemble those of fainter nonbelt stars of all ages. The unresolveddiscrepancy between the expansion observed in the youngest nearby starsand the predictions of simple models of expansion from a point isattributed to the inhomogeneous distribution of interstellar matter.
| Effective temperature of AP stars - A comparison between several photometric estimators
Effective temperatures are inferred for 28 Bp stars with theBlackwell-Shallis method. They are compared to photometric estimations,which are made with Delta-m(2100), (B-V)j, (B2-G), (R-I)c, and (J-K). Itis shown that (B2-G) and (R-I)c are the most interesting temperatureestimators for peculiar stars. A new calibration of (B2-G)0 is thenprovided for Si and He weak stars. Linear radii of Bp stars are found tobe similar to those of main sequence late B-type stars.
| Six clusters in Puppis-Vela
Intermediate band and H-beta observations of stars in the clusters NGC2451, Cr 140, Cr 135, Cr 173, IC 2391, and Cr 132 in Puppis-Vela arediscussed. Photometric and astrometric parameters for the stars areshown and discussed, along with light and color curves, color-luminosityarrays, histograms for reddening and luminosity, and stellardistributions. Cr 132 consists mainly of members of CMa OB2 plus a fewstars that may be an extension of Cr 140. The latter is an elongatedcluster of some 20 stars, 450 pc distant, 30 million years old, and with(U,V,W) = (+27,-6,-16) km/s. Cr 135 has only eight members, including aK2 Ib star, some 30 million years old, 310 pc distant, and (U,V,W) =(+13,-11,-12) km/s. For NGC 2451, extensive photometry reveals nocluster. Cr 173 is in the Vela sheet and may contain Gamma Vel and thecepheid AH Vel. IC 2391 is a very extended cluster including about twodozen stars brighter than the sun, some 30 million years old and 165 pcdistant, with (U,V,W) = (+20,-19,-3) km/s.
| Infrared magnitudes (JHKLM) for 105 chemically peculiar A- and B-stars
Observational data for IR surveys of 105 CP stars in the JHKLM regionsare reported. The data were taken with the ESO 1 m telescope duringthree separate viewing periods, with four observations being made ofeach object. The measurements were treated for the mean SNR ratio, thestandard deviation of the mean signal, and the accuracy of thezero-point. Duplicate measurements were made of objects which weresuspected of being variable, and mean values were derived.
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