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Searching for links between magnetic fields and stellar evolution: II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations
Context: The evolution of magnetic fields in Ap stars during the mainsequence phase is presently mostly unconstrained by observation becauseof the difficulty of assigning accurate ages to known field Ap stars. Aims: We are carrying out a large survey of magnetic fields in clusterAp stars with the goal of obtaining a sample of these stars withwell-determined ages. In this paper we analyse the information availablefrom the survey as it currently stands. Methods: We select from theavailable observational sample the stars that are probably (1) clusteror association members and (2) magnetic Ap stars. For the stars in thissubsample we determine the fundamental parameters T{eff},L/L_ȯ, and M/M_ȯ. With these data and the cluster ages weassign both absolute age and fractional age (the fraction of the mainsequence lifetime completed). For this purpose we have derived newbolometric corrections for Ap stars. Results: Magnetic fields arepresent at the surfaces of Ap stars from the ZAMS to the TAMS.Statistically for the stars with M > 3 M_ȯ the fields declinewith advancing age approximately as expected from flux conservationtogether with increased stellar radius, or perhaps even faster than thisrate, on a time scale of about 3×107 yr. In contrast,lower mass stars show no compelling evidence for field decrease even ona timescale of several times 108 yr. Conclusions: Study ofmagnetic cluster stars is now a powerful tool for obtaining constraintson evolution of Ap stars through the main sequence. Enlarging the sampleof known cluster magnetic stars, and obtaining more precise rms fields,will help to clarify the results obtained so far. Further fieldobservations are in progress.Tables 2 and 3 are only available in electronic form athttp://www.aanda.org
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| 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.
| 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
| 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
| 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 λ 3984 feature in late-B spectroscopic binaries.
We report about a spectroscopic study aimed at searching the line Hg IIλ 3984 in a sample of 28 spectroscopic binaries with primaries inthe same temperature range as HgMn stars. One system, HD 75642, showsthis line very prominently. In another system, HD 41040, Hg may besomewhat overabundant, while HD 87751 shows marginal Mn and Penhancement. The present study does not bring a definitive answer to thequestion of the relation between binarity, slow rotation, and theappearance of HgMn anomalies in late B stars, possibly because othereffects (like the influence of a magnetic field), which could not betaken into consideration, are important too. In the course of thisstudy, 11 new double-lined spectroscopic binaries were discovered, 10 ofwhich were previously known as single-lined systems. Of particularinterest among them are 4 stars that may have the Si or He weakpeculiarities.
| NGC 2451 - What is it?
During the last decade, a debate has arisen whether the grouping ofstars called NGC 2451 is a real star cluster. We investigate thecorresponding area of the sky kinematically, using proper motions of thePPM catalogue. The group of stars that was originally called NGC 2451shows a large scatter of their proper motions, which proves that they donot form a cluster. However, close-by we find a group of 24 PPM starswith common proper motion. A colour-magnitude diagram of 19 of theseshows an extremely well-defined main sequence, giving a distance ofabout 220 pc. This cluster is not NGC 2451. We tentatively call itPuppis Moving Group (PMG). There is strong indication for a secondcluster at about 400 pc. Photometry, radial velocities and propermotions of stars fainter than the magnitude limit of PPM are needed tofurther investigate the matter.
| Liste des étoiles Ap et Am dans les amas ouverts (édition révisée)
Not Available
| Liste des étoiles Ap et Am dans les amas ouverts (Edition révisée)
Not Available
| 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.
| 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.
| 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
Not Available
| Evidence of decay of the magnetic fields of AP stars
Data obtained in the Geneva photometric system (Rufener, 1981) andappropriate calibrations of this system in terms of surface magneticfield and gravity are used to provide, on the basis of 708 field andcluster Ap stars, observational evidence that these stars undergo decayof their magnetic field on an evolutionary timescale. Justifications aregiven for the application of a photometric gravity calibration topeculiar stars. The dependence of the photometrically estimated surfacemagnetic field on gravity is found to differ markedly from availabletheoretical calculations. HgMn stars are found to show the same trend,strengthening the impression that they might be slightly magnetic.He-weak stars do not.
| The common origin of some open clusters
Eight open clusters have been observed with uvby-beta photometry and forseven of them the metal abundances have been determined. Six of thesehave similar metallicities. They are close in space and are also knownto have similar ages and radial velocities. These clusters are suspectedof having a common origin. Some studies are made of the reliability ofthe photometric system. A large gain in limiting magnitude can beachieved against an unimportant loss of accuracy if interference filtersare removed.
| 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.
| Photometric properties of AP stars in the Geneva system
An examination of the properties in some photometric diagrams of morethan 600 Ap stars measured in the Geneva photometric system confirm thatthe Balmer discontinuity is smaller than for normal stars, along withthe link between a proposed peculiarity parameter and both rotationalvelocity and effective magnetic field. It is shown that the peculiarityparameter is sensitive to interstellar reddening, and it is foundthrough examination of the standard deviations for visual magnitudesthat cool CP 2 stars without Eu peculiarity have the greatestamplitudes. Rapid rotators have a mild peculiarity, while positivecorrelation exists for Si and SrCr stars.
| Relative radial velocities from objective prism spectra in the region of nine southern open star clusters and a star field at Eta Carinae
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1980A&AS...41..245G&db_key=AST
| The frequency of peculiar A and metallic-line stars in open clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976ApJ...205..807H&db_key=AST
| Spectral types in the open cluster NGC 2451.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975PASP...87..823L&db_key=AST
| Rotational Velocities in Short-Period A-Type Binaries
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971ApJ...163..333A&db_key=AST
| Photoelectric Photometry of NGC 2451
Not Available
| The Open Cluster NGC 2451
Not Available
| Photometry of NGC 2451
Not Available
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