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Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| 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.
| Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS
We have performed a new and homogeneous analysis of all the Li dataavailable in the literature for main sequence stars (spectral-types fromlate F to K) in open clusters. In the present paper we focus on adetailed investigation of MS Li depletion and its time scales for starsin the 6350-5500 K effective temperature range. For the first time, wewere able to constrain the age at which non-standard mixing processes,driving MS Li depletion, appear. We have also shown that MS Li depletionis not a continuous process and cannot be simply described by at-α law. We confirm that depletion becomes ineffectivebeyond an age of 1-2 Gyr for the majority of the stars, leading to a Liplateau at old ages. We compared the empirical scenario of Li as afunction of age with the predictions of three non-standard models. Wefound that models including only gravity waves as main mixing processare not able to fit the Li vs. age pattern and thus this kind of mixingcan be excluded as the predominant mechanism responsible for Lidepletion. On the other hand, models including slow mixing induced byrotation and angular momentum loss, and in particular those includingalso diffusive processes not related to rotation, can explain to someextent the empirical evidence. However, none of the currently proposedmodels can fit the plateau at old ages.
| The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters
Spectra have been obtained of 21 white dwarfs (WDs) in the direction ofthe young, rich open star cluster NGC 2099. This represents anappreciable fraction (>30%) of the cluster's total WD population. Themean derived mass of the sample is 0.8 Msolar-about 0.2Msolar larger than the mean seen among field WDs. Asurprising result is that all of the NGC 2099 WDs have hydrogen-richatmospheres (DAs); none exhibit helium-rich ones (DBs) or any otherspectral class. The number ratio in the field at the temperatures of theNGC 2099 WDs is DA/DB ~ 3.5. While the probability of seeing no DB WDsin NGC 2099 solely by chance is ~2%, if we include WDs in other openclusters of similar age it then becomes highly unlikely that the dearthof DB WDs in young open clusters is just a statistical fluctuation. Weexplore possible reasons for the lack of DBs in these clusters andconclude that the most promising scenario for the DA/DB number ratiodiscrepancy in young clusters is that hot, high-mass WDs do not developlarge enough helium convection zones to allow helium to be brought tothe surface and turn a hydrogen-rich WD into a helium-rich one.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
| Kinematic Peculiarities of Gould Belt Stars
We analyzed the space velocities of Gould Belt stars younger than 125Myr located at heliocentric distances < 650 pc. We determined therotation and expansion parameters of the Gould Belt by assuming theexistence of a single kinematic center whose direction was found to bethe following: lo = 128 deg and Ro = 150 pc. Thelinear velocities reach their maximum at a distance of ~300 pc from thecenter and are -6 km/s for the rotation (whose direction coincides withthe Galactic rotation) and +4 km/s for the expansion. The stellarrotation model used here is shown to give a more faithful description ofthe observed velocity field than the linear model based on the Oortconstants AG and BG. We present evidence that theyoung clusters beta Pic, Tuc/HorA, and TWA belong to the Gould Beltstructure.
| 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.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| 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
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| The origin of the young stellar population in the solar neighborhood -- A link to the formation of the Local Bubble?
We have analyzed the trajectories of moving stellar groups in the solarneighborhood in an attempt to estimate the number of supernovaexplosions in our local environment during the past 20 million years.Using Hipparcos stellar distances and the results of kinematicalanalyses by Asiain et al. (\cite{Asiain1999a}a) on the Pleiades movinggroups, we are able to show that subgroup B1, consisting of early type Bstars up to 10 Msun, but lacking more massive objects, haspassed through the local interstellar medium within less than 100 pc.Comparing the stellar content of B1 with the initial mass functionderived from the analysis of galactic OB associations, we estimate thenumber of supernova explosions and find that about 20 supernovae musthave occurred during the past ~ 10 - 20 million years, which issuggested to be the age of the Local Bubble; the age of the star clusteris about ~ 20 - 30 million years. For the first time, this providesstrong evidence that the Local Bubble must have been created and shapedby multi-supernova explosions and presumably been reheated more than 1million years ago, consistent with recent findings of an excess of60Fe in a deep ocean ferromanganese crust. Calculatingsimilarity solutions of an expanding superbubble for time-dependentenergy input, we show that the number of explosions is sufficient toexplain the size of the Local Bubble. The present energy input rate isabout dot ESN ~ 5 x 1036 erg/s, in good agreementwith the estimated local soft X-ray photon output rate. It seemsplausible that the origin of the Local Bubble is also linked to theformation of the Gould Belt, which originated about 30-60 Myrs ago.
| The CFHT Open Star Cluster Survey. III. The White Dwarf Cooling Age of the Rich Open Star Cluster NGC 2099 (M37)
We present deep CCD photometry of the very rich, intermediate-age(similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-Vcolor-magnitude diagram (CMD) for the cluster shows an extremelywell-populated and very tightly constrained main sequence extending over12 mag from the turnoff. Both a well-defined main-sequence turnoff and ared giant population are also clearly evident. The CFH12K photometry forthis cluster is faint enough (V~24.5) to detect the remnants of the mostmassive progenitor cluster stars under the Type I SNe limit. Therefore,the CMD of the cluster also exhibits a well defined white dwarf``clump'' caused by the decreased rate of cooling of these stars as theyage, and a subsequent gap with very few objects. By using sourceclassification to eliminate faint blue galaxies and a statisticalsubtraction technique to eliminate foreground/background objects, wehave determined the age of the star cluster from the termination point(MV=11.95+/-0.30) in the white dwarf luminosity function. Thewhite dwarf cooling age is found to be566+/-154176 Myr from comparisons with white dwarfcooling models and is in excellent agreement with the main-sequenceturnoff isochrone age (520 Myr). After carefully accounting for theuncertainties in the white dwarf limiting magnitude, we show that thecooling age confirms that models including convective core overshootingare preferred for young-intermediate-aged clusters. This is particularlyimportant in the case of NGC 2099 as the age is similar to that of theHyades cluster, for which current models have difficulties inreproducing the details of the main-sequence turnoff. We also derive thereddening [E(B-V)=0.21+/-0.03] and distance[(m-M)V=11.55+/-0.13] to NGC 2099 by matching main-sequencefeatures in the cluster to a new fiducial main sequence for the Hyades,after correcting for small metallicity differences. As a continuing partof the goals of the CFHT Open Star Cluster Survey to better understanddynamical processes of open clusters, we also fit a King model to thecluster density distribution and investigate the cluster main-sequenceluminosity and mass functions in increasing concentric annuli. We findsome evidence for mass segregation within the boundary of NGC 2099 asexpected given the cluster's age relative to the dynamical age. Thepresent global mass function for the cluster is found to be shallowerthan a Salpeter IMF.
| Arc-Shaped and Spheroidal Stellar Complexes
Complexes of young clusters and high-luminosity stars in the shape ofregular, circular arcs have been found in a number of galaxies, firstand foremost the LMC, NGC 6946, and M83. These shapes are found even instrongly inclined galaxies, suggesting that the observed arcs areprojections of partial spherical shells. Obviously, these stellar shellsmust have formed from gaseous shells swept up by some source of centralpressure and become gravitationally unstable. The power of this sourcecorresponds to several dozen supernova explosions; however, its natureremains unclear. A central cluster providing a source of O stars andsupernovae is usually absent. The presence of multiple arcs locatedclose to each other can be explained by the fall of a swarm of fragmentsor by the progenitor stars originating in a single peculiar starcluster, implying the existence of stellar objects capable of givingrise to explosions with energies an order of magnitude higher than thoseof individual supernovae. The same objects may be responsible forgamma-ray bursts. It may be that only the most massive clusters withfrequent or especially powerful supernova explosions are capable ofproducing HI supershells. Otherwise, it is impossible to explain why nosupershells have been found around numerous clusters that should becapable of producing them according to current theories. The presence ofstar clusters in shell-like structures provides extremely importantinformation about the physical conditions in and the ages of the initialgaseous shells, making stellar arcs the best available laboratory forstudies of triggered star formation.
| 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.
| Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue
We present mean absolute proper motions of 112 open clusters, determinedusing the data from the Tycho2 Catalogue. For 28 clusters, this is thefirst determination of proper motion. The measurements made use of alarge number of stars (usually several tens) for each cluster. The totalnumber of stars studied in the fields of the 164 open clusters is 5016,of which 4006 were considered members. The mean proper motions of theclusters and membership probability of individual stars were obtainedfrom the proper motion data by applying the statistical method proposedby Sanders (\cite{Sanders71}). Based on observations of the ESAHipparcos satellite. Tables 1, 2 and 5 to 117 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/441
| Hipparcos Trigonometric Parallaxes and the Distance Scale for Open Star Clusters
Hipparcos trigonometric parallaxes are used to estimate the distances tothe maximum possible number of open star clusters (OSC); distance moduliare estimated for 45 clusters with maximum heliocentric distances ofabout 1000 pc. The latter value can serve as an estimate of the limit towhich it still makes sense to use Hipparcos trigonometric parallaxes todetermine the distances to small groups composed of 6-10 sufficientlybright stars. A systematic correction to the distance moduli of clustersfrom the homogeneous catalog of OSC parameters (Loktin et al. 1997,2000) is estimated, which turns out to be independent of the clusterage.
| The IMF of open star clusters with Tycho-2
We studied the fields of nine nearby open star clusters based on theTycho-2 catalogue. We determined membership probabilities for the starsin the cluster fields from the stellar proper motions and used theTycho-2 photometry to compute the initial mass function (IMF) of theclusters from the main sequence turn-off point down to approx. 1 M_sun.We found IMF slopes ranging from Gamma =-0.69 down to Gamma =-2.27 (whenthe Salpeter \cite{salpeter} value would be Gamma =-1.35). We alsostudied the membership of individual stars of special astrophysicalinterest. In some cases previous results had to be revised. As aby-product, we investigated some general properties of the Tycho-2catalogue; we confirmed that the Tycho-2 proper motions show onlymarginal deviations from the Hipparcos catalogue. On the other hand, insome regions the completeness of the catalogue seems to decrease atmagnitudes brighter than claimed by Høg et al. (\cite{tycho2}).Based on observations of the ESA Hipparcos satellite.
| The White Dwarf Cooling Age of the Open Cluster NGC 2420
We have used deep HST WFPC2 observations of two fields in NGC 2420 toproduce a cluster color-magnitude diagram down to V~27. After imposingmorphological selection criteria we find eight candidate white dwarfs inNGC 2420. Our completeness estimates indicate that we have found theterminus of the WD cooling sequence. We argue that the cluster distancemodulus is likely to be close to 12.10 with E(B-V)=0.04. With theseparameters we find a white dwarf cooling age for NGC 2420 of 2.0+/-0.20(1 σ) Gyr. The 0.20 Gyr uncertainty includes errors in thephotometry, sequence fitting, precursor timescales, and theoreticalwhite dwarf cooling timescales. Comparing the cluster white dwarfcooling age to ages derived from stellar isochrone fitting we find apreference for ages derived from models incorporating convectiveovershoot. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| Kinematics of young stars. I. Local irregularities
The local velocity field of young stars is dominated by the galacticrotation, the kinematics of the Gould Belt and the nearest OBassociations and open clusters, and the kinematics of the spiralstructure. We re-examined here this local velocity field by using alarge sample of nearby O and B stars from the Hipparcos Catalogue. Thehigh quality astrometric data are complemented with a carefulcompilation of radial velocities and Strömgren photometry, whichallows individual photometric distances and ages to be derived. TheGould Belt extends up to 600 pc from the Sun with an inclination withrespect to the galactic plane of iG = 16-22degr and theascending node placed at Omega G = 275-295degr .Approximately 60% of the stars younger than 60 Myr belong to thisstructure. The values found for the Oort constants when differentsamples selected by age or distance were used allowed us to interpretthe systematic trends observed as signatures induced by the kinematicbehaviour of the Gould Belt. The contribution of Sco-Cen and Ori OB1complexes in the characterization of the expansion of the Gould Beltsystem is also discussed. We found that a positive K-term remains whenthese aggregates are excluded. From the kinematic behaviour of the starsand their spatial distribution we derive an age for the Gould Beltsystem in the interval 30-60 Myr. Based on data from the Hipparcosastrometry satellite (European Space Agency)}
| Stellar Activity in the Open Cluster NGC 2451
We present first results of the analysis of X-ray data (ROSAT PSPC andHRI pointings) and optical observations of the open cluster NGC 2451.This object consists of two different clusters A and B in the sameline-of-sight at distances of 190 and 360 pc, respectively. NGC 2451A isone of the 10 nearest clusters, but has been little studied because ofstrong contamination by field stars. In order to improve this situation,we used X-ray observations to identify cluster members, a method provento be very succesful at several other clusters. To identify the X-raysources with optical counterparts we performed CCD photometry with theESO 0.9m Dutch telescope. Altogether, 188 X-ray sources were found downto X-ray luminosities of a few 1028 erg s-1. Basedon positional coincidence (i.e., offset < 25'') 122 of them could beidentified with optical counterparts with magnitudes between V = 3 and20, most of them ranging from 11 to 18. Of these 122 stars, 105 arepossible members of clusters NGC 2451A or NGC 2451B, as derived fromtheir position in the Colour-Magnitude-Diagram. Four stars showed strongflares during the X-ray observations, and we present a short analysis ofthe flare parameters. NGC 2451 A/B is especially interesting since it isa rather young (50 Myrs) and possibly metal-deficient cluster very closeto us.
| Long-term changes of the Be star OT Geminorum
We investigate the character of light variability of the bright Be starOT Gem . Two phases of the long-term light changes were distinguished: -a quiet phase which is characterized by mild light changes; and - anactive phase in which the sudden increases of brightness up to 0. (v (r)m m) 4 in V occur. During the active phases, a cyclic oscillation ofbrightness of a period between 70 and 80 days is present, with the mostprobable value being 71. (v (r) m d) 89. A positive correlation wasfound between the light and colour variations and the emission strengthin both phases.
| Astrometric radial velocities. I. Non-spectroscopic methods for measuring stellar radial velocity
High-accuracy astrometry permits the determination of not only stellartangential motion, but also the component along the line-of-sight. Suchnon-spectroscopic (i.e. astrometric) radial velocities are independentof stellar atmospheric dynamics, spectral complexity and variability, aswell as of gravitational redshift. Three methods are analysed: (1)changing annual parallax, (2) changing proper motion and (3) changingangular extent of a moving group of stars. All three have significantpotential in planned astrometric projects. Current accuracies are stillinadequate for the first method, while the second is marginally feasibleand is here applied to 16 stars. The third method reaches high accuracy(<1 km s(-1) ) already with present data, although for some clustersan accuracy limit is set by uncertainties in the cluster expansion rate.Based (in part) on observations by the ESA Hipparcos satellite
| 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
| HIPPARCOS distance calibrations for 9 open clusters
Parallaxes have been determined for 9 nearby open clusters in order toderive a composite, age resolved, HR diagram. An age-luminosity relationwas found to exist for main-sequence stars. A suggested 1 mas error inthe Hipparcos parallax for the Pleiades, to explain the difference withthe ground-based photometric distance estimate, seems unlikely: 5 otheryoung open clusters examined (IC 2602, IC 2391, alpha Per, NGC 2451 andBlanco 1) all share the Pleiades main sequence locus in the HR diagram.The 3 older open clusters (Coma Ber, Praesepe and NGC 6475) were founddisplaced towards brighter magnitudes. These trends are more pronouncedthan predicted by theoretical stellar evolution models, and affectdistance, metallicity and possibly also age determinations for open- andglobular clusters through the use of theoretical isochrones. Based onobservations obtained with the Hipparcos 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
| X-ray observations of the open cluster NGC 2451
First results of our combined X-ray and optical study of the opencluster NGC 2451 are presented. NGC 2451 actually consists of twodifferent clusters situated along the same line-of-sight at distances of190 and 360 pc. Due to strong contamination by field stars this objecthas been little studied in the optical range, although the 190 pccluster ranks among the 10 nearest clusters! Since X-ray observationshave been proven to be a very efficient method to identify clustermembers, we performed a detailed study of NGC 2451 by means of ROSATobservations and CCD photometry. In two ROSAT PSPC and one HRI pointingsmore than 100 X-ray sources are detected in the cluster region. By meansof identifying the X-ray sources with stellar counterparts we providethe first study of the low-mass star population of this cluster. NGC2451 A/B is especially interesting since it is a rather young (50 Myrs)and metal-deficient cluster very close to us.
| 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.
| A Search for Star Clusters from the HIPPARCOS Data
We present results of a search for nearby star clusters and associationsusing Hipparcos Catalogue data, restricting the sample to stars withparallaxes above 2 mas (d <~ 500 pc). Two new OB associations havebeen identified in the Carina-Vela and Cepheus-Cygnus-Lyra-Vulpecularegions. A very probable new open cluster has been discovered in Carina.The cluster, a Car, named after its brightest member, is young (60 Myr)and nearby (d = 132 pc). However, only seven bona fide members can bedrawn from the Hipparcos data. We report a detection of nine opencluster candidates in the distance range of 150 to 400 pc, and sixpossible associations almost all located within the Gould belt, althoughslightly older than the known nearby associations. In all cases, wepresent Yale theoretical isochrone fits to the color-magnitude diagrams,which indicate a moderate spread of ages between 60 to 200 Myr.Evidently, these young open cluster and association candidates arerelated to the overall distribution of young OB and A-type stars in thesolar neighborhood.
| The overlapping open clusters NGC 1750 and NGC 1758. III. Cluster-field segregation and clusters physical parameters
Cluster-field segregation and determination of physical parameters ofthe open clusters NGC 1750 and NGC 1758 were performed from CCD andphotographic photometric and astrometric surveys previously published bythe authors. Membership probabilities were computed to a brightnesslimit of R=15 mag through a non-standard method in a fullynon-parametric approach, using proper motions, positions and photometry.This study allowed several parameters for NGC 1750 and NGC 1758 to bedetermined, including position, size, density profile, absorption,distance, age, luminosity function and mass. The common colour excess ofboth clusters is E(B-V)=0.34 mag. NGC 1750 is slightly younger (log t =8.3), closer (V-M_v=9.0 mag) and less concentrated than NGC 1758 (log t= 8.60 and V-M_v=9.4 mag). We also conclude that NGC 1746 is anerroneous assignation (not corresponding to any real open cluster), andthat NGC 1750 and NGC 1758 are overlapping clusters, but they do notconstitute a gravitationally bounded system.
| Contribution of White Dwarfs to Cluster Masses
I have undertaken a literature search through 1997 July 31 of whitedwarfs (WDs) in open and globular clusters. I have tried to make acareful evaluation in each case of the likelihood that the object is aWD and that it is a cluster member. The results are presented for 13open clusters and 11 globular clusters. Currently there are 36 singleWDs and five WDs in binaries known among the open clusters, and 340single WDs and 11 WDs in binaries known among the globular clusters.From these data, I have calculated WD mass fractions for four openclusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and oneglobular cluster (NGC 6121). I develop a simple model of clusterevolution that incorporates stellar evolution but not dynamicalevolution to interpret the WD mass fractions. I augment the results ofmy simple model by turning to sophisticated N-body simulationsincorporating stellar evolution. I find that even though these clustersundergo a range of degrees of kinematic evolution, from moderate (thePleiades, NGC 2168, and NGC 3532) to strong (the Hyades and NGC 6121),the WD mass fraction is relatively insensitive to kinematic evolutionand little changed from a model incorporating only stellar evolutionwith a Salpeter-like initial mass function. By comparing the clustermass functions with that of the Galactic disk, and incorporatingplausibility arguments for the mass function of the Galactic halo, Iestimate the WD mass fraction in these two field populations. I assumethe Galactic disk is ~10 Gyr old and that the Galactic halo is ~12 Gyrold, although the WD mass fraction is insensitive to age within thisregime. I find that the Galactic halo should contain from 8%-9% (alpha =-2.35) to perhaps as much as 15%-17% (alpha = -2.0) of its stellar massin the form of WDs. The Galactic disk WD mass fraction should be 6% to7% (for a median stellar age of 5 to 7 Gyr and alpha = -2.35),consistent with the empirical estimates of 3% to 7%.
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