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The evolution of binary star clusters and the nature of NGC 2136/NGC 2137
We study the evolution of bound pairs of star clusters by means ofdirect N-body simulations. Our simulations include mass loss by stellarevolution. The initial conditions are selected to mimic the observedbinary star clusters, NGC2136 and 2137, in the Large Magellanic Cloud.Based on their rather old ages (~100Myr), masses, sizes and projectedseparation, we conclude that the cluster pair must have been born withan initial separation of 15-20 pc. Clusters with a smaller initialseparation tend to merge in <~60Myr due to loss of angular momentumfrom escaping stars. Clusters with a larger initial separation tend tobecome even more widely separated due to mass loss from the evolvingstellar populations. The early orbital evolution of a binary cluster isgoverned by mass loss from the evolving stellar population and by lossof angular momentum from escaping stars. Mass loss by stellar winds andsupernovae explosions in the first ~30Myr causes the binary to expandand the orbit to become eccentric. The initially less massive clusterexpands more quickly than the binary separation increases, and istherefore bound to initiate mass transfer to the more massive cluster.This process is quite contrary to stellar binaries in which the moremassive star tends to initiate mass transfer. Since mass transferproceeds on a thermal time-scale from the less massive to the moremassive cluster, this semidetached phase is quite stable, even in aneccentric orbit until the orbital separation reaches the gyration radiusof the two clusters, at which point both clusters merge to one.
| Chemically peculiar stars in the Large Magellanic Cloud
Context: .The detection of magnetic chemically peculiar (CP2) stars inopen clusters of extragalactic systems can give observational answers tomany unsolved questions. For example, one can study the influence ofdifferent global as well local environments on the lack of and presenceof peculiarities. Aims: .The mean percentage of CP2 stars in theMilky Way is of the order of 5% for the spectral range from early B- toF-type, luminosity class V objects. The origin of the CP2 phenomenonseems to be closely connected to the overall metallicity and globalmagnetic field environment. The theoretical models are still only testedby observations in the Milky Way. It is therefore essential to providehigh quality observations in rather different global environments.Methods: .The young clusters NGC 2136/7 were observed in the Δ aphotometric system. This intermediate band photometric system samplesthe depth of the 520 nm flux depression by comparing the flux at thecenter with the adjacent regions with bandwidths of 11 nm to 23 nm. TheΔ a photometric system is most suitable for detecting CP2 starswith high efficiency, but is also capable of detecting a smallpercentage of non-magnetic CP objects. Furthermore, the groups of(metal-weak) λ Bootis, as well as classical Be/shell stars, canbe successfully investigated. Results: .We present high precisionphotometric Δ a observations of 417 objects in NGC 2136/7 and itssurrounding field, of which five turned out to be bona fide magnetic CPstars. In addition, we discovered two Be/Ae stars. Conclusions:.From our investigations of NGC 1711, NGC 1866, NGC 2136/7, theirsurroundings, and one independent field of the LMC population, we derivean occurrence of classical chemically peculiar stars of 2.2(6)% in theLMC, which is only half the value found in the Milky Way. The mass andage distribution of the photometrically detected CP stars is notdifferent from that of similar objects in galactic open clusters.
| OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic 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/391/547
| Research in Concepción on globular cluster systems and galaxy formation, and the extragalactic distance scale
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| Large Magellanic Cloud stellar clusters. I. 21 HST colour magnitude diagrams
We present WFPC2 photometry of 21 stellar clusters of the LargeMagellanic Cloud obtained on images retrieved from the Hubble SpaceTelescope archive. The derived colour magnitude diagrams (CMDs) arepresented and discussed. This database provides a sample of CMDsrepresenting, with reliable statistics, simple stellar populations witha large spread of age. The stars in the core of the clusters are allresolved and measured at least down to the completeness limit; themagnitudes of the main sequence terminations and of the red giant clumpare also evaluated for each cluster, together with the radius at halfmaximum of the star density. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. Table 1 is only available in electronic form athttp://www.edpsciences.org
| Age and metallicity for six LMC clusters and their surrounding field population
We investigate, on the basis of CCD Strömgren photometry, the agesand metallicities of six LMC clusters together with their surroundingfield population. The clusters and metallicities are: NGC 1651 (in therange [Fe/H] = -0.65 dex to -0.41 dex), NGC 1711 (-0.57 ∓ 0.17dex), NGC 1806 (-0.71 ∓ 0.23 dex), NGC 2031 (-0.52 ∓ 0.21dex) and NGC 2136/37 (-0.55 ∓ 0.23 dex) and NGC 2257 (-1.63∓ 0.21 dex). The metallicities for NGC 1651, NGC 1711, NGC 1806and NGC 2031 have been determined for the first time (NGC 2031 and NGC2136/37 are interesting for the Cepheid distance scale). In the clustersurroundings, we found about 650 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR). Ourmethod is to estimate ages for individual stars on the basis ofStrömgren isochrones with individually measured metallicities. Withthis method we are able to sample the AMR of the field population up to8 Gyr. Our metallicity data are incompatible with models predicting manymetal-poor stars (G-dwarf problem). The metallicity of the fieldpopulation increased by a factor of six, starting around 2 Gyr ago. Theproposed AMR is consistent with the AMR of the LMC cluster system(including ESO 121 SC03 and three clusters with an age of 4 Gyr). Theproposed AMR is incompatible with the recently proposed AMR by Pagel& Tautvaisiene.
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
| Chemical evolution and star formation history in the LMC from cluster and field stars
We investigate, on the basis of CCD Strömgren photometry, the agesand metallicities of six LMC clusters together with their surroundingfield population. The clusters and metallicities are: NGC 1651 (in therange [Fe/H]=-0.65 dex to -0.41 dex), NGC 1711 (-0.64+/-0.18 dex), NGC1806 (-0.67+/-0.24 dex), NGC 2031 (-0.40+/-0.21 dex) and NGC 2136/37(-0.43+/-0.23 dex). The metallicities for NGC 1651, NGC 1711, NGC 1806and NGC 2031 have been determined for the first time (NGC 2031 and NGC2136/37 are interesting for the Cepheid distance scale). In the clustersurroundings, we found about 700 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR) and a starformation history (SFH). Our method was to estimate ages for individualstars on the basis of Strömgren isochrones with individuallymeasured metallicities. With this method we are able to sample the AMRand SFH of the field population up to 8 Gy. Our metallicity data areincompatible with models predicting many metal-poor stars. Themetallicity of the field population increased by a factor of seven,starting around 2 Gy ago. This increase was preceded by an increase ofthe star formation rate 3-4 Gy ago. The results support the SFH found inearlier investigations of the LMC field population. The absence of starclusters between 4 Gy and 10 Gy agrees very well with a low rate of thestar formation during that time.
| Dynamical studies of cluster pairs in the Magellanic Clouds
We performed N-body simulations of star cluster encounters withHernquist's TREECODE in a CRAY YMP-2E computer under different initialconditions (relative positions and velocities, cluster sizes, masses andconcentration degrees). The total number of particles per simulationranged from 1024 to 20480. These models are compared with a series ofisodensity maps of cluster pairs in the Magellanic Clouds. Evidence isfound that during the interactions, transient morphological effects suchas an expanded halo, isophotal deformation and isophotal twisting canoccur as a result of tidal effects and dynamical friction. Thesimulations also show that different outcomes are possible depending onthe initial parameters: (i) long-standing changes of concentrationdegree can occur after the collision; (ii) one member can disaggregate;or (iii) the pair can coalesce into a single cluster with a distinctstructure compared with the original ones. These simulations canreproduce a wide range of morphological structures in observed clusterpairs.
| Star Clusters Driven to Form by Strong Collisions Between Gas Clouds in High-Velocity Random Motion
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113..249F
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Kugelsternhaufen in Galaxien.
Not Available
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| The binary, (triple?) star cluster NGC 2136/37 in the Large Magellanic Cloud
We present Stroemgren CCD photometry for the binary cluster NGC 2136/37in the LMC and for its neighbouring field population. The common originof NGC 2136 and 2137 is supported not only by their identical ages, butalso by their common Stroemgren metallicity of -0.55dex. A third clustermay also have a common origin with NGC 2136/37, indicated by the factthat the three clusters are connected by regions of enhanced stardensities. No counterpart of such a configuration has yet been found inthe Milky Way.
| The power of Stroemgren CCD photometry in the LMC.
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| CCD Stroemgren photometry in the Magellanic Clouds. The binary star cluster NGC 2136/37.
Not Available
| Integrated CCD photometry of binary clusters in the Large Magellanic Cloud
Integrated CCD magnitudes and colors of 10 pairs of clusters, with acenter-to-center separation of less than 40 pc, have been obtained.Statistical analysis shows that the mean color difference is: (1) smallenough not to have arisen due to chance superposition of singleclusters, thus implying that the clusters of any pair are likely to bephysically associated; (2) large enough to rule out the possibility thatthe individual components of the pairs have identical ages, implyingthat the clusters have a small but finite age difference.
| A catalogue of binary star cluster candidates in the Large Magellanic Cloud
A photographic atlas of close pairs of star clusters in the LargeMagellanic Cloud is presented here. The criterion for inclusion ofcluster pairs in the atlas was an upper limit of 18.7 pc for theprojected separation between the centers of the clusters in each pair.Accurate coordinates for the clusters, the projected separations andestimates of the diameters and positional angles are given and some ofthe global properties of the cluster-pair population of the LMC arediscussed. It is found that the individual clusters in pairspreferentially have nearly equal sizes.
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| The stellar content of binary star clusters in the LMC
The bright stellar content of fifteen binary star clusters in the LMCwas estimated, using film copies of plates taken with the 1.2 U.K.Schmidt telescope to derive the spectral types of the stars in theregion. It was found that all classified stars are brighter than V =17.5 mag and are situated in the large areas around each pair and in aneighboring field. The binary clusters were of different ages, from 8 x10 to the 6th to greater than 6 x 10 to the 8th yr, indicating that theformation of binary clusters occurred at different stages of theGalaxy's dynamical history. Seven of the pairs, the brightest and themost populous, are young clusters. For these, both members of each pairexhibit the same stellar content.
| Synchronization instability and merging of binary globular clusters
In Magellanic Clouds many binary globular clusters have been observed.Companion clusters exert tidal forces on each other, stripping off theiroutlying stars and finally merging into a single cluster. Such processescan be simulated using an N-body code. The result shows that mergingbegins suddenly when the orbital separation between the componentsreaches a critical value which is practically independent of theirinitial orbital parameters. Such a situation causes the merged clustersto have similar physical states regarding spin angular momentum andinternal structure, i.e., density and angular velocity distributions.The reason for this is theoretically analyzed using a roughapproximation in which the tidal effects bring about a circularizationof the orbital and a synchronization of the spins of each cluster withorbital rotation. It is found that the loss of angular momentum from thesystem leads to instability at a critical separation because the orbitalangular momentum becomes insufficient to supply the necessary angularmomentum for spin synchronization. The observational as well astheoretical implications of such uniqueness of the mergers are alsodiscussed.
| Binary star clusters in the Large Magellanic Cloud
In a survey of the LMC cluster system, double clusters with acenter-to-center separation of less than 1.3 arcmin (18 pc) have beenidentified. It is inferred that a considerable fraction of these doubleclusters must be binaries since the calculated projection effects canaccount for only 31 of them. This inference is strongly supported by thefact that the ages available for some of the culsters of the sample (asdetermined from UBV photometry) are less than the computed times ofmerger or disruption of the binary cluster system. Furthermore, thespace distribution of these pairs indicates that these clusters belongto a very young or young population.
| A Catalogue of Clusters in The LMC
Not Available
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