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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.
| Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| Structure and Mass Segregation in h and χ Persei
We use V magnitudes and spectral types to examine the density structureof h and χ Per. We describe an automatic method for derivingspectral types and compare classifications for observations made at twodifferent facilities. With these data, we measure an extinction to theclusters of E(B-V)=0.52+/-0.07, consistent with other authors. However,there appears to be a correlation between the spectral types of thestars used and the resulting value of the extinction. We compareextinction values measured by different authors using different numbersof stars and reproduce their values by imposing different cuts in the Vmagnitude. This variation in color excess versus spectral type suggeststhat the standard intrinsic colors for the earliest type stars are bluerthan the stars in h and χ Per. We measure centers for h and χPer at α(2000)=2h18m56.4s+/-3.0s,δ(2000)=57deg8'25''+/-23''and α(2000)=2h22m4.3s+/-2.9s,δ(2000)=57deg8'35''+/-25'',respectively. We fit the density structure of the clusters and find coreradii of 1.9 and 2.4 pc, respectively. Integration of the Miller-Scaloinitial mass function suggests overall cluster masses of 5500 and 4300Msolar and central densities of 27 and 50 Msolarpc-3, respectively. We find strong evidence of masssegregation in h Per but not in χ Per. Examination of the dynamicaltimescales, as well as comparisons between the two clusters, suggestthat the mass segregation is partly primordial.
| Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
| 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
| 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.
| Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.
| Differences in the fractions of Be stars in galaxies
The number ratios Be/(B+Be) of Be to B-type stars in young, well studiedclusters of the Galaxy, the LMC and SMC are examined. In order todisentangle age and metallicity effects we choose clusters in the sameage interval and for which reliable photometric and spectroscopic dataare available. Number counts are made for various magnitude intervals,and the results are found to be stable with respect to this choice. Inthe magnitude interval MV = -5 to -1.4 (i.e. O9 to B3) weobtained a ratio Be/(B+Be) = 0.11, 0.19, 0.23, 0.39 for 21 clusterslocated in the interior of the Galaxy, the exterior of the Galaxy, theLMC and the SMC, respectively. Various hypotheses for these differencesare examined. An interesting possibility is that the average rotation isfaster at low metallicities as a result of star formation processes. Themuch higher relative N-enrichment found by Venn et al. (\cite{vencar})in A-type supergiants of the SMC, compared to galactic supergiants, alsostrongly supports the presence of more rotational mixing at lowmetallicities. We discuss whether high rotational mixing may be thesource of primary nitrogen in the early chemical evolution of galaxies.
| 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.
| The cluster pair SL 538 / NGC 2006 (SL 537)
We studied in detail the binary cluster candidates SL 538 and NGC 2006in the Large Magellanic Cloud (LMC). This cluster pair is located in thenorthwestern part of the large OB association LH 77 in supergiant shellLMC 4. A third star cluster, KMHK 1019, is located within 5arcmin fromthe cluster pair. Our study is the first age determination of SL 538 andNGC 2006 that is based on CMDs. We derived an age of 18+/-2 Myr for SL538, 22.5+/-2.5 Myr for NGC 2006, and 16 Myr for KMHK 1019. Thus thethree clusters are (nearly) coeval. We identified Be star candidates andfind the same ratio N(Be)/N(B) for the components of the binary cluster(12%) while the amount of Be stars detected in KMHK 1019 (5%) and in thesurrounding field (2%) is considerably lower. Since Be stars are usuallyrapid rotators this may indicate intrinsically higher rotationalvelocities in the components of the cluster pair. Also the IMF derivedfrom the CMDs shows the same slope for both SL 538 and NGC 2006 and isconsistent with a Salpeter IMF. An estimation of the cluster massesbased on the IMF slopes showed that both clusters have similar totalmasses. These findings support joint, near-simultaneous formation of thecluster pair in the same giant molecular cloud. Based on observationstaken at the European Southern Observatory, La Silla, Chile, during timeallocated by the MPIA, Heidelberg.
| Recent Star Formation in Shapley Constellation III in the Large Magellanic Cloud
We present UBV photometry of four fields within Shapley ConstellationIII and one field on the edge of the shell. Our fields cover roughly 20%of the region, mostly in the southern half. Determinations are made ofages of the fields, the star formation densities, and the initial massfunction (IMF) slopes. The field-age determinations inside theconstellation show ages between 12 and 16 Myr uncorrelated with distancefrom the center, while the age of the field on the edge of theconstellation shows an age of around 6-7 Myr. The southern part of theconstellation shows star formation densities and IMF slopes typical ofOB associations and giant H ii regions, while the northern part showssignificantly fewer intermediate-mass stars and a steeper IMF slope. Wecompare these properties of Constellation III with those of 30 Doradus,another LMC star-forming region of comparable size to Constellation III.Although the regions formed from roughly the same amount of gas, weestimate that 30 Doradus formed a few times more stars thanConstellation III.
| Studies of binary clusters: SL 538 / NGC 2006
The properties of gravitationally bound pairs of star clusters areimportant for the understanding of formation and evolution of starclusters. Since the probability of tidal capture of one cluster byanother one is very small, we can assume that the components of a truebinary star cluster have a common origin, thus having the same or atleast similar ages and metallicities. In the Magellanic Clouds a largenumber of pairs was found, exceeding the statistically expected numberof chance superpositions. We have started a project to study in detailbinary cluster candidates. Here we present first results for the youngclusters SL 538 and NGC 2006. This cluster pair is located near thelarge OB association LH 77 in the supergiant shell LMC 4 in thenorthwest part of the LMC. By comparing CMDs with isochrones we findsimilar ages for both clusters. Investigating the stellar density aroundthe clusters we see indications for a stellar bridge between the twocomponents, which may imply that the merger process has begun. Weidentify Be-star candidates and find the same ratio N(Be)/N(B) for bothclusters. Also the slopes of the IMFs are similar and both consistentwith a Salpeter IMF. All these results suggest that SL 538 and NGC 2006constitute a true binary cluster that formed at the same time in thesame giant molecular cloud.
| 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.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| 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.
| Indications for common origin and gravitational interaction in three binary LMC clusters
Three close pair clusters of the LMC, NGC 2006/SL 538, NGC 2011 a/b, andNGC 2042 a/b, have been studied in order to establish their binarity.The stellar content in the outer region of each cluster has beeninvestigated by means of low resolution objective UK Schmidt prismspectra, the cores of the clusters have been examined using lowresolution integrated IUE spectra, whereas their density profiles andtheir observed dynamical parameters have been derived by means of starcounts. The integrated spectra of their cores and the stellar content oftheir outer cluster regions have shown a common origin and a very youngage (less than 2 x 10 exp 7 yr) for each member of the pairs, whereastheir dynamical study has shown that they are gravitationallyinteracting. Comparing the age of their stellar content with theirdynamical and relaxation times, it has been found that these clustersare physically associated and had no time to relax by stellarencounters, no time to merge, and no time to be destroyed by dynamicalfriction.
| 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.
| Blue Magellanic clusters - Near-infrared spectral evolution
New integrated spectra in the range 5600-10,000 A are presented for 28LMC and 3 SMC young star clusters. The equivalent widths (W) ofprominent features and the continuum distribution are measured. Theanalysis, supplemented by 8 additional LMC clusters from previousstudies, indicates that the red supergiant phase is indeed verytime-peaked, occuring from 7 to 12 Myr. In addition to the previous caseof NGC 2004, it is found that NGC 1805, NGC 1994, NGC 2002, NGC 2098,and NGC 2100 (as well as NGC 2011 to a lesser extent) are undergoingthis phase. The red supergiant phase is clearly denoted by strong TiObands and Ca II triplet as well as a flat continuum or (in extremecases) a continuum with positive slope above 6000 A.
| 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.
| Integrated UV magnitudes of the Large Magellanic Cloud associations
UV photographs (2600 A, 350 A passband) of the LMC have been obtained bythe S183 experiment during a Skylab mission. The background is estimatedand a method for deriving the integrated fluxes is presented. Theintegrated magnitudes of about 50 associations and isocontours of theirintensities are given, along with the B and V integrated magnitudes of13 associations.
| 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.
| The stellar populations of Shapley constellation III
A V-I color-magnitude diagram is presented for a 0.6-sq deg fieldencompassing part of the LMC's Shapley III star-formation region. Thepronounced luminosity function peak exhibited by the main-sequence starsis identified with the turnoff of the first star-forming burst, and thenused as an age indicator with which to compare stellar evolutionarymodels with the dynamical age estimate determined by Dopita et al.(1985); the initial luminosity and mass functions are derived. Thedynamical clock in Shapley III is in better agreement with the stellarevolutionary clock if models without convective overshoot are adopted.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
| A Catalogue of Clusters in The LMC
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