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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
| The photometric evolution of dissolving star clusters. I. First predictions
The broad-band photometric evolution of unresolved star clusters iscalculated in a simplified way, including the preferential loss oflow-mass stars due to mass segregation. The stellar mass function of acluster evolves due to three effects: (a) the evolution of the massivestars reduces their number; (b) tidal effects before cluster-wide masssegregation reduce the mass function homogeneously, i.e. independentlyof the stellar mass; (c) after mass segregation has finished, tidaleffects preferentially remove the lowest-mass stars from the cluster.These effects result in a narrowing of the stellar mass range. Theseeffects are described quantitatively, following the results of N-bodysimulations, and are taken into account in the calculation of thephotometric history, based on the galev cluster evolution models forsolar metallicity and a Salpeter mass function. We find the followingresults: (1) During the first ~40% of the lifetime of a cluster itscolour evolution is adequately described by the standard galev models(without mass segregation) but the cluster becomes fainter due to theloss of stars by tidal effects. (2) Between ~40 and ~80% of its lifetime(independent of the total lifetime), the cluster becomes bluer due tothe loss of low-mass stars. This will result in an underestimate of theage of clusters if standard cluster evolution models are used. (3) After~80% of the total lifetime of a cluster it will rapidly become redder.This will result in an overestimate of the age of clusters if standardcluster evolution models are used. (4) Clusters with mass segregationand the preferential loss of low-mass stars evolve along almost the sametracks in colour-colour diagrams as clusters without mass segregation.Only if the total lifetime of clusters can be estimated can the coloursbe used to give reliable age estimates. (5) The changes in the colourevolution of unresolved clusters due to the preferential loss oflow-mass stars will affect the determination of the star formationhistories of galaxies if they are derived from clusters that have atotal lifetime of less than about 30 Gyr. (6) The preferential loss oflow-mass stars might explain the presence of old (~13 Gyr) clusters inNGC 4365 which are photometrically disguised as intermediate-ageclusters (2-5 Gyr), if the expected total lifetime of these clusters isbetween 16 and 32 Gyr.
| 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.
| The Low End of the Initial Mass Function in Young Clusters. II. Evidence for Primordial Mass Segregation in NGC 330 in the Small Magellanic Cloud
As part of a larger program aimed at investigating the universality ofthe initial mass function (IMF) at low masses in a number of youngclusters in the LMC and SMC, we present a new study of the low end ofthe stellar IMF of NGC 330, the richest young star cluster in the SMC,from deep broadband V and I images obtained with HST/WFPC2. We detectstars down to a limiting magnitude of m555=24.9, whichcorresponds to stellar masses of ~0.8Msolar at the distanceof the SMC. A comparison of the cluster color-magnitude diagram withtheoretical evolutionary tracks indicates an age of ~30 Myr for NGC 330,in agreement with previous published results. We derive the clusterluminosity function, which we correct for background contamination usingan adjacent SMC field, and construct the mass function in the1-7Msolar mass range. Given the young cluster age, the MF canwell approximate the IMF. We find that the IMF in the central clusterregions (within 30") is well reproduced by a power law with a slopeconsistent with Salpeter's. In addition, the richness of the clusterallows us to investigate the IMF as a function of radial distance fromthe center. We find that the IMF becomes steeper at increasing distancesfrom the cluster center (between 30" and 90"), with the number ofmassive stars (>5Msolar) decreasing from the core to theoutskirts of the cluster 5 times more rapidly than the less-massiveobjects (~=1Msolar). We believe the observed mass segregationto be of a primordial nature rather than dynamical since the age of NGC330 is 10 times shorter than the expected relaxation time of thecluster. Based on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated byAURA for NASA under contract NAS5-26555.
| Mass segregation in young compact star clusters in the Large Magellanic Cloud - I. Data and luminosity functions
We have undertaken a detailed analysis of HST/WFPC2 and STIS imagingobservations, and of supplementary wide-field ground-based observationsobtained with the ESO New Technology Telescope (NTT) of two young(~10-25Myr) compact star clusters in the LMC, NGC 1805 and 1818. Theultimate goal of our work is to improve our understanding of the degreeof primordial mass segregation in star clusters. This is crucial for theinterpretation of observational luminosity functions (LFs) in terms ofthe initial mass function (IMF), and for constraining the universalityof the IMF. We present evidence for strong luminosity segregation inboth clusters. The LF slopes steepen with cluster radius; in both NGC1805 and 1818 the LF slopes reach a stable level well beyond the core ofthe clusters or half-light radii. In addition, the brightest clusterstars are strongly concentrated within the inner ~4Rhl. Theglobal cluster LF, although strongly non-linear, is fairly wellapproximated by the core or half-light LF; the (annular) LFs at theseradii are dominated by the segregated high-luminosity stars, however. Wepresent tentative evidence for the presence of an excess number ofbright stars surrounding NGC 1818, for which we argue that they are mostprobably massive stars that have been collisionally ejected from thecluster core. We therefore suggest that the cores of massive young starsclusters undergo significant dynamical evolution, even on time-scales asshort as ~25Myr.
| M82-F: a doomed super star cluster?
We present high-dispersion echelle spectroscopy of the very luminous,young super star cluster (SSC) `F' in M82, obtained with the 4.2-mWilliam Herschel Telescope, for the purpose of deriving its dynamicalmass and assessing whether it will survive to become an old globularcluster. In addition to the stellar lines, the spectrum contains complexNai absorption and broad emission lines from the ionized gas. We measurea stellar velocity dispersion of 13.4+/-0.7kms-1 and aprojected half-light radius of 2.8+/-0.3pc from archival HST/WFPC2images, and derive a dynamical mass of1.2+/-0.1×106Msolar, demonstrating thatM82-F is a very massive, compact cluster. We determine that the currentluminosity-to-mass ratio (LV/M)solar for M82-F is45+/-13. Comparison with spectral synthesis models shows that(LV/M)solar is a factor of ~5 higher than thatpredicted for a standard Kroupa initial mass function (IMF) at thewell-determined age for M82-F of 60+/-20Myr. This high value of(LV/M)solar indicates a deficit of low-mass starsin M82-F; the current mass function (MF) evidently is `top-heavy'. Wefind that a lower mass cut-off of 2-3Msolar is required tomatch the observations for a MF with a slope α=2.3. Since thecluster apparently lacks long-lived low-mass stars, it will not becomean old globular cluster, but probably will dissolve at an age of <=2Gyr. We also derive updated luminosity-to-mass ratios for the youngerSSCs NGC 1569A and 1705-1. For the first object, the observations areconsistent with a slightly steeper MF (α=2.5), whereas for NGC1705-1 the observed ratio requires the MF to be truncated near2Msolar for a slope of α=2.3. We discuss theimplications of our findings in the context of large-scale IMFvariations; with the present data the top-heavy MF could reflect a localmass segregation effect during the birth of the cluster. M82-F probablyformed in a dense molecular cloud; however, its high radial velocitywith respect to the centre of M82 (~-175kms-1) suggests thatit is on an eccentric orbit and now far from its birthplace, so theenvironment of its formation is unknown.
| The Stellar Initial Mass Function from Random Sampling in Hierarchical Clouds. II. Statistical Fluctuations and a Mass Dependence for Starbirth Positions and Times
Observed variations in the slope of the stellar initial mass function(IMF) are shown to be consistent with a previously introduced model inwhich the protostellar gas is randomly sampled from clouds with aself-similar hierarchical structure. Root mean square variations in theIMF slope around the Salpeter value are +/-0.4 when only 100 stars areobserved, and +/-0.1 when 1000 stars are observed. Similar variationsshould be present in other stochastic models as well. The hierarchicalsampling model reproduces the tendency for massive stars to form closerto the center of a cloud at a time somewhat later than the formationtime of the lower mass stars. The systematic variation in birth positionresults from the tendency for the trunk and larger branches of thehierarchical tree of cloud structure to lie closer to the cloud center,while the variations in birth order result from the relative infrequencyof stars with larger masses. The hierarchical cloud sampling model hasnow reproduced most of the reliably observed features of the clusterIMF. The power-law part of the IMF comes from cloud hierarchicalstructure that is sampled during various star formation processes with arelative rate proportional to the square root of the local density.These processes include turbulence compression, magnetic diffusion,gravitational collapse, and clump or wavepacket coalescence, all ofwhich have about this rate dependence. The low-mass flattening comesfrom the inability of gas to form stars below the thermal Jeans mass attypical temperatures and pressures. The thermal Jeans mass is the onlyrelevant scale in the problem. Considerations of heating and coolingprocesses indicate why the thermal Jeans mass should be nearly constantin normal environments and why this mass might increase in starburstregions. In particular, the relative abundance of high-mass stars shouldincrease where the average density of the interstellar medium is verylarge; accompanying this increase should be an increase in the averagetotal efficiency of star formation. Alternative models in which the rateof star formation is independent of density and the local efficiencydecreases systematically with increasing stellar mass can also reproducethe IMF, but this is an adjustable result and not a fundamental propertyof hierarchical cloud structure, as is the preferred model. The steepIMF in the extreme field is not explained by the model, but otherorigins are suggested, including one in which massive stars inlow-pressure environments halt star formation in their clouds. In thiscase, the slope of the extreme field IMF is independent of the slope ofeach component cluster IMF and is given by (gamma-1)/alpha for a cloudmass function slope, -gamma~-2, and a power-law relation, M_L~M^alpha_c,between the largest star in a low-pressure cloud, M_L, and the cloudmass, M_c. A value of alpha~1/4 is required to explain the extreme fieldIMF as a superposition of individual cluster IMFs; cloud destruction byionizing has this property. We note that the similarity between clusterIMFs and the average IMF from global studies of galaxies implies thatmost stars form in clusters and that massive stars do not generally haltstar formation in the same cloud.
| Interacting star clusters in the Large Magellanic Cloud. Overmerging problem solved by cluster group formation
We present the tidal tail distributions of a sample of candidate binaryclusters located in the bar of the Large Magellanic Cloud (LMC). Oneisolated cluster, SL 268, is presented in order to study the effect ofthe LMC tidal field. All the candidate binary clusters show tidal tails,confirming that the pairs are formed by physically linked objects. Thestellar mass in the tails covers a large range, from 1.8x 10(3) to 3x10(4) \msun. We derive a total mass estimate for SL 268 and SL 356. Atlarge radii, the projected density profiles of SL 268 and SL 356 falloff as r(-gamma ) , with gamma = 2.27 and gamma =3.44, respectively. Outof 4 pairs or multiple systems, 2 are older than the theoreticalsurvival time of binary clusters (going from a few 10(6) years to 10(8)years). A pair shows too large age difference between the components tobe consistent with classical theoretical models of binary clusterformation (Fujimoto & Kumai \cite{fujimoto97}). We refer to this asthe ``overmerging'' problem. A different scenario is proposed: theformation proceeds in large molecular complexes giving birth to groupsof clusters over a few 10(7) years. In these groups the expected clusterencounter rate is larger, and tidal capture has higher probability.Cluster pairs are not born together through the splitting of the parentcloud, but formed later by tidal capture. For 3 pairs, we tentativelyidentify the star cluster group (SCG) memberships. The SCG formation,through the recent cluster starburst triggered by the LMC-SMC encounter,in contrast with the quiescent open cluster formation in the Milky Waycan be an explanation to the paucity of binary clusters observed in ourGalaxy. Based on observations collected at the European SouthernObservatory, La Silla, Chile}
| 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.
| Mass segregation in two young clusters in the Large Magellanic Cloud: SL 666 and NGC 2098
The age and dynamics of the young LMC clusters SL 666 and NGC 2098 wereinvestigated using CCD observations obtained with the 3.9mAnglo-Australian Telescope. SL 666 was found to have an age of 1 - 1.25x 10(8) yr, while NGC 2098 is somewhat younger, with an age of 6.3 -7.9*E(7) yr. In the case of SL 666 the good quality of the acquired dataallowed the construction of the main sequence luminosity function as afunction of radius, which revealed mass segregation, also confirmed fromstar counts. Since the age of SL 666 is significantly lower than themean relaxation time of at least 5 x 10(8) yr calculated for thiscluster and since the mass range of the cluster members does not permitus to assume faster energy equipartition, the observed mass segregationseems to be due to the way stars form in the cluster. Indications on astar formation scenario are examined. Disruption time is also discussedand there is strong evidence that the cluster is bound. NGC 2098 showsstrong evidence of mass segregation, but further observations are neededin this case.
| 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.
| The structure of the halo of the star cluster NGC 2070
The structure of the halo of the star cluster NGC 2070 in the LargeMagellanic Cloud is investigated on the basis of published magnitudesand positions of its stars. The cluster halo has been found to becomposed of a large number of stellar subclusters (about 30) having from10 to 150 stars. In most cases, the subclusters appear as filaments orarcs; they form ring-shaped structures, 10 - 20 pc in diameter, thatcoincide with ionized-gas filaments in the central part of the Tarantulanebula. The color-magnitude diagrams for the subclusters reveal apossible difference in age. The subclusters differ in stellarcomposition: the farther the subgroup from the cluster center, the lowerthe upper limit on the brightness of its stars. This can be explainedeither by the difference in the age of the subclusters or by adifference in the physical conditions in the original gas cloud.
| Colour Evolution Models and the Distribution of Large Magellanic Cloud Clusters in the Integrated UBV Plane
We present a comparison between photometric cluster models, based onclassical and with-overshooting stellar tracks, and the enlarged sampleof 624 LMC clusters recently gathered in integrated UBV photometry byBica et al. Models based on Maeder and Meynet's tracks present twotemporary red phases: the first at age 10 Myr, caused by a clump of redsupergiants; the second at ~100 Myr due to the combined effect of boththe progressive reduction of the blue loop of core He-burning stars, andtheir fading relative to top-MS stars. The 100 Myr red phase does notoccur in models without overshooting. Taking into account stochasticeffects on the mass distribution of stars, the models describe well thegeneral distribution of clusters in the (U - B) vs. (B - V) diagram,except for the oldest, SWB types V-VII, clusters. The dispersion ofcluster colours due to stochastic effects is found to be stronglyvariable along the ageing sequence: the general trend is a decrease withage due to the increasing population of post-MS phases, but thedispersion increases in the temporary red phases and is expected toincrease again after the red giant branch phase transition due to theappearance of extended RGBs and carbon stars. We also study the LMCclusters age distribution function, based on the age frequency ofclusters of equal initial masses, taking into account different valuesfor the IMF slope.
| Near-infrared spectral evolution of blue LMC clusters : a comparison with galactic open clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1990RMxAA..21..202B
| 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.
| An automated search for star clusters in the Magellanic Clouds. I - Description of the technique and application to a 6 square degree field near the bar of the LMC
Abstract image available at:http://adsabs.harvard.edu/abs/1989A&A...211....9B
| 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.
| Distribution of spectral types in the LMC clusters
The distribution of spectral types in 42 LMC globular star clusterscovering all evolutionary ages was determined using objective prismspectra taken with the 1.2-m U.K. Schmidt Telescope in Australia. Thederived spectral type distributions show that the clusters can bedivided into five age categories from about 10 to the 7th to more than10 to the 9th yr. Several clusters were found to contain carbon starswith C/M ratios ranging from 0.07 to 0.4. These ratios were comparedwith those found for the SMC clusters and the Milky Way. It was foundthat the stars of the LMC exhibit a smaller range of C/M ratios than inthe SMC, but larger than in the Galaxy, thus providing an additionaltest of the theoretical models predicting a correlation between the C/Mratio and metal content. It was also found that the majority of youngclusters were embedded in older fields, while the intrmediate clusterswere embeded in younger fields, and the remote old clusters wereembedded in a stellar content of similar age.
| A preliminary survey of collapsed cores in the Magellanic Clouds' globular clusters
A preliminary report on a surface photometry survey for collapsed coresin the Magellanic Clouds' globular clusters is presented. Coremorphology classifications are given for the 33 globular clustersexamined so far. One cluster, NGC 2019, shows definite signs of acollapsed core, and two others, NGC 1774 and NGC 1951, appear as strongcandidates. This detection of postcollapse cores outside the Milky Wayopens some interesting prospects for future dynamical studies. However,the fraction of collapsed-core clusters appears to be smaller in theMagellanic Clouds than in the Galaxy. This may be due in part to theiryounger ages, or to the limitations imposed by the seeing effects. It isalso possible that the relative scarcity reflects the physicaldifference in the tidal field environments between the Galaxy and theClouds, in agreement with a trend found earlier, viz., that the tidalshocks from disk passages accelerate dynamical evolution and enhance thepropensity for core collapse.
| Observed dynamical parameters of the disk clusters of the LMC. I
A study of the observed dynamical parameters of 32 globular clusters ofthe LMC disk has been carried out by means of star counts. The clusterswere measured on a set of three plates (J, V, I) taken with the 1.2 mU.K. Schmidt Telescope. The derived tidal radii were all found to belarge within a very narrow range. As a consequence the range of thetotal masses was found to be very narrow as well. These two parametersare large in comparison to those of the disk young clusters of thegalaxy but they are similar to the dimensions of the halo galacticglobulars.
| Spectral classification of bright stars in LMC clusters
Identification charts and spectral classification catalogs for ten LMCglobular clusters and their adjoining fields are presented here. Filmcopies of plates taken with the 1.2 m U.K. Schmidt telescope wereexamined in Athens by means of a binocular microscope. The studiedclusters are of various ages, from very young to old, and are located inthe northern part of the galaxy. All classified stars are brighter thanV of about 17.5 mag and situated within the cluster's tidal radii.
| 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.
| An ellipticity - age relation for globular clusters in the Large Magellanic Cloud. I - Measurements
It is pointed out that the rich star clusters of the Magellanic Cloudsresemble the globular clusters of the Galaxy. The present investigationhas the objective to determine the shapes of these clusters and theirdependence on age. The study has been restricted to the Large MagellanicCloud (LMC) because the Small Magellanic Cloud (SMC) does not provide alarge enough sample for the statistical analysis. The shapes of globularclusters are usually expressed in terms of ellipticities. Attention isgiven to the measurement of ellipticities with the aid of a ruler and agraduated magnifying glass, star count data on 12 LMC clusters, and ageestimates. It is found that estimates of the ellipticities of globularclusters made by eye are in excellent agreement with those based on starcounts. The ellipticity-age relation is probably explained mostnaturally by internal evolution in the structure of globular clusters.
| A Catalogue of Clusters in The LMC
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