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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.
| The Optical Gravitational Lensing Experiment. Catalog of Star Clusters from the Large Magellanic Cloud
We present the catalog of star clusters found in the area of about 5.8square degree in the central regions of the Large Magellanic Cloud. Itcontains data for 745 clusters. 126 of them are new objects. For eachcluster equatorial coordinates, radius, approximate number of membersand cross-identification are provided. Photometric data for all clusterspresented in the catalog and Atlas consisting of finding charts andcolor-magnitude diagrams are available electronically from the OGLEInternet archive.
| 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.
| The ellipticities of Galactic and Large Magellanic Cloud globular clusters
The correlations between the ellipticity and the age and mass of LMCglobular clusters are examined, and both are found to be weak. It isconcluded that neither of these properties is mainly responsible for theobserved differences in the LMC and Galactic globular clusterellipticity distributions. Most importantly, age cannot be the primaryfactor in the LMC-Galaxy ellipticity differences, even if there is arelationship, as even the oldest LMC clusters are more elliptical thantheir Galactic counterparts. The strength of the tidal field of theparent galaxy is proposed as the dominant factor in determining theellipticities of that galaxy's globular clusters. A strong tidal fieldrapidly destroys velocity anisotropies in initially triaxial, rapidlyrotating elliptical globular clusters. A weak tidal field, however, isunable to remove these anisotropies and the clusters remain close totheir initial shapes.
| 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.
| 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.
| Moment analysis applied to LMC star clusters
Statistical moment-based ellipse fitting is performed on observations ofLarge Magellanic Cloud clusters, confirming that trends are evident intheir position angles and ellipticities, as had been reported in theliterature. Artificial cluster images with known parameters aregenerated, and subjected to the same analysis techniques, revealingapparent trends caused by stochastic processes. Caution should thereforebe exercised in the interpretation of observational trends in young LMCclusters.
| Ultraviolet colors of old LMC clusters
New ultraviolet spectra for five red LMC globular clusters have beenobtained with IUE. These have been supplemented with archival spectrafor eleven old LMC clusters. These data strengthen and extend theUV-color versus age relation for clusters older than about 10 exp 9 yr,but do not offer much precision in age determination, presumably becausethe ultraviolet colors of the oldest clusters depend strongly on thehorizontal-branch morphology. Comparison of LMC data with UV colors forthe brightest M31 clusters suggests their ages might be only a fewgigayears.
| Bar star clusters in the LMC - Formation history from UBV integrated photometry
The sample of star clusters in the LMC Bar region with integrated UBVphotometry was enlarged by approximately a factor four, totaling 129objects. The (B-V) histogram gap between blue and red clustersdisappears with this deeper sample. Age groups in terms of equivalentSWB types were derived and their spatial distribution studied. Clustersyounger than t about 200 Myr are not homogeneously distributed throughthe bar. In particular a strong star forming event at t about 100 Myrwas detected in the eastern part of the Bar, consisting of a compactgrouping of seven coeval clusters around NGC 2058 and NGC 2065. Also, 11close pairs and two trios are analyzed, and the colors indicate thatonly four pairs are clearly not coeval.
| Spectroscopy of giants in LMC clusters. II - Kinematics of the cluster sample
Velocities for 83 star clusters in the LMC are analyzed, based onindividual stellar velocities measured at the Calcium triplet. One-halfof the clusters are objects in the outer parts of the LMC which had noprevious velocity determinations. Published velocities for intermediateand old clusters are shown to have had systematic errors. These newvelocities with various rotation curve analyses of the LMC, and testaspects of the twisted disk model proposed by Freeman et al. (1983).When the transverse motion of the LMC is taken into account, a singlerotating disk solution fits the old and intermediate-aged clusters andother tracers (i.e., there is no need for an additional 'tilted disk'system).
| The evolution of carbon stars in the Magellanic Clouds
This study presents JHK photometric data for over 100 field stars in theSMC and for 10 in the Large Cloud together with spectroscopic resultsfor about half of them. In the Small Cloud carbon stars were found athigher temperatures and lower luminosities than previously observed. Thefaintest are below the top of the red giant branch. The medium- andlow-luminosity C stars in the M-C transition zone have a low C2 content.At these luminosities, most of the J-type stars are found close to theC2-poor stars in the HR diagram. Their C2 content is about as high as inthe coolest, most evolved C stars. The present observations of carbonstars in the SMC show that they cover a range in M(bo) from -3 to 5.9mag. The transitions from M to C via S appear to occur in both Clouds ata rather well-defined range in M(bol) for SWB and classes IV and V.
| Spectroscopy of giants in LMC clusters. I - Velocities, abundances, and the age-metallicity relation
Velocities and equivalent widths are presented for a large sample of LMCclusters. The calcium abundance is found to be a sensitive abundanceindicator over a very wide range of (Fe/H) between 0.0 and -2.2. Theage-metallicity relation is constructed for the inner and outer parts ofthe LMC. This relationsip can be characterized by a simple one-zoneenrichment model. The abundances for the inner and outer clusters at anage of 2 Gyr are nearly identical, so that little radial abundancegradient is evident in the cluster system.
| Ellipticities at R(h) of LMC star clusters
The projected ellipticities of 53 populous LMC star clusters have beenderived by means of PDS 1010A scans and a computer interactive method ofreduction implemented on an Apollo 570 workstation. Film copies of apair of J and U plates taken with the 1.2 m UK Schmidt Telescope inAustralia were used. The ellipticities derived here agree with thosefound by previous investigators, when comparisons were possible at thesame radius. Ellipticity variations within individual globular clustersare seen to be a common phenomenon, so the ellipticities e(h) at adistance corresponding to the half-mass radius R(h) from the center wereadopted to represent the cluster's flatness. Using these values for theLMC clusters, it is found that LMC clusters are more elliptical thanthose of the Galaxy. Although the young LMC globular clusters show atendency to be more elliptical than the old ones, there is no strongevidence for a significant difference among them. Finally, e(h) wasfound to increase with the total mass of the clusters, possiblyindicating that high-mass clusters have higher angular momentum, or havemore difficulty in shedding angular momentum, than do low mass clusters,and remain longer in their initial flattened shape.
| Observed dynamical parameters of the disk clusters of the Large Magellanic Cloud. II
The structural parameters and density profiles for 28 LMC globularclusters (located within 5 kpc from the rotation center) have beenderived by means of star counts. The clusters were measured on plates offour different colors (U, J, V, I) taken with the 1.2-m UK Schmidttelescope. The tidal radii are found to be within 40-65 pc and theirdynamical masses from 10,000 to 100,000 solar masses. Comparing thedynamical parameters of these clusters with those studied by Kontizas etal. (1987), it is found that the most extended and massive clusters ofthis galaxy are in the innermost area, at distances not exceeding 3 kpcfrom the rotation center; the distances have been corrected for theinclination of the LMC.
| The evolution of the Magellanic Clouds. I - The ages of globular clusters
Theoretical and observed maximum luminosities of AGB stars in theMagellanic Cloud clusters are compared in order to obtain cluster ageestimations. The ages of 10 clusters in the SMC and 25 in the LMC areconsidered for the cases of several rates of mass loss by AGB stars. Itis demonstrated that discrepancies between ages derived from AGB peakluminosities and from the Main-Sequence turn off and maximum luminositycan be accounted for by the intensive mass loss during the AGBevolutionary phase.
| 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.
| Magellanic Cloud globular cluster ages
Comparison of peak luminosities observed for asymptotic giant branch(AGB) stars in Magellanic Cloud globular clusters against theoreticalvalues yields age-estimates for 12 SMC and 22 LMC clusters. Theallowance for intensive mass loss during the AGB evolutionary phasebrings these ages into agreement with those based on the clustercolor-magnitude diagrams. Clusters have developed differently in the twoClouds.
| Ages and metallicities of LMC and SMC red clusters through H-beta and G band photometry
Narrow band integrated photometry of the H-beta and G band absorptionfeatures for 41 LMC and 10 SMC red star clusters is presented. Anage-metallicity calibration is provided for the color-color diagram. SWBtypes between IV and VII are derived for 23 unclassified clusters, andtheir distribution in the age versus metallicity plane is discussed. Astudy of chemical evolution of the Magellanic Clouds has shown that theLMC presents a steeper chemical enrichment slope. An intrinsicmetallicity dispersion is found in the LMC chemical evolution,indicating that the gas has been inhomogeneous at any time, with localenrichment prevailing over a global one. One zone model describes theevolution of both clouds, the efficiency of star cluster formation beinglarger in the LMC. The LMC presents a burst of star cluster formation att = 4.5 x 10 to the 9th yr. New B - V data for fainter SMC clusters arealso presented, providing an essentially complete color histogram forclusters with globular cluster appearance.
| 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.
| The kinematics of globular clusters in the Large Magellanic Cloud
Velocities for 35 globular clusters in the LMC have been combined withdata from other sources to yield velocities for a total of 59 clustersthat range in age from 100 million to 10 billion years. Clusters youngerthan one billion years are noted to have motions similar to the gas intheir vicinity and to share the rotation solution previously found onthe basis of H I velocity maps and H II region velocities. These youngclusters therefore constitute a flattened system having a lowline-of-sight velocity dispersion, consistent with that found inprevious kinematic and photometric studies. The older clusters are alsoflattened to a disk-like system, although both the systematic velocityand position angle of the line of nodes are significantly different forthese older clusters. The data presented also suggest that, unlike theMilky Way, there is no evidence for a kinematic halo population amongglobular clusters in the LMG.
| The extended giant branches of intermediate age globular clusters in the Magellanic Clouds. III
The latest findings of a photographic near-IR survey of the red globularclusters in the Magellanic Clouds for upper asymptotic giant branchstars are reported. New IR (JHK) photometry for some 80 cluster stars isalso presented. These results combined with earlier data are used toderive age estimates for a nearly complete sample of Cloud clustershaving an integrated absolute magnitude less than -7. The agedistribution of clusters in the Large Cloud, which shows a pronouncedpeak at 4 Gyr, may be different from that in the Small Cloud. This peakcould be a result of luminosity evolution of clusters, however, and aconstant rate of cluster formation in the Large Cloud cannot be ruledout. A cluster age-metallicity relation clearly exists in the LargeCloud, althoug the degree of scatter about this relation is somewhatuncertain and may be significant.
| 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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