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|Fundamental parameters of the LMC clusters NGC 1836, NGC 1860, NGC 1865, SL 444, LW 224 and SL 548|
Complementing our recent Washington photometric studies on intermediateage and young Large Magellanic Cloud (LMC) clusters, we now turn ourattention to six previously unstudied star clusters in the transitionrange 200-700 Myr. We study NGC 1836, 1860 and 1865, which are projectedon the LMC bar; SL 444, also located in the central disc but outside thebar; and LW 224 and SL 548, both located in the outer disc. We deriveages and metallicities from extracted T1 versusC-T1 colour-magnitude diagrams (CMDs), using theoreticalisochrones recently computed for the Washington photometric system. Forthe metallicity determinations, these CMDs are particularly sensitive.We also estimate ages and metallicities of the surrounding fields of NGC1860 and 1865 by employing the δT1 index defined inGeisler et al. (1997, AJ, 114, 1920) and theoretical isochrones. Byadding the present cluster sample to those of our previous studies, wenow gather 37 LMC clusters with homogeneous parameter determinations,which are employed to probe the chemical enrichment of the LMC and itsspatial distribution. On average, inner disc clusters turned out to benot only younger than the outer ones, but also more metal-rich; somehave solar metal content. Furthermore, inner clusters located to thewest of the LMC centre are younger and more metal-rich than theireastern counterparts. We propose that a bursting formation mechanism,with an important formation event centred at ~2.0 Gyr, provides a betterdescription of the cluster age-metallicity relation than a closed-boxchemical evolution model. In the outer disc, the field star formationseems to have lasted until 2 Gyr ago while it continued in the innerdisc for almost 1 Gyr longer.
|Young star clusters immersed in intermediate-age fields in the Large Magellanic Cloud bar|
We present Washington System photometry for 11 star clusters immersed inthe north-west part of the Large Magellanic Cloud (LMC) bar, centred onthe intermediate-age clusters NGC 1865 and SL 244. The fields areheavily populated by the intermediate-age component of the LMC bar. Wesucceeded in disentangling cluster colour-magnitude diagrams from thoseof the fields and in deriving reddening and ages for five clusters - SL218, BRHT4b, and NGC 1839, 1838 and 1863 - with the aid of recentWashington System theoretical isochrones. The resulting cluster agesrange between 50 and 125 Myr. Despite their proximity, NGC 1836 andBRHT4b have very different ages. Thus the possibility of these twoobjects being a binary cluster is very unlikely, although a capturecannot be ruled out a priori. Our results suggest that for eachintermediate-age cluster remaining in the LMC bar region, a number ofrobust young blue star clusters occurs in the same region.
|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.
|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.
|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.
|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.
|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.
|A Catalogue of Clusters in The LMC|
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