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Spectral evolution of star clusters in the Large Magellanic Cloud. I. Blue concentrated clusters in the age range 40-300 Myr
Aims.Integrated spectroscopy of a sample of 17 blue concentrated LargeMagellanic Cloud (LMC) clusters is presented and its spectral evolutionstudied. The spectra span the range ≈(3600-6800) Å with aresolution of ≈14 Å FWHM, being used to determine cluster agesand, in connection with their spatial distribution, to explore the LMCstructure and cluster formation history. Methods.Cluster reddeningvalues were estimated by interpolation, using the available extinctionmaps. We used two methods to derive cluster ages: (i) template matching,in which line strengths and continuum distribution of the clusterspectra were compared and matched to those of template clusters withknown astrophysical properties, and (ii) equivalent width (EW) method,in which new age/metallicity calibrations were used together withdiagnostic diagrams involving the sum of EWs of selected spectral lines(K Ca II, G band (CH), Mg I, Hδ, Hγ and Hβ).Results.The derived cluster ages range from 40 Myr (NGC 2130and SL 237) to 300 Myr (NGC 1932and SL 709), a good agreement between the results ofthe two methods being obtained. Combining the present sample withadditional ones indicates that cluster deprojected distances from theLMC center are related to age in the sense that inner clusters tend tobe younger. Conclusions.Spectral libraries of star clusters are usefuldatasets for spectral classifications and extraction of parameterinformation for target star clusters and galaxies. The present clustersample complements previous ones, in an effort to gather a spectrallibrary with several clusters per age bin.
| 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.
| 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
| The Optical Gravitational Lensing Experiment. Cepheids in Star Clusters from the Magellanic Clouds
We present Cepheids located in the close neighborhood of star clustersfrom the Magellanic Clouds. 204 and 132 such stars were found in the LMCand SMC, respectively. The lists of objects were constructed based oncatalogs of Cepheids and star clusters, recently published by theOGLE-II collaboration. Location of selected Cepheids on the skyindicates that many of them are very likely cluster members. Photometricdata of Cepheids and clusters are available from the OGLE Internetarchive.
| 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.
| 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.
| 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 BVRI photometric study of star clusters in the BOK region of the Large Magellanic Cloud
BVRI photometric observations of 14 LMC star clusters, obtained onphotographic plates with the 1.5-m telescope at Tololo in 1972, with the2.5-m telescope at Las Campanas in 1978-1981, and with the 3.6-mtelescope at ESO in 1982-1983, are reported. The data are presented in aseries of tables and color-color and color-magnitude diagrams, andindividually derived reddenings and mean-age estimates (mainly in therange 15-63 Myr, with one cluster 138 + or - 25 Myr old) are included.
| A Catalogue of Clusters in The LMC
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