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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.
| 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
| Studies of binary star cluster candidates in the bar of the LMC. II
Binary clusters account for more than 10% of the cluster population inthe Magellanic Clouds. Statistically fewer than 50% of the found pairsare expected to be chance superpositions. We estimated the clusterencounter rate and suggest that tidal capture is an unlikely formationscenario for the formation of binary clusters. Thus, most true binaryclusters can be expected to have formed together. Here we present astudy of three binary cluster candidates which are located in the bar ofthe LMC. NGC 1971 & NGC 1972 are situated in the association LH 59in the eastern part of the bar. A third star cluster, NGC 1969, is closeenough to this pair that all three objects may constitute a triplesystem. We present the first age determination that is based on CMDs forthese star clusters. Our findings suggest that all three clusters areyoung (40-70 Myr) and may have been formed in the same GMC. It cannotclearly decided whether the clusters are physically interacting or not.NGC 1894 & SL 341 are located at the south-western rim of the LMCbar. This pair is studied in detail for the first time: The isopleths ofboth clusters reveal an elliptical shape. Whether this might beinterpreted as a sign of interaction or is a peculiarity which is sharedwith a large amount of LMC star clusters which show higher ellipticitiesthan their counterparts in the Milky Way remains unclear. From our agedetermination we find that both clusters are coeval with an age of55+/-5 Myr. This makes a formation from the same GMC a likely scenario.SL 385 & SL 387 are a close pair in the western part of the LMC bar.We derived ages of 170+/-30 Myr for SL 385 and >=250 for SL 387. Thelarge age difference makes it unlikely that these two clusters formed inthe same GMC. Based on observations taken at the European SouthernObservatory, La Silla, Chile in program 54.D-0730 and during timeallocated by the MPIA, Heidelberg.
| 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.
| 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}
| 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.
| 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.
| A catalog of 255 new clusters in the Large Magellanic Cloud
A catalog of 255 newly discovered clusters has resulted from a search often fields in the LMC. Most of the clusters are faint, small, and old.The equatorial coordinates, sizes, and estimates of the B magnitudes ofthe brightest stars are given for all of the clusters. The mean diameterfor all LMC clusters in the fields is shown to be 7.7 pc. A correlationis found between size and distance from the center of the LMC, with morelarge clusters in the remote outer parts of the LMC. The total clusterpopulation of the LMC is estimated at about 4200.
| 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.
| A Catalogue of Clusters in The LMC
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