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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 Star Formation History and Mass Function of the Double Cluster h and χ Persei
The h and χ Per ``double cluster'' is examined using wide-field(0.98d×0.98d) CCD UBV imaging supplemented by optical spectra ofseveral hundred of the brightest stars. Restricting our analysis to nearthe cluster nuclei, we find identical reddenings [E(B-V)=0.56+/-0.01],distance moduli (11.85+/-0.05), and ages (12.8+/-1.0 Myr) for the twoclusters. In addition, we find an initial mass function slope for eachof the cluster nuclei that is quite normal for high-mass stars,Γ=-1.3+/-0.2, indistinguishable from a Salpeter value. We derivemasses of 3700 Msolar (h) and 2800 Msolar (χ)integrating the present-day mass function from 1 to 120Msolar. There is evidence of mild mass segregation within thecluster cores. Our data are consistent with the stars having formed at asingle epoch; claims to the contrary are very likely due to theinclusion of the substantial population of early-type stars located atsimilar distances in the Perseus spiral arm, in addition tocontamination by G and K giants at various distances. We discuss theuniqueness of the double cluster, citing other examples of suchstructures in the literature but concluding that the nearly identicalnature of the two cluster cores is unusual. We fail to settle thelong-standing controversy regarding whether or not the double cluster isthe core of the Per OB1 association and argue that this may beunanswerable with current techniques. We also emphasize the need forfurther work on the pre-main-sequence population of this nearby andhighly interesting region.
| 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. 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.
| Star Clusters Driven to Form by Strong Collisions Between Gas Clouds in High-Velocity Random Motion
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....113..249F
| 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.
| The Orion star-forming region - Far-infrared and radio molecular observations
New J = 1-0 CO and far-infrared maps of the Orion star-forming regionare presented and discussed. The total infrared luminosity of the Orionstar-forming ridge is 250,000 solar luminosities. The material that isemitting strongly at 60 microns is traced and found to be highlycentrally concentrated. However, the majority of the extended emissionfrom this region comes from dust that is ultimately heated by thevisible Trapezium cluster stars. The luminosity of IRc 2, the mostluminous member of the infrared cluster, is estimated to be40,000-50,000 solar luminosities. A schematic drawing of the Ori MC 1region is presented.
| Catalog of CO radial velocities toward galactic H II regions
This is a catalog of 242 molecular cloud complexes which are associatedwith optical H II regions. CO observations were made toward all but fiveof the H II regions in the Sharpless catalog and toward 62 additionalsuspected H II regions, 33 of which are previously uncataloged. Radialvelocities are tabulated for each molecular cloud complex found to beassociated with an H II region. The CO antenna temperature and linewidth are given for the most intense CO line seen toward each source.The catalog also summarizes previous CO observations as well as theoptical distances to the stars exciting the H II regions. Radio-quiet HII regions (those with 1.4 GHz flux densities less than 100 mJy) arefound to be well correlated with objects having no associated CO. A listof kinematically distinct complexes is tabulated to facilitateinvestigations of the motions of the complexes.
| A Catalogue of Clusters in The LMC
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