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Classical Cepheid Pulsation Models. X. The Period-Age Relation
We present new period-age (PA) and period-age-color (PAC) relations forfundamental and first-overtone classical Cepheids. Current predictionsrely on homogeneous sets of evolutionary and pulsation models covering abroad range of stellar masses and chemical compositions. We found thatPA and PAC relations present a mild dependence on metal content.Moreover, the use of different PA and PAC relations for fundamental andfirst-overtone Cepheids improves the accuracy of age estimates in theshort-period (logP<1) range (old Cepheids), because they presentsmaller intrinsic dispersions. At the same time, the use of the PACrelations improves the accuracy in the long-period (logP>=1) range(young Cepheids), since they account for the position of individualobjects inside the instability strip. We performed a detailed comparisonbetween evolutionary and pulsation ages for a sizable sample of LMC (15)and SMC (12) clusters which host at least two Cepheids. In order toavoid deceptive uncertainties in the photometric absolute zero point, weadopted the homogeneous set of B, V, and I data for clusters andCepheids collected by OGLE. We also adopted the same reddening scale.The different age estimates agree at the level of 20% for LMC clustersand of 10% for SMC clusters. We also performed the same comparison fortwo Galactic clusters (NGC 6067, NGC 7790), and the difference in age issmaller than 20%. These findings support the use of PA and PAC relationsto supply accurate estimates of individual stellar ages in the Galaxyand in external Galaxies. The main advantage of this approach is itsindependence from the distance.
| Cepheids in LMC Clusters and the Period-Age Relation
We have made a new comparison of the positions of Cepheids and clustersin the LMC and constructed a new empirical period-age relation takinginto account all available data on Cepheids in the LMC bar provided bythe OGLE project. The most probable relation is logT=8.50-0.65 logP, inreasonably good agreement with theoretical expectations. NumerousCepheids in rich clusters of the LMC provide the best data for comparingtheories of stellar evolution and pulsation and the dynamical evolutionof clusters with observations. These data suggest that stars undergoingtheir first crossing of the instability strip are first-overtonepulsators, though the converse is true of only a small fraction offirst-overtone stars. Several rich clusters with suitable ages have noCepheids—a fact that is not understood and requires verification.Differences in the concentration of Cepheids toward their clustercenters probably reflect the fact that the clusters are at differentstages of their dynamical evolution, with the Cepheids in clustercoronas being ejected from the cluster cores during dynamicalinteractions between stars.
| 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 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.
| 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.
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| Long-period variables in the Large Magellanic Cloud. I - Search and discovery
A search for long-period variables has been made in the bar and southernregions of the LMC using a series of I band UKST plates, resulting inthe discovery of 471 Mira variables and 572 SRa variables. By usingmainly automated methods of determining periods and amplitudes ofvariability, corrections for incompleteness have been estimated. TheMiras show a trend toward larger amplitudes and brighter luminositieswith period, both of which should contribute to increase mass-lossrates. The period distribution falls abruptly longward of about 420 daysand shortward of about 140 days, whereas the corresponding limits in thesolar neighborhood are about 450 and about 220 days, suggestingdifferent histories of star formation in the LMC and the Galaxy. Inparticular, there appear to be relatively more old stars in the LMC.
| Population-I Pulsating Stars. VI - Ages of Star Clusters and Associations
On the basis of our age estimations of Population I pulsating stars inour Galaxy (Tsvetkov, 1986a), the mean ages of 6 open star clusterscontaining 21 Delta Scuti-variables and of 8 star clusters andassociations containing 13 classical cepheids, have been evaluated.These mean cluster age estimations weighted according to theprobabilities for different evolutionary phases of the pulsating stars,are obtained in the evolutionary track systems of Iben (1967) andPaczyñski (1970); the cluster ages are larger in theformer system. Our results are compared with those obtained from variousmethods by other authors. Clusters with classical cepheids and withDelta Scuti-stars have ages, respectively, in the ranges 107_108 yearsand 106_109 years. It is shown that the use of simpleperiod-age(-colour) relations for Population I pulsating stars givessufficiently accurate cluster age estimations. By use of our period-agerelations for classical cepheids (Tsvetkov, 1986a), the mean ages of 56other star clusters and associations in our Galaxy, the MagellanicClouds, and M 31 galaxy have been estimated in both systems of tracks.The results are generally in agreement with those obtained from variousmethods by other authors. The use of Population I pulsating stars instar clusters and associations is one of the simplest and most easilyapplied methods for determining cluster ages; but there are somelimitations in its application
| A Catalogue of Clusters in The LMC
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