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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.
| 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
| MSX, 2MASS, and the LARGE MAGELLANIC CLOUD: A Combined Near- and Mid-Infrared View
The Large Magellanic Cloud (LMC) has been observed by the MidcourseSpace Experiment (MSX) in the mid-infrared and the Two Micron All SkySurvey (2MASS) in the near-infrared. We have performed across-correlation of the 1806 MSX catalog sources and nearly 1.4 million2MASS cataloged point and extended sources and find 1664 matches. Usingthe available color information, we identify a number of stellarpopulations and nebulae, including main-sequence stars, giant stars, redsupergiants, carbon- and oxygen-rich asymptotic giant branch (AGB)stars, planetary nebulae, H II regions, and other dusty objects likelyassociated with early-type stars. A total of 731 of these sources haveno previous identification. We compile a listing of all objects, whichincludes photometry and astrometry. The 8.3 μm MSX sensitivity is thelimiting factor for object detection: only the brighter red objects,specifically the red supergiants, AGB stars, planetary nebulae, and H IIregions, are detected in the LMC. The remaining objects are likely inthe Galactic foreground. The spatial distribution of the infrared LMCsources may contribute to understanding stellar formation and evolutionand the overall galactic evolution. We demonstrate that a combined mid-and near-infrared photometric baseline provides a powerful means ofidentifying new objects in the LMC for future ground-based andspace-based follow-up observations.
| 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.
| Cepheids in MC Clusters: New Observations
Not Available
| 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.
| Detection of the helium flash gap in the integrated (U - B) versus (B - V) diagram for 624 Large Magellanic Cloud clusters
The number of Large Magellanic Cloud clusters with integrated UBVphotometry has been increased by more than a factor of 4, now totaling624 objects. A gap in the cluster distribution through the color-colordiagram appears in the region of the equivalent SWB type IV. Theamplitude of the gap is about 0.1 mag in both colors. The turnoff agesof a few clusters near the gap edges perfectly fit theoreticalpredictions of a red giant branch phase transition. This jump is due tothe first appearance of stars suffering the helium flash, which form abright and populous red giant branch that persists that the subsequentcluster evolution. As an additional result, evidence is found that Hodge7 (SL 735) might be a classical globular cluster.
| 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
Not Available
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