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Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters
Using available data for C and M giants with M_bol<-3.6 in MagellanicCloud clusters, we derive limits to the lifetimes for the correspondingevolutionary phases, as a function of stellar mass. The C-star phase isfound to have a duration between 2 and 3 Myr for stars in the mass rangefrom ~1.5 to 2.8 M_ȯ. There is also an indication that the peak ofC-star lifetime shifts to lower masses (from slightly above to slightlybelow 2 Mȯ) as we move from LMC to SMC metallicities.The M-giant lifetimes also peak at ~2 Mȯ in the LMC,with a maximum value of about 4 Myr, whereas in the SMC their lifetimesappear much shorter, but, actually, they are poorly constrained by thedata. These numbers constitute useful constraints to theoretical modelsof the TP-AGB phase. We show that several models in the literatureunderestimate the duration of the C-star phase at LMC metallicities.
| Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds
We present accurate integrated-light photometry in Johnson/Cousins V, Rand I for a sample of 28 globular clusters in the Magellanic Clouds. Themajority of the clusters in our sample have reliable age and metallicityestimates available in the literature. The sample encompasses agesbetween 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between -1.5 and0.0 dex. The sample is dominated by clusters of ages between roughly 0.5and 2 Gyr, an age range during which the bolometric luminosity of simplestellar populations is dominated by evolved red giant branch stars andthermally pulsing asymptotic giant branch (TP-AGB) stars whosetheoretical colours are rather uncertain. The VRI colours presented inthis paper have been used to calibrate stellar population synthesismodel predictions.
| 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.
| Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
| 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.
| The Nature of Peculiar Stellar Complexes
The nature of stellar complexes with peculiar populations andmorphologies is investigated. The existence in the LMC of complexes madeup of isolated stars, on the one hand, and consisting exclusively ofclusters, on the other hand, could be due to different turbulencepatterns in the initial gaseous medium. Arc-shaped stellar complexes areunlikely to be the result of star formation in a gaseous shell swept upby a central source of pressure, and instead probably reflect the shapeof a bow shock that develops when a sufficiently dense cloud is subjectto dynamical pressure. A peculiar arc-shaped complex in NGC 6946, whichcontains a young, massive cluster, may be the result of an obliqueinfall of a high-velocity cloud onto a region of the gaseous disk of theGalaxy with a strong, regular magnetic field; the properties of thiscomplex can be explained as the result of a collision of the resultingshocks. The arc-shaped complexes in the LMC were also probably producedby high-velocity clouds moving obliquely through the more tenuous gas ofthe LMC disk. A similar complex in NGC 300 may owe its origin to theeffect produced on a dense cloud by the shock from an extremely powerfulexternal explosion, whose stellar remnant may have survived as an X-raysource now located along the line of symmetry of the arc of the complex.The rareness of such structures can be explained by the narrow range ofconditions under which they can develop.
| Calibration of the MACHO Photometry Database
The MACHO Project is a microlensing survey that monitors thebrightnesses of ~60 million stars in the Large Magellanic Cloud (LMC),Small Magellanic Cloud, and Galactic bulge. Our database presentlycontains about 80 billion photometric measurements, a significantfraction of all astronomical photometry. We describe the calibration ofMACHO two-color photometry and transformation to the standardKron-Cousins V and R system. Calibrated MACHO photometry may be properlycompared with all other observations on the Kron-Cousins standardsystem, enhancing the astrophysical value of these data. For ~9 millionstars in the LMC bar, independent photometric measurements of ~20,000stars with V<~18 mag in field-overlap regions demonstrate an internalprecision σV=0.021, σR=0.019,σV-R=0.028 mag. The accuracy of the zero point in thiscalibration is estimated to be +/-0.035 mag for stars with colors in therange -0.1 mag
| 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 comparative study of the spatial distributions of Cepheids and star clusters in the Large Magellanic Cloud
A new simple method for the comparison of two-dimensional distributionsis elaborated and applied to the observed spatial distributions ofCepheids and open clusters in the LMC. This method is particularlysuited to pick out the clusterings within non-uniform fields. Thecomparative study of the spatial distributions for objects with knownages provides useful hints on the dominant mode of large scale starformation. We found that only one clump, out of four evident groups ofopen clusters coeval with the observed Cepheids (i.e. logt ~ 7.5/8.5)coincides with a local density enhancement of Cepheids. A relationbetween the age range inside a clump and its size is found; this isconsistent with the theory of star formation in a turbulent medium.
| 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.
| The evolution of theV-Kcolours of single stellar populations
Models of evolutionary population synthesis of galaxies rely on theproperties of the so-called single stellar populations (SSP). In thispaper, we discuss how the integrated near-infrared colours, andespecially V-K, of SSPs evolve with age and metallicity. Some of theuncertainties associated with the properties of the underlying stellarmodels are thoroughly discussed. Our models include all the relevantstellar evolutionary phases, with particular attention being dedicatedto the asymptotic giant branch (AGB), which plays a fundamental role inthe evolution of the near-infrared part of the spectrum. First, wepresent the effects that different formulations for the mass-loss ratesproduce on the final remnant mass (i.e., on the initial-final massrelation), and hence on the AGB-termination luminosity and the relativecontribution of these stars to the integrated light. The results for theevolution of the V-K colour are very different depending on the choiceof the mass-loss prescription; the same is true also for the B-V colourin the case of low-metallicity SSPs. Secondly, we describe the changesoccurring in the integrated colours at the onset of the AGB and redgiant (RGB) branches. According to the classical formalism for the AGBevolution, the onset of this evolutionary phase is marked by a colourjump to the red, the amplitude of which is shown here to be highlydependent on the metallicity and mass-loss rates adopted in the models.We then consider the effect of the overluminosity with respect to thestandard core mass-luminosity relation that occurs in the most massiveAGB stars. Different simplified formulations for this effect are testedin the models; they cause a smoothing of the colour evolution in the agerange at which the AGB starts to develop, rather than a splitting of thecolour jump into two separate events. On the other hand, we find that atemporary red phase takes place ~1.5x10^8 yr after the RGB develops.Thanks to the transient nature of this feature, the onset of the RGB isprobably not able to cause marked features in the spectral evolution ofgalaxies. We then discuss the possible reasons for the transition of V-Kcolours (from ~1.5 to 3) that takes place in LMC clusters of SWB typeIV. A revision of the ages attributed to the single clusters revealsthat the transition may not be as fast as originally suggested. Thecomparison of the data with the models indicates that the transitionresults mainly from the development of the AGB. A gradual (or delayed)transition of the colours, as predicted by models which include theoverluminosity of the most massive AGB stars, seems to describe the databetter than the sudden colour jump predicted by classical models.
| Pulsation modes of Mira stars and questioning of linear modelling: indications from HIPPARCOS and the LMC
Thorough discussion of the previous theoretical works on the pulsationof Long Period Variables leads us to the conclusion that the modeperiods predicted by linear models must significantly differ from thereality, and that, if one nevertheless relies upon such a modelling, itis at least necessary to change the mixing length. The hypothesis thatthe so-derived mode periods be reasonably reliable is supported byconfrontation between a model grid based on these grounds and theluminosities of LPVs in the Large Magellanic Cloud and in LMC clusters,as well as the luminosities and effective temperatures of Miras in thesolar neighbourhood. A wide majority of the Miras appear probablypulsating on the first overtone, and the sample Semi-Regulars on thesecond. However, a significant proportion of Miras seem to befundamental pulsators. Individual masses are derived. A few stars areprobably undergoing hot bottom burning, while two seem to have apeculiar dust envelope. Based on data from the HIPPARCOS astrometrysatellite
| Carbon stars in LMC clusters revisited.
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
| On the pulsation mode of Mira variables: evidence from the Large Magellanic Cloud
Recent angular diameter measurements for Mira variables suggest that theradii of these stars are very large and consistent with pulsation in thefirst-overtone mode rather than the fundamental mode. On the other hand,non-linear pulsation models of Mira variables suggest that the observedpulsation velocity amplitudes can only be achieved duringfundamental-mode pulsation, at least for stellar masses <~2.0Msolar. Here, we present some new observations of long-period variables(LPVs) in the LMC which show that the LPVs lie on two (K, logP)sequences, one sequence being the well-known Mira sequence and the otherbeing a sequence parallel to the Mira sequence but separated from it byDeltalogP ~ 0.35. The LPVs on the Mira sequence have a wide range ofamplitudes (0.1 < DeltaI < 3) while those on the secondsequence have relatively small amplitudes (DeltaI < 0.5). Thepreviously known LPVs of large amplitude (DeltaI > 0.5) in the LMClie almost always on the Mira sequence. Theoretical models of LPVspredict a ratio of fundamental to first-or second-overtone period ofDeltalogP~ 0.3-0.4, and overtone pulsators are expected to have smallerlimiting amplitudes than fundamental-mode pulsators. Hence the aboveobservations can be easily understood if the LPVs on the Mira sequenceare fundamental-mode pulsators while LPVs on the second sequence areovertone pulsators. A second test of the pulsation mode is obtained bycomputing pulsation periods for model stars on the LMC old giant branchand comparing these periods with those of observed Mira variables. Onceagain, the fundamental-mode pulsators have periods consistent with thoseseen in the LMC Miras while the overtone periods are too short. Theabove results strongly suggest that Mira variables are fundamental-modepulsators.
| Mid-infrared properties of globular clusters using the IRAS data base
We present an analysis of the mid-IR properties of 18 globular clusters(GCs) [15 in the Galaxy and three in the Large Magellanic Cloud (LMC)]using the IRAS photometric data at 12 and 25 mum. Eight of the nineGalactic GCs with central escape velocities greater than 50 km s^-1 haveIRAS sources within a radius of 60 arcsec from the centre, in agreementwith the expectation that interstellar gas and dust should indeed bepresent in the central regions of the most massive clusters owing tomass-loss processes occurring in the late stages of the stellarevolution. No other significant correlation is found between IRAS sourceincidence and any intrinsic GC parameters. Warm dust (T~300K) isdetectable mostly around unresolved giant stars, but in three massiveGCs it is also present as diffuse emission. However, most of the dustmight be cold (T<50K) and it was thus notdetected by IRAS because of its limited sensitivity at 60 and 100 mum.The inferred mass-loss rates and statistical considerations arecompatible with a non-steady mass-loss process with several episodes ofejection lasting a few times 10^5 yr.
| 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.
| Star Complexes and Associations: Fundamental and Elementary Cells of Star Formation
This paper discusses data on young star groups that imply the existenceof two distinct scales of star formation. The largest scale is that ofthe star complexes which we assume to be the initial scale, connectedwith the gas superclouds. The smaller scale is for the well known OBassociations and young clusters, which relate to the elementary scale ofstar formation, that connected with individual giant molecular clouds.An intermediate scale, that of aggregates (groups of a fewassociations), probably also exists and corresponds to clusters of a fewGMCs. The great majority of associations and young clusters are foundinside huge star complexes, which include individual older stars(plausible former members of dissolved associations). This hierarchicalstructure of young star groups, together with the generally differentluminosities of individual stars within the different scale aggregatesexplain the very different estimates of sizes of these groups (all ofwhich have previously been called "associations") in galaxies atdifferent distances and therefore resolution. Concentrations of gasclouds, young associations, and stars within vast complexes areconsistent with a top-down scenario of star formation, implying thatsuperclouds producing star complexes are the initial structures, whichformed faster than clouds of smaller scale in gaseous galactic disks andespecially within spiral density waves, mainly owing to large scalegravitational instability. Sheared star complexes form a flocculentspiral structure without older stars in spiraling patches. Probably allyoung galactic disks consist of units with masses of about 10^7^ insolar units. Most of the specimens of star complexes- the huge groups ofclusters, associations, and high luminosity stars in our Galaxy,luminous patches in unresolved galaxies, and the vast star clouds innearby galaxies-are not accidental agglomerations but physical entitiesdeserving careful investigation.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| The MACHO project LMC variable star inventory. 1: Beat Cepheids-conclusive evidence for the excitation of the second overtone in classical Cepheids
We report the discovery of 45 beat Cepheids in the Large MegellanicCloud (LMC) using the Massive Compact Halo Object (MACHO) shown to breakcleanly into two period-ratio groups, providing the first unambiguousevidence that the second overtone is indeed excited in Cepheids. 30stars are beating in the fundamental and first overtone mode (F/1H, witha period ratio in the neighborhood of 0.72), and 15 stars are beating inthe first and second overtone (1H/2H, with a period ratio near 0.80).The F/1H period ratios are systematically higher than known Galacticbeat Cepeids, indicating a metallicity dependence whose sense is inagreement with theory. Beat Cepheids in the LMC are found to select the1H/2H mode for principal periods shorter than 1.25 days. We find thefraction of Cepheids excited in two modes to be about 20% for stars withfundamental periods shorter than 2.5 days. We fail to confirm any of theproposed beat Cepheid candidates common to our sample from the surveysof Andreasen (1987) and Andreasen & Petersen (1987). We also presentfinder charts and find several of the beat Cepheids to be in or near LMCclusters. In addition, we find three double Cepheids -unresolved pairsof Cepheids which may be physically related.
| 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.
| Near-infrared spectral evolution of blue LMC clusters : a comparison with galactic open clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1990RMxAA..21..202B
| Blue Magellanic clusters - Near-infrared spectral evolution
New integrated spectra in the range 5600-10,000 A are presented for 28LMC and 3 SMC young star clusters. The equivalent widths (W) ofprominent features and the continuum distribution are measured. Theanalysis, supplemented by 8 additional LMC clusters from previousstudies, indicates that the red supergiant phase is indeed verytime-peaked, occuring from 7 to 12 Myr. In addition to the previous caseof NGC 2004, it is found that NGC 1805, NGC 1994, NGC 2002, NGC 2098,and NGC 2100 (as well as NGC 2011 to a lesser extent) are undergoingthis phase. The red supergiant phase is clearly denoted by strong TiObands and Ca II triplet as well as a flat continuum or (in extremecases) a continuum with positive slope above 6000 A.
| The asymptotic giant branch of Magellanic Cloud clusters
The present search for carbon and M-type asymptotic giant branch (AGB)stars in the 39 clusters of the Magellanic Clouds has yieldedidentifications and near-IR photometry for about 400 such stars. TheSearle et al. (1980) cluster-age-related classification scheme is abasic element of the present analysis of these data. In a C-M diagram,the cluster M stars shift steadily redward as one proceeds from clustersof SWB type I to VI, due to the increasing age of the clusters along thesequence. Luminous carbon stars are present only in SWB IV-VI clusters,and are easily distinguished from M stars by their color and luminosity.
| 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
| LMC clusters - Age calibration and age distribution revisited
The empirical age relation for star clusters in the Large MagellanicCloud presented by Elson and Fall (1985) are reexamined using ages basedonly on main-sequence turnoffs. The present sample includes 57 clusters,24 of which have color-magnitude diagrams published since 1985. The newcalibration is very similar to that found previously, and the scatter inthe relation corresponds to uncertainties of about a factor of 2 in age.The age distribution derived from the new calibration does not differsignificantly from that derived in earlier work. It is compared with agedistributions estimated by other authors for different samples ofclusters, and the results are discussed.
| Integrated colours and ages of LMC clusters - The nature of the bimodal distribution of the (B-V) colours
Integrated (B-V)0 and (U-B)0 colors and synthetic HR diagrams of stellarclusters are obtained as a function of age with the aid of stellarmodels that incorporate the effects of convective overshoot all overtheir major evolutionary phases. The possibility that a large spread inthe colors of real clusters exists due to stochastic fluctuations in themass distribution of stars is discussed. The calibrating relationshipsare used to date a few test clusters. The color gap is addressed, fixingits real boundaries and examining whether it can be simply attributed tosudden transitions in the integrated properties of stellar clusters. Theage distribution of the LMC clusters is obtained, and no evidence isfound of periods of inactivity in the cluster formation history. Thecolor distribution function is calculated and compared with the observedone. It is concluded that the color gap is not caused by phasetransitions occurring in the stellar content of individual clusters, butprimarily reflects cluster formation versus disruption activity thatoccurred in the LMC cluster population.
| Observed dynamical parameters of the disk clusters of the LMC. I
A study of the observed dynamical parameters of 32 globular clusters ofthe LMC disk has been carried out by means of star counts. The clusterswere measured on a set of three plates (J, V, I) taken with the 1.2 mU.K. Schmidt Telescope. The derived tidal radii were all found to belarge within a very narrow range. As a consequence the range of thetotal masses was found to be very narrow as well. These two parametersare large in comparison to those of the disk young clusters of thegalaxy but they are similar to the dimensions of the halo galacticglobulars.
| Magellanic Cloud star clusters: The problems of age determination, metallicity - Age relationship and AGB star luminosity function
Abstract image available at:http://adsabs.harvard.edu/abs/1986A&A...165...84C
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