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|Physical parameters of 15 intermediate-age LMC clusters from modelling of HST colour-magnitude diagrams|
Aims.We analyzed HST/WFPC2 colour-magnitude diagrams (CMDs) of 15populous Large Magellanic Cloud (LMC) stellar clusters with ages between~0.3 Gyr and ~3 Gyr. These (V, B-V) CMDs are photometrically homogeneousand typically reach V ˜ 22. Accurate and self-consistent physicalparameters (age, metallicity, distance modulus and reddening) wereextracted for each cluster by comparing the observed CMDs with syntheticones. Methods: These determinations involved simultaneous statisticalcomparisons of the main-sequence fiducial line and the red clumpposition, offering objective and robust criteria to determine the bestmodels. The models explored a regular grid in the parameter spacecovered by previous results found in the literature. Control experimentswere used to test our approach and to quantify formal uncertainties. Results: In general, the best models show a satisfactory fit to thedata, constraining well the physical parameters of each cluster. Theage-metallicity relation derived by us presents a lower spread thansimilar results found in the literature for the same clusters. Ourresults are in accordance with the published ages for the oldestclusters, but reveal a possible underestimation of ages by previousauthors for the youngest clusters. Our metallicity results in generalagree with the ones based on spectroscopy of giant stars and with recentworks involving CMD analyses. The derived distance moduli implied by themost reliable solutions, correlate with the reddening values, asexpected from the non-negligible three-dimensional distribution of theclusters within the LMC. Conclusions: .The inferred spatialdistribution for these clusters is roughly aligned with the LMC disk,being also more scattered than recent numerical predictions, indicatingthat they were not formed in the LMC disk. The set of ages andmetallicities homogeneously derived here can be used to calibrateintegrated light studies applied to distant galaxies.
|Red Giant Stars in the Large Magellanic Cloud Clusters|
We present deep J, H, and Ks photometry and accurate colormagnitude diagrams down to K~18.5 for a sample of 13 globular clustersin the Large Magellanic Cloud. This data set combined with the previoussample of six clusters published by our group gives the opportunity tostudy the properties of giant stars in clusters with different ages(ranging from ~80 Myr up to 3.5 Gyr). Quantitative estimates of starpopulation ratios (by number and luminosity) in the asymptotic giantbranch (AGB), the red giant branch (RGB), and the He clump have beenobtained and compared with theoretical models in the framework ofprobing the so-called phase transitions. The AGB contribution to thetotal luminosity starts to be significant at ~200 Myr and reaches itsmaximum at 500-600 Myr, when the RGB phase transition is starting. At~900 Myr the full development of an extended and well-populated RGB hasbeen completed. The occurrences of both the AGB and RGB phasetransitions are sharp events, lasting a few hundred megayears only.These empirical results agree very well with the theoretical predictionsof simple stellar population models based on canonical tracks and thefuel-consumption approach.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m NTT, within the observing programs64.N-0038 and 68.D-0287.
|A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters|
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
|Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters|
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
|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.
|Probing the Red Giant Branch Phase Transition: Near-Infrared Photometry of Six Intermediate-Age Large Magellanic Cloud Clusters|
This is the first of a series of papers devoted to a global study of thephotometric properties of the red stellar sequences in a complete sampleof the Large Magellanic Cloud clusters, by means of near-infrared arrayphotometry. Deep J, H, Ks photometry and accuratecolor-magnitude diagrams down to K~18.5, i.e., ~1.5 mag below the red Heclump, for six intermediate-age clusters (namely, NGC 1987, NGC 2108,NGC 2190, NGC 2209, NGC 2231, NGC 2249) are presented. A quantitativeestimate of the population ratios (by number and luminosity) between redgiant branch (RGB) and He-clump stars for each target cluster isprovided and discussed in the framework of probing the so-called RGBphase transition (Ph-T). By using the Elson & Fall s-parameter as anage indicator, the observed RGB population shows a sharp enhancement (inboth number and luminosity) at s=36. Obviously, the correspondingabsolute age strictly depends on the details of theoretical modelsadopted to calibrate the s-parameter. Curiously, the currently availablecalibrations of the s-parameter in terms of age based on canonical (byElson & Fall) and overshooting (Girardi and coworkers) modelsprovide ages that well agree within 10%, suggesting that the fulldevelopment of the RGB occurs at t~700 Myr and is a relatively fastevent (δt~300 Myr). However, the RGB Ph-T epoch derived from theovershooting calibration of the s-parameter turns out to besignificantly earlier than the epoch provided by the recent evolutionarytracks by Girardi and coworkers. A new calibration of the s-parameterbased on high-quality color-magnitude diagrams and updated models isurged to address the origin of this discrepancy and finally establishthe epoch of the RGB Ph-T.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m New Technology Telescope, withinthe observing programs 64.N-0038 and 68.D-0287.
|Analyzing Starbursts Using Magellanic Cloud Star Clusters as Simple Stellar Populations|
Integrated spectra have been obtained of 31 star clusters in theMagellanic Clouds (MC) and of four Galactic globular clusters. Thespectra cover the wavelength range 3500-4700 Å at a resolution of3.2 Å FWHM. The MC clusters primarily cover the age range fromless than 108 to about 3 Gyr and hence are well-suited to anempirical study of aging poststarburst stellar populations. Anage-dating method is presented that relies on two spectral absorptionfeature indices, Hδ/Fe I λ4045 and Ca II, as well as anindex measuring the strength of the Balmer discontinuity. We compare thebehavior of the spectral indices in the observed integrated spectra ofthe MC clusters with that of indices generated from theoreticalevolutionary synthesis models of varying age and metal abundance. Thesynthesis models are based on those of Worthey, when coupled with thecombination of an empirical library of stellar spectra by Jones for thecooler stars and synthetic spectra, generated from Kurucz modelatmospheres, for the hotter stars. Overall, we find good agreementbetween the ages of the MC clusters derived from our integrated spectra(and the evolutionary synthesis modelling of the spectral indices) andages derived from analyses of the cluster color-magnitude diagrams, asfound in the literature. Hence, the principal conclusion of this studyis that ages of young stellar populations can be reliably measured frommodelling of their integrated spectra.
|Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. I. The Data|
This is the first of a series of three papers devoted to the calibrationof a few parameters of crucial importance in the modeling of theevolution of intermediate-mass stars, giving special attention to theamount of convective core overshoot. To this end we acquired deep V andR photometry for three globular clusters of the Large Magellanic Cloud,namely, NGC 2173, SL 556, and NGC 2155, in the age interval 1-3 Gyr. Inthis first paper, we describe the aim of the project and Very LargeTelescope observations and data reduction and we make preliminarycomparisons of the color-magnitude diagrams with both the Padova andYonsei-Yale isochrones. Two following papers in this series present theresults of a detailed analysis of these data, independently carried outby members of the Yale and Padova stellar evolution groups. This allowsus to compare both sets of models and discuss their main differences, aswell as the systematic effects that they would have on the determinationof the ages and metallicities of intermediate-age single-stellarpopulations.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO 64.L-0385).
|Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud|
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
|A Large and Homogeneous Sample of CMDs of LMC Stellar Clusters|
We present the photometric results of 21 stellar clusters of the LargeMagellanic Cloud. The WFPC2 images were retrieved from the HST archive.Simple stellar populations in a large spread of age are well representedin the sample of color-magnitude diagrams shown here.
|Large Magellanic Cloud stellar clusters. I. 21 HST colour magnitude diagrams|
We present WFPC2 photometry of 21 stellar clusters of the LargeMagellanic Cloud obtained on images retrieved from the Hubble SpaceTelescope archive. The derived colour magnitude diagrams (CMDs) arepresented and discussed. This database provides a sample of CMDsrepresenting, with reliable statistics, simple stellar populations witha large spread of age. The stars in the core of the clusters are allresolved and measured at least down to the completeness limit; themagnitudes of the main sequence terminations and of the red giant clumpare also evaluated for each cluster, together with the radius at halfmaximum of the star density. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. Table 1 is only available in electronic form athttp://www.edpsciences.org
|A secondary clump of red giant stars: why and where|
Based on the results of detailed population synthesis models, Girardi etal. recently claimed that the clump of red giants in thecolour-magnitude diagram (CMD) of composite stellar populations shouldpresent an extension to lower luminosities, which goes down to about0.4mag below the main clump. This feature is made of stars just massiveenough to have ignited helium in non-degenerate conditions, andtherefore corresponds to a limited interval of stellar masses and ages.In the present models, which include moderate convective overshooting,it corresponds to ~1Gyr old populations. In this paper, we go into moredetail about the origin and properties of this feature. We first comparethe clump theoretical models with data for clusters of different agesand metallicities, basically confirming the predicted behaviour. We thenrefine the previous models in order to show the following behaviour. (i)The faint extension is expected to be clearly separated from the mainclump in the CMD of metal-rich populations, defining a `secondary clump'by itself. (ii) It should be present in all galactic fields containing~1Gyr old stars and with mean metallicities higher than about Z=0.004.(iii) It should be particularly strong, if compared with the main redclump, in galaxies that have increased their star formation rate in thelast Gyr or so of their evolution. In fact, secondary clumps similar tothe model predictions are observed in the CMD of nearby stars fromHipparcos data, and in those of some Large Magellanic Cloud fieldsobserved to date. There are also several reasons why this secondaryclump may be missing or hidden in other observed CMDs of galaxy fields.For instance, it becomes indistinguishable from the main clump if thephotometric errors or differential absorption are larger than about0.2mag. None the less, this structure may provide important constraintson the star formation history of Local Group galaxies. We comment alsoon the intrinsic luminosity variation and dispersion of clump stars,which may limit their use as either absolute or relative distanceindicators, respectively.
|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.
|A Search for Old Star Clusters in the Large Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
|Carbon stars in LMC clusters revisited.|
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
|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.
|Age distribution of LMC clusters from their integrated UBV colors: history of star formation.|
In this paper we revise the relationship between ages and metallicitiesof LMC star clusters and their integrated UBV colors. The study standson the catalog of UBV colors of the Large Magellanic Cloud (LMC)clusters by Bica et al. (1994; BCDSP) and the photometric models ofsingle stellar populations (SSP) calculated by Bertelli et al. (1994).These photometric models nicely describe the color distribution of LMCclusters in the (U-B) vs. (B-V) plane together with the observeddispersion of the colors and the existence of a gap in a certain regionof this diagram. In the case of blue clusters, most of the dispersion inthe colors can be accounted for by the presence of stochastic effects onthe mass distribution of stars, whereas for the red ones additionaldispersion's of ~0.2dex in metallicity and of ~0.05mag in color excessare needed. From comparing the observed distribution of integratedcolors in the (U-B) vs. (B-V) diagram with the theoretical models, itturns out that: 1) The data are consistent with the presence of a gap(period of quiescence) in the history of cluster formation. If theage-metallicity relation (AMR) for the LMC obeys the simple model ofchemical evolution, the gap is well evident and corresponds to the ageinterval ~3Gyr to (12-15)Gyr. On the contrary, if the chemicalenrichment has been much slower than in the simple model, so thatintermediate age clusters are less metal rich, the gap is expected tooccur over a much narrower color range and to be hidden by effects ofcolor dispersion. 2) The bimodal distribution of B-V colors can bereproduced by a sequence of clusters almost evenly distributed in thelogarithm of the age, whose metallicity is governed by a normal AMR. Noneed is found of the so-called phase transitions in the integratedcolors of a cluster taking place at suitable ages (Renzini & Buzzoni1986). 3) The gap noticed by BCDSP in the (U-B) vs. (B-V) plane can beexplained by the particular direction along which cluster colors aredispersed in that part of the (U-B) vs. (B-V) diagram. Also in thiscase, no sudden changes in the integrated properties of clusters must beinvoked. The results of this analysis are used to revise the empiricalmethod proposed by Elson & Fall (1985, EF85) to attribute ages toLMC clusters according to their integrated UBV colors. We show that theEF85 method does not provide the correct relation between ages andcolors for clusters of low metallicity and hence its inability to datethe old clusters. We propose two modifications to the definition of theparameter S of EF85 such that the age sequence of red clusters issuitably described, and the intrinsic errors on ages caused by the heavypresence of various effects dispersing the colors are reduced to aminimum. The age sequence is calibrated on 24 template clusters forwhich ages were independently derived from recent color-magnitudediagrams (CMD). Finally, we attribute ages to all clusters present inBCDSP catalog, and derive the global age distribution function (ADF) forLMC clusters. The ADF presents new features that were not clear inprevious analyses of UBV data, but were already suggested by a number ofindependent observational studies. The features in question are periodsof enhanced cluster formation at ~100Myr and 1-2Gyr, and a gap in thecluster formation history between ~3 and (12-15)Gyr. The peaks observedin the distribution of B-V colors are found to be sensitive to thepresence of these periods of enhanced cluster formation and the lack ofextremely red clusters caused by the age gap between intermediate-ageand old clusters.
|Globular clusters in the Magellanic Clouds - II. IR-array photometry for 12 globular clusters and contributions to the integrated cluster light|
We report JHK results of observations of 12 globular clusters in theLarge Magellanic Cloud (LMC), and present colour-magnitude diagrams downto K=16 (corresponding to M_K~-2.6) for ~450 stars in these clusters. Wemerge our data with BV photometry for 11 LMC clusters, previouslypublished in Paper I of this series, and use the merged data to studythe evolution of integrated magnitudes and colours of simple stellarpopulations (SSPs), which are samples of coeval and chemicallyhomogeneous stars. In particular, we examine the effect of phasetransitions (ph-ts), which signal the appearance of the RGB or AGB inSSPs of increasing age. We find that the AGB contributes ~60 per cent ofthe integrated cluster light at K, while the contribution from thebright RGB stars (i.e., K_0<14.3, log L/L_~2.66) is correlated withthe s parameter (Elson & Fall) ranging from ~0 per cent for s=0 upto ~20 per cent for s>35. The age at which the RGB ph-t actuallytakes place (i.e., the calibration of s with age) depends on the detailsof stellar evolutionary models. In 'classical' models (those withoutovershooting), the RGB ph-t occurs at ~(6+/-2)x10^8 yr and lasts for2.9x10^8 yr. In models with overshooting, the occurrence of the RGB ph-tis later [at ~(1.5+/-0.3)x10^9 yr] and the duration is longer (4.3x10^8yr). While the age and duration of the RGB ph-t depend on the treatmentof mixing, both classical and overshooting models yield the samefractional contribution of RGB stars to the integrated cluster lightbefore and after the RGB ph-t, in agreement with the Fuel ComsumptionTheorem (Renzini & Buzzoni). We report extensive experiments whichshow that the variations of the integrated colours of the LMC clustersfrom s=31 to 43 are controlled by the complex interplay of variousfactors, different from colour to colour and frequently dominated by thestochastic noise induced by a few very bright objects. The overallpicture that emerges is consistent with the early conclusions drawn byPersson et al. and Frogel et al. that the J-K colour is mostly driven bythe AGB stars, that V-K is substantially controlled by AGB and RGB stars(AGB stars being slightly more important), and that B-Vis partiallyinfluenced by the whole population of red stars brighter than the bulkof the RGB clump, but is also quite strongly dependent on theprogressive fading and reddening of the turn-off stars due to ageincrease.
|Globular clusters in the Magellanic Clouds - I. BV CCD photometry for 11 clusters.|
Abstract image available at:http://adsabs.harvard.edu/abs/1994MNRAS.271..385C
|A study of the color-magnitude diagrams and luminosity functions of the two LMC clusters NGC 2134 and NGC 2249 with the new radiative opacities|
We present Johnson CCD photometry in the B and V passbands for thestellar content of two intermediate age star clusters of the LargeMagellanic Cloud (LMC), NGC 2134 and NGC 2249. In total 1613 stars aremeasured in NGC 2134 and 850 in NGC 2249. With the aid of two grids ofstellar models calculated with convective overshoot (both from the coreand the envelope) and different radiative opacities, we examine thecolor-magnitude diagrams (CMDs) and luminosity functions (LFs) of theclusters and estimate reddening, metallicity, and age. The opacities inusage are either the classical ones by Huebner et al. (1977)(therinafter LAOL) or those by Iglesias et al. (1992) (therinafterOPAL). Adopting the distance modulus of (m - M0) = 18.5 mag,we find that stellar models with metallicity Z = 0.008 are best suitedto matech the main features of the observed CMDs. Accordingly, we getE(B-V) = 0.22 and the age of 1.9 x 108 yr for NGC2134, and E(B-V) = 0.25 and the age of 5.5 x 108yr for NGC 2249. Similar analysis is made using both the opacity source(OPAL or LAOL) bringing into evidence the major differences. Finally,for the sake of comparison we also examine the CMDs and LFs by means ofmodels with identical opacities but the classical scheme of mixing. TheLFs indicate that classical models with OPAL require unusually steepslopes of the initial mass function (IMF), x = 3.35 or greater, whereasmodels with overshoot and OPAL are consistent with the classical slope,x = 2.35.
|CCD and IR photometry of intermediate-age Magellanic Clouds clusters|
The clusters of the Magellanic Clouds (MCs) are studied with a completedataset for the intermediate-age clusters to determine the integratedcolors and spectral energy distribution of the distant galaxies.Observations of a wide sample of MC clusters are conducted in differentspectral bands with attention given to the range in which observedintegrated color variations were most evident. The CCD and IRphotometric data are reduced, and color-magnitude diagrams are given for9 clusters in V and (B-V) and for 9 clusters in the the IR bands. Theobservational data yield important clues regarding the extension andlifetime increase of the RGB and the evolutionary status of theclusters. The RGB evolutionary phase transition and an increase in thenumber of evolved giant stars are found within the age range where theMC clusters show a color change.
|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.
|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.
|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.
|CMDs for the LMC clusters NGC 2249 and NGC 2241|
Color-magnitude diagrams are derived for two LMC clusters, NGC-2249 andNGC 2241. A technique is introduced for 'subtracting' the field-starcontribution from the cluster CMDs using CMDs from nearby LMC fields.The possibility of a 'gap' in the main sequence of NGC 2249 isdiscussed, and, by identifying the gap as the point of core hydrogenexhaustion for stars with convective cores, the isochrones are found tobe well placed relative to the turnoff. Based on these isochrones, NGC2249 is thought to be 550 to 700 x 10 to the 6th years old, depending onthe reddening and distance modulus used. NGC 2241, by virtue of itssubgiant branch and giant clump and by comparison with NGC 2506, isthought to be between 3 and 4 x 10 to the 9th years old.
|A theoretical and observational study of the Red Giant Branch phase transition in Magellanic Cloud clusters - A progress report|
Preliminary results are reported for an investigation comparingtheoretical models of the sudden appearance of an extended RGB (and itseffects on the spectral energy distributions of stellar populations)with data from ESO CCD observations of clusters in the LMC and SMC.Isochrones for the entire RGB are being constructed on the basis of 100new evolutionary sequences (calculated using the evolution code ofSweigart and Gross, 1976 and 1978) to permit determination of syntheticcolors and spectral energy distributions. The observations so farindicate a main sequence about 0.1 mag redder than that predicted by thepresent models or by the isochrones of VandenBerg and Bell (1985), andfail to show a B-V color difference at the RGB phase transition.
|Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud|
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
|Ages of Two LMC Clusters and an LMC Distance Determination Using Features of an LMC Field CMD|
|An ellipticity - age relation for globular clusters in the Large Magellanic Cloud. I - Measurements|
It is pointed out that the rich star clusters of the Magellanic Cloudsresemble the globular clusters of the Galaxy. The present investigationhas the objective to determine the shapes of these clusters and theirdependence on age. The study has been restricted to the Large MagellanicCloud (LMC) because the Small Magellanic Cloud (SMC) does not provide alarge enough sample for the statistical analysis. The shapes of globularclusters are usually expressed in terms of ellipticities. Attention isgiven to the measurement of ellipticities with the aid of a ruler and agraduated magnifying glass, star count data on 12 LMC clusters, and ageestimates. It is found that estimates of the ellipticities of globularclusters made by eye are in excellent agreement with those based on starcounts. The ellipticity-age relation is probably explained mostnaturally by internal evolution in the structure of globular clusters.
|Instrumental color-magnitude diagrams for 24 Large Magellanic Cloud star clusters|
Abstract image available at:http://adsabs.harvard.edu/abs/1976ApJS...32..283H
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