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Distances to Populous Clusters in the Large Magellanic Cloud via the K-band Luminosity of the Red Clump
We present results from a study of the distances and distribution of asample of intermediate-age clusters in the Large Magellanic Cloud (LMC).Using deep near-infrared photometry obtained with ISPI on the CTIO 4 m,we have measured the apparent K-band magnitude of the corehelium-burning red clump stars in 17 LMC clusters. We combine clusterages and metallicities with the work of Grocholski and Sarajedini topredict each cluster's absolute K-band red-clump magnitude and therebycalculate absolute cluster distances. An analysis of these data showsthat the cluster distribution is in good agreement with the thick,inclined-disk geometry of the LMC, as defined by its field stars. Wealso find that the old globular clusters follow the same distribution,suggesting that the LMC's disk formed at about the same time as theglobular clusters, ~13 Gyr ago. Finally, we have used our clusterdistances in conjunction with the disk geometry to calculate thedistance to the LMC center, for which we find(m-M)0=18.40+/-0.04 (random)+/-0.08 (systematic), orD0=47.9+/-0.9+/-1.8 kpc.
| 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.
| Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
| 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.
| 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.
| The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting
Seventy-two grids of stellar evolutionary tracks, along with the meansto generate isochrones and luminosity/color functions from them, arepresented in this investigation. Sixty of them extend (and encompass)the sets of models reported by VandenBerg et al. for 17 [Fe/H] valuesfrom -2.31 to -0.30 and α-element abundances corresponding to[α/Fe]=0.0, 0.3, and 0.6 (at each iron abundance) to the solarmetallicity and to sufficiently high masses (up to ~2.2Msolar) that isochrones may be computed for ages as low as 1Gyr. The remaining grids contain tracks for masses from 0.4 to 4.0Msolar and 12 [Fe/H] values between -0.60 and +0.49 (assumingsolar metal-to-hydrogen number abundance ratios): in this case,isochrones may be calculated down to ~0.2 Gyr. The extent of convectivecore overshooting has been modeled using a parameterized version of theRoxburgh criterion, in which the value of the free parameter at a givenmass and its dependence on mass have been determined from analyses ofbinary star data and the observed color-magnitude diagrams for severalopen clusters. Because the calculations reported herein satisfy manyempirical constraints, they should provide useful probes into theproperties of both simple and complex stellar populations.All of the model grids may be obtained from the Canadian Astronomy DataCenter(http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cvo/community/VictoriaReginaModels/).Included in this archive are (1) the interpolation software (FORTRAN 77)to produce isochrones, isochrone probability functions, luminosityfunctions, and color functions, along with instructions on how toimplement and use the software, (2) BVRI (VandenBerg & Clem 2003)and uvby (Clem et al. 2004) color-temperature relations, and (3)zero-age horizontal branch loci for all of the chemical compositionsconsidered.
| 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.
| Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| Y2 Isochrones with an Improved Core Overshoot Treatment
Convective core overshoot affects stellar evolution rates and the datingof stellar populations. In this paper, we provide a patch to theY2 isochrones with an improved treatment of convective coreovershoot. The new tracks cover the transition mass range from noconvective core to a fully developed convective core. We compare theimproved isochrones to CMDs of a few well-observed open star clusters inthe Galaxy and the Large Magellanic Cloud. Finally, we discuss futureprospects for improving the treatment of core overshoot with the help ofasteroseismology.
| 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.
| 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.
| Near-infrared color evolution of LMC clusters
We present here the digital aperture photometry for 28 LMC clusterswhose ages are between 5 Myr and 12 Gyr. This photometry is based on ourimaging observations in JHK and contains integrated magnitudes andcolors as a function of aperture radius. In contrast to optical colors,our near-infrared colors do not show any strong dependence on clusterages.Tables 2 and 3 and Fig. 2 are only available in electronic form athttp://www.edpsciences.org
| Fundamental parameters of the LMC clusters NGC 1836, NGC 1860, NGC 1865, SL 444, LW 224 and SL 548
Complementing our recent Washington photometric studies on intermediateage and young Large Magellanic Cloud (LMC) clusters, we now turn ourattention to six previously unstudied star clusters in the transitionrange 200-700 Myr. We study NGC 1836, 1860 and 1865, which are projectedon the LMC bar; SL 444, also located in the central disc but outside thebar; and LW 224 and SL 548, both located in the outer disc. We deriveages and metallicities from extracted T1 versusC-T1 colour-magnitude diagrams (CMDs), using theoreticalisochrones recently computed for the Washington photometric system. Forthe metallicity determinations, these CMDs are particularly sensitive.We also estimate ages and metallicities of the surrounding fields of NGC1860 and 1865 by employing the δT1 index defined inGeisler et al. (1997, AJ, 114, 1920) and theoretical isochrones. Byadding the present cluster sample to those of our previous studies, wenow gather 37 LMC clusters with homogeneous parameter determinations,which are employed to probe the chemical enrichment of the LMC and itsspatial distribution. On average, inner disc clusters turned out to benot only younger than the outer ones, but also more metal-rich; somehave solar metal content. Furthermore, inner clusters located to thewest of the LMC centre are younger and more metal-rich than theireastern counterparts. We propose that a bursting formation mechanism,with an important formation event centred at ~2.0 Gyr, provides a betterdescription of the cluster age-metallicity relation than a closed-boxchemical evolution model. In the outer disc, the field star formationseems to have lasted until 2 Gyr ago while it continued in the innerdisc for almost 1 Gyr longer.
| Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. III. Padova Results
The color-magnitude diagrams (CMDs) of three intermediate-age LargeMegallanic Cloud clusters, NGC 2173, SL 556, and NGC 2155, are analyzedto determine their age and metallicity based on Padova stellar models.Synthetic CMDs are compared with cluster data. The best match isobtained using two fitting functions based on star counts in thedifferent bins of the cluster CMD. Two different criteria are used. Oneof them takes into account the uncertainties in the color of the redclump stars. Given the uncertainties on the experimental values of theclusters metallicity, we provide a set of acceptable solutions. Theydefine the corresponding values of metallicity, age, reddening anddistance modulus (for the assumed initial mass function). The comparisonwith Padova models suggests for NGC 2173 a prolonged star formation(spanning a period of about 0.3 Gyr), beginning 1.7 Gyr and ending 1.4Gyr ago. The metallicity Z is in the range 0.0016-0.003. For SL 556 anage of 2.0 Gyr is obtained. The metallicity value is in the range0.002-0.004, depending on the adopted comparison criterion. The derivedage for NGC 2155 is 2.8 Gyr, and its metallicity Z is in the range0.002-0.003. The CMD features of this cluster suggest that a moreefficient overshoot should be adopted in the evolutionary models. Aperiod of extended star formation is not required to fit the SL 556 andNGC 2155 observations.Based on observations collected at the European Southern Observatory,Paranal, Chile.
| Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. II. Analysis with the Yale Models
Using Yale stellar evolution models, we present an analysis of thecolor-magnitude diagrams (CMDs) of three intermediate-age LMC clusters,namely, NGC 2173, SL 556, and NGC 2155, obtained with the Very LargeTelescope. The main goal of our project is to investigate the amount ofconvective core overshoot necessary to reproduce the CMDs of relativelymetal-poor, intermediate-age stellar populations, to check whether theextrapolation that is usually made from solar metallicity is valid. Inthe process, we obtained values for the binary fraction of each cluster,together with refined age estimates. Our method involved the comparisonof the observed CMDs with synthetic CMDs computed using various valuesof the overshoot parameter and binary fraction. We conclude that amoderate amount of overshoot and some fraction of binary stars areessential for reproducing the observed shapes around the turnoff in theCMDs of all three clusters: unresolved binary stars fill in the expectedcore contraction gap and make a unique sequence near the gap, whichcannot be reproduced by single stars alone, even with a larger amount ofovershoot. We utilize ratios of the number of stars in different areasaround the core contraction gap to constrain the binary fraction, whichis around 10%-20% (for primary-to-secondary mass ratio >=0.7) in allthree clusters. Even if binary stars contaminate the core contractiongap, it is shown that the overshoot parameter can be inferred from thecolor dispersion of the stars around the contraction gap, regardless ofthe assumed binary fraction. From our overall analysis such as shape ofisochrones, star counts, color distribution, and synthetic CMDcomparisons, we conclude that overshoot ~20% of the local pressure scaleheight best reproduces the CMD properties of all three clusters. Thebest age estimates are 1.5, 2.1, and 2.9 Gyr for NGC 2173, SL 556, andNGC 2155, respectively.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO 64.L-0385).
| 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.
| Magellanic Clouds stellar clusters. II. New B,V CM diagrams for 6 LMC and 10 SMC clusters
We present new CCD photometry for 6 LMC and 10 SMC stellar clusterstaken at the ESO 1.54-m Danish Telescope in La Silla, to extend aprevious investigation on Magellanic Clouds clusters based on HSTsnapshots. Thanks to the much larger area covered by the Danishdetector, we investigate the spatial distribution of cluster stars,giving V, (B-V) CM diagrams for both clusters and surrounding fields.Evidence of a complex history of star formation in the Clouds isoutlined, showing that old field populations in both Clouds havemetallicities much lower than normally adopted for them (Z = 0.008 and Z= 0.004 for LMC and SMC respectively), with SMC field stars more metalpoor than in the LMC. Observational data concerning the red clump offield stars in both Clouds are briefly discussed. Based on observationscarried out at the European Southern Observatory, La Silla, Chile.
| Constraining the LMC cluster age gap: Washington photometry of NGC 2155 and SL 896 (LW 480)
We carried out Washington system photometry of the intermediate-ageLarge Magellanic Cloud (LMC) star clusters NGC2155 and SL896 (LW480). Wederive ages and metallicities from the T1 versusC-T1 colour-magnitude diagrams (CMDs). For the first time anage has been obtained for SL896, 2.3+/-0.5Gyr. For NGC2155 we derive3.6+/-0.7Gyr. The two clusters basically define the lower age limit ofthe LMC age gap. In particular, NGC2155 is confirmed as the oldestintermediate-age LMC cluster so far studied. The derived metallicitiesare [Fe/H]=-0.9+/-0.2 and -0.6+/-0.2 for NGC2155 and SL896,respectively. We also studied the CMDs of the surrounding fields, whichhave a dominant turn-off comparable to that of the clusters themselves,and similar metallicity, showing that one is dealing with anintermediate-age disc where clusters and field stars have the sameorigin. We inserted the present clusters in the LMC and Small MagellanicCloud (SMC) age-metallicity relations, using a set of homogeneousdeterminations with the same method as in our previous studies, nowtotalling 15 LMC clusters and four SMC clusters, together with someadditional values from the literature. The LMC and SMC age-metallicityrelations appear to be remarkably complementary, since the SMC wasactively star-forming during the LMC quiescent age gap epoch.
| CCD Washington Photometry of the Oldest Intermediate-Age Clusters NGC 2121, NGC 2155 and SL896 (lw 480) in the Large Magellanic Cloud
Not Available
| 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.
| The Star Cluster Systems of the Magellanic Clouds
The characteristics of the cluster systems of the Magellanic Clouds, asinferred from integrated properties, are compared with those fromindividual cluster studies and from the field population. The agreementis generally satisfactory though in the case of the LMC, the lack ofclusters older than ˜3 Gyr is not reflected in the fieldpopulation. The possible origin(s) for this cluster ``age-gap'' arediscussed. The SMC cluster age-metallicity relation is also presentedand discussed.
| Ages and metallicities of five intermediate-age star clusters projected towards the Small Magellanic Cloud
Colour-magnitude diagrams are presented for the first time for L32, L38,K28 (L43), K44 (L68) and L116, which are clusters projected on to theouter parts of the Small Magellanic Cloud (SMC). The photometry wascarried out in the Washington system C and T1 filters,allowing the determination of ages by means of the magnitude differencebetween the red giant clump and the main-sequence turn-off, andmetallicities from the red giant branch locus. The clusters have ages inthe range 2-6Gyr, and metallicities in the range-1.65<[Fe/H]<-1.10, increasing the sample of intermediate-ageclusters in the SMC. L116, the outermost cluster projected on to theSMC, is a foreground cluster, and somewhat closer to us than the LargeMagellanic Cloud. Our results, combined with those for other clusters inthe literature, show epochs of sudden chemical enrichment in theage-metallicity plane, which favour a bursting star formation history asopposed to a continuous one for the SMC.
| New Photometry for the Intermediate-Age Large Magellanic Cloud Globular Cluster NGC 2121 and the Nature of the LMC Age Gap
We report new photometry for the cluster NGC 2121 in the LargeMagellanic Cloud, which shows a prominent hydrogen core exhaustion gapat the turnoff and a descending subgiant branch reminiscent of Galacticopen clusters. We achieve an excellent fit using the Girardi isochrones,finding an age of 3.2+/-0.5 Gyr, with [Fe/H]=-0.6+/-0.2. The isochronesfit the color and shape of the turnoff and subgiant branch so preciselythat we can constrain the metallicity, as well as the age. The sameisochrones also fit SL 663 and NGC 2155, although our photometry forthese clusters has much larger errors. We find these clusters to be 0.8Gyr younger and 0.4 dex more metal-rich than recently reported in theliterature. Consequently, we argue that NGC 2121, NGC 2155, and SL 663are not properly assigned to the age gap in the LMC, but instead areamong the first clusters to be have formed in the relatively metal-rich,younger group of LMC clusters. We propose a new definition of the LMCage gap as extending from 3.2 to 13 Gyr, with ESO 121-SC03 still theonly remaining candidate for membership in the age gap. Based onobservations with the NASA/ESA Hubble Space Telescope (HST) obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| 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
| Age and metallicity for six LMC clusters and their surrounding field population
We investigate, on the basis of CCD Strömgren photometry, the agesand metallicities of six LMC clusters together with their surroundingfield population. The clusters and metallicities are: NGC 1651 (in therange [Fe/H] = -0.65 dex to -0.41 dex), NGC 1711 (-0.57 ∓ 0.17dex), NGC 1806 (-0.71 ∓ 0.23 dex), NGC 2031 (-0.52 ∓ 0.21dex) and NGC 2136/37 (-0.55 ∓ 0.23 dex) and NGC 2257 (-1.63∓ 0.21 dex). The metallicities for NGC 1651, NGC 1711, NGC 1806and NGC 2031 have been determined for the first time (NGC 2031 and NGC2136/37 are interesting for the Cepheid distance scale). In the clustersurroundings, we found about 650 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR). Ourmethod is to estimate ages for individual stars on the basis ofStrömgren isochrones with individually measured metallicities. Withthis method we are able to sample the AMR of the field population up to8 Gyr. Our metallicity data are incompatible with models predicting manymetal-poor stars (G-dwarf problem). The metallicity of the fieldpopulation increased by a factor of six, starting around 2 Gyr ago. Theproposed AMR is consistent with the AMR of the LMC cluster system(including ESO 121 SC03 and three clusters with an age of 4 Gyr). Theproposed AMR is incompatible with the recently proposed AMR by Pagel& Tautvaisiene.
| The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems
Prompted by a new catalogue of M31 globular clusters, we have collectedtogether individual metallicity values for globular clusters in theLocal Group. Although we briefly describe the globular cluster systemsof the individual Local Group galaxies, the main thrust of our paper isto examine the collective properties. In this way we are simulating thedissipationless merger of the Local Group, into presumably an ellipticalgalaxy. Such a merger is dominated by the Milky Way and M31, whichappear to be fairly typical examples of globular cluster systems ofspiral galaxies. The Local Group `Elliptical' has about 700 +/- 125globular clusters, with a luminosity function resembling the `universal'one. The metallicity distribution has peaks at [Fe/H] ~ -1.55 and -0.64with a metal-poor to metal-rich ratio of 2.5:1. The specific frequencyof the Local Group Elliptical is initially about 1 but rises to about 3,when the young stellar populations fade and the galaxy resembles an oldelliptical. The metallicity distribution and stellar populationcorrected specific frequency are similar to that of some known earlytype galaxies. Based on our results, we briefly speculate on the originof globular cluster systems in galaxies.
| Hubble Space Telescope Observations of the Large Magellanic Cloud Field around SN 1987A: Distance Determination with Red Clump and Tip of the Red Giant Branch Stars
We have used Hubble Space Telescope (HST) WFPC2 multiband observationsof a field around SN 1987A in the Large Magellanic Cloud to measure itsdistance from the Sun. The observations allowed us to carefullydetermine the interstellar extinction along the line of sight to a largenumber of stars and to measure the LMC distance by using two stellardistance indicators: the red clump (RC) and the tip of the red giantbranch (TRGB). From an application of the red clump method, we obtain adistance modulus of(m-M)LMC0,RC=18.59+/-0.04+/-0.08 mag (statisticalplus systematic error), in good agreement with the distance derived byusing the TRGB stars, namely,(m-M)LMC0,TRGB=18.69+/-0.25+/-0.06 mag(statistical plus systematic error). Both values agree well with thedistance to SN 1987A as determined from a study of its inner ringfluorescent echo [(m-M)SN1987A=18.55+/-0.05 mag], thusexcluding distance moduli lower than 18.43 to a 99.7% significancelevel. Differences with respect to previous results obtained using thesame distance indicators are discussed.
| 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.
| Statistics of Stellar Populations of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud
A comparative analysis of Washington color-magnitude diagrams (CMDs) for14 star clusters and respective surrounding fields in the LargeMagellanic Cloud (LMC) outer disk is presented. Each CCD frame includingfield and the respective cluster covers an area of 185 arcmin^2. Thestellar population sampled is of intermediate age and metallicity. CMDradial analysis involving star count ratios, morphologies, andintegrated light properties are carried out. Luminosity functions (LFs)are also presented. The two main results are, (1) within the range 4kpc
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