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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.
| On the Age and Metallicity Estimation of Spiral Galaxies Using Optical and Near-Infrared Photometry
In integrated light, some color-color diagrams that use optical andnear-infrared photometry show surprisingly orthogonal grids as age andmetallicity are varied, and they are coming into common usage forestimating the average age and metallicity of spiral galaxies. In thispaper we reconstruct these composite grids using simple stellarpopulation models from several different groups convolved with someplausible functional forms of star formation histories at fixedmetallicity. We find that the youngest populations present (t<2 Gyr)dominate the light, and because of their presence the age-metallicitydegeneracy can be partially broken with broadband colors, unlike olderpopulations. The scatter among simple stellar population models bydifferent authors is, however, large at ages t<2 Gyr. The dominantuncertainties in stellar population models arise from convective coreovershoot assumptions and the treatment of the thermally pulsingasymptotic giant branch phase and helium abundance may play asignificant role at higher metallicities. Real spiral galaxies areunlikely to have smooth, exponential star formation histories, andburstiness will cause a partial reversion to the single-burst case,which has even larger model-to-model scatter. Finally, it is emphasizedthat the current composite stellar population models need someimplementation of chemical enrichment histories for the proper analysisof the observational data.
| 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.
| 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.
| 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.
| ISOCAM Observations of Globular Clusters in the Magellanic Clouds: The Data
Seventeen globular clusters in the Large and Small Magellanic Cloudswere observed in the mid-infrared wavelength region with the ISOCAMinstrument on board the Infrared Space Observatory (ISO). Observationswere made using the broadband filters LW1, LW2, and LW10, correspondingto the effective wavelengths of 4.5, 6.7, and 12 μm, respectively. Wepresent the photometry of point sources in each cluster, as well astheir precise positions and finding charts.Based on observations with ISO, an ESA project with instruments fundedby ESA Member states (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with participation of ISAS andNASA.
| 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.
| 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.
| 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.
| Spectral and photometric evolution of simple stellar populations at various metallicities
A new set of evolutionary synthesis spectra are presented for SimpleStellar Populations (SSPs) covering ranges in metallicity from 0.02<= Z/Zsun <= 2.5 and ages from 4x 106 yr<= t <= 16 Gyr. They are based on the most recent isochrones fromthe Padova group that extend earlier models by the inclusion of thethermal pulsing AGB phase for stars in the mass range 2 Msun<= m <= 7 Msun in accordance with the fuel consumptiontheorem. We show that with respect to earlier models, inclusion of theTP-AGB phase leads to significant changes in the (V-I) and (V-K) colorsof SSPs in the age range from 108 to 109 yr. Usingmodel atmosphere spectra from Lejeune et al. (\cite{lej2}, \cite{lej}),we calculate the spectral evolution of single burst populations ofvarious metallicities covering the wavelength range from 90 Åthrough 160 mu m. Isochrone spectra are convolved with filter responsefunctions to describe the time evolution of luminosities and colors inJohnson, Thuan & Gunn, Koo, HST, Washington and Strömgrenfilters. The models and their results are not only intended for use inthe interpretation of star clusters but also for combination with anykind of dynamical galaxy formation and/or evolution model that containsa star formation criterion. Moreover, the evolution of these singleburst single metallicity stellar populations is readily folded with anykind of star formation - and eventually chemical enrichment - history todescribe the evolutionary spectral synthesis of composite stellarpopulations like galaxies of any type with continuous or discontinuousstar formation. For these latter purposes we also present the timeevolution of ejection rates for gas and metals for two different InitialMass Functions (IMFs) as well as cosmological and evolutionarycorrections for all the filters as a function of redshift for 0 <= z<= 5 and two different cosmologies. Extensive data files are providedin the electronic version, at CDS, and at our above www-address.
| An Upper Limit to the Age of the Galactic Bar
Using data from the Two Micron All Sky Survey, we identify a populationof infrared carbon stars with J-KS>=2 in the Milky Way.These stars are shown to trace the stellar bar previously identified inIR and optical surveys. The properties of C stars strongly suggest thatthey are of intermediate age. We conclude that the bar is likely to haveformed more recently than 3 Gyr ago and must be younger than 6 Gyr.Implications and further tests of this conclusion are briefly discussed.
| 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
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| 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
| The Metallicity Distribution Function of Red Giants in the Large Magellanic Cloud
We report new metallicity determinations for 39 red giants in a 220arcmin2 region, 1.8d southwest of the bar of the LargeMagellanic Cloud. These abundance measurements are based on spectroscopyof the Ca II infrared triplet. We have carefully considered the effectsof abundance ratios, the physics of Ca II line formation, the variationof red clump magnitude, and the contamination by foreground stars in ourabundance analyses. The metallicity distribution function (MDF) shows astrong peak at [Fe/H]=-0.57+/-0.04 a tail to abundances at least as lowas [Fe/H]~-1.6 brings the average abundance down to [Fe/H]=-0.64+/-0.02.Half the red giants in our field fall within the range-0.83<=[Fe/H]<=-0.41. The MDF appears to be truncated at[Fe/H]~-0.25 the exact value of the maximum abundance is subject to ~0.1dex uncertainty in the calibration of the Ca II IR triplet for young,metal-rich stars. We find a striking contrast in the shape of the MDFbelow [Fe/H]<=-1 between our inner disk field and the distant outerfield studied by Olszewski: red giants deficient by more than a factorof 10 in heavy elements relative to the Sun are extremely scarce in theinner disk of the LMC. Our field star sample does not reproduce the fullMDF of the LMC star clusters but seems similar to that of theintermediate-age (1-3 Gyr) clusters. We have also obtained abundanceestimates using Strömgren photometry for ~103 red giantsin the same field. Photometry is the only practical way to measureabundances for the large numbers of stars necessary to liftage-metallicity degeneracy from our high-precision color-magnitudediagrams. The Strömgren measurements, which are sensitive to acombination of cyanogen and iron lines, correlate well with the Ca IImeasurements, but a metallicity-dependent offset is found. The offsetmay be due either to variations in the elemental abundance ratios due togalactic chemical evolution or to a metal-dependent mixing mechanism inRGB stars. An empirical relation between photometric and spectroscopicabundance estimates is derived. This will allow photometric abundancemeasurements to be placed on a consistent metallicity scale withspectroscopic metallicities, for very large numbers of stars. Based onobservations obtained at Cerro Tololo Inter-American Observatory, adivision of the National Optical Astronomy Observatories, which areoperated by the Association of Universities for Research in Astronomy,Inc. under cooperative agreement with the National Science Foundation.
| The Evolved Red Stellar Content of M32
Near-infrared images obtained with the Canada-France-Hawaii Telescope(CFHT) Adaptive Optics Bonnette (AOB) are used to investigate thestellar content of the Local Group compact elliptical galaxy M32.Observations of a field 2.3′ from the galaxy center reveal a largepopulation of asymptotic giant branch (AGB) stars, and comparisons withmodels indicate that these objects have an agelog(tGyr)<=9.3. The AGB population is very homogeneous,with Δlog(tGyr)<=+/-0.1 dex andΔ[M/H]<=+/-0.3 dex. The reddest AGB stars have J-K<=1.5, andit is suggested that the very red stars seen in earlier, less deep,surveys are the result of large photometric errors. The bolometric AGBluminosity function (LF) of this field is in excellent agreement withthat of the Galactic bulge. Based on the integrated brightness of AGBstars brighter than the red giant branch tip, which occurs at K=17.8, itis concluded that intermediate-age stars account for roughly 25% of thetotal K light and 10%+/-5% of the total mass in this field. A fieldclose to the center of M32 was also observed. The brightest stars withina few arcseconds of the nucleus have K=15.5, and the density of theseobjects is consistent with that predicted from the outer regions of thegalaxy after scaling according to surface brightness. Moreover, the Kluminosity function (LF) of bright sources between 20" and 30" of thenucleus is well matched by the LF of the outer regions of the galaxyafter accounting for differences in surface brightness and correctingfor the effects of crowding. It is concluded that the relative size ofthe intermediate-age component with respect to other populations doesnot change with radius over much of the galaxy. However, the integratedJ-K color and 2.3 μm CO index change with radius within a few tenthsof an arcsecond of the galaxy center, indicating that, contrary to whatmight be inferred from observations at visible wavelengths, theintegrated photometric properties of the central regions of M32 differfrom those of the surrounding galaxy.
| 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
| 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.
| Discovery of intrared stars in globular clusters in the Magellanic Clouds and their light variations.
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