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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.
| 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.
| 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.
| 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.
| 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.
| The Dwarf Irregular/Wolf-Rayet Galaxy NGC 4214. I. A New Distance, Stellar Content, and Global Parameters
We present the results of a detailed optical and near-IR study of thenearby star-forming dwarf galaxy NGC 4214. We discuss the stellarcontent, drawing particular attention to the intermediate-age and/or oldfield stars, which are used as a distance indicator. On images obtainedwith the Hubble Space Telescope Wide Field Planetary Camera 2 andNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) instrumentsin the equivalents of the V, R, I, J and H bands, the galaxy is wellresolved into stars. We achieve limiting magnitudes of F814W~27 in theWF chips and F110W~25 in the NICMOS 2 camera. The optical andnear-infrared color-magnitude diagrams confirm a core-halo galaxymorphology: an inner, high surface brightness, young population within~1.5′ (~1 kpc) from the center of the galaxy, where the stars areconcentrated in bright complexes along a barlike structure, and arelatively low surface brightness, field star population extending outto at least 8' (7 kpc). The color-magnitude diagrams of the core regionshow evidence of blue and red supergiants, main-sequence stars,asymptotic giant branch stars, and blue loop stars. We identify somecandidate carbon stars from their extreme near-IR color. The field-starpopulation is dominated by the ``red tangle,'' which contains the redgiant branch. We use the I-band luminosity function to determine thedistance based on the tip of the red giant branch method: 2.7+/-0.3 Mpc.This is much closer than the values usually assumed in the literature,and we provide revised distance-dependent parameters such as physicalsize, luminosity, H I mass, and star formation rate. From the mean colorof the red giant branch in V and I, we estimate the mean metal abundanceof this population to be [Fe/H]~=-1.7 dex, with a large internalabundance spread characterized by σint([Fe/H])~1 dex.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| 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
| NIR Census of the Stellar Content of Nearby Blue Compact Dwarf Galaxies with HST
We resolved the brightest stars in 5 nearby blue compact dwarf galaxieswith NICMOS aboard HST in J and H. In all galaxies, RGB stars weredetected. This finding allows an improved estimate of the distances ofthese galaxies and sets a lower limit to the age of the old stellarcomponent of 1 Gyr. Further, we detected a pronounced AGB population,including - in all but one - also TP-AGB stars. Our data do not supportthe hypothesis that very low metallicity BCD galaxies may have formedtheir first stars only recently.
| A Near-Infrared Stellar Census of the Blue Compact Dwarf Galaxy VII ZW 403
We present near-infrared single-star photometry for the low-metallicityblue compact dwarf galaxy VII Zw 403. We achieve limiting magnitudes ofF110W~25.5 and F160W~24.5 using one of the NICMOS cameras with theHubble Space Telescope equivalents of the ground-based J and H filters.The data have a high photometric precision (0.1 mag) and are more than95% complete down to magnitudes of about 23, far deeper than previousground-based studies in the near-IR. The color-magnitude diagramcontains about 1000 point sources. We provide a preliminarytransformation of the near-IR photometry into the ground system. Weinvestigate the tip of the red giant branch method in the J and H bandsto provide an empirical distance calibration. Combining our result withglobular cluster data as well as stellar-evolution models, we recommendMH,TRGB=-5.5(+/-0.1) for -2.3<[Fe/H]<-1.5. We proceed todiscuss the stellar content of VII Zw 403 using evolutionary tracks aswell as a classification scheme based on optical and near-IR colors, andwe comment on the detection of asymptotic giant branch stars and theblue Hertzprung gap. We use MH as an indicator ofMbol for red stars after evaluating BCH at lowmetallicity. We calculate the fractional contribution of individualstars from our color-magnitude diagram to the integrated light of VII Zw403 and determine which red stellar population dominates the integratedcolors. We find that young red supergiants, and young andintermediate-age asymptotic giants, together provide about 50% of thelight in I, J, and H bands, whereas the old red giant stars contributeless than 15%. Young, main-sequence stars and blue supergiants accountfor the remaining light and dominate in V. This explains thedifficulties in discerning the nature of blue compact dwarf galaxieswhen only integrated photometry is available.
| 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.
| The evolution of theV-Kcolours of single stellar populations
Models of evolutionary population synthesis of galaxies rely on theproperties of the so-called single stellar populations (SSP). In thispaper, we discuss how the integrated near-infrared colours, andespecially V-K, of SSPs evolve with age and metallicity. Some of theuncertainties associated with the properties of the underlying stellarmodels are thoroughly discussed. Our models include all the relevantstellar evolutionary phases, with particular attention being dedicatedto the asymptotic giant branch (AGB), which plays a fundamental role inthe evolution of the near-infrared part of the spectrum. First, wepresent the effects that different formulations for the mass-loss ratesproduce on the final remnant mass (i.e., on the initial-final massrelation), and hence on the AGB-termination luminosity and the relativecontribution of these stars to the integrated light. The results for theevolution of the V-K colour are very different depending on the choiceof the mass-loss prescription; the same is true also for the B-V colourin the case of low-metallicity SSPs. Secondly, we describe the changesoccurring in the integrated colours at the onset of the AGB and redgiant (RGB) branches. According to the classical formalism for the AGBevolution, the onset of this evolutionary phase is marked by a colourjump to the red, the amplitude of which is shown here to be highlydependent on the metallicity and mass-loss rates adopted in the models.We then consider the effect of the overluminosity with respect to thestandard core mass-luminosity relation that occurs in the most massiveAGB stars. Different simplified formulations for this effect are testedin the models; they cause a smoothing of the colour evolution in the agerange at which the AGB starts to develop, rather than a splitting of thecolour jump into two separate events. On the other hand, we find that atemporary red phase takes place ~1.5x10^8 yr after the RGB develops.Thanks to the transient nature of this feature, the onset of the RGB isprobably not able to cause marked features in the spectral evolution ofgalaxies. We then discuss the possible reasons for the transition of V-Kcolours (from ~1.5 to 3) that takes place in LMC clusters of SWB typeIV. A revision of the ages attributed to the single clusters revealsthat the transition may not be as fast as originally suggested. Thecomparison of the data with the models indicates that the transitionresults mainly from the development of the AGB. A gradual (or delayed)transition of the colours, as predicted by models which include theoverluminosity of the most massive AGB stars, seems to describe the databetter than the sudden colour jump predicted by classical models.
| The relation between the initial and final masses of stars with different chemical compositions
We present the results of calculations for the relations between theinitial and final masses M_i-M_f of low- and moderate-mass stars forvarious initial heavy-element abundances Z. For Z = 0.02 and Z = 0.001,the resulting differences in the final masses for white dwarfs reach0.1M_solar for initial masses from 1.5 to 4M_solar. These differencesare primarily due to the dependence of the initial masses of thecarbon-oxygen cores of asymptotic giant branch stars on their chemicalcompositions. We study the roles of various assumptions about mass lossof stars in the final stages of their evolution. The population of whitedwarfs is modeled, and their mass distribution is obtained for variousassumptions about the initial chemical composition of the stars.
| A Search for Old Star Clusters in the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
| 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
| Stellar evolution on the asymptotic giant branch: some actualities.
Not Available
| Spectroscopy of giants in LMC clusters. II - Kinematics of the cluster sample
Velocities for 83 star clusters in the LMC are analyzed, based onindividual stellar velocities measured at the Calcium triplet. One-halfof the clusters are objects in the outer parts of the LMC which had noprevious velocity determinations. Published velocities for intermediateand old clusters are shown to have had systematic errors. These newvelocities with various rotation curve analyses of the LMC, and testaspects of the twisted disk model proposed by Freeman et al. (1983).When the transverse motion of the LMC is taken into account, a singlerotating disk solution fits the old and intermediate-aged clusters andother tracers (i.e., there is no need for an additional 'tilted disk'system).
| CCD BVR photometry of the open cluster IC 1311
Six CCD frames for two fields in the area of the rich, presumably old,and unstudied open cluster IC 1311 have been obtained in the BVR bands.The photometric analysis leads to an estimated age of between 1 Gyr(standard models) and 2 Gyr (overshooting models), placing the clusterat a distance of about 4.8 kpc from the sun. Comparison of theobservational color-magnitude diagram (CMD) with model isochronesproduces a good fit for a metallicity of Z = 0.006-0.008. Somemorphological features displayed by the CMD of this cluster are betterunderstood in terms of convective overshooting rather than by classicaltheories. The location of IC 1311 on the age-metallicity diagramsuggests an ecological environment for its formation more similar tothat found in the LMC than in the solar vicinity.
| 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.
| The evolution of carbon stars in the Magellanic Clouds
This study presents JHK photometric data for over 100 field stars in theSMC and for 10 in the Large Cloud together with spectroscopic resultsfor about half of them. In the Small Cloud carbon stars were found athigher temperatures and lower luminosities than previously observed. Thefaintest are below the top of the red giant branch. The medium- andlow-luminosity C stars in the M-C transition zone have a low C2 content.At these luminosities, most of the J-type stars are found close to theC2-poor stars in the HR diagram. Their C2 content is about as high as inthe coolest, most evolved C stars. The present observations of carbonstars in the SMC show that they cover a range in M(bo) from -3 to 5.9mag. The transitions from M to C via S appear to occur in both Clouds ata rather well-defined range in M(bol) for SWB and classes IV and V.
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