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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.
| 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.
| 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.
| The Chemical Properties of Milky Way and M31 Globular Clusters. I. A Comparative Study
A comparative analysis is performed between high-quality integratedspectral indices of 30 M31 globular clusters, 20 Milky Way globularclusters, and a sample of field and cluster elliptical galaxies. We findthat the Lick CN indices in the M31 and Galactic clusters are enhancedrelative to the bulges of the Milky Way, M31, and elliptical spheroids,in agreement with Burstein and coworkers. Although not particularlyevident in the Lick CN indices, the near-UV cyanogen feature(λ3883) is strongly enhanced with respect to the Galacticglobular clusters at metallicities -1.5<[Fe/H]<-0.3. Carbon showssigns of varying among these two groups. For [Fe/H]>-0.8, we observeno systematic differences in the Hδ, Hγ, or Hβ indicesbetween the M31 and Galactic globular clusters, in contrast to previousstudies. The elliptical galaxy sample lies offset from the loci of theglobular clusters in both the cyanogen-[MgFe] and Balmer-line-[MgFe]planes. Six of the M31 clusters appear young and are projected onto theM31 disk. Population synthesis models suggest that these are metal-richclusters with ages 100-800 Myr, metallicities -0.20<=[Fe/H]<=0.35,and masses 0.7-~7.0×104 Msolar. Two otheryoung clusters are Hubble V in NGC 205, observed as a template, and anolder (~3 Gyr) cluster some 7 kpc away from the plane of the disk. Thesix clusters projected onto the disk show signs of rotation similar tothe H I gas in M31, and three clusters exhibit thin disk kinematics,according to Morrison and coworkers. Dynamical mass estimates anddetailed structural parameters are required for these objects todetermine whether they are massive open clusters or globular clusters.If they are the latter, our findings suggest globular clusters may tracethe buildup of galaxy disks. In either case, we conclude that theseclusters are part of a young, metal-rich disk cluster system in M31,possibly as young as 1 Gyr old.
| 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.
| Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects
The properties of ~939 star clusters in the Large and Small MagellanicClouds were determined from ground-based CCD images in UBVR passbands.The areal coverage was extensive, corresponding to 11.0 kpc2in the LMC and 8.3 kpc2 in the SMC. After corrections forreddening, the colors and magnitudes of the clusters were converted toages and masses, and the resulting mass distributions were searched forthe effects of fading, evaporation, and size-of-sample bias. The datashow a clear signature of cluster fading below the detection threshold.The initial cluster mass function (ICMF) was determined by fitting themass and age distributions with cluster population models. These modelssuggest a new method to determine the ICMF that is nearly independent offading or disruption and is based on the slope of a correlation betweenage and the maximum cluster mass in equally spaced intervals of log age.For a nearly uniform star formation rate, this correlation has a slopeequal to 1/(α-1) for an ICMF of dn(M)/dM~M-α. Wedetermine that α is between 2 and 2.4 for the LMC and SMC usingthis method plus another method in which models are fitted to the massdistribution integrated over age and to the age distribution integratedover mass. The maximum mass method also suggests that the clusterformation rate in the LMC age gap between 3 and 13 Gyr is about a factorof 10 below that in the period from 0.1 to 1 Gyr. The oldest clusterscorrespond in age and mass to halo globular clusters in the Milky Way.They do not fit the trends for lower mass clusters but appear to be aseparate population that either had a very high star formation rate andbecame depleted by evaporation or formed with only high masses.
| 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
| Testing stellar population models with star clusters in the Large Magellanic Cloud
We present high signal-to-noise ratio integrated spectra of 24 starclusters in the Large Magellanic Cloud (LMC), obtained using the FLAIRspectrograph at the UK Schmidt telescope. The spectra have been placedon to the Lick/IDS system in order to test the calibration of SimpleStellar Population (SSP) models. We have compared the SSP-predictedmetallicities of the clusters with those from the literature,predominantly taken from the Ca-triplet spectroscopy of Olszewski et al.(1991). We find that there is good agreement between the metallicitiesin the range -2.10 <=[Fe/H]<= 0. However, the Mg2 index(and to a lesser degree Mg b) systematically predict highermetallicities (up to +0.5 dex higher) than . Among thepossible explanations for this are that the LMC clusters possess[α/Fe] > 0. Metallicities are presented for eleven LMC clusterswhich have no previous measurements. We compare SSP ages for theclusters, derived from the Hβ, Hγ and Hδ Lick/IDSindices, with the available literature data, and find good agreement forthe vast majority. This includes six old globular clusters in oursample, which have ages consistent with their HST colour-magnitudediagram (CMD) ages and/or integrated colours. However, two globularclusters, NGC 1754 and NGC 2005, identified as old (~15 Gyr) on thebasis of HST CMDs, have Hβ line-strengths which lead ages that aretoo low (~8 and ~6 Gyr respectively). These findings are inconsistentwith their CMD-derived values at the 3σ level. Comparison betweenthe horizontal branch morphology and the Balmer line strengths of theseclusters suggests that the presence of blue horizontal branch stars hasincreased their Balmer indices by up to ~1.0 Å. We conclude thatthe Lick/IDS indices, used in conjunction with contemporary SSP models,are able to reproduce the ages and metallicities of the LMC clustersreassuringly well. The required extrapolations of the fitting functionsand stellar libraries in the models to lower ages and low metallicitiesdo not lead to serious systematic errors. However, owing to thesignificant contribution of horizontal branch stars to Balmer indices,SSP model ages derived for metal-poor globular clusters are ambiguouswithout a priori knowledge of horizontal branch morphology.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| 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.
| Magellanic Cloud Periphery Carbon Stars. IV. The SMC
The kinematics of 150 carbon stars observed at moderate dispersion onthe periphery of the Small Magellanic Cloud are compared with themotions of neutral hydrogen and early-type stars in the intercloudregion. The distribution of radial velocities implies a configuration ofthese stars as a sheet inclined at 73°+/-4° to the plane of thesky. The near side, to the south, is dominated by a stellar component;to the north, the far side contains fewer carbon stars and is dominatedby the neutral gas. The upper velocity envelope of the stars is closelythe same as that of the gas. This configuration is shown to beconsistent with the known extension of the SMC along the line of sightand is attributed to a tidally induced disruption of the SMC thatoriginated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago.The dearth of gas on the near side of the sheet is attributed toablation processes akin to those inferred in 1996 by Weiner &Williams to collisional excitation of the leading edges of MagellanicStream clouds. Comparison with the 1989 kinematic data of Hardy,Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the1986 and 1988 data of Mathewson et al. leaves little doubt that forcesother than gravity play a role in the dynamics of the H I.
| 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.
Not Available
| Chemical evolution and star formation history in the LMC from cluster and field stars
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 (-0.64+/-0.18 dex), NGC1806 (-0.67+/-0.24 dex), NGC 2031 (-0.40+/-0.21 dex) and NGC 2136/37(-0.43+/-0.23 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 700 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR) and a starformation history (SFH). Our method was to estimate ages for individualstars on the basis of Strömgren isochrones with individuallymeasured metallicities. With this method we are able to sample the AMRand SFH of the field population up to 8 Gy. Our metallicity data areincompatible with models predicting many metal-poor stars. Themetallicity of the field population increased by a factor of seven,starting around 2 Gy ago. This increase was preceded by an increase ofthe star formation rate 3-4 Gy ago. The results support the SFH found inearlier investigations of the LMC field population. The absence of starclusters between 4 Gy and 10 Gy agrees very well with a low rate of thestar formation during that time.
| 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.
| A digital photometric survey of the magellanic clouds: First results from one million stars.
We present the first results from, and a complete description of, ourongoing UBVI digital photometric survey of the Magellanic Clouds. Inparticular, we discuss the photometric quality and automated reductionof a CCD survey (magnitude limits, completeness, and astrometricaccuracy) that covers the central 8(deg) x 8(deg) of the LargeMagellanic Cloud (LMC) and 4(deg) x 4(deg) of the Small Magellanic Cloud(SMC). We discuss photometry of over 1 million stars from the initialsurvey observations (an area northwest of the LMC bar covering ~ 2(deg)x 1.5(deg) ) and present a deep stellar cluster catalog that containsabout 45% more clusters than previously identified within this region.Of the 68 clusters found, only 12 are also identified as concentrationsof ``old'', red clump stars. Furthermore, only three clusters areidentified solely on the basis of a concentration of red clump stars,rather than as a concentration of luminous (V < 21) main sequencestars. Extrapolating from the current data, we expect to obtain B and Vphotometry for 25 million stars, and U and I photometry for 10 and 20million stars, respectively, over the entire survey area.
| The ellipticities of Galactic and Large Magellanic Cloud globular clusters
The correlations between the ellipticity and the age and mass of LMCglobular clusters are examined, and both are found to be weak. It isconcluded that neither of these properties is mainly responsible for theobserved differences in the LMC and Galactic globular clusterellipticity distributions. Most importantly, age cannot be the primaryfactor in the LMC-Galaxy ellipticity differences, even if there is arelationship, as even the oldest LMC clusters are more elliptical thantheir Galactic counterparts. The strength of the tidal field of theparent galaxy is proposed as the dominant factor in determining theellipticities of that galaxy's globular clusters. A strong tidal fieldrapidly destroys velocity anisotropies in initially triaxial, rapidlyrotating elliptical globular clusters. A weak tidal field, however, isunable to remove these anisotropies and the clusters remain close totheir initial shapes.
| Duration of the superwind phase of asymptotic giant branch stars
Near the ends of their lives, low- and intermediate-mass stars gothrough a phase of evolution known as the asymptotic giantbranch1,2. This luminous red-giant phase is thought to beterminated by a period of intense mass loss in the form of asuperwind3, which leads to the formation of a planetarynebula. Although the effects of mass loss have been studied extensivelyin many stars, the duration of this phase is not well constrained,because of uncertainties in the distances, masses, ages, and absoluteluminosities of the observed stars. On the other hand, the properties ofstars in the globular clusters associated with the Magellanic Clouds arenot subject to these uncertainties, and so provide an excellentopportunity for studying mass-loss phenomena in a quantitative way. Herewe report the discovery of two infrared stars in Magellanic Cloudglobular clusters that are undergoing a period of intense mass loss.Those observations, together with those of a previously discoveredinfrared star, confirm that asymptotic giant branch stars go through asuperwind phase, and constrain the duration of this phase to be about100,000 years.
| Extreme Infrared Stars Discovered in Magellanic Cloud Globular Clusters
We report preliminary results of our systematic survey for infraredstars in the globular clusters of the Magellanic Clouds. In the courseof an ISOCAM survey for AGB stars in the intermediate-age clusters, wehave discovered extremely red AGB stars in NGC 419 and NGC 1978. Fromtheir colours and luminosities, they are thought to be experiencingintense mass-loss and to be in the final or superwind phase of the AGBevolution. However, they seem to be of somewhat lower luminosity thanthe corresponding visible AGB stars when only the mid-infrared data aretaken into account. This suggests that hitherto unobserved infraredexcesses may exist at longer wavelengths.
| Carbon stars in LMC clusters revisited.
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| 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.
| Moment analysis applied to LMC star clusters
Statistical moment-based ellipse fitting is performed on observations ofLarge Magellanic Cloud clusters, confirming that trends are evident intheir position angles and ellipticities, as had been reported in theliterature. Artificial cluster images with known parameters aregenerated, and subjected to the same analysis techniques, revealingapparent trends caused by stochastic processes. Caution should thereforebe exercised in the interpretation of observational trends in young LMCclusters.
| 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.
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