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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.
| Infrared Surface Brightness Fluctuations of Magellanic Star Clusters
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
| Probing the Red Giant Branch Phase Transition: Near-Infrared Photometry of Six Intermediate-Age Large Magellanic Cloud Clusters
This is the first of a series of papers devoted to a global study of thephotometric properties of the red stellar sequences in a complete sampleof the Large Magellanic Cloud clusters, by means of near-infrared arrayphotometry. Deep J, H, Ks photometry and accuratecolor-magnitude diagrams down to K~18.5, i.e., ~1.5 mag below the red Heclump, for six intermediate-age clusters (namely, NGC 1987, NGC 2108,NGC 2190, NGC 2209, NGC 2231, NGC 2249) are presented. A quantitativeestimate of the population ratios (by number and luminosity) between redgiant branch (RGB) and He-clump stars for each target cluster isprovided and discussed in the framework of probing the so-called RGBphase transition (Ph-T). By using the Elson & Fall s-parameter as anage indicator, the observed RGB population shows a sharp enhancement (inboth number and luminosity) at s=36. Obviously, the correspondingabsolute age strictly depends on the details of theoretical modelsadopted to calibrate the s-parameter. Curiously, the currently availablecalibrations of the s-parameter in terms of age based on canonical (byElson & Fall) and overshooting (Girardi and coworkers) modelsprovide ages that well agree within 10%, suggesting that the fulldevelopment of the RGB occurs at t~700 Myr and is a relatively fastevent (δt~300 Myr). However, the RGB Ph-T epoch derived from theovershooting calibration of the s-parameter turns out to besignificantly earlier than the epoch provided by the recent evolutionarytracks by Girardi and coworkers. A new calibration of the s-parameterbased on high-quality color-magnitude diagrams and updated models isurged to address the origin of this discrepancy and finally establishthe epoch of the RGB Ph-T.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m New Technology Telescope, withinthe observing programs 64.N-0038 and 68.D-0287.
| 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.
| 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.
| 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.
| The Supergiant Shell LMC 2. I. The Kinematics and Physical Structure
LMC 2 has the brightest, most coherent filamentary structure of allknown supergiant shells in the Large Magellanic Cloud. The opticalemission-line images show active star formation regions along thewestern edge and long filaments to the east. ROSAT PSPC and HRI imagesshow bright X-ray emission from within the shell boundary, indicatingthe presence of hot gas. Counterintuitively, neither high-resolutionechelle spectra in the Hα line nor aperture synthesis H I 21 cmemission-line observations show LMC 2 to have the kinematics expected ofan expanding shell. Rather, LMC 2 appears to consist of hot gas confinedbetween H I sheets. The interior surfaces of these sheets are ionized bythe UV flux of massive stars in the star formation regions along theperiphery of LMC 2, while the heating is provided by outflows of hot gasfrom the star formation regions and by SNRs interior to LMC 2. We havecompared LMC 2 to other supergiant shells in the LMC and in more distantgalaxies. When the spatial resolution of our data are degraded, we findthat LMC 2 resembles supergiant shells observed at a distance of 4 Mpcthat have previously been interpreted as expanding shells. Therefore,great caution should be exercised in the analysis and interpretation ofthe kinematics of distant supergiant shells to prevent overestimates oftheir velocities and total kinetic energies.
| 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.
| 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.
| Carbon stars in LMC clusters revisited.
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Age distribution of LMC clusters from their integrated UBV colors: history of star formation.
In this paper we revise the relationship between ages and metallicitiesof LMC star clusters and their integrated UBV colors. The study standson the catalog of UBV colors of the Large Magellanic Cloud (LMC)clusters by Bica et al. (1994; BCDSP) and the photometric models ofsingle stellar populations (SSP) calculated by Bertelli et al. (1994).These photometric models nicely describe the color distribution of LMCclusters in the (U-B) vs. (B-V) plane together with the observeddispersion of the colors and the existence of a gap in a certain regionof this diagram. In the case of blue clusters, most of the dispersion inthe colors can be accounted for by the presence of stochastic effects onthe mass distribution of stars, whereas for the red ones additionaldispersion's of ~0.2dex in metallicity and of ~0.05mag in color excessare needed. From comparing the observed distribution of integratedcolors in the (U-B) vs. (B-V) diagram with the theoretical models, itturns out that: 1) The data are consistent with the presence of a gap(period of quiescence) in the history of cluster formation. If theage-metallicity relation (AMR) for the LMC obeys the simple model ofchemical evolution, the gap is well evident and corresponds to the ageinterval ~3Gyr to (12-15)Gyr. On the contrary, if the chemicalenrichment has been much slower than in the simple model, so thatintermediate age clusters are less metal rich, the gap is expected tooccur over a much narrower color range and to be hidden by effects ofcolor dispersion. 2) The bimodal distribution of B-V colors can bereproduced by a sequence of clusters almost evenly distributed in thelogarithm of the age, whose metallicity is governed by a normal AMR. Noneed is found of the so-called phase transitions in the integratedcolors of a cluster taking place at suitable ages (Renzini & Buzzoni1986). 3) The gap noticed by BCDSP in the (U-B) vs. (B-V) plane can beexplained by the particular direction along which cluster colors aredispersed in that part of the (U-B) vs. (B-V) diagram. Also in thiscase, no sudden changes in the integrated properties of clusters must beinvoked. The results of this analysis are used to revise the empiricalmethod proposed by Elson & Fall (1985, EF85) to attribute ages toLMC clusters according to their integrated UBV colors. We show that theEF85 method does not provide the correct relation between ages andcolors for clusters of low metallicity and hence its inability to datethe old clusters. We propose two modifications to the definition of theparameter S of EF85 such that the age sequence of red clusters issuitably described, and the intrinsic errors on ages caused by the heavypresence of various effects dispersing the colors are reduced to aminimum. The age sequence is calibrated on 24 template clusters forwhich ages were independently derived from recent color-magnitudediagrams (CMD). Finally, we attribute ages to all clusters present inBCDSP catalog, and derive the global age distribution function (ADF) forLMC clusters. The ADF presents new features that were not clear inprevious analyses of UBV data, but were already suggested by a number ofindependent observational studies. The features in question are periodsof enhanced cluster formation at ~100Myr and 1-2Gyr, and a gap in thecluster formation history between ~3 and (12-15)Gyr. The peaks observedin the distribution of B-V colors are found to be sensitive to thepresence of these periods of enhanced cluster formation and the lack ofextremely red clusters caused by the age gap between intermediate-ageand old clusters.
| 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.
| 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
| Surface brightness profiles for five rich star clusters in the Large Magellanic Cloud
Surface profiles are presented for five rich star clusters in the LMC(NGC 1783, 1856, 1987, 2107, and 2108), with ages between 10 exp 8 and10 exp 9 yr. The profiles are well represented by King-like models, andthe core radii follow the trend of increasing core radius with age thatwas found in previous work. If this trend reveals expansion due to massloss through stellar evolution, then the new data reinforce the ideathat the slope of the initial mass function, which determines the rateof expansion, is consistent with the Salpeter value (x = 1.35), and isnot generally as steep as x = 2.5. A sample of clusters and surfacebrightness profiles for them are presented in tabular and graphic form.
| 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.
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| The asymptotic giant branch of Magellanic Cloud clusters
The present search for carbon and M-type asymptotic giant branch (AGB)stars in the 39 clusters of the Magellanic Clouds has yieldedidentifications and near-IR photometry for about 400 such stars. TheSearle et al. (1980) cluster-age-related classification scheme is abasic element of the present analysis of these data. In a C-M diagram,the cluster M stars shift steadily redward as one proceeds from clustersof SWB type I to VI, due to the increasing age of the clusters along thesequence. Luminous carbon stars are present only in SWB IV-VI clusters,and are easily distinguished from M stars by their color and luminosity.
| The evolution of the Magellanic Clouds. I - The ages of globular clusters
Theoretical and observed maximum luminosities of AGB stars in theMagellanic Cloud clusters are compared in order to obtain cluster ageestimations. The ages of 10 clusters in the SMC and 25 in the LMC areconsidered for the cases of several rates of mass loss by AGB stars. Itis demonstrated that discrepancies between ages derived from AGB peakluminosities and from the Main-Sequence turn off and maximum luminositycan be accounted for by the intensive mass loss during the AGBevolutionary phase.
| Ages and metallicities of LMC and SMC red clusters through H-beta and G band photometry
Narrow band integrated photometry of the H-beta and G band absorptionfeatures for 41 LMC and 10 SMC red star clusters is presented. Anage-metallicity calibration is provided for the color-color diagram. SWBtypes between IV and VII are derived for 23 unclassified clusters, andtheir distribution in the age versus metallicity plane is discussed. Astudy of chemical evolution of the Magellanic Clouds has shown that theLMC presents a steeper chemical enrichment slope. An intrinsicmetallicity dispersion is found in the LMC chemical evolution,indicating that the gas has been inhomogeneous at any time, with localenrichment prevailing over a global one. One zone model describes theevolution of both clouds, the efficiency of star cluster formation beinglarger in the LMC. The LMC presents a burst of star cluster formation att = 4.5 x 10 to the 9th yr. New B - V data for fainter SMC clusters arealso presented, providing an essentially complete color histogram forclusters with globular cluster appearance.
| A theoretical and observational study of the Red Giant Branch phase transition in Magellanic Cloud clusters - A progress report
Preliminary results are reported for an investigation comparingtheoretical models of the sudden appearance of an extended RGB (and itseffects on the spectral energy distributions of stellar populations)with data from ESO CCD observations of clusters in the LMC and SMC.Isochrones for the entire RGB are being constructed on the basis of 100new evolutionary sequences (calculated using the evolution code ofSweigart and Gross, 1976 and 1978) to permit determination of syntheticcolors and spectral energy distributions. The observations so farindicate a main sequence about 0.1 mag redder than that predicted by thepresent models or by the isochrones of VandenBerg and Bell (1985), andfail to show a B-V color difference at the RGB phase transition.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
| The extended giant branches of intermediate age globular clusters in the Magellanic Clouds. IV
A complete survey is available for asymptotic giant-branch stars in therich star clusters of the Magellanic Clouds. Although data on themain-sequence turnoffs of these clusters are still incomplete, somesystematic properties of these stars emerge, when grouped by clusterage. Clusters younger than approximately 8 billion years have carbonstars at the tip of the giant branch, produced by the third dredge-upmechanism. Clusters younger than approximately 0.8 billion years havegiant branches populated by M stars. It is suggested that in stars ofthis mass range thermal pulses have not commenced before mass losscompletely erodes the stellar envelope. Cluster stars of 5 solar mass(turnoff approximately 80 million years) suffer about 80-percent massloss in the course of their evolution, compared with approximately 30percent for the oldest stars.
| The distribution of carbon and M-type giants in the Magellanic Clouds
Small-dispersion near-infrared surveys for carbon and M giants in theLMC and SMC have been completed for a large number of sample areas, andthe observational techniques and results are presented. The surfacedistribution of the carbon stars and M giants correlates reasonably wellwith the red surface brightness of the clouds but not with thedistribution of neutral hydrogen. The C/M ratio between the surfacefrequency of carbon stars and that of type M6 giants or later is 2.2 +or - 0.1 throughout the LMC. In the SMC this ratio varies from 19.2 + or- 0.8 at the center to 4.7 + or - 0.4 at the periphery. Upper mainsequence stars contribute a major fraction of the total surfaceluminosity in a peripheral LMC region while that contributed by thecarbon stars is small. It is concluded that in the MC star formation mayhave occurred in discrete bursts rather than in a smooth and continuousprocess.
| Photometric studies of composite stellar systems. V - Infrared photometry of star clusters in the Magellanic clouds
Abstract image available at:http://adsabs.harvard.edu/abs/1983ApJ...266..105P
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