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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.
| Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
| A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
| Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds
We present accurate integrated-light photometry in Johnson/Cousins V, Rand I for a sample of 28 globular clusters in the Magellanic Clouds. Themajority of the clusters in our sample have reliable age and metallicityestimates available in the literature. The sample encompasses agesbetween 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between -1.5 and0.0 dex. The sample is dominated by clusters of ages between roughly 0.5and 2 Gyr, an age range during which the bolometric luminosity of simplestellar populations is dominated by evolved red giant branch stars andthermally pulsing asymptotic giant branch (TP-AGB) stars whosetheoretical colours are rather uncertain. The VRI colours presented inthis paper have been used to calibrate stellar population synthesismodel predictions.
| The 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.
| 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.
| 0.8-13.5 Micron Spectroscopy of IRAS 07077+1536: A Dusty Carbon Star
Contemporaneous near-infrared (0.8-2.5 μm) and infrared (3-14 μm)spectroscopy is presented of the previously uncharacterized IRAS source07077+1536. The data show it to be a carbon star embedded in acircumstellar dust envelope. The near-infrared spectrum displaysmolecular absorption features of C2, CN, and CO, while the 11μm emission feature from SiC is present in the thermal infrared. Thespectral energy distribution of IRAS 07077+1536 is similar to theso-called extreme carbon stars in that it consists primarily of areddened stellar photosphere and thermal dust emission. However, theextinction is much less and the dust significantly warmer (~900 K vs.~300 K) than in extreme carbon stars such as AFGL 3068. IRAS 07077+1536is also unusually bright shortward of 0.5 μm, an aspect that could bedue to variability but may indicate the presence of a binary companionor a foreground star.
| 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.
| 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.
| 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 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.
| 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.
| 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.
| 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).
| 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.
| Spectroscopy of giants in LMC clusters. I - Velocities, abundances, and the age-metallicity relation
Velocities and equivalent widths are presented for a large sample of LMCclusters. The calcium abundance is found to be a sensitive abundanceindicator over a very wide range of (Fe/H) between 0.0 and -2.2. Theage-metallicity relation is constructed for the inner and outer parts ofthe LMC. This relationsip can be characterized by a simple one-zoneenrichment model. The abundances for the inner and outer clusters at anage of 2 Gyr are nearly identical, so that little radial abundancegradient is evident in the cluster system.
| 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.
| Ellipticities at R(h) of LMC star clusters
The projected ellipticities of 53 populous LMC star clusters have beenderived by means of PDS 1010A scans and a computer interactive method ofreduction implemented on an Apollo 570 workstation. Film copies of apair of J and U plates taken with the 1.2 m UK Schmidt Telescope inAustralia were used. The ellipticities derived here agree with thosefound by previous investigators, when comparisons were possible at thesame radius. Ellipticity variations within individual globular clustersare seen to be a common phenomenon, so the ellipticities e(h) at adistance corresponding to the half-mass radius R(h) from the center wereadopted to represent the cluster's flatness. Using these values for theLMC clusters, it is found that LMC clusters are more elliptical thanthose of the Galaxy. Although the young LMC globular clusters show atendency to be more elliptical than the old ones, there is no strongevidence for a significant difference among them. Finally, e(h) wasfound to increase with the total mass of the clusters, possiblyindicating that high-mass clusters have higher angular momentum, or havemore difficulty in shedding angular momentum, than do low mass clusters,and remain longer in their initial flattened shape.
| 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.
| Distribution of spectral types in the LMC clusters
The distribution of spectral types in 42 LMC globular star clusterscovering all evolutionary ages was determined using objective prismspectra taken with the 1.2-m U.K. Schmidt Telescope in Australia. Thederived spectral type distributions show that the clusters can bedivided into five age categories from about 10 to the 7th to more than10 to the 9th yr. Several clusters were found to contain carbon starswith C/M ratios ranging from 0.07 to 0.4. These ratios were comparedwith those found for the SMC clusters and the Milky Way. It was foundthat the stars of the LMC exhibit a smaller range of C/M ratios than inthe SMC, but larger than in the Galaxy, thus providing an additionaltest of the theoretical models predicting a correlation between the C/Mratio and metal content. It was also found that the majority of youngclusters were embedded in older fields, while the intrmediate clusterswere embeded in younger fields, and the remote old clusters wereembedded in a stellar content of similar age.
| Morphology of LMC clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1987RMxAA..14..172Z
| Observed dynamical parameters of the disk clusters of the LMC. I
A study of the observed dynamical parameters of 32 globular clusters ofthe LMC disk has been carried out by means of star counts. The clusterswere measured on a set of three plates (J, V, I) taken with the 1.2 mU.K. Schmidt Telescope. The derived tidal radii were all found to belarge within a very narrow range. As a consequence the range of thetotal masses was found to be very narrow as well. These two parametersare large in comparison to those of the disk young clusters of thegalaxy but they are similar to the dimensions of the halo galacticglobulars.
| Spectral classification of bright stars in LMC clusters
Identification charts and spectral classification catalogs for ten LMCglobular clusters and their adjoining fields are presented here. Filmcopies of plates taken with the 1.2 m U.K. Schmidt telescope wereexamined in Athens by means of a binocular microscope. The studiedclusters are of various ages, from very young to old, and are located inthe northern part of the galaxy. All classified stars are brighter thanV of about 17.5 mag and situated within the cluster's tidal radii.
| Magellanic Cloud globular cluster ages
Comparison of peak luminosities observed for asymptotic giant branch(AGB) stars in Magellanic Cloud globular clusters against theoreticalvalues yields age-estimates for 12 SMC and 22 LMC clusters. Theallowance for intensive mass loss during the AGB evolutionary phasebrings these ages into agreement with those based on the clustercolor-magnitude diagrams. Clusters have developed differently in the twoClouds.
| 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.
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