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Distances to Populous Clusters in the Large Magellanic Cloud via the K-band Luminosity of the Red Clump
We present results from a study of the distances and distribution of asample of intermediate-age clusters in the Large Magellanic Cloud (LMC).Using deep near-infrared photometry obtained with ISPI on the CTIO 4 m,we have measured the apparent K-band magnitude of the corehelium-burning red clump stars in 17 LMC clusters. We combine clusterages and metallicities with the work of Grocholski and Sarajedini topredict each cluster's absolute K-band red-clump magnitude and therebycalculate absolute cluster distances. An analysis of these data showsthat the cluster distribution is in good agreement with the thick,inclined-disk geometry of the LMC, as defined by its field stars. Wealso find that the old globular clusters follow the same distribution,suggesting that the LMC's disk formed at about the same time as theglobular clusters, ~13 Gyr ago. Finally, we have used our clusterdistances in conjunction with the disk geometry to calculate thedistance to the LMC center, for which we find(m-M)0=18.40+/-0.04 (random)+/-0.08 (systematic), orD0=47.9+/-0.9+/-1.8 kpc.
| Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
| A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| Near-infrared surface brightness fluctuations and optical colours of Magellanic star clusters
This work continues our efforts to calibrate model surface brightnessfluctuation luminosities for the study of unresolved stellarpopulations, through a comparison with the data of Magellanic Cloud starclusters. We present here the relation between absoluteKs-band fluctuation magnitude and (V-I) integrated colour,using data from the Two-Micron All-Sky Survey (2MASS) and the DeepNear-Infrared Southern Sky Survey (DENIS), and from the literature. Wecompare the star cluster sample with the sample of early-type galaxiesand spiral bulges studied by Liu et al. We find that intermediate-age toold star clusters lie along a linear correlation with the same slope,within the errors, of that defined by the galaxies in the versus (V-I)diagram. While the calibration by Liu et al. was determined in thecolour range 1.05 < (V-IC)0 < 1.25, oursholds in the interval . This implies, according to Bruzual-Charlot andMouhcine-Lançon models, that the star clusters and the lateststar formation bursts in the galaxies and bulges constitute an agesequence. At the same time, a slight offset between the galaxies and thestar clusters [the latter are ~0.7 mag fainter than the former at agiven value of (V-I)], caused by the difference in metallicity ofroughly a factor of 2, confirms that the versus (V-I) plane maycontribute to break the age-metallicity degeneracy in intermediate-ageand old stellar populations. The confrontation between models and galaxydata also suggests that galaxies with Ks fluctuationmagnitudes that are brighter than predicted, given their (V-I) colour,might be explained in part by longer lifetimes of thermally pulsingasymptotic giant branch stars. A preliminary comparison between the H2MASS data of the Magellanic star clusters and the sample of 47early-type galaxies and spiral bulges observed by Jensen et al. throughthe F160WHubble Space Telescope filter leads to the same basicconclusions: galaxies and star clusters lie along correlations with thesame slope, and there is a slight offset between the star cluster sampleand the galaxies, caused by their different metallicities. Magellanicstar clusters are single populations, while galaxies are compositestellar systems; moreover, the objects analysed live in differentenvironments. Therefore, our findings mean that the relationship betweenfluctuation magnitudes in the near-infrared, and (V-I) might be a fairlyrobust tool for the study of stellar population ages and metallicities,could provide additional constraints on star formation histories, andaid in the calibration of near-infrared surface brightness fluctuationsfor cosmological distance measurements.
| 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.
| Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
| New Photometry for the Intermediate-Age Large Magellanic Cloud Globular Cluster NGC 2121 and the Nature of the LMC Age Gap
We report new photometry for the cluster NGC 2121 in the LargeMagellanic Cloud, which shows a prominent hydrogen core exhaustion gapat the turnoff and a descending subgiant branch reminiscent of Galacticopen clusters. We achieve an excellent fit using the Girardi isochrones,finding an age of 3.2+/-0.5 Gyr, with [Fe/H]=-0.6+/-0.2. The isochronesfit the color and shape of the turnoff and subgiant branch so preciselythat we can constrain the metallicity, as well as the age. The sameisochrones also fit SL 663 and NGC 2155, although our photometry forthese clusters has much larger errors. We find these clusters to be 0.8Gyr younger and 0.4 dex more metal-rich than recently reported in theliterature. Consequently, we argue that NGC 2121, NGC 2155, and SL 663are not properly assigned to the age gap in the LMC, but instead areamong the first clusters to be have formed in the relatively metal-rich,younger group of LMC clusters. We propose a new definition of the LMCage gap as extending from 3.2 to 13 Gyr, with ESO 121-SC03 still theonly remaining candidate for membership in the age gap. Based onobservations with the NASA/ESA Hubble Space Telescope (HST) obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| The Line-of-Sight Depth of Populous Clusters in the Small Magellanic Cloud
We present an analysis of age, metal abundance, and positional data onpopulous clusters in the Small Magellanic Cloud (SMC) with the ultimateaim of determining the line-of-sight (LOS) depth of the SMC by usingthese clusters as proxies. Our data set contains 12 objects and islimited to clusters with the highest-quality data for which the ages andabundances are best known and can be placed on an internally consistentscale. We have analyzed the variation of the clusters' properties withposition on the sky and with line-of-sight depth. Based on thisanalysis, we draw the following conclusions: (1) The observational dataindicate that the eastern side of the SMC (facing the Large MagellanicCloud) contains younger and more metal-rich clusters as compared withthe western side. This is not a strong correlation because our data setof clusters is necessarily limited, but it is suggestive and warrantsfurther study. (2) Depending on how the reddening is computed to ourclusters, we find a mean distance modulus that ranges from(m-M)0=18.71+/-0.06 to 18.82+/-0.05. (3) The intrinsic +/-1σ LOS depth of the SMC populous clusters in our study is between~6 and ~12 kpc, depending primarily on whether we adopt the Burstein& Heiles reddenings or those from Schlegel et al. (4) Viewing theSMC as a triaxial galaxy with declination, right ascension, and LOSdepth as the three axes, we find axial ratios of approximately 1:2:4.Taken together, these conclusions largely agree with those of previousinvestigators and underscore the utility of populous star clusters asprobes of the structure of the Small Magellanic Cloud.
| Globular clusters in NGC 5128
We used the Wide Field Planetary Camera 2 aboard the Hubble SpaceTelescope to search for globular clusters in the inner regions of thenearby giant elliptical galaxy NGC 5128. This galaxyis believed to be the product of a merger between a large ellipticalgalaxy and a small late-type spiral between 160 and 500 Myr ago. Weidentified 21 globular cluster candidates and measured their core radii,tidal radii, half-mass radii, ellipticities, position angles, and V - Icolors. We find evidence that the NGC 5128 globularcluster candidates are systematically more elliptical than are those ofthe Milky Way. Approximately half of the candidateshave {(V - I)}_0 colors that are consistent with their being either old,unreddened globular clusters, similar to those found in theMilky Way, or young, reddened globular clusters thatmay have formed during the recent merger event. Most of the rest havecolors that are consistent with their being old globular clusterssimilar to those found in the Milky Way. We find oneblue object with {(V - I)}_0 < 0.26 +/- 0.09. The color, reddening,and integrated magnitude of this object are consistent with its being asmall globular cluster with an age of phantom {0}im 100 Myr and a mass(based on its integrated luminosity) of <= 4000 M_sun. We find noevidence for bimodality in the colors of the globular cluster candidatesin our sample beyond what can be explained by uncertainties in thedifferential reddening. Based in observations with the NASA/ESA {\itHubble Space Telescope}, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555.
| 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.
| Three Populous Clusters Discovered in the Large Magellanic Cloud Age Gap
In the process of searching the Hubble Space Telescope archive, we haveserendipitously discovered three populous Large Magellanic Cloud (LMC)clusters with ages that place them in the LMC ``age gap.'' Theseclusters-NGC 2155, SL 663, and NGC 2121-turn out to have [Fe/H] ~ -1.0and ages of ~4 Gyr. This puts them in the ``age gap'' between theintermediate-age LMC clusters, the oldest of which are ~2.5 Gyr old, andESO 121-SC03, which has an age of ~9 Gyr. The addition of these threeclusters to the LMC age-metallicity relation has reduced the discrepancybetween the age distribution of the LMC clusters and the field stars.Furthermore, it indicates that searches to find more clusters older than~2.5 Gyr in the LMC are crucial to a better understanding of its globalstar formation history. Based on observations with the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| A Search for Old Star Clusters in the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| NGC 1978 in the LMC: the cluster and surrounding field.
We present deep BVRI CCD photometry of the Large Magellanic Cloudcluster NGC 1978 and of fields immediately to the west. From the studyof the colour-magnitude diagram and the luminosity function we derivefor NGC 1978 an age of slightly more than 2Gyr and a metallicity of[Fe/H]=-0.4. NGC 1978 is, for a globular cluster, rather ellipticalwhich had been explained by a merger origin. No age structure inside NGC1978 was found, which limits this possibility to a merger of roughlycoeval subunits. In the field indications for a higher star formationrate at 1x10^8^ and 6x10^8^yr exist 8' west of the cluster, while nosuch phases can be found near the cluster. At ages older than 2Gyr thestar formation rate appears to have been the same in all analysedfields. On a broader scale, NGC 1978 seems to be one of the oldest ofthe LMC intermediate age clusters. A coherent redetermination of theages of the oldest clusters of this age group underlines a suddenenhancement of the cluster formation rate in the LMC about 2 Gyr ago.
| 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 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 Magellanic Clouds - Their evolution, structure and composition
Recent data related to the history of the Magellanic Clouds as galaxiesare described, and attempts to determine accurate distances to theMagellanic Clouds are discussed, with special attention given to thegeometry of the Magellanic Clouds and different methods of distancedeterminations. Consideration is given to the various star generationspresent in the Clouds (i.e., the oldest generation, of greater than 10Gyr; the intermediate-age generations, between 7 and 0.2 Gyr, and theyoungest generation, the formation of which started only about 50 Myrago) and to their occurrences in the LMC and SMC populations, as well asto the interstellar medium in the Magellanic Clouds. The structure ofthe Magellanic System, which comprises the Magellanic Clouds, theIntercloud Region, and the Magellanic Stream is described, withparticualr consideration given to the complex structure of the LMC andSMC and the kinematics of their populations.
| LMC clusters - Age calibration and age distribution revisited
The empirical age relation for star clusters in the Large MagellanicCloud presented by Elson and Fall (1985) are reexamined using ages basedonly on main-sequence turnoffs. The present sample includes 57 clusters,24 of which have color-magnitude diagrams published since 1985. The newcalibration is very similar to that found previously, and the scatter inthe relation corresponds to uncertainties of about a factor of 2 in age.The age distribution derived from the new calibration does not differsignificantly from that derived in earlier work. It is compared with agedistributions estimated by other authors for different samples ofclusters, and the results are discussed.
| 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.
| 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.
| M and S stars in LMC globular clusters
Spectroscopic observations of 16 oxygen-rich AGB stars in LMC clustersreveal 12 M and four S stars, all radial velocity members of the LMC.Twenty S stars are known in the intermediate age clusters of the LMC.These data, together with other information on the clusters, confirm anearlier finding that the M-S and S-C transitions occur at higherluminosity in the younger clusters. The correlation between age andmetal abundance of LMC clusters creates ambiguity but there is evidencethat metal abundance is important. The lifetime of the S star stage ofevolution is a substantial fraction of the life of a carbon staralthough a prediction that S stars will be more common relative to Cstars at higher metal content is in accordance with observation. Theabsence of pure S, SC or CS stars may be in conflict with the currentcalibration of the S and C spectral types in terms of C/O. Four old openclusters in the Galaxy were searched for S stars without success.
| M and S stars in the Magellanic Clouds
The present consideration of digital spectra for 46 red stars in theSmall Magellanic Cloud (SMC), as well as in globular clusters of bothMagellanic Clouds, has yielded identifications of eight K stars, 18 Mstars, 19 early S stars, and a foreground dwarf. K, M, or S types arefound in the SMC among stars with B-V values of about 2, and most of thenoncarbon stars brighter than M(bol) of -4.3 in the clusters are foundto be S stars which evidently represent an intermediate stage in themodification of atmospheric composition. Tentative systematic trendswith cluster age indicate that the M-S and S-C transitions occur athigher luminosity and lower surface temperature in the younger, moremassive stars.
| The kinematics of globular clusters in the Large Magellanic Cloud
Velocities for 35 globular clusters in the LMC have been combined withdata from other sources to yield velocities for a total of 59 clustersthat range in age from 100 million to 10 billion years. Clusters youngerthan one billion years are noted to have motions similar to the gas intheir vicinity and to share the rotation solution previously found onthe basis of H I velocity maps and H II region velocities. These youngclusters therefore constitute a flattened system having a lowline-of-sight velocity dispersion, consistent with that found inprevious kinematic and photometric studies. The older clusters are alsoflattened to a disk-like system, although both the systematic velocityand position angle of the line of nodes are significantly different forthese older clusters. The data presented also suggest that, unlike theMilky Way, there is no evidence for a kinematic halo population amongglobular clusters in the LMG.
| The age-metallicity relationship for the clusters of the Large Magellanic Cloud
Moderate dispersion spectrophotometric scans with an intensified Reticonarray have been obtained using the du Pont telescope of the Las CampanasObservatory for 38 stars expected to be members of 15 clusters in theLarge Magellanic Cloud. Ages for these clusters are deduced from atransformation of their classification in the scheme of Searle,Wilkinson, and Bagnuolo (1980). Abundances are derived from the scansusing a crude analysis applied to computer-generated pseudoequivalentwidths calibrated by identical observations of 42 stars in six galacticglobular clusters and by several galactic supergiants. A strongage-metallicity relationship is found and the chemical history of theLMC is discussed. Unlike the solar neighborhood or galactic halo, simplemodels of chemical evolution are adequate to fit the derivedage-abundance correlation.
| The late-type stellar content of Magellanic Cloud clusters
Visual spectrophotometry and new broad-band photometric data obtainedfor 48 late-type giants in clusters in the MC are combined withpublished data to predict various evolutionary schemes for cool,luminous, carbon and oxygen rich stars. The MC cluster C stars have arange in spectral energy distributions quite similar to that of MC fieldC stars, and the LMC field contains M giants, which are redder and moreluminous than any so far found in LMC or SMC clusters. This isattributed to the presence in the LMC field of a significant populationof stars that are younger and/or more metal rich than the stars in thecluster sample. The locations of the NGC 1841 stars in a C-M diagramappear to be anomalous in the sense that its brightest stars haveluminosities greater than the tips of giant branches of metal poorgalactic globular clusters.
| The extended giant branches of intermediate age globular clusters in the Magellanic Clouds
Vidicon spectra and infrared JHK photometry are presented for stars nearthe tip of the giant branch in a sample of red globular clusters in theMagellanic Clouds. The coverage extends considerably that of our earlierspectroscopic survey. Numerous carbon stars and some M stars are foundwhose luminosities place them on the upper asymptotic giant branch(i.e., above the luminosity of the helium flash). Effective temperaturesbased on J-K colors are given for all stars. A simplified theory ofasymptotic giant branch evolution is applied to calculate ages for thefull sample of clusters studied to date.
| Instrumental color-magnitude diagrams for 24 Large Magellanic Cloud star clusters
Abstract image available at:http://adsabs.harvard.edu/abs/1976ApJS...32..283H
| A Catalogue of Clusters in The LMC
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