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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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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 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.
| 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 extended giant branches of intermediate age globular clusters in the Magellanic Clouds. II
In order to obtain a complete sample of upper asymptotic giant branch(AGB) stars in the red globular clusters of the Magellanic Clouds, aphotographic near-infrared survey of the clusters was conducted. Theresults are compared with previous photometry and the problem of errorarising from variability of carbon stars is addressed. Stars withoutspectra are tentatively classified based on their JHK colors. Apparentand absolute bolometric magnitudes and effective temperatures werecalculated from the IR colors, allowing for the location of the redstars and of the cluster giant branches in the physical H-R diagram tobe determined. Stellar evolution on the AGB is discussed, leading toimproved estimates of the extent of the upper AGB. A carbon star censusis presented and the ages of the clusters is derived with suitablycomplete photometry. On this basis, the chemical enrichment history ofthe Clouds is discussed.
| 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.
| 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
Not Available
| A catalogue of clusters in the outer parts of the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/abs/1963MNRAS.127...31L
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