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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.
|On the Iron Content of NGC 1978 in the LMC: A Metal-rich, Chemically Homogeneous Cluster|
We present a detailed abundance analysis of giant stars in NGC 1978, amassive, intermediate-age stellar cluster in the Large Magellanic Cloud,characterized by a high ellipticity and suspected to have a metallicityspread. We analyzed 11 giants, all cluster members, by usinghigh-resolution spectra acquired with UVES/FLAMES at the ESO Very LargeTelescope. We find an iron content of [Fe/H] = -0.38 dex with very lowσ[Fe/H]=0.07 dex dispersion, a mean heliocentric radialvelocity vr=293.1+/-0.9 km s-1, and a velocitydispersion σvr=3.1 km s-1, thusexcluding the presence of a significant metallicity, as well asvelocity, spread within the cluster.Based on observations collected at the Very Large Telescope of theEuropean Southern Observatory (ESO), Cerro Paranal, Chile, underprograms 072.D-0342 and 074.D-0369.
|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.
|Chemical Abundances in 12 Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy|
High-resolution infrared spectra (λ/Δλ=50,000) havebeen obtained for 12 red giant members of the Large Magellanic Cloud(LMC) with the Gemini South 8.3 m telescope and Phoenix spectrometer.Two wavelength regions, at 15540 and 23400 Å, were observed.Quantitative chemical abundances of carbon (both 12C and13C), nitrogen, and oxygen were derived from molecular linesof CO, CN, and OH, while sodium, scandium, titanium, and iron abundanceswere obtained from neutral atomic lines. The 12 LMC red giants span ametallicity range from [Fe/H]=-1.1 to [Fe/H]=-0.3. It is found thatvalues for both [Na/Fe] and [Ti/Fe] in the LMC giants fall below theircorresponding Galactic values (at these same [Fe/H] abundances) by about~0.1-0.5 dex; this effect is similar to abundance patterns found in thefew dwarf spheroidal galaxies with published abundances. The program redgiants all show evidence of first dredge-up mixing of material exposedto the CN cycle, that is, low 12C/13C ratios andlower 12C with higher 14N abundances. The carbonand nitrogen trends are similar to what is observed in samples ofGalactic red giants, although the LMC red giants seem to show smaller12C/13C ratios for a given stellar mass. Thisrelatively small difference in the carbon isotope ratios between LMC andGalactic red giants could be due to increased extra mixing in stars oflower metallicity, as suggested previously in the literature.Comparisons of the oxygen-to-iron ratios in the LMC and the Galaxyindicate that the trend of [O/Fe] versus [Fe/H] in the LMC falls about0.2 dex below the Galactic trend. Such an offset can be modeled as dueto an overall lower rate of supernovae per unit mass in the LMC relativeto the Galaxy, as well as a slightly lower ratio of supernovae of TypeII to supernovae of Type Ia. Based on observations obtained at theGemini Observatory, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under a cooperative agreement with theNSF on behalf of the Gemini partnership: the National Science Foundation(US), the Particle Physics and Astronomy Research Council (UK), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
|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 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.
|A Search for Old Star Clusters in the Large Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
|Research programs at the Carter Observatory: an overview|
An outline is given of the main research programmes currently inprogress at the Carter Observatory. These include the establishment of aset of standard star magnitudes and colours in the Vilnius seven colourphotometric system; the study of galactic and extragalactic starclusters using Vilnius and broadband photometries; binary stars and thedevelopment of APTs; and the history of Australasian astronomy. The roleof Carter Observatory Honorary Research Associates is described andmention is made of the joint New Zealand/Japan programme to observegravitational micro-lensing effects, discover variable stars and patrolselected clusters of galaxies for supernovae.
|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 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.
|Star clusters in the clouds of Magellan|
Star clusters in the LMC and SMC are compared with those in the Galaxy.It is shown that Cloud clusters have radii about three times larger thanthose of comparable Galactic clusters, and it is speculated that thisdifference may be due to the fact that Cloud clusters formed in a lowerdensity environment than Galactic clusters. Young clusters in the Cloudsseem to remain embedded in the remnants of associations for extendedperiods because tidal forces in the Clouds are weaker than they are inthe Galaxy. Luminous clusters of all ages in the Clouds are on averagesignificantly flatter than Galactic clusters. The LMC is surrounded byat least seven real globular cluster with ages greater than 10 Gyrwhereas the SMC has only one such companion. These clusters haveluminosities and mass-to-light ratios similar to those of Galacticclusters. In both Clouds old clusters are more widely distributed thanare younger clusters.
|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 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.
|A catalog of LMC star clusters outside the Hodge-Wright atlas|
The paper presents a catalog of 156 clusters outside the boundaries ofthe Hodge and Wright (1967) LMC atlas. The catalog contains coordinatesaccurate to 1-2 arcsec, offsets from the edge of the appropriate SRCJplates, cross references to previous identifications, and finding chartsof the brighter clusters. As defined by the clusters, the Hodge andWright atlas is found to represent the extent of the LMC to the west,and reasonably well to the east. To the north and the south, the clustersystem extends substantially beyond the boundaries of the atlas. Thesouthern clusters delineate a portion of the 'spiral arm' noted by deVaucouleurs (1955).
|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.
|Some studies of Magellanic Cloud clusters|
Four photoelectric sequences in the UBV system for areas in the Largeand Small Magellanic Clouds are presented. The stars range in brightnessfrom V = 8.33 to V = 19.42 and in color from (B-V) = 0.08 to (B-V) = +1.80. Photographic color-magnitude diagrams for seven clusters in theClouds are derived from CTIO 4-m telescope plates.
|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.
|SIT Vidicon photometry for four old Magellanic Cloud clusters|
SIT Vidicon photometry in B and V from the CTIO 4-m telescope ispresented for NGC 1466, NGC 2203, NGC 2210, and NGC 2257 in the LargeMagellanic Cloud. The deepest data are for NGC 2257, where the mainsequence turnoff is reached at V = 22.4, confirming Stryker's (1983)inference from photographic data that the cluster has an age comparableto those of the Galactic globular clusters. NGC 2203 is found to be anintermediate-age cluster with a C-M diagram similar to that of NGC 7789in the Galaxy. For NGC 1466 and NGC 2210 the present data are compatiblewith previous studies that have found these to be genuinely old clustersin the Galactic globular cluster sense.
|A Catalogue of Clusters in The LMC|
|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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