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|Red Giant Stars in the Large Magellanic Cloud Clusters|
We present deep J, H, and Ks photometry and accurate colormagnitude diagrams down to K~18.5 for a sample of 13 globular clustersin the Large Magellanic Cloud. This data set combined with the previoussample of six clusters published by our group gives the opportunity tostudy the properties of giant stars in clusters with different ages(ranging from ~80 Myr up to 3.5 Gyr). Quantitative estimates of starpopulation ratios (by number and luminosity) in the asymptotic giantbranch (AGB), the red giant branch (RGB), and the He clump have beenobtained and compared with theoretical models in the framework ofprobing the so-called phase transitions. The AGB contribution to thetotal luminosity starts to be significant at ~200 Myr and reaches itsmaximum at 500-600 Myr, when the RGB phase transition is starting. At~900 Myr the full development of an extended and well-populated RGB hasbeen completed. The occurrences of both the AGB and RGB phasetransitions are sharp events, lasting a few hundred megayears only.These empirical results agree very well with the theoretical predictionsof simple stellar population models based on canonical tracks and thefuel-consumption approach.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m NTT, within the observing programs64.N-0038 and 68.D-0287.
|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.
|Probing the Red Giant Branch Phase Transition: Near-Infrared Photometry of Six Intermediate-Age Large Magellanic Cloud Clusters|
This is the first of a series of papers devoted to a global study of thephotometric properties of the red stellar sequences in a complete sampleof the Large Magellanic Cloud clusters, by means of near-infrared arrayphotometry. Deep J, H, Ks photometry and accuratecolor-magnitude diagrams down to K~18.5, i.e., ~1.5 mag below the red Heclump, for six intermediate-age clusters (namely, NGC 1987, NGC 2108,NGC 2190, NGC 2209, NGC 2231, NGC 2249) are presented. A quantitativeestimate of the population ratios (by number and luminosity) between redgiant branch (RGB) and He-clump stars for each target cluster isprovided and discussed in the framework of probing the so-called RGBphase transition (Ph-T). By using the Elson & Fall s-parameter as anage indicator, the observed RGB population shows a sharp enhancement (inboth number and luminosity) at s=36. Obviously, the correspondingabsolute age strictly depends on the details of theoretical modelsadopted to calibrate the s-parameter. Curiously, the currently availablecalibrations of the s-parameter in terms of age based on canonical (byElson & Fall) and overshooting (Girardi and coworkers) modelsprovide ages that well agree within 10%, suggesting that the fulldevelopment of the RGB occurs at t~700 Myr and is a relatively fastevent (δt~300 Myr). However, the RGB Ph-T epoch derived from theovershooting calibration of the s-parameter turns out to besignificantly earlier than the epoch provided by the recent evolutionarytracks by Girardi and coworkers. A new calibration of the s-parameterbased on high-quality color-magnitude diagrams and updated models isurged to address the origin of this discrepancy and finally establishthe epoch of the RGB Ph-T.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m New Technology Telescope, withinthe observing programs 64.N-0038 and 68.D-0287.
|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.
|A secondary clump of red giant stars: why and where|
Based on the results of detailed population synthesis models, Girardi etal. recently claimed that the clump of red giants in thecolour-magnitude diagram (CMD) of composite stellar populations shouldpresent an extension to lower luminosities, which goes down to about0.4mag below the main clump. This feature is made of stars just massiveenough to have ignited helium in non-degenerate conditions, andtherefore corresponds to a limited interval of stellar masses and ages.In the present models, which include moderate convective overshooting,it corresponds to ~1Gyr old populations. In this paper, we go into moredetail about the origin and properties of this feature. We first comparethe clump theoretical models with data for clusters of different agesand metallicities, basically confirming the predicted behaviour. We thenrefine the previous models in order to show the following behaviour. (i)The faint extension is expected to be clearly separated from the mainclump in the CMD of metal-rich populations, defining a `secondary clump'by itself. (ii) It should be present in all galactic fields containing~1Gyr old stars and with mean metallicities higher than about Z=0.004.(iii) It should be particularly strong, if compared with the main redclump, in galaxies that have increased their star formation rate in thelast Gyr or so of their evolution. In fact, secondary clumps similar tothe model predictions are observed in the CMD of nearby stars fromHipparcos data, and in those of some Large Magellanic Cloud fieldsobserved to date. There are also several reasons why this secondaryclump may be missing or hidden in other observed CMDs of galaxy fields.For instance, it becomes indistinguishable from the main clump if thephotometric errors or differential absorption are larger than about0.2mag. None the less, this structure may provide important constraintson the star formation history of Local Group galaxies. We comment alsoon the intrinsic luminosity variation and dispersion of clump stars,which may limit their use as either absolute or relative distanceindicators, respectively.
|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
|Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud|
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
|Age distribution of LMC clusters from their integrated UBV colors: history of star formation.|
In this paper we revise the relationship between ages and metallicitiesof LMC star clusters and their integrated UBV colors. The study standson the catalog of UBV colors of the Large Magellanic Cloud (LMC)clusters by Bica et al. (1994; BCDSP) and the photometric models ofsingle stellar populations (SSP) calculated by Bertelli et al. (1994).These photometric models nicely describe the color distribution of LMCclusters in the (U-B) vs. (B-V) plane together with the observeddispersion of the colors and the existence of a gap in a certain regionof this diagram. In the case of blue clusters, most of the dispersion inthe colors can be accounted for by the presence of stochastic effects onthe mass distribution of stars, whereas for the red ones additionaldispersion's of ~0.2dex in metallicity and of ~0.05mag in color excessare needed. From comparing the observed distribution of integratedcolors in the (U-B) vs. (B-V) diagram with the theoretical models, itturns out that: 1) The data are consistent with the presence of a gap(period of quiescence) in the history of cluster formation. If theage-metallicity relation (AMR) for the LMC obeys the simple model ofchemical evolution, the gap is well evident and corresponds to the ageinterval ~3Gyr to (12-15)Gyr. On the contrary, if the chemicalenrichment has been much slower than in the simple model, so thatintermediate age clusters are less metal rich, the gap is expected tooccur over a much narrower color range and to be hidden by effects ofcolor dispersion. 2) The bimodal distribution of B-V colors can bereproduced by a sequence of clusters almost evenly distributed in thelogarithm of the age, whose metallicity is governed by a normal AMR. Noneed is found of the so-called phase transitions in the integratedcolors of a cluster taking place at suitable ages (Renzini & Buzzoni1986). 3) The gap noticed by BCDSP in the (U-B) vs. (B-V) plane can beexplained by the particular direction along which cluster colors aredispersed in that part of the (U-B) vs. (B-V) diagram. Also in thiscase, no sudden changes in the integrated properties of clusters must beinvoked. The results of this analysis are used to revise the empiricalmethod proposed by Elson & Fall (1985, EF85) to attribute ages toLMC clusters according to their integrated UBV colors. We show that theEF85 method does not provide the correct relation between ages andcolors for clusters of low metallicity and hence its inability to datethe old clusters. We propose two modifications to the definition of theparameter S of EF85 such that the age sequence of red clusters issuitably described, and the intrinsic errors on ages caused by the heavypresence of various effects dispersing the colors are reduced to aminimum. The age sequence is calibrated on 24 template clusters forwhich ages were independently derived from recent color-magnitudediagrams (CMD). Finally, we attribute ages to all clusters present inBCDSP catalog, and derive the global age distribution function (ADF) forLMC clusters. The ADF presents new features that were not clear inprevious analyses of UBV data, but were already suggested by a number ofindependent observational studies. The features in question are periodsof enhanced cluster formation at ~100Myr and 1-2Gyr, and a gap in thecluster formation history between ~3 and (12-15)Gyr. The peaks observedin the distribution of B-V colors are found to be sensitive to thepresence of these periods of enhanced cluster formation and the lack ofextremely red clusters caused by the age gap between intermediate-ageand old clusters.
|Globular clusters in the Magellanic Clouds - II. IR-array photometry for 12 globular clusters and contributions to the integrated cluster light|
We report JHK results of observations of 12 globular clusters in theLarge Magellanic Cloud (LMC), and present colour-magnitude diagrams downto K=16 (corresponding to M_K~-2.6) for ~450 stars in these clusters. Wemerge our data with BV photometry for 11 LMC clusters, previouslypublished in Paper I of this series, and use the merged data to studythe evolution of integrated magnitudes and colours of simple stellarpopulations (SSPs), which are samples of coeval and chemicallyhomogeneous stars. In particular, we examine the effect of phasetransitions (ph-ts), which signal the appearance of the RGB or AGB inSSPs of increasing age. We find that the AGB contributes ~60 per cent ofthe integrated cluster light at K, while the contribution from thebright RGB stars (i.e., K_0<14.3, log L/L_~2.66) is correlated withthe s parameter (Elson & Fall) ranging from ~0 per cent for s=0 upto ~20 per cent for s>35. The age at which the RGB ph-t actuallytakes place (i.e., the calibration of s with age) depends on the detailsof stellar evolutionary models. In 'classical' models (those withoutovershooting), the RGB ph-t occurs at ~(6+/-2)x10^8 yr and lasts for2.9x10^8 yr. In models with overshooting, the occurrence of the RGB ph-tis later [at ~(1.5+/-0.3)x10^9 yr] and the duration is longer (4.3x10^8yr). While the age and duration of the RGB ph-t depend on the treatmentof mixing, both classical and overshooting models yield the samefractional contribution of RGB stars to the integrated cluster lightbefore and after the RGB ph-t, in agreement with the Fuel ComsumptionTheorem (Renzini & Buzzoni). We report extensive experiments whichshow that the variations of the integrated colours of the LMC clustersfrom s=31 to 43 are controlled by the complex interplay of variousfactors, different from colour to colour and frequently dominated by thestochastic noise induced by a few very bright objects. The overallpicture that emerges is consistent with the early conclusions drawn byPersson et al. and Frogel et al. that the J-K colour is mostly driven bythe AGB stars, that V-K is substantially controlled by AGB and RGB stars(AGB stars being slightly more important), and that B-Vis partiallyinfluenced by the whole population of red stars brighter than the bulkof the RGB clump, but is also quite strongly dependent on theprogressive fading and reddening of the turn-off stars due to ageincrease.
|Globular clusters in the Magellanic Clouds - I. BV CCD photometry for 11 clusters.|
Abstract image available at:http://adsabs.harvard.edu/abs/1994MNRAS.271..385C
|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).
|CCD BVR photometry of the open cluster IC 1311|
Six CCD frames for two fields in the area of the rich, presumably old,and unstudied open cluster IC 1311 have been obtained in the BVR bands.The photometric analysis leads to an estimated age of between 1 Gyr(standard models) and 2 Gyr (overshooting models), placing the clusterat a distance of about 4.8 kpc from the sun. Comparison of theobservational color-magnitude diagram (CMD) with model isochronesproduces a good fit for a metallicity of Z = 0.006-0.008. Somemorphological features displayed by the CMD of this cluster are betterunderstood in terms of convective overshooting rather than by classicaltheories. The location of IC 1311 on the age-metallicity diagramsuggests an ecological environment for its formation more similar tothat found in the LMC than in the solar vicinity.
|CCD and IR photometry of intermediate-age Magellanic Clouds clusters|
The clusters of the Magellanic Clouds (MCs) are studied with a completedataset for the intermediate-age clusters to determine the integratedcolors and spectral energy distribution of the distant galaxies.Observations of a wide sample of MC clusters are conducted in differentspectral bands with attention given to the range in which observedintegrated color variations were most evident. The CCD and IRphotometric data are reduced, and color-magnitude diagrams are given for9 clusters in V and (B-V) and for 9 clusters in the the IR bands. Theobservational data yield important clues regarding the extension andlifetime increase of the RGB and the evolutionary status of theclusters. The RGB evolutionary phase transition and an increase in thenumber of evolved giant stars are found within the age range where theMC clusters show a color change.
|The evolution of carbon stars in the Magellanic Clouds|
This study presents JHK photometric data for over 100 field stars in theSMC and for 10 in the Large Cloud together with spectroscopic resultsfor about half of them. In the Small Cloud carbon stars were found athigher temperatures and lower luminosities than previously observed. Thefaintest are below the top of the red giant branch. The medium- andlow-luminosity C stars in the M-C transition zone have a low C2 content.At these luminosities, most of the J-type stars are found close to theC2-poor stars in the HR diagram. Their C2 content is about as high as inthe coolest, most evolved C stars. The present observations of carbonstars in the SMC show that they cover a range in M(bo) from -3 to 5.9mag. The transitions from M to C via S appear to occur in both Clouds ata rather well-defined range in M(bol) for SWB and classes IV and V.
|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 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.
|On precise ZAMSs, the solar color, and pre-main-sequence lithium depletion|
This paper describes a semiempirical main-sequence-fitting method forthe determination of distances to stellar systems, which uses a ZAMSlocus carefully normalized to the sun, and whose shape is defined by aquartic over the color range for (B-V)0 values between 0.2 and 1.0 suchthat the morphology of the Pleiades C-M diagram is accuratelyreproduced. Using this technique, distances were derived for a number ofstar clusters. It was found that the observed depletion of lithium amongcool main-sequence stars in the Hyades and Pleiades can be matched quitewell by the present models. Calculations also show that the depletion ofLi at a fixed T(eff) along the main sequence is a sensitive function ofFe/H.
|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).
|Cepheids as distance indicators|
The current status of astronomical distance estimation based onobservations of Cepheids is surveyed. Consideration is given to theproblems of reddening and absorption correction, the forms of the BVIperiod-luminosity (PL) and period-luminosity-color (PLC) relations,abundance effects, the IR PL/PLC relations, PL/PLC relations at otherwavelengths, shortcut methods, PL/PLC zero points, long-period Cepheids,and Cepheid distances to galaxies. Diagrams, graphs, and extensivetables of numerical data are provided.
|E2 - an intermediate-age LMC cluster|
A color-magnitude diagram is presented for the faint star cluster E2,located near the tip of the Magellanic Stream. The main-sequenceturnoff, a few giants, and a giant clump can be discerned. Comparisonswith VandenBerg models show E2 to be a 1.5 Gyr old cluster withabundance Z of about 0.01 at the distance of the LMC, and thus similarto the more populous intermediate-age LMC clusters. The existence ofcluster formation at a large distance from the LMC center (E2 is atalmost 8 deg radius) is discussed in the context of the star-formationhistory, structure, and kinematics of the outer 'halo' of the LargeMagellanic Cloud.
|The distance modulus of the Large Magellanic Cloud|
The ages of clusters NGC 2162 and NGC 2190 and the distance modulus ofthe LMC are determined using the deep CCD photometric data of Schommeret al. (1984) and intermediate-mass stellar models with convectiveovershooting (Bertelli et al., 1985). The choice of reddening,metallicity (Z), and H content (X) is discussed. For Z = 0.020 and X =0.700, the cluster ages and distance modulus are found to be about 1 Gyrand 18.4 mag, respectively; for solar Z, the modulus is about 18.6 mag.
|Magellanic Cloud star clusters: The problems of age determination, metallicity - Age relationship and AGB star luminosity function|
Abstract image available at:http://adsabs.harvard.edu/abs/1986A&A...165...84C
|The luminosity and kinematics of RR Lyraes. II - RR Lyraes and the distance to the LMC|
Statistical parallaxes derived from maximum-likelihood analyses ofvariables in the LMC are used to re-evaluate previous estimates ofdistance to the LMC, which differ by up to 0.6 mag. The calculations areperformed using the proper motions and radial velocities of thevariables. RR Lyraes in the SMC are determined to be 53-62 kpc away.When combined with distance estimates for Cepheids, clustermain-sequence stars and Mira variables in the LMC, the distance to theLMC is estimated to be 45-49 kpc. A Hubble constant of approximately 90km/sc per MPc is also derived.
|The evolution of asymptotic giant branch stars in the Large Magellanic Cloud. II - Spectroscopy of a complete sample|
Spectra of 113 asymptotic giant branch (AGB) star candidatesconstituting six magnitude-limited, area-complete samples in the outerregions of the northern LMC have been obtained. Luminosity functionsconstructed from these data are well represented by an underlyingintermediate-mass AGB population, the product of continuous starformation over the last 3-4 Gyr, supplemented in some areas by moremassive red giant and supergiant stars, with a typical age of about 10to the 8th yr. Most stars with Mbol between -5 and -5.5 show evidencefor dredge-up of s-process elements, as do a smaller proportion in therange Mbol between -4.5 and -5. Only 10 percent of the sample, however,are C stars, and these are concentrated toward the Bar of the LMC. Noevidence has been found for envelope burning in any of the stars in thesample.
|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.
|Color Magnitude Diagrams of LMC Clusters|
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