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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.
|A double main-sequence turn-off in the rich star cluster NGC 1846 in the Large Magellanic Cloud|
We report on Hubble Space Telescope/ACS photometry of the richintermediate-age star cluster NGC 1846 in the Large Magellanic Cloud,which clearly reveals the presence of a double main-sequence turn-off inthis object. Despite this, the main-sequence, subgiant branch and redgiant branch are all narrow and well defined, and the red clump iscompact. We examine the spatial distribution of turn-off stars anddemonstrate that all belong to NGC 1846 rather than to any field starpopulation. In addition, the spatial distributions of the two sets ofturn-off stars may exhibit different central concentrations and someasymmetries. By fitting isochrones, we show that the properties of thecolour-magnitude diagram can be explained if there are two stellarpopulations of equivalent metal abundance in NGC 1846, differing in ageby ~300 Myr. The absolute ages of the two populations are ~1.9 and ~2.2Gyr, although there may be a systematic error of up to +/-0.4 Gyr inthese values. The metal abundance inferred from isochrone fitting is[M/H] ~ -0.40, consistent with spectroscopic measurements of [Fe/H]. Wepropose that the observed properties of NGC 1846 can be explained ifthis object originated via the tidal capture of two star clusters formedseparately in a star cluster group in a single giant molecular cloud.This scenario accounts naturally for the age difference and uniformmetallicity of the two member populations, as well as the differences intheir spatial distributions.
|The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters|
Using available data for C and M giants with M_bol<-3.6 in MagellanicCloud clusters, we derive limits to the lifetimes for the correspondingevolutionary phases, as a function of stellar mass. The C-star phase isfound to have a duration between 2 and 3 Myr for stars in the mass rangefrom ~1.5 to 2.8 M_ȯ. There is also an indication that the peak ofC-star lifetime shifts to lower masses (from slightly above to slightlybelow 2 Mȯ) as we move from LMC to SMC metallicities.The M-giant lifetimes also peak at ~2 Mȯ in the LMC,with a maximum value of about 4 Myr, whereas in the SMC their lifetimesappear much shorter, but, actually, they are poorly constrained by thedata. These numbers constitute useful constraints to theoretical modelsof the TP-AGB phase. We show that several models in the literatureunderestimate the duration of the C-star phase at LMC metallicities.
|Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters|
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
|A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters|
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
|Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds|
We present accurate integrated-light photometry in Johnson/Cousins V, Rand I for a sample of 28 globular clusters in the Magellanic Clouds. Themajority of the clusters in our sample have reliable age and metallicityestimates available in the literature. The sample encompasses agesbetween 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between -1.5 and0.0 dex. The sample is dominated by clusters of ages between roughly 0.5and 2 Gyr, an age range during which the bolometric luminosity of simplestellar populations is dominated by evolved red giant branch stars andthermally pulsing asymptotic giant branch (TP-AGB) stars whosetheoretical colours are rather uncertain. The VRI colours presented inthis paper have been used to calibrate stellar population synthesismodel predictions.
|Evolutionary population synthesis: models, analysis of the ingredients and application to high-z galaxies|
Evolutionary population synthesis models for a wide range ofmetallicities, ages, star formation histories, initial mass functionsand horizontal branch morphologies, including blue morphologies at highmetallicity, are computed. The model output comprises spectral energydistributions, colours, stellar M/L ratios, bolometric corrections andnear-infrared (IR) spectral line indices. The energetics of the postmain sequence evolutionary phases are evaluated with the fuelconsumption theorem. The impact on the models of the stellarevolutionary tracks (in particular with and without overshooting) isassessed. We find modest differences in synthetic broad-band colours asinduced by the use of different tracks in our code [e.g. Δ(V-K) ~0.08 mag, Δ(B-V) ~ 0.03 mag]. Noticeably, these differences aresubstantially smaller than the scatter among other models in theliterature, even when the latter adopt the same evolutionary tracks. Themodels are calibrated with globular cluster data from the Milky Way forold ages, and the Magellanic clouds plus the merger remnant galaxy NGC7252, both for young ages of ~0.1-2Gyr, in a large wavelength range fromthe U band to the K band. Particular emphasis is put on the contributionfrom the thermally pulsing asymptotic giant branch (TP-AGB) phase. Weshow that this evolutionary phase is crucial for the modelling of youngstellar populations by direct comparison with observed spectral energydistributions of Magellanic cloud clusters, which are characterized byrelatively high fluxes, both blueward and redward of the V band. We findthat the combination of the near-IR spectral indices C2 andH2O can be used to determine the metallicity of ~1 Gyrstellar populations. As an illustrative application, we re-analyse thespectral energy distributions of some of the high-z galaxies (2.4<~z<~ 2.9) observed with the Spitzer Space Telescope by Yan et al.Their high rest-frame near-IR fluxes is reproduced very well with themodels including TP-AGB stars for ages in the range ~0.6-1.5Gyr,suggesting formation redshifts for these objects around z~ 3-6.
|Evolutionary stellar population synthesis at high spectral resolution: optical wavelengths|
We present the single stellar population (SSP) synthesis results of ournew synthetic stellar atmosphere models library with a spectral samplingof 0.3 Å, covering the wavelength range from 3000 to 7000Åfor a wide range of metallicities (twice solar, solar, half solarand 1/10 solar). The stellar library is composed of 1650 spectracomputed with the latest improvements in stellar atmospheres. Inparticular, it incorporates non-local thermodynamic equilibrium (LTE)line-blanketed models for hot (Teff>= 27500 K), and LTEline-blanketed models (Phoenix) for cool (3000 <=Teff<=4500 K) stars. Because of the high spectral resolution of this library,evolutionary synthesis models can be used to predict the strength ofnumerous weak absorption lines and the evolution of the profiles of thestrongest lines over a wide range of ages. The SSP results have beencalculated for ages from 1 Myr to 17 Gyr using the stellar evolutionarytracks provided by the Geneva and Padova groups. For young stellarpopulations, our results have a very detailed coverage ofhigh-temperature stars with similar results for the Padova and Genevaisochrones. For intermediate and old stellar populations, our results,once degraded to a lower resolution, are similar to the ones obtained byother groups (limitations imposed by the stellar evolutionary physicsnotwidthstanding). The limitations and advantages of our models for theanalysis of integrated populations are described. The full set of thestellar library and the evolutionary models are available for retrievalat the websites http://www.iaa.csic.es/~rosa andhttp://www.iaa.csic.es/~mcs/sed@, or on request from the first twoauthors.
|ISOCAM Observations of Globular Clusters in the Magellanic Clouds: The Data|
Seventeen globular clusters in the Large and Small Magellanic Cloudswere observed in the mid-infrared wavelength region with the ISOCAMinstrument on board the Infrared Space Observatory (ISO). Observationswere made using the broadband filters LW1, LW2, and LW10, correspondingto the effective wavelengths of 4.5, 6.7, and 12 μm, respectively. Wepresent the photometry of point sources in each cluster, as well astheir precise positions and finding charts.Based on observations with ISO, an ESA project with instruments fundedby ESA Member states (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with participation of ISAS andNASA.
|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 Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer|
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
|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.
|Testing stellar population models with star clusters in the Large Magellanic Cloud|
We present high signal-to-noise ratio integrated spectra of 24 starclusters in the Large Magellanic Cloud (LMC), obtained using the FLAIRspectrograph at the UK Schmidt telescope. The spectra have been placedon to the Lick/IDS system in order to test the calibration of SimpleStellar Population (SSP) models. We have compared the SSP-predictedmetallicities of the clusters with those from the literature,predominantly taken from the Ca-triplet spectroscopy of Olszewski et al.(1991). We find that there is good agreement between the metallicitiesin the range -2.10 <=[Fe/H]<= 0. However, the Mg2 index(and to a lesser degree Mg b) systematically predict highermetallicities (up to +0.5 dex higher) than . Among thepossible explanations for this are that the LMC clusters possess[α/Fe] > 0. Metallicities are presented for eleven LMC clusterswhich have no previous measurements. We compare SSP ages for theclusters, derived from the Hβ, Hγ and Hδ Lick/IDSindices, with the available literature data, and find good agreement forthe vast majority. This includes six old globular clusters in oursample, which have ages consistent with their HST colour-magnitudediagram (CMD) ages and/or integrated colours. However, two globularclusters, NGC 1754 and NGC 2005, identified as old (~15 Gyr) on thebasis of HST CMDs, have Hβ line-strengths which lead ages that aretoo low (~8 and ~6 Gyr respectively). These findings are inconsistentwith their CMD-derived values at the 3σ level. Comparison betweenthe horizontal branch morphology and the Balmer line strengths of theseclusters suggests that the presence of blue horizontal branch stars hasincreased their Balmer indices by up to ~1.0 Å. We conclude thatthe Lick/IDS indices, used in conjunction with contemporary SSP models,are able to reproduce the ages and metallicities of the LMC clustersreassuringly well. The required extrapolations of the fitting functionsand stellar libraries in the models to lower ages and low metallicitiesdo not lead to serious systematic errors. However, owing to thesignificant contribution of horizontal branch stars to Balmer indices,SSP model ages derived for metal-poor globular clusters are ambiguouswithout a priori knowledge of horizontal branch morphology.
|The Evolved Red Stellar Content of M32|
Near-infrared images obtained with the Canada-France-Hawaii Telescope(CFHT) Adaptive Optics Bonnette (AOB) are used to investigate thestellar content of the Local Group compact elliptical galaxy M32.Observations of a field 2.3′ from the galaxy center reveal a largepopulation of asymptotic giant branch (AGB) stars, and comparisons withmodels indicate that these objects have an agelog(tGyr)<=9.3. The AGB population is very homogeneous,with Δlog(tGyr)<=+/-0.1 dex andΔ[M/H]<=+/-0.3 dex. The reddest AGB stars have J-K<=1.5, andit is suggested that the very red stars seen in earlier, less deep,surveys are the result of large photometric errors. The bolometric AGBluminosity function (LF) of this field is in excellent agreement withthat of the Galactic bulge. Based on the integrated brightness of AGBstars brighter than the red giant branch tip, which occurs at K=17.8, itis concluded that intermediate-age stars account for roughly 25% of thetotal K light and 10%+/-5% of the total mass in this field. A fieldclose to the center of M32 was also observed. The brightest stars withina few arcseconds of the nucleus have K=15.5, and the density of theseobjects is consistent with that predicted from the outer regions of thegalaxy after scaling according to surface brightness. Moreover, the Kluminosity function (LF) of bright sources between 20" and 30" of thenucleus is well matched by the LF of the outer regions of the galaxyafter accounting for differences in surface brightness and correctingfor the effects of crowding. It is concluded that the relative size ofthe intermediate-age component with respect to other populations doesnot change with radius over much of the galaxy. However, the integratedJ-K color and 2.3 μm CO index change with radius within a few tenthsof an arcsecond of the galaxy center, indicating that, contrary to whatmight be inferred from observations at visible wavelengths, theintegrated photometric properties of the central regions of M32 differfrom those of the surrounding galaxy.
|Magellanic Cloud Periphery Carbon Stars. IV. The SMC|
The kinematics of 150 carbon stars observed at moderate dispersion onthe periphery of the Small Magellanic Cloud are compared with themotions of neutral hydrogen and early-type stars in the intercloudregion. The distribution of radial velocities implies a configuration ofthese stars as a sheet inclined at 73°+/-4° to the plane of thesky. The near side, to the south, is dominated by a stellar component;to the north, the far side contains fewer carbon stars and is dominatedby the neutral gas. The upper velocity envelope of the stars is closelythe same as that of the gas. This configuration is shown to beconsistent with the known extension of the SMC along the line of sightand is attributed to a tidally induced disruption of the SMC thatoriginated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago.The dearth of gas on the near side of the sheet is attributed toablation processes akin to those inferred in 1996 by Weiner &Williams to collisional excitation of the leading edges of MagellanicStream clouds. Comparison with the 1989 kinematic data of Hardy,Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the1986 and 1988 data of Mathewson et al. leaves little doubt that forcesother than gravity play a role in the dynamics of the H I.
|The Optical Gravitational Lensing Experiment. Catalog of Star Clusters from the Large Magellanic Cloud|
We present the catalog of star clusters found in the area of about 5.8square degree in the central regions of the Large Magellanic Cloud. Itcontains data for 745 clusters. 126 of them are new objects. For eachcluster equatorial coordinates, radius, approximate number of membersand cross-identification are provided. Photometric data for all clusterspresented in the catalog and Atlas consisting of finding charts andcolor-magnitude diagrams are available electronically from the OGLEInternet archive.
|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.
|Discovery of intrared stars in globular clusters in the Magellanic Clouds and their light variations.|
|The evolution of theV-Kcolours of single stellar populations|
Models of evolutionary population synthesis of galaxies rely on theproperties of the so-called single stellar populations (SSP). In thispaper, we discuss how the integrated near-infrared colours, andespecially V-K, of SSPs evolve with age and metallicity. Some of theuncertainties associated with the properties of the underlying stellarmodels are thoroughly discussed. Our models include all the relevantstellar evolutionary phases, with particular attention being dedicatedto the asymptotic giant branch (AGB), which plays a fundamental role inthe evolution of the near-infrared part of the spectrum. First, wepresent the effects that different formulations for the mass-loss ratesproduce on the final remnant mass (i.e., on the initial-final massrelation), and hence on the AGB-termination luminosity and the relativecontribution of these stars to the integrated light. The results for theevolution of the V-K colour are very different depending on the choiceof the mass-loss prescription; the same is true also for the B-V colourin the case of low-metallicity SSPs. Secondly, we describe the changesoccurring in the integrated colours at the onset of the AGB and redgiant (RGB) branches. According to the classical formalism for the AGBevolution, the onset of this evolutionary phase is marked by a colourjump to the red, the amplitude of which is shown here to be highlydependent on the metallicity and mass-loss rates adopted in the models.We then consider the effect of the overluminosity with respect to thestandard core mass-luminosity relation that occurs in the most massiveAGB stars. Different simplified formulations for this effect are testedin the models; they cause a smoothing of the colour evolution in the agerange at which the AGB starts to develop, rather than a splitting of thecolour jump into two separate events. On the other hand, we find that atemporary red phase takes place ~1.5x10^8 yr after the RGB develops.Thanks to the transient nature of this feature, the onset of the RGB isprobably not able to cause marked features in the spectral evolution ofgalaxies. We then discuss the possible reasons for the transition of V-Kcolours (from ~1.5 to 3) that takes place in LMC clusters of SWB typeIV. A revision of the ages attributed to the single clusters revealsthat the transition may not be as fast as originally suggested. Thecomparison of the data with the models indicates that the transitionresults mainly from the development of the AGB. A gradual (or delayed)transition of the colours, as predicted by models which include theoverluminosity of the most massive AGB stars, seems to describe the databetter than the sudden colour jump predicted by classical models.
|A digital photometric survey of the magellanic clouds: First results from one million stars.|
We present the first results from, and a complete description of, ourongoing UBVI digital photometric survey of the Magellanic Clouds. Inparticular, we discuss the photometric quality and automated reductionof a CCD survey (magnitude limits, completeness, and astrometricaccuracy) that covers the central 8(deg) x 8(deg) of the LargeMagellanic Cloud (LMC) and 4(deg) x 4(deg) of the Small Magellanic Cloud(SMC). We discuss photometry of over 1 million stars from the initialsurvey observations (an area northwest of the LMC bar covering ~ 2(deg)x 1.5(deg) ) and present a deep stellar cluster catalog that containsabout 45% more clusters than previously identified within this region.Of the 68 clusters found, only 12 are also identified as concentrationsof ``old'', red clump stars. Furthermore, only three clusters areidentified solely on the basis of a concentration of red clump stars,rather than as a concentration of luminous (V < 21) main sequencestars. Extrapolating from the current data, we expect to obtain B and Vphotometry for 25 million stars, and U and I photometry for 10 and 20million stars, respectively, over the entire survey area.
|The ellipticities of Galactic and Large Magellanic Cloud globular clusters|
The correlations between the ellipticity and the age and mass of LMCglobular clusters are examined, and both are found to be weak. It isconcluded that neither of these properties is mainly responsible for theobserved differences in the LMC and Galactic globular clusterellipticity distributions. Most importantly, age cannot be the primaryfactor in the LMC-Galaxy ellipticity differences, even if there is arelationship, as even the oldest LMC clusters are more elliptical thantheir Galactic counterparts. The strength of the tidal field of theparent galaxy is proposed as the dominant factor in determining theellipticities of that galaxy's globular clusters. A strong tidal fieldrapidly destroys velocity anisotropies in initially triaxial, rapidlyrotating elliptical globular clusters. A weak tidal field, however, isunable to remove these anisotropies and the clusters remain close totheir initial shapes.
|Faint carbon stars from the evolution of close binaries.|
The assumption that faint carbon stars in the Magellanic Clouds are onthe early asymptotic giant branch (E-AGB) evolutionary stage, isexamined using population simulation techniques. It is assumed thatthese stars are formed as a result of the mass transfer in close binarysystems while the primary is a carbon star on the thermally-pulsing AGB(TP-AGB) stage. The populations of carbon stars resulting from bothsingle-star evolution and mass transfer in close binary systems havebeen calculated. For the heavy element abundance by mass Z=0.002, theexpected amount of E-AGB carbon stars is comparable with the amount ofthose in the TP-AGB stage. The theoretically obtained and observedluminosity functions of E-AGB carbon stars are similar. Examplesillustrating the importance of correct identification of star'sevolutionary stage for the interpretation of observations are given. Theignorance of the fact that AGB consists of two stages of the evolutionleads to wrong cluster ages resulting from the luminosities of AGBstars.
|Duration of the superwind phase of asymptotic giant branch stars|
Near the ends of their lives, low- and intermediate-mass stars gothrough a phase of evolution known as the asymptotic giantbranch1,2. This luminous red-giant phase is thought to beterminated by a period of intense mass loss in the form of asuperwind3, which leads to the formation of a planetarynebula. Although the effects of mass loss have been studied extensivelyin many stars, the duration of this phase is not well constrained,because of uncertainties in the distances, masses, ages, and absoluteluminosities of the observed stars. On the other hand, the properties ofstars in the globular clusters associated with the Magellanic Clouds arenot subject to these uncertainties, and so provide an excellentopportunity for studying mass-loss phenomena in a quantitative way. Herewe report the discovery of two infrared stars in Magellanic Cloudglobular clusters that are undergoing a period of intense mass loss.Those observations, together with those of a previously discoveredinfrared star, confirm that asymptotic giant branch stars go through asuperwind phase, and constrain the duration of this phase to be about100,000 years.
|Extreme Infrared Stars Discovered in Magellanic Cloud Globular Clusters|
We report preliminary results of our systematic survey for infraredstars in the globular clusters of the Magellanic Clouds. In the courseof an ISOCAM survey for AGB stars in the intermediate-age clusters, wehave discovered extremely red AGB stars in NGC 419 and NGC 1978. Fromtheir colours and luminosities, they are thought to be experiencingintense mass-loss and to be in the final or superwind phase of the AGBevolution. However, they seem to be of somewhat lower luminosity thanthe corresponding visible AGB stars when only the mid-infrared data aretaken into account. This suggests that hitherto unobserved infraredexcesses may exist at longer wavelengths.
|Carbon stars in LMC clusters revisited.|
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
|Planetary nebulae associated with star clusters in the LMC.|
The catalogues of LMC clusters and PN both based on surveys of UKSchmidt Telescope plates were searched to identify those PN that arefound near to cluster centres and could be genuine cluster members.Statistical tests were carried out to study chance superpositioneffects. For a number of clusters which were bright enough, spectralclassification of individual stars was carried out on objective-prismplates taken with the UK Schmidt Telescope, in order to trace M- andC-stars which are believed to be PN progenitors. There are 48 clusterswith PNs close enough to be actual cluster members. Various testssuggest that about half of these are expected to belong to the clusters.
|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.
|High-luminosity carbon stars in the early asymptotic giant branch phase|
There are high-luminosity carbon stars in the Large Magellanic Cloud(LMC) whose effective temperatures are well above those of ordinaryN-type stars. To elucidate the evolutionary stage of these stars, thepopulations of carbon stars formed as a result of both single-starevolution and mass transfer in close binary systems have beentheoretically modeled by the method of synthetic evolution. It is shownthat high-luminosity carbon stars in the LMC with effective temperaturesgreater than those of most of the LMC stars are in the early asymptoticgiant branch (AGB) phase, while most of the carbon stars withsignificantly lower effective temperatures are in the phase ofhelium-shell flashes. This conclusion is confirmed by the observation ofcarbon and S-type LMC stars in clusters where these stars are clearlyseparated into two groups according to their effective temperature. Itappears that such stars cannot be present in the Galaxy because of largeheavy-element abundances, intermediate-mass stars in the early AGB phasedo not reach high luminosities.
|Ultraviolet spectral evolution of star clusters in the IUE library.|
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
|Moment analysis applied to LMC star clusters|
Statistical moment-based ellipse fitting is performed on observations ofLarge Magellanic Cloud clusters, confirming that trends are evident intheir position angles and ellipticities, as had been reported in theliterature. Artificial cluster images with known parameters aregenerated, and subjected to the same analysis techniques, revealingapparent trends caused by stochastic processes. Caution should thereforebe exercised in the interpretation of observational trends in young LMCclusters.
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