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Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
| The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90
We present the results of our investigation of the intermediate-age starcluster BS 90, located in the vicinity of the H II region N66 in theSMC, observed with HST ACS. The high-resolution data provide a uniqueopportunity for a very detailed photometric study performed on one ofthe rare intermediate-age rich SMC clusters. The complete set ofobservations is centered on the association NGC 346 and contains almost100,000 stars down to V~=28 mag. In this study we focus on the northernpart of the region, which covers almost the whole stellar content of BS90. We construct its stellar surface density profile and derivestructural parameters. Isochrone fits on the CMD of the cluster resultsin an age of about 4.5 Gyr. The luminosity function is constructed andthe present-day mass function of BS 90 has been obtained using themass-luminosity relation, derived from the isochrone models. We found aslope between -1.30 and -0.95, comparable to or somewhat shallower thana typical Salpeter IMF. Examination of the radial dependence of the massfunction shows a steeper slope at larger radial distances, indicatingmass segregation in the cluster. The derived half-mass relaxation timeof 0.95 Gyr suggests that the cluster is mass segregated due to itsdynamical evolution. From the isochrone model fits we derive ametallicity for BS 90 of [Fe/H]=-0.72, which adds an important point tothe age-metallicity relation of the SMC. We discuss our findings on thisrelation in comparison to other SMC clusters.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
| 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.
| A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| The Star Clusters of the Small Magellanic Cloud: Age Distribution
We present age measurements for 195 star clusters in the SmallMagellanic Cloud based on comparison of integrated colors measured fromthe Magellanic Clouds Photometric Survey with models of simple stellarpopulations. We find that the modeled nonuniform changes of clustercolors with age can lead to spurious age peaks in the cluster agedistribution; that the observed numbers of clusters with age t declinessmoothly as t-2.1 that for an assumed initial cluster massfunction scaling as M-2, the dependence of the clusterdisruption time on mass is proportional to M0.48; thatdespite the apparent abundance of young clusters, the dominant epoch ofcluster formation was the initial one; and that there are significantdifferences in the spatial distribution of clusters of different ages.Because of limited precision in our age measurements, we cannot addressthe question of detailed correspondence between the cluster age functionand the field star formation history. However, this sample provides aninitial guide as to which clusters to target in more detailed studies ofspecific age intervals.
| Classical Cepheid Pulsation Models. X. The Period-Age Relation
We present new period-age (PA) and period-age-color (PAC) relations forfundamental and first-overtone classical Cepheids. Current predictionsrely on homogeneous sets of evolutionary and pulsation models covering abroad range of stellar masses and chemical compositions. We found thatPA and PAC relations present a mild dependence on metal content.Moreover, the use of different PA and PAC relations for fundamental andfirst-overtone Cepheids improves the accuracy of age estimates in theshort-period (logP<1) range (old Cepheids), because they presentsmaller intrinsic dispersions. At the same time, the use of the PACrelations improves the accuracy in the long-period (logP>=1) range(young Cepheids), since they account for the position of individualobjects inside the instability strip. We performed a detailed comparisonbetween evolutionary and pulsation ages for a sizable sample of LMC (15)and SMC (12) clusters which host at least two Cepheids. In order toavoid deceptive uncertainties in the photometric absolute zero point, weadopted the homogeneous set of B, V, and I data for clusters andCepheids collected by OGLE. We also adopted the same reddening scale.The different age estimates agree at the level of 20% for LMC clustersand of 10% for SMC clusters. We also performed the same comparison fortwo Galactic clusters (NGC 6067, NGC 7790), and the difference in age issmaller than 20%. These findings support the use of PA and PAC relationsto supply accurate estimates of individual stellar ages in the Galaxyand in external Galaxies. The main advantage of this approach is itsindependence from the distance.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| 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.
| Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects
The properties of ~939 star clusters in the Large and Small MagellanicClouds were determined from ground-based CCD images in UBVR passbands.The areal coverage was extensive, corresponding to 11.0 kpc2in the LMC and 8.3 kpc2 in the SMC. After corrections forreddening, the colors and magnitudes of the clusters were converted toages and masses, and the resulting mass distributions were searched forthe effects of fading, evaporation, and size-of-sample bias. The datashow a clear signature of cluster fading below the detection threshold.The initial cluster mass function (ICMF) was determined by fitting themass and age distributions with cluster population models. These modelssuggest a new method to determine the ICMF that is nearly independent offading or disruption and is based on the slope of a correlation betweenage and the maximum cluster mass in equally spaced intervals of log age.For a nearly uniform star formation rate, this correlation has a slopeequal to 1/(α-1) for an ICMF of dn(M)/dM~M-α. Wedetermine that α is between 2 and 2.4 for the LMC and SMC usingthis method plus another method in which models are fitted to the massdistribution integrated over age and to the age distribution integratedover mass. The maximum mass method also suggests that the clusterformation rate in the LMC age gap between 3 and 13 Gyr is about a factorof 10 below that in the period from 0.1 to 1 Gyr. The oldest clusterscorrespond in age and mass to halo globular clusters in the Milky Way.They do not fit the trends for lower mass clusters but appear to be aseparate population that either had a very high star formation rate andbecame depleted by evaporation or formed with only high masses.
| 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.
| The faint Cepheids of the Small Magellanic Cloud: An evolutionary selection effect?
Two problems concerning the faintest Small Magellanic Cloud (SMC)Cepheids are addressed. On the one hand evolutionary tracks fail tocross the Cepheid Instability Strip for the highest magnitudes (i.e.I-mag ~ 17) where Cepheids are observed; mass-luminosity relations(ML) obtained from evolutionary tracks disagree with mass-luminosityrelations derived from observations. We find that the above failuresconcern models built with standard input physics as well as withnon-standard ones. The present work suggests that towards highestmagnitudes, Cepheids stars undergo a selection effect caused byevolution: only the most metal poor stars cross the Instability Stripduring the ``blue loop'' phase and are therefore the only ones that canbe observed at low luminosity. This solution enables us to reproduce theshape of the lower part of the Instability Strip and improves theagreement between observed and theoretical ML-relations. Some issues arediscussed, among them Beat Cepheid results that argue strongly in favorof our hypothesis.
| Surface brightness profiles and structural parameters for 10 rich stellar clusters in the Small Magellanic Cloud
As a follow-up to our recent study of a large sample of Large MagellanicCloud (LMC) clusters, we have conducted a similar study of thestructures of 10 Small Magellanic Cloud (SMC) clusters, using archivalHubble Space Telescope snapshot data. We present surface brightnessprofiles for each cluster and derive structural parameters, includingcore radii and luminosity and mass estimates, using exactly the sameprocedure as for the LMC sample. Because of the small sample size, theSMC results are not as detailed as for the larger LMC sample. We do notobserve any post-core-collapse clusters (although we did not expect to),and there is little evidence for any double clusters in our sample. Nonethe less, despite the small sample size, we show for the first time thatthe SMC clusters follow almost exactly the same trend in core radiuswith age observed for the LMC system, including the apparent bifurcationat several hundred Myr. This further strengthens our argument that thisrelationship represents true physical evolution in these clusters, withsome developing significantly expanded cores due to an as yetunidentified physical process. Additional data, both observational andfrom N-body simulations, are still required to clarify many issues.
| The Optical Gravitational Lensing Experiment. Catalog of RRLyr Stars from the Small Magellanic Cloud
We present the catalog of RRLyrae stars from 2.4 square degrees ofcentral parts of the Small Magellanic Cloud (SMC). The photometric datawere collected during four years of the OGLE-II microlensing survey.Photometry of each star was obtained using the Difference Image Analysis(DIA) method. The catalog contains 571 objects, including 458RRab, 56RRcvariables, and 57 double mode RRLyr stars (RRd). Additionally we attacha list of variables with periods between 0.18-0.26 days, which areprobably delta Sct stars. Period, BVI photometry, astrometry, amplitude,and parameters of the Fourier decomposition of the I-band light curveare provided for each object. We also present the Petersen diagram fordouble mode pulsators.We found that the SMC RRLyr stars are fairly uniformly distributed overthe studied area, with no clear correlation to any location. The mostpreferred periods for RRab and RRc stars are 0.589 and 0.357 days,respectively. We noticed significant excess of stars with periods ofabout 0.29 days, which might be second-overtone RRLyr stars (RRe). Themean extinction free magnitudes derived for RRab stars are 18.97, 19.45and 19.73 mag for the I, V and B-band, respectively.All presented data, including individual BVI observations, are availablefrom the OGLE Internet archive.
| Dating star clusters in the Small Magellanic Cloud by means of integrated spectra
In this study flux-calibrated integrated spectra in the range(3600-6800) Å are presented for 16 concentrated star clusters inthe Small Magellanic Cloud (SMC), approximately half of which constituteunstudied objects. We have estimated ages and foreground interstellarreddening values from the comparison of the line strengths and continuumdistribution of the cluster spectra with those of template clusterspectra with known parameters. Most of the sample clusters are youngblue clusters (6-50 Myr), while L 28, NGC 643 and L 114 are found to beintermediate-age clusters (1-6 Gyr). One well known SMC cluster (NGC416) was observed for comparison purposes. The sample includes clustersin the surroundings and main body of the SMC, and the derived foregroundreddening values are in the range 0.00 <= E(B-V) <= 0.15. Thepresent data also make up a cluster spectral library at SMC metallicity.Based on observations made at Complejo Astronómico El Leoncito,which is operated under agreement between the Consejo Nacional deInvestigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba and San Juan, Argentina.
| An Upper Limit to the Age of the Galactic Bar
Using data from the Two Micron All Sky Survey, we identify a populationof infrared carbon stars with J-KS>=2 in the Milky Way.These stars are shown to trace the stellar bar previously identified inIR and optical surveys. The properties of C stars strongly suggest thatthey are of intermediate age. We conclude that the bar is likely to haveformed more recently than 3 Gyr ago and must be younger than 6 Gyr.Implications and further tests of this conclusion are briefly discussed.
| Constraining the LMC cluster age gap: Washington photometry of NGC 2155 and SL 896 (LW 480)
We carried out Washington system photometry of the intermediate-ageLarge Magellanic Cloud (LMC) star clusters NGC2155 and SL896 (LW480). Wederive ages and metallicities from the T1 versusC-T1 colour-magnitude diagrams (CMDs). For the first time anage has been obtained for SL896, 2.3+/-0.5Gyr. For NGC2155 we derive3.6+/-0.7Gyr. The two clusters basically define the lower age limit ofthe LMC age gap. In particular, NGC2155 is confirmed as the oldestintermediate-age LMC cluster so far studied. The derived metallicitiesare [Fe/H]=-0.9+/-0.2 and -0.6+/-0.2 for NGC2155 and SL896,respectively. We also studied the CMDs of the surrounding fields, whichhave a dominant turn-off comparable to that of the clusters themselves,and similar metallicity, showing that one is dealing with anintermediate-age disc where clusters and field stars have the sameorigin. We inserted the present clusters in the LMC and Small MagellanicCloud (SMC) age-metallicity relations, using a set of homogeneousdeterminations with the same method as in our previous studies, nowtotalling 15 LMC clusters and four SMC clusters, together with someadditional values from the literature. The LMC and SMC age-metallicityrelations appear to be remarkably complementary, since the SMC wasactively star-forming during the LMC quiescent age gap epoch.
| The Star Cluster Systems of the Magellanic Clouds
The characteristics of the cluster systems of the Magellanic Clouds, asinferred from integrated properties, are compared with those fromindividual cluster studies and from the field population. The agreementis generally satisfactory though in the case of the LMC, the lack ofclusters older than ˜3 Gyr is not reflected in the fieldpopulation. The possible origin(s) for this cluster ``age-gap'' arediscussed. The SMC cluster age-metallicity relation is also presentedand discussed.
| Ages and metallicities of five intermediate-age star clusters projected towards the Small Magellanic Cloud
Colour-magnitude diagrams are presented for the first time for L32, L38,K28 (L43), K44 (L68) and L116, which are clusters projected on to theouter parts of the Small Magellanic Cloud (SMC). The photometry wascarried out in the Washington system C and T1 filters,allowing the determination of ages by means of the magnitude differencebetween the red giant clump and the main-sequence turn-off, andmetallicities from the red giant branch locus. The clusters have ages inthe range 2-6Gyr, and metallicities in the range-1.65<[Fe/H]<-1.10, increasing the sample of intermediate-ageclusters in the SMC. L116, the outermost cluster projected on to theSMC, is a foreground cluster, and somewhat closer to us than the LargeMagellanic Cloud. Our results, combined with those for other clusters inthe literature, show epochs of sudden chemical enrichment in theage-metallicity plane, which favour a bursting star formation history asopposed to a continuous one for the SMC.
| The Line-of-Sight Depth of Populous Clusters in the Small Magellanic Cloud
We present an analysis of age, metal abundance, and positional data onpopulous clusters in the Small Magellanic Cloud (SMC) with the ultimateaim of determining the line-of-sight (LOS) depth of the SMC by usingthese clusters as proxies. Our data set contains 12 objects and islimited to clusters with the highest-quality data for which the ages andabundances are best known and can be placed on an internally consistentscale. We have analyzed the variation of the clusters' properties withposition on the sky and with line-of-sight depth. Based on thisanalysis, we draw the following conclusions: (1) The observational dataindicate that the eastern side of the SMC (facing the Large MagellanicCloud) contains younger and more metal-rich clusters as compared withthe western side. This is not a strong correlation because our data setof clusters is necessarily limited, but it is suggestive and warrantsfurther study. (2) Depending on how the reddening is computed to ourclusters, we find a mean distance modulus that ranges from(m-M)0=18.71+/-0.06 to 18.82+/-0.05. (3) The intrinsic +/-1σ LOS depth of the SMC populous clusters in our study is between~6 and ~12 kpc, depending primarily on whether we adopt the Burstein& Heiles reddenings or those from Schlegel et al. (4) Viewing theSMC as a triaxial galaxy with declination, right ascension, and LOSdepth as the three axes, we find axial ratios of approximately 1:2:4.Taken together, these conclusions largely agree with those of previousinvestigators and underscore the utility of populous star clusters asprobes of the structure of the Small Magellanic Cloud.
| Cepheid mass-luminosity relations from the Magellanic Clouds
The OGLE data base is used in conjunction with Kurucz atmosphere modelsto generate sets of period, effective temperature and luminosity forfundamental and overtone Magellanic Cloud Cepheids. The Floridapulsation code (with linear turbulent convection) is then used tocompute masses for these stars, assuming an average composition of(X=0.716, Z=0.010) for the LMC and of (X=0.726, Z=0.004) for the SMC.The average M-L relation for the fundamental Cepheids matches closelythat for the first overtone Cepheids for each Magellanic Cloud. Neitherthe SMC nor the LMC average Log M-Log L relations are straight, but havea noticeable curvature. In view of the uncertainties in distance andreddening we have adopted three different choices for these quantities.The results based on the ``long'' distance scale to the clouds give abetter agreement between theory and and observations than the ``short''one. All the current evolutionary tracks predict systematically largermasses for given luminosities than our observationally derived ones,especially at the high end. Moreover, our study confirms that theevolutionary tracks of the low mass stars in SMC are not in agreementwith the observations as they do not extend sufficiently blueward and donot penetrate deep enough into the instability strip, or not at all. Theinference of masses directly from the observational database yields anovel and strong constraint on evolutionary calculations.
| The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems
Prompted by a new catalogue of M31 globular clusters, we have collectedtogether individual metallicity values for globular clusters in theLocal Group. Although we briefly describe the globular cluster systemsof the individual Local Group galaxies, the main thrust of our paper isto examine the collective properties. In this way we are simulating thedissipationless merger of the Local Group, into presumably an ellipticalgalaxy. Such a merger is dominated by the Milky Way and M31, whichappear to be fairly typical examples of globular cluster systems ofspiral galaxies. The Local Group `Elliptical' has about 700 +/- 125globular clusters, with a luminosity function resembling the `universal'one. The metallicity distribution has peaks at [Fe/H] ~ -1.55 and -0.64with a metal-poor to metal-rich ratio of 2.5:1. The specific frequencyof the Local Group Elliptical is initially about 1 but rises to about 3,when the young stellar populations fade and the galaxy resembles an oldelliptical. The metallicity distribution and stellar populationcorrected specific frequency are similar to that of some known earlytype galaxies. Based on our results, we briefly speculate on the originof globular cluster systems in galaxies.
| Updating the Census of Star Clusters in the Small Magellanic Cloud
Surveys using CCD detectors are retrieving bright and faint catalogedclusters and revealing new ones in the Magellanic Clouds. This paperdiscusses the contribution of the OGLE Survey to the overall census ofstar clusters in the Small Magellanic Cloud (SMC). A detailedcross-identification indicates that the new objects in the SMC OGLEcatalog are 46. The increase in the number of cataloged clusters is ~7%,the total sample being ~700. This updated census includes embeddedclusters in H II regions and a density range attaining loose systems.
| Two Groups of Nearly Coeval Star Clusters in the Small Magellanic Cloud
We report new photometry and main-sequence turnoff ages for sevenpopulous star clusters in the SMC with MV<-6 and age greaterthan 1 Gyr, using the Wide Field Planetary Camera 2 on board the HubbleSpace Telescope. In contrast to the accepted picture, these clustersappear to have formed in two brief intervals, the oldest 8+/-2 Gyr agoand one during a more recent burst 2+/-0.5 Gyr ago. When the ridgelinesof the four clusters (NGC 339, 361, and 416 and Kron 3) in the 8 Gyrburst are aligned, the dispersion in turnoff luminosities is less than0.2 mag, corresponding to a maximum age spread of +/-0.7 Gyr. When theridgelines of three clusters (NGC 152, 411, and 419) in the 2 Gyr burstare aligned, the maximum dispersion of 0.2 mag in turnoff luminositycorresponds to a permitted age spread of +/-0.2 Gyr. Within each groupof clusters, the entire cluster loci (including red giant branches andclumps) are nearly identical, consistent with a very small spread inmetallicity and age. In contrast to the wide dispersion in agespreviously reported in the literature, our sample with more precisephotometry and age measurements supports a burst-punctuated rather thana continuous cluster formation history for the oldest SMC clusters.
| The Optical Gravitational Lensing Experiment. Cepheids in Star Clusters from the Magellanic Clouds
We present Cepheids located in the close neighborhood of star clustersfrom the Magellanic Clouds. 204 and 132 such stars were found in the LMCand SMC, respectively. The lists of objects were constructed based oncatalogs of Cepheids and star clusters, recently published by theOGLE-II collaboration. Location of selected Cepheids on the skyindicates that many of them are very likely cluster members. Photometricdata of Cepheids and clusters are available from the OGLE Internetarchive.
| 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.
| The Optical Gravitational Lensing Experiment. Age of Star Clusters from the SMC
We present determination of age of clusters from 2.4 square degreeregion of the SMC bar. The photometric data were taken from the BVI mapsof the SMC and catalog of clusters in this galaxy obtained during theOGLE-II microlensing survey. For 93 well populated SMC clusters theirage is derived with the standard procedure of isochrone fitting. Thedistribution of age of cluster from the SMC is presented. It indicateseither non-uniform process of cluster formation or very effectivedisruption of clusters.
| The Optical Gravitational Lensing Experiment. Variable Stars in Star Clusters of the Magellanic Clouds. I.Eclipsing Systems in the Clusters of the SMC
The list of 127 eclipsing stars in optical coincidence with starclusters from the SMC is presented. It was prepared using the catalogsof eclipsing systems and star clusters from the SMC based onobservations collected during the OGLE-II microlensing project. Locationof 12 eclipsing stars in the color-magnitude diagram of clusters allowsto exclude their membership. Photometric data of 73 systems supporttheir membership. The remaining 42 objects were found in loose, faintclusters and therefore no conclusive statement about their membershipcan be made. All presented data are available from the OGLE archive.
| The Age-dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump
Color-magnitude diagrams of globular clusters usually exhibit aprominent horizontal branch (HB) and may also show features such as thered giant branch (RGB) bump and the asymptotic giant branch (AGB) bump.Stellar evolution theory predicts that the luminosities of thesefeatures will depend on the metallicity and age of the cluster. Wecalculate theoretical lines of 2-12 Gyr constant age RGB bumps and AGBbumps in the DeltaV^HB_Bump-[Fe/H] diagram, which shows the brightnessdifference between the bump and the HB as a function of metallicity. Inorder to test the predictions, we identify giant branch bumps in newHubble Space Telescope color-magnitude diagrams for eight SMC clusters.First, we conclude that the SMC cluster bumps are RGB bumps. The datafor clusters younger than ~6 Gyr are in fair agreement with ourpredictions for the relative age-dependent luminosities of the HB andRGB bump. The DeltaV^HB_Bump data for clusters older than ~6 Gyrdemonstrate a less satisfactory agreement with our calculations. Weconclude that ~6 Gyr is the lower bound to the age of clusters for whichthe Galactic globular cluster, age-independent DeltaV^HB_Bump-[Fe/H]calibration is valid. Application of the DeltaV^HB_Bump-[Fe/H] diagramto stellar population studies is discussed.
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