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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.
| 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.
| 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.
| 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.
| Tracing the formation history of intermediate-age star clusters in the Small Magellanic Cloud
Colour-magnitude diagrams (CMDs) are presented for the first time for 10star clusters projected on to the Small Magellanic Cloud (SMC). Thephotometry was carried out in the Washington system C and T1filters allowing the determination of ages by means of the magnitudedifference between the red giant clump and the main-sequence turnoff(MSTO), and metallicities from the red giant branch (RGB) locus. Theclusters all have ages in the range 1.5-4 Gyr and metallicities between-1.3 < [Fe/H] < -0.6, with respective errors of ~0.5 Gyr and 0.3dex. This increases substantially the sample of intermediate-ageclusters in the SMC with well-derived parameters. We combine our resultswith those for other clusters in the literature to derive as large andhomogeneous a data base as possible (totalling 26 clusters) in order tostudy global effects. We find evidence for two peaks in the agedistribution of SMC clusters, at ~6.5 and 2.5 Gyr, in good agreementwith previous hints involving smaller samples. The most recent peakoccurs at a time that corresponds to a very close encounter between theLarge Magellanic Cloud (LMC) and the SMC according to the recentdynamical models of Bekki et al. that they used to explain theenhancement of LMC clusters with this age. It appears cluster formationmay have been similarly stimulated in the SMC by this encounter as well.We also find very good agreement between cluster ages and metallicitiesand the prediction from a bursting model from Pagel andTautvaišienė with a burst that occurred 3 Gyr ago. These twolines of evidence together favour a bursting cluster formation historyas opposed to a continuous one for the SMC.
| 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.
| 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.
| Spectroscopy of a globular cluster in the Local Group dwarf irregular NGC 6822
We present low-resolution Keck spectroscopy for the globular cluster(GC) HVIII in the Local Group dwarf irregular galaxy NGC 6822. We findthe metallicity of the cluster to be [Fe/H]=-1.58 +/- 0.28 and the ageof the cluster to be 3-4 Gyr, slightly older than but consistent withprevious age estimates. HVIII seems to be more metal-poor than mostintermediate-age GCs in the Local Group, and appears most similar to theanomalous Small Magellanic Cloud clusters Lindsay 113 and NGC 339.
| 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.
| 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.
| UBVI Color-Magnitude Diagrams in Baade's Window Metallicity Range, Implications for the Red Clump Method, Color "Anomaly" and the Distances to the Galactic Center and the Large Magellanic Cloud
We analyze the UBVI color-magnitude diagrams towards the Galactic bulgein a relatively low-reddening region of Baade's Window. The dereddenedred giant branch is very wide [approx 1.0 mag in (U-B)_0 and approx 0.4mag in (B-V)_0 and (V-I)_0], indicating a significant dispersion ofstellar metallicities, which by comparison with the theoreticalisochrones and data for Galactic clusters we estimate to lie betweenapproximately -0.7<[Fe/H]<+0.3, i.e. spanning a bout 1dex inmetallicity, in good agreement with earlier spectroscopic studies. Wealso discuss the metallicity dependence of the red clump I-bandbrightness and we show that it is between 0.1-0.2 mag/dex. This agreeswell with the previous empirical determinations and the models ofstellar evolution. The dereddened (V-I)_0 color of the red clump in theobserved bulge field is <(V-I)_0>=1.066, sigma_(V-I)_0=0.14, i.e.,0.056 mag redder than the local stars with good parallaxes measured byHipparcos. It seems that the large "color anomaly" of approx 0.2 magnoticed by Paczynski and Stanek and discussed in many recent papers wasmostly due to earlier problems with photometric calibration. When we useour data to re-derive the red clump distance to the Galactic center, weobtain the Galactocentric distance modulus mu_{0,GC}=14.69+/-0.1 mag(R_0=8.67+/-0.4 kpc), with error dominated by the systematics ofphotometric calibration. We then discuss the systematics of the redclump method and how they affect the red clump distance to the LargeMagellanic Cloud. We argue that the value of distance modulusmu_{0,LMC}=18.24+/-0.08 (44.5+/-1.7 kpc), recently refined by Udalski,is currently the most secure and robust of all LMC distance estimates.This has the effect of increasing any LMC-tied Hubble constant by about12%, including the recent determinations by the HST Key Project andSandage et al. The UBVI photometry is available through the anonymousftp service.
| 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.
| 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.
| 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 chemical composition of the young, Inter-Cloud population
High-resolution AAT spectroscopy and lower resolution spectrophotometryare presented for three early B-type stars that are members of theyoung, Inter-Cloud population between the Magellanic Clouds. Thesespectra have been analyzed using LTE model-atmosphere techniques, toderive the stellar atmospheric parameters and photospheric chemicalcompositions. The latter should reflect that of the present-dayinterstellar medium (ISM) within the Inter-Cloud Region (ICR). From adifferential analysis, the three ICR stars appear to have a mean metalabundance of ~ 1.1 dex lower than their Population i Galactic analogues,and 0.5 dex lower than the SMC star (AV 304). Hence, the ICR gas doesnot reflect the present-day composition of either the SMC (or LMC) ISM.Age (and distance) estimates were obtained using the theoreticalisochrones of Bertelli et al. (\cite{ber94}); these imply that theyoung, Inter-Cloud population has an age dispersion of at least 10-40Myr, and provide evidence for a distance gradient across the ICR. Wediscuss our results within the context of recent numerical simulationsof the gravitational interactions between the Galaxy-LMC-SMC, thatpredict that the ICR was tidally disrupted from the SMC some 200 Myrago. If the SMC was chemically homogeneous, a comparison of the ICRabundance determinations with the SMC age-metallicity relationship wouldthen imply that the formation of the ICR must have occurred ~ 8.5 Gyrago. Alternatively and more plausible, we postulate that the ICR gasformed from a mixture of SMC gas and an unenriched component. This isconsistent with model-predictions that both a halo and disc componentshould have contributed to the material within the ICR during the tidaldisruption.
| The third dredge-up and the carbon star luminosity functions in the Magellanic Clouds
We investigate the formation of carbon stars as a function of thestellar mass and parent metallicity. Theoretical modelling is based onan improved scheme for treating the third dredge-up in syntheticcalculations of thermally pulsing asymptotic giant branch (TP-AGB)stars. In this approach, the usual criterion (based on a constantminimum core mass for the occurrence of dredge-up, M_c() min) isreplaced by one on the minimum temperature at the base of the convectiveenvelope, T_b() dred, at the stage of the post-flash luminosity maximum.Envelope integrations then allow determination of M_c() min as afunction of stellar mass, metallicity, and pulse strength (see Wood1981), thus inferring if and when dredge-up first occurs. Moreover, thefinal possible shut down of the process is predicted. Extensive grids ofTP-AGB models were computed by Marigo (1998a,b) using this scheme. Inthis paper, we present and discuss the calibration of the two dredge-upparameters (i.e. efficiency lambda and T_b() dred) aimed at reproducingthe carbon star luminosity function (CSLF) in the LMC, using TP-AGBmodels with original metallicity Z=0.008. In addition to this, theeffects of different input quantities on the theoretical CSLF areanalyzed. It turns out that the faint tail is almost insensitive to thehistory of star formation rate (SFR) in the parent galaxy, in contrastto the bright wing which may be more affected by the details of therecent history. Actually, we find that the faint end of the CSLF isessentially determined by the temperature parameter T_b() dred. Once thefaint end is reproduced, the peak location is a stringent calibrator ofthe efficiency parameter lambda . The best fit to the observed CSLF inthe LMC is obtained with lambda =0.50, log T_b() dred = 6.4, and aconstant SFR up to an age of about 5 x 10(8) yr. This recent drop of theSFR is invoked to remove a slight excess of bright carbon starsotherwise predicted. A good fit to the observed CSLF in the SMC is theneasily derived from the Z=0.004 models, with a single choice ofparameters lambda =0.65, log T_b() dred = 6.4, and a constant SFR overthe entire significant age interval. The result for lambda is consistentwith the theoretical expectation that the third dredge-up is moreefficient at lower metallicities.
| 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.
| WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud
We present our analysis of archival Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) observations in F450W (~B) and F555W (~V) ofthe intermediate-age populous star clusters NGC 121, NGC 339, NGC 361,NGC 416, and Kron 3 in the Small Magellanic Cloud. We use publishedphotometry of two other SMC populous star clusters, Lindsay 1 andLindsay 113, to investigate the age sequence of these seven starclusters in order to improve our understanding of the formationchronology of the SMC. We analyzed the V versus B-V and M_V versus(B-V)_0 color-magnitude diagrams of these populous Small MagellanicCloud star clusters using a variety of techniques and determined theirages, metallicities, and reddenings. These new data enable us to improvethe age-metallicity relation of star clusters in the Small MagellanicCloud. In particular, we find that a closed-box continuous starformation model does not reproduce the age-metallicity relationadequately. However, a theoretical model punctuated by bursts of starformation is in better agreement with the observational data presentedherein.
| A catalogue of Mg_2 indices of galaxies and globular clusters
We present a catalogue of published absorption-line Mg_2 indices ofgalaxies and globular clusters. The catalogue is maintained up-to-datein the HYPERCAT database. The measurements are listed together with thereferences to the articles where the data were published. A codeddescription of the observations is provided. The catalogue gathers 3541measurements for 1491 objects (galaxies or globular clusters) from 55datasets. Compiled raw data for 1060 galaxies are zero-point correctedand transformed to a homogeneous system. Tables 1, 3, and 4 areavailable in electronic form only at the CDS, Strasbourg, via anonymousftp 130.79.128.5. Table 2 is available both in text and electronic form.
| 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.
| The Optical Gravitational Lensing Experiment. Population Effects on the Mean Brightness of the Red Clump Stars
We present an empirical test indicating that the mean I-band magnitudeof the red clump stars, used as the standard candle in the recentdistance determinations to the Magellanic Clouds and other objects, isage independent for intermediate age (2-10Gyr) stars. Fifteen starclusters of age approx 1.5-12 Gyr from the LMC and SMC (ESO121SC03,SL663, NGC2155, NGC2121, SL388, SL862, NGC121, L1, KRON3, NGC416, L113,NGC339, L11, NGC419, NGC411) were observed and their color-magnitudediagrams are presented. The mean I-band brightness of the red clump inthese clusters is constant and its mean extinction-free magnitude is:I_0=17.88+/-0.05 mag and I_0=18.31+/-0.07 mag at the mean metallicity of-0.8 dex and -1.2 dex for the LMC and SMC clusters, respectively. Forolder objects (>10Gyr) the brightness of the red clump, whichconverts into the red part of the horizontal branch, fades by about0.3-0.4mag, setting an important limitation on the red clump starsmethod of distance determination. The red clump distance moduli to theMagellanic Clouds from the new independent data set are:m-M=18.18+/-0.06 mag and m-M=18.65+/-0.08 mag for the LMC and SMC,respectively, in very good agreement with previous determinations. Weakdependence of the mean I-band brightness of the red clump on metallicityand its independence of age for intermediate age population (2-10 Gyr)of stars as well as the most precise calibration as compared to otherstandard candle candidates makes the red clump stars method one of themost accurate steps in the distance scale ladder.
| Ca II Triplet Spectroscopy of Giants in Small Magellanic Cloud Star Clusters: Abundances, Velocities, and the Age-Metallicity Relation
We have obtained spectra at the Ca ii triplet of individual red giantsin seven Small Magellanic Cloud (SMC) star clusters whose ages rangefrom ~4 to 12 Gyr. The spectra have been used to determine meanabundances for six of the star clusters to a typical precision of 0.12dex. When combined with existing data for other objects, the resultingSMC age-metallicity relation is generally consistent with that for asimple model of chemical evolution, scaled to the present-day SMC meanabundance and gas mass fraction. Two of the clusters (Lindsay 113 andNGC 339), however, have abundances that are ~0.5 dex lower than thatexpected from the mean age-metallicity relation. It is suggested thatthe formation of these clusters, which have ages of ~5 Gyr, may haveinvolved the infall of unenriched gas, perhaps from the MagellanicStream. The spectra also yield radial velocities for the seven clusters.The resulting velocity dispersion is 16 +/- 4 km s^-1, consistent withthose of the SMC planetary nebula and carbon star populations.
| Age and metallicity of star clusters in the Magellanic Clouds
Spectral indices as defined by the Lick group (Hβ , Mg_2, Fe5270,Fe5335 and NaD) were measured for a sample of 14 clusters in theMagellanic Clouds. Iron and hydrogen indices, together with theintegrated colours (B-V) and (V-K) for these clusters, were used toestimate their ages and metallicities, using calibrations based onsingle stellar population models. The resulting ages and metallicitiesare in agreement with ages estimated from colour-magnitude diagrams, andmetallicities derived from spectroscopic analises or narrow bandphotometry. Based on observations collected at the European SouthernObservatory, La Silla, Chile and the Laboratório Nacional deAstrof\'{\i
| WFPC2 Observations of the Small Magellanic Cloud Intermediate-Age Populous Cluster NGC 416
We present our analysis of archival Hubble Space Telescope Wide FieldPlanetary Camera 2 observations in F555W (~V) and F450W (~B) of theintermediate-age populous star cluster NGC 416 in the Small MagellanicCloud (SMC). We use published photometry of two other SMC populous starclusters, Lindsay 1 and Lindsay 113, to investigate the age sequence ofthese three star clusters. We estimate that these clusters have ageratios of {age}_{{NGC 416}}/{age}_{{L1}}~0.73+/-0.05 and{age}_{{L113}}/{age}_{{L1}}~0.52+/-0.09, using an extrapolation of thed_{(B-V)} method (which uses the color difference between the redhorizontal branch and the red giant branch as an age indicator) ofSarajedini and coworkers. These age ratios provide absolute ageestimates of 6.6+/-0.5 Gyr and 4.7+/-0.8 Gyr for NGC 416 and Lindsay113, respectively, assuming that Lindsay 1 is 9 Gyr old. Metallicitiesof [Fe/H] = - 1.44+/-0.12 , - 1.35+/-0.08 , and - 1.24+/-0.11 dex, andreddenings of E(B-V)=0.08+/-0.03 , 0.06+/-0.02 , and 0.00+/-0.02 mag forNGC 416, Lindsay 1, and Lindsay 113, respectively, were determined usingthe simultaneous reddening and metallicity (SRM) method of Sarajediniand Layden. Accurate (relative) ages for the intermediate-age populousclusters in the SMC (e.g., via deep main-sequence photometry) wouldallow the d_{(B-V)} method to be recalibrated with star clusters thatare significantly younger than 7 Gyr. An extended d_{(B-V)} method couldprove to be a very useful age diagnostic for future studies of theintermediate-age metal-poor stellar populations in Local Group galaxies,where accurate main-sequence turnoff photometry at MV~+4 magis currently not possible or practical.
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