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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).
| Old Main-Sequence Turnoff Photometry in the Small Magellanic Cloud. I. Constraints on the Star Formation History in Different Fields
We present ground-based B- and R-band color-magnitude diagrams (CMDs)reaching the oldest main-sequence (MS) turnoffs with good photometricaccuracy for 12 fields in the Small Magellanic Cloud (SMC). Our fields,located between ~1° and ~4° from the center of the galaxy, aresituated in different parts of the SMC such as the ``Wing'' area andtoward the west and south. In this paper we perform a first analysis ofthe stellar content in our SMC fields through comparison withtheoretical isochrones and color functions (CFs). We find that theunderlying spheroidally distributed population is composed of bothintermediate-age and old stars and that its age composition does notshow strong galactocentric gradients. The three fields situated towardthe east, in the Wing region, show very active current star formation.However, only in the eastern field closest to the center do we find anenhancement of recent star formation with respect to a constant SFR(t).The fields corresponding to the western side of the SMC present a muchless populated young MS, and the CF analysis indicates that the SFR(t)greatly diminished around 2 Gyr ago in these parts. Field smc0057, theclosest to the center of the galaxy and located in the southern part,shows recent star formation, while the rest of the southern fieldspresent few bright MS stars. The structure of the red clump in all theCMDs is consistent with the large amount of intermediate-age starsinferred from the CMDs and color functions. None of the SMC fieldspresented here are dominated by old stellar populations, a fact that isin agreement with the lack of a conspicuous horizontal branch in allthese SMC CMDs. This could indicate that a disk population is rulingover a possible old halo in all the observed fields.
| Three clusters of the SMC from ACS/WFC HST archive data: NGC 265, K 29 and NGC 290 and their field population
Aims.We determine the age, metallicity and initial mass function ofthree clusters, namely NGC 265, K29, NGC 290, located in the main body ofthe Small Magellanic Cloud. In addition, we derive the history of starformation in the companion fields. Methods: We make use of ACS/WFC HSTarchive data. For the clusters, the age and metallicity are derivedfitting the integrated luminosity function with single synthetic stellarpopulation by means of the χ2 minimization. For thecompanion fields, the history of star formation is derived using theχ2 minimization together with the downhill-simplexmethod. Results: For the clusters we find the following ages andmetallicities: NGC 265 has log(Age)=8.5±0.3 yrand metallicity 0.004±0.003 (or [ Fe/H]=-0.62); K29 has log(Age)=8.2±0.2 yr and metallicityZ=0.003±0.002 (or [ Fe/H]=-0.75); NGC 290 haslog(Age)=7.8±0.5 yr and metallicity 0.003±0.002 (or [Fe/H]=-0.75). The superior quality of the data allows the study of theinitial mass function down to M ˜ 0.7 Mȯ. Theinitial mass function turns out to be in agreement with the standardKroupa model. The comparison of the NGC 265luminosity function with the theoretical ones from stellar models bothtaking overshoot from the convective core into account and neglectingit, seems to suggest that a certain amount of convective overshoot isrequired. However, this conclusion is not a strong one because thiscluster has a certain amount of mass segregation which makes itdifficult to choose a suitable area for this comparison. The starformation rate of the field population presents periods of enhancementsat 300-400 Myr, 3-4 Gyr and finally 6 Gyr. However it is relativelyquiescent at ages older than 6 Gyr. This result suggests that at olderages, the tidal interaction between the Magellanic Clouds and the MilkyWay was not able to trigger significant star formation events.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| 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.
| ACS Photometry of Extended, Luminous Globular Clusters in the Outskirts of M31
A new population of extended, luminous globular clusters has recentlybeen discovered in the outskirts of M31. These objects have luminositiestypical of classical globular clusters, but much larger half-lightradii. We report the first results from deep ACS imaging of four suchclusters, one of which is a newly discovered example lying at aprojected distance of ~60 kpc from M31. Our F606W, F814W color-magnitudediagrams extend ~3 mag below the horizontal branch level, and clearlydemonstrate, for the first time, that all four clusters are composed of>~10 Gyr old, metal-poor stellar populations. No evidence formultiple populations is observed. From a comparison with Galacticglobular cluster fiducials we estimate metallicities in the range-2.2<~[Fe/H]<~-1.8. The observed horizontal branch morphologiesshow a clear second parameter effect between the clusters. Preliminaryradial luminosity profiles suggest integrated magnitudes in the range-7.7<~MV<~-6.6, near the median value of the globularcluster luminosity function. Our results confirm that these four objectsare bona fide old, metal-poor globular clusters, albeit with combinedstructures and luminosities unlike those observed for any other globularclusters in the Local Group or beyond.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith program 10394.
| 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.
| AGB stars in the Magellanic Clouds. III. The rate of star formation across the Small Magellanic Cloud
Aims.This article compares the Ks magnitude distribution ofSmall Magellanic Cloud asymptotic giant branch stars obtained from theDENIS and 2MASS data with theoretical distributions. Methods:.Theoretical Ks magnitude distributions have been constructedusing up-to-date stellar evolution calculations for low- andintermediate-mass stars, and in particular for thermally pulsingasymptotic giant branch stars. Separate fits of the magnitudedistributions of carbon- and oxygen-rich stars allowed us to constrainthe metallicity distribution across the galaxy and its star formationrate. Results: .The Small Magellanic Cloud stellar population isfound to be on average 7-9 Gyr old but older stars are present at itsperiphery and younger stars are present in the direction of thecompanion galaxy the Large Magellanic Cloud. The metallicitydistribution traces a ring-like structure that is more metal rich thanthe inner region of the galaxy. Conclusions: .The C/M ratiodiscussed in Paper I is a tracer of the metallicity distribution only ifthe underlying stellar population is of intermediate-age.
| Age distribution of young clusters and field stars in the Small Magellanic Cloud
Aims.In this paper we discuss the cluster and field star formation inthe central part of the Small Magellanic Cloud. The main goal is tostudy the correlation between young objects and their interstellarenvironment. Methods: . The ages of about 164 associations and 311clusters younger than 1 Gyr are determined using isochrone fitting. Thespatial distribution of the clusters is compared with the HI maps, withthe HI velocity dispersion field, with the location of the CO clouds andwith the distribution of young field stars. Results: .The clusterage distribution supports the idea that clusters formed in the last 1Gyr of the SMC history in a roughly continuous way with periods ofenhancements. The two super-shells 37A and 304A detected in the HIdistribution are clearly visible in the age distribution of theclusters: an enhancement in the cluster formation rate has taken placefrom the epoch of the shell formation. A tight correlation between youngclusters and the HI intensity is found. The degree of correlation isdecreasing with the age of the clusters. Clusters older than 300 Myr arelocated away from the HI peaks. Clusters and associations younger than10 Myr are related to the CO clouds in the SW region of the SMC disk. Apositive correlation between the location of the young clusters and thevelocity dispersion field of the atomic gas is derived only for theshell 304A, suggesting that the cloud-cloud collision is probably notthe most important mechanism of cluster formation. Evidence ofgravitational triggered episode due to the most recent close interactionbetween SMC and LMC is found both in the cluster andfield star distribution.
| 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.
| Stellar Populations in the Wing of the Small Magellanic Cloud from Hubble Space Telescope Photometry
We have used the Wide Field Planetary Camera 2 on the Hubble SpaceTelescope to image a star field in the wing of the Small MagellanicCloud (SMC), near the H II region N81. The images were taken in theF336W, F547M, F675W, and F814W filters. From photometry of stars in thisfield, we construct color-magnitude diagrams (CMDs) for about 4200 starsand compare them with theoretical isochrones. In one CCD frame, weidentify an open cluster with an approximate age between 400 and 560Myr. After statistically subtracting the cluster stars, the remainingfield star CMD shows both a strong upper main sequence and awell-developed red giant branch. The brightest main-sequence starscorrespond to at most an age of 100 Myr. We also see a substantialnumber of turnoff stars with V magnitudes between 20 and 22,corresponding to an age range of 1.3 Gyr to at most 12 Gyr. We discussthe effects of the SMC's extended depth on the analysis. From acomparison of the observed CMD with Monte Carlo simulations, we findthat the star formation history for this field is not consistent with aconstant rate over the last 12 Gyr. We find that the CMD is consistentwith increased star formation from 4-12 Gyr ago and over the past 1.7Gyr. However, we find reduced star formation rates during the period1.7-4 Gyr ago, in contrast to studies of star clusters that havesuggested that the SMC experienced a burst of star formation 2 Gyr ago.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Integrated spectral analysis of 18 concentrated star clusters in the Small Magellanic Cloud
We present in this study flux-calibrated integrated spectra in the range(3600-6800) Å for 18 concentrated SMC clusters. Cluster reddeningvalues were estimated by interpolation between the extinction maps ofBurstein & Heiles (1982, AJ, 87, 1165) and Schlegel et al. (1998,ApJ, 500, 525). The cluster parameters were derived from the templatematching procedure by comparing the line strengths and continuumdistribution of the cluster spectra with those of template clusterspectra with known parameters and from the equivalent width (EW) method.In this case, new calibrations were used together with diagnosticdiagrams involving the sum of EWs of selected spectral lines. A verygood agreement between ages derived from both methods was found. Thefinal cluster ages obtained from the weighted average of values takenfrom the literature and the present measured ones range from 15 Mr (e.g.L 51) to 7 Gyr (K 3). Metal abundances have been derived for only 5clusters from the present sample, while metallicity values directlyaveraged from published values for other 4 clusters have been adopted.Combining the present cluster sample with 19 additional SMC clusterswhose ages and metal abundances were put onto a homogeneous scale, weanalyse the age and metallicity distributions in order to explore theSMC star formation history and its spatial extent. By considering thedistances of the clusters from the SMC centre instead of theirprojections onto the right ascension and declination axes, the presentage-position relation suggests that the SMC inner disk could have beenrelated to a cluster formation episode which reached the peak ~2.5 Gyrago. Evidence for an age gradient in the inner SMC disk is alsopresented.
| From young massive star cluster to old globular: the LV-σ0 relationship as a diagnostic tool
We present a new analysis of the properties of the young massive starclusters (YMCs) forming profusely in intense starburst environments,which demonstrates that these objects are plausible progenitors of theold globular clusters (GCs) seen abundantly in the Local Group. Themethod is based on the tight relationship for old GCs between theirV-band luminosities, LV, and (central) velocity dispersions,σ0. We improve the significance of the relationship byincreasing the GC sample size and find that its functional form,LV/Lsolar~σ1.57+/-0.100(km s-1), is fully consistent with previous determinationsfor smaller Galactic and M31 GC samples. The tightness of therelationship for a GC sample drawn from environments as diverse as thosefound in the Local Group implies that its origin must be sought inintrinsic properties of the GC formation process itself. We evolve theluminosities of those YMCs in the local Universe which have velocitydispersion measurements to an age of 12 Gyr, adopting a variety ofinitial mass function (IMF) descriptions, and find that most YMCs willevolve to loci close to, or to slightly fainter luminosities than theimproved GC relationship. In the absence of significant externaldisturbances, this implies that these objects may potentially survive tobecome old GC-type objects over a Hubble time. The main advantage of ournew method is its simplicity. Whereas alternative methods, based ondynamical mass estimates, require one to obtain accurate size estimatesand to make further assumptions, the only observables required here arethe system's velocity dispersion and luminosity. The most importantfactor affecting the robustness of our conclusions is the adopted formof the IMF. We use the results of N-body simulations to confirm thatdynamical evolution of the clusters does not significantly alter ourconclusions about the likelihood of individual clusters surviving tolate times. Finally, we find that our youngest observed clusters areconsistent with having evolved from a relation of the form . Thisrelation may actually correspond to the origin of the GC fundamentalplane.
| 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.
| 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.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| 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 Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-masssystems will be the first sites of star formation. As these objects haveshallow gravitational potential wells, the subsequent growth of theirstellar populations may be halted by heating and gas loss due toreionization. This effect has been suggested to have profoundlyinfluenced properties of present-day dwarf galaxies, including theirstellar populations and even survival as visible galaxies. In thisLetter we draw on results from quantitative studies of Local Group dwarfgalaxy star formation histories, especially for Milky Way satellites, toshow that no clear signature exists for a widespread evolutionary impactfrom reionization. All nearby dwarf galaxies studied in sufficientdetail contain ancient populations indistinguishable in age from theoldest Galactic globular clusters. Ancient star formation activityproceeded over several gigayears, and some dwarf spheroidal galaxieseven experienced fairly continuous star formation until just a fewgigayears ago. Despite their uniformly low masses, their star formationhistories differ considerably. The evolutionary histories of nearbydwarf galaxies appear to reflect influences from a variety of localprocesses rather than a dominant effect from reionization.
| The Star Formation History of the Small Magellanic Cloud
We present the spatially resolved star formation and chemical enrichmenthistory of the Small Magellanic Cloud (SMC) across the entire central4deg×4.5d area of the main body, based on UBVIphotometry from our Magellanic Clouds Photometric Survey. We find that(1) approximately 50% of the stars that ever formed in the SMC formedprior to 8.4 Gyr ago (z>1.2 for WMAP cosmology); (2) the SMC formedrelatively few stars between 8.4 and 3 Gyr ago; (3) there was a rise inthe mean star formation rate during the most recent 3 Gyr punctuated by``bursts'' at ages of 2.5, 0.4, and 0.06 Gyr; (4) the bursts at 2.5 and0.4 Gyr are temporally coincident with past perigalactic passages of theSMC with the Milky Way; (5) there is preliminary evidence for alarge-scale annular structure in the 2.5 Gyr burst; and (6) the chemicalenrichment history derived from our analysis is in agreement with theage-metallicity relation of the SMC's star clusters. Consistentinterpretation of the data required an ad hoc correction of 0.1-0.2 magto the B-V colors of 25% of the stars; the cause of this anomaly isunknown, but we show that it does not strongly influence our results.
| 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.
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/967
| BV Photometry of Variable Stars in the Northeast Arm of the Small Magellanic Cloud
B and V photometry has been obtained for variable stars in the northeastarm of the Small Magellanic Cloud (SMC). Periods and light curves havebeen determined for 237 periodic variables, including 201 Cepheids, 68of which are newly discovered. Fundamental-mode Cepheids andfirst-overtone-mode Cepheids are generally well separated in the BVcolor-magnitude diagram, with the latter having bluer mean colors thanthe former. The Cepheid period-color relationship for this outlying SMCfield is indistinguishable from that seen in more centrally located SMCfields and is bluer than theoretical predictions. The red edge to thepopulated portion of the instability strip shifts to bluer colors forfainter Cepheids. There is support from our sample for a previouslyreported steepening in the slope of the period-luminosity relation forfundamental-mode Cepheids near a period of 2 days. The Cepheids of thenortheast arm may be closer to us than are those of the main body of theSMC, but the difference is smaller than or equal to about 4 kpc,comparable to the tidal radius of the SMC.
| Globular clusters and the Mira period-luminosity relation
A globular cluster distance scale based on Hipparcos parallaxes ofsubdwarfs has been used to derive estimates of MK for clusterMiras, including one in the Small Magellanic Cloud (SMC) globularcluster NGC 121. These lead to a zero-point of the Mira infraredperiod-luminosity (PL) relation, PL(K), in good agreement with thatderived from Hipparcos parallaxes of nearby field Miras. The mean ofthese two estimates together with data on LMC Miras yields a LargeMagellanic Cloud (LMC) distance modulus of 18.60+/-0.10 in evidentagreement with a metallicity-corrected Cepheid modulus (18.59+/-~0.10).The use of luminous asymptotic giant branch (AGB) stars as extragalacticpopulation indicators is also discussed.
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