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|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.
|Physical parameters of 15 intermediate-age LMC clusters from modelling of HST colour-magnitude diagrams|
Aims.We analyzed HST/WFPC2 colour-magnitude diagrams (CMDs) of 15populous Large Magellanic Cloud (LMC) stellar clusters with ages between~0.3 Gyr and ~3 Gyr. These (V, B-V) CMDs are photometrically homogeneousand typically reach V ˜ 22. Accurate and self-consistent physicalparameters (age, metallicity, distance modulus and reddening) wereextracted for each cluster by comparing the observed CMDs with syntheticones. Methods: These determinations involved simultaneous statisticalcomparisons of the main-sequence fiducial line and the red clumpposition, offering objective and robust criteria to determine the bestmodels. The models explored a regular grid in the parameter spacecovered by previous results found in the literature. Control experimentswere used to test our approach and to quantify formal uncertainties. Results: In general, the best models show a satisfactory fit to thedata, constraining well the physical parameters of each cluster. Theage-metallicity relation derived by us presents a lower spread thansimilar results found in the literature for the same clusters. Ourresults are in accordance with the published ages for the oldestclusters, but reveal a possible underestimation of ages by previousauthors for the youngest clusters. Our metallicity results in generalagree with the ones based on spectroscopy of giant stars and with recentworks involving CMD analyses. The derived distance moduli implied by themost reliable solutions, correlate with the reddening values, asexpected from the non-negligible three-dimensional distribution of theclusters within the LMC. Conclusions: .The inferred spatialdistribution for these clusters is roughly aligned with the LMC disk,being also more scattered than recent numerical predictions, indicatingthat they were not formed in the LMC disk. The set of ages andmetallicities homogeneously derived here can be used to calibrateintegrated light studies applied to distant galaxies.
|Red Giant Stars in the Large Magellanic Cloud Clusters|
We present deep J, H, and Ks photometry and accurate colormagnitude diagrams down to K~18.5 for a sample of 13 globular clustersin the Large Magellanic Cloud. This data set combined with the previoussample of six clusters published by our group gives the opportunity tostudy the properties of giant stars in clusters with different ages(ranging from ~80 Myr up to 3.5 Gyr). Quantitative estimates of starpopulation ratios (by number and luminosity) in the asymptotic giantbranch (AGB), the red giant branch (RGB), and the He clump have beenobtained and compared with theoretical models in the framework ofprobing the so-called phase transitions. The AGB contribution to thetotal luminosity starts to be significant at ~200 Myr and reaches itsmaximum at 500-600 Myr, when the RGB phase transition is starting. At~900 Myr the full development of an extended and well-populated RGB hasbeen completed. The occurrences of both the AGB and RGB phasetransitions are sharp events, lasting a few hundred megayears only.These empirical results agree very well with the theoretical predictionsof simple stellar population models based on canonical tracks and thefuel-consumption approach.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m NTT, within the observing programs64.N-0038 and 68.D-0287.
|A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters|
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
|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.
|New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations|
We present theoretical surface brightness fluctuation (SBF) amplitudesfor single-burst stellar populations of young and intermediate age (25Myr<=t<=5 Gyr) and metallicities Z=0.0003, 0.001, 0.004, 0.008,0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expectedin the Johnson-Cousins (UBVRIJHK) photometric system are provided. Wepay attention to the contribution of thermally pulsating asymptoticgiant branch (TP-AGB) stars. The sensitivity of the predicted SBF tochanges in the mass-loss scenario along the TP-AGB phase is examined.Below 0.6-1 Gyr both optical and near-IR SBF models exhibit a strongdependence on age and mass loss. We also evaluate SBF amplitudes usingMonte Carlo techniques to reproduce the random variation in the numberof stars experiencing bright and fast evolutionary phases (red giantbranch, AGB, TP-AGB). On these grounds we provide constraints on thefaintest integrated flux of real stellar populations required to derivereliable and meaningful SBF measurements. We analyze a technique forderiving SBF amplitudes of star clusters from the photometry ofindividual stars and estimate the uncertainty due to statisticaleffects, which may impinge on the procedure. The first optical SBFmeasurements for 11 Large Magellanic Cloud (LMC) star-rich clusters-withages ranging from a few megayears to several gigayears-are derived usingHubble Space Telescope observations. The measurements are compared toour SBF predictions, providing a good agreement with models ofmetallicity Z=0.0001-0.01. Our results suggest that, for TP-AGB stars, amass loss as a power-law function of the star luminosity is required inorder to properly reproduce the optical SBF data of the LMC clusters.Finally, near-IR models have been compared to available data, thusshowing that the general trend is well fitted. We suggest how toovercome the general problem of SBF models in reproducing the details ofthe near-IR SBF measurements of the Magellanic Cloud star clusters.
|Constraints on the star formation history of the Large Magellanic Cloud|
We present the analysis of deep colour-magnitude diagrams (CMDs) of 6stellar fields in the LMC. The data were obtained using HST/WFPC2 in theF814W (˜I) and F555W (˜V) filters, reaching V555˜ 26.5. We discuss and apply a method of correcting CMDs forphotometric incompleteness. A method to generate artificial CMDs basedon a model star formation history is also developed. This methodincorporates photometric error effects, unresolved binaries, reddeningand allows use of different forms of the initial mass function and ofthe SFH itself. We use the Partial Models Method, as presented byGallart and others, for CMD modelling, and include control experimentsto prove its validity in a search for constraints on the LargeMagellanic Cloud star formation history in different regions. Reliablestar formation histories for each field are recovered by this method. Inall fields, a gap in star formation with τ ˜ 700 Myr isobserved. Field-to-field variations have also been observed. The twofields near the LMC bar present some significant star forming events,having formed both young (τ ⪉ 1 Gyr) and old (τ ⪆ 10Gyr) stars, with a clear gap from 3-6 Gyr. Two other fields displayquite similar SFHs, with increased star formation having taken place atτ ≃ 2-3 Gyr and 6 ⪉ τ ⪉ 10 Gyr. The remaining twofields present star formation histories closer to uniform, with no clearevent of enhanced star formation.
|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.
|Probing the Red Giant Branch Phase Transition: Near-Infrared Photometry of Six Intermediate-Age Large Magellanic Cloud Clusters|
This is the first of a series of papers devoted to a global study of thephotometric properties of the red stellar sequences in a complete sampleof the Large Magellanic Cloud clusters, by means of near-infrared arrayphotometry. Deep J, H, Ks photometry and accuratecolor-magnitude diagrams down to K~18.5, i.e., ~1.5 mag below the red Heclump, for six intermediate-age clusters (namely, NGC 1987, NGC 2108,NGC 2190, NGC 2209, NGC 2231, NGC 2249) are presented. A quantitativeestimate of the population ratios (by number and luminosity) between redgiant branch (RGB) and He-clump stars for each target cluster isprovided and discussed in the framework of probing the so-called RGBphase transition (Ph-T). By using the Elson & Fall s-parameter as anage indicator, the observed RGB population shows a sharp enhancement (inboth number and luminosity) at s=36. Obviously, the correspondingabsolute age strictly depends on the details of theoretical modelsadopted to calibrate the s-parameter. Curiously, the currently availablecalibrations of the s-parameter in terms of age based on canonical (byElson & Fall) and overshooting (Girardi and coworkers) modelsprovide ages that well agree within 10%, suggesting that the fulldevelopment of the RGB occurs at t~700 Myr and is a relatively fastevent (δt~300 Myr). However, the RGB Ph-T epoch derived from theovershooting calibration of the s-parameter turns out to besignificantly earlier than the epoch provided by the recent evolutionarytracks by Girardi and coworkers. A new calibration of the s-parameterbased on high-quality color-magnitude diagrams and updated models isurged to address the origin of this discrepancy and finally establishthe epoch of the RGB Ph-T.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m New Technology Telescope, withinthe observing programs 64.N-0038 and 68.D-0287.
|Fundamental parameters of the LMC clusters NGC 1836, NGC 1860, NGC 1865, SL 444, LW 224 and SL 548|
Complementing our recent Washington photometric studies on intermediateage and young Large Magellanic Cloud (LMC) clusters, we now turn ourattention to six previously unstudied star clusters in the transitionrange 200-700 Myr. We study NGC 1836, 1860 and 1865, which are projectedon the LMC bar; SL 444, also located in the central disc but outside thebar; and LW 224 and SL 548, both located in the outer disc. We deriveages and metallicities from extracted T1 versusC-T1 colour-magnitude diagrams (CMDs), using theoreticalisochrones recently computed for the Washington photometric system. Forthe metallicity determinations, these CMDs are particularly sensitive.We also estimate ages and metallicities of the surrounding fields of NGC1860 and 1865 by employing the δT1 index defined inGeisler et al. (1997, AJ, 114, 1920) and theoretical isochrones. Byadding the present cluster sample to those of our previous studies, wenow gather 37 LMC clusters with homogeneous parameter determinations,which are employed to probe the chemical enrichment of the LMC and itsspatial distribution. On average, inner disc clusters turned out to benot only younger than the outer ones, but also more metal-rich; somehave solar metal content. Furthermore, inner clusters located to thewest of the LMC centre are younger and more metal-rich than theireastern counterparts. We propose that a bursting formation mechanism,with an important formation event centred at ~2.0 Gyr, provides a betterdescription of the cluster age-metallicity relation than a closed-boxchemical evolution model. In the outer disc, the field star formationseems to have lasted until 2 Gyr ago while it continued in the innerdisc for almost 1 Gyr longer.
|Core radius evolution of star clusters|
We use N-body simulations of star clusters to investigate the possibledynamical origins of the observed spread in core radius amongintermediate-age and old star clusters in the Large Magellanic Cloud(LMC). Two effects are considered, a time-varying external tidal fieldand variations in primordial hard binary fraction. Simulations ofclusters orbiting a point-mass galaxy show similar core radius evolutionfor clusters on both circular and elliptical orbits and we thereforeconclude that the tidal field of the LMC has not yet significantlyinfluenced the evolution of the intermediate-age clusters. The presenceof large numbers of hard primordial binaries in a cluster leads to coreradius expansion; however, the magnitude of the effect is insufficientto explain the observations. Furthermore, the range of binary fractionsrequired to produce significant core radius growth is inconsistent withthe observational evidence that all the LMC clusters have similarstellar luminosity functions.
|Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. I. The Data|
This is the first of a series of three papers devoted to the calibrationof a few parameters of crucial importance in the modeling of theevolution of intermediate-mass stars, giving special attention to theamount of convective core overshoot. To this end we acquired deep V andR photometry for three globular clusters of the Large Magellanic Cloud,namely, NGC 2173, SL 556, and NGC 2155, in the age interval 1-3 Gyr. Inthis first paper, we describe the aim of the project and Very LargeTelescope observations and data reduction and we make preliminarycomparisons of the color-magnitude diagrams with both the Padova andYonsei-Yale isochrones. Two following papers in this series present theresults of a detailed analysis of these data, independently carried outby members of the Yale and Padova stellar evolution groups. This allowsus to compare both sets of models and discuss their main differences, aswell as the systematic effects that they would have on the determinationof the ages and metallicities of intermediate-age single-stellarpopulations.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO 64.L-0385).
|Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud|
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
|Mass segregation in young compact clusters in the Large Magellanic Cloud - III. Implications for the initial mass function|
The distribution of core radii of rich clusters in the Large MagellanicCloud (LMC) systematically increases in both upper limit and spread withincreasing cluster age. Cluster-to-cluster variations in the stellarinitial mass function (IMF) have been suggested as an explanation. Wediscuss the implications of the observed degree of mass segregation inour sample clusters for the shape of the initial mass function. Ourresults are based on Hubble Space Telescope/WFPC2 observations of sixrich star clusters in the LMC, selected to include three pairs ofclusters of similar age, metallicity and distance from the LMC centre,and exhibiting a large spread in core radii between the clusters in eachpair. All clusters show clear evidence of mass segregation: (i) theirluminosity function slopes steepen with increasing cluster radius, and(ii) the brighter stars are characterized by smaller core radii. For allsample clusters, both the slope of the luminosity function in thecluster centres and the degree of mass segregation are similar to eachother, within observational errors of a few tenths of power-law slopefits to the data. This implies that their initial mass functions musthave been very similar, down to ~0.8-1.0 Msolar. We thereforerule out variations in the IMF of the individual sample clusters as themain driver of the increasing spread of cluster core radii with clusterage.
|On the nature of a secondary main-sequence turn-off in the rich LMC cluster NGC 1868|
Evidence for a second main-sequence turn-off in a deep colour-magnitudediagram (CMD) of NGC 1868 is presented. The data were obtained withHubble Space Telescope (HST) WFPC2 and reach down to m555~=25 mag. Besides the usual τ~= 0.8 Gyr turn-off found in previousanalyses, another possible turn-off is seen at m555~= 21 mag(MV~= 2.5), which is consistent with an age of τ~= 3 Gyr.This CMD feature stands out clearly especially when contaminating fieldLarge Magellanic Cloud (LMC) stars are statistically removed. Thebackground subtracted CMD also visibly displays a red subgiant branchextending about 1.5 mag below the younger turn-off and the clump of redgiants. The significance of the secondary turn-off in NGC 1868 wasconfirmed with Monte Carlo simulations and bootstrapping techniques.Star counts in selected regions in the cluster CMD indicate a mass ratioof old population/young population in the range 5<~Mold/Myoung<~ 12 per cent, depending onthe mass-function slope. The existence of such a subpopulation in NGC1868 is significant even in the presence of uncertainties in backgroundsubtraction. The possibility that the secondary turn-off is associatedwith the field star population was examined by searching for similarfeatures in CMDs of field stars. Statistically significant excesses ofstars redwards of the main sequence were found in all such fields in therange 20 <~m555<~ 22 mag. These however are muchbroader features that do not resemble the main-sequence termination of asingle population. We also discuss other alternative explanations forthe feature at m555~= 21 mag, such as unresolved binarism,peculiar stars or CMD discontinuities associated with theBöhm-Vitense gap.
|A Large and Homogeneous Sample of CMDs of LMC Stellar Clusters|
We present the photometric results of 21 stellar clusters of the LargeMagellanic Cloud. The WFPC2 images were retrieved from the HST archive.Simple stellar populations in a large spread of age are well representedin the sample of color-magnitude diagrams shown here.
|Deep colour-magnitude diagrams of LMC field stars imaged with HST|
We present deep photometry (V<~26) in V and I bands obtained with theWide Field and Planetary Camera 2 on board the Hubble Space Telescopefor 7 fields ~5° away from the Large Magellanic Cloud centre. Thefields contain, typically, 2000 stars each. Isochrones were fitted tothe colour-magnitude diagrams in order to identify different starpopulations in these fields. An old population (τ>10Gyr) has beenfound in all fields. Some events of enhanced star formation, with agesbetween 2 and 4Gyr, were identified in the fields localized in the northto north-west regions. Luminosity functions of low-mass stars were alsoobtained for all fields. Kolmogorov Smirnov test results suggestdifferences smaller than 30 per cent in the mixture of stellarpopulations contributing to the fields. Finally, density profiles werederived for old and intermediate-age stars. The former shows a slightlysteeper decline than the latter.
|Large Magellanic Cloud stellar clusters. I. 21 HST colour magnitude diagrams|
We present WFPC2 photometry of 21 stellar clusters of the LargeMagellanic Cloud obtained on images retrieved from the Hubble SpaceTelescope archive. The derived colour magnitude diagrams (CMDs) arepresented and discussed. This database provides a sample of CMDsrepresenting, with reliable statistics, simple stellar populations witha large spread of age. The stars in the core of the clusters are allresolved and measured at least down to the completeness limit; themagnitudes of the main sequence terminations and of the red giant clumpare also evaluated for each cluster, together with the radius at halfmaximum of the star density. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. Table 1 is only available in electronic form athttp://www.edpsciences.org
|Stellar luminosity functions of rich star clusters in the Large Magellanic Cloud|
We show the results of deep V and I HST photometry of 6 rich starclusters in the Large Magellanic Cloud with different ages andmetallicities. The number of stars with measured magnitudes in eachcluster varies from about 3000 to 10 000 stars. We build stellar densityand surface brightness profiles for the clusters and extract half-lightradii and other structural parameters for each. We also obtainluminosity functions, Phi (M_V), down to M_V =~ 6 (M/Msun>~ 0.9), and investigate their dependence with distance from thecluster centre well beyond their half-light radius. In all clusters wefind a systematic increase in the luminosity functions slope, Delta logPhi (M_V) / Delta (M_V), with radial distance from the centre. Among theclusters displaying significant mass segregation are the 2 youngest inthe sample: NGC 1805, NGC 1818. For these two clusters we obtain presentday mass functions. The NGC 1818 mass function is in excellent agreementwith that derived by other authors, also using HST data. The youngcluster mass function slopes differ, that of NGC 1805 beingsystematically steeper than NGC 1818. Since these are very young stellarsystems (tau <~ 40 Myrs), these variations may reflect the initialconditions rather than evolution due to internal dynamics.
|A New Giant Branch Clump Structure in the Large Magellanic Cloud|
We present Washington C and T1 CCD photometry of 21 fieldslocated in the northern part of the Large Magellanic Cloud (LMC), andspread over a region of more than 2.5 deg2 approximately 6deg from the bar. The surveyed areas were chosen on the basis of theirproximity to SL 388 and SL 509, whose fields showed the presence of asecondary giant clump, observationally detected by Bica et al. We alsoobserved NGC 2209, located ~14 deg away from SL 509. From the collecteddata, we found that most of the observed field CMDs do not show aseparate secondary clump, but rather reveal a continuous verticalstructure (VS), which is clearly seen for the first time. The VS alsoappears in the field of NGC 2209. Its position and size are nearly thesame throughout the surveyed regions: it lies below the red giant clump(RGC) and extends from the bottom of the RGC to ~0.45 mag fainter,spanning the bluest color range of the RGC. In two fields in addition tothe NGC 2209 field the RGC is slightly tilted, following approximatelythe reddening vector, while the VS maintains its verticality. We foundthat the number of stars in the VS box defined byΔ(C-T1)=1.45-1.55 mag andΔT1=18.75-19.15 mag has a strong spatial variation,reaching the highest VS star density just northeast of SL 509. Moreover,the more numerous the VS stars in a field, the larger the number of LMCgiants in the same zone. We also found that, in addition to SL 509, tworelatively massive star clusters, SL 515 and NGC 2209, separated by morethan 10 deg from each other, develop giant clumps with a considerablenumber of VS stars. This result demonstrates that VS stars belong to theLMC and are most likely the result of some kind of evolutionary processin the LMC, particularly in those LMC regions with a noticeable largegiant population. Our results are successfully predicted by the modelsof Girardi in the sense that a large proportion of 1-2 Gyr old starsmixed with older stars and with metallicities higher than [Fe/H]~=-0.7should result in a fainter and bluer secondary clump near the mass atwhich degenerate core He burning takes place. However, our resultsapparently suggest that in order to trigger the formation of VS stars,there should be other conditions in addition to the appropriate age,metallicity, and the necessary red giant star density. Indeed, starssatisfying the requisites mentioned above are commonly found throughoutthe LMC, but the VS phenomenon is only clearly seen in some isolatedregions. Finally, the fact that clump stars have an intrinsic luminositydispersion further constrains the use of the clump magnitude as areliable distance indicator.
|A secondary clump of red giant stars: why and where|
Based on the results of detailed population synthesis models, Girardi etal. recently claimed that the clump of red giants in thecolour-magnitude diagram (CMD) of composite stellar populations shouldpresent an extension to lower luminosities, which goes down to about0.4mag below the main clump. This feature is made of stars just massiveenough to have ignited helium in non-degenerate conditions, andtherefore corresponds to a limited interval of stellar masses and ages.In the present models, which include moderate convective overshooting,it corresponds to ~1Gyr old populations. In this paper, we go into moredetail about the origin and properties of this feature. We first comparethe clump theoretical models with data for clusters of different agesand metallicities, basically confirming the predicted behaviour. We thenrefine the previous models in order to show the following behaviour. (i)The faint extension is expected to be clearly separated from the mainclump in the CMD of metal-rich populations, defining a `secondary clump'by itself. (ii) It should be present in all galactic fields containing~1Gyr old stars and with mean metallicities higher than about Z=0.004.(iii) It should be particularly strong, if compared with the main redclump, in galaxies that have increased their star formation rate in thelast Gyr or so of their evolution. In fact, secondary clumps similar tothe model predictions are observed in the CMD of nearby stars fromHipparcos data, and in those of some Large Magellanic Cloud fieldsobserved to date. There are also several reasons why this secondaryclump may be missing or hidden in other observed CMDs of galaxy fields.For instance, it becomes indistinguishable from the main clump if thephotometric errors or differential absorption are larger than about0.2mag. None the less, this structure may provide important constraintson the star formation history of Local Group galaxies. We comment alsoon the intrinsic luminosity variation and dispersion of clump stars,which may limit their use as either absolute or relative distanceindicators, respectively.
|A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud|
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
|The evolution of theV-Kcolours of single stellar populations|
Models of evolutionary population synthesis of galaxies rely on theproperties of the so-called single stellar populations (SSP). In thispaper, we discuss how the integrated near-infrared colours, andespecially V-K, of SSPs evolve with age and metallicity. Some of theuncertainties associated with the properties of the underlying stellarmodels are thoroughly discussed. Our models include all the relevantstellar evolutionary phases, with particular attention being dedicatedto the asymptotic giant branch (AGB), which plays a fundamental role inthe evolution of the near-infrared part of the spectrum. First, wepresent the effects that different formulations for the mass-loss ratesproduce on the final remnant mass (i.e., on the initial-final massrelation), and hence on the AGB-termination luminosity and the relativecontribution of these stars to the integrated light. The results for theevolution of the V-K colour are very different depending on the choiceof the mass-loss prescription; the same is true also for the B-V colourin the case of low-metallicity SSPs. Secondly, we describe the changesoccurring in the integrated colours at the onset of the AGB and redgiant (RGB) branches. According to the classical formalism for the AGBevolution, the onset of this evolutionary phase is marked by a colourjump to the red, the amplitude of which is shown here to be highlydependent on the metallicity and mass-loss rates adopted in the models.We then consider the effect of the overluminosity with respect to thestandard core mass-luminosity relation that occurs in the most massiveAGB stars. Different simplified formulations for this effect are testedin the models; they cause a smoothing of the colour evolution in the agerange at which the AGB starts to develop, rather than a splitting of thecolour jump into two separate events. On the other hand, we find that atemporary red phase takes place ~1.5x10^8 yr after the RGB develops.Thanks to the transient nature of this feature, the onset of the RGB isprobably not able to cause marked features in the spectral evolution ofgalaxies. We then discuss the possible reasons for the transition of V-Kcolours (from ~1.5 to 3) that takes place in LMC clusters of SWB typeIV. A revision of the ages attributed to the single clusters revealsthat the transition may not be as fast as originally suggested. Thecomparison of the data with the models indicates that the transitionresults mainly from the development of the AGB. A gradual (or delayed)transition of the colours, as predicted by models which include theoverluminosity of the most massive AGB stars, seems to describe the databetter than the sudden colour jump predicted by classical models.
|The relation between the initial and final masses of stars with different chemical compositions|
We present the results of calculations for the relations between theinitial and final masses M_i-M_f of low- and moderate-mass stars forvarious initial heavy-element abundances Z. For Z = 0.02 and Z = 0.001,the resulting differences in the final masses for white dwarfs reach0.1M_solar for initial masses from 1.5 to 4M_solar. These differencesare primarily due to the dependence of the initial masses of thecarbon-oxygen cores of asymptotic giant branch stars on their chemicalcompositions. We study the roles of various assumptions about mass lossof stars in the final stages of their evolution. The population of whitedwarfs is modeled, and their mass distribution is obtained for variousassumptions about the initial chemical composition of the stars.
|A Search for Old Star Clusters in the Large Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
|The Star-Formation History of LGS 3|
LGS 3 presents the optical appearance of a dwarf spheroidal galaxy, buthas a significant neutral hydrogen component. Photometry of itsbrightest stars with COSMIC at the prime focus of the Hale telescopeshows a 10 (superscript 8) year old population and a 10 (superscript 10)year old population. Either we are seeing a recent burst of starformation in a dwarf spheroidal galaxy, or, if intermediate agepopulations are also present, we are witnesing recent transition from adwarf irregular to a dwarf spheroidal galaxy. (SECTION: Galaxies)
|Carbon stars in LMC clusters revisited.|
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
|Globular Clusters in the Inner Regions of NGC 5128 (Centaurus A)|
We have identified 26 new globular cluster candidates in the inner 3 kpcof NGC 5128 (Centaurus A), the nearest known large galaxy that is theprobable product of a merger. The clusters are selected on the basis oftheir structural parameters (observed core diameters and ellipticities),as measured from archival Wide Field Planetary Camera (WFPC) HubbleSpace Telescope (HST) images. IR photometry obtained with IRAC2B at theESO/MPI 2.2 m telescope is combined with the optical HST photometry.Most of these clusters have normal colors typical of old globularclusters like those found in the Milky Way and M31. We estimate theirmetal abundances based on the R-K_0_ color, confirming the existence ofa metallicity gradient in the inner regions of NGC 5128. The presence ofmetal-rich globular clusters suggests that one of the colliding galaxieswas a bulge-dominated galaxy (E or early S). A few clusters have colorsand magnitudes similar to intermediate-age clusters containing carbonstars in the Magellanic Clouds. If the intermediate-age clusters wereformed during a merger, then this episode must have occurred a fewgigayears ago. Alternatively, we are looking at the cluster members ofone of the colliding galaxies, which would then have been a late-typedisk galaxy.
|Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud|
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
|Age distribution of LMC clusters from their integrated UBV colors: history of star formation.|
In this paper we revise the relationship between ages and metallicitiesof LMC star clusters and their integrated UBV colors. The study standson the catalog of UBV colors of the Large Magellanic Cloud (LMC)clusters by Bica et al. (1994; BCDSP) and the photometric models ofsingle stellar populations (SSP) calculated by Bertelli et al. (1994).These photometric models nicely describe the color distribution of LMCclusters in the (U-B) vs. (B-V) plane together with the observeddispersion of the colors and the existence of a gap in a certain regionof this diagram. In the case of blue clusters, most of the dispersion inthe colors can be accounted for by the presence of stochastic effects onthe mass distribution of stars, whereas for the red ones additionaldispersion's of ~0.2dex in metallicity and of ~0.05mag in color excessare needed. From comparing the observed distribution of integratedcolors in the (U-B) vs. (B-V) diagram with the theoretical models, itturns out that: 1) The data are consistent with the presence of a gap(period of quiescence) in the history of cluster formation. If theage-metallicity relation (AMR) for the LMC obeys the simple model ofchemical evolution, the gap is well evident and corresponds to the ageinterval ~3Gyr to (12-15)Gyr. On the contrary, if the chemicalenrichment has been much slower than in the simple model, so thatintermediate age clusters are less metal rich, the gap is expected tooccur over a much narrower color range and to be hidden by effects ofcolor dispersion. 2) The bimodal distribution of B-V colors can bereproduced by a sequence of clusters almost evenly distributed in thelogarithm of the age, whose metallicity is governed by a normal AMR. Noneed is found of the so-called phase transitions in the integratedcolors of a cluster taking place at suitable ages (Renzini & Buzzoni1986). 3) The gap noticed by BCDSP in the (U-B) vs. (B-V) plane can beexplained by the particular direction along which cluster colors aredispersed in that part of the (U-B) vs. (B-V) diagram. Also in thiscase, no sudden changes in the integrated properties of clusters must beinvoked. The results of this analysis are used to revise the empiricalmethod proposed by Elson & Fall (1985, EF85) to attribute ages toLMC clusters according to their integrated UBV colors. We show that theEF85 method does not provide the correct relation between ages andcolors for clusters of low metallicity and hence its inability to datethe old clusters. We propose two modifications to the definition of theparameter S of EF85 such that the age sequence of red clusters issuitably described, and the intrinsic errors on ages caused by the heavypresence of various effects dispersing the colors are reduced to aminimum. The age sequence is calibrated on 24 template clusters forwhich ages were independently derived from recent color-magnitudediagrams (CMD). Finally, we attribute ages to all clusters present inBCDSP catalog, and derive the global age distribution function (ADF) forLMC clusters. The ADF presents new features that were not clear inprevious analyses of UBV data, but were already suggested by a number ofindependent observational studies. The features in question are periodsof enhanced cluster formation at ~100Myr and 1-2Gyr, and a gap in thecluster formation history between ~3 and (12-15)Gyr. The peaks observedin the distribution of B-V colors are found to be sensitive to thepresence of these periods of enhanced cluster formation and the lack ofextremely red clusters caused by the age gap between intermediate-ageand old clusters.
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