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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.
| 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
| 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.
| A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
| Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds
We present accurate integrated-light photometry in Johnson/Cousins V, Rand I for a sample of 28 globular clusters in the Magellanic Clouds. Themajority of the clusters in our sample have reliable age and metallicityestimates available in the literature. The sample encompasses agesbetween 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between -1.5 and0.0 dex. The sample is dominated by clusters of ages between roughly 0.5and 2 Gyr, an age range during which the bolometric luminosity of simplestellar populations is dominated by evolved red giant branch stars andthermally pulsing asymptotic giant branch (TP-AGB) stars whosetheoretical colours are rather uncertain. The VRI colours presented inthis paper have been used to calibrate stellar population synthesismodel predictions.
| Mass segregation in rich LMC clusters from modelling of deep HST colour-magnitude diagrams
Aims.We used the deep colour-magnitude diagrams (CMDs) of five rich LMCclusters (NGC 1805, NGC 1818, NGC 1831, NGC 1868, and Hodge 14) observedwith HST/WFPC2 to derive their present day mass function (PDMF) and itsvariation with position within the cluster. Methods: .The PDMFwas parameterized as a power law in the available main-sequence massrange of each cluster, typically 0.9 ⪉ m/Mȯ ⪉2.5; its slope was determined at different positions spanning from thevery centre out to several core radii. The CMDs in the central regionsof the clusters were carefully studied earlier, resulting in accurateage, metallicity, distance modulus, and reddening values. The slopeα (where Salpeter is 2.35) was determined in annuli by followingtwo distinct methods: 1) a power law fit to the PDMF obtained from thesystemic luminosity function (LF); 2) a statistical comparison betweenobserved and model CMDs. In the second case, α is a free inputparameter in the CMD modelling process where we incorporate photometricerrors and the effect of binarity as a fraction of unresolved binaries(f{bin}=100%) with random pairing of masses from the samePDMF. Results: .In all clusters, significant mass segregation isfound from the positional dependence of the PDMF slope: α ⪉1.8 for R ≤ 1.0 R{core} and α ˜ Salpeterinside R=2 ˜ 3 R{core} (except for Hodge 14, whereα ˜ Salpeter for R ˜ 4 R{core}). Theresults are robust in the sense that they hold true for both methodsused. The CMD method reveals that unresolved binaries flatten the PDMFobtained form the systemic LF, but this effect is smaller than theuncertainties in the α determination. For each cluster weestimated dynamical ages inside the core and for the entire system. Inboth cases we found a trend in the sense that older clusters haveflatter PDMF, consistent with a dynamical mass segregation and stellarevaporation.
| Core Radius-Mass Evolution of Globular Clusters
Some dynamical features of present day globular clusters seem to be theresult of the effects produced at the epoch of formation, both by therate of primordial binary stars and the formation and destruction of newones. Even a mass segregation and a cluster evaporation driven by thepopulation of binary stars are possible. The spread in the core radiusamong intermediate age and old stars clusters, observed e.g. in the LMC,could be generated by these two effects. In this contribution somepreliminary results are shown.
| 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.
| Physical parameters of rich LMC clusters from modeling of deep HST colour-magnitude diagrams
We present the analysis of deep colour-magnitude diagrams (CMDs) of fiverich LMC clusters. The data were obtained with HST/WFPC2 in the F555W(~V) and F814W (~I) filters, reaching V555 ˜ 25. Thesample of clusters is composed of NGC 1805 and NGC 1818, the youngestones (τ < 100 Myr), NGC 1831 and NGC 1868, of intermediate-age (400 < τ < 1000 Myr), and Hodge 14, the oldest (τ > 1200Myr). We discuss and apply a statistical method for correcting the CMDfor sampling incompleteness and field star contamination. Efficient useof the CMD data was made by means of direct comparisons of the observedto model CMDs. The CMD modeling process generates a synthetic MainSequence (MS), where we introduce as model inputs the information aboutage, chemical composition, present day mass function (PDMF), fraction ofunresolved binaries, distance modulus and light extinction. Thephotometric uncertainties were empirically determined from the data andincorporated into the model as well. Statistical techniques of CMDcomparisons using 1 and 2 dimensions are presented and applied as anobjective method to assess the compatibility between model and dataCMDs. By modeling the CMDs from the central region we infer themetallicity (Z), the intrinsic distance modulus ((m-M)0) andthe reddening value (E(B-V)) for each cluster. We also determined theage for the clusters with τ > 400 Myr. By means oftwo-dimensional CMD comparisons we infer the following values: for NGC1805, Z=0.007 ± 0.003, (m-M)0=18.50 ± 0.11,E(B-V)=0.03 ± 0.01; for NGC 1818, Z=0.005 ± 0.002,(m-M)0=18.49 ± 0.14, E(B-V) ˜ 0.00; for NGC1831, Z=0.012 ± 0.002, log(τ/yr)=8.70 ± 0.03,(m-M)0=18.70 ± 0.03, E(B-V) ˜ 0.00; for NGC1868, Z=0.008 ± 0.002, log(τ/yr)=8.95 ± 0.03,(m-M)0=18.70 ± 0.03, E(B-V) ˜ 0.00; for Hodge14, Z=0.008 ± 0.004, log(τ/yr)=9.23 ± 0.10,(m-M)0=18.51 ± 0.13, E(B-V)=0.02 ± 0.02. Takinginto account the uncertainties, these values are in accordance with theones obtained applying the one-dimensional CMD analysis, addingreliability to these determinations.
| 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.
| 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.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| Star Clusters in Virgo and Fornax Dwarf Irregular Galaxies
We present the results of a search for clusters in dwarf irregulargalaxies in the Virgo and Fornax Clusters using Hubble Space Telescope(HST) WFPC2 snapshot data. The galaxy sample includes 28 galaxies, 11 ofwhich are confirmed members of the Virgo and Fornax Clusters. In the 11confirmed members, we detect 237 cluster candidates and determine theirV magnitudes, V-I colors, and core radii. After statistical subtractionof background galaxies and foreground stars, most of the clustercandidates have V-I colors of -0.2 and 1.4, V magnitudes lying between20 and 25 mag, and core radii between 0 and 6 pc. Using Hαobservations, we find that 26% of the blue cluster candidates are mostlikely H II regions. The rest of the cluster candidates are most likelymassive (>104 Msolar) young and old clusters. Acomparison between the red cluster candidates in our sample and theMilky Way globular clusters shows that they have similar luminositydistributions but that the red cluster candidates typically have largercore radii. Assuming that the red cluster candidates are in factglobular clusters, we derive specific frequencies (SN)ranging from ~0 to 9 for the galaxies. Although the values areuncertain, seven of the galaxies appear to have specific frequenciesgreater than 2. These values are more typical of elliptical andnucleated dwarf elliptical galaxies than they are of spiral or LocalGroup dwarf irregular galaxies.
| Analyzing Starbursts Using Magellanic Cloud Star Clusters as Simple Stellar Populations
Integrated spectra have been obtained of 31 star clusters in theMagellanic Clouds (MC) and of four Galactic globular clusters. Thespectra cover the wavelength range 3500-4700 Å at a resolution of3.2 Å FWHM. The MC clusters primarily cover the age range fromless than 108 to about 3 Gyr and hence are well-suited to anempirical study of aging poststarburst stellar populations. Anage-dating method is presented that relies on two spectral absorptionfeature indices, Hδ/Fe I λ4045 and Ca II, as well as anindex measuring the strength of the Balmer discontinuity. We compare thebehavior of the spectral indices in the observed integrated spectra ofthe MC clusters with that of indices generated from theoreticalevolutionary synthesis models of varying age and metal abundance. Thesynthesis models are based on those of Worthey, when coupled with thecombination of an empirical library of stellar spectra by Jones for thecooler stars and synthetic spectra, generated from Kurucz modelatmospheres, for the hotter stars. Overall, we find good agreementbetween the ages of the MC clusters derived from our integrated spectra(and the evolutionary synthesis modelling of the spectral indices) andages derived from analyses of the cluster color-magnitude diagrams, asfound in the literature. Hence, the principal conclusion of this studyis that ages of young stellar populations can be reliably measured frommodelling of their integrated spectra.
| Near-infrared color evolution of LMC clusters
We present here the digital aperture photometry for 28 LMC clusterswhose ages are between 5 Myr and 12 Gyr. This photometry is based on ourimaging observations in JHK and contains integrated magnitudes andcolors as a function of aperture radius. In contrast to optical colors,our near-infrared colors do not show any strong dependence on clusterages.Tables 2 and 3 and Fig. 2 are only available in electronic form athttp://www.edpsciences.org
| 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.
| 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.
| Mass segregation in young compact star clusters in the Large Magellanic Cloud - I. Data and luminosity functions
We have undertaken a detailed analysis of HST/WFPC2 and STIS imagingobservations, and of supplementary wide-field ground-based observationsobtained with the ESO New Technology Telescope (NTT) of two young(~10-25Myr) compact star clusters in the LMC, NGC 1805 and 1818. Theultimate goal of our work is to improve our understanding of the degreeof primordial mass segregation in star clusters. This is crucial for theinterpretation of observational luminosity functions (LFs) in terms ofthe initial mass function (IMF), and for constraining the universalityof the IMF. We present evidence for strong luminosity segregation inboth clusters. The LF slopes steepen with cluster radius; in both NGC1805 and 1818 the LF slopes reach a stable level well beyond the core ofthe clusters or half-light radii. In addition, the brightest clusterstars are strongly concentrated within the inner ~4Rhl. Theglobal cluster LF, although strongly non-linear, is fairly wellapproximated by the core or half-light LF; the (annular) LFs at theseradii are dominated by the segregated high-luminosity stars, however. Wepresent tentative evidence for the presence of an excess number ofbright stars surrounding NGC 1818, for which we argue that they are mostprobably massive stars that have been collisionally ejected from thecluster core. We therefore suggest that the cores of massive young starsclusters undergo significant dynamical evolution, even on time-scales asshort as ~25Myr.
| 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
| Synthetic Spectra of H Balmer and HE I Absorption Lines. II. Evolutionary Synthesis Models for Starburst and Poststarburst Galaxies
We present evolutionary stellar population synthesis models to predictthe spectrum of a single-metallicity stellar population, with a spectralsampling of 0.3 Å in five spectral regions between 3700 and 5000Å. The models, which are optimized for galaxies with active starformation, synthesize the profiles of the hydrogen Balmer series(Hβ, Hγ, Hδ, H8, H9, H10, H11, H12, and H13) and theneutral helium absorption lines (He I λ4922, He I λ4471,He I λ4388, He I λ4144, He I λ4121, He Iλ4026, He I λ4009, and He I λ3819) for a burst withan age ranging from 106 to 109 yr, and differentassumptions about the stellar initial mass function (IMF). Continuousstar formation models lasting for 1 Gyr are also presented. The inputstellar library includes non-LTE absorption profiles for stars hotterthan 25,000 K and LTE profiles for lower temperatures. The temperatureand gravity coverage is 4000 K<=Teff<=50,000 K and0.0<=logg<=5.0, respectively. The metallicity is solar. It is found thatthe Balmer and He I line profiles are sensitive to the age, exceptduring the first 4 Myr of the evolution, when the equivalent widths ofthese lines are constant. At these early stages of the evolution, theprofiles of the lines are also sensitive to the IMF. However, strong HBalmer and He I lines are predicted even when the low-mass cutoff of theIMF is as high as 10 Msolar. The equivalent widths of theBalmer lines range from 2 to 16 Å and those of the He I lines from0.2 to 1.2 Å. During the nebular phase (cluster younger than about10 Myr), Hβ ranges from 2 to 5 Å and He I λ4471 rangesbetween 0.5 and 1.2 Å. The strength of the lines is maximum whenthe cluster is a few hundred (for the Balmer lines) and a few tens (forthe He I lines) of Myr old. In the continuous star formation scenario,the strength of the Balmer and He I lines increases monotonically withtime until 500 and 100 Myr, respectively. However, the lines are weakerthan in the burst models owing to the dilution of the Balmer and He Ilines by the contribution from very massive stars. The high spectralresolution of the profiles is useful to reproduce the absorption wingsobserved in regions of recent star formation and to estimate the effectof the underlying absorption on the nebular emission lines. The strengthof the nebular emission Balmer and He I lines compared with the stellarabsorption components indicates that Hδ and the higher order termsof the Balmer series and He I are dominated by the stellar absorptioncomponent if an instantaneous burst is older than ~=5 Myr. Some of theHe I lines (e.g., He I λ3819 and He I λ4922) are morefavorable than others (e.g., He I λ4471) for the detection ofstellar features in the presence of nebular emission. We estimate thatthe correction to the He I λ4471 nebular emission line due to thestellar absorption is between 5% and 25%, if the nebular emission hasequivalent width between 10 and 2.5 Å (corresponding to a burstage between 1 and 3 Myr). The models can be used to date starburst andpoststarburst galaxies until 1 Gyr. They have been tested on data forclusters in the LMC, the super-star cluster B in the starburst galaxyNGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous``E+A'' galaxy. The full data set is available for retrieval at ourwebsites or on request from the authors.
| 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.
| Keck Spectroscopy of Candidate 97Proto-Globular Clusters in NGC 1275
Keck spectroscopy of five proto-globular cluster candidates in NGC 1275has been combined with Hubble Space Telescope Wide Field PlanetaryCamera 2 photometry to explore the nature and origin of these objectsand discriminate between merger and cooling-flow scenarios for globularcluster formation. The objects we have studied are not H ii regions, butrather star clusters, yet their integrated spectral properties do notresemble young or intermediate-age Magellanic Cloud clusters or MilkyWay open clusters. The clusters' Balmer absorption appears to be toostrong to be consistent with any of the standard Bruzual & Charlotevolutionary models at any metallicity. If the Bruzual & Charlotmodels are adopted, an initial mass function (IMF) that is skewed tohigh masses provides a better fit to the data of the proto-globularcluster candidates. A truncated IMF with a mass range of 2-3 M_ȯreproduces the observed Balmer equivalent widths and colors at ~450 Myr.Formation in a continuous cooling flow appears to be ruled out since theage of the clusters is much larger than the cooling time, the spatialscale of the clusters is much smaller than the cooling-flow radius, andthe deduced star formation rate in the cooling flow favors a steeprather than a flat IMF. A merger would have to produce clusters only inthe central few kiloparsecs, presumably from gas in the merging galaxiesthat was channeled rapidly to the center. Widespread shocks in merginggalaxies cannot have produced these clusters. If these objects areconfirmed to have a relatively flat, or truncated, IMF, it is unclearwhether they will evolve into objects that we would regard as bona fideglobular clusters. Based on observations obtained at the W. M. KeckObservatory, which is operated jointly by the California Institute ofTechnology and the University of California.
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