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Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
| The evolution of binary star clusters and the nature of NGC 2136/NGC 2137
We study the evolution of bound pairs of star clusters by means ofdirect N-body simulations. Our simulations include mass loss by stellarevolution. The initial conditions are selected to mimic the observedbinary star clusters, NGC2136 and 2137, in the Large Magellanic Cloud.Based on their rather old ages (~100Myr), masses, sizes and projectedseparation, we conclude that the cluster pair must have been born withan initial separation of 15-20 pc. Clusters with a smaller initialseparation tend to merge in <~60Myr due to loss of angular momentumfrom escaping stars. Clusters with a larger initial separation tend tobecome even more widely separated due to mass loss from the evolvingstellar populations. The early orbital evolution of a binary cluster isgoverned by mass loss from the evolving stellar population and by lossof angular momentum from escaping stars. Mass loss by stellar winds andsupernovae explosions in the first ~30Myr causes the binary to expandand the orbit to become eccentric. The initially less massive clusterexpands more quickly than the binary separation increases, and istherefore bound to initiate mass transfer to the more massive cluster.This process is quite contrary to stellar binaries in which the moremassive star tends to initiate mass transfer. Since mass transferproceeds on a thermal time-scale from the less massive to the moremassive cluster, this semidetached phase is quite stable, even in aneccentric orbit until the orbital separation reaches the gyration radiusof the two clusters, at which point both clusters merge to one.
| Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
| The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters
Using available data for C and M giants with M_bol<-3.6 in MagellanicCloud clusters, we derive limits to the lifetimes for the correspondingevolutionary phases, as a function of stellar mass. The C-star phase isfound to have a duration between 2 and 3 Myr for stars in the mass rangefrom ~1.5 to 2.8 M_ȯ. There is also an indication that the peak ofC-star lifetime shifts to lower masses (from slightly above to slightlybelow 2 Mȯ) as we move from LMC to SMC metallicities.The M-giant lifetimes also peak at ~2 Mȯ in the LMC,with a maximum value of about 4 Myr, whereas in the SMC their lifetimesappear much shorter, but, actually, they are poorly constrained by thedata. These numbers constitute useful constraints to theoretical modelsof the TP-AGB phase. We show that several models in the literatureunderestimate the duration of the C-star phase at LMC metallicities.
| 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.
| The Star Clusters of the Small Magellanic Cloud: Structural Parameters
We present structural parameters for 204 stellar clusters in the SmallMagellanic Cloud derived from fitting King and Elson, Fall, and Freeman(EFF) model profiles to the V-band surface brightness profiles asmeasured from the Magellanic Clouds Photometric Survey images. Both Kingand EFF profiles are satisfactory fits to the majority of the profiles,although King profiles are generally slightly superior to the softenedpower-law profiles of EFF and provide statistically acceptable fits to~90% of the sample. We find no correlation between the preferred modeland cluster age. The only systematic deviation in the surface brightnessprofiles that we identify is a lack of a central concentration in asubsample of clusters, which we designate as ``ring'' clusters. Inagreement with previous studies, we find that the clusters in the SMCare significantly more elliptical than those in the Milky Way. However,given the mean age difference and the rapid destruction of thesesystems, the comparison between the SMC and the Milky Way should notdirectly be interpreted as a difference in either the initial clusterproperties or their subsequent evolution. We find that clusterellipticity correlates with cluster mass more strongly than with clusterage. We identify several other correlations (central surface brightnessvs. local background density, core radius vs. tidal force, and size vs.distance) that can be used to constrain models of cluster evolution inthe SMC.
| 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.
| 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.
| Cepheids in LMC Clusters and the Period-Age Relation
We have made a new comparison of the positions of Cepheids and clustersin the LMC and constructed a new empirical period-age relation takinginto account all available data on Cepheids in the LMC bar provided bythe OGLE project. The most probable relation is logT=8.50-0.65 logP, inreasonably good agreement with theoretical expectations. NumerousCepheids in rich clusters of the LMC provide the best data for comparingtheories of stellar evolution and pulsation and the dynamical evolutionof clusters with observations. These data suggest that stars undergoingtheir first crossing of the instability strip are first-overtonepulsators, though the converse is true of only a small fraction offirst-overtone stars. Several rich clusters with suitable ages have noCepheids—a fact that is not understood and requires verification.Differences in the concentration of Cepheids toward their clustercenters probably reflect the fact that the clusters are at differentstages of their dynamical evolution, with the Cepheids in clustercoronas being ejected from the cluster cores during dynamicalinteractions between stars.
| 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
| 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.
| Lithium during the AGB evolution in young open clusters of the Large Magellanic Cloud
We present the results of mid-resolution spectroscopy in the Li I 6708Å spectral region of Asymptotic Giant Branch (AGB) stars belongingto young open clusters of the Large Magellanic Cloud. Most stars belongto the clusters NGC 1866 and NGC2031, which have an age of =~ 150 Myr. Lithium lines ofdifferent strength are detected in the spectra of stars evolving alongthe AGB, in agreement with theoretical predictions. According to stellarevolutionary models, at the start of the AGB the stars should all show alow residual lithium abundance as a consequence of dilution during theprevious evolutionary phases. The most luminous and cooler thermallypulsating AGB stars, if they are massive enough, once in the AGB gofirst through a phase of Li destruction, which is followed by a phase ofstrong lithium production and further destruction. The production oflithium on the AGB is in particular explained by the onset of the ``HotBottom Burning'' (HBB) process. Our most conclusive results are obtainedfor the populous cluster NGC 1866 where: the``early-AGB'' stars show a weak Li line, which can be attributed to thedilution of the initial abundance; one of the two luminous stars seem tohave completely depleted lithium, as no line is detected; the second oneshows a deep lithium line, whose strength can be explained by lithiumproduction. The bolometric magnitude of these stars are consistent withthe values predicted by the theory, for the mass evolving on the AGB ofNGC 1866, at which lithium is first destroyed andthen produced (Mbol =~ -6). We also analyze the infraredluminosities (ISOCAM data) of these stars, to discuss if theirevolutionary phase precedes or follows the lithium production stage.More intriguing and less clear results are obtained for the mostluminous stars in NGC 2031: the brightest star seemsas well to have destroyed lithium, while the second one shows a stronglithium line. However its bolometric luminosity - derived from thenear-IR photometry, is much lower (Mbol =~ -5.2 +/- 0.15)than that expected from HBB models. Although low luminosity lithium richAGB stars are also known, whose appearance is attributed tonon-canonical mixing processes, it is not clear why two almost coevalclusters show such a different behaviour. It is also possible that thisstar does not belong to NGC 2031. Finally we suggestthe observational tests that could shed further light on this matter.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| 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 statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| Morphologies and ages of star cluster pairs and multiplets in the Small Magellanic Cloud
An isophotal atlas of 75 star cluster pairs and multiplets in the SmallMagellanic Cloud is presented, comprising 176 objects. They areconcentrated in the SMC main body. The isophotal contours were made fromDigitized Sky Survey* images and showed relevant structuralfeatures possibly related to interactions in about 25% of the sample.Previous N-body simulations indicate that such shapes could be due totidal tails, bridges or common envelopes. The diameter ratio between themembers of a pair is preferentially in the range 1 - 2, with a peak at1. The projected separation is in the range ~ 3 - 22 pc with apronounced peak at ~ 13 pc. For 91 objects it was possible to deriveages from Colour-Magnitude Diagrams using the OGLE-II photometricsurvey. The cluster multiplets in general occur in OB stellarassociations and/or HII region complexes. This indicates a common originand suggests that multiplets coalesce into pairs or single clusters in ashort time scale. Pairs in the SMC appear to be mostly coeval andconsequently captures are a rare phenomenon. We find evidence that starcluster pairs and multiplets may have had an important role in thedynamical history of clusters presently seen as large single objects.The images in this study are based on photographic data obtained usingthe UK Schmidt Telescope, which was operated by the Royal ObservatoryEdinburgh, with funding from the UK Science and Engineering ResearchCouncil, until 1988 June, and thereafter by the Anglo-AustralianObservatory. Original plate material is copyright by the RoyalObservatory Edinburgh and the Anglo-Australian Observatory. The plateswere processed into the present compressed digital form with theirpermission. The Digitized Sky Survey was produced at the Space TelescopeScience Institute under US Government grant NAG W-2166.
| Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.
| 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.
| Suitability of the King law for the globular clusters NGC 6171, NGC 5466, and M92
We have studied the suitability of a King law for the distributions ofthe apparent stellar density in the three globular clusters NGC 6171,NGC 5466, and M92 based on star counts to various limiting stellarmagnitudes B. The density distributions cannot be adequately describedby the King law. The parameters r_t (the tidal radius of the cluster)and r_c (the core radius) depend on the limiting B magnitude for countsof faint cluster stars below the transition region between the subgiantbranch and the main-sequence turnoff in the (V, B_V) diagram. Variationof these parameters is similar to variation of the sizes of the clusterspatial zones introduced by Kholopov.
| Young star clusters of our galaxy and the Large Magellanic Cloud as test for stellar evolutionary models.
Not Available
| 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.
| Hierarchical star formation from the time-space distribution of star clusters in the Large Magellanic Cloud
The average age difference between pairs of star clusters in the LargeMagellanic Cloud (LMC) increases with their separation as the ~0.35power. This suggests that star formation is hierarchical in space and intime. Small regions form stars quickly and large regions, which oftencontain the small regions, form stars over a longer period. A similarresult found previously for Cepheid variables is statistically lesscertain than the cluster result.
| 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.
| Constraints on the initial conditions of globular clusters
N-body simulations are made with a variety of initial conditions, inparticular clumpy and flattened distributions, to attempt to constrainthe possible initial conditions of globular clusters, using theobservations that young LMC globular clusters appear relaxed after only20 to 40 Myr. It is found that violent relaxation is able to erase mostof the initial substructure in only approximately six crossing times.However, initially very clumpy distributions (not greater than about 100clumps) form clusters that are too concentrated to resemble realglobular clusters. Such clusters also often have large clumps inlong-lasting (not less than about 30 crossing times) orbits which do notappear in observed cluster profiles. It is also found that even modestamounts of initial flattening produce clusters that are too ellipticalto resemble real globular clusters. In such a scenario, cloud-cloudcollisions and similar energetic processes would be unlikely to producesufficiently spherical globular clusters. It is suggested that globularclusters form from roughly spherical initial conditions with starformation occurring either smoothly or in many small clumps.
| The initial conditions of young globular clusters in the Large Magellanic Cloud
N-body simulations are used to model the early evolution of globularclusters. These simulations include residual gas which was not turnedinto stars, which is expelled from the globular cluster by the actionsof massive (>8Msolar) stars. The results of these simulations arecompared with observations of eight LMC globular clusters less than 100Myr old. These observations are used to constrain the initial conditionsthat may have produced these clusters. It is found that the observationscan be accounted for in a model where the globular clusters form fromproto-cluster clouds similar to Plummer models with length-scales in therange 1
| Internal dynamics of globular clusters
Galactic globular clusters, which are ancient building blocks of ourGalaxy, represent a very interesting family of stellar systems in whichsome fundamental dynamical processes have taken place on time scalesshorter than the age of the universe. In contrast with galaxies, theseclusters represent unique laboratories for learning about two-bodyrelaxation, mass segregation from equipartition of energy, stellarcollisions, stellar mergers, and core collapse. In the present review,we summarize the tremendous developments, as much theoretical asobservational, that have taken place during the last two decades, andwhich have led to a quantum jump in our understanding of these beautifuldynamical systems.
| Carbon stars in LMC clusters revisited.
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
| Research programs at the Carter Observatory: an overview
An outline is given of the main research programmes currently inprogress at the Carter Observatory. These include the establishment of aset of standard star magnitudes and colours in the Vilnius seven colourphotometric system; the study of galactic and extragalactic starclusters using Vilnius and broadband photometries; binary stars and thedevelopment of APTs; and the history of Australasian astronomy. The roleof Carter Observatory Honorary Research Associates is described andmention is made of the joint New Zealand/Japan programme to observegravitational micro-lensing effects, discover variable stars and patrolselected clusters of galaxies for supernovae.
| The fundamental plane of early-type galaxies: stellar populations and mass-to-light ratio.
We analyse the residuals to the fundamental plane (FP) of ellipticalgalaxies as a function of stellar-population indicators; these are basedon the line-strength parameter Mg_2_ and on UBVRI broad-band colors, andare partly derived from new observations. The effect of the stellarpopulations accounts for approximately half the observed variation ofthe mass-to-light ratio responsible for the FP tilt. The residual tiltcan be explained by the contribution of two additional effects: thedependence of the rotational support, and possibly that of the spatialstructure, on the luminosity. We conclude to a constancy of thedynamical-to-stellar mass ratio. This probably extends to globularclusters as well, but the dominant factor would be here the luminositydependence of the structure rather than that of the stellar population.This result also implies a constancy of the fraction of dark matter overall the scalelength covered by stellar systems. Our compilation ofinternal stellar kinematics of galaxies is appended.
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