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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
| 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.
| The Chemical Properties of Milky Way and M31 Globular Clusters. I. A Comparative Study
A comparative analysis is performed between high-quality integratedspectral indices of 30 M31 globular clusters, 20 Milky Way globularclusters, and a sample of field and cluster elliptical galaxies. We findthat the Lick CN indices in the M31 and Galactic clusters are enhancedrelative to the bulges of the Milky Way, M31, and elliptical spheroids,in agreement with Burstein and coworkers. Although not particularlyevident in the Lick CN indices, the near-UV cyanogen feature(λ3883) is strongly enhanced with respect to the Galacticglobular clusters at metallicities -1.5<[Fe/H]<-0.3. Carbon showssigns of varying among these two groups. For [Fe/H]>-0.8, we observeno systematic differences in the Hδ, Hγ, or Hβ indicesbetween the M31 and Galactic globular clusters, in contrast to previousstudies. The elliptical galaxy sample lies offset from the loci of theglobular clusters in both the cyanogen-[MgFe] and Balmer-line-[MgFe]planes. Six of the M31 clusters appear young and are projected onto theM31 disk. Population synthesis models suggest that these are metal-richclusters with ages 100-800 Myr, metallicities -0.20<=[Fe/H]<=0.35,and masses 0.7-~7.0×104 Msolar. Two otheryoung clusters are Hubble V in NGC 205, observed as a template, and anolder (~3 Gyr) cluster some 7 kpc away from the plane of the disk. Thesix clusters projected onto the disk show signs of rotation similar tothe H I gas in M31, and three clusters exhibit thin disk kinematics,according to Morrison and coworkers. Dynamical mass estimates anddetailed structural parameters are required for these objects todetermine whether they are massive open clusters or globular clusters.If they are the latter, our findings suggest globular clusters may tracethe buildup of galaxy disks. In either case, we conclude that theseclusters are part of a young, metal-rich disk cluster system in M31,possibly as young as 1 Gyr old.
| 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.
| Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models
Motivated by new sounding-rocket wide-field polarimetric images of theLarge Magellanic Cloud (reported simultaneously by Cole et al.), we haveused a three-dimensional Monte Carlo radiation transfer code toinvestigate the escape of near-ultraviolet photons from young stellarassociations embedded within a disk of dusty material (i.e., a galaxy).As photons propagate through the disk, they may be scattered or absorbedby dust. Scattered photons are polarized and tracked until they escapethe dust layer, allowing them to be observed; absorbed photons heat thedust, which radiates isotropically in the far-infrared where the galaxyis optically thin. The code produces four output images: near-UV andfar-IR flux, and near-UV images in the linear Stokes parameters Q and U.From these images we construct simulated UV polarization maps of theLMC. We use these maps to place constraints on the star+dust geometry ofthe LMC and the optical properties of its dust grains. By tuning themodel input parameters to produce maps that match the observedpolarization maps, we derive information about the inclination of theLMC disk to the plane of the sky and about the scattering phase functiong. We compute a grid of models with i=28 deg, 36 deg, and 45 deg, andg=0.64, 0.70, 0.77, 0.83, and 0.90. The model that best reproduces theobserved polarization maps has i=36 deg+2-5 andg~0.7. Because of the low signal-to-noise in the data, we cannot placefirm constraints on the value of g. The highly inclined models do notmatch the observed centrosymmetric polarization patterns around brightOB associations or the distribution of polarization values. Our modelsapproximately reproduce the observed ultraviolet photopolarimetry of thewestern side of the LMC; however, the output images depend on many inputparameters and are nonunique. We discuss some of the limitations of themodels and outline future steps to be taken; our models make somepredictions regarding the polarization properties of diffuse lightacross the rest of the LMC.
| Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. I. Observations
We have used the rocketborne Wide-Field Imaging Survey Polarimeter(WISP) to image a 1.5dx4.8d area of the western side of the LargeMagellanic Cloud (LMC) at a wavelength of λ=2150 Å and aresolution of 1'x1.5′. These are the first wide-field ultravioletpolarimetric images in astronomy. We find the UV background light of theLMC to be linearly polarized at levels ranging from our sensitivitylimit of 4% to as high as ~40%. In general, the polarization in a pixelincreases as the flux decreases; the weighted mean value of polarizationacross the WISP field is 12.6%+/-2.3%. The LMC's diffuse UV background,in uncrowded areas, rises from a minimum of (5.6+/-3.1)x10-8ergs s-1 cm-2 Å-1 sr-1(23.6+/-0.5 mag arcsec-2) to (9.3+/-1.1)x10-8 ergss-1 cm-2 Å-1 sr-1(23.1+/-0.2 mag arcsec-2) in regions near the brightassociations. We use our polarization maps to investigate the geometryof the interstellar medium in the LMC and to search for evidence of asignificant contribution of scattered light from OB associations to thediffuse galactic light of the LMC. Through a statistical analysis of ourpolarization map, we identify nine regions of intense UV emission whichmay be giving rise to scattering halos in our image. We find thatstarlight from the N11 complex and the LH 15 association are thestrongest contributors to the scattered light component of the LMC'sdiffuse galactic light. This region of the northwestern LMC can bethought of as a kiloparsec-scale reflection nebula in which OB starsilluminate distant dust grains that scatter the light into our sightline. In contrast, the polarization map does not support the scatteringof light from the large B2 complex in the southern WISP field; thiseffect may be astrophysical, or it may be the result of bias in ouranalysis.
| 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.
| 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.
| Clusters in the west side of the bar of the Large Magellanic Cloud: interacting pairs?
In this paper, we present the VI-CCD photometry of 11 unstudied clusterslocated in the bar of the Large Magellanic Cloud (LMC), as a part of aproject aimed to infer the star formation history of this galaxy. Wederive the ages of these clusters by means of isochrone fitting. Threeclose pairs of clusters are included in the sample, namely NGC 1903-SL357, SL 349-SL 353, SL 387-SL 385. We discuss the surface photometry ofthese objects: the distortion in the isophotal contours is regarded as asign of interactions between pairs of physically connected clusters.While the systems SL 349-SL 353 and SL 387-SL 385 are likely pairs ofnearly coeval clusters, NGC 1903-SL 357 is not because of the large agedifference between the two. Several possible mechanisms for theformation of this peculiar pair are examined in the context ofinteractions between the Large and Small Magellanic Cloud (SMC).
| The ellipticities of Galactic and Large Magellanic Cloud globular clusters
The correlations between the ellipticity and the age and mass of LMCglobular clusters are examined, and both are found to be weak. It isconcluded that neither of these properties is mainly responsible for theobserved differences in the LMC and Galactic globular clusterellipticity distributions. Most importantly, age cannot be the primaryfactor in the LMC-Galaxy ellipticity differences, even if there is arelationship, as even the oldest LMC clusters are more elliptical thantheir Galactic counterparts. The strength of the tidal field of theparent galaxy is proposed as the dominant factor in determining theellipticities of that galaxy's globular clusters. A strong tidal fieldrapidly destroys velocity anisotropies in initially triaxial, rapidlyrotating elliptical globular clusters. A weak tidal field, however, isunable to remove these anisotropies and the clusters remain close totheir initial shapes.
| Ultraviolet ages of young clusters in the Magellanic Clouds.
Following a previous investigation on the integrated UV colours ofstellar clusters (Barbero et al. 1990), we study the calibration of theultraviolet colour index C(15-31) in terms of cluster age, usingobservations by the International Ultraviolet Explorer of 29 young andpopulous clusters of the Large Magellanic Cloud (LMC), and of the SmallMagellanic Cloud (SMC). The study is limited to the range of ages5x10^6^ to 8x10^8^yr, which is free from contamination by HorizontalBranch stars. It is shown that in this range of ages the theoreticalsequence C(15-31) vs. age agrees well with the one derived by combiningthe observed colour index C(15-31) with the ages determined viaisochrone fitting to the colour-magnitude diagrams while systematicdifferences, which are discussed on here, exist with respect to the agecalibration by Meurer, Cacciari and Freeman (1990). The present agecalibration C(15-31) vs. log(t), provided in an analytical form, isfinally used to determine the ages of the 29 clusters in our sample,including 13 objects for which no determination was available via theisochrone fitting method.
| 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.
| Blue-violet spectral evolution of young Magellanic Cloud clusters
We study the integrated spectral evolution in the blue-violet range of97 blue star clusters in the Magellanic Clouds, from those associatedwith gas emission to those as old as a few hundred Myr. Some clustersare dominated by the flux of those massive stars that pass throughevolutionary stages such as Wolf-Rayet, Luminous Blue Variable, Be, andsupergiant stars of different temperatures. The relationships amongspectral features such as absorption and emission lines, Balmerdiscontinuity and Balmer continuum are used to study the spectralevolution of the clusters. Finally, we sort into groups spectra ofsimilar evolutionary stages, creating a template spectral library withpossible applications in stellar populations syntheses of star-forminggalaxies and in the spectral simulation of bursts of star formation withdifferent mean ages and durations.
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| The structure and evolution of rich star clusters in the Large Magellanic Cloud
The present evaluation of surface brightness profiles andcolor-magnitude diagrams for 18 rich star clusters in the LMC, whoseages range from 10 million to 1 billion years, notes that while theprofiles of the older clusters are representable by models withKing-like cores, those of many younger clusters resist such modeling invirtue of bumps, sharp 'shoulders', and central dips. If the clustershave undergone violent relaxation, then the small cores of the youngestones may be indicative of formation from relatively 'cool' initialconditions. The sharp shoulders would then point point toward 'warmer'initial conditions, although they are alternatively explainable assignatures of merging subcondensations.
| Analysis of the UV spectra of young clusters of the Large Magellanic Cloud
UV and visual spectral energy distributions of young generations havebeen synthesized from evolutionary tracks in the HR diagram and modelatmospheres. The influence of several parameters has also been analyzed.Their UV portions have been checked with the UV spectra of 24 clustersof the LMC obtained by Cassatella et al. (1987). The models generallyagree well with the observations and make it possible to derive ages forvarious assumed abundances. Ages thus derived have been compared, in anage-age diagram, with those obtained from the integrated UBV photometry.While there are no systematic differences between these two agedeterminations, a fraction of clusters displays a large scatter, largerthan what is expected from the observational errors alone. Possiblecauses for this scatter are briefly analyzed.
| Near-infrared spectral evolution of blue LMC clusters : a comparison with galactic open clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1990RMxAA..21..202B
| The cluster system of the Large Magellanic Cloud
A new catalog of clusters in the Large Magellanic Cloud has beenconstructed from searches of the IIIa-J component of the ESO/SERCSouthern Sky Atlas. The catalog contains coordinate and diametermeasurements of 1762 clusters in a 25 deg x 25 deg area of sky centeredon the LMC, but excluding the very crowded 3.5 sq deg region around theBar. The distribution of these clusters appears as two superimposedelliptical systems. The higher density inner system extends over about 8deg; the lower density outer system can be represented by a 13 deg x 10deg disk inclined at 42 deg to the line of sight. There are suggestionsof two weak 'arms' in the latter.
| Blue Magellanic clusters - Near-infrared spectral evolution
New integrated spectra in the range 5600-10,000 A are presented for 28LMC and 3 SMC young star clusters. The equivalent widths (W) ofprominent features and the continuum distribution are measured. Theanalysis, supplemented by 8 additional LMC clusters from previousstudies, indicates that the red supergiant phase is indeed verytime-peaked, occuring from 7 to 12 Myr. In addition to the previous caseof NGC 2004, it is found that NGC 1805, NGC 1994, NGC 2002, NGC 2098,and NGC 2100 (as well as NGC 2011 to a lesser extent) are undergoingthis phase. The red supergiant phase is clearly denoted by strong TiObands and Ca II triplet as well as a flat continuum or (in extremecases) a continuum with positive slope above 6000 A.
| Ultraviolet colors as age indicators for LMC clusters
Empirical correlations are found between log ages and the intrinsicultraviolet colors for 27 LMC clusters. The problems and limitations ofusing these correlations as age indicators for LMC clusters and otherstellar populations are discussed. The correlations are used to estimatethe ages of two LMC clusters of unknown age (NGC 1968 and NGC 1974) andthe nuclei of two nearby blue compact dwarf galaxies (NGC 1705 and NGC5253). For the latter two objects optical- and ultraviolet-based ageestimates are in good agreement.
| The age calibration of integrated ultraviolet colors and young stellar clusters in the Large Magellanic Cloud
Integrated colors in selected far-UV bands are presented for a largesample of Large Magellanic Cloud (LMC) clusters. Theoreticalcalculations of these integrated colors are derived and discussed. Thelocation in the two-color diagram C(18-28), C(15-31) is expected to be asensitive but smooth function of cluster age for ages in the range 5 to800 million yr. Theoretical results appear in very good agreement withthe observed colors of LMC clusters. From this comparison, the gap inthe observed colors is suggested to be caused by the lack of LMCclusters in the range of ages between 200 million to one billion yr. Thetwo-color location of old globulars is discussed, also in connectionwith available data for the M31 clusters.
| Core expansion in young star clusters in the Large Magellanic Cloud
The core radii of 18 rich star clusters in the LMC with ages from 10 Myrto 1 Gyr. Data for an additional 17 clusters with ages from 1 Myr to 10Gyr are available in the literature. The combined sample shows that thecore radii increase from about 0 to about 5 pc between about 1 Myr and 1Gyr, and then begin to decrease again. The expansion of the cores isprobably driven by mass loss from evolving stars. Models of clusterevolution show that the rate of increase in core radius is sensitive tothe slope of the initial mass function. The observed core radius-agerelation for the LMC clusters favors an intial mass function with slopeslightly flatter than the Salpeter value.
| Ellipticities at R(h) of LMC star clusters
The projected ellipticities of 53 populous LMC star clusters have beenderived by means of PDS 1010A scans and a computer interactive method ofreduction implemented on an Apollo 570 workstation. Film copies of apair of J and U plates taken with the 1.2 m UK Schmidt Telescope inAustralia were used. The ellipticities derived here agree with thosefound by previous investigators, when comparisons were possible at thesame radius. Ellipticity variations within individual globular clustersare seen to be a common phenomenon, so the ellipticities e(h) at adistance corresponding to the half-mass radius R(h) from the center wereadopted to represent the cluster's flatness. Using these values for theLMC clusters, it is found that LMC clusters are more elliptical thanthose of the Galaxy. Although the young LMC globular clusters show atendency to be more elliptical than the old ones, there is no strongevidence for a significant difference among them. Finally, e(h) wasfound to increase with the total mass of the clusters, possiblyindicating that high-mass clusters have higher angular momentum, or havemore difficulty in shedding angular momentum, than do low mass clusters,and remain longer in their initial flattened shape.
| Observed dynamical parameters of the disk clusters of the Large Magellanic Cloud. II
The structural parameters and density profiles for 28 LMC globularclusters (located within 5 kpc from the rotation center) have beenderived by means of star counts. The clusters were measured on plates offour different colors (U, J, V, I) taken with the 1.2-m UK Schmidttelescope. The tidal radii are found to be within 40-65 pc and theirdynamical masses from 10,000 to 100,000 solar masses. Comparing thedynamical parameters of these clusters with those studied by Kontizas etal. (1987), it is found that the most extended and massive clusters ofthis galaxy are in the innermost area, at distances not exceeding 3 kpcfrom the rotation center; the distances have been corrected for theinclination of the LMC.
| The ultraviolet spectra of M31 globular clusters
Ultraviolet spectra of 11 of the brightest globular clusters in M31 showthat some exhibit residual flux below 3000 A, greater than that expectedfrom the bright, evolved stars in the cluster. There seems to be noapparent correlation of the strength of this ultraviolet flux withparameters such as metallicity, U-B color, visual magnitude, X-rayemission, or location within the parent galaxy. However, comparison ofthe ultraviolet colors of the M31 globular clusters with those in theGalaxy and in the Large Magellanic Cloud suggests that the M31 clustersmay contain a high percentage of blue horizontal-branch stars or thatsome clusters could be as young as about 2 x 10 to the 9th yr.
| Morphology of LMC clusters.
Abstract image available at:http://adsabs.harvard.edu/abs/1987RMxAA..14..172Z
| Ultraviolet observations by the IUE of 31 clusters of the Large Magellanic Cloud
Ultraviolet observations of 31 star clusters of the Large MagellanicCloud obtained by the International Ultraviolet Explorer are presented.The clusters, mostly globular, span the range of ages from about 10 tothe 7th to 10 to the 10th yr. The sample includes several young globularclusters, which have no counterpart in the Galaxy. The paper is devotedto the study of the basic properties of the clusters in the ultraviolet,such as the interstellar reddening, the spatial extension in theultraviolet, and the ultraviolet color indices. These data are essentialfor a later study of the evolutionary stage of the clusters. Oneimportant result of this study is that a discontinuity exists in the UVtwo-color diagram, a fact which deserves further investigation, since itmight support the suggestion made by other authors that an active starformation phase took place in the Large Magellanic Cloud starting about100 million yr ago.
| Age calibration and age distribution for rich star clusters in the Large Magellanic Cloud
An empirical relation is presented for estimating the ages of rich starclusters in the Large Magellanic Cloud (LMC), to within a factor ofabout 2, from their integrated UBV colors. The calibration is based onpublished ages for 58 LMC clusters derived from main-sequencephotometry, integrated spectra, or the extent of the asymptotic giantbranches. Using stellar population models, a sample of LMC clusters moremassive than about 10,000 solar masses is isolated, which is correctedfor incompleteness as a function of magnitude. An unbiased agedistribution for three clusters is then determined. The number ofclusters decreases with increasing age in a manner that is qualitativelysimilar to the age distribution for the open clusters in our Galaxy. TheLMC age distribution is, however, flatter, and the median age of theclusters is greater. If the formation rate has been approximatelyconstant over the history of the two galaxies, then the age distributionobtained here implies that clusters are disrupted more slowly in theLMC. The results contain no evidence for bursts in the formation ofclusters, although fluctuations on small time scales and slow variationsover the lifetime of the LMC cannot be ruled out.
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