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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.
| Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
| 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.
| Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| ISOCAM Observations of Globular Clusters in the Magellanic Clouds: The Data
Seventeen globular clusters in the Large and Small Magellanic Cloudswere observed in the mid-infrared wavelength region with the ISOCAMinstrument on board the Infrared Space Observatory (ISO). Observationswere made using the broadband filters LW1, LW2, and LW10, correspondingto the effective wavelengths of 4.5, 6.7, and 12 μm, respectively. Wepresent the photometry of point sources in each cluster, as well astheir precise positions and finding charts.Based on observations with ISO, an ESA project with instruments fundedby ESA Member states (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with participation of ISAS andNASA.
| Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
| 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.
| Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - I. The old Large Magellanic Cloud clusters NGC 1928, 1939 and Reticulum
We present the results of photometric measurements from images of theLarge Magellanic Cloud (LMC) globular clusters NGC 1928, 1939 andReticulum taken with the Advanced Camera for Surveys on the Hubble SpaceTelescope. Exposures through the F555W and F814W filters result inhigh-accuracy colour-magnitude diagrams (CMDs) for these three clusters.This is the first time that CMDs for NGC 1928 and 1939 have beenpublished. All three clusters possess CMDs with features indicating themto be >10 Gyr old, including main-sequence turn-offs at V~ 23 andwell-populated horizontal branches (HBs). We use the CMDs to obtainmetallicity and reddening estimates for each cluster. NGC 1939 is ametal-poor cluster, with [Fe/H]=-2.10 +/- 0.19, while NGC 1928 issignificantly more metal rich, with [Fe/H]=-1.27 +/- 0.14. The abundanceof Reticulum is intermediate between the two, with [Fe/H]=-1.66 +/-0.12- a measurement which matches well with previous estimates. Allthree clusters are moderately reddened, with values ranging from E(V-I)= 0.07 +/- 0.02 for Reticulum and E(V-I) = 0.08 +/- 0.02 for NGC 1928,to E(V-I) = 0.16 +/- 0.03 for NGC 1939. After correcting the CMDs forextinction we estimate the HB morphology of each cluster. NGC 1928 and1939 possess HBs consisting almost exclusively of stars to the blue ofthe instability strip, with NGC 1928 in addition showing evidence for anextended blue HB. In contrast, Reticulum has an intermediate HBmorphology, with stars across the instability strip. Using a variety ofdating techniques we show that these three clusters are coeval with eachother and the oldest Galactic and LMC globular clusters, to within ~2Gyr. The census of known old LMC globular clusters therefore now numbers15 plus the unique, younger cluster ESO121-SC03. The NGC 1939 fieldcontains another cluster in the line of sight, NGC 1938. A CMD for thisobject shows it to be less than ~400 Myr old, and it is thereforeunlikely to be physically associated with NGC 1939.
| The Stellar Halo in the Large Magellanic Cloud: Mass, Luminosity, and Microlensing Predictions
Recently obtained kinematic data have shown that the Large MagellanicCloud (LMC) possesses an old stellar halo. In order to furthercharacterize the properties of this halo, parametric King models arefitted to the surface density of RR Lyrae stars. Using data from boththe MACHO and OGLE II microlensing surveys, the model fits yield thecenter of their distribution at α=5h21.1'+/-0.8',δ=-69deg45'+/-6' (J2000.0) and acore radius of 1.42+/-0.12 kpc. As a check, the halo model is comparedwith RR Lyrae star counts in fields near the LMC's periphery previouslysurveyed with photographic plates. These data, however, require acautious interpretation. Several topics regarding the LMC stellar haloare discussed. First, the properties of the halo imply a globalmass-to-light ratio of M/LV=5.3+/-2.1 and a total mass of(1.6+/-0.6)×1010 Msolar for the LMC, in goodagreement with estimates based on the rotation curve. Second, althoughthe LMC's disk and halo are kinematically distinct, the shape of thesurface density profile of the halo is remarkably similar to that of theyoung disk. For example, the best-fit exponential scale length for theRR Lyrae stars is 1.47+/-0.08 kpc, which compares to 1.46 kpc for theLMC's blue light. In the Galaxy, the halo and disk do not resemble eachother like this. Finally, a local maximum in the LMC's microlensingoptical depth due to halo-on-disk stellar self-lensing is predicted. Forthe parameters of the stellar halo obtained, this maximum is locatednear MACHO events LMC-4 and LMC-23 and is large enough to possiblyaccount for these two events but not for all of the observedmicrolensing.
| 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.
| 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 globular clusters in the Fornax and Sagittarius dwarf spheroidal galaxies
We present radial surface brightness profiles for all five globularclusters in the Fornax dwarf spheroidal galaxy, and for the four presentmembers of the Sagittarius dwarf spheroidal galaxy. These profiles arederived from archival Hubble Space Telescope observations, and have beencalculated using the same techniques with which we measured profiles inour previous studies of Large and Small Magellanic Cloud (LMC and SMC)clusters, apart from some small modifications. From the surfacebrightness profiles, we have determined structural parameters for eachcluster, including core radii and luminosity and mass estimates. We alsoprovide a brief summary of literature measurements of other parametersfor these clusters, including their ages, metallicities and distances.Our core radius measurements are mostly in good agreement with thosefrom previous lower resolution studies, although for several clustersour new values are significantly different. The profile for Fornaxcluster 5 does not appear to be well fitted by a King-type model and wesuggest that it is a post-core-collapse candidate. We examine thedistribution of cluster core radii in each of the two dwarf galaxysystems and compare these with the distribution of core radii for oldLMC clusters. The three distributions match within the limits ofmeasurement errors and the small-sample sizes. We discuss theimplications of this in the context of the radius-age trend we havepreviously highlighted for the Magellanic Cloud clusters.
| Distance to the Large Magellanic Cloud: The RR Lyrae Stars
New photometry and spectroscopy for more than a hundred RR Lyrae starsin two fields located close to the bar of the Large Magellanic Cloud(LMC) are used to derive new accurate estimates of the averagemagnitude, the local reddening, the luminosity-metallicity relation, andthe distance to the LMC. The average apparent luminosity of the RRLyraes with complete V and B light curves is=19.412+/-0.019 (σ=0.153),=19.807+/-0.022 (σ=0.172) in our field A (62 stars)and =19.320+/-0.023 (σ=0.159),=19.680+/-0.024 (σ=0.163) in our field B (46 stars).The average V apparent luminosity of the clump stars in the same areasis 0.108 and 0.029 mag brighter than the RR Lyrae level(=19.304+/-0.002 and 19.291+/-0.003, in fieldA: 6728 stars, and B: 3851 stars, respectively). Metallicities fromlow-resolution spectra obtained with the Very Large Telescope have beenderived for 101 RR Lyrae stars, finding an average value of[Fe/H]=-1.48+/-0.03 (σ=0.29, on the Harris metallicity scale). Anestimate of the reddening within the two fields was obtained (1) fromthe Sturch method applied to the fundamental-mode pulsators (RRab's)with known metal abundance and (2) from the colors of the edges of theinstability strip defined by the full sample of RR Lyrae variable stars.We obtained E(B-V)=0.116+/-0.017 and 0.086+/-0.017 mag in fields A andB, respectively, with a clear-cut indication of a 0.03 mag differentialreddening between the two fields. We find that reddening in field A is0.028 mag smaller than derived by OGLE-II in the same area. On average,the new reddenings are also 0.035 mag larger than derived from Cepheidswith projected distances within 2° from the centers of our fields.The new metallicities were combined with the apparent averageV0 luminosities to determine the slope of theluminosity-metallicity relation for the RR Lyrae stars. We derivedΔMV(RR)/Δ [Fe/H]=0.214+/-0.047, with no clearevidence for the change in slope at [Fe/H]=-1.5, as recently suggestedby evolutionary/pulsation and horizontal-branch models.The dereddened apparent average luminosity of the RR Lyraes defined bythe present photometry is 0=19.064+/-0.064 at[Fe/H]=-1.5. When coupled with the absolute magnitude derived from theBaade-Wesselink and the statistical parallaxes methods(MV(RR)=0.68+/-0.15 and 0.76+/-0.13 mag at [Fe/H]=-1.5), bothmethods known to favor the short distance scale, this value leads todistance moduli for the LMC of μLMC=18.38+/-0.16 andμLMC=18.30+/-0.14, respectively. If we use instead theabsolute magnitude from the new main-sequence fitting of Galacticglobular clusters from Gratton et al. [MV(RR)=0.61+/-0.07 magat [Fe/H]=-1.5], we derive μLMC=18.45+/-0.09.The average I apparent luminosity of the clump stars derived by thepresent photometry is =18.319+/-0.002 and18.307+/-0.003, in field A (σ=0.190, 6728 stars) and B(σ=0.184, 3851 stars), respectively. These values, once correctedfor our new reddening estimates, lead to0=18.12+/-0.06 mag and move the clump distancemodulus to the LMC to 18.42+/-0.07 and 18.45+/-0.07 when Udalski orPopowski metallicity-I luminosity relations for the clump stars areadopted.All these values are only 1 σ shorter than provided by thePopulation I distance indicators and make it possible to reconcile theshort- and long-distance scale on a common value for the distancemodulus of the LMC of μLMC=18.515+/-0.085 mag.Based on observations collected at the European Southern Observatory,proposals 62.N-0802, 66.A-0485, and 68.D-0466.
| 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.
| The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI
We have surveyed Andromeda VI, a dwarf spheroidal galaxy companion toM31, for variable stars by using F450W and F555W observations obtainedwith the Hubble Space Telescope. A total of 118 variables were found,including 111 RR Lyrae stars, six anomalous Cepheids, and one variablethat we were unable to classify. We find that the Andromeda VI anomalousCepheids have properties consistent with those of anomalous Cepheids inother dwarf spheroidal galaxies. We revise the existingperiod-luminosity relations for these variables. Further, using theseand other available data, we show that there is no clear differencebetween fundamental and first-overtone anomalous Cepheids in aperiod-amplitude diagram at shorter periods, unlike the RR Lyrae stars.For the Andromeda VI RR Lyrae stars, we find that they lie close to theOosterhoff type I Galactic globular clusters in the period-amplitudediagram, although the mean period of the RRab stars,=0.588 days, is slightly longer than that of thetypical Oosterhoff type I cluster. The mean V magnitude of the RR Lyraestars in Andromeda VI is 25.29+/-0.03, resulting in a distance 815+/-25kpc on the Lee, Demarque, & Zinn distance scale. This is consistentwith the distance derived from the I magnitude of the tip of the redgiant branch. Similarly, the properties of the RR Lyrae stars indicate amean abundance for Andromeda VI that is consistent with that derivedfrom the mean red giant branch color. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555.
| Constraining the LMC cluster age gap: Washington photometry of NGC 2155 and SL 896 (LW 480)
We carried out Washington system photometry of the intermediate-ageLarge Magellanic Cloud (LMC) star clusters NGC2155 and SL896 (LW480). Wederive ages and metallicities from the T1 versusC-T1 colour-magnitude diagrams (CMDs). For the first time anage has been obtained for SL896, 2.3+/-0.5Gyr. For NGC2155 we derive3.6+/-0.7Gyr. The two clusters basically define the lower age limit ofthe LMC age gap. In particular, NGC2155 is confirmed as the oldestintermediate-age LMC cluster so far studied. The derived metallicitiesare [Fe/H]=-0.9+/-0.2 and -0.6+/-0.2 for NGC2155 and SL896,respectively. We also studied the CMDs of the surrounding fields, whichhave a dominant turn-off comparable to that of the clusters themselves,and similar metallicity, showing that one is dealing with anintermediate-age disc where clusters and field stars have the sameorigin. We inserted the present clusters in the LMC and Small MagellanicCloud (SMC) age-metallicity relations, using a set of homogeneousdeterminations with the same method as in our previous studies, nowtotalling 15 LMC clusters and four SMC clusters, together with someadditional values from the literature. The LMC and SMC age-metallicityrelations appear to be remarkably complementary, since the SMC wasactively star-forming during the LMC quiescent age gap epoch.
| Ages and metallicities of five intermediate-age star clusters projected towards the Small Magellanic Cloud
Colour-magnitude diagrams are presented for the first time for L32, L38,K28 (L43), K44 (L68) and L116, which are clusters projected on to theouter parts of the Small Magellanic Cloud (SMC). The photometry wascarried out in the Washington system C and T1 filters,allowing the determination of ages by means of the magnitude differencebetween the red giant clump and the main-sequence turn-off, andmetallicities from the red giant branch locus. The clusters have ages inthe range 2-6Gyr, and metallicities in the range-1.65<[Fe/H]<-1.10, increasing the sample of intermediate-ageclusters in the SMC. L116, the outermost cluster projected on to theSMC, is a foreground cluster, and somewhat closer to us than the LargeMagellanic Cloud. Our results, combined with those for other clusters inthe literature, show epochs of sudden chemical enrichment in theage-metallicity plane, which favour a bursting star formation history asopposed to a continuous one for the SMC.
| The Ages of Globular Clusters
We examine the luminosity levels of the main-sequence turnoffs,MTOv, and horizontal branches, Mv(HB),in 16 globular clusters. An entirely new approach of inferring theluminosity levels by utilizing high-amplitude δ Scuti variables(HADS) is introduced. When the MTOv values arecompared with theoretical values inferred from models, we find all 16clusters (metal-strong to metal-poor) are coeval with an average age of~11.3 Gyr. A considerable scatter of Mv(HB) values ofclusters at similar [Fe/H] values is found. A trend for clusters withblue horizontal branches to have brighter Mv(HB) thanclusters with blue-red horizontal branches is suggested by the data. TheMv(HB) values appear to depend on another or other parametersin addition to the [Fe/H] values. In spite of this problem, we derive anequation relating Mv(HB) values of globular clusters to their[Fe/H] values. We also derive an equation relating theMTOv values of clusters to their [Fe/H] values.Both of these equations can be utilized to find cluster distances. Thedistance modulus of the LMC is found to be 18.66 from the VTOvalues of three LMC globular clusters; RR Lyrae stars in seven globularclusters yield 18.61, and RR Lyrae stars in the LMC bar yield 18.64.
| Age-metallicity relation and chemical evolution of the LMC from UVES spectra of Globular Cluster giants
We report on the first high-resolution spectroscopy of 10 giants in LMCGlobular Clusters in a wide age range, obtained with the newlycommissioned spectrograph UVES at VLT UT2. These observations are usedto derive oxygen and iron content of these clusters, and the abundancesare then used to cast a more precise view, not only on theage-metallicity relation in the LMC, but also on the chemical evolutionof this dwarf irregular galaxy. Based on observations made at the ESOTelescopes in Chile
| The Metallicity Distribution Function of Red Giants in the Large Magellanic Cloud
We report new metallicity determinations for 39 red giants in a 220arcmin2 region, 1.8d southwest of the bar of the LargeMagellanic Cloud. These abundance measurements are based on spectroscopyof the Ca II infrared triplet. We have carefully considered the effectsof abundance ratios, the physics of Ca II line formation, the variationof red clump magnitude, and the contamination by foreground stars in ourabundance analyses. The metallicity distribution function (MDF) shows astrong peak at [Fe/H]=-0.57+/-0.04 a tail to abundances at least as lowas [Fe/H]~-1.6 brings the average abundance down to [Fe/H]=-0.64+/-0.02.Half the red giants in our field fall within the range-0.83<=[Fe/H]<=-0.41. The MDF appears to be truncated at[Fe/H]~-0.25 the exact value of the maximum abundance is subject to ~0.1dex uncertainty in the calibration of the Ca II IR triplet for young,metal-rich stars. We find a striking contrast in the shape of the MDFbelow [Fe/H]<=-1 between our inner disk field and the distant outerfield studied by Olszewski: red giants deficient by more than a factorof 10 in heavy elements relative to the Sun are extremely scarce in theinner disk of the LMC. Our field star sample does not reproduce the fullMDF of the LMC star clusters but seems similar to that of theintermediate-age (1-3 Gyr) clusters. We have also obtained abundanceestimates using Strömgren photometry for ~103 red giantsin the same field. Photometry is the only practical way to measureabundances for the large numbers of stars necessary to liftage-metallicity degeneracy from our high-precision color-magnitudediagrams. The Strömgren measurements, which are sensitive to acombination of cyanogen and iron lines, correlate well with the Ca IImeasurements, but a metallicity-dependent offset is found. The offsetmay be due either to variations in the elemental abundance ratios due togalactic chemical evolution or to a metal-dependent mixing mechanism inRGB stars. An empirical relation between photometric and spectroscopicabundance estimates is derived. This will allow photometric abundancemeasurements to be placed on a consistent metallicity scale withspectroscopic metallicities, for very large numbers of stars. Based onobservations obtained at Cerro Tololo Inter-American Observatory, adivision of the National Optical Astronomy Observatories, which areoperated by the Association of Universities for Research in Astronomy,Inc. under cooperative agreement with the National Science Foundation.
| The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems
Prompted by a new catalogue of M31 globular clusters, we have collectedtogether individual metallicity values for globular clusters in theLocal Group. Although we briefly describe the globular cluster systemsof the individual Local Group galaxies, the main thrust of our paper isto examine the collective properties. In this way we are simulating thedissipationless merger of the Local Group, into presumably an ellipticalgalaxy. Such a merger is dominated by the Milky Way and M31, whichappear to be fairly typical examples of globular cluster systems ofspiral galaxies. The Local Group `Elliptical' has about 700 +/- 125globular clusters, with a luminosity function resembling the `universal'one. The metallicity distribution has peaks at [Fe/H] ~ -1.55 and -0.64with a metal-poor to metal-rich ratio of 2.5:1. The specific frequencyof the Local Group Elliptical is initially about 1 but rises to about 3,when the young stellar populations fade and the galaxy resembles an oldelliptical. The metallicity distribution and stellar populationcorrected specific frequency are similar to that of some known earlytype galaxies. Based on our results, we briefly speculate on the originof globular cluster systems in galaxies.
| A catalogue of helium abundance indicators from globular cluster photometry
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
| Hubble Space Telescope Observations of the Oldest Star Clusters in the Large Magellanic Cloud
We present V, V-I color-magnitude diagrams (CMDs) for three old starclusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257, andHodge 11. Our data extend ~3 mag below the main-sequence turnoff,allowing us to determine accurate relative ages and the blue stragglerfrequencies. Based on a differential comparison of the CMDs, any agedifference between the three LMC clusters is less than 1.5 Gyr.Comparing their CMDs to those of M92 and M3, the LMC clusters, unlesstheir published metallicities are significantly in error, are the sameage as the old Galactic globulars. The similar ages to Galacticglobulars are shown to be consistent with hierarchial clustering modelsof galaxy formation. The blue straggler frequencies are also similar tothose of Galactic globular clusters. We derive a true distance modulusto the LMC of (m-M)0=18.46+/-0.09 [assuming(m-M)0=14.61 for M92] using these three LMC clusters.
| On the Sensitivity of the Cepheid Period-Luminosity Relation to Variations of Metallicity
Predictions are made of the effect of variations in the [Fe/H]metallicity on the zero point of the Cepheid period-luminosity relationin bolometric, B, V, and I magnitudes. Theoretical evolutionary tracksin the H-R diagram, computed by three independent groups at Geneva,Padua, and Basel, are combined with the positions of the blue and rededges of the instability strip in the relevant H-R diagrams to give thepredicted P-L relations for [Fe/H] metallicities of 0.0, -0.4, -0.7,-1.3, and -1.7. The predictions are based on the pulsation equation,P(M,L,T_e,Z,Y), calculated at the points where the tracks of a givenmass for each metallicity intersect the instability strip in the H-Rdiagrams. New model atmospheres and synthetic spectra are computedgiving sets of grids of the bolometric corrections and B-V, V-R, R-I,and V-I colors for temperatures between 7500 and 5000 K, gravitiesbetween log g=3.0 and log g=0.75, and metallicities between [Fe/H]=0.0and [Fe/H]=-1.7. Interpolation in the grids at the relevanttemperatures, gravities, and metallicities of the Cepheid instabilitystrip give theoretical P-L relations on the Cape Cousins BVI photometricsystem at the blue and red edges of the strip. The metallicitydependence of the P-L relations, read at P=10 days, are dM/d[Fe/H]=0.00mag dex^-1 in M(bol), +0.03 mag dex^-1 in B, -0.08 mag dex^-1 in V, and-0.10 mag dex^-1 in I in the sense that lower metallicities meanbrighter magnitudes in B and fainter ones in V and I. Similardependencies are found at P=31.6 days. Confirmation that the zero pointsof the Cepheid P-L relations are not steeper functions of [Fe/H] thanthe theoretical values derived here is found by comparing the distancemoduli of the LMC, the SMC, and IC 1613 based on the P-L relation forGalactic Cepheids (<[Fe/H]>=0) with the distance moduli determinedfor these galaxies using RR Lyrae variables with the steep absolutemagnitude-metallicity calibration of M_V(RR)=0.30[Fe/H]+0.94. Applyingthis bright calibration, based on the Oosterhoff-Arp-Preston (OAP)metallicity effect, to the RR Lyrae variables in the LMC, the SMC, andIC 1613 gives individual distance moduli that agree to within 0.10 magwith the Cepheid distance moduli for [Fe/H]=0 for each galaxy,confirming that no metallicity dependence of the Cepheid P-L relationcan be detected at this level with the present observational data if thebright RR Lyrae M_V ([Fe/H]) calibration is used. Using the statedcalibrations with the Cepheid data from the literature gives thedistance modulusof the LMC to be (m-M)_0=18.57. The modulus for the SMC,corrected for the mild metallicity effect derived here, reconciles thedeviant individual B, V, and I moduli to within +/-0.01 mag,giving(m-M)_0=18.94 for the SMC.
| The absolute magnitudes of RR Lyrae stars from BT HIPPARCOS parallaxes parallaxes
Using the method of ``reduced parallaxes'' for the Halo RR Lyrae starsin the hipparcos catalogue we derive a zero point of 0.77 +/- 0.26 magfor an assumed slope of 0.18 in the M_V-[Fe/H] relation. This is 0.28magnitude brighter than the value Fernley et al. (1998a) derived byemploying the method of statistical parallax for the identical sampleand using the same slope. We point out that a similar difference existsbetween the ``reduced parallaxes'' method and the statistical parallaxmethod for the Cepheids in the hipparcos catalogue. We also determinethe zero point for the M_K-log P0 relation, and obtain avalue of -1.16 +/- 0.27 mag (for a slope of -2.33). The distance modulito the hipparcos RR Lyrae stars derived from the two relations agreewell. The derived distance scale is in good agreement with the resultsfrom the Main Sequence fitting distances of Galactic globular clustersand with the results of theoretical Horizontal Branch models, andimplies a distance modulus to the LMC of 18.61 +/- 0.28 mag. Based ondata from the ESA Hipparcos astrometry satellite.}
| Spectroscopic analysis of the candidate globular clusters NGC 1928 and 1939 in the Large Magellanic Cloud
The integrated spectral properties in the range 3600-6700 A of thecandidate old clusters NGC 1928 and 1939 in the LMC bar are comparedwith those of old- and intermediate-age reference LMC clusters, theproperties of which are better established. It has been possible toinfer the age of the sample clusters by means of absorption features andthe continuum distribution, in particular in the plane W_M x W_B (whereW_B is the average of Hdelta, Hγ and H beta equivalent widths, andW_M that of Ca II K, G band and Mg i). The results indicate that NGC1928 and 1939 are compatible with old clusters. The metallicity isderived with respect to galactic globular cluster templates: [Fe/H]~-1.2 and -2.0 for NGC 1928 and 1939, respectively. We also discuss thecensus of Population II clusters in the LMC, their spatial distributionand the possibility of a LMC core and a transient morphologicalclassification for interacting late-type disc galaxies.
| 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.
| HSTcolour-magnitude diagrams of six old globular clusters in the LMC
We report on HST observations of six candidate old globular clusters inthe Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 and2019. Deep exposures with the F555W and F814W filters provide us withcolour-magnitude diagrams that reach to an apparent magnitude in V of~25, well below the main-sequence turn-off. These particular clustersare associated with significantly high LMC field star densities and carewas taken to subtract the field stars from the cluster colour-magnitudediagrams accurately. In two cases there is significant variablereddening across at least part of the image, but only for NGC 1916 doesthe differential reddening preclude accurate measurements of the CMDcharacteristics. The morphologies of the colour-magnitude diagrams matchwell those of Galactic globular clusters of similar metallicity. All sixhave well-developed horizontal branches, while four clearly have starson both sides of the RR Lyrae gap. The abundances obtained frommeasurements of the height of the red giant branch above the level ofthe horizontal branch are 0.3 dex higher, on average, than previouslymeasured spectroscopic abundances. Detailed comparisons with Galacticglobular cluster fiducials show that all six clusters are old objects,very similar in age to classical Galactic globulars such as M5, withlittle age spread among the clusters. This result is consistent withages derived by measuring the magnitude difference between thehorizontal branch and main-sequence turn-off. We also find a similarchronology by comparing the horizontal branch morphologies andabundances with the horizontal branch evolutionary tracks of Lee,Demarque & Zinn. Our results imply that the LMC formed at the sametime as the Milky Way Galaxy.
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