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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.
|Distances to Populous Clusters in the Large Magellanic Cloud via the K-band Luminosity of the Red Clump|
We present results from a study of the distances and distribution of asample of intermediate-age clusters in the Large Magellanic Cloud (LMC).Using deep near-infrared photometry obtained with ISPI on the CTIO 4 m,we have measured the apparent K-band magnitude of the corehelium-burning red clump stars in 17 LMC clusters. We combine clusterages and metallicities with the work of Grocholski and Sarajedini topredict each cluster's absolute K-band red-clump magnitude and therebycalculate absolute cluster distances. An analysis of these data showsthat the cluster distribution is in good agreement with the thick,inclined-disk geometry of the LMC, as defined by its field stars. Wealso find that the old globular clusters follow the same distribution,suggesting that the LMC's disk formed at about the same time as theglobular clusters, ~13 Gyr ago. Finally, we have used our clusterdistances in conjunction with the disk geometry to calculate thedistance to the LMC center, for which we find(m-M)0=18.40+/-0.04 (random)+/-0.08 (systematic), orD0=47.9+/-0.9+/-1.8 kpc.
|The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters|
Using available data for C and M giants with M_bol<-3.6 in MagellanicCloud clusters, we derive limits to the lifetimes for the correspondingevolutionary phases, as a function of stellar mass. The C-star phase isfound to have a duration between 2 and 3 Myr for stars in the mass rangefrom ~1.5 to 2.8 M_ȯ. There is also an indication that the peak ofC-star lifetime shifts to lower masses (from slightly above to slightlybelow 2 Mȯ) as we move from LMC to SMC metallicities.The M-giant lifetimes also peak at ~2 Mȯ in the LMC,with a maximum value of about 4 Myr, whereas in the SMC their lifetimesappear much shorter, but, actually, they are poorly constrained by thedata. These numbers constitute useful constraints to theoretical modelsof the TP-AGB phase. We show that several models in the literatureunderestimate the duration of the C-star phase at LMC metallicities.
|Physical parameters of 15 intermediate-age LMC clusters from modelling of HST colour-magnitude diagrams|
Aims.We analyzed HST/WFPC2 colour-magnitude diagrams (CMDs) of 15populous Large Magellanic Cloud (LMC) stellar clusters with ages between~0.3 Gyr and ~3 Gyr. These (V, B-V) CMDs are photometrically homogeneousand typically reach V ˜ 22. Accurate and self-consistent physicalparameters (age, metallicity, distance modulus and reddening) wereextracted for each cluster by comparing the observed CMDs with syntheticones. Methods: These determinations involved simultaneous statisticalcomparisons of the main-sequence fiducial line and the red clumpposition, offering objective and robust criteria to determine the bestmodels. The models explored a regular grid in the parameter spacecovered by previous results found in the literature. Control experimentswere used to test our approach and to quantify formal uncertainties. Results: In general, the best models show a satisfactory fit to thedata, constraining well the physical parameters of each cluster. Theage-metallicity relation derived by us presents a lower spread thansimilar results found in the literature for the same clusters. Ourresults are in accordance with the published ages for the oldestclusters, but reveal a possible underestimation of ages by previousauthors for the youngest clusters. Our metallicity results in generalagree with the ones based on spectroscopy of giant stars and with recentworks involving CMD analyses. The derived distance moduli implied by themost reliable solutions, correlate with the reddening values, asexpected from the non-negligible three-dimensional distribution of theclusters within the LMC. Conclusions: .The inferred spatialdistribution for these clusters is roughly aligned with the LMC disk,being also more scattered than recent numerical predictions, indicatingthat they were not formed in the LMC disk. The set of ages andmetallicities homogeneously derived here can be used to calibrateintegrated light studies applied to distant galaxies.
|Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters|
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
|Red Giant Stars in the Large Magellanic Cloud Clusters|
We present deep J, H, and Ks photometry and accurate colormagnitude diagrams down to K~18.5 for a sample of 13 globular clustersin the Large Magellanic Cloud. This data set combined with the previoussample of six clusters published by our group gives the opportunity tostudy the properties of giant stars in clusters with different ages(ranging from ~80 Myr up to 3.5 Gyr). Quantitative estimates of starpopulation ratios (by number and luminosity) in the asymptotic giantbranch (AGB), the red giant branch (RGB), and the He clump have beenobtained and compared with theoretical models in the framework ofprobing the so-called phase transitions. The AGB contribution to thetotal luminosity starts to be significant at ~200 Myr and reaches itsmaximum at 500-600 Myr, when the RGB phase transition is starting. At~900 Myr the full development of an extended and well-populated RGB hasbeen completed. The occurrences of both the AGB and RGB phasetransitions are sharp events, lasting a few hundred megayears only.These empirical results agree very well with the theoretical predictionsof simple stellar population models based on canonical tracks and thefuel-consumption approach.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m NTT, within the observing programs64.N-0038 and 68.D-0287.
|A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters|
The (rest-frame) near-IR domain contains important stellar populationdiagnostics and is often used to estimate masses of galaxies at low, aswell as high, redshifts. However, many stellar population models arestill relatively poorly calibrated in this part of the spectrum. Toallow an improvement of this calibration we present a new database ofintegrated near-IR JHKs magnitudes for 75 star clusters inthe Magellanic Clouds, using the Two Micron All Sky Survey (2MASS). Themajority of the clusters in our sample have robust age and metallicityestimates from color-magnitude diagrams available in the literature, andpopulate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H]from -2.17 to +0.01 dex. A comparison with matched star clusters in the2MASS Extended Source Catalog (XSC) reveals that the XSC only provides agood fit to the unresolved component of the cluster stellar population.We also compare our results with the often-cited single-channel JHKphotometry of Persson and coworkers and find significant differences,especially for their 30" diameter apertures, up to ~2.5 mag in the Kband, more than 1 mag in J-K, and up to 0.5 mag in H-K. Usingsimulations to center apertures based on maximum light throughput (asperformed by Persson et al.), we show that these differences can beattributed to near-IR-bright cluster stars (e.g., carbon stars) locatedaway from the true center of the star clusters. The wide age andmetallicity coverage of our integrated JHKs photometry sampleconstitute a fundamental data set for testing population synthesis modelpredictions and for direct comparison with near-IR observations ofdistant stellar populations.
|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.
|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.
|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.
|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
|Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. I. The Data|
This is the first of a series of three papers devoted to the calibrationof a few parameters of crucial importance in the modeling of theevolution of intermediate-mass stars, giving special attention to theamount of convective core overshoot. To this end we acquired deep V andR photometry for three globular clusters of the Large Magellanic Cloud,namely, NGC 2173, SL 556, and NGC 2155, in the age interval 1-3 Gyr. Inthis first paper, we describe the aim of the project and Very LargeTelescope observations and data reduction and we make preliminarycomparisons of the color-magnitude diagrams with both the Padova andYonsei-Yale isochrones. Two following papers in this series present theresults of a detailed analysis of these data, independently carried outby members of the Yale and Padova stellar evolution groups. This allowsus to compare both sets of models and discuss their main differences, aswell as the systematic effects that they would have on the determinationof the ages and metallicities of intermediate-age single-stellarpopulations.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO 64.L-0385).
|Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud|
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
|K-Band Red Clump Distances to the Large Magellanic Cloud Clusters Hodge 4 and NGC 1651|
We present near-infrared color-magnitude diagrams (CMDs) for the LargeMagellanic Cloud (LMC) star clusters Hodge 4 and NGC 1651, revealing thehelium-burning red clump (RC) stars for the first time in the JKpassbands. From these diagrams, which extend to K~19, and existingoptical CMDs that reveal the clusters' main-sequence turnoffs, wedetermine the following cluster parameters. For Hodge 4, we estimate ametallicity of [Fe/H]=-0.17+/-0.04, an age of 1.7+/-0.3 Gyr, and a RCabsolute magnitude of MK(RC)=-1.64+/-0.17. Along with anadopted reddening of E(J-K)=0.03+/-0.01 based on the Burstein &Heiles and Schlegel et al. reddening maps, we find a distance of(m-M)0=18.52+/-0.17 for Hodge 4. In the case of NGC 1651, wederive [Fe/H]=-0.07+/-0.10, an age of 1.8+/-0.3 Gyr,MK(RC)=-1.56+/-0.12, E(J-K)=0.06+/-0.01, and(m-M)0=18.55+/-0.12, all measured in the same manner as forHodge 4. Based on these two clusters, we calculate a mean LMC distanceof (m-M)0=18.54+/-0.10.
|A Large and Homogeneous Sample of CMDs of LMC Stellar Clusters|
We present the photometric results of 21 stellar clusters of the LargeMagellanic Cloud. The WFPC2 images were retrieved from the HST archive.Simple stellar populations in a large spread of age are well representedin the sample of color-magnitude diagrams shown here.
|Large Magellanic Cloud stellar clusters. I. 21 HST colour magnitude diagrams|
We present WFPC2 photometry of 21 stellar clusters of the LargeMagellanic Cloud obtained on images retrieved from the Hubble SpaceTelescope archive. The derived colour magnitude diagrams (CMDs) arepresented and discussed. This database provides a sample of CMDsrepresenting, with reliable statistics, simple stellar populations witha large spread of age. The stars in the core of the clusters are allresolved and measured at least down to the completeness limit; themagnitudes of the main sequence terminations and of the red giant clumpare also evaluated for each cluster, together with the radius at halfmaximum of the star density. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. Table 1 is only available in electronic form athttp://www.edpsciences.org
|Age and metallicity for six LMC clusters and their surrounding field population|
We investigate, on the basis of CCD Strömgren photometry, the agesand metallicities of six LMC clusters together with their surroundingfield population. The clusters and metallicities are: NGC 1651 (in therange [Fe/H] = -0.65 dex to -0.41 dex), NGC 1711 (-0.57 ∓ 0.17dex), NGC 1806 (-0.71 ∓ 0.23 dex), NGC 2031 (-0.52 ∓ 0.21dex) and NGC 2136/37 (-0.55 ∓ 0.23 dex) and NGC 2257 (-1.63∓ 0.21 dex). The metallicities for NGC 1651, NGC 1711, NGC 1806and NGC 2031 have been determined for the first time (NGC 2031 and NGC2136/37 are interesting for the Cepheid distance scale). In the clustersurroundings, we found about 650 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR). Ourmethod is to estimate ages for individual stars on the basis ofStrömgren isochrones with individually measured metallicities. Withthis method we are able to sample the AMR of the field population up to8 Gyr. Our metallicity data are incompatible with models predicting manymetal-poor stars (G-dwarf problem). The metallicity of the fieldpopulation increased by a factor of six, starting around 2 Gyr ago. Theproposed AMR is consistent with the AMR of the LMC cluster system(including ESO 121 SC03 and three clusters with an age of 4 Gyr). Theproposed AMR is incompatible with the recently proposed AMR by Pagel& Tautvaisiene.
|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.
|Chemical evolution and star formation history in the LMC from cluster and field stars|
We investigate, on the basis of CCD Strömgren photometry, the agesand metallicities of six LMC clusters together with their surroundingfield population. The clusters and metallicities are: NGC 1651 (in therange [Fe/H]=-0.65 dex to -0.41 dex), NGC 1711 (-0.64+/-0.18 dex), NGC1806 (-0.67+/-0.24 dex), NGC 2031 (-0.40+/-0.21 dex) and NGC 2136/37(-0.43+/-0.23 dex). The metallicities for NGC 1651, NGC 1711, NGC 1806and NGC 2031 have been determined for the first time (NGC 2031 and NGC2136/37 are interesting for the Cepheid distance scale). In the clustersurroundings, we found about 700 field stars that were suitable to beused for a determination of an age-metallicity relation (AMR) and a starformation history (SFH). Our method was to estimate ages for individualstars on the basis of Strömgren isochrones with individuallymeasured metallicities. With this method we are able to sample the AMRand SFH of the field population up to 8 Gy. Our metallicity data areincompatible with models predicting many metal-poor stars. Themetallicity of the field population increased by a factor of seven,starting around 2 Gy ago. This increase was preceded by an increase ofthe star formation rate 3-4 Gy ago. The results support the SFH found inearlier investigations of the LMC field population. The absence of starclusters between 4 Gy and 10 Gy agrees very well with a low rate of thestar formation during that time.
|The HIPPARCOS Hertzsprung-Russell diagram of S stars: probing nucleosynthesis and dredge-up|
HIPPARCOS trigonometrical parallaxes make it possible to compare thelocation of Tc-rich and Tc-poor S stars in the Hertzsprung-Russell (HR)diagram: Tc-rich S stars are found to be cooler and intrinsicallybrighter than Tc-poor S stars. The comparison with the Genevaevolutionary tracks reveals that the line marking the onset of thermalpulses on the asymptotic giant branch (AGB) matches well the observedlimit between Tc-poor and Tc-rich S stars. Tc-rich S stars are, asexpected, identified with thermally-pulsing AGB stars of low andintermediate masses, whereas Tc-poor S stars comprise mostly low-massstars (with the exception of 57 Peg) located either on the red giantbranch or on the early AGB. Like barium stars, Tc-poor S stars are knownto belong exclusively to binary systems, and their location in the HRdiagram is consistent with the average mass of 1.6+/-0.2 Msb ȯderived from their orbital mass-function distribution (Jorissen et al.1997, A&A, submitted). A comparison with the S stars identified inthe Magellanic Clouds and in the Fornax dwarf elliptical galaxy revealsthat they have luminosities similar to the galactic Tc-rich S stars.However, most of the surveys of S stars in the external systems did notreach the lower luminosities at which galactic Tc-poor S stars arefound. The deep Westerlund survey of carbon stars in the SMC uncovered afamily of faint carbon stars that may be the analogues of thelow-luminosity, galactic Tc-poor S stars. Based on data from theHIPPARCOS astrometry satellite
|A Search for Old Star Clusters in the Large Magellanic Cloud|
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1920G
|The Age of the Large Magellanic Cloud Cluster NGC 1651|
The age of NGC 1651 is 1.6 +/- 0.4 billion yr. This letter shows thataccurate age determinations of young LMC clusters are achievable withaperture photometry of Wide Field Planetary Camera 2 (WFPC2) imagestaken in a single orbit of the HST.
|Carbon stars in LMC clusters revisited.|
Abstract image available at:http://adsabs.harvard.edu/abs/1996A&A...316L...1M
|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.
|NGC 1978 in the LMC: the cluster and surrounding field.|
We present deep BVRI CCD photometry of the Large Magellanic Cloudcluster NGC 1978 and of fields immediately to the west. From the studyof the colour-magnitude diagram and the luminosity function we derivefor NGC 1978 an age of slightly more than 2Gyr and a metallicity of[Fe/H]=-0.4. NGC 1978 is, for a globular cluster, rather ellipticalwhich had been explained by a merger origin. No age structure inside NGC1978 was found, which limits this possibility to a merger of roughlycoeval subunits. In the field indications for a higher star formationrate at 1x10^8^ and 6x10^8^yr exist 8' west of the cluster, while nosuch phases can be found near the cluster. At ages older than 2Gyr thestar formation rate appears to have been the same in all analysedfields. On a broader scale, NGC 1978 seems to be one of the oldest ofthe LMC intermediate age clusters. A coherent redetermination of theages of the oldest clusters of this age group underlines a suddenenhancement of the cluster formation rate in the LMC about 2 Gyr ago.
|Moment analysis applied to LMC star clusters|
Statistical moment-based ellipse fitting is performed on observations ofLarge Magellanic Cloud clusters, confirming that trends are evident intheir position angles and ellipticities, as had been reported in theliterature. Artificial cluster images with known parameters aregenerated, and subjected to the same analysis techniques, revealingapparent trends caused by stochastic processes. Caution should thereforebe exercised in the interpretation of observational trends in young LMCclusters.
|Spectroscopy of giants in LMC clusters. II - Kinematics of the cluster sample|
Velocities for 83 star clusters in the LMC are analyzed, based onindividual stellar velocities measured at the Calcium triplet. One-halfof the clusters are objects in the outer parts of the LMC which had noprevious velocity determinations. Published velocities for intermediateand old clusters are shown to have had systematic errors. These newvelocities with various rotation curve analyses of the LMC, and testaspects of the twisted disk model proposed by Freeman et al. (1983).When the transverse motion of the LMC is taken into account, a singlerotating disk solution fits the old and intermediate-aged clusters andother tracers (i.e., there is no need for an additional 'tilted disk'system).
|The evolution of carbon stars in the Magellanic Clouds|
This study presents JHK photometric data for over 100 field stars in theSMC and for 10 in the Large Cloud together with spectroscopic resultsfor about half of them. In the Small Cloud carbon stars were found athigher temperatures and lower luminosities than previously observed. Thefaintest are below the top of the red giant branch. The medium- andlow-luminosity C stars in the M-C transition zone have a low C2 content.At these luminosities, most of the J-type stars are found close to theC2-poor stars in the HR diagram. Their C2 content is about as high as inthe coolest, most evolved C stars. The present observations of carbonstars in the SMC show that they cover a range in M(bo) from -3 to 5.9mag. The transitions from M to C via S appear to occur in both Clouds ata rather well-defined range in M(bol) for SWB and classes IV and V.
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