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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
| 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.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
| Circumstellar masers in the Magellanic Clouds
Results are presented of a search for 22 GHz H_2O616->523, 43 GHz SiOv=1(J=1->0),86 GHz SiOv=1(J=2->1) and 129 GHzSiOv=1(J=3->2) maser emission from bright IRAS pointsources in the Small and Large Magellanic Clouds - mostly circumstellarenvelopes around obscured red supergiants and Asymptotic Giant Branchstars (OH/IR stars). The aim of this effort was to test whether thekinematics of the mass loss from these stars depends on metallicity.H_2O maser emission was detected in the red supergiants IRAS 04553-6825and IRAS 05280-6910, and tentatively in the luminous IR object IRAS05216-6753 and the AGB star IRAS 05329-6708, all in the LMC.SiOv=1(J=2->1) maser emission was detected in IRAS04553-6825. The double-peaked H_2O maser line profiles of IRAS04553-6825 and IRAS 05280-6910, in combination with the OH (and SiO)maser line profiles, yield the acceleration of the outflows from thesestars. The outflow velocity increases between the H_2O masing zone nearthe dust-formation region and the more distant OH masing zone from v ~18 to 26 km s-1 for IRAS 04553-6825 and from v ~ 6 to 17 kms-1 for IRAS 05280-6910. The total sample of LMC targets isanalysed in comparison with circumstellar masers in the Galactic Centre.The photon fluxes of circumstellar masers in the LMC are found to bevery similar to those in the Galactic Centre. The expansion velocitiesin the LMC appear to be ~ 20% lower than for similarly bright OH masersin the Galactic Centre, but the data are still consistent with nodifference in expansion velocity. OH/IR stars in the LMC appear to haveslower accelerating envelopes than OH/IR stars in the Galactic Centre.The masers in the LMC have blue-asymmetric emission profiles. This maybe due to the amplification of stellar and/or free-free radiation,rather than the amplification of dust emission, and may be morepronounced in low metallicity envelopes. The SiO maser strengthincreases with the photometric amplitude at 2.2 mu m but is independentof the photometric amplitude at 10 mu m. This suggests a strongconnection between shocks in the dust-free SiO masing zone and the dustformation process. The LMC masers obey the same trend as the GalacticCentre masers. Appendices describe H_2O maser emission from themoderately mass-losing AGB star R Dor in the Milky Way, optical echellespectroscopy of IRAS 04553-6825, and the properties of circumstellarmasers in the Galactic Centre.
| Evolutionary Synthesis Modeling of Red Supergiant Features in the Near-Infrared
We present evolutionary synthesis models applied to near-infraredspectral features observed in the spectra of young Magellanic Cloudclusters and starburst galaxies. The temporal evolution of the first andsecond overtones of CO at 2.29 μm (2-0 band head) and 1.62 μm (6-3band head) and of the U-B, B-V, and J-K colors are investigated. We findthat the current evolutionary tracks of massive stars with subsolarchemical composition in the red supergiant phase are not reliable forany synthesis of the temporal evolution of infrared stellar features.The high sensitivity of the selected infrared features to theatmospheric parameters of cool stars allows us to place constraints onthe temperature and the fraction of time spent in the red part of theHertzsprung-Russell diagram by massive stars during their core heliumburning phase. We derive a set of empirically calibratedspectrophotometric models by adjusting the red supergiant parameters sothat the properties of the observed templates are reproduced.
| 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.
| Dynamical studies of cluster pairs in the Magellanic Clouds
We performed N-body simulations of star cluster encounters withHernquist's TREECODE in a CRAY YMP-2E computer under different initialconditions (relative positions and velocities, cluster sizes, masses andconcentration degrees). The total number of particles per simulationranged from 1024 to 20480. These models are compared with a series ofisodensity maps of cluster pairs in the Magellanic Clouds. Evidence isfound that during the interactions, transient morphological effects suchas an expanded halo, isophotal deformation and isophotal twisting canoccur as a result of tidal effects and dynamical friction. Thesimulations also show that different outcomes are possible depending onthe initial parameters: (i) long-standing changes of concentrationdegree can occur after the collision; (ii) one member can disaggregate;or (iii) the pair can coalesce into a single cluster with a distinctstructure compared with the original ones. These simulations canreproduce a wide range of morphological structures in observed clusterpairs.
| IR spectra of young Magellanic Cloud clusters and starburst galaxies: constraints on the temperature of red supergiants and new estimates of metallicity in young stellar populations
Infrared spectra of young stellar clusters in the Magellanic Clouds areused to derive information on the red supergiants dominating their 1.6mu m emission, and to obtain a new and independent estimate of theirmetallicities. The most striking result is that red supergiants with lowmetallicity appear to be much cooler than predicted by evolutionarymodels, and this most probably reflects uncertainties in the calibrationof the mixing-length parameter in the outermost layers of the stellarenvelopes. The metallicity [Fe/H] can be estimated from the W_lambda(1.62) index which is here calibrated using synthetic stellar spectra,and the new scale is also applied to eight starburst galaxies. Theresulting values of [Fe/H] range between -1.3 for the SMC cluster NGC330(in excellent agreement with previous estimates) to -0.2 for the LMCcluster NGC1994. Starburst galaxies have metallicities ranging between-1.0 (NGC6240) and -0.5 (NGC7552). The spectra are also used to estimatethe Carbon depletion which in MC clusters is found compatible with a`standard' value of [C/Fe] =~ -0.3. Interestingly, our spectra showpossible evidence of significant variations of Carbon depletion in somestarburst galaxies. Finally, the Silicon relative abundance is estimatedfrom the W_lambda (1.59) index. In MC clusters we find [Si/Fe]~+0.5,i.e. values similar to those of old clusters in our galaxy andcompatible with primordial Si-enhancement by type II supernovae. Basedon observations collected at the European Southern Observatory, LaSilla, Chile
| Obscured AGB stars in the Magellanic Clouds. I. IRAS candidates
We have selected 198 IRAS sources in the Large Magellanic Cloud, and 11in the Small Magellanic Cloud, which are the best candidates to bemass--loosing AGB stars (or possibly post--AGB stars). We used thecatalogues of \cite[Schwering \& Israel (1990)]{ref42} and\cite[Reid et al. (1990)]{ref36}. They are based on the IRAS pointedobservations and have lower detection limits than the Point SourceCatalogue. We also made cross-identifications between IRAS sources andoptical catalogues. Our resulting catalogue is divided in 7 tables.Table \ref{tab1} lists optically known red supergiants and AGB stars forwhich we found an IRAS counterpart (7 and 52 stars in the SMC and LMC,respectively). Table \ref{tab2} lists ``obscured'' (or ``cocoon'') AGBstars or late-type supergiants which have been identified as such inprevious works through their IRAS counterpart and JHKLM photometry (2SMC and 34 LMC sources; no optical counterparts). Table \ref{tab3} listsknown planetary nebulae with an IRAS counterpart (4 SMC and 19 LMC PNe).Table \ref{tab4} lists unidentified IRAS sources that we believe to begood AGB or post--AGB or PNe candidates (11 SMC and 198 LMC sources).Table~\ref{tab5} lists unidentified IRAS sources which could be any typeof object (23 SMC and 121 LMC sources). Table \ref{tab6} lists IRASsources associated with foreground stars (29 SMC and 135 LMC stars).Table \ref{tab7} lists ruled out IRAS sources associated with HIIregions, hot stars, etc... We show that the sample of IRAS AGB stars inthe Magellanic Clouds is very incomplete. Only AGB stars more luminousthan typically 10^4 L_\odot and with a mass-loss rate larger thantypically 5 10^{-6} M_\odot/yr could be detected by the IRAS satellite.As a consequence, one expects to find very few carbon stars in the IRASsample. We also expect that most AGB stars with intermediate mass--lossrates have not been discovered yet, neither in optical surveys, nor inthe IRAS survey. Tables 1 to 8 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Mid-infrared properties of globular clusters using the IRAS data base
We present an analysis of the mid-IR properties of 18 globular clusters(GCs) [15 in the Galaxy and three in the Large Magellanic Cloud (LMC)]using the IRAS photometric data at 12 and 25 mum. Eight of the nineGalactic GCs with central escape velocities greater than 50 km s^-1 haveIRAS sources within a radius of 60 arcsec from the centre, in agreementwith the expectation that interstellar gas and dust should indeed bepresent in the central regions of the most massive clusters owing tomass-loss processes occurring in the late stages of the stellarevolution. No other significant correlation is found between IRAS sourceincidence and any intrinsic GC parameters. Warm dust (T~300K) isdetectable mostly around unresolved giant stars, but in three massiveGCs it is also present as diffuse emission. However, most of the dustmight be cold (T<50K) and it was thus notdetected by IRAS because of its limited sensitivity at 60 and 100 mum.The inferred mass-loss rates and statistical considerations arecompatible with a non-steady mass-loss process with several episodes ofejection lasting a few times 10^5 yr.
| 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.
| Red supergiants as starburst tracers in galactic nuclei.
We present infrared spectral observations of CO (1.62, 2.29μm) and Si(1.59μm) stellar absorption features in 23 galaxies including 14calibrators (normal ellipticals/spirals and well known HII galaxies) and9 Seyferts. Our main aim was to define and calibrate an observationalmethod for tracing red supergiants (i.e. starbursts) in active galaxies.Additional spectra were also obtained around the Brγ hydrogenrecombination line. The measured equivalent widths of the absorptionfeatures in old stellar systems and HII galaxies are found to beremarkably similar and the CO and Si indices do not therefore providereliable diagnostics for distinguishing red supergiants (i.e.starbursts) from metallic red giants (i.e. old stellar systems). A moresensitive quantity is found to be the light to mass ratio (L_H_/M) asinferred from the observed 1.65μm stellar luminosities and velocitydispersions. All HII galaxies and several of the Seyfert 2's exhibitL_H_/M values a factor =~5 larger than normal ellipticals and spirals.Both the L_H_/M ratios and the Brγ equivalent widths in Sy2's areconsistent with the presence of starbursts which are older than those inHII galaxies. A large fraction of the near infrared (<2μm)continuum in Sy1's also appears to be stellar but associated with evenolder starburst activity or normal red giants in the galaxy bulge. Theseresults therefore provide additional support for evolutionary models inwhich Seyfert activity is related to the presence of a black hole whichis formed/fueled by the remnants of a pre-cursor starburst. The originof the non-stellar 2μm continuum in Seyferts and the metallicity ofellipticals are also discussed.
| 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.
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
| Near-IR Spectroscopy Indices and the Stellar Content of Galaxies
Not Available
| Colour Evolution Models and the Distribution of Large Magellanic Cloud Clusters in the Integrated UBV Plane
We present a comparison between photometric cluster models, based onclassical and with-overshooting stellar tracks, and the enlarged sampleof 624 LMC clusters recently gathered in integrated UBV photometry byBica et al. Models based on Maeder and Meynet's tracks present twotemporary red phases: the first at age 10 Myr, caused by a clump of redsupergiants; the second at ~100 Myr due to the combined effect of boththe progressive reduction of the blue loop of core He-burning stars, andtheir fading relative to top-MS stars. The 100 Myr red phase does notoccur in models without overshooting. Taking into account stochasticeffects on the mass distribution of stars, the models describe well thegeneral distribution of clusters in the (U - B) vs. (B - V) diagram,except for the oldest, SWB types V-VII, clusters. The dispersion ofcluster colours due to stochastic effects is found to be stronglyvariable along the ageing sequence: the general trend is a decrease withage due to the increasing population of post-MS phases, but thedispersion increases in the temporary red phases and is expected toincrease again after the red giant branch phase transition due to theappearance of extended RGBs and carbon stars. We also study the LMCclusters age distribution function, based on the age frequency ofclusters of equal initial masses, taking into account different valuesfor the IMF slope.
| 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.
| 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
| 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.
| LMC clusters - Age calibration and age distribution revisited
The empirical age relation for star clusters in the Large MagellanicCloud presented by Elson and Fall (1985) are reexamined using ages basedonly on main-sequence turnoffs. The present sample includes 57 clusters,24 of which have color-magnitude diagrams published since 1985. The newcalibration is very similar to that found previously, and the scatter inthe relation corresponds to uncertainties of about a factor of 2 in age.The age distribution derived from the new calibration does not differsignificantly from that derived in earlier work. It is compared with agedistributions estimated by other authors for different samples ofclusters, and the results are discussed.
| Integrated UV magnitudes of the Large Magellanic Cloud associations
UV photographs (2600 A, 350 A passband) of the LMC have been obtained bythe S183 experiment during a Skylab mission. The background is estimatedand a method for deriving the integrated fluxes is presented. Theintegrated magnitudes of about 50 associations and isocontours of theirintensities are given, along with the B and V integrated magnitudes of13 associations.
| M supergiants in the Milky Way and the Magellanic Clouds Colors, spectral types, and luminosities
The differences in metal abundances between the Milky Way, LargeMagellanic Cloud (LMC), and Small Magellanic Cloud (SMC) affect most ofthe observable properties of the M supergiants in these galaxies; thosein the SMC (which has the lowest metal abundance) have the earliest meanspectral type, while those of the Milky Way exhibit the latest meanspectral type. This is presently interpreted as a combination of twoeffects of differing metal abundance on the supergiant atmospheres:first, lower abundance stars of a given effective temperature haveearlier MK spectral types due to reduced TiO abundance; second, theHayashi track is shifted to hotter effective temperature at reducedmetal abundance, thereby shifting the mean spectral type still earlier.The fact that the 10-micron excess decreases linearly with metalabundance suggests that mass loss rates are roughly the same for starsin all three galaxies, with the dust-to-gas ratio proportional to metalabundance.
| The age-metallicity relationship for the clusters of the Large Magellanic Cloud
Moderate dispersion spectrophotometric scans with an intensified Reticonarray have been obtained using the du Pont telescope of the Las CampanasObservatory for 38 stars expected to be members of 15 clusters in theLarge Magellanic Cloud. Ages for these clusters are deduced from atransformation of their classification in the scheme of Searle,Wilkinson, and Bagnuolo (1980). Abundances are derived from the scansusing a crude analysis applied to computer-generated pseudoequivalentwidths calibrated by identical observations of 42 stars in six galacticglobular clusters and by several galactic supergiants. A strongage-metallicity relationship is found and the chemical history of theLMC is discussed. Unlike the solar neighborhood or galactic halo, simplemodels of chemical evolution are adequate to fit the derivedage-abundance correlation.
| The extended giant branches of intermediate age globular clusters in the Magellanic Clouds. II
In order to obtain a complete sample of upper asymptotic giant branch(AGB) stars in the red globular clusters of the Magellanic Clouds, aphotographic near-infrared survey of the clusters was conducted. Theresults are compared with previous photometry and the problem of errorarising from variability of carbon stars is addressed. Stars withoutspectra are tentatively classified based on their JHK colors. Apparentand absolute bolometric magnitudes and effective temperatures werecalculated from the IR colors, allowing for the location of the redstars and of the cluster giant branches in the physical H-R diagram tobe determined. Stellar evolution on the AGB is discussed, leading toimproved estimates of the extent of the upper AGB. A carbon star censusis presented and the ages of the clusters is derived with suitablycomplete photometry. On this basis, the chemical enrichment history ofthe Clouds is discussed.
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