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|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 Globular Cluster NGC 1978 in the Large Magellanic Cloud|
We have used deep, high-resolution Hubble Space Telescope ACSobservations to image the cluster NGC 1978 in the Large MagellanicCloud. This high-quality photometric data set allowed us to confirm thehigh ellipticity (ε~0.30+/-0.02) of this stellar system. Thederived color-magnitude diagram allowed a detailed study of the mainevolutionary sequences; in particular, we have detected the so-calledbump along the red giant branch (at V555=19.10+/-0.10). Thisis the first detection of this feature in an intermediate-age cluster.Moreover, the morphology of the evolutionary sequence and the populationratios have been compared with the expectations of different theoreticalmodels (namely, BaSTI, Pisa Evolutionary Library [PEL], and Padua) inorder to quantify the effect of convective overshooting. The bestagreement (in terms of both morphology and star counts) has been foundwithin the PEL isochrone, with Z=0.008 (consistent with the most recentdetermination of the cluster metallicity, [M/H]=-0.37 dex) and a mildovershooting efficiency (Λos=0.1). By adopting thistheoretical set an age of τ=1.9+/-0.1 Gyr has been obtained.
|Luminosities and mass-loss rates of carbon stars in the Magellanic Clouds|
Dust radiative transfer models are presented for 60 carbon stars in theMagellanic Clouds (MCs) for which 5-35 μm Spitzer infraredspectrograph (IRS) spectra and quasi-simultaneous ground-based JHKLphotometry are available. From the modelling, the luminosity andmass-loss rate are derived (under the assumption of a fixed expansionvelocity and dust-to-gas ratio), and the ratio of silicon carbide (SiC)to amorphous carbon (AMC) dust is also derived. This ratio is smallerthan observed in Galactic carbon stars, as has been noted before. Lightcurves for 36 objects can be retrieved from the massive compact haloobject (MACHO) and optical gravitational lensing experiment (OGLE) databases, and periods can be derived for all but two of these. Includingdata from the literature, periods are available for 53 stars.There is significant scatter in a diagram where the mass-loss rates areplotted against luminosity, and this is partly due to the fact that theluminosities are derived from single-epoch data. The mass-loss rates forthe MC objects roughly scatter around the mean relation for GalacticC-stars.The situation is better defined when the mass-loss rate is plottedagainst pulsation period. For a given period, most of the LargeMagellanic Cloud (LMC) and Small Magellanic Cloud (SMC) stars havemass-loss rates that are in agreement with that observed in Galacticcarbon stars (under the assumption that these objects have an expansionvelocity and dust-to-gas ratio typical of the mean observed in Galacticcarbon Miras).For some SMC sources only, the IRS spectrum at longer wavelengths fallsclearly below the model flux predicted by a constant mass-loss rate. Analternative model with a substantial increase of the mass-loss rate toits present-day value over a time-scale of a few tens of years is ableto explain the spectral energy distribution (SED) and IRS spectra ofthese sources. However, the probability to have two such cases in asample of 60 is small, and makes this not a likely explanation (andtestable by re-observing these objects near the end of the lifetime ofSpitzer). Alternative explanations are (ad hoc) changes to the dustemissivity at longer wavelengths, and/or deviations from sphericalsymmetry.
|Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models|
Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. Wepresent a comprehensive study designed to extract the maximuminformation from spectra of data quality typical in large galaxysurveys. These techniques are not intended for detailed stellarpopulation studies that use high-quality spectra. We test techniques ona sample of globular clusters, which should consist of single stellarpopulations and provide good test cases, using the Bruzual-Charlothigh-resolution stellar population synthesis models to simultaneouslyestimate the ages and metallicities of 101 globular clusters in M31 andthe Magellanic Clouds. The clusters cover a wide range of ages andmetallicities, 4 Myr
|The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters|
Using available data for C and M giants with M_bol<-3.6 in MagellanicCloud clusters, we derive limits to the lifetimes for the correspondingevolutionary phases, as a function of stellar mass. The C-star phase isfound to have a duration between 2 and 3 Myr for stars in the mass rangefrom ~1.5 to 2.8 M_ȯ. There is also an indication that the peak ofC-star lifetime shifts to lower masses (from slightly above to slightlybelow 2 Mȯ) as we move from LMC to SMC metallicities.The M-giant lifetimes also peak at ~2 Mȯ in the LMC,with a maximum value of about 4 Myr, whereas in the SMC their lifetimesappear much shorter, but, actually, they are poorly constrained by thedata. These numbers constitute useful constraints to theoretical modelsof the TP-AGB phase. We show that several models in the literatureunderestimate the duration of the C-star phase at LMC metallicities.
|An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near-Infrared Color-Magnitude Diagrams|
We present an empirical method to derive photometric metallicity,reddening, and distance to old stellar populations by using a few majorfeatures of the red giant branch (RGB) in near-IR color-magnitudediagrams. We combine the observed RGB features with a set of equationslinking the global metallicity [M/H] to suitable RGB parameters (colors,magnitudes, and slope), as calibrated from a homogeneous sample ofGalactic globular clusters with different metallicities. This techniquecan be applied to efficiently derive the main population parameters ofold stellar systems, with the goal of using ground-based adaptive opticsand space facilities to probe the stellar content of remote galaxies.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG), operated on the island La Palma bythe Fundacion Galileo Galilei of INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud|
We present a Spitzer Space Telescope spectroscopic survey of mass-losingcarbon stars (and one oxygen-rich star) in the Large Magellanic Cloud(LMC). The stars represent the superwind phase on the asymptotic giantbranch (AGB), which forms a major source of dust for the interstellarmedium (ISM) in galaxies. Bolometric magnitudes indicate progenitormasses of 1.5-2.5Msolar. The spectra cover the wavelengthrange 5-38μm. They show varying combinations of dust continuum, dustemission features (SiC, MgS) and molecular absorption bands(C2H2, HCN). A 10-μm absorption feature isattributed to C3. A weak band at 5.8μm is suggestive ofcarbonyl. The circumstellar 7.5-μm C2H2 band isfound to be stronger at lower metallicity, explained by higher C/Oratios at low metallicity. The J - K versus K - A colours, used toselect the sample, are shown to be relatively insensitive in separatingcarbon versus oxygen-rich AGB stars. The predominance of carbon starstherefore indicates that in the range 1.5-2.5Msolar, LMC AGBstars become carbon-rich before onset of the superwind. A set of fournarrow bands, dubbed the Manchester system, is used to define theinfrared continuum for dusty carbon stars. We investigate the strengthand central wavelength of the SiC and MgS dust bands as a function ofcolour and metallicity. The line-to-continuum ratio of these bands showssome indication of being lower at low metallicity. The MgS band is onlyseen at dust temperatures below 600K. Metal-poor carbon stars can formamorphous carbon dust from self-produced carbon. The formationefficiency of oxygen-rich dust depends more strongly on metallicity. Inlower-metallicity environments, the dust input into the ISM by AGB starsmay be strongly biased towards carbonaceous dust.
|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.
|On the Iron Content of NGC 1978 in the LMC: A Metal-rich, Chemically Homogeneous Cluster|
We present a detailed abundance analysis of giant stars in NGC 1978, amassive, intermediate-age stellar cluster in the Large Magellanic Cloud,characterized by a high ellipticity and suspected to have a metallicityspread. We analyzed 11 giants, all cluster members, by usinghigh-resolution spectra acquired with UVES/FLAMES at the ESO Very LargeTelescope. We find an iron content of [Fe/H] = -0.38 dex with very lowσ[Fe/H]=0.07 dex dispersion, a mean heliocentric radialvelocity vr=293.1+/-0.9 km s-1, and a velocitydispersion σvr=3.1 km s-1, thusexcluding the presence of a significant metallicity, as well asvelocity, spread within the cluster.Based on observations collected at the Very Large Telescope of theEuropean Southern Observatory (ESO), Cerro Paranal, Chile, underprograms 072.D-0342 and 074.D-0369.
|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.
|Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud|
We present ESO/VLT spectra in the 2.9-4.1 μm range for a large sampleof infrared stars in the Large Magellanic Cloud (LMC), selected on thebasis of MSX and 2MASS colours to be extremely dust-enshrouded AGB starcandidates. Out of 30 targets, 28 are positively identified as carbonstars, significantly adding to the known population of opticallyinvisible carbon stars in the LMC. We also present spectra for sixIR-bright stars in or near three clusters in the LMC, identifying fourof them as carbon stars and two as oxygen-rich supergiants. We analysethe molecular bands of C2H2 at 3.1 and 3.8 μm, HCN at 3.57 μm, andsharp absorption features in the 3.70-3.78 μm region that weattribute to C2H2. There is evidence for a generally high abundance ofC2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundanceratios at the low metallicity in the LMC. The low initial metallicity isalso likely to have resulted in less abundant HCN and CS. The sample ofIR carbon stars exhibits a range in C2H2:HCN abundance ratio. We do notfind strong correlations between the properties of the molecularatmosphere and circumstellar dust envelope, but the observed differencesin the strengths and shapes of the absorption bands can be explained bydifferences in excitation temperature. High mass-loss rates and strongpulsation would then be seen to be associated with a large scale heightof the molecular atmosphere.
|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.
|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.
|Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra|
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
|Infrared Surface Brightness Fluctuations of Magellanic Star Clusters|
We present surface brightness fluctuations (SBFs) in the near-IR for 191Magellanic star clusters available in the Second Incremental and All SkyData releases of the Two Micron All Sky Survey (2MASS) and compare themwith SBFs of Fornax Cluster galaxies and with predictions from stellarpopulation models as well. We also construct color-magnitude diagrams(CMDs) for these clusters using the 2MASS Point Source Catalog (PSC).Our goals are twofold. The first is to provide an empirical calibrationof near-IR SBFs, given that existing stellar population synthesis modelsare particularly discrepant in the near-IR. Second, whereas mostprevious SBF studies have focused on old, metal-rich populations, thisis the first application to a system with such a wide range of ages(~106 to more than 1010 yr, i.e., 4 orders ofmagnitude), at the same time that the clusters have a very narrow rangeof metallicities (Z~0.0006-0.01, i.e., 1 order of magnitude only). Sincestellar population synthesis models predict a more complex sensitivityof SBFs to metallicity and age in the near-IR than in the optical, thisanalysis offers a unique way of disentangling the effects of age andmetallicity. We find a satisfactory agreement between models and data.We also confirm that near-IR fluctuations and fluctuation colors aremostly driven by age in the Magellanic cluster populations and that inthis respect they constitute a sequence in which the Fornax Clustergalaxies fit adequately. Fluctuations are powered by red supergiantswith high-mass precursors in young populations and by intermediate-massstars populating the asymptotic giant branch in intermediate-agepopulations. For old populations, the trend with age of both fluctuationmagnitudes and colors can be explained straightforwardly by evolution inthe structure and morphology of the red giant branch. Moreover,fluctuation colors display a tendency to redden with age that can befitted by a straight line. For the star clusters only,(H-Ks)=(0.21+/-0.03)log(age)-(1.29+/-0.22) once galaxies areincluded, (H-Ks)=(0.20+/-0.02)log(age)-(1.25+/-0.16).Finally, we use for the first time a Poissonian approach to establishthe error bars of fluctuation measurements, instead of the customaryMonte Carlo simulations.This research has made use of the NASA/ IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under contract with the National Aeronautics and SpaceAdministration.
|Superassociations and Stellar Complexes in Galaxies|
The basic characteristics of stellar complexes and superassociations, aswell as the differences between these kinds of gigantic groups of youngstars, are discussed. The main difference is that superassociations arethe result of induced (triggered) star formation, while the stars andclusters in stellar complexes develop as a result of the spontaneousprocesses typical of galactic gaseous disks.
|The Star Formation History of the Small Magellanic Cloud|
We present the spatially resolved star formation and chemical enrichmenthistory of the Small Magellanic Cloud (SMC) across the entire central4deg×4.5d area of the main body, based on UBVIphotometry from our Magellanic Clouds Photometric Survey. We find that(1) approximately 50% of the stars that ever formed in the SMC formedprior to 8.4 Gyr ago (z>1.2 for WMAP cosmology); (2) the SMC formedrelatively few stars between 8.4 and 3 Gyr ago; (3) there was a rise inthe mean star formation rate during the most recent 3 Gyr punctuated by``bursts'' at ages of 2.5, 0.4, and 0.06 Gyr; (4) the bursts at 2.5 and0.4 Gyr are temporally coincident with past perigalactic passages of theSMC with the Milky Way; (5) there is preliminary evidence for alarge-scale annular structure in the 2.5 Gyr burst; and (6) the chemicalenrichment history derived from our analysis is in agreement with theage-metallicity relation of the SMC's star clusters. Consistentinterpretation of the data required an ad hoc correction of 0.1-0.2 magto the B-V colors of 25% of the stars; the cause of this anomaly isunknown, but we show that it does not strongly influence our results.
|A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer|
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
|The Stellar Populations of dE Galaxies in Nearby Groups|
In this contribution Gemini-North NIRI J,K-observations are used toinvestigate the upper-Asymptotic Giant Branch (AGB) intermediate-agepopulation in the M81 Group dwarf elliptical (dE) F8D1. Hubble SpaceTelescope (HST) `snapshot' V,I-observations are also analysed toinvestigate the upper-AGB populations in two other M81 Group dEs, DDO 71and kk077. In all three dEs, significant intermediate-age populationsare found. Further, there are sizeable dE-to-dE differences in thesepopulations: F8D1 contains relatively more, and relatively moreluminous, upper-AGB stars. These results are compared with existinginformation for Local Group and Sculptor group dwarfs. It is suggestedthat `environmental harrassment' plays an important role in governingdwarf galaxy evolution.
|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
|Obscured asymptotic giant branch variables in the Large Magellanic Cloud and the period-luminosity relation|
The characteristics of oxygen-rich and carbon-rich, large-amplitude(ΔK > 0.4 mag), asymptotic giant branch (AGB) variables in theLarge Magellanic Clouds are discussed, with an emphasis on thoseobscured by dust. Near-infrared photometry, obtained over about 8 yr, iscombined with published mid-infrared observations from IRAS and ISO todetermine bolometric magnitudes for 42 stars. Pulsation periods of theO-rich stars are in the range 116 < P < 1393 d, while those forC-rich stars have 298 < P < 939 d. In addition to the regularpulsations, one O-rich star and four C-rich stars show large-amplitude,ΔK > 0.6 mag, secular or very long-period variations, which maybe associated with changes in their mass-loss rates. We discuss andcompare various methods of determining the bolometric magnitudes andshow, perhaps surprisingly, that most of the very long-period stars seemto follow an extrapolation of the period-luminosity relation determinedfor stars with shorter periods - although the details do depend on howthe bolometric magnitudes are calculated.Three stars with thin shells, which are clearly more luminous than theobscured AGB stars, are undergoing hot bottom burning, while other starswith similar luminosities have yet to be investigated in sufficientdetail to determine their status in this regard. We suggest that anapparent change in slope of the period-luminosity relation around400-420 d is caused by variables with luminosities brighter than thepredictions of the core-mass-luminosity relation, owing to excess fluxfrom hot bottom burning.
|Measuring cosmological parameters with the SDSS QSO spatial power spectrum analysis to test the cosmological principle|
In this paper we emphasize the importance of the Sloan Digital SkySurvey (SDSS) quasi-stellar object (QSO) clustering statistics as aunique probe of the Universe. Because the complete SDSS QSO samplecovers a quarter of the observable Universe, cosmological parametersestimated from the clustering statistics have an implication as a testof the cosmological principle, by comparing with those from localgalaxies and other cosmological observations. Using an analyticalapproach to the power spectrum for the QSO sample, we assess theaccuracy with which the cosmological parameters can be determined.Arguments based on the Fisher matrix approach demonstrate that the SDSSQSO sample might have a potential to provide useful constraints on thedensity parameters as well as the cosmic equation of state.
|Ages and metallicities of eight star clusters and their surrounding fields in the inner disc of the Large Magellanic Cloud|
We present Washington system colour-magnitude diagrams for 8 starclusters and their surrounding fields which, with one exception, liewithin the inner parts of the Large Magellanic Cloud (LMC) disc. Carefulattention is paid to separating out the cluster and field stardistributions. Ages and metallicities are then determined in aconsistent manner for both populations in two different ways. We firstcompare the colour-magnitude diagrams (CMDs) with new theoreticalisochrones in the Washington system. We also derive ages using themagnitude difference between the red clump and the turnoff, and derivemetallicities by comparing the giant branches to standard calibratingclusters. For this latter metallicity derivation, we presentage-dependent metallicity corrections for intermediate age clusters(IACs) based on the new isochrones. The two methods for both age andmetallicity determination are in good agreement with each other. Allclusters are found to be IACs (1-3 Gyr), with [Fe/H] from -0.4 to -0.9.We find that the stellar population of each star cluster is generallyquite similar to that of the field where it is embedded, sharing itsmean age and metallicity. Combining the present sample with a revisionof that of Bica et al. studied similarly, we find that our metallicitiesfor IACs are intermediate in metallicity to those for clusters ofsimilar age studied by Olszewski et al. and by Beasley, Hoyle &Sharples. A combined age-metallicity relation is presented which showsthat LMC clusters formed between 1-3 Gyr ago with a mean metallicity(-0.5 dex) and metallicity spread (0.23 dex) independent of age. Goodagreement is found with the bursting model of Pagel & Tautvaisiene.No evidence for a metallicity gradient is found.
|VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies. II. Implications for Understanding Galaxy Evolution|
We have used the Ultraviolet Visual-Echelle Spectrograph (UVES) onKueyen (UT2) of the Very Large Telescope to take spectra of 15individual red giant stars in the centers of four nearby dwarfspheroidal galaxies (dSph's): Sculptor, Fornax, Carina, and Leo I. Wemeasure the abundance variations of numerous elements in these low-massstars with a range of ages (1-15 Gyr old). This means that we caneffectively measure the chemical evolution of these galaxies with time.Our results show a significant spread in metallicity with age, but anoverall trend consistent with what might be expected from a closed- (orperhaps leaky-) box chemical evolution scenario over the last 10-15 Gyr.We make comparisons between the properties of stars observed in dSph'sand in our Galaxy's disk and halo, as well as globular clusterpopulations in our Galaxy and in the Large Magellanic Cloud. We alsolook for the signature of the earliest star formation in the universe,which may have occurred in these small systems. We notice that each ofthese galaxies show broadly similar abundance patterns for all elementsmeasured. This suggests a fairly uniform progression of chemicalevolution with time, despite quite a large range of star formationhistories. It seems likely that these galaxies had similar initialconditions, and that they evolve in a similar manner with star formationoccurring at a uniformly low rate, even if at different times. With ouraccurate measurements we find evidence for small variations inabundances, which seem to be correlated to variations in star formationhistories between different galaxies. The α-element abundancessuggest that dSph chemical evolution has not been affected by very highmass stars (>15-20 Msolar). The abundance patterns we measurefor stars in dSph's are significantly different from those typicallyobserved in the disk, bulge, and inner halo of our Galaxy. This meansthat, as far as we can tell from the (limited) data available to date,it is impossible to construct a significant fraction of our disk, innerhalo, or bulge from stars formed in dSph's such as we see today, whichsubsequently merged into our own. Any merger scenario involving dSph'shas to occur in the very early universe while they are still gas-rich,so the majority of mass transfer is gas and few stars.Based on Ultraviolet-Visual Echelle Spectrograph observations collectedat the European Southern Observatory, proposals 66.B-0320, 65.N-0378,62.N-0653, and 61.A-0275.
|Pulsation at the tip of the first giant branch?|
The first results of our ongoing near-infrared (NIR) survey of thevariable red giants in the Large Magellanic Cloud, using the InfraredSurvey Facility (IRSF) and the SIRIUS infrared camera, are presented.Many very red stars were detected and we found that most of them arevariables. In the observed colour-magnitude diagram (J-K, K) and thestellar K magnitude distribution, the tip of the first giant branch(TRGB), where helium burning in the core starts, is clearly seen. Apartfrom the genuine AGB variables, we found many variable stars atluminosities around the TRGB. From this result, we infer that asubstantial fraction of them are RGB variables.
|0.8-13.5 Micron Spectroscopy of IRAS 07077+1536: A Dusty Carbon Star|
Contemporaneous near-infrared (0.8-2.5 μm) and infrared (3-14 μm)spectroscopy is presented of the previously uncharacterized IRAS source07077+1536. The data show it to be a carbon star embedded in acircumstellar dust envelope. The near-infrared spectrum displaysmolecular absorption features of C2, CN, and CO, while the 11μm emission feature from SiC is present in the thermal infrared. Thespectral energy distribution of IRAS 07077+1536 is similar to theso-called extreme carbon stars in that it consists primarily of areddened stellar photosphere and thermal dust emission. However, theextinction is much less and the dust significantly warmer (~900 K vs.~300 K) than in extreme carbon stars such as AFGL 3068. IRAS 07077+1536is also unusually bright shortward of 0.5 μm, an aspect that could bedue to variability but may indicate the presence of a binary companionor a foreground star.
|Testing stellar population models with star clusters in the Large Magellanic Cloud|
We present high signal-to-noise ratio integrated spectra of 24 starclusters in the Large Magellanic Cloud (LMC), obtained using the FLAIRspectrograph at the UK Schmidt telescope. The spectra have been placedon to the Lick/IDS system in order to test the calibration of SimpleStellar Population (SSP) models. We have compared the SSP-predictedmetallicities of the clusters with those from the literature,predominantly taken from the Ca-triplet spectroscopy of Olszewski et al.(1991). We find that there is good agreement between the metallicitiesin the range -2.10 <=[Fe/H]<= 0. However, the Mg2 index(and to a lesser degree Mg b) systematically predict highermetallicities (up to +0.5 dex higher) than . Among thepossible explanations for this are that the LMC clusters possess[α/Fe] > 0. Metallicities are presented for eleven LMC clusterswhich have no previous measurements. We compare SSP ages for theclusters, derived from the Hβ, Hγ and Hδ Lick/IDSindices, with the available literature data, and find good agreement forthe vast majority. This includes six old globular clusters in oursample, which have ages consistent with their HST colour-magnitudediagram (CMD) ages and/or integrated colours. However, two globularclusters, NGC 1754 and NGC 2005, identified as old (~15 Gyr) on thebasis of HST CMDs, have Hβ line-strengths which lead ages that aretoo low (~8 and ~6 Gyr respectively). These findings are inconsistentwith their CMD-derived values at the 3σ level. Comparison betweenthe horizontal branch morphology and the Balmer line strengths of theseclusters suggests that the presence of blue horizontal branch stars hasincreased their Balmer indices by up to ~1.0 Å. We conclude thatthe Lick/IDS indices, used in conjunction with contemporary SSP models,are able to reproduce the ages and metallicities of the LMC clustersreassuringly well. The required extrapolations of the fitting functionsand stellar libraries in the models to lower ages and low metallicitiesdo not lead to serious systematic errors. However, owing to thesignificant contribution of horizontal branch stars to Balmer indices,SSP model ages derived for metal-poor globular clusters are ambiguouswithout a priori knowledge of horizontal branch morphology.
|The brightest asymptotic giant branch stars in the Leo I dwarf spheroidal galaxy|
The first results of a study of the dwarf spheroidal galaxy Leo I usingthe new Nagoya-South African Infrared Survey Facility (IRSF) arepresented. J, H and Ks observations show that most, if notall, of at least the top magnitude of the asymptotic giant branch (AGB)in Ks is populated by carbon stars. In addition there arefive very red objects which are believed to be dust-enshrouded AGBstars. One of these is, remarkably, well outside the main body of thegalaxy. Three of these obscured stars and five known carbon stars showvariability in observations 11 months apart. One of the obscured starshas ΔKs= 0.87, making it highly likely that it, atleast, is a Mira variable. The tip of the AGB is atMbol~-5.1, but further variability studies are necessary toobtain a definitive value. Comparison with carbon stars, both Miras andnon-Miras, in Magellanic Cloud clusters, and taking into account otherevidence on the ages and metallicities of Leo I populations, suggeststhat these obscured stars belong to the youngest significant populationof Leo I and have ages of ~2 Gyr.
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