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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.
|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.
|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.
|Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452|
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
|Long period variables in globular clusters .|
We present results from our survey of long period variables (LPVs) inglobular clusters of the Milky Way and the LMC. Based on a large sampleof previously known and newly discoverd LPVs in 47 Tuc we produce aP-L-diagram for the LPVs in this cluster and compare it with pulsationmodels. For the first time measurement of masses of AGB stars have shownthat mass loss of the order of 0.3 Msun occurs on the RGBand AGB. We discuss the pulsation mode along the evolutionary path upthe giant branch. Results from two further clusters are brieflysummarized.
|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.
|Evolutionary population synthesis: models, analysis of the ingredients and application to high-z galaxies|
Evolutionary population synthesis models for a wide range ofmetallicities, ages, star formation histories, initial mass functionsand horizontal branch morphologies, including blue morphologies at highmetallicity, are computed. The model output comprises spectral energydistributions, colours, stellar M/L ratios, bolometric corrections andnear-infrared (IR) spectral line indices. The energetics of the postmain sequence evolutionary phases are evaluated with the fuelconsumption theorem. The impact on the models of the stellarevolutionary tracks (in particular with and without overshooting) isassessed. We find modest differences in synthetic broad-band colours asinduced by the use of different tracks in our code [e.g. Δ(V-K) ~0.08 mag, Δ(B-V) ~ 0.03 mag]. Noticeably, these differences aresubstantially smaller than the scatter among other models in theliterature, even when the latter adopt the same evolutionary tracks. Themodels are calibrated with globular cluster data from the Milky Way forold ages, and the Magellanic clouds plus the merger remnant galaxy NGC7252, both for young ages of ~0.1-2Gyr, in a large wavelength range fromthe U band to the K band. Particular emphasis is put on the contributionfrom the thermally pulsing asymptotic giant branch (TP-AGB) phase. Weshow that this evolutionary phase is crucial for the modelling of youngstellar populations by direct comparison with observed spectral energydistributions of Magellanic cloud clusters, which are characterized byrelatively high fluxes, both blueward and redward of the V band. We findthat the combination of the near-IR spectral indices C2 andH2O can be used to determine the metallicity of ~1 Gyrstellar populations. As an illustrative application, we re-analyse thespectral energy distributions of some of the high-z galaxies (2.4<~z<~ 2.9) observed with the Spitzer Space Telescope by Yan et al.Their high rest-frame near-IR fluxes is reproduced very well with themodels including TP-AGB stars for ages in the range ~0.6-1.5Gyr,suggesting formation redshifts for these objects around z~ 3-6.
|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.
|The Stellar Halo in the Large Magellanic Cloud: Mass, Luminosity, and Microlensing Predictions|
Recently obtained kinematic data have shown that the Large MagellanicCloud (LMC) possesses an old stellar halo. In order to furthercharacterize the properties of this halo, parametric King models arefitted to the surface density of RR Lyrae stars. Using data from boththe MACHO and OGLE II microlensing surveys, the model fits yield thecenter of their distribution at α=5h21.1'+/-0.8',δ=-69deg45'+/-6' (J2000.0) and acore radius of 1.42+/-0.12 kpc. As a check, the halo model is comparedwith RR Lyrae star counts in fields near the LMC's periphery previouslysurveyed with photographic plates. These data, however, require acautious interpretation. Several topics regarding the LMC stellar haloare discussed. First, the properties of the halo imply a globalmass-to-light ratio of M/LV=5.3+/-2.1 and a total mass of(1.6+/-0.6)×1010 Msolar for the LMC, in goodagreement with estimates based on the rotation curve. Second, althoughthe LMC's disk and halo are kinematically distinct, the shape of thesurface density profile of the halo is remarkably similar to that of theyoung disk. For example, the best-fit exponential scale length for theRR Lyrae stars is 1.47+/-0.08 kpc, which compares to 1.46 kpc for theLMC's blue light. In the Galaxy, the halo and disk do not resemble eachother like this. Finally, a local maximum in the LMC's microlensingoptical depth due to halo-on-disk stellar self-lensing is predicted. Forthe parameters of the stellar halo obtained, this maximum is locatednear MACHO events LMC-4 and LMC-23 and is large enough to possiblyaccount for these two events but not for all of the observedmicrolensing.
|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).
|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.
|The Optical Gravitational Lensing Experiment. Catalog of RR Lyr Stars in the Large Magellanic Cloud|
We present the catalog of RR Lyr stars discovered in a 4.5 squaredegrees area in the central parts of the Large Magellanic Cloud (LMC).Presented sample contains 7612 objects, including 5455 fundamental modepulsators (RRab), 1655 first-overtone (RRc), 272 second-overtone (RRe)and 230 double-mode RR Lyr stars (RRd). Additionally we attach alist ofseveral dozen other short-period pulsating variables. The catalog datainclude astrometry, periods, BVI photometry, amplitudes, and parametersof the Fourier decomposition of the I-band light curve of each object.We present density map of RR Lyr stars in the observed fields whichshows that the variables are strongly concentrated toward the LMCcenter. The modal values of the period distribution for RRab, RRc andRRe stars are 0.573, 0.339 and 0.276 days, respectively. Theperiod-luminosity diagrams for BVI magnitudes and for extinctioninsensitive index W_I are constructed. We provide the log P-I, log P-Vand log P-W_I relations for RRab, RRc and RRe stars. The mean observedV-band magnitudes of RR Lyr stars in the LMC are 19.36 mag and 19.31 magfor ab and c types, respectively, while the extinction free values are18.91 mag and 18.89 mag.We found a large number of RR Lyr stars pulsating in two modes closelyspaced in the power spectrum. These stars are believed to exhibitnon-radial pulsating modes. We discovered three stars whichsimultaneously reveal RR Lyr-type and eclipsing-type variability. If anyof these objects were an eclipsing binary system containing RR Lyr star,then for the first time the direct determination of the mass of RR Lyrvariable would be possible.We provide a list of six LMC star clusters which contain RR Lyr stars.The richest cluster, NGC 1835, hosts 84 RR Lyr variables. The perioddistribution of these stars suggests that NGC1835 shares features ofOosterhoff type I and type II groups.All presented data, including individual BVI observations and findingcharts are available from the OGLE Internet archive.
|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.
|The Dwarf Irregular/Wolf-Rayet Galaxy NGC 4214. I. A New Distance, Stellar Content, and Global Parameters|
We present the results of a detailed optical and near-IR study of thenearby star-forming dwarf galaxy NGC 4214. We discuss the stellarcontent, drawing particular attention to the intermediate-age and/or oldfield stars, which are used as a distance indicator. On images obtainedwith the Hubble Space Telescope Wide Field Planetary Camera 2 andNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) instrumentsin the equivalents of the V, R, I, J and H bands, the galaxy is wellresolved into stars. We achieve limiting magnitudes of F814W~27 in theWF chips and F110W~25 in the NICMOS 2 camera. The optical andnear-infrared color-magnitude diagrams confirm a core-halo galaxymorphology: an inner, high surface brightness, young population within~1.5′ (~1 kpc) from the center of the galaxy, where the stars areconcentrated in bright complexes along a barlike structure, and arelatively low surface brightness, field star population extending outto at least 8' (7 kpc). The color-magnitude diagrams of the core regionshow evidence of blue and red supergiants, main-sequence stars,asymptotic giant branch stars, and blue loop stars. We identify somecandidate carbon stars from their extreme near-IR color. The field-starpopulation is dominated by the ``red tangle,'' which contains the redgiant branch. We use the I-band luminosity function to determine thedistance based on the tip of the red giant branch method: 2.7+/-0.3 Mpc.This is much closer than the values usually assumed in the literature,and we provide revised distance-dependent parameters such as physicalsize, luminosity, H I mass, and star formation rate. From the mean colorof the red giant branch in V and I, we estimate the mean metal abundanceof this population to be [Fe/H]~=-1.7 dex, with a large internalabundance spread characterized by σint([Fe/H])~1 dex.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
|An Upper Limit to the Age of the Galactic Bar|
Using data from the Two Micron All Sky Survey, we identify a populationof infrared carbon stars with J-KS>=2 in the Milky Way.These stars are shown to trace the stellar bar previously identified inIR and optical surveys. The properties of C stars strongly suggest thatthey are of intermediate age. We conclude that the bar is likely to haveformed more recently than 3 Gyr ago and must be younger than 6 Gyr.Implications and further tests of this conclusion are briefly discussed.
|Evolution of the dust mass loss with luminosity along the giant branch of the globular cluster 47 Tucanae|
The present paper investigates the properties of the dust mass loss instars populating the giant branch of the globular cluster 47 Tuc, bycombining ISOCAM and DENIS data. Raster maps of 5 fields covering areasranging from 4x4 to 15x15 arcmin2 at different distances fromthe center of the cluster have been obtained with ISOCAM at 11.5 mu m(LW10 filter). The covered fields include most of the red variablesknown in this cluster. A detection threshold of about 0.2 mJy isachieved, allowing us to detect giant stars at 11.5 mu m all the waydown to the horizontal branch. No dust-enshrouded asymptotic giantbranch stars have been found in the observed fields, contrary to thesituation encountered in LMC/SMC globular clusters with larger turnoffmasses. The color index - (based on the ISO 11.5 mu m flux and onthe DENIS Ks magnitude) is used as a diagnostic of dustemission (and hence dust mass loss). Its evolution with luminosity alongthe giant branch reveals that dust mass loss is only present in V3 (theonly cluster Mira variable observed in the present study) and in V18, astar presenting intermittent variability. This conclusion confirms theimportance of stellar pulsations in the dust formation and ensuing massloss. Table 3 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/372/85
|Kinematics, ages and metallicities of star clusters in NGC 1316: a 3-Gyr-old merger remnant|
We report on multi-object spectroscopy in the red spectral region of 37candidate star clusters in an ~8×8arcmin2 field centredon the giant early-type radio galaxy NGC 1316 (Fornax A), the brightestgalaxy in the Fornax cluster. Out of this sample, 24 targets are foundto be genuine star clusters associated with NGC 1316, and 13 targets areGalactic foreground stars. For the star cluster sample, we measure amean heliocentric velocity vhel=1698+/-46kms-1 anda velocity dispersion σ=227+/-33kms-1 within agalactocentric radius of 24kpc. Partly responsible for the velocitydispersion is a significant rotation in the star cluster system, with amean velocity of ~175+/-70kms-1 along a position angle of~6°+/-18°. Using the projected mass estimator and assumingisotropic orbits, the estimated total mass is(6.6+/-1.7)×1011Msolar within a radius of24kpc. The mass is uncertain by about a factor of 2, depending on theorbital assumptions. The implied M/LB ratio is in the range3-6. Four star clusters in our sample are exceptionally luminous(MV<-12.3). This means that (1) at least this manyclusters in NGC 1316 are up to an order of magnitude more luminous thanthe most luminous star cluster in our Galaxy or M31, and that (2) theS/N ratio of their spectra allows us to measure line strengths with goodaccuracy. By comparing the measured colours and equivalent widths ofHα and the Caii triplet (λλ 8498, 8542, 8662Å)absorption lines for those bright star clusters in our sample with thoseof single-burst population models (the Bruzual & Charlot models), wefind that they are coeval with an age of 3.0+/-0.5Gyr. Theirmetallicities are found to be solar to within +/-0.15dex. We discuss theproperties of the main body of NGC 1316, and conclude that they areconsistent with having hosted a major merger 3Gyr ago as well. Thepresence of intermediate-age globular clusters in NGC 1316 shows onceagain that globular clusters with near-solar metallicity do form duringgalactic mergers, and, moreover, that they can survive disruptionprocesses taking place during the merger (e.g., dynamical friction,tidal disruption), as well as evaporation. In this respect, NGC 1316provides a hitherto `missing' evolutionary link between young mergerremnants of age ~0.5Gyr such as NGC 3597, 3921 and 7252 on one side andolder giant ellipticals featuring bimodal colour distributions on theother side.
|Structure of the Large Magellanic Cloud from 2MASS|
We derive structural parameters and evidence for extended tidal debrisfrom star count and preliminary standard candle analyses of the LargeMagellanic Cloud based on Two-Micron All-Sky Survey (2MASS) data. Thefull-sky coverage and low extinction in Ks present an idealsample for structural analysis of the LMC. The star count surfacedensities and deprojected inclination for both young and olderpopulations are generally consistent with previous work. We fit theprojected density with an exponential disk or spherical power-law modelwith an optional two-dimensional elliptical bar. The derived disk scalelength is R=1.42+/-0.01 kpc for models without the bar and R=2.15+/-0.01kpc for models with the bar included. The median value for the bar axisratio is q=3.4 for intermediate and old LMC populations and q=4.1 foryoung populations. The radial profile shows evidence for disk truncationnear 3 kpc. We use the full areal coverage and large ``LMCdiameter''/``Galactocentric'' distance ratio to infer the diskinclination based on perspective. The values for inclination derivedfrom different populations range from 22° to 29°, with thevariance weighted average i=24.0d+/-0.3d. The inclinations derived frommodels including the bar result in a larger estimate, i=38.2d+/-0.4d. Astandard candle analysis based on a sample of carbon long-periodvariables (LPV) in a narrow color range, 1.6
|NIR Census of the Stellar Content of Nearby Blue Compact Dwarf Galaxies with HST|
We resolved the brightest stars in 5 nearby blue compact dwarf galaxieswith NICMOS aboard HST in J and H. In all galaxies, RGB stars weredetected. This finding allows an improved estimate of the distances ofthese galaxies and sets a lower limit to the age of the old stellarcomponent of 1 Gyr. Further, we detected a pronounced AGB population,including - in all but one - also TP-AGB stars. Our data do not supportthe hypothesis that very low metallicity BCD galaxies may have formedtheir first stars only recently.
|The Metallicity Distribution Function of Red Giants in the Large Magellanic Cloud|
We report new metallicity determinations for 39 red giants in a 220arcmin2 region, 1.8d southwest of the bar of the LargeMagellanic Cloud. These abundance measurements are based on spectroscopyof the Ca II infrared triplet. We have carefully considered the effectsof abundance ratios, the physics of Ca II line formation, the variationof red clump magnitude, and the contamination by foreground stars in ourabundance analyses. The metallicity distribution function (MDF) shows astrong peak at [Fe/H]=-0.57+/-0.04 a tail to abundances at least as lowas [Fe/H]~-1.6 brings the average abundance down to [Fe/H]=-0.64+/-0.02.Half the red giants in our field fall within the range-0.83<=[Fe/H]<=-0.41. The MDF appears to be truncated at[Fe/H]~-0.25 the exact value of the maximum abundance is subject to ~0.1dex uncertainty in the calibration of the Ca II IR triplet for young,metal-rich stars. We find a striking contrast in the shape of the MDFbelow [Fe/H]<=-1 between our inner disk field and the distant outerfield studied by Olszewski: red giants deficient by more than a factorof 10 in heavy elements relative to the Sun are extremely scarce in theinner disk of the LMC. Our field star sample does not reproduce the fullMDF of the LMC star clusters but seems similar to that of theintermediate-age (1-3 Gyr) clusters. We have also obtained abundanceestimates using Strömgren photometry for ~103 red giantsin the same field. Photometry is the only practical way to measureabundances for the large numbers of stars necessary to liftage-metallicity degeneracy from our high-precision color-magnitudediagrams. The Strömgren measurements, which are sensitive to acombination of cyanogen and iron lines, correlate well with the Ca IImeasurements, but a metallicity-dependent offset is found. The offsetmay be due either to variations in the elemental abundance ratios due togalactic chemical evolution or to a metal-dependent mixing mechanism inRGB stars. An empirical relation between photometric and spectroscopicabundance estimates is derived. This will allow photometric abundancemeasurements to be placed on a consistent metallicity scale withspectroscopic metallicities, for very large numbers of stars. Based onobservations obtained at Cerro Tololo Inter-American Observatory, adivision of the National Optical Astronomy Observatories, which areoperated by the Association of Universities for Research in Astronomy,Inc. under cooperative agreement with the National Science Foundation.
|Magellanic Cloud Periphery Carbon Stars. IV. The SMC|
The kinematics of 150 carbon stars observed at moderate dispersion onthe periphery of the Small Magellanic Cloud are compared with themotions of neutral hydrogen and early-type stars in the intercloudregion. The distribution of radial velocities implies a configuration ofthese stars as a sheet inclined at 73°+/-4° to the plane of thesky. The near side, to the south, is dominated by a stellar component;to the north, the far side contains fewer carbon stars and is dominatedby the neutral gas. The upper velocity envelope of the stars is closelythe same as that of the gas. This configuration is shown to beconsistent with the known extension of the SMC along the line of sightand is attributed to a tidally induced disruption of the SMC thatoriginated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago.The dearth of gas on the near side of the sheet is attributed toablation processes akin to those inferred in 1996 by Weiner &Williams to collisional excitation of the leading edges of MagellanicStream clouds. Comparison with the 1989 kinematic data of Hardy,Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the1986 and 1988 data of Mathewson et al. leaves little doubt that forcesother than gravity play a role in the dynamics of the H I.
|The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud|
We present a 9 million star color-magnitude diagram (9M CMD) of theLarge Magellanic Cloud (LMC) bar. The 9M CMD reveals a complexsuperposition of different-age and -metallicity stellar populations,with important stellar evolutionary phases occurring over 3 orders ofmagnitude in number density. First, we count the nonvariable red andblue supergiants and the associated Cepheid variables and measure thestellar effective temperatures defining the Cepheid instability strip.Lifetime predictions of stellar evolution theory are tested, withimplications for the origin of low-luminosity Cepheids. The highlyevolved asymptotic giant branch (AGB) stars in the 9M CMD have a bimodaldistribution in brightness, which we interpret as discrete oldpopulations (>~1 Gyr). The faint AGB sequence may be metal-poor andvery old. Comparing the mean properties of giant branch andhorizontal-branch (HB) stars in the 9M CMD with those of clusters, weidentify NGC 411 and M3 as templates for the admixture of old stellarpopulations in the bar. However, there are several indications that theold and metal-poor field population has a red HB morphology: the RRLyrae variables lie preferentially on the red edge of the instabilitystrip, the AGB bump is very red, and the ratio of AGB bump stars to RRLyrae variables is quite large. If the HB second parameter is age, theold and metal-poor field population in the bar likely formed after theoldest LMC clusters. Lifetime predictions of stellar evolution theorylead us to associate a significant fraction of the ~1 million red HBclump giants in the 9M CMD with the same old and metal-poor populationproducing the RR Lyrae stars and the AGB bump. In this case, comparedwith the age-dependent luminosity predictions of stellar evolutiontheory, the red HB clump is too bright relative to the RR Lyrae starsand AGB bump. Last, we show that the surface density profile of RR Lyraevariables is fitted by an exponential, favoring a disklike rather than aspheroidal distribution. We conclude that the age of the LMC disk isprobably similar to the age of the Galactic disk.
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