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Surface Brightness Profiles for a Sample of LMC, SMC, and Fornax Galaxy Globular Clusters
We use Hubble Space Telescope archival images to measure central surfacebrightness profiles of globular clusters around satellite galaxies ofthe Milky Way. We report results for 21 clusters around the LMC, fivearound the SMC, and four around the Fornax dwarf galaxy. The profileswere obtained using a recently developed technique based on measuringintegrated light, which is tested on an extensive simulated data set.Our results show that for 70% of the sample, the central photometricpoints of our profiles are brighter than previous measurements usingstar counts with deviations as large as 2 mag arcsec-2. About40% of the objects have central profiles deviating from a flat centralcore, with central logarithmic slopes continuously distributed between-0.2 and -1.2. These results are compared with those found for a sampleof Galactic clusters using the same method. We confirm the knowncorrelation in which younger clusters tend to have smaller core radii,and we find that they also have brighter central surface brightnessvalues. This seems to indicate that globular clusters might be bornrelatively concentrated, and that a profile with an extended flat coremight not be the ideal choice for initial profiles in theoreticalmodels.
| Be stars and binaries in the field of the SMC open cluster NGC 330 with VLT-FLAMES
Aims.Observations of hot stars belonging to the young cluster SMC-NGC330 and its surrounding region were obtained with the VLT-GIRAFFEfacilities in MEDUSA mode. We investigated the B and Be star propertiesand proportions in this environment of low metallicity. We also searchedfor rapid variability in Be stars using photometric databases. Methods:Using spectroscopic measurements, we characterized the emission andproperties of Be stars. By cross-correlation with photometric databasessuch as MACHO and OGLE, we searched for binaries in our sample of hotstars, as well as for short-term variability in Be stars. Results: Wereport on the global characteristics of the Be star sample (131objects). We find that the proportion of early Be stars with a largeequivalent width of the Hα emission line is higher in the SMC thanin the LMC and MW. We find a slight increase in the proportion of Bestars compared to B-type stars with decreasing metallicity. We alsodiscovered spectroscopic and photometric binaries, and for the latter wegive their orbital period. We identify 13 Be stars with short-termvariability. We determine their period(s) and find that 9 Be stars aremultiperiodic.Figures 1, 5-20, and Tables 1, 6, 7 are only available in electronicform at http://www.aanda.org
| The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90
We present the results of our investigation of the intermediate-age starcluster BS 90, located in the vicinity of the H II region N66 in theSMC, observed with HST ACS. The high-resolution data provide a uniqueopportunity for a very detailed photometric study performed on one ofthe rare intermediate-age rich SMC clusters. The complete set ofobservations is centered on the association NGC 346 and contains almost100,000 stars down to V~=28 mag. In this study we focus on the northernpart of the region, which covers almost the whole stellar content of BS90. We construct its stellar surface density profile and derivestructural parameters. Isochrone fits on the CMD of the cluster resultsin an age of about 4.5 Gyr. The luminosity function is constructed andthe present-day mass function of BS 90 has been obtained using themass-luminosity relation, derived from the isochrone models. We found aslope between -1.30 and -0.95, comparable to or somewhat shallower thana typical Salpeter IMF. Examination of the radial dependence of the massfunction shows a steeper slope at larger radial distances, indicatingmass segregation in the cluster. The derived half-mass relaxation timeof 0.95 Gyr suggests that the cluster is mass segregated due to itsdynamical evolution. From the isochrone model fits we derive ametallicity for BS 90 of [Fe/H]=-0.72, which adds an important point tothe age-metallicity relation of the SMC. We discuss our findings on thisrelation in comparison to other SMC clusters.Research supported by the Deutsche Forschungsgemeinschaft (GermanResearch Foundation).
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| Effects of metallicity, star-formation conditions, and evolution in B and Be stars. II. Small Magellanic Cloud, field of NGC 330
Aims.We search for the effects of metallicity on B and Be stars in theSmall and Large Magellanic Clouds (SMC and LMC) and in the Milky Way(MW), by extending our previous analysis of B and Be star populations inthe LMC to the SMC. The rotational velocities of massive stars and theevolutionary status of Be stars are examined with respect to theirenvironments. Methods: Spectroscopic observations of hot starsbelonging to the young cluster SMC-NGC 330 and its surrounding regionwere obtained with the VLT-GIRAFFE facilities in MEDUSA mode. Wedetermined fundamental parameters for B and Be stars with the GIRFITcode, taking the effect of fast rotation and the age of observedclusters into account. We compared the mean V sin i obtained by spectraltype- and mass-selection for field and cluster B and Be stars in the SMCwith the one in the LMC and MW. Results: We find that (i) B and Bestars rotate faster in the SMC than in the LMC and in the LMC than inthe MW; (ii) at a given metallicity, Be stars begin their main sequencelife with a higher initial rotational velocity than B stars.Consequently, only a fraction of the B stars that reach the ZAMS with asufficiently high initial rotational velocity can become Be stars; (iii)the distributions of initial rotational velocities at the ZAMS for Bestars in the SMC, LMC, and MW are mass- and metallicity-dependent; (iv)the angular velocities of B and Be stars are higher in the SMC than inthe LMC and MW; (v) in the SMC and LMC, massive Be stars appear in thesecond part of the main sequence, in contrast to massive Be stars in theMW.Tables 1-6, 8 and 11 are only available in electronic form athttp://www.aanda.org
| 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
| Past and Present Star Formation in the SMC: NGC 346 and its Neighborhood
In the quest to understand how star formation occurs and propagates inthe low-metallicity environment of the Small Magellanic Cloud (SMC), weacquired deep F555W (~V) and F814W (~I) Hubble Space Telescope ACSimages of the young and massive star-forming region NGC 346. Theseimages and their photometric analysis provide us with a snapshot of thestar formation history of the region. We find evidence for starformation extending from ~10 Gyr in the past until ~150 Myr in the fieldof the SMC. The youngest stellar population (~3+/-1 Myr) is associatedwith the NGC 346 cluster. It includes a rich component of low-masspre-main-sequence stars mainly concentrated in a number of subclustersspatially colocated with CO clumps previously detected by Rubio andcoworkers. Within our analysis uncertainties, these subclusters appearcoeval with each other. The most massive stars appear concentrated inthe central subclusters, indicating possible mass segregation. A numberof embedded clusters are also observed. This finding, combined with theoverall wealth of dust and gas, could imply that star formation is stillactive. An intermediate-age star cluster, BS 90, formed ~4.3+/-0.1 Gyrago, is also present in the region. Thus, this region of the SMC hassupported star formation with varying levels of intensity over much ofthe cosmic time.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
| The VLT-FLAMES survey of massive stars: mass loss and rotation of early-type stars in the SMC
We have studied the optical spectra of a sample of 31 O-and early B-typestars in the Small Magellanic Cloud, 21 of which are associated with theyoung massive cluster NGC 346. Stellar parameters are determined usingan automated fitting method (Mokiem et al. 2005, A&A, 441, 711),which combines the stellar atmosphere code FASTWIND (Puls et al. 2005,A&A, 435, 669) with the genetic algorithm based optimisation routinePIKAIA (Charbonneau 1995, ApJS, 101, 309). Comparison with predictionsof stellar evolution that account for stellar rotation does not resultin a unique age, though most stars are best represented by an age of 1-3Myr. The automated method allows for a detailed determination of theprojected rotational velocities. The present day v_r sin i distributionof the 21 dwarf stars in our sample is consistent with an underlyingrotational velocity (v_r) distribution that can be characterised by amean velocity of about 160 - 190 km s-1 and an effective halfwidth of 100 - 150 km s-1. The vr distributionmust include a small percentage of slowly rotating stars. If predictionsof the time evolution of the equatorial velocity for massive starswithin the environment of the SMC are correct (Maeder & Meynet 2001,A&A, 373, 555), the young age of the cluster implies that thisunderlying distribution is representative for the initial rotationalvelocity distribution. The location in the Hertzsprung-Russell diagramof the stars showing helium enrichment is in qualitative agreement withevolutionary tracks accounting for rotation, but not for those ignoringv_r. The mass loss rates of the SMC objects having luminosities of logL*/Lȯ ≳ 5.4 are in excellent agreementwith predictions by Vink et al. (2001, A&A, 369, 574). However, forlower luminosity stars the winds are too weak to determine dot{M}accurately from the optical spectrum. Three targets were classifiedas Vzstars, two of which are located close to the theoretical zero-age mainsequence. Three lower luminosity targets that were not classified as Vzstars are also found to lie near the ZAMS. We argue that this is relatedto a temperature effect inhibiting cooler from displaying the spectralfeatures required for the Vz luminosity class.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| 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.
| Age distribution of young clusters and field stars in the Small Magellanic Cloud
Aims.In this paper we discuss the cluster and field star formation inthe central part of the Small Magellanic Cloud. The main goal is tostudy the correlation between young objects and their interstellarenvironment. Methods: . The ages of about 164 associations and 311clusters younger than 1 Gyr are determined using isochrone fitting. Thespatial distribution of the clusters is compared with the HI maps, withthe HI velocity dispersion field, with the location of the CO clouds andwith the distribution of young field stars. Results: .The clusterage distribution supports the idea that clusters formed in the last 1Gyr of the SMC history in a roughly continuous way with periods ofenhancements. The two super-shells 37A and 304A detected in the HIdistribution are clearly visible in the age distribution of theclusters: an enhancement in the cluster formation rate has taken placefrom the epoch of the shell formation. A tight correlation between youngclusters and the HI intensity is found. The degree of correlation isdecreasing with the age of the clusters. Clusters older than 300 Myr arelocated away from the HI peaks. Clusters and associations younger than10 Myr are related to the CO clouds in the SW region of the SMC disk. Apositive correlation between the location of the young clusters and thevelocity dispersion field of the atomic gas is derived only for theshell 304A, suggesting that the cloud-cloud collision is probably notthe most important mechanism of cluster formation. Evidence ofgravitational triggered episode due to the most recent close interactionbetween SMC and LMC is found both in the cluster andfield star distribution.
| Near-infrared spectroscopy of a young super-star cluster in NGC 6946: chemical abundances and abundance patterns*
Using the NIRSPEC spectrograph at Keck II, we have obtained H- andK-band echelle spectra for a young (~10-15 Myr), luminous(MV~-13.2) super-star cluster in the nearby spiral galaxy NGC6946. From spectral synthesis and equivalent width measurements weobtain for the first time accurate abundances and abundance patterns inan extragalactic super-star cluster. We find [Fe/H]=-0.45 +/- 0.08 dex,an average α-enhancement of ~+0.22 +/- 0.1 dex, and a relativelylow 12 C/13 C ~ 8 +/- 2 isotopic ratio. We alsomeasure a velocity dispersion of ~9.1 km s-1, in agreementwith previous estimates. We conclude that integrated high-dispersionspectroscopy of massive star clusters is a promising alternative toother methods for abundance analysis in extragalactic young stellarpopulations.
| Bump Cepheids in the Magellanic Clouds: Metallicities, the Distances to the LMC and SMC, and the Pulsation-Evolution Mass Discrepancy
We use nonlinear pulsation models to reproduce the observed light andcolor curves for two samples of bump Cepheid variables, 19 from theLarge Magellanic Cloud and 9 from the Small Magellanic Cloud. Thisanalysis determines the fundamental parameters mass, luminosity,effective temperature, metallicity, distance, and reddening for thesample of stars. The use of the light-curve shape alone to determinemetallicity is a new modeling technique introduced here. Themetallicity, distance, and reddening distributions for the two samplesare in agreement with those of similar stellar populations in theliterature. The distance modulus of the Large Magellanic Cloud isdetermined to be 18.54+/-0.018, and the distance modulus of the SmallMagellanic Cloud is determined to be 18.93+/-0.024. The mean Cepheidmetallicities are Z=0.0091+/-0.0007 and 0.0050+/-0.0005 for the LMC andSMC, respectively. The masses derived from pulsation analysis aresignificantly less than those predicted by stellar evolutionary modelswith no or mild convective core overshoot. We show that this discrepancycannot be accounted for by uncertainties in our input opacities or inmass-loss physics. We interpret the observed mass discrepancy in termsof enhanced internal mixing in the vicinity of the convective coreduring the main-sequence lifetime and find that the overshoot parameterΛc rises from 0.688+/-0.009Hp at the meanLMC metallicity to 0.746+/-0.009Hp in the SMC.
| The VLT-FLAMES survey of massive stars.
Not Available
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| Dust-enshrouded giants in clusters in the Magellanic Clouds
We present the results of an investigation of post-Main Sequence massloss from stars in clusters in the Magellanic Clouds, based around animaging survey in the L'-band (3.8 μm) performed with the VLT at ESO.The data are complemented with JHKs (ESO and 2MASS) andmid-IR photometry (TIMMI2 at ESO, ISOCAM on-board ISO, and data fromIRAS and MSX). The goal is to determine the influence of initialmetallicity and initial mass on the mass loss and evolution during thelatest stages of stellar evolution. Dust-enshrouded giants areidentified by their reddened near-IR colours and thermal-IR dust excessemission. Most of these objects are Asymptotic Giant Branch (AGB) carbonstars in intermediate-age clusters, with progenitor masses between 1.3and ~5 M_ȯ. Red supergiants with circumstellar dust envelopes arefound in young clusters, and have progenitor masses between 13 and 20M_ȯ. Post-AGB objects (e.g., Planetary Nebulae) and massive starswith detached envelopes and/or hot central stars are found in severalclusters. We model the spectral energy distributions of the cluster IRobjects, in order to estimate their bolometric luminosities andmass-loss rates. The IR objects are the most luminous cluster objects,and have luminosities as expected for their initial mass andmetallicity. They experience mass-loss rates in the range from a few10-6 up to 10-4 M_ȯ yr-1 (ormore), with most of the spread being due to evolutionary effects andonly a weak dependence on progenitor mass and/or initial metallicity.About half of the mass lost by 1.3-3 M_ȯ stars is shed during thesuperwind phase, which lasts of order 105 yr. Objects withdetached shells are found to have experienced the highest mass-lossrates, and are therefore interpreted as post-superwind objects. We alsopropose a simple method to measure the cluster mass from L'-band images.
| Integrated spectral analysis of 18 concentrated star clusters in the Small Magellanic Cloud
We present in this study flux-calibrated integrated spectra in the range(3600-6800) Å for 18 concentrated SMC clusters. Cluster reddeningvalues were estimated by interpolation between the extinction maps ofBurstein & Heiles (1982, AJ, 87, 1165) and Schlegel et al. (1998,ApJ, 500, 525). The cluster parameters were derived from the templatematching procedure by comparing the line strengths and continuumdistribution of the cluster spectra with those of template clusterspectra with known parameters and from the equivalent width (EW) method.In this case, new calibrations were used together with diagnosticdiagrams involving the sum of EWs of selected spectral lines. A verygood agreement between ages derived from both methods was found. Thefinal cluster ages obtained from the weighted average of values takenfrom the literature and the present measured ones range from 15 Mr (e.g.L 51) to 7 Gyr (K 3). Metal abundances have been derived for only 5clusters from the present sample, while metallicity values directlyaveraged from published values for other 4 clusters have been adopted.Combining the present cluster sample with 19 additional SMC clusterswhose ages and metal abundances were put onto a homogeneous scale, weanalyse the age and metallicity distributions in order to explore theSMC star formation history and its spatial extent. By considering thedistances of the clusters from the SMC centre instead of theirprojections onto the right ascension and declination axes, the presentage-position relation suggests that the SMC inner disk could have beenrelated to a cluster formation episode which reached the peak ~2.5 Gyrago. Evidence for an age gradient in the inner SMC disk is alsopresented.
| A Be star with a low nitrogen abundance in the SMC cluster NGC 330
High-resolution UVES/VLT spectra of B 12, an extreme pole-on Be star inthe SMC cluster NGC 330, have been analysed using non-LTE modelatmospheres to obtain its chemical composition relative to the SMCstandard star AV 304. We find a general underabundance of metals whichcan be understood in terms of an extra contribution to the stellarcontinuum due to emission from a disk which we estimate to be at the~25% level. When this is corrected for, the nitrogen abundance for B 12shows no evidence of enhancement by rotational mixing as has been foundin other non-Be B-type stars in NGC 330, and is inconsistent withevolutionary models which include the effects of rotational mixing. Asecond Be star, NGC 330-B 17, is also shown to have no detectablenitrogen lines. Possible explanations for the lack of rotational mixingin these rapidly rotating stars are discussed, one promising solutionbeing the possibility that magnetic fields might inhibit rotationalmixing.
| The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611
We introduce a new survey of massive stars in the Galaxy and theMagellanic Clouds using the Fibre Large Array Multi-Element Spectrograph(FLAMES) instrument at the Very Large Telescope (VLT). Here we presentobservations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph(R ≃ 25 000), in fields centered on the open clusters NGC 3293,NGC 4755 and NGC 6611. These data are supplemented by a further 50targets observed with the Fibre-Fed Extended Range Optical Spectrograph(FEROS, R = 48 000). Following a description of our scientificmotivations and target selection criteria, the data reduction methodsare described; of critical importance the FLAMES reduction pipeline isfound to yield spectra that are in excellent agreement with lessautomated methods. Spectral classifications and radial velocitymeasurements are presented for each star, with particular attention paidto morphological peculiarities and evidence of binarity. Theseobservations represent a significant increase in the known spectralcontent of NGC 3293 and NGC 4755, and will serve as standards againstwhich our subsequent FLAMES observations in the Magellanic Clouds willbe compared.
| The Star Clusters of the Small Magellanic Cloud: Age Distribution
We present age measurements for 195 star clusters in the SmallMagellanic Cloud based on comparison of integrated colors measured fromthe Magellanic Clouds Photometric Survey with models of simple stellarpopulations. We find that the modeled nonuniform changes of clustercolors with age can lead to spurious age peaks in the cluster agedistribution; that the observed numbers of clusters with age t declinessmoothly as t-2.1 that for an assumed initial cluster massfunction scaling as M-2, the dependence of the clusterdisruption time on mass is proportional to M0.48; thatdespite the apparent abundance of young clusters, the dominant epoch ofcluster formation was the initial one; and that there are significantdifferences in the spatial distribution of clusters of different ages.Because of limited precision in our age measurements, we cannot addressthe question of detailed correspondence between the cluster age functionand the field star formation history. However, this sample provides aninitial guide as to which clusters to target in more detailed studies ofspecific age intervals.
| B-type supergiants in the SMC: Chemical compositions and comparison of static and unified models
High-resolution UCLES/AAT spectra are presented for nine B-typesupergiants in the SMC, chosen on the basis that they may show varyingamounts of nuclear-synthetically processed material mixed to theirsurface. These spectra have been analysed using a new grid ofapproximately 12 000 non-LTE line blanketed tlusty model atmospheres toestimate atmospheric parameters and chemical composition. The abundanceestimates for O, Mg and Si are in excellent agreement with those deducedfrom other studies, whilst the low estimate for C may reflect the use ofthe C II doublet at 4267 Å. The N estimates are approximately anorder of magnitude greater than those found in unevolved B-type stars orH II regions but are consistent with the other estimates in AB-typesupergiants. These results have been combined with results from aunified model atmosphere analysis of UVES/VLT spectra of B-typesupergiants (Trundle et al. 2004, A&A, 417, 217) to discuss theevolutionary status of these objects. For two stars that are in commonwith those discussed by Trundle et al., we have undertaken a carefulcomparison in order to try to understand the relative importance of thedifferent uncertainties present in such analyses, includingobservational errors and the use of static or unified models. We findthat even for these relatively luminous supergiants, tlusty models yieldatmospheric parameters and chemical compositions similar to thosededuced from the unified code fastwind.
| Understanding B-type supergiants in the low metallicity environment of the SMC II
Despite a resurgence of effort over the last decade in the area ofmassive stars there is still ambiguity over their evolutionary path,contamination of their surface abundances and the behaviour of theirstellar winds. Here 10 SMC B-type supergiants are analysed applying aunified model atmosphere code fastwind to intermediate resolutionspectra from the ESO Multi Mode Instrument (emmi) on the NTT telescope.Combined with the 8 targets analysed in Paper I (Trundle et al. 2004),this work provides observational results on the properties of the windsand chemical compositions of B-type supergiants in the SMC. This paperemphasizes and substantiates the implications for stellar evolution fromPaper I; that current theoretical models need to produce larger degreesof surface nitrogen enhancements at lower rotational velocities. Inaddition a significant discrepancy between theoretical and observedmass-loss rates is discussed which will have important implications forthe rotational velocities obtained from stellar evolution calculations.Furthermore, an initial calibration of the wind-momentum luminosityrelationship for B-type supergiants in a low metallicity environment (Z= 0.004) is presented.
| The Initial Mass Function toward the Low-Mass End in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations
We present V- and I-equivalent Hubble Space Telescope (HST) WFPC2photometry of two areas in the Large Magellanic Cloud: the southern partof the stellar association LH 52, located on the western edge of thesupershell LMC 4, and a field between two associations, which is locatedon the southwestern edge of the shell and accounts for the generalbackground field of the galaxy. The HST WFPC2 observations reachmagnitudes as faint as V=25 mag, much deeper than have been observedearlier in stellar associations in the LMC. We determine the massfunction (MF) for main-sequence stars in the areas. Its slope in bothareas is steeper for stars with masses M<~2 Msolar(-4<~Γ<~-6) than for stars of M>~2 Msolar(-1<~Γ<~-2). Thus, as far as the field of the LMC isconcerned, the MF does not have a uniform slope throughout its observedmass range. The MF of the general field of the LMC was found previouslyto be steeper than the MF of a stellar association for massive starswith M>~5 Msolar. We conclude that this seems to also bethe case toward lower masses down to M~1 Msolar. Our dataallow us to construct the field-subtracted, incompleteness-corrected,main-sequence MF of the southwestern part of the young stellarassociation LH 52, which accounts for the initial mass function (IMF) ofthe system. Its mean slope is found to be comparable to, but moreshallow than, a typical Salpeter IMF (Γ~=-1.12+/-0.24) for massesdown to ~1 Msolar. We found indications that the IMF of theassociation probably is ``top heavy,'' owing to the large number ofintermediate-mass stars in the field of the system, while the generalLMC field is found to be responsible for the low-mass population, withM<~2 Msolar, observed in both fields. This findingsuggests that the local conditions seem to favor the formation of highermass stars in associations, and not in the background field. No evidencefor flattening of the IMF toward the low-mass regime or for a lower masscutoff in the IMF was detected in our data.
| A Photometric Method to Search for Be Stars in Open Clusters
We describe a technique to identify Be stars in open clusters usingStrömgren b, y, and narrowband Hα photometry. We firstidentify the B-type stars of the cluster using a theoretical isochronefit to the (b-y, y) color-magnitude diagram. The strongest Be stars areeasily identified in a (b-y, y-Hα) color-color diagram, but thosewith weaker Hα emission (classified as possible Be stardetections) may be confused with evolved or foreground stars. Here wepresent such photometry plus Hα spectroscopy of members of thecluster NGC 3766 to demonstrate the accuracy of our technique.Statistical results on the relative numbers of Be and B-type stars inadditional clusters will be presented in a future paper.
| Mass Segregation and the Initial Mass Function of Super Star Cluster M82-F
We investigate the initial mass function and mass segregation in superstar cluster M82-F with high-resolution Keck NIRSPEC echellespectroscopy. Cross-correlation with template supergiant spectraprovides the velocity dispersion of the cluster, enabling measurement ofthe kinematic (virial) mass of the cluster when combined with sizes fromNICMOS and Advanced Camera for Surveys (ACS) images. We find a mass of6.6+/-0.9×105Msolar based on near-IR lightand 7.0+/-1.2×105Msolar based on opticallight. Using PSF-fitting photometry, we derive the cluster'slight-to-mass (L/M) ratio in both near-IR and optical light and compareto population-synthesis models. The ratios are inconsistent with anormal stellar initial mass function for the adopted age of 40-60 Myr,suggesting a deficiency of low-mass stars within the volume sampled.King model light profile fits to new Hubble Space Telescope ACS imagesof M82-F, in combination with fits to archival near-IR images, indicatemass segregation in the cluster. As a result, the virial mass representsa lower limit on the mass of the cluster.Based on observations made at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
| 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.
| The Ultraviolet and Optical Spectra of Luminous B-Type Stars in the Small Magellanic Cloud
We present ultraviolet spectra from the Space Telescope ImagingSpectrograph (STIS) of 12 early B-type stars in the Small MagellanicCloud (SMC), composed of nine supergiants and three giants. Amorphological comparison with Galactic analogs is made using archivaldata from the International Ultraviolet Explorer (IUE). In general, theintensity of the P Cygni emission in the UV resonance lines is greaterand is seen to later spectral types in the Galactic spectra than intheir metal-deficient SMC counterparts. We attribute these effects asmost likely arising from weaker stellar winds in the SMC targets, aspredicted by radiatively driven wind theory. We also include unpublishedSTIS observations of two late O-type stars in the SMC. In combinationwith published O-type STIS data, we now have an extensive ultravioletspectral library of metal-deficient stars to use in the study ofunresolved starbursts and high-redshift star-forming galaxies. In thiscontext, we present empirical measurements for the B-type spectra of thenew ``1978 index'' suggested by Rix et al. as a probe of metallicity insuch systems.
| A 2dF survey of the Small Magellanic Cloud
We present a catalogue of new spectral types for hot, luminous stars inthe Small Magellanic Cloud (SMC). The catalogue contains 4161 objects,giving an order-of-magnitude increase in the number of SMC stars withpublished spectroscopic classifications. The targets are primarily B-and A-type stars (2862 and 853 objects respectively), with oneWolf-Rayet, 139 O-type and 306 FG stars, sampling the main sequence to~mid-B. The selection and classification criteria are described, andobjects of particular interest are discussed, including UV-selectedtargets from the Ultraviolet Imaging Telescope (UIT) experiment, Be andB[e] stars, `anomalous A supergiants' and composite-spectrum systems. Weexamine the incidence of Balmer-line emission, and the relationshipbetween Hγ equivalent width and absolute magnitude for BA stars.
| 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.
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