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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.
| The Magnetic Field Structure of the LMC 2 Supershell: NGC 2100
We present UBVRI imaging polarimetry of NGC 2100 and its surroundingenvironment, which comprise a part of the LMC 2 supershell. Themorphology of the observed position angle distribution provides a tracerof the projected magnetic field in this environment. Our polarizationmaps detail regions exhibiting similarly aligned polarization positionangles, as well as more complex position angle patterns. We observeregions of coherent fields on spatial scales of 42×24 to104×83 pc, and infer projected field strengths of ~14-30 μG. Wepropose that the superposition of global outflows from the LMC 2environment, as well as outflows created within NGC 2100, produce theunique field geometry in the region.
| A hypervelocity star from the Large Magellanic Cloud
We study the acceleration of the star HE0437-5439to hypervelocity anddiscuss its possible origin in the Large Magellanic Cloud (LMC). Thestar has a radial velocity of 723kms-1 and is located at adistance of 61 kpc from the Sun. With a mass of about8Msolar, the traveltime from the Galactic Centre is about 100Myr, much longer than its main-sequence lifetime. Given the relativelysmall distance to the LMC (18 kpc), we consider it likely thatHE0437-5439originated in the Cloud rather than in the Galactic Centrelike the other hypervelocity stars. The minimum ejection velocityrequired to travel from the LMC to its current location within itslifetime is about 500kms-1. Such a high velocity can only beobtained in a dynamical encounter with a massive black hole. We performthree-body scattering simulations in which a stellar binary encounters amassive black hole, and find that a black hole more massive than103Msolar is necessary to explain the highvelocity of HE0437-5439. We look for possible parent clusters forHE0437-5439, and find that NGC2100 and 2004 are young enough to hoststars coeval to HE0437-5439and dense enough to produce anintermediate-mass black hole able to eject an 8-Msolar starwith hypervelocity.
| 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
| 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.
| The photometric evolution of dissolving star clusters. I. First predictions
The broad-band photometric evolution of unresolved star clusters iscalculated in a simplified way, including the preferential loss oflow-mass stars due to mass segregation. The stellar mass function of acluster evolves due to three effects: (a) the evolution of the massivestars reduces their number; (b) tidal effects before cluster-wide masssegregation reduce the mass function homogeneously, i.e. independentlyof the stellar mass; (c) after mass segregation has finished, tidaleffects preferentially remove the lowest-mass stars from the cluster.These effects result in a narrowing of the stellar mass range. Theseeffects are described quantitatively, following the results of N-bodysimulations, and are taken into account in the calculation of thephotometric history, based on the galev cluster evolution models forsolar metallicity and a Salpeter mass function. We find the followingresults: (1) During the first ~40% of the lifetime of a cluster itscolour evolution is adequately described by the standard galev models(without mass segregation) but the cluster becomes fainter due to theloss of stars by tidal effects. (2) Between ~40 and ~80% of its lifetime(independent of the total lifetime), the cluster becomes bluer due tothe loss of low-mass stars. This will result in an underestimate of theage of clusters if standard cluster evolution models are used. (3) After~80% of the total lifetime of a cluster it will rapidly become redder.This will result in an overestimate of the age of clusters if standardcluster evolution models are used. (4) Clusters with mass segregationand the preferential loss of low-mass stars evolve along almost the sametracks in colour-colour diagrams as clusters without mass segregation.Only if the total lifetime of clusters can be estimated can the coloursbe used to give reliable age estimates. (5) The changes in the colourevolution of unresolved clusters due to the preferential loss oflow-mass stars will affect the determination of the star formationhistories of galaxies if they are derived from clusters that have atotal lifetime of less than about 30 Gyr. (6) The preferential loss oflow-mass stars might explain the presence of old (~13 Gyr) clusters inNGC 4365 which are photometrically disguised as intermediate-ageclusters (2-5 Gyr), if the expected total lifetime of these clusters isbetween 16 and 32 Gyr.
| 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.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| The Variation of Integrated Star Initial Mass Functions among Galaxies
The integrated galaxial initial mass function (IGIMF) is the relevantdistribution function containing the information on the distribution ofstellar remnants, the number of supernovae, and the chemical enrichmenthistory of a galaxy. Since most stars form in embedded star clusterswith different masses, the IGIMF becomes an integral of the assumed(universal or invariant) stellar IMF over the embedded star cluster massfunction (ECMF). For a range of reasonable assumptions about the IMF andthe ECMF we find the IGIMF to be steeper (containing fewer massive starsper star) than the stellar IMF, but below a few solar masses it isinvariant and identical to the stellar IMF for all galaxies. However,the steepening sensitively depends on the form of the ECMF in thelow-mass regime. Furthermore, observations indicate a relation betweenthe star formation rate of a galaxy and the most massive young stellarcluster in it. The assumption that this cluster mass marks the upper endof a young-cluster mass function leads to a connection of the starformation rate and the slope of the IGIMF above a few solar masses. TheIGIMF varies with the star formation history of a galaxy. Notably, largevariations of the IGIMF are evident for dE, dIrr, and LSB galaxies witha small to modest stellar mass. We find that for any galaxy the numberof supernovae per star (NSNS) is suppressed relative to that expectedfor a Salpeter IMF. Dwarf galaxies have a smaller NSNS than massivegalaxies. For dwarf galaxies the NSNS varies substantially depending onthe galaxy assembly history and the assumptions made about the low-massend of the ECMF. The findings presented here may be of some consequencefor the cosmological evolution of the number of supernovae per low-massstar and the chemical enrichment of galaxies of different mass.
| 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.
| 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 Kinematic Map of the Ionized Gas of the Supergiant Shell LMC SGS 2
A complete kinematic map of much of the supergiant shell LMC SGS 2 hasbeen established by scanning Fabry-Pérot observations at Hαwavelength. We present velocity determinations for this large-diametersupergiant shell. We confirm some motions already observed by otherauthors in previous local interferometric and spectrographicobservations. However, our results give a better insight into the wholekinematics of this supergiant shell because of our complete spatialcoverage of the two-dimensional velocities. We find that no globalexpansion pattern is present in this supergiant shell. On the contrary,we find line-splitting of the velocity profile even outside thefilamentary boundaries of LMC SGS 2. Indeed, we detect faint Hαdiffuse emission as far as ~6' outside the long filaments of theboundary of the supershell, with several velocity components. We alsofind that the outermost northern filament displays two velocitycomponents (also contradicting a simple radial expansion of the shell),while the filamentary boundary to the east occasionally has a faintsecondary velocity component. Inside the supergiant shell, the diffuseemission at the center of LMC SGS 2 is the most coherent, while someinterior filaments display two velocity components. The velocity patternfound agrees more with two gas layers seen projected along the same lineof sight, with local expansions of small wind bubbles formed by stellarassociations inside to LMC SGS 2.Based on observations done at La Silla (ESO), Chile.
| Mass segregation in young Magellanic Cloud star clusters: Four clusters observed with HST
We present the results of our investigation on the phenomenon of masssegregation in young star clusters in the Magellanic Clouds. HST/WFPC2observations on NGC 1818, NGC 2004 and NGC 2100 in the Large MagellanicCloud and NGC 330 in the Small Magellanic Cloud have been used for theapplication of diagnostic tools for mass segregation: i) the radialdensity profiles of the clusters for various mass groups and ii) theirmass functions (MFs) at various radii around their centres. All fourclusters are found to be mass segregated, but each one in a differentmanner. Specifically not all the clusters in the sample show the samedependence of their density profiles on the selected magnitude range,with NGC 1818 giving evidence of a strong relation and NGC 330 showingonly a hint of the phenomenon. NGC 2004 did not show any significantsignature of mass segregation in its density profiles either. The MFsradial dependence provides clear proof of the phenomenon for NGC 1818,NGC 2100 and NGC 2004, while for NGC 330 it gives only indications. Aninvestigation of the constraints introduced by the application of bothdiagnostic tools is presented. We also discuss the problems related tothe construction of a reliable MF for a cluster and their impact on theinvestigation of the phenomenon of mass segregation. We find that theMFs of these clusters as they were constructed with two methods arecomparable to Salpeter's IMF. A discussion is given on the dynamicalstatus of the clusters and a test is applied on the equipartition amongseveral mass groups in them. Both showed that the observed masssegregation in the clusters is of primordial nature.
| Rotation of Early B-type Stars in the Large Magellanic Cloud: The Role of Evolution and Metallicity
I present measurements of the projected rotational velocities of asample of 100 early B-type main-sequence stars in the Large MagellanicCloud (LMC). This is the first extragalactic study of the distributionof stellar rotational velocities. The sample is drawn from two sources:from the vicinity of the main-sequence turnoff of young clusters (ages1-3×107yr) and from the general field. I find that thecluster population exhibits significantly more rapid rotation than thatseen in the field. I have drawn analogous Galactic cluster and fieldsamples from the literature. Comparison of these samples reveals thesame effect. I propose that the observed difference between cluster andfield populations can be explained by a scenario of evolutionaryenhancement of the surface angular momentum over the main-sequencelifetime. A comparison is made between the cluster and field populationsof the LMC and the Galaxy in order to explore the effects ofmetallicity. This shows that the stars of the LMC are more rapidrotators than their Galactic counterparts.
| OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.
| Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud
We have compiled a pseudo-snapshot data set of two-colour observationsfrom the Hubble Space Telescope archive for a sample of 53 rich LMCclusters with ages of 106-1010 yr. We presentsurface brightness profiles for the entire sample, and derive structuralparameters for each cluster, including core radii, and luminosity andmass estimates. Because we expect the results presented here to form thebasis for several further projects, we describe in detail the datareduction and surface brightness profile construction processes, andcompare our results with those of previous ground-based studies. Thesurface brightness profiles show a large amount of detail, includingirregularities in the profiles of young clusters (such as bumps, dipsand sharp shoulders), and evidence for both double clusters andpost-core-collapse (PCC) clusters. In particular, we find power-lawprofiles in the inner regions of several candidate PCC clusters, withslopes of approximately -0.7, but showing considerable variation. Weestimate that 20 +/- 7 per cent of the old cluster population of theLarge Magellanic Cloud (LMC) has entered PCC evolution, a similarfraction to that for the Galactic globular cluster system. In addition,we examine the profile of R136 in detail and show that it is probablynot a PCC cluster. We also observe a trend in core radius with age thathas been discovered and discussed in several previous publications bydifferent authors. Our diagram has better resolution, however, andappears to show a bifurcation at several hundred Myr. We argue that thisobserved relationship reflects true physical evolution in LMC clusters,with some experiencing small-scale core expansion owing to mass loss,and others large-scale expansion owing to some unidentifiedcharacteristic or physical process.
| Early-type variables in the Magellanic Clouds. I. beta Cephei stars in the LMC bar
A thorough analysis of the OGLE-II time-series photometry of the LargeMagellanic Cloud bar supplemented by similar data from the MACHOdatabase led us to the discovery of three beta Cephei-type stars. Theseare the first known extragalactic beta Cephei-type stars. Two of thethree stars are multiperiodic. Two stars have inferred masses of about10 M_sun while the third is about 2 mag brighter and at least twice asmassive. All three variables are located in or very close to the massiveand young LMC associations (LH 41, 59 and 81). It is therefore veryprobable that the variables have higher than average metallicities. Thiswould reconcile our finding with theoretical predictions of the shapeand location of the beta Cephei instability strip in the H-R diagram.The low number of beta Cephei stars found in the LMC is anotherobservational confirmation of strong dependence of the mechanism drivingpulsations in these variables on metallicity. Follow-up spectroscopicdetermination of the metallicities in the discovered variables willprovide a good test for the theory of pulsational stability in massivemain-sequence stars.
| The blue to red supergiant ratio in young clusters at various metallicities
We present new determinations of the blue to red supergiant ratio (B/R)in young open clusters at various metallicities. For this purpose, weexamine the HR diagrams of 45 clusters in the Galaxy and of 4 clustersin the Magellanic Clouds. The identification of supergiants is based onspectroscopic measurements (with photometric counts to check theresults). The new counts confirm the increase of the B/R ratio when themetallicity increases with the following normalized relation:(B/R)/((B/R)sun) =~ 0.05* e3(Z)/(Zsun)}, where Zsun=0.02 and(B/R)sun is the value of B/R at Zsun which dependson the definition of B and R and on the age interval considered (e.g.for spectroscopic counts including clusters with log age between 6.8 and7.5, (B/R)sun =~ 3 when B includes O, B and A supergiants).
| Mass segregation in young compact star clusters in the Large Magellanic Cloud - I. Data and luminosity functions
We have undertaken a detailed analysis of HST/WFPC2 and STIS imagingobservations, and of supplementary wide-field ground-based observationsobtained with the ESO New Technology Telescope (NTT) of two young(~10-25Myr) compact star clusters in the LMC, NGC 1805 and 1818. Theultimate goal of our work is to improve our understanding of the degreeof primordial mass segregation in star clusters. This is crucial for theinterpretation of observational luminosity functions (LFs) in terms ofthe initial mass function (IMF), and for constraining the universalityof the IMF. We present evidence for strong luminosity segregation inboth clusters. The LF slopes steepen with cluster radius; in both NGC1805 and 1818 the LF slopes reach a stable level well beyond the core ofthe clusters or half-light radii. In addition, the brightest clusterstars are strongly concentrated within the inner ~4Rhl. Theglobal cluster LF, although strongly non-linear, is fairly wellapproximated by the core or half-light LF; the (annular) LFs at theseradii are dominated by the segregated high-luminosity stars, however. Wepresent tentative evidence for the presence of an excess number ofbright stars surrounding NGC 1818, for which we argue that they are mostprobably massive stars that have been collisionally ejected from thecluster core. We therefore suggest that the cores of massive young starsclusters undergo significant dynamical evolution, even on time-scales asshort as ~25Myr.
| The Effects of Dust in Simple Environments: Large Magellanic Cloud H II Regions
We investigate the effects of dust on Large Magellanic Cloud (LMC)H II region spectral energy distributions usingarcminute-resolution far-ultraviolet (FUV), Hα, far-infrared(FIR), and radio images. Widely used indicators of the amount of lightlost to dust (attenuation) at Hα and in the FUV correlate witheach other, although often with substantial scatter. There are twointeresting systematic discrepancies: First, Hα attenuationsestimated from the Balmer decrement are lower than those estimated fromthe Hα-to-thermal radio luminosity ratio. Our data, at this stage,cannot unambiguously identify the source of this discrepancy. Second,the attenuation at 1500 Å and the UV spectral slope, β,correlate, although the slope and scatter are substantially differentfrom the correlation first derived for starbursting galaxies by Calzettiet al. Combining our result with those of Meurer et al. forultraluminous infrared galaxies and Calzetti et al. for starburstinggalaxies, we conclude that no single relation between β and 1500Å attenuation is applicable to all star-forming systems.
| A Large and Homogeneous Sample of CMDs of LMC Stellar Clusters
We present the photometric results of 21 stellar clusters of the LargeMagellanic Cloud. The WFPC2 images were retrieved from the HST archive.Simple stellar populations in a large spread of age are well representedin the sample of color-magnitude diagrams shown here.
| Young Populous Clusters in the Magellanic Clouds
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| Large Magellanic Cloud stellar clusters. I. 21 HST colour magnitude diagrams
We present WFPC2 photometry of 21 stellar clusters of the LargeMagellanic Cloud obtained on images retrieved from the Hubble SpaceTelescope archive. The derived colour magnitude diagrams (CMDs) arepresented and discussed. This database provides a sample of CMDsrepresenting, with reliable statistics, simple stellar populations witha large spread of age. The stars in the core of the clusters are allresolved and measured at least down to the completeness limit; themagnitudes of the main sequence terminations and of the red giant clumpare also evaluated for each cluster, together with the radius at halfmaximum of the star density. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under the NASA contract NAS5-26555. Table 1 is only available in electronic form athttp://www.edpsciences.org
| Young star clusters in the Large Magellanic Cloud: NGC 1805 and 1818
We present colour-magnitude diagrams for two rich(~104Msolar) Large Magellanic Cloud star clusterswith ages ~107yr, constructed from optical and near-infrareddata obtained with the Hubble Space Telescope. These data are part of anHST project to study LMC clusters with a range of ages. In this paper weinvestigate the massive star content of the young clusters, anddetermine the cluster ages and metallicities, paying particularattention to Be-star and blue-straggler populations and evidence of agespreads. We compare our data with detailed stellar-populationsimulations to investigate the turn-off structure of ~25Myr stellarsystems, highlighting the complexity of the blue-straggler phenomenon.
| Resolving Sirius-like binaries with the Hubble Space Telescope
We present initial results from a Hubble Space Telescope ultravioletimaging survey of stars known to have hot white dwarf companions whichare unresolved from the ground. The hot companions, discovered throughtheir EUV or UV emission, are hidden by the overwhelming brightnesses ofthe primary stars at visible wavelengths. Out of 17 targets observed, wehave resolved eight of them with the Wide Field Planetary Camera 2,using various ultraviolet filters. Most of the implied orbital periodsfor the resolved systems are hundreds to thousands of years, but in atleast three cases (56 Persei, ζ Cygni and RE J1925-566) it shouldbe possible to detect the orbital motions within the next few years, andthey may eventually yield new dynamically determined masses for thewhite dwarf components. The 56 Persei and 14 Aurigae systems are foundto be quadruple and quintuple, respectively, including the known opticalcomponents as well as the newly resolved white dwarf companions. Themild barium star ζ Cygni, known to have an 18-year spectroscopicperiod, is marginally resolved. All of these newly resolved Sirius-typebinaries will be useful in determining gravitational redshifts andmasses of the white dwarf components.
| Young Clusters in the Magellanic Clouds. II.
We present the results of a quantitative study of the degree ofextension to the boundary of the classical convective core withinintermediate-mass stars. The basis of our empirical study is the stellarpopulation of four young populous clusters in the Magellanic Clouds thathas recently been detailed by Keller, Bessell, & Da Costa. Thesample affords a meaningful comparison with theoretical scenarios withvarying degrees of convective core overshoot and binary star fraction.Two critical properties of the population, the main-sequence luminosityfunction and the number of evolved stars, form the basis of ourcomparison between the observed data set and that simulated from thestellar evolutionary models. On the basis of this comparison we concludethat the case of no convective core overshoot is excluded at a 2 σlevel. Based on observations 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.
| Spectroscopy of Star Cluster Candidates and H II Regions in NGC 6822
We present optical spectroscopy of four star clusters and six H IIregions in the nearby, dwarf irregular galaxy NGC 6822. From the data,we estimate ages of2.5+1.5-1.0×107,2.0+3.0-1.0×108,1.4+1.1-0.6×109, and10+/-2×109 yr for clusters H VI, C21, H VIII, and HVII, respectively. For clusters H VI and H VII we estimate[Fe/H]~-1.46+/-0.26 and ~-2.0, lower than Large and Small MagellanicCloud clusters at similar ages. Mass estimates for H VI, H VII, and HVIII demonstrate that clusters with typical globular cluster masses(>104 Msolar) have formed over the lifetime of NGC6822. For six H II regions, ionic abundances are derived for elementspecies of N, O, S, and Ne. These show that there is a correlation ofabundance with position, the highest oxygen abundance H II regions beinglocated off the main body of the galaxy. Based on Observations with theCerro Tololo Inter-American Observatory.
| Beauty and astrophysics
Spectacular colour images have been made by combining CCD images inthree different passbands using Adobe Photoshop. These beautiful imageshighlight a variety of astrophysical phenomena and should be a valuableresource for science education and public awareness of science. The widefield images were obtained at the Siding Spring Observatory (SSO) bymounting a Hasselblad or Nikkor telephoto lens in front of a 2K by 2KCCD. Options of more than 30 degrees or 6 degrees square coverage areproduced in a single exposure in this way. Narrow band or broad bandfilters were placed between lens and CCD enabling deep, linear images ina variety of passbands to be obtained. We have mapped the LMC and SMCand are mapping the Galactic Plane for comparison with the MolongloRadio Survey. Higher resolution images have also been made with the 40inch telescope of galaxies and star forming regions in the Milky Way.
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