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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.
| The evolution of binary star clusters and the nature of NGC 2136/NGC 2137
We study the evolution of bound pairs of star clusters by means ofdirect N-body simulations. Our simulations include mass loss by stellarevolution. The initial conditions are selected to mimic the observedbinary star clusters, NGC2136 and 2137, in the Large Magellanic Cloud.Based on their rather old ages (~100Myr), masses, sizes and projectedseparation, we conclude that the cluster pair must have been born withan initial separation of 15-20 pc. Clusters with a smaller initialseparation tend to merge in <~60Myr due to loss of angular momentumfrom escaping stars. Clusters with a larger initial separation tend tobecome even more widely separated due to mass loss from the evolvingstellar populations. The early orbital evolution of a binary cluster isgoverned by mass loss from the evolving stellar population and by lossof angular momentum from escaping stars. Mass loss by stellar winds andsupernovae explosions in the first ~30Myr causes the binary to expandand the orbit to become eccentric. The initially less massive clusterexpands more quickly than the binary separation increases, and istherefore bound to initiate mass transfer to the more massive cluster.This process is quite contrary to stellar binaries in which the moremassive star tends to initiate mass transfer. Since mass transferproceeds on a thermal time-scale from the less massive to the moremassive cluster, this semidetached phase is quite stable, even in aneccentric orbit until the orbital separation reaches the gyration radiusof the two clusters, at which point both clusters merge to one.
| 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.
| Gas expulsion and the destruction of massive young clusters
We examine the luminosity and dynamical mass estimates for young massivestellar clusters. For many young (<50 Myr) clusters, the luminosityand dynamical mass estimates differ by a significant amount. We explainthis as being due to many young clusters being out of virial equilibrium(which is assumed in dynamical mass estimates) because the clusters areundergoing violent relaxation after expelling gas not used in starformation. We show that, if we assume that luminous mass estimates arecorrect (for a standard initial mass function), at least 50 per cent ofyoung clusters for which dynamical masses are known are likely to bedestroyed within a few tens of Myr of their formation. Even clusterswhich will retain a bound core may lose a large fraction of theirstellar mass. We also show that the core radius and other structuralparameters change significantly during the violent relaxation thatfollows gas expulsion and that they should be considered instantaneousvalues only, not necessarily reflecting the final state of the cluster.In particular we note that the increasing core radii observed in youngLMC/SMC clusters can be well explained as an effect of rapid gas loss.
| Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
We present observations of two LMC supernova remnants (SNRs), DEM L238and DEM L249, with the Chandra and XMM-Newton X-ray satellites. Brightcentral emission, surrounded by a faint shell, is present in bothremnants. The central emission has an entirely thermal spectrumdominated by strong Fe L-shell lines, with the deduced Fe abundance inexcess of solar and not consistent with the LMC abundance. This Feoverabundance leads to the conclusion that DEM L238 and DEM L249 areremnants of thermonuclear (Type Ia) explosions. The shell emissionoriginates in gas swept up and heated by the blast wave. A standardSedov analysis implies about 50 Msolar in both swept-upshells, SNR ages between 10,000 and 15,000 yr, low (<~0.05cm-3) preshock densities, and subluminous explosions withenergies of 3×1050 ergs. The central Fe-rich supernovaejecta are close to collisional ionization equilibrium. Their presenceis unexpected, because standard Type Ia SNR models predict faint ejectaemission with short ionization ages. Both SNRs belong to a previouslyunrecognized class of Type Ia SNRs characterized by bright interioremission. Denser than expected ejecta and/or a dense circumstellarmedium around the progenitors are required to explain the presence ofFe-rich ejecta in these SNRs. Substantial amounts of circumstellar gasare more likely to be present in explosions of more massive Type Iaprogenitors. DEM L238, DEM L249, and similar SNRs could be remnants of``prompt'' Type Ia explosions with young (~100 Myr old) progenitors.
| The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks. I. New Candidate Be Stars in the LMC, SMC, and Milky Way
We present B, V, R, and Hα photometry of eight clusters in theSmall Magellanic Cloud, five in the Large Magellanic Cloud, and threeGalactic clusters and use two-color diagrams (2-CDs) to identifycandidate Be star populations in these clusters. We find evidence thatthe Be phenomenon is enhanced in low-metallicity environments, based onthe observed fractional early-type candidate Be star content of clustersof age 10-25 Myr. Numerous candidate Be stars of spectral types B0-B5were identified in clusters of age 5-8 Myr, challenging the suggestionof Fabregat & Torrejon that classical Be stars should only be foundin clusters at least 10 Myr old. These results suggest that asignificant number of B-type stars must emerge onto the zero-age mainsequence as rapid rotators. We also detect an enhancement in thefractional content of early-type candidate Be stars in clusters of age10-25 Myr, suggesting that the Be phenomenon does become more prevalentwith evolutionary age. We briefly discuss the mechanisms that mightcontribute to such an evolutionary effect. A discussion of thelimitations of using the 2-CD technique to investigate the roleevolutionary age and/or metallicity play in the development of the Bephenomenon is offered, and we provide evidence that other B-type objectsof very different nature, such as candidate Herbig Ae/Be stars, maycontaminate the claimed detections of Be stars via 2-CDs.
| 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.
| Equilibrium Star Cluster Formation
We argue that rich star clusters take at least several local dynamicaltimes to form and so are quasi-equilibrium structures during theirassembly. Observations supporting this conclusion include morphologiesof star-forming clumps, momentum flux of protostellar outflows fromforming clusters, age spreads of stars in the Orion Nebula cluster (ONC)and other clusters, and the age of a dynamical ejection event from theONC. We show that these long formation timescales are consistent withthe expected star formation rate in turbulent gas, as recently evaluatedby Krumholz & McKee. Finally, we discuss the implications of thesetimescales for star formation efficiencies, the disruption of gas bystellar feedback, mass segregation of stars, and the longevity ofturbulence in molecular clumps.
| 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.
| Supernova Remnants in the Magellanic Clouds. VI. The DEM L316 Supernova Remnants
The DEM L316 system contains two shells, both with the characteristicsignatures of supernova remnants (SNRs). We analyze Chandra andXMM-Newton data for DEM L316, investigating its spatial and spectralX-ray features. Our Chandra observations resolve the structure of thenortheastern SNR (shell A) as a bright inner ring and a set of ``arcs''surrounded by fainter diffuse emission. The spectrum is well fit by athermal plasma model with a temperature of ~1.4 keV; we do not findsignificant spectral differences for different regions of this SNR. Thesouthwestern SNR (shell B) exhibits an irregular X-ray outline, with abrighter interior ring of emission including a bright knot of emission.Overall, the emission of the SNR is well described by a thermal plasmaof temperature ~0.6 keV. The bright knot, however, is spectrallydistinct from the rest of the SNR, requiring the addition of ahigh-energy spectral component consistent with a power-law spectrum ofphoton index 1.6-1.8. We confirm the findings of Nishiuchi and coworkersthat the spectra of these shells are notably different, with shell Arequiring a high iron abundance for a good spectral fit, implying a TypeIa origin. We further explicitly compare abundance ratios to modelpredictions for Type Ia and Type II supernovae. The low ratios for shellA (O/Fe of 1.5 and Ne/Fe of 0.2) and the high ratios for shell B (O/Feof 30-130 and Ne/Fe of 8-16) are consistent with Type Ia and Type IIorigins, respectively. The difference between the SNR progenitor typescasts some doubt on the suggestion that these SNRs are interacting withone another.
| 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.
| Classical Cepheid Pulsation Models. X. The Period-Age Relation
We present new period-age (PA) and period-age-color (PAC) relations forfundamental and first-overtone classical Cepheids. Current predictionsrely on homogeneous sets of evolutionary and pulsation models covering abroad range of stellar masses and chemical compositions. We found thatPA and PAC relations present a mild dependence on metal content.Moreover, the use of different PA and PAC relations for fundamental andfirst-overtone Cepheids improves the accuracy of age estimates in theshort-period (logP<1) range (old Cepheids), because they presentsmaller intrinsic dispersions. At the same time, the use of the PACrelations improves the accuracy in the long-period (logP>=1) range(young Cepheids), since they account for the position of individualobjects inside the instability strip. We performed a detailed comparisonbetween evolutionary and pulsation ages for a sizable sample of LMC (15)and SMC (12) clusters which host at least two Cepheids. In order toavoid deceptive uncertainties in the photometric absolute zero point, weadopted the homogeneous set of B, V, and I data for clusters andCepheids collected by OGLE. We also adopted the same reddening scale.The different age estimates agree at the level of 20% for LMC clustersand of 10% for SMC clusters. We also performed the same comparison fortwo Galactic clusters (NGC 6067, NGC 7790), and the difference in age issmaller than 20%. These findings support the use of PA and PAC relationsto supply accurate estimates of individual stellar ages in the Galaxyand in external Galaxies. The main advantage of this approach is itsindependence from the distance.
| Eclipsing Binaries in the Young Large Magellanic Cloud Cluster NGC 1850
I present light curves for two detached eclipsing binary stars in theregion of the LMC cluster NGC 1850, which is possibly a young globularcluster still in formation. One, a likely O-type star, is a newlydetected eclipsing binary in the region of the very young subcluster NGC1850A. This binary is among a small number of highly massive O-typestars in binary systems found in LMC clusters. These two eclipsingbinaries are the first to be discovered in the well-studied NGC 1850,and the O-type star is the first eclipsing binary found in NGC 1850A.Light curves for two Cepheid variables in the NGC 1850 region are alsoshown. The discovery of two eclipsing binaries in the youngglobular-like cluster NGC 1850 is discussed in terms of the importanceof the binary fraction to globular cluster evolution.
| 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.
| Eclipsing Binaries in the Young LMC Globular-Like Cluster NGC 1850
I present light curves for two detached eclipsing binary stars in theregion of the LMC cluster NGC 1850, which may be a young globularcluster. One, a likely spectral type O star, is a newly detectedeclipsing binary in the region of the very young subcluster NGC 1850A.This binary is among a small number of highly massive O-type stars inbinary systems found in LMC clusters. These two eclipsing binaries arethe first discovered in the well studied NGC 1850, and the O-type staris the first eclisping binary found in NGC 1850A. Light curves for twoNGC 1850 region Cepheid variables are also shown. Discovering twoeclipsing binaries in the young globlular-like cluster NGC 1850 isdiscussed in terms of the importance of the binary fraction to globularcluster evolution.
| 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.
| An Intriguing X-Ray Arc Surrounding the X-Ray Source RX J053335-6854.9 toward the Large Magellanic Cloud
ROSAT observations of the Large Magellanic Cloud have revealed a largediffuse X-ray arc around the point source RX J053335-6854.9. Therelative locations of the diffuse and point sources suggest that theymight originate from a common supernova explosion. We have analyzed thephysical properties of the diffuse X-ray emission and determined that itis most likely a supernova remnant in a low-density medium in the LMC.We have also analyzed the X-ray and optical observations of RXJ053335-6854.9 and concluded that it is a foreground dMe star in thesolar neighborhood. Therefore, despite their positional coincidence,these two X-ray sources are physically unrelated.
| Cepheids in LMC Clusters and the Period-Age Relation
We have made a new comparison of the positions of Cepheids and clustersin the LMC and constructed a new empirical period-age relation takinginto account all available data on Cepheids in the LMC bar provided bythe OGLE project. The most probable relation is logT=8.50-0.65 logP, inreasonably good agreement with theoretical expectations. NumerousCepheids in rich clusters of the LMC provide the best data for comparingtheories of stellar evolution and pulsation and the dynamical evolutionof clusters with observations. These data suggest that stars undergoingtheir first crossing of the instability strip are first-overtonepulsators, though the converse is true of only a small fraction offirst-overtone stars. Several rich clusters with suitable ages have noCepheids—a fact that is not understood and requires verification.Differences in the concentration of Cepheids toward their clustercenters probably reflect the fact that the clusters are at differentstages of their dynamical evolution, with the Cepheids in clustercoronas being ejected from the cluster cores during dynamicalinteractions between stars.
| Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects
The properties of ~939 star clusters in the Large and Small MagellanicClouds were determined from ground-based CCD images in UBVR passbands.The areal coverage was extensive, corresponding to 11.0 kpc2in the LMC and 8.3 kpc2 in the SMC. After corrections forreddening, the colors and magnitudes of the clusters were converted toages and masses, and the resulting mass distributions were searched forthe effects of fading, evaporation, and size-of-sample bias. The datashow a clear signature of cluster fading below the detection threshold.The initial cluster mass function (ICMF) was determined by fitting themass and age distributions with cluster population models. These modelssuggest a new method to determine the ICMF that is nearly independent offading or disruption and is based on the slope of a correlation betweenage and the maximum cluster mass in equally spaced intervals of log age.For a nearly uniform star formation rate, this correlation has a slopeequal to 1/(α-1) for an ICMF of dn(M)/dM~M-α. Wedetermine that α is between 2 and 2.4 for the LMC and SMC usingthis method plus another method in which models are fitted to the massdistribution integrated over age and to the age distribution integratedover mass. The maximum mass method also suggests that the clusterformation rate in the LMC age gap between 3 and 13 Gyr is about a factorof 10 below that in the period from 0.1 to 1 Gyr. The oldest clusterscorrespond in age and mass to halo globular clusters in the Milky Way.They do not fit the trends for lower mass clusters but appear to be aseparate population that either had a very high star formation rate andbecame depleted by evaporation or formed with only high masses.
| 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
| 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.
| Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. III. Padova Results
The color-magnitude diagrams (CMDs) of three intermediate-age LargeMegallanic Cloud clusters, NGC 2173, SL 556, and NGC 2155, are analyzedto determine their age and metallicity based on Padova stellar models.Synthetic CMDs are compared with cluster data. The best match isobtained using two fitting functions based on star counts in thedifferent bins of the cluster CMD. Two different criteria are used. Oneof them takes into account the uncertainties in the color of the redclump stars. Given the uncertainties on the experimental values of theclusters metallicity, we provide a set of acceptable solutions. Theydefine the corresponding values of metallicity, age, reddening anddistance modulus (for the assumed initial mass function). The comparisonwith Padova models suggests for NGC 2173 a prolonged star formation(spanning a period of about 0.3 Gyr), beginning 1.7 Gyr and ending 1.4Gyr ago. The metallicity Z is in the range 0.0016-0.003. For SL 556 anage of 2.0 Gyr is obtained. The metallicity value is in the range0.002-0.004, depending on the adopted comparison criterion. The derivedage for NGC 2155 is 2.8 Gyr, and its metallicity Z is in the range0.002-0.003. The CMD features of this cluster suggest that a moreefficient overshoot should be adopted in the evolutionary models. Aperiod of extended star formation is not required to fit the SL 556 andNGC 2155 observations.Based on observations collected at the European Southern Observatory,Paranal, Chile.
| 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.
| The Radial Structure of Supernova Remnant N103B
We report on the results from a Chandra ACIS observation of the young,compact, supernova remnant N103B. The unprecedented spatial resolutionof Chandra reveals subarcsecond structure, in both the brightness andspectral variations. Underlying these small-scale variations is asurprisingly simple radial structure in the equivalent widths of thestrong Si and S emission lines. We investigate these radial variationsthrough spatially resolved spectroscopy, using a plane-parallel,nonequilibrium ionization model with multiple components. The majorityof the emission arises from components with a temperature of 1 keV: afully ionized hydrogen component; a high ionization timescale(net>1012 s cm-3) componentcontaining Si, S, Ar, Ca, and Fe; and a low ionization timescale(net~1011 s cm-3) O, Ne, and Mgcomponent. To reproduce the strong Fe Kα line, it is necessary toinclude additional Fe in a hot (> 2 keV), low ionization timescale(net~1010.8 s cm-3) component. This hotFe might be in the form of hot Fe bubbles, formed in the radioactivedecay of clumps of 56Ni. We find no radial variation in theionization timescales or temperatures of the various components. Rather,the Si and S equivalent widths increase at large radii because theselines, as well as those of Ar and Ca, are formed in a shell occupyingthe outer half of the remnant. A shell of hot Fe is located interior tothis, but there is a large region of overlap between these two shells.In the inner 30% of the remnant, there is a core of cooler, 1 keV Fe. Wefind that the distribution of the ejecta and the yields of theintermediate-mass species are consistent with model prediction for TypeIa events.
| The unexpected youth of globular clusters.
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| The Star Formation History and Mass Function of the Double Cluster h and χ Persei
The h and χ Per ``double cluster'' is examined using wide-field(0.98d×0.98d) CCD UBV imaging supplemented by optical spectra ofseveral hundred of the brightest stars. Restricting our analysis to nearthe cluster nuclei, we find identical reddenings [E(B-V)=0.56+/-0.01],distance moduli (11.85+/-0.05), and ages (12.8+/-1.0 Myr) for the twoclusters. In addition, we find an initial mass function slope for eachof the cluster nuclei that is quite normal for high-mass stars,Γ=-1.3+/-0.2, indistinguishable from a Salpeter value. We derivemasses of 3700 Msolar (h) and 2800 Msolar (χ)integrating the present-day mass function from 1 to 120Msolar. There is evidence of mild mass segregation within thecluster cores. Our data are consistent with the stars having formed at asingle epoch; claims to the contrary are very likely due to theinclusion of the substantial population of early-type stars located atsimilar distances in the Perseus spiral arm, in addition tocontamination by G and K giants at various distances. We discuss theuniqueness of the double cluster, citing other examples of suchstructures in the literature but concluding that the nearly identicalnature of the two cluster cores is unusual. We fail to settle thelong-standing controversy regarding whether or not the double cluster isthe core of the Per OB1 association and argue that this may beunanswerable with current techniques. We also emphasize the need forfurther work on the pre-main-sequence population of this nearby andhighly interesting region.
| High-Resolution X-ray imaging and spectroscopy of N 103B
The X-ray emission from the young supernova remnant (SNR) N 103B ismeasured and analysed using the high-resolution cameras andspectrometers on board XMM-Newton and Chandra. The spectrum from theentire remnant is reproduced very well with three plasma components ofkTe = 0.55, 0.65, and 3.5 keV, corresponding roughly to lineemission by the O-K, Fe-L, and Fe-K species, respectively. Narrow bandimages reveal different morphologies for each component. ThekTe = 0.65 keV component, which dominates the emissionmeasure (4.5*E65 m-3), is in ionisationequilibrium. This provides a lower limit of 1200 yrs to the age of theremnant, which is in agreement with the previously assumed age of theremnant (1500 yrs). Based on the measured energy of the Fe-K feature at6.5 keV, the hot (3.5 keV) component is found to be recently shocked ( ~200 yrs) and still ionising. The high elemental abundances of O and Neand the low abundance of Fe could imply that N 103B originated from atype II supernova (SN) rather than a type Ia SN as previously thought.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
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