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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.
| 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.
| 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
| 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 MODEST questions: Challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellarcluster research, including young clusters, globular clusters, andgalactic nuclei. Topics considered include: primordial mass segregationand runaway mergers, expulsion of gas from clusters, the production ofstellar exotica seen in some clusters (e.g., blue stragglers and extremehorizontal-branch stars), binary populations within clusters, theblack-hole population within stellar clusters, the final parsec problem,stellar dynamics around a massive black hole, and stellar collisions.The Modest Questions posed here are the outcome of discussions whichtook place at the Modest-6A workshop held in Lund, Sweden, in December,2005. Modest-6A was organised as part of the activities of the ModestCollaboration (see www.manybody.org for further details).
| 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.
| Mass segregation in rich LMC clusters from modelling of deep HST colour-magnitude diagrams
Aims.We used the deep colour-magnitude diagrams (CMDs) of five rich LMCclusters (NGC 1805, NGC 1818, NGC 1831, NGC 1868, and Hodge 14) observedwith HST/WFPC2 to derive their present day mass function (PDMF) and itsvariation with position within the cluster. Methods: .The PDMFwas parameterized as a power law in the available main-sequence massrange of each cluster, typically 0.9 ⪉ m/Mȯ ⪉2.5; its slope was determined at different positions spanning from thevery centre out to several core radii. The CMDs in the central regionsof the clusters were carefully studied earlier, resulting in accurateage, metallicity, distance modulus, and reddening values. The slopeα (where Salpeter is 2.35) was determined in annuli by followingtwo distinct methods: 1) a power law fit to the PDMF obtained from thesystemic luminosity function (LF); 2) a statistical comparison betweenobserved and model CMDs. In the second case, α is a free inputparameter in the CMD modelling process where we incorporate photometricerrors and the effect of binarity as a fraction of unresolved binaries(f{bin}=100%) with random pairing of masses from the samePDMF. Results: .In all clusters, significant mass segregation isfound from the positional dependence of the PDMF slope: α ⪉1.8 for R ≤ 1.0 R{core} and α ˜ Salpeterinside R=2 ˜ 3 R{core} (except for Hodge 14, whereα ˜ Salpeter for R ˜ 4 R{core}). Theresults are robust in the sense that they hold true for both methodsused. The CMD method reveals that unresolved binaries flatten the PDMFobtained form the systemic LF, but this effect is smaller than theuncertainties in the α determination. For each cluster weestimated dynamical ages inside the core and for the entire system. Inboth cases we found a trend in the sense that older clusters haveflatter PDMF, consistent with a dynamical mass segregation and stellarevaporation.
| Core Radius-Mass Evolution of Globular Clusters
Some dynamical features of present day globular clusters seem to be theresult of the effects produced at the epoch of formation, both by therate of primordial binary stars and the formation and destruction of newones. Even a mass segregation and a cluster evaporation driven by thepopulation of binary stars are possible. The spread in the core radiusamong intermediate age and old stars clusters, observed e.g. in the LMC,could be generated by these two effects. In this contribution somepreliminary results are shown.
| 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.
| New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations
We present theoretical surface brightness fluctuation (SBF) amplitudesfor single-burst stellar populations of young and intermediate age (25Myr<=t<=5 Gyr) and metallicities Z=0.0003, 0.001, 0.004, 0.008,0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expectedin the Johnson-Cousins (UBVRIJHK) photometric system are provided. Wepay attention to the contribution of thermally pulsating asymptoticgiant branch (TP-AGB) stars. The sensitivity of the predicted SBF tochanges in the mass-loss scenario along the TP-AGB phase is examined.Below 0.6-1 Gyr both optical and near-IR SBF models exhibit a strongdependence on age and mass loss. We also evaluate SBF amplitudes usingMonte Carlo techniques to reproduce the random variation in the numberof stars experiencing bright and fast evolutionary phases (red giantbranch, AGB, TP-AGB). On these grounds we provide constraints on thefaintest integrated flux of real stellar populations required to derivereliable and meaningful SBF measurements. We analyze a technique forderiving SBF amplitudes of star clusters from the photometry ofindividual stars and estimate the uncertainty due to statisticaleffects, which may impinge on the procedure. The first optical SBFmeasurements for 11 Large Magellanic Cloud (LMC) star-rich clusters-withages ranging from a few megayears to several gigayears-are derived usingHubble Space Telescope observations. The measurements are compared toour SBF predictions, providing a good agreement with models ofmetallicity Z=0.0001-0.01. Our results suggest that, for TP-AGB stars, amass loss as a power-law function of the star luminosity is required inorder to properly reproduce the optical SBF data of the LMC clusters.Finally, near-IR models have been compared to available data, thusshowing that the general trend is well fitted. We suggest how toovercome the general problem of SBF models in reproducing the details ofthe near-IR SBF measurements of the Magellanic Cloud star clusters.
| Structure and Mass Segregation in h and χ Persei
We use V magnitudes and spectral types to examine the density structureof h and χ Per. We describe an automatic method for derivingspectral types and compare classifications for observations made at twodifferent facilities. With these data, we measure an extinction to theclusters of E(B-V)=0.52+/-0.07, consistent with other authors. However,there appears to be a correlation between the spectral types of thestars used and the resulting value of the extinction. We compareextinction values measured by different authors using different numbersof stars and reproduce their values by imposing different cuts in the Vmagnitude. This variation in color excess versus spectral type suggeststhat the standard intrinsic colors for the earliest type stars are bluerthan the stars in h and χ Per. We measure centers for h and χPer at α(2000)=2h18m56.4s+/-3.0s,δ(2000)=57deg8'25''+/-23''and α(2000)=2h22m4.3s+/-2.9s,δ(2000)=57deg8'35''+/-25'',respectively. We fit the density structure of the clusters and find coreradii of 1.9 and 2.4 pc, respectively. Integration of the Miller-Scaloinitial mass function suggests overall cluster masses of 5500 and 4300Msolar and central densities of 27 and 50 Msolarpc-3, respectively. We find strong evidence of masssegregation in h Per but not in χ Per. Examination of the dynamicaltimescales, as well as comparisons between the two clusters, suggestthat the mass segregation is partly primordial.
| 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.
| Physical parameters of rich LMC clusters from modeling of deep HST colour-magnitude diagrams
We present the analysis of deep colour-magnitude diagrams (CMDs) of fiverich LMC clusters. The data were obtained with HST/WFPC2 in the F555W(~V) and F814W (~I) filters, reaching V555 ˜ 25. Thesample of clusters is composed of NGC 1805 and NGC 1818, the youngestones (τ < 100 Myr), NGC 1831 and NGC 1868, of intermediate-age (400 < τ < 1000 Myr), and Hodge 14, the oldest (τ > 1200Myr). We discuss and apply a statistical method for correcting the CMDfor sampling incompleteness and field star contamination. Efficient useof the CMD data was made by means of direct comparisons of the observedto model CMDs. The CMD modeling process generates a synthetic MainSequence (MS), where we introduce as model inputs the information aboutage, chemical composition, present day mass function (PDMF), fraction ofunresolved binaries, distance modulus and light extinction. Thephotometric uncertainties were empirically determined from the data andincorporated into the model as well. Statistical techniques of CMDcomparisons using 1 and 2 dimensions are presented and applied as anobjective method to assess the compatibility between model and dataCMDs. By modeling the CMDs from the central region we infer themetallicity (Z), the intrinsic distance modulus ((m-M)0) andthe reddening value (E(B-V)) for each cluster. We also determined theage for the clusters with τ > 400 Myr. By means oftwo-dimensional CMD comparisons we infer the following values: for NGC1805, Z=0.007 ± 0.003, (m-M)0=18.50 ± 0.11,E(B-V)=0.03 ± 0.01; for NGC 1818, Z=0.005 ± 0.002,(m-M)0=18.49 ± 0.14, E(B-V) ˜ 0.00; for NGC1831, Z=0.012 ± 0.002, log(τ/yr)=8.70 ± 0.03,(m-M)0=18.70 ± 0.03, E(B-V) ˜ 0.00; for NGC1868, Z=0.008 ± 0.002, log(τ/yr)=8.95 ± 0.03,(m-M)0=18.70 ± 0.03, E(B-V) ˜ 0.00; for Hodge14, Z=0.008 ± 0.004, log(τ/yr)=9.23 ± 0.10,(m-M)0=18.51 ± 0.13, E(B-V)=0.02 ± 0.02. Takinginto account the uncertainties, these values are in accordance with theones obtained applying the one-dimensional CMD analysis, addingreliability to these determinations.
| 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.
| Constraints on the star formation history of the Large Magellanic Cloud
We present the analysis of deep colour-magnitude diagrams (CMDs) of 6stellar fields in the LMC. The data were obtained using HST/WFPC2 in theF814W (˜I) and F555W (˜V) filters, reaching V555˜ 26.5. We discuss and apply a method of correcting CMDs forphotometric incompleteness. A method to generate artificial CMDs basedon a model star formation history is also developed. This methodincorporates photometric error effects, unresolved binaries, reddeningand allows use of different forms of the initial mass function and ofthe SFH itself. We use the Partial Models Method, as presented byGallart and others, for CMD modelling, and include control experimentsto prove its validity in a search for constraints on the LargeMagellanic Cloud star formation history in different regions. Reliablestar formation histories for each field are recovered by this method. Inall fields, a gap in star formation with τ ˜ 700 Myr isobserved. Field-to-field variations have also been observed. The twofields near the LMC bar present some significant star forming events,having formed both young (τ ⪉ 1 Gyr) and old (τ ⪆ 10Gyr) stars, with a clear gap from 3-6 Gyr. Two other fields displayquite similar SFHs, with increased star formation having taken place atτ ≃ 2-3 Gyr and 6 ⪉ τ ⪉ 10 Gyr. The remaining twofields present star formation histories closer to uniform, with no clearevent of enhanced star formation.
| Massive Stars in Clusters and the Field
What is the relation between field massive stars and clusters? Do theyrepresent an extreme in the universal, power-law relation for stellarclustering? Or do they represent a substantially different mode of starformation? What is the origin of the clustering law itself? We examinethe massive star population of the Small Magellanic Cloud and find acontinuous, power-law relation between field stars and clusters. Thisimplies that the fraction of field massive stars ranges from about 35%to 7% for most astrophysical situations, with a weak dependence on thegalaxy size and/or star formation rate. We also examine the starformation history of the Galactic complex W3/W4, which is a system ofthree generations of hierarchical, triggered star formation. This lendssome of the strongest evidence to date that superbubbles indeed triggerstar formation. We speculatively link this hierarchical process to thepower-law clustering of stars.
| 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.
| Variability in the stellar initial mass function at low and high mass: three-component IMF models
Three-component models of the initial mass function (IMF) are made toconsider possible origins for the observed relative variations in thenumbers of brown dwarfs, solar-to-intermediate-mass stars and high-massstars. The differences between the IMFs observed for clusters, field andremote field are also discussed. Three distinct physical processes thatshould dominate the three stellar mass regimes are noted. Thecharacteristic mass for most star formation is identified with thethermal Jeans mass in the molecular cloud core, and this presumablyleads to the middle mass range by the usual collapse and accretionprocesses. Pre-stellar condensations (PSCs) observed in millimetre-wavecontinuum studies presumably form at this mass. Significantly smallerself-gravitating masses require much larger pressures and may arisefollowing dynamical processes inside these PSCs, including discformation, tight-cluster ejection, and photoevaporation as studiedelsewhere, but also gravitational collapse of shocked gas in collidingPSCs. Significantly larger stellar masses form in relatively lowabundance by normal cloud processes, possibly leading to steep IMFs inlow-pressure field regions, but this mass range can be significantlyextended in high-pressure cloud cores by gravitationally focused gasaccretion on to PSCs and by the coalescence of PSCs. These modelssuggest that the observed variations in brown dwarf,solar-to-intermediate-mass and high-mass populations are the result ofdynamical effects that depend on environmental density and velocitydispersion. They accommodate observations ranging from shallow IMFs incluster cores to Salpeter IMFs in average clusters and whole galaxies tosteep and even steeper IMFs in field and remote field regions. They alsosuggest how the top-heavy IMFs in some starburst clusters may originateand they explain bottom-heavy IMFs in low surface brightness galaxies.
| 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.
| On the determination of age and mass functions of stars in young open star clusters from the analysis of their luminosity functions
We construct the observed luminosity functions of the remote young openclusters NGC 2383, 2384, 4103, 4755, 7510 and Hogg 15 from CCDobservations of them. The observed LFs are corrected for field starcontamination determined with the help of a Galactic star count model.In the case of Hogg 15 and NGC 2383 we also consider the additionalcontamination from neighbouring clusters NGC 4609 and 2384,respectively. These corrections provide a realistic pattern of clusterLF in the vicinity of the main-sequence (MS) turn-on point and atfainter magnitudes reveal the so-called H-feature arising as a result ofthe transition of the pre-MS phase to the MS, which is dependent on thecluster age. The theoretical LFs are constructed representing a clusterpopulation model with continuous star formation for a short time-scaleand a power-law initial mass function (IMF), and these are fitted to theobserved LF. As a result, we are able to determine for each cluster aset of parameters describing the cluster population (the age, durationof star formation, IMF slope and percentage of field starcontamination). It is found that in spite of the non-monotonic behaviourof observed LFs, cluster IMFs can be described as power-law functionswith slopes similar to Salpeter's value. The present main-sequenceturn-on cluster ages are several times lower than those derived from thefitting of theoretical isochrones to the turn-off region of the uppermain sequences.
| Core radius evolution of star clusters
We use N-body simulations of star clusters to investigate the possibledynamical origins of the observed spread in core radius amongintermediate-age and old star clusters in the Large Magellanic Cloud(LMC). Two effects are considered, a time-varying external tidal fieldand variations in primordial hard binary fraction. Simulations ofclusters orbiting a point-mass galaxy show similar core radius evolutionfor clusters on both circular and elliptical orbits and we thereforeconclude that the tidal field of the LMC has not yet significantlyinfluenced the evolution of the intermediate-age clusters. The presenceof large numbers of hard primordial binaries in a cluster leads to coreradius expansion; however, the magnitude of the effect is insufficientto explain the observations. Furthermore, the range of binary fractionsrequired to produce significant core radius growth is inconsistent withthe observational evidence that all the LMC clusters have similarstellar luminosity functions.
| 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.
| Mass segregation in young compact clusters in the Large Magellanic Cloud - III. Implications for the initial mass function
The distribution of core radii of rich clusters in the Large MagellanicCloud (LMC) systematically increases in both upper limit and spread withincreasing cluster age. Cluster-to-cluster variations in the stellarinitial mass function (IMF) have been suggested as an explanation. Wediscuss the implications of the observed degree of mass segregation inour sample clusters for the shape of the initial mass function. Ourresults are based on Hubble Space Telescope/WFPC2 observations of sixrich star clusters in the LMC, selected to include three pairs ofclusters of similar age, metallicity and distance from the LMC centre,and exhibiting a large spread in core radii between the clusters in eachpair. All clusters show clear evidence of mass segregation: (i) theirluminosity function slopes steepen with increasing cluster radius, and(ii) the brighter stars are characterized by smaller core radii. For allsample clusters, both the slope of the luminosity function in thecluster centres and the degree of mass segregation are similar to eachother, within observational errors of a few tenths of power-law slopefits to the data. This implies that their initial mass functions musthave been very similar, down to ~0.8-1.0 Msolar. We thereforerule out variations in the IMF of the individual sample clusters as themain driver of the increasing spread of cluster core radii with clusterage.
| On the nature of a secondary main-sequence turn-off in the rich LMC cluster NGC 1868
Evidence for a second main-sequence turn-off in a deep colour-magnitudediagram (CMD) of NGC 1868 is presented. The data were obtained withHubble Space Telescope (HST) WFPC2 and reach down to m555~=25 mag. Besides the usual τ~= 0.8 Gyr turn-off found in previousanalyses, another possible turn-off is seen at m555~= 21 mag(MV~= 2.5), which is consistent with an age of τ~= 3 Gyr.This CMD feature stands out clearly especially when contaminating fieldLarge Magellanic Cloud (LMC) stars are statistically removed. Thebackground subtracted CMD also visibly displays a red subgiant branchextending about 1.5 mag below the younger turn-off and the clump of redgiants. The significance of the secondary turn-off in NGC 1868 wasconfirmed with Monte Carlo simulations and bootstrapping techniques.Star counts in selected regions in the cluster CMD indicate a mass ratioof old population/young population in the range 5<~Mold/Myoung<~ 12 per cent, depending onthe mass-function slope. The existence of such a subpopulation in NGC1868 is significant even in the presence of uncertainties in backgroundsubtraction. The possibility that the secondary turn-off is associatedwith the field star population was examined by searching for similarfeatures in CMDs of field stars. Statistically significant excesses ofstars redwards of the main sequence were found in all such fields in therange 20 <~m555<~ 22 mag. These however are muchbroader features that do not resemble the main-sequence termination of asingle population. We also discuss other alternative explanations forthe feature at m555~= 21 mag, such as unresolved binarism,peculiar stars or CMD discontinuities associated with theBöhm-Vitense gap.
| 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.
| Mass segregation in young compact star clusters in the Large Magellanic Cloud - II. Mass functions
We review the complications involved in the conversion of stellarluminosities into masses and apply a range of mass-to-luminosityrelations to our Hubble Space Telescope observations of the young LargeMagellanic Cloud (LMC) star clusters NGC 1805 and 1818. Both the radialdependence of the mass function (MF) and the dependence of the clustercore radii on mass indicate clear mass segregation in both clusters atradii of r<~20-30arcsec, for masses in excess of~1.6-2.5Msolar. This result does not depend on the mass rangeused to fit the slopes or the metallicity assumed. It is clear that thecluster MFs, at any radius, are not simple power laws. The global andthe annular MFs near the core radii appear to be characterized bysimilar slopes in the mass range(-0.15<=logm/Msolar<=0.85), and the MFs beyondr>~30arcsec have significantly steeper slopes. We estimate that whilethe NGC 1818 cluster core is between ~5 and ~30 crossing times old, thecore of NGC 1805 is likely to be <~3-4 crossing times old. However,since strong mass segregation is observed out to ~6Rcore and~3Rcore in NGC 1805 and 1818, respectively, it is most likelythat significant primordial mass segregation was present in bothclusters, particularly in NGC 1805.
| 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.
| MSX, 2MASS, and the LARGE MAGELLANIC CLOUD: A Combined Near- and Mid-Infrared View
The Large Magellanic Cloud (LMC) has been observed by the MidcourseSpace Experiment (MSX) in the mid-infrared and the Two Micron All SkySurvey (2MASS) in the near-infrared. We have performed across-correlation of the 1806 MSX catalog sources and nearly 1.4 million2MASS cataloged point and extended sources and find 1664 matches. Usingthe available color information, we identify a number of stellarpopulations and nebulae, including main-sequence stars, giant stars, redsupergiants, carbon- and oxygen-rich asymptotic giant branch (AGB)stars, planetary nebulae, H II regions, and other dusty objects likelyassociated with early-type stars. A total of 731 of these sources haveno previous identification. We compile a listing of all objects, whichincludes photometry and astrometry. The 8.3 μm MSX sensitivity is thelimiting factor for object detection: only the brighter red objects,specifically the red supergiants, AGB stars, planetary nebulae, and H IIregions, are detected in the LMC. The remaining objects are likely inthe Galactic foreground. The spatial distribution of the infrared LMCsources may contribute to understanding stellar formation and evolutionand the overall galactic evolution. We demonstrate that a combined mid-and near-infrared photometric baseline provides a powerful means ofidentifying new objects in the LMC for future ground-based andspace-based follow-up observations.
| Deep colour-magnitude diagrams of LMC field stars imaged with HST
We present deep photometry (V<~26) in V and I bands obtained with theWide Field and Planetary Camera 2 on board the Hubble Space Telescopefor 7 fields ~5° away from the Large Magellanic Cloud centre. Thefields contain, typically, 2000 stars each. Isochrones were fitted tothe colour-magnitude diagrams in order to identify different starpopulations in these fields. An old population (τ>10Gyr) has beenfound in all fields. Some events of enhanced star formation, with agesbetween 2 and 4Gyr, were identified in the fields localized in the northto north-west regions. Luminosity functions of low-mass stars were alsoobtained for all fields. Kolmogorov Smirnov test results suggestdifferences smaller than 30 per cent in the mixture of stellarpopulations contributing to the fields. Finally, density profiles werederived for old and intermediate-age stars. The former shows a slightlysteeper decline than the latter.
| 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.
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