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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.
| Three clusters of the SMC from ACS/WFC HST archive data: NGC 265, K 29 and NGC 290 and their field population
Aims.We determine the age, metallicity and initial mass function ofthree clusters, namely NGC 265, K29, NGC 290, located in the main body ofthe Small Magellanic Cloud. In addition, we derive the history of starformation in the companion fields. Methods: We make use of ACS/WFC HSTarchive data. For the clusters, the age and metallicity are derivedfitting the integrated luminosity function with single synthetic stellarpopulation by means of the χ2 minimization. For thecompanion fields, the history of star formation is derived using theχ2 minimization together with the downhill-simplexmethod. Results: For the clusters we find the following ages andmetallicities: NGC 265 has log(Age)=8.5±0.3 yrand metallicity 0.004±0.003 (or [ Fe/H]=-0.62); K29 has log(Age)=8.2±0.2 yr and metallicityZ=0.003±0.002 (or [ Fe/H]=-0.75); NGC 290 haslog(Age)=7.8±0.5 yr and metallicity 0.003±0.002 (or [Fe/H]=-0.75). The superior quality of the data allows the study of theinitial mass function down to M ˜ 0.7 Mȯ. Theinitial mass function turns out to be in agreement with the standardKroupa model. The comparison of the NGC 265luminosity function with the theoretical ones from stellar models bothtaking overshoot from the convective core into account and neglectingit, seems to suggest that a certain amount of convective overshoot isrequired. However, this conclusion is not a strong one because thiscluster has a certain amount of mass segregation which makes itdifficult to choose a suitable area for this comparison. The starformation rate of the field population presents periods of enhancementsat 300-400 Myr, 3-4 Gyr and finally 6 Gyr. However it is relativelyquiescent at ages older than 6 Gyr. This result suggests that at olderages, the tidal interaction between the Magellanic Clouds and the MilkyWay was not able to trigger significant star formation events.
| 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.
| Extended star formation history of the star cluster NGC 2154 in the Large Magellanic Cloud
The colour-magnitude diagram (CMD) of the intermediate-age LargeMagellanic Cloud star cluster NGC 2154 and its adjacent field has beenanalysed using Padova stellar models to determine the cluster'sfundamental parameters and its star formation history. Deep BR CCDphotometry, together with synthetic CMDs and integrated luminosityfunctions, has allowed us to infer that the cluster experienced anextended star formation period of about 1.2 Gyr, which beganapproximately 2.3 Gyr ago and ended 1.1 Gyr ago. However, the physicalreality of such a prolonged period of star formation is questionable,and could be the result of inadequacies in the stellar evolutionarytracks themselves. A substantial fraction of binaries (70 per cent)seems to exist in NGC 2154.
| 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
| Physical parameters of 15 intermediate-age LMC clusters from modelling of HST colour-magnitude diagrams
Aims.We analyzed HST/WFPC2 colour-magnitude diagrams (CMDs) of 15populous Large Magellanic Cloud (LMC) stellar clusters with ages between~0.3 Gyr and ~3 Gyr. These (V, B-V) CMDs are photometrically homogeneousand typically reach V ˜ 22. Accurate and self-consistent physicalparameters (age, metallicity, distance modulus and reddening) wereextracted for each cluster by comparing the observed CMDs with syntheticones. Methods: These determinations involved simultaneous statisticalcomparisons of the main-sequence fiducial line and the red clumpposition, offering objective and robust criteria to determine the bestmodels. The models explored a regular grid in the parameter spacecovered by previous results found in the literature. Control experimentswere used to test our approach and to quantify formal uncertainties. Results: In general, the best models show a satisfactory fit to thedata, constraining well the physical parameters of each cluster. Theage-metallicity relation derived by us presents a lower spread thansimilar results found in the literature for the same clusters. Ourresults are in accordance with the published ages for the oldestclusters, but reveal a possible underestimation of ages by previousauthors for the youngest clusters. Our metallicity results in generalagree with the ones based on spectroscopy of giant stars and with recentworks involving CMD analyses. The derived distance moduli implied by themost reliable solutions, correlate with the reddening values, asexpected from the non-negligible three-dimensional distribution of theclusters within the LMC. Conclusions: .The inferred spatialdistribution for these clusters is roughly aligned with the LMC disk,being also more scattered than recent numerical predictions, indicatingthat they were not formed in the LMC disk. The set of ages andmetallicities homogeneously derived here can be used to calibrateintegrated light studies applied to distant galaxies.
| 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.
| On the mass of dense star clusters in starburst galaxies from spectrophotometry
The mass of unresolved young star clusters derived fromspectrophotometric data may well be off by a factor of 2 or more oncethe migration of massive stars driven by mass segregation is accountedfor. We quantify this effect for a large set of cluster parameters,including variations in the stellar initial mass function (IMF), theintrinsic cluster mass, and mean mass density. Gas-dynamical modelscoupled with the Cambridge stellar evolution tracks allow us to derive ascheme to recover the real cluster mass given measured half-lightradius, one-dimensional velocity dispersion and age. We monitor theevolution with time of the ratio of real to apparent mass through theparameter η. When we compute η for rich star clusters, we findnon-monotonic evolution in time when the IMF stretches beyond a criticalcut-off mass of 25.5Msolar. We also monitor the rise ofcolour gradients between the inner and outer volume of clusters: we findtrends in time of the stellar IMF power indices overlapping well withthose derived for the Large Magellanic Cloud cluster NGC 1818 at an ageof 30Myr. We argue that the core region of massive Antennae clustersshould have suffered from much segregation despite their low ages. Weapply these results to a cluster mass function, and find that the peakof the mass distribution would appear to observers shifted to lowermasses by as much as 0.2dex. The star formation rate derived for thecluster population is then underestimated by from 20 to 50 per cent.
| 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.
| The Low-Mass Initial Mass Function of the Field Population in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations
We present V- and I-equivalent HST WFPC2 stellar photometry of an areain the Large Magellanic Cloud (LMC), located to the west of the bar ofthe galaxy, which accounts for the general background field of its innerdisk. The WFPC2 observations reach magnitudes as faint as V=25 mag, andthe large sample of more than 80,000 stars allows us to determine indetail the present-day mass function (PDMF) of the detectedmain-sequence stars, which is identical to the initial mass function(IMF) for masses M<~1 Msolar. The low-mass main-sequencemass function of the LMC field is found not to have a uniform slopethroughout the observed mass range; i.e., the slope does not follow asingle power law. This slope changes at about 1 Msolar tobecome more shallow for stars with smaller masses down to the lowestobserved mass of ~0.7 Msolar, giving clear indications offlattening for even smaller masses. We verified statistically that forstars with M<~1 Msolar the IMF has a slope Γ around-2, with an indicative slope Γ~=-1.4 for0.7<~M/Msolar<~0.9, while for more massive stars themain-sequence mass function becomes much steeper with Γ~=-5. Themain-sequence luminosity function (LF) of the observed field is in verygood agreement with the Galactic LF as it was previously found. Takinginto account several assumptions concerning evolutionary effects, whichshould have changed through time the stellar content of the observedfield, we reconstruct qualitatively its IMF for the whole observed massrange (0.7<~M/Msolar<~2.3), and we find that the numberof observed evolved stars is not large enough to have affectedsignificantly the form of the IMF, which thus is found almost identicalto the observed PDMF.
| 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.
| 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.
| From young massive star cluster to old globular: the LV-σ0 relationship as a diagnostic tool
We present a new analysis of the properties of the young massive starclusters (YMCs) forming profusely in intense starburst environments,which demonstrates that these objects are plausible progenitors of theold globular clusters (GCs) seen abundantly in the Local Group. Themethod is based on the tight relationship for old GCs between theirV-band luminosities, LV, and (central) velocity dispersions,σ0. We improve the significance of the relationship byincreasing the GC sample size and find that its functional form,LV/Lsolar~σ1.57+/-0.100(km s-1), is fully consistent with previous determinationsfor smaller Galactic and M31 GC samples. The tightness of therelationship for a GC sample drawn from environments as diverse as thosefound in the Local Group implies that its origin must be sought inintrinsic properties of the GC formation process itself. We evolve theluminosities of those YMCs in the local Universe which have velocitydispersion measurements to an age of 12 Gyr, adopting a variety ofinitial mass function (IMF) descriptions, and find that most YMCs willevolve to loci close to, or to slightly fainter luminosities than theimproved GC relationship. In the absence of significant externaldisturbances, this implies that these objects may potentially survive tobecome old GC-type objects over a Hubble time. The main advantage of ournew method is its simplicity. Whereas alternative methods, based ondynamical mass estimates, require one to obtain accurate size estimatesand to make further assumptions, the only observables required here arethe system's velocity dispersion and luminosity. The most importantfactor affecting the robustness of our conclusions is the adopted formof the IMF. We use the results of N-body simulations to confirm thatdynamical evolution of the clusters does not significantly alter ourconclusions about the likelihood of individual clusters surviving tolate times. Finally, we find that our youngest observed clusters areconsistent with having evolved from a relation of the form . Thisrelation may actually correspond to the origin of the GC fundamentalplane.
| 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.
| 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.
| Evolutionary stellar population synthesis at high spectral resolution: optical wavelengths
We present the single stellar population (SSP) synthesis results of ournew synthetic stellar atmosphere models library with a spectral samplingof 0.3 Å, covering the wavelength range from 3000 to 7000Åfor a wide range of metallicities (twice solar, solar, half solarand 1/10 solar). The stellar library is composed of 1650 spectracomputed with the latest improvements in stellar atmospheres. Inparticular, it incorporates non-local thermodynamic equilibrium (LTE)line-blanketed models for hot (Teff>= 27500 K), and LTEline-blanketed models (Phoenix) for cool (3000 <=Teff<=4500 K) stars. Because of the high spectral resolution of this library,evolutionary synthesis models can be used to predict the strength ofnumerous weak absorption lines and the evolution of the profiles of thestrongest lines over a wide range of ages. The SSP results have beencalculated for ages from 1 Myr to 17 Gyr using the stellar evolutionarytracks provided by the Geneva and Padova groups. For young stellarpopulations, our results have a very detailed coverage ofhigh-temperature stars with similar results for the Padova and Genevaisochrones. For intermediate and old stellar populations, our results,once degraded to a lower resolution, are similar to the ones obtained byother groups (limitations imposed by the stellar evolutionary physicsnotwidthstanding). The limitations and advantages of our models for theanalysis of integrated populations are described. The full set of thestellar library and the evolutionary models are available for retrievalat the websites http://www.iaa.csic.es/~rosa andhttp://www.iaa.csic.es/~mcs/sed@, or on request from the first twoauthors.
| 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.
| 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.
| The astrophysics of cool white dwarfs
Electronic Article Available from Elsevier Science.
| 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.
| 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.
| 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
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