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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
| 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.
| Red Giant Stars in the Large Magellanic Cloud Clusters
We present deep J, H, and Ks photometry and accurate colormagnitude diagrams down to K~18.5 for a sample of 13 globular clustersin the Large Magellanic Cloud. This data set combined with the previoussample of six clusters published by our group gives the opportunity tostudy the properties of giant stars in clusters with different ages(ranging from ~80 Myr up to 3.5 Gyr). Quantitative estimates of starpopulation ratios (by number and luminosity) in the asymptotic giantbranch (AGB), the red giant branch (RGB), and the He clump have beenobtained and compared with theoretical models in the framework ofprobing the so-called phase transitions. The AGB contribution to thetotal luminosity starts to be significant at ~200 Myr and reaches itsmaximum at 500-600 Myr, when the RGB phase transition is starting. At~900 Myr the full development of an extended and well-populated RGB hasbeen completed. The occurrences of both the AGB and RGB phasetransitions are sharp events, lasting a few hundred megayears only.These empirical results agree very well with the theoretical predictionsof simple stellar population models based on canonical tracks and thefuel-consumption approach.Based on observations collected at the European Southern Observatory, LaSilla, Chile, using SOFI at the 3.5 m NTT, within the observing programs64.N-0038 and 68.D-0287.
| 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.
| 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.
| 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.
| 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.
| 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.
| Population and dynamical state of the η Chamaeleontis sparse young open cluster
We report new results in our continuing study of the unique compact (1pc extent), nearby (d= 97 pc), young (t= 9 Myr) stellar clusterdominated by the B9 star η Chamaeleontis. An optical photometricsurvey spanning 1.3 × 1.3 pc adds two M5-M5.5 weak-lined T Tauri(WTT) stars to the cluster inventory which is likely to be significantlycomplete for primaries with masses M > 0.15 Msolar. Thecluster now consists of 17 primaries and ~=9 secondaries lying within100 au of their primaries. The apparent distribution of 9:7:1single:binary:triple systems shows 2-4 times higher multiplicity than inthe field main-sequence stars, and is comparable to that seen in otherpre-main-sequence populations. The initial mass function (IMF) isconsistent with that of rich young clusters and field stars. Byextending the cluster IMF to lower masses, we predict 10-14 additionallow-mass stars with 0.08 < M < 0.15 Msolar and 10-15brown dwarfs with 0.025 < M < 0.08 Msolar remain to bediscovered. The η Cha cluster extends the established stellardensity and richness relationship for young open clusters. The radialdistribution of stars is consistent with an isothermal sphere, but masssegregation is present with >50 per cent of the stellar mass residingin the inner 6 arcmin (0.17 pc). Considering that the η Cha clusteris sparse, diffuse and young, the cluster may be an ideal laboratory fordistinguishing between mass segregation that is primordial in nature, orarising from dynamical interaction processes.
| 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.
| 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
| 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 Low End of the Initial Mass Function in Young Clusters. II. Evidence for Primordial Mass Segregation in NGC 330 in the Small Magellanic Cloud
As part of a larger program aimed at investigating the universality ofthe initial mass function (IMF) at low masses in a number of youngclusters in the LMC and SMC, we present a new study of the low end ofthe stellar IMF of NGC 330, the richest young star cluster in the SMC,from deep broadband V and I images obtained with HST/WFPC2. We detectstars down to a limiting magnitude of m555=24.9, whichcorresponds to stellar masses of ~0.8Msolar at the distanceof the SMC. A comparison of the cluster color-magnitude diagram withtheoretical evolutionary tracks indicates an age of ~30 Myr for NGC 330,in agreement with previous published results. We derive the clusterluminosity function, which we correct for background contamination usingan adjacent SMC field, and construct the mass function in the1-7Msolar mass range. Given the young cluster age, the MF canwell approximate the IMF. We find that the IMF in the central clusterregions (within 30") is well reproduced by a power law with a slopeconsistent with Salpeter's. In addition, the richness of the clusterallows us to investigate the IMF as a function of radial distance fromthe center. We find that the IMF becomes steeper at increasing distancesfrom the cluster center (between 30" and 90"), with the number ofmassive stars (>5Msolar) decreasing from the core to theoutskirts of the cluster 5 times more rapidly than the less-massiveobjects (~=1Msolar). We believe the observed mass segregationto be of a primordial nature rather than dynamical since the age of NGC330 is 10 times shorter than the expected relaxation time of thecluster. Based on observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated byAURA for NASA under contract NAS5-26555.
| 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.
| HUBBLE SPACE TELESCOPE Survey of Clusters in Nearby Galaxies. I. Detection and Photometry
We have developed photometric techniques that can be applied to imageswith highly variable backgrounds, as well as to slightly extendedobjects (object size comparable to or smaller than point-spread function[PSF] size). We have shown that ordinary stellar PSF-fitting photometrycan be applied to slightly extended objects provided that one applies asystematic correction to the photometry that is a function primarily ofthe observed sharpness. Applying these techniques to the Cepheid targetNGC 3627, we find that we are successfully able to photometer the starsand clusters, as well as discriminate the cluster population with anegligible number of false detections.
| M82-F: a doomed super star cluster?
We present high-dispersion echelle spectroscopy of the very luminous,young super star cluster (SSC) `F' in M82, obtained with the 4.2-mWilliam Herschel Telescope, for the purpose of deriving its dynamicalmass and assessing whether it will survive to become an old globularcluster. In addition to the stellar lines, the spectrum contains complexNai absorption and broad emission lines from the ionized gas. We measurea stellar velocity dispersion of 13.4+/-0.7kms-1 and aprojected half-light radius of 2.8+/-0.3pc from archival HST/WFPC2images, and derive a dynamical mass of1.2+/-0.1×106Msolar, demonstrating thatM82-F is a very massive, compact cluster. We determine that the currentluminosity-to-mass ratio (LV/M)solar for M82-F is45+/-13. Comparison with spectral synthesis models shows that(LV/M)solar is a factor of ~5 higher than thatpredicted for a standard Kroupa initial mass function (IMF) at thewell-determined age for M82-F of 60+/-20Myr. This high value of(LV/M)solar indicates a deficit of low-mass starsin M82-F; the current mass function (MF) evidently is `top-heavy'. Wefind that a lower mass cut-off of 2-3Msolar is required tomatch the observations for a MF with a slope α=2.3. Since thecluster apparently lacks long-lived low-mass stars, it will not becomean old globular cluster, but probably will dissolve at an age of <=2Gyr. We also derive updated luminosity-to-mass ratios for the youngerSSCs NGC 1569A and 1705-1. For the first object, the observations areconsistent with a slightly steeper MF (α=2.5), whereas for NGC1705-1 the observed ratio requires the MF to be truncated near2Msolar for a slope of α=2.3. We discuss theimplications of our findings in the context of large-scale IMFvariations; with the present data the top-heavy MF could reflect a localmass segregation effect during the birth of the cluster. M82-F probablyformed in a dense molecular cloud; however, its high radial velocitywith respect to the centre of M82 (~-175kms-1) suggests thatit is on an eccentric orbit and now far from its birthplace, so theenvironment of its formation is unknown.
| Jet-induced Star Formation in Centaurus A
The inner part of the northeast middle radio lobe of the radio galaxyCentaurus A is the site of complex interactions. This area contains alarge H I cloud as well as filaments of ionized gas and associated blueknots, several of which exist along the northeastern edge of theradio-emitting zones. We observed the filaments and blue knots with theHubble Space Telescope using WFPC2, and the ionized gas from the ground.Our sensitive, high angular resolution WFPC2 images reveal the presenceof young stars, many concentrated in what appear to be OB associations,superimposed on a background sheet of older stars that is typical of theCen A halo. The ages of the OB associations are estimated to be lessthan 15 Myr from a comparison of color-magnitude diagrams with those forthe Large Magellanic Cloud star cluster NGC 2004, and younger stellargroups may be nearer regions of Hα emission. We discuss our datain the context of models for star formation stimulated by interactionsbetween the radio jet and gas cloud. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.
| Synthetic Spectra of H Balmer and HE I Absorption Lines. II. Evolutionary Synthesis Models for Starburst and Poststarburst Galaxies
We present evolutionary stellar population synthesis models to predictthe spectrum of a single-metallicity stellar population, with a spectralsampling of 0.3 Å in five spectral regions between 3700 and 5000Å. The models, which are optimized for galaxies with active starformation, synthesize the profiles of the hydrogen Balmer series(Hβ, Hγ, Hδ, H8, H9, H10, H11, H12, and H13) and theneutral helium absorption lines (He I λ4922, He I λ4471,He I λ4388, He I λ4144, He I λ4121, He Iλ4026, He I λ4009, and He I λ3819) for a burst withan age ranging from 106 to 109 yr, and differentassumptions about the stellar initial mass function (IMF). Continuousstar formation models lasting for 1 Gyr are also presented. The inputstellar library includes non-LTE absorption profiles for stars hotterthan 25,000 K and LTE profiles for lower temperatures. The temperatureand gravity coverage is 4000 K<=Teff<=50,000 K and0.0<=logg<=5.0, respectively. The metallicity is solar. It is found thatthe Balmer and He I line profiles are sensitive to the age, exceptduring the first 4 Myr of the evolution, when the equivalent widths ofthese lines are constant. At these early stages of the evolution, theprofiles of the lines are also sensitive to the IMF. However, strong HBalmer and He I lines are predicted even when the low-mass cutoff of theIMF is as high as 10 Msolar. The equivalent widths of theBalmer lines range from 2 to 16 Å and those of the He I lines from0.2 to 1.2 Å. During the nebular phase (cluster younger than about10 Myr), Hβ ranges from 2 to 5 Å and He I λ4471 rangesbetween 0.5 and 1.2 Å. The strength of the lines is maximum whenthe cluster is a few hundred (for the Balmer lines) and a few tens (forthe He I lines) of Myr old. In the continuous star formation scenario,the strength of the Balmer and He I lines increases monotonically withtime until 500 and 100 Myr, respectively. However, the lines are weakerthan in the burst models owing to the dilution of the Balmer and He Ilines by the contribution from very massive stars. The high spectralresolution of the profiles is useful to reproduce the absorption wingsobserved in regions of recent star formation and to estimate the effectof the underlying absorption on the nebular emission lines. The strengthof the nebular emission Balmer and He I lines compared with the stellarabsorption components indicates that Hδ and the higher order termsof the Balmer series and He I are dominated by the stellar absorptioncomponent if an instantaneous burst is older than ~=5 Myr. Some of theHe I lines (e.g., He I λ3819 and He I λ4922) are morefavorable than others (e.g., He I λ4471) for the detection ofstellar features in the presence of nebular emission. We estimate thatthe correction to the He I λ4471 nebular emission line due to thestellar absorption is between 5% and 25%, if the nebular emission hasequivalent width between 10 and 2.5 Å (corresponding to a burstage between 1 and 3 Myr). The models can be used to date starburst andpoststarburst galaxies until 1 Gyr. They have been tested on data forclusters in the LMC, the super-star cluster B in the starburst galaxyNGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous``E+A'' galaxy. The full data set is available for retrieval at ourwebsites or on request from the authors.
| The Stellar Mass Spectrum in the Young Populous Cluster NGC 1866
The young populous cluster NGC 1866 in the Large Magellanic Cloud (LMC),which is probably one of the most massive objects formed in the LMCduring the last ~3 Gyr, appears to have an unexpectedly highmass-to-light ratio. From its velocity dispersion Fischer and coworkersfind its mass to be (1.35+/-0.25)x10^5 M_solar. The luminosity of thiscluster is M_v=-8.93+/-0.13, corresponding to L_v=(3.2+/-0.4)x10^5L_v,solar. This yields M/L_v=0.42+/-0.09 in solar units. For a clusterof age 0.1 Gyr, such a relatively high mass-to-light ratio requires amass spectrum with an exponent x=1.72+/-0.09, or x=1.75+/-0.09 if massloss by evolving stars is taken into account.
| Star Clusters in Local Group Galaxies
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| Astrophysics in 1998
From Alpha (Orionis and the parameter in mixing-length theory) to Omega(Centauri and the density of the universe), the Greeks had a letter forit. In between, we look at the Sun and planets, some very distantgalaxies and nearby stars, neutrinos, gamma rays, and some of theanomalies that arise in a very large universe being studied by roughlyone astronomer per 10^7 Galactic stars.
| The Stellar Initial Mass Function from Random Sampling in Hierarchical Clouds. II. Statistical Fluctuations and a Mass Dependence for Starbirth Positions and Times
Observed variations in the slope of the stellar initial mass function(IMF) are shown to be consistent with a previously introduced model inwhich the protostellar gas is randomly sampled from clouds with aself-similar hierarchical structure. Root mean square variations in theIMF slope around the Salpeter value are +/-0.4 when only 100 stars areobserved, and +/-0.1 when 1000 stars are observed. Similar variationsshould be present in other stochastic models as well. The hierarchicalsampling model reproduces the tendency for massive stars to form closerto the center of a cloud at a time somewhat later than the formationtime of the lower mass stars. The systematic variation in birth positionresults from the tendency for the trunk and larger branches of thehierarchical tree of cloud structure to lie closer to the cloud center,while the variations in birth order result from the relative infrequencyof stars with larger masses. The hierarchical cloud sampling model hasnow reproduced most of the reliably observed features of the clusterIMF. The power-law part of the IMF comes from cloud hierarchicalstructure that is sampled during various star formation processes with arelative rate proportional to the square root of the local density.These processes include turbulence compression, magnetic diffusion,gravitational collapse, and clump or wavepacket coalescence, all ofwhich have about this rate dependence. The low-mass flattening comesfrom the inability of gas to form stars below the thermal Jeans mass attypical temperatures and pressures. The thermal Jeans mass is the onlyrelevant scale in the problem. Considerations of heating and coolingprocesses indicate why the thermal Jeans mass should be nearly constantin normal environments and why this mass might increase in starburstregions. In particular, the relative abundance of high-mass stars shouldincrease where the average density of the interstellar medium is verylarge; accompanying this increase should be an increase in the averagetotal efficiency of star formation. Alternative models in which the rateof star formation is independent of density and the local efficiencydecreases systematically with increasing stellar mass can also reproducethe IMF, but this is an adjustable result and not a fundamental propertyof hierarchical cloud structure, as is the preferred model. The steepIMF in the extreme field is not explained by the model, but otherorigins are suggested, including one in which massive stars inlow-pressure environments halt star formation in their clouds. In thiscase, the slope of the extreme field IMF is independent of the slope ofeach component cluster IMF and is given by (gamma-1)/alpha for a cloudmass function slope, -gamma~-2, and a power-law relation, M_L~M^alpha_c,between the largest star in a low-pressure cloud, M_L, and the cloudmass, M_c. A value of alpha~1/4 is required to explain the extreme fieldIMF as a superposition of individual cluster IMFs; cloud destruction byionizing has this property. We note that the similarity between clusterIMFs and the average IMF from global studies of galaxies implies thatmost stars form in clusters and that massive stars do not generally haltstar formation in the same cloud.
| Collapse and evolution of flattened star clusters
We investigate the dynamical evolution of point-mass systems startingfrom spheroidal geometry. Such structures may arise as a result ofviolent compression which results from collisions between clouds ofinterstellar gas. We use N-body calculations to seek out traces of theinitial conditions in their relaxed structures. For initially cold andthick, or thin but hot, systems, the phase of collapse is well describedby adiabatic scaling: we show that the axis ratio of stellar clustersgrows ~ R^-1/3 during infall, where R is the cluster cylindrical radius.The adiabatic track provides an adequate description of the collapse,provided only that the initial scaleheight of the spheroid, h, is wellresolved by the stars: when the mean interparticle distance is largerthan h, we find that two-body effects develop rapidly over a singlecrossing time. Following the phase of collapse, the clusters `bounce'and quickly establish a core--halo structure. The central region relaxesto near-spherical shape in a few crossing times; however, the enveloperemains more elliptical, since two-body effects are reduced there. Thisellipticity changes only slightly from its value at the time of thebounce over several crossing times of evolution. As cluster morphologyat the bounce is deduced from adiabatic invariance, itself fixed by theinitial conditions, we relate relaxed configurations to initialconditions through a simple analytic expression.
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
| The evolution of theV-Kcolours of single stellar populations
Models of evolutionary population synthesis of galaxies rely on theproperties of the so-called single stellar populations (SSP). In thispaper, we discuss how the integrated near-infrared colours, andespecially V-K, of SSPs evolve with age and metallicity. Some of theuncertainties associated with the properties of the underlying stellarmodels are thoroughly discussed. Our models include all the relevantstellar evolutionary phases, with particular attention being dedicatedto the asymptotic giant branch (AGB), which plays a fundamental role inthe evolution of the near-infrared part of the spectrum. First, wepresent the effects that different formulations for the mass-loss ratesproduce on the final remnant mass (i.e., on the initial-final massrelation), and hence on the AGB-termination luminosity and the relativecontribution of these stars to the integrated light. The results for theevolution of the V-K colour are very different depending on the choiceof the mass-loss prescription; the same is true also for the B-V colourin the case of low-metallicity SSPs. Secondly, we describe the changesoccurring in the integrated colours at the onset of the AGB and redgiant (RGB) branches. According to the classical formalism for the AGBevolution, the onset of this evolutionary phase is marked by a colourjump to the red, the amplitude of which is shown here to be highlydependent on the metallicity and mass-loss rates adopted in the models.We then consider the effect of the overluminosity with respect to thestandard core mass-luminosity relation that occurs in the most massiveAGB stars. Different simplified formulations for this effect are testedin the models; they cause a smoothing of the colour evolution in the agerange at which the AGB starts to develop, rather than a splitting of thecolour jump into two separate events. On the other hand, we find that atemporary red phase takes place ~1.5x10^8 yr after the RGB develops.Thanks to the transient nature of this feature, the onset of the RGB isprobably not able to cause marked features in the spectral evolution ofgalaxies. We then discuss the possible reasons for the transition of V-Kcolours (from ~1.5 to 3) that takes place in LMC clusters of SWB typeIV. A revision of the ages attributed to the single clusters revealsthat the transition may not be as fast as originally suggested. Thecomparison of the data with the models indicates that the transitionresults mainly from the development of the AGB. A gradual (or delayed)transition of the colours, as predicted by models which include theoverluminosity of the most massive AGB stars, seems to describe the databetter than the sudden colour jump predicted by classical models.
| Keck Spectroscopy of Candidate 97Proto-Globular Clusters in NGC 1275
Keck spectroscopy of five proto-globular cluster candidates in NGC 1275has been combined with Hubble Space Telescope Wide Field PlanetaryCamera 2 photometry to explore the nature and origin of these objectsand discriminate between merger and cooling-flow scenarios for globularcluster formation. The objects we have studied are not H ii regions, butrather star clusters, yet their integrated spectral properties do notresemble young or intermediate-age Magellanic Cloud clusters or MilkyWay open clusters. The clusters' Balmer absorption appears to be toostrong to be consistent with any of the standard Bruzual & Charlotevolutionary models at any metallicity. If the Bruzual & Charlotmodels are adopted, an initial mass function (IMF) that is skewed tohigh masses provides a better fit to the data of the proto-globularcluster candidates. A truncated IMF with a mass range of 2-3 M_ȯreproduces the observed Balmer equivalent widths and colors at ~450 Myr.Formation in a continuous cooling flow appears to be ruled out since theage of the clusters is much larger than the cooling time, the spatialscale of the clusters is much smaller than the cooling-flow radius, andthe deduced star formation rate in the cooling flow favors a steeprather than a flat IMF. A merger would have to produce clusters only inthe central few kiloparsecs, presumably from gas in the merging galaxiesthat was channeled rapidly to the center. Widespread shocks in merginggalaxies cannot have produced these clusters. If these objects areconfirmed to have a relatively flat, or truncated, IMF, it is unclearwhether they will evolve into objects that we would regard as bona fideglobular clusters. Based on observations obtained at the W. M. KeckObservatory, which is operated jointly by the California Institute ofTechnology and the University of California.
| Mass segregation in young Large Magellanic Cloud clusters. I - NGC 2157
We have carried out Wide Field Planetary Camera 2 V- and I-band imagingof the young LMC cluster NGC 2157. Construction of a color-magnitudediagram and isochrone fitting yield an age of = 10 exp 8 yr, a reddeningE(B - V) = 0.1, and a distance modulus of 18.4 mag. Our data cover themass range 0.75-5.1 M(solar). We find that the cluster mass functionchanges significantly from the inner regions to the outer regions,becoming steeper at larger radii. The age of NGC 2157 is comparable toits two-body relaxation timescale only in the cluster core. The observedsteepening of the mass function at larger radii is therefore most likelyan initial condition of the cluster stars. Such initial conditions arepredicted in models of cluster star formation in which dissipativeprocesses act more strongly upon more massive stars.
| Cepheids in MC Clusters: New Observations
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