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Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| 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.
| Blue Stragglers in Low-Luminosity Star Clusters
We examine the blue straggler populations of 13 low-luminosity(MVt>~-6) globular clusters and two old openclusters. These clusters test blue straggler formation in environmentsintermediate between higher luminosity (and usually higher density)clusters and the Galactic field. The anticorrelation between therelative frequency of blue stragglers(FBSS=NBSS/NHB) and cluster luminositycontinues to the lowest luminosity clusters, which have frequenciesmeeting or exceeding that of field stars. In addition, we find that theanticorrelation between straggler frequency and central densitydisappears for clusters with density less than about 300LV,solar pc-3, although this appears to be anartifact of the correlation between cluster luminosity and centraldensity. We argue on observational (wide, eccentric binaries containingblue stragglers in M67, and the existence of very bright stragglers inmost of the clusters in our sample) and theoretical grounds that stellarcollisions still produce a significant fraction of the blue stragglersin low-luminosity star clusters, due to the long-term survival of widebinaries.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
| A Globular Cluster Metallicity Scale Based on the Abundance of Fe II
Assuming that in the atmospheres of low-mass, metal-poor red giantstars, one-dimensional models based on local thermodynamic equilibriumaccurately predict the abundance of iron from Fe II, we derive aglobular cluster metallicity scale based on the equivalent widths of FeII lines measured from high-resolution spectra of giants in 16 keyclusters lying in the abundance range-2.4<[Fe/H]II<-0.7. We base the scale largely on theanalysis of spectra of 149 giant stars in 11 clusters by the Lick-Texasgroup supplemented by high-resolution studies of giants in five otherclusters. We also derive ab initio the true distance moduli for certainkey clusters (M5, M3, M13, M92, and M15) as a means of setting stellarsurface gravities. Allowances are made for changes in the abundancescale if one employs (1) Kurucz models with and without convectiveovershooting to represent giant star atmospheres in place of MARCSmodels and (2) the Houdashelt et al. color-temperature scale in place ofthe Alonso et al. scale.We find that [Fe/H]II is correlated linearly withW', the reduced strength of the near-infrared Ca II tripletdefined by Rutledge et al., although the actual correlation coefficientsdepend on the atmospheric model employed. The correlations, limited tothe range -2.4<[Fe/H]II<-0.7, are as follows:1.[Fe/H]II=0.531W'-3.279(MARCS),2.[Fe/H]II=0.537W'-3.225 (Kurucz withconvective overshooting),3.[Fe/H]II=0.562W'-3.329 (Kurucz withoutconvective overshooting).We also discuss how to estimate [X/Fe] ratios. We suggest that C, N, andO, as well as elements appearing in the spectrum in the singly ionizedstate, e.g., Ti, Sc, Ba, La, and Eu, should be normalized to theabundance of Fe II. Other elements, which appear mostly in the neutralstate, but for which the dominant species is nevertheless the ionizedstate, are probably best normalized to Fe I, but uncertainties remain.
| Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms
We present homogeneous age determinations for a large sample of 55Galactic globular clusters, which constitute about 30% of the totalGalactic population. A study of their age distribution reveals that allclusters from the most metal poor ones up to intermediate metallicitiesare coeval, whereas at higher [Fe/H] an age spread exists, together withan age-metallicity relationship. At the same time, all clusters within acertain galactocentric distance appear coeval, whereas an age spread ispresent further away from the Galactic centre, without any correlationwith distance. The precise value of [Fe/H] and galactocentric distancefor the onset of the age spread and the slope of the age-metallicityrelationship are strongly affected by the as yet uncertain [Fe/H] scale.We discuss how differences in the adopted [Fe/H] scale and clustersample size may explain discrepant results about the clusters agedistribution reached by different authors. Taking advantage of the largenumber of objects included in our sample, we also tested the possibilitythat age is the global second parameter which determines the HorizontalBranch morphology, and found indications that age could explain theglobal behaviour of the second parameter effect.
| Logarithmic Density Range and the Percentage of Stars in the Cores of Various Subsystems of the Globular Clusters M56, M12, NGC 6535, NGC 6171, NGC 5466, and M92
Lower limits for the percentages of stars with various luminosities inthe cores of six globular clusters are derived using stellar spatialdensity distributions f(r) to deep limiting B magnitudes obtainedearlier. For NGC 6535 and NGC 5466, the logarithmic density range andKholopov parameters D f and D r are also determined. These twoparameters are correlated with the mean masses of stars of varioussubsystems and the total mass (number) of stars in the cluster.
| Spatial Structure of the Old Open Cluster NGC 2420
The spatial structure of the intermediate and outer regions of the oldopen cluster NGC 2420 is analyzed using data from the catalog ofPaparó. The differential and integrated distributions of theprojected [ΔF(r) and F(r)] and spatial [Δf(r) and f(r)]stellar densities are obtained for various subsystems of cluster starsusing Kholopov’s star-count method. Analysis of these curves showsthat: (1) the cluster has at least three distinct spatial zones withdifferent stellar-density gradients, (2) the each cluster subsystem hasa layered structure, and (3) the spatial structure of the clusterchanges systematically in the transition to subsystems containingfainter main-sequence stars. Empirical relations describing thestructure of the cluster are also derived. Similar structural featurescan also be found in other globular and open clusters.
| Spatial Structure of the Globular Cluster M15
The spatial structure of the metal-poor globular cluster M15 is studiedas a function of magnitude interval ΔB and the limiting Bmagnitude of star counts. Astrometric and photometric measurements oftwo plates obtained with the 2-m reflector of the National Academy ofSciences of Bulgaria were used. Analysis of the differential andintegrated apparent (ΔF(r) and F(r)) and spatial (Δf(r) andf(r)) stellar density distributions in different ΔB intervals andto different limiting magnitudes reaching B=21.5m indicates that thecluster structure changes systematically as we consider fainter stars,beginning with the transition region between the subgiant branch and themain-sequence turnoff in the (V, B-V) diagram. This variation ismanifest in two ways: a homological growth in the radii of spatial zonesof the cluster and of the cluster radius in accordance with a singlelaw, and variations in the stellar density, with the rate of increase inthe number of faint stars growing toward the outer zones of the cluster.Empirical relations describing these variations and parametersdetermining the cluster structure are obtained.
| Photometric Investigation of the Galaxy in the Direction of Serpens Cauda. A Catalog of Extinctions and Distances
A catalog of spectral types, color excesses, interstellar extinctionsand distances of 402 stars located in the Serpens Cauda dark cloudcomplex and the new results of photoelectric photometry in the Vilniussystem of 56 fainter stars in the same area are presented.
| Horizontal-Branch Morphology and Dense Environments: Hubble Space Telescope Observations of Globular Clusters NGC 2298, 5897, 6535, and 6626
HST observations of the core of four galactic globular clusters allowedus to obtain interesting clues about the influence of the environment onthe star distribution along the horizontal branch. The chemicalcomposition and the age of the target clusters have been derived througha detailed analysis of the (V,V-I) color-magnitude diagrams. Thephotometric metallicities have been found in close agreement with therecent spectroscopic determinations. The four clusters are coeval withinthe uncertainties. Hence, age and metallicity cannot account for thedifference in the horizontal-branch morphologies, as shown by acomparison with theoretical zero-age horizontal-branch sequences andsynthetic horizontal branches taken from the literature. Our analysisfurther supports the hypothesis according to which dense environmentscause the occurrence of the so-called ``blue tails'' in the horizontalbranches of galactic globular clusters. The use of HST, as well as ofground-based data for some clusters, enabled us to study the radialdistribution of the various evolved populations, starting from the verycenter. We thus discovered that blue stragglers are, in general, moreconcentrated while the horizontal and the red giant branch do notdisplay a markedly different trend with the distance from the clustercenter, although the red giant branch of NGC 6626 is more concentratedthan the horizontal branch. Based on observations with the NASA/ESAHubble 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, and onobservations obtained at the European Southern Observatory at La Silla,Chile.
| Globular Cluster Subsystems in the Galaxy
Data from the literature are used to construct a homogeneous catalog offundamental astrophysical parameters for 145 globular clusters of theMilky Way Galaxy. The catalog is used to analyze the relationshipsbetween chemical composition, horizontal-branch morphology, spatiallocation, orbital elements, age, and other physical parameters of theclusters. The overall globular-cluster population is divided by a gap inthe metallicity function at [Fe/H]=-1.0 into two discrete groups withwell-defined maxima at [Fe/H]=-1.60±0.03 and -0.60±0.04.The mean spatial-kinematic parameters and their dispersions changeabruptly when the metallicity crosses this boundary. Metal-poor clustersoccupy a more or less spherical region and are concentrated toward theGalactic center. Metal-rich clusters (the thick disk subsystem), whichare far fewer in number, are concentrated toward both the Galacticcenter and the Galactic plane. This subsystem rotates with an averagevelocity of V rot=165±28 km/s and has a very steep negativevertical metallicity gradient and a negligible radial gradient. It is,on average, the youngest group, and consists exclusively of clusterswith extremely red horizontal branches. The population ofspherical-subsystem clusters is also inhomogeneous and, in turn, breaksup into at least two groups according to horizontal-branch morphology.Clusters with extremely blue horizontal branches occupy a sphericalvolume of radius 9 kpc, have high rotational velocities (Vrot=77±33 km/s), have substantial and equal negative radial andvertical metallicity gradients, and are, on average, the oldest group(the old-halo subsystem). The vast majority of clusters withintermediate-type horizontal branches occupy a more or less sphericalvolume ≈18 kpc in radius, which is slightly flattened perpendicularto the Z direction and makes an angle of ≈30° to the X-axis. Onaverage, this population is somewhat younger than the old-halo clusters(the young-halo subsystem), and exhibits approximately the samemetallicity gradients as the old halo. As a result, since theirGalactocentric distance and distance from the Galactic plane are thesame, the young-halo clusters have metallicities that are, on average,Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. Theyoung-halo subsystem, which apparently consists of objects captured bythe Galaxy at various times, contains many clusters with retrogradeorbits, so that its rotational velocity is low and has large errors, Vrot=-23±54 km/s. Typical parameters are derived for all thesubsystems, and the mean characteristics of their member globularclusters are determined. The thick disk has a different nature than boththe old and young halos. A scenario for Galactic evolution is proposedbased on the assumption that only the thick-disk and old-halo subsystemsare genetically associated with the Galaxy. The age distributions ofthese two subsystems do not overlap. It is argued that heavy-elementenrichment and the collapse of the proto-Galactic medium occurred mainlyin the period between the formation of the old-halo and thick-disksubsystems.
| Photometric catalog of nearby globular clusters. II. A large homogeneous (V,I) color-magnitude diagram data-base
In this paper we present the second and final part of a large andphotometrically homogeneous CCD color-magnitude diagram (CMD) data base,comprising 52 nearby Galactic globular clusters (GGC) imaged in the Vand I bands. The catalog has been collected using only two telescopes(one for each hemisphere). The observed clusters represent 75% of theknown Galactic globulars with (m-M)_V<= 16.15 mag, cover most of theglobular cluster metallicity range (-2.2 <= [Fe/H]<= -0.4), andspan Galactocentric distances from ~ 1.2 to ~ 18.5 kpc. In particular,here we present the CMDs for the 13 GGCs observed in the Northernhemisphere. The remaining 39 Southern hemisphere clusters of the cataloghave been presented in a companion paper (Rosenberg et al.\cite{rosenberg00}). We present the first CCD color magnitude diagramfor NGC 6779 (M 56). All the CMDs extend from at least ~ 2 magnitudesbelow the turn-off (i.e. V_lim>=22) to the tip of the red giantbranch. The calibration has been done using a large number of standardstars, and the absolute calibration is reliable to a ~ 0.02 mag level inboth filters. This catalog, because of its homogeneity, is expected torepresent a useful data base for the measurement of the main absoluteand relative parameters characterizing the CMD of GGCs. Based onobservations made with the 1~m Jacobus Kapteyn Telescope operated on theisland of La Palma by the ING in the Spanish Observatorio del Roque deLos Muchachos of the Instituto de Astrofìsica de Canarias.
| Tidal tails around 20 Galactic globular clusters. Observational evidence for gravitational disk/bulge shocking
Large-field multi-color images of 20 galactic globular clusters are usedto investigate the presence of tidal tails around these stellar systems.Field and cluster stars are sorted with the help of color-magnitudediagrams, and star-count analysis is performed on the selected clusterstars in order to increase the signal-to-noise ratio of their surfacedensity. We study the overdensities of these stars using the wavelettransform of the star counts in order to filter the background densitynoise and to detect the weak structures, at large scale, formed by thenumerous stars previously members of the clusters. We associate thesestellar overdensities with the stars evaporated from the clustersbecause of dynamical relaxation and/or tidal stripping from the clustersby the galactic gravitational field. We take into account the strongobservational biases induced by the clustering of galactic field starsand of background galaxies, along with the fluctuations of thebackground due to dust extinction. Most of the globular clusters in oursample display strong evidence of tidal interactions with the galacticplane in the form of large and extended deformations. These tidal tailsexhibit projected directions preferentially towards the galactic center.All the clusters observed, which do not suffer from strong observationalbiases, present such tidal tails, tracing their dynamical evolution(evaporation, tidal shocking, tidal torquing, and bulge shocking) in theGalaxy. The clusters exhibit different regimes of mass loss rate,detected using the radial density slope in the outer parts of theclusters. For NGC 5139 (omega Centauri), we estimate, taking intoaccount the possible presence of mass segregation in its outer parts,that about 0.6 to 1% of its mass has been lost during the current diskshocking event. In the case of NGC 6254, we tentatively estimate, in thecluster reference frame, for the radial diffusion velocity of the starsstripped, a value of the order of the velocity dispersion in the clusteritself. The sizes and orientations of these observed tidal tails areperfectly reproduced by N-body simulations of globular clusters in thegalactic potential well. We present these results in a companion paper(Combes et al. 1999). As a by-product of this study, we detect severalnew galaxy clusters towards the different fields studied at highgalactic latitude. The estimation of the tidal radius of some of theglobular clusters could have been overestimated because of these galaxyclusters.Based on observations made at the European Southern Observatory, LaSilla, ChilePlate scanning done with the MAMA (Machine Automatique à Mesurerpour l'Astronomie), a facility developed and operated by the INSU(Institut National des Sciences de l'Univers) at the Observatoire deParis, FranceFigs.~9, 11, 15--18, 22--24 are only available electronically with theOn-Line publication athttp://link.springer.de/link/service/journals/00230/
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way
Not Available
| Galactic Globular Cluster Relative Ages
Based on a new large, homogeneous photometric database of 34 Galacticglobular clusters (GGCs plus Palomar 12), a set of distance- andreddening- independent relative age indicators has been measured. Theobserved δ(V-I)2.5 andΔVHBTO versus metallicity relations havebeen compared with the relations predicted by two recently updatedlibraries of isochrones. Using these models and two independent methods,we have found that self-consistent relative ages can be estimated forour GGC sample. In turn, this demonstrates that the models areinternally self-consistent. Based on the relative age versus metallicitydistribution, we conclude that (1) there is no evidence of an age spreadfor clusters with [Fe/H]<-1.2, all the clusters of our sample in thisrange being old and coeval; (2) for the intermediate-metallicity group(-1.2<=[Fe/H]<-0.9), there is a clear evidence of age dispersion,with clusters up to ~25% younger than the older members; and (3) theclusters within the metal-rich group ([Fe/H]>=-0.9) seem to be coevalwithin the uncertainties (except Pal 12) but younger (~17%) than thebulk of the GGCs. The latter result is totally model dependent. From theGalactocentric distribution of the GGC ages, we can divide the GGCs intwo groups: the old, coeval clusters and the young clusters. The secondgroup can be divided into two subgroups: the ``really young clusters''and the ``young, but model dependent,'' which are within theintermediate- and high-metallicity groups, respectively. From thisdistribution, we can present a possible scenario for the Milky Way'sformation: The globular cluster formation process started at the samezero age throughout the halo, at least out to ~20 kpc from the Galacticcenter. According to the present stellar evolution models, themetal-rich clusters are formed at a later time (~17% lower age).Finally, significantly younger halo GGCs are found at anyRGC>8 kpc. For these, a possible scenario associated withmergers of dwarf galaxies to the Milky Way is suggested.
| The Giant, Horizontal, and Asymptotic Branches of Galactic Globular Clusters. I. The Catalog, Photometric Observables, and Features
A catalog including a set of the most recent color-magnitude diagrams(CMDs) is presented for a sample of 61 Galactic globular clusters(GGCs). We used this database to perform a homogeneous systematicanalysis of the evolved sequences (namely, the red giant branch [RGB],horizontal branch [HB], and asymptotic giant branch [AGB]). Based onthis analysis, we present (1) a new procedure to measure the level ofthe zero-age horizontal branch (V_ZAHB) and a homogeneous set ofdistance moduli obtained by adopting the HB as standard candle; (2) anindependent estimate for RGB metallicity indicators and new calibrationsof these parameters in terms of both spectroscopic ([Fe/H]_CG97) andglobal metallicity ([M/H], including also the α-elementenhancement), such that the set of equations presented can be used tosimultaneously derive a photometric estimate of the metal abundance andthe reddening from the morphology and the location of the RGB in the(V,B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and theAGB bump (in nine GGCs). The dependence of these features on metallicityis discussed. We find that by using the latest theoretical models andthe new metallicity scales, the earlier discrepancy between theory andobservations (~0.4 mag) completely disappears.
| Suitability of the King law for the globular clusters NGC 6171, NGC 5466, and M92
We have studied the suitability of a King law for the distributions ofthe apparent stellar density in the three globular clusters NGC 6171,NGC 5466, and M92 based on star counts to various limiting stellarmagnitudes B. The density distributions cannot be adequately describedby the King law. The parameters r_t (the tidal radius of the cluster)and r_c (the core radius) depend on the limiting B magnitude for countsof faint cluster stars below the transition region between the subgiantbranch and the main-sequence turnoff in the (V, B_V) diagram. Variationof these parameters is similar to variation of the sizes of the clusterspatial zones introduced by Kholopov.
| The spatial structure of the globular cluster M92
We have studied the spatial structure of the globular cluster M92 as afunction of various intervals of stellar magnitude Delta B and limitingmagnitudes B used in star counts. We used astrometric and photometricmeasurements of a photographic plate obtained with the 2-m reflector ofthe National Astronomical Observatory of Bulgaria. Differential andintegrated apparent (Delta F(r) and F(r)) and spatial (Delta f(r) andf(r)) stellar-density functions were considered for the intervals DeltaB = (13 -15.1^m), (15.1-16^m), (16 -17^m), (17-18^m), (18 -18.5^m),(18.5 -19^m), (19 -19.5^m), (19.5 - 20^m), (20 - 20.5^m), and (20.5 -21^m) and for the limiting magnitudes B = 15.1, 16, 17, 18, 18.5, 19,19.5, 20, 20.5, and 21^m. Analysis of these functions indicates that thestructure of the cluster varies systematically as we proceed towardfainter stars, starting from the transition region between the subgiantbranch to the turnoff point of the main sequence in the (V, B -V)diagram. This variation is twofold. First, the radii of spatial zones ofthe cluster and the radius of the cluster as a whole homologouslyincrease according to the same law. Second, in the transition to theouter zones of the cluster, the rate of increase of the number of faintstars increases with B. We derived empirical relationships describingthese variations, together with parameters determining the structure ofthe cluster.
| Calibration of the Faber--Jackson relation for M31 globular clusters using HIPPARCOS data
In this paper we present data analysis regarding globular clusters aspossible extragalactic distance indicators. For this purpose, wecollected all velocity dispersion measurements published for Galacticand M31 globular clusters. The slope and zero-point of theFaber--Jackson relation were calibrated using Hipparcos distancemeasurements, and the relation was applied to extragalactic globularclusters in M31. A distance modulus of 24.12 +/- 0.45 mag was found.This is consistent with those found by fitting the red giant branches ofglobular clusters (24.47 +/- 0.07) and from the peak of the globularcluster luminosity function (24.03 +/- 0.23), but is lower than thevalues of 24.84 +/- 0.2 mag and 24.77 +/- 0.11 mag obtained by usingHipparcos data to calibrate the Cepheid period--luminosity relation.This calibrated Faber--Jackson relation can now be used directly forother Sc galaxies with resolved globular clusters, as soon as largenumbers of spectra become available, e.g. through the Very LargeTelescope (VLT).
| WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud
We present our analysis of archival Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) observations in F450W (~B) and F555W (~V) ofthe intermediate-age populous star clusters NGC 121, NGC 339, NGC 361,NGC 416, and Kron 3 in the Small Magellanic Cloud. We use publishedphotometry of two other SMC populous star clusters, Lindsay 1 andLindsay 113, to investigate the age sequence of these seven starclusters in order to improve our understanding of the formationchronology of the SMC. We analyzed the V versus B-V and M_V versus(B-V)_0 color-magnitude diagrams of these populous Small MagellanicCloud star clusters using a variety of techniques and determined theirages, metallicities, and reddenings. These new data enable us to improvethe age-metallicity relation of star clusters in the Small MagellanicCloud. In particular, we find that a closed-box continuous starformation model does not reproduce the age-metallicity relationadequately. However, a theoretical model punctuated by bursts of starformation is in better agreement with the observational data presentedherein.
| The Age of Globular Clusters in Light of Hipparcos: Resolving the Age Problem?
We review five independent techniques that are used to set the distancescale to globular clusters, including subdwarf main-sequence fittingutilizing the recent Hipparcos parallax catalog. These data together allindicate that globular clusters are farther away than previouslybelieved, implying a reduction in age estimates. We now adopt a best-fitvalue Mv (RR Lyrae stars) = 0.39 +/- 0.08 (statistical) at [Fe/H] = -1.9with an additional uniform systematic uncertainty of ^{+0.13}_{-0.18} .This new distance scale estimate is combined with a detailed numericalMonte Carlo study (previously reported by Chaboyer et al.) designed toassess the uncertainty associated with the theoretical age-turnoffluminosity relationship in order to estimate both the absolute age anduncertainty in age of the oldest globular clusters. Our best estimatefor the mean age of the oldest globular clusters is now 11.5 +/- 1.3Gyr, with a one-sided 95% confidence level lower limit of 9.5 Gyr. Thisrepresents a systematic shift of over 2 sigma compared to our earlierestimate, owing completely to the new distance scale--a shift which weemphasize results not only from the Hipparcos data. This now provides alower limit on the age of the universe that is consistent with either anopen universe or with a flat matter-dominated universe (the latterrequiring H0 <= 67 km s-1 Mpc-1). Our new study also explicitlyquantifies how remaining uncertainties in the distance scale and stellarevolution models translate into uncertainties in the derived globularcluster ages. Simple formulae are provided that can be used to updateour age estimate as improved determinations for various quantitiesbecome available. Formulae are also provided that can be used to derivethe age and its uncertainty for a globular cluster, given the absolutemagnitude of the turnoff or the point on the subgiant branch 0.05 magredder than the turnoff.
| On the calibration of Red Giant Branch metallicity indicators in globular clusters: new relations based on an improved abundance scale
We present an improved calibration of photometric metallicityindicators, derived from the new metallicity scale for Globular Clusterspresented by Carretta & Gratton (1997) and based on direct highresolution spectroscopy of 160 stars in 24 globular clusters. We havecarefully recalibrated the traditional abundance indices based upon thered giant branch (RGB) morphology, both in the V,B-V and V,V-I planes,namely the dereddened colour at the luminosity level of the horizontalbranch (HB), and the magnitude difference between the HB and the RBG ata given dereddened colour. Finally, we give new accurate relations toemploy in the Simultaneous Metallicity Reddening method by Sarajedini(1994), also tied to the Carretta & Gratton (1997) abundance scale.
| The Relative Ages of Galactic Globular Clusters
We present a review of the present state of knowledge regarding therelative ages of Galactic globular clusters. First, we discuss therelevant galaxy formation models and describe the detailed predictionsthey make with respect to the formation timescale and chemical evolutionof the globular clusters. Next, the techniques used to estimate globularcluster ages are described and evaluated with particular emphasis on theadvantages and disadvantages of each method. With these techniques as afoundation, we present arguments in favor of the following assertions:1) The age of a globular cluster is the likeliest candidate to be theglobal second parameter, which along with metal abundance, controls themorphology of the horizonal branch. 2) A total age range of as much as ~5 Gyr exists among the bulk of the Galactic globulars. 3) There is asignificant relation between age and metallicity among the Galacticglobular clusters if the slope of the \mvrr-\feh\ relation is less than~ 0.23. These conclusions along with other supporting evidence favor aformation scenario in which the inner regions of the Galactic halocollapsed in a monotonic fashion over a short time period much less than1 Gyr. In contrast, the outer regions of the halo fragmented andcollapsed in a chaotic manner over several Gyrs. (SECTION: InvitedReview Articles)
| Standard Stars for CCD Photometry in the Vilnius System
The results of seven-color photometry of 73 stars of magnitudes 9--13are given in 11 areas. The areas are of 10times 10 arcmin size at lowgalactic latitudes in the transparent parts of the Milky Way in theconstellations of Serpens, Aquila, Vulpecula, Lyra and Cygnus. Thesestars can be used as zero-point standards for future CCD photometry.
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