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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.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
| 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.
| On the horizontal branch of the galactic globular cluster NGC 2808
We present new UV (F218W) data for stars in the central region of theGalactic globular cluster NGC 2808, collected with the WFPC2 camera onboard the Hubble Space Telescope. These data together with F439W andF555W-band data and previous ground-based observations provide amultifrequency coverage of the cluster stellar population extending upto a distance of 1.7 times the cluster core radius. We discuss thiscomplete sample of stars, which includes 764 Red Giant Branch (RGB)stars brighter than the Horizontal Branch (HB) luminosity level, 1239 HBstars, 119 Asymptotic Giant Branch (AGB), and 22 AGB-manquéstellar structures. As already known, we find that blue HB starsseparate into three distinct groups. However, our multiband photometryindicates that several stars in the two hotter HB groups show a flatspectrum, thus suggesting the binarity of these objects. Artificial starexperiments suggest that at most 50% of them might be photometricblends. Moreover, at variance with previous claims one finds thatcanonical Zero Age Horizontal Branch (ZAHB) models do reach effectivetemperatures typical of observed hot HB stars. We also show that theratio between HB and RGB stars brighter than the HB luminosity levelsteadly increases when moving from the cluster center to the periphery,passing from R=1.37±0.14 in the cluster core toR=1.95±0.26 in the outer cluster regions. We discuss the possibleorigin of such a radial gradient in the context of the Blue Tailsphenomenon, advancing some suggestions concerning the clumpy stellardistribution along the HB.
| 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.
| 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.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way
We present an analysis of the asymmetries in the population of GalacticM-giant stars present in the 2MASS All Sky catalogue. Severallarge-scale asymmetries are detected, the most significant of which is astrong elliptical-shaped stellar overdensity, close to the Galacticplane at (l= 240°, b=-8°), in the constellation of Canis Major.A small grouping of globular clusters (NGC 1851, 1904, 2298 and 2808),coincident in position and radial velocity, surround this structure, asdo a number of open clusters. The population of M-giant stars in thisoverdensity is similar in number to that in the core of the Sagittariusdwarf galaxy. We argue that this object is the likely dwarf galaxyprogenitor of the ring-like structure that has recently been found atthe edge of the Galactic disc. A numerical study of the tidal disruptionof an accreted dwarf galaxy is presented. The simulated debris fits theextant position, distance and velocity information on the Galactic`Ring', as well as that of the M-giant overdensities, suggesting thatall these structures are the consequence of a single accretion event.The disrupted dwarf galaxy stream orbits close to the Galactic plane,with a pericentre at approximately the solar circle, an orbitaleccentricity similar to that of stars in the Galactic thick disc, aswell as a vertical scaleheight similar to that of the thick disc. Thisfinding strongly suggests that the Canis Major dwarf galaxy is abuilding block of the Galactic thick disc, that the thick disc iscontinually growing, even up to the present time, and that thick discglobular clusters were accreted on to the Milky Way from dwarf galaxiesin co-planar orbits.
| 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.
| Does the mixing length parameter depend on metallicity?. Further tests of evolutionary sequences using homogeneous databases
This paper is a further step in the investigation of the morphology ofthe color-magnitude diagram of Galactic globular clusters, and thefine-tuning of theoretical models, made possible by the recentobservational efforts to build homogeneous photometric databases. Inparticular, we examine here the calibration of the morphologicalparameter WHB vs. metallicity, originally proposed by Brocatoet al. (\cite{brocatoEtal98}; B98), which essentially measures the colorposition of the red-giant branch. We show that the parameter can be usedto have a first-order estimate of the cluster metallicity, since thedispersion around the mean trend with [Fe/H] is compatible with themeasurement errors. The tight WHB-[Fe/H] relation is thenused to show that variations in helium content or age do not affect theparameter, whereas it is strongly influenced by the mixing-lengthparameter alpha (as expected). This fact allows us, for the first time,to state that there is no trend of alpha with the metal content of acluster. A thorough examination of the interrelated questions of thealpha -elements enhancement and the color-Tefftransformations, highlights that there is an urgent need for anindependent assessment of which of the two presently acceptedmetallicity scales is the true indicator of a cluster's iron content.Whatever scenario is adopted, it also appears that a deep revision ofthe V-I-temperature relations is needed.
| An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way
One of the long-standing problems in modern astronomy is the curiousdivision of Galactic globular clusters, the ``Oosterhoff dichotomy,''according to the properties of their RR Lyrae stars. Here, we find thatmost of the lowest metallicity ([Fe/H] 〈 -2.0) clusters, whichare essential to an understanding of this phenomenon, display a planaralignment in the outer halo. This alignment, combined with evidence fromkinematics and stellar population, indicates a captured origin from asatellite galaxy. We show that, together with the horizontal-branchevolutionary effect, the factor producing the dichotomy could be a smalltime gap between the cluster-formation epochs in the Milky Way and thesatellite. The results oppose the traditional view that themetal-poorest clusters represent the indigenous and oldest population ofthe Galaxy.
| 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.
| On the Distribution of Orbital Poles of Milky Way Satellites
In numerous studies of the outer Galactic halo some evidence foraccretion has been found. If the outer halo did form in part or whollythrough merger events, we might expect to find coherent streams of starsand globular clusters following orbits similar to those of their parentobjects, which are assumed to be present or former Milky Way dwarfsatellite galaxies. We present a study of this phenomenon by assessingthe likelihood of potential descendant ``dynamical families'' in theouter halo. We conduct two analyses: one that involves a statisticalanalysis of the spatial distribution of all known Galactic dwarfsatellite galaxies (DSGs) and globular clusters, and a second, morespecific analysis of those globular clusters and DSGs for which fullphase space dynamical data exist. In both cases our methodology isappropriate only to members of descendant dynamical families that retainnearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) isconsidered a paradigm for the type of merger/tidal interaction event forwhich we are searching, we also undertake a case study of the Sgr systemand identify several globular clusters that may be members of itsextended dynamical family. In our first analysis, the distribution ofpossible orbital poles for the entire sample of outer(Rgc>8 kpc) halo globular clusters is tested forstatistically significant associations among globular clusters and DSGs.Our methodology for identifying possible associations is similar to thatused by Lynden-Bell & Lynden-Bell, but we put the associations on amore statistical foundation. Moreover, we study the degree of possibledynamical clustering among various interesting ensembles of globularclusters and satellite galaxies. Among the ensembles studied, we findthe globular cluster subpopulation with the highest statisticallikelihood of association with one or more of the Galactic DSGs to bethe distant, outer halo (Rgc>25 kpc), second-parameterglobular clusters. The results of our orbital pole analysis aresupported by the great circle cell count methodology of Johnston,Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC6229, NGC 7006, and Pyxis are predicted to be among those most likely toshow the clusters to be following stream orbits, since these clustersare responsible for the majority of the statistical significance of theassociation between outer halo, second-parameter globular clusters andthe Milky Way DSGs. In our second analysis, we study the orbits of the41 globular clusters and six Milky Way-bound DSGs having measured propermotions to look for objects with both coplanar orbits and similarangular momenta. Unfortunately, the majority of globular clusters withmeasured proper motions are inner halo clusters that are less likely toretain memory of their original orbit. Although four potential globularcluster/DSG associations are found, we believe three of theseassociations involving inner halo clusters to be coincidental. While thepresent sample of objects with complete dynamical data is small and doesnot include many of the globular clusters that are more likely to havebeen captured by the Milky Way, the methodology we adopt will becomeincreasingly powerful as more proper motions are measured for distantGalactic satellites and globular clusters, and especially as resultsfrom the Space Interferometry Mission (SIM) become available.
| Mass segregation of different populations inside the cluster NGC 6101
We have used ESO telescopes at La Silla and the Hubble Space Telescope(HST) in order to obtain accurate B, V, I CCD photometry for the starslocated within 200arcsec ( =~ 2 half-mass radii, rh =1.71arcmin ) from the center of the cluster NGC 6101. Color-MagnitudeDiagrams (CMDs) extending from the red-giant tip to about 5 magnitudesbelow the main-sequence turnoff MSTO (V = 20.05 +/- 0.05) have beenconstructed. The following results have been obtained from the analysisof the CMDs: a) The overall morphology of the main branches confirmsprevious results from the literature, in particular the existence of asizeable population of 73 ``blue stragglers'' (BSS), which had beenalready partly detected (27). They are considerably more concentratedthan either the subgiant branch (SGB) or the main sequence (MS) stars,and have the same spatial distribution as the horizontal branch (HB)stars (84% probability from K-S test). An hypothesis on the possible BSSprogeny is also presented. b) The HB is narrow and the bulk of stars isblue, as expected for a typical metal-poor globular cluster. c) Thederived magnitudes for the HB and the MSTO, VZAHB = 16.59 +/-0.10, VTO = 20.05 +/- 0.05, coupled with the values E(B-V) =0.1, [Fe/H] = -1.80, Y = 0.23 yield a distance modulus (m-M)V= 16.23 and an age similar to other ``old'' metal-poor globularclusters. In particular, from the comparison with theoreticalisochrones, we derive for this cluster an age of 13 Gyrs. d) By usingthe large statistical sample of Red Giant Branch (RGB) stars, wedetected with high accuracy the position of the bump in the RGBluminosity function. This observational feature has been compared withtheoretical prescriptions, yielding a good agreement within the currenttheoretical and observational uncertainties. Based on observationscollected at the European Southern Observatory, La Silla Chile and onHST observations. Tables with the x, y coordinates, V magnitudes and(V-I), (B-V) colors (for both ground and HST-data), are only availablein electronic form at the CDS via anonymous ftp cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/478
| Variable Stars in Galactic Globular Clusters
Based on a search of the literature up to 2001 May, the number of knownvariable stars in Galactic globular clusters is approximately 3000. Ofthese, more than 2200 have known periods and the majority (approximately1800) are of the RR Lyrae type. In addition to the RR Lyrae population,there are approximately 100 eclipsing binaries, 120 SX Phoenicisvariables, 60 Cepheids (including Population II Cepheids, anomalousCepheids and RV Tauri), and 120 SR/red variables. The mean period of thefundamental mode RR Lyrae variables is 0.585 days, for the overtonevariables it is 0.342 days (0.349 days for the first-overtone pulsatorsand 0.296 days for the second-overtone pulsators) and approximately 30%are overtone pulsators. These numbers indicate that about 65% of RRLyrae variables in Galactic globular clusters belong to Oosterhoff typeI systems. The mean period of the RR Lyrae variables in the Oosterhofftype I clusters seems to be correlated with metal abundance in the sensethat the periods are longer in the more metal poor clusters. Such acorrelation does not exist for the Oosterhoff type II clusters. Most ofthe Cepheids are in clusters with blue horizontal branches.
| Ages and Metallicities of Fornax Dwarf Elliptical Galaxies
Narrowband photometry is presented on 27 dwarf ellipticals in the Fornaxcluster. Calibrated with Galactic globular cluster data andspectrophotometric population models, the colors indicated that dwarfellipticals have a mean [Fe/H] of -1.00+/-0.28 ranging from -1.6 to-0.4. The mean age of dwarf ellipticals, also determinedphotometrically, is estimated at 10+/-1 Gyr compared with 13 Gyr forbright Fornax ellipticals. Comparison of our metallicity color andMg2 indices demonstrates that the [Mg/Fe] ratio is lower indwarf ellipticals than their more massive cousins, which is consistentwith a longer duration of initial star formation to explain theiryounger ages. There is a increase in dwarf metallicity with distancefrom the Fornax cluster center, where core galaxies are on average 0.5dex more metal-poor than halo dwarfs. In addition, we find the halodwarfs are younger in mean age compared with core dwarfs. One possibleexplanation is that the intracluster medium ram pressure strips the gasfrom dwarf ellipticals, halting star formation (old age) and stoppingenrichment (low metallicity) as they enter the core.
| 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.
| What Are These Blue Metal-Poor Stars?
The radial velocity behavior and chemical compositions of sixty-two bluemetal-poor (BMP) stars have been established from more than 1200 echellespectra obtained at Las Campanas Observatory from 1992 through 1999.Analysis of survey spectra provides abundances for this sample, which weuse to calibrate the K line versus B-V relation. Forty-four of the starshave [Fe/H]<-1, while eighteen lie on -1<[Fe/H]<0. One star,the SX Phe variable CS 22966-043, appears to be the most extreme exampleof a rare abundance class characterized by α-element deficiencies,high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and[Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities,while 42 are definite or probable members of binary systems. The binaryfraction of BMP stars appears to be independent of chemical composition.The high binary fraction fBMP~0.6 of BMP stars compared withthat found for the F- and G-type stars near the Sun, the systematicallylow mass functions of these binaries, and the paucity of double-linedbinaries among them lead us to suggest that at least half of the BMPbinaries are blue stragglers and that these blue stragglers are formedby McCrea mass transfer rather than by the various merger processes thatare currently believed to produce most blue stragglers in globularclusters. This conclusion is supported by the abnormally high proportionof BMP binaries with long periods and small orbital eccentricities,properties these binaries share with McClure's carbon star binaries. Thegreat majority of field blue stragglers (BSs) probably are created byRoche-lobe overflow during red giant branch evolution. Primaries of morewidely separated binaries that survive this phase of stellar evolutionmay engage in mass transfer during subsequent asymptotic giant branchevolution to form s-process abundance enhanced carbon stars. Our resultrequires a major downward revision of the fraction of BMP starsattributed to a captured intermediate-age population of metal-poor fieldstars. The high original estimate of the size of this component probablyarose from improper use of the globular cluster BS specific frequency,SBS=n(BS)/n(HB)~1, to estimate the halo BS space density. Weuse a simple model to calculate the specific frequency of BSs producedby McCrea mass transfer in an old metal-poor population with a givenprimordial binary fraction fB. Our model calculations returnvalues of SBS~5 for fB=0.15, much more like ourvalue for the field blue stragglers. We suggest that globular clusterseither destroy the primordial binaries that produce long period BSbinaries like those in the Galactic field reported here, or they neverpossessed them.
| 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.
| Photometric catalog of nearby globular clusters. I. A large homogeneous (V,I) color-magnitude diagram data-base
We present the first part of the first large and homogeneous CCDcolor-magnitude diagram (CMD) data base, comprising 52 nearby Galacticglobular clusters (GGC) imaged in the V and I bands using only twotelescopes (one for each hemisphere). The observed clusters represent75% of the known Galactic globulars with (m-M)_V<= 16.15 mag, covermost of the globular cluster metallicity range (-2.2 <= [Fe/H]<=-0.4), and span Galactocentric distances from ~ 1.2 to ~ 18.5 kpc. Inthis paper, the CMDs for the 39 GGCs observed in the southern hemisphereare presented. The remaining 13 northern hemisphere clusters of thecatalog are presented in a companion paper. For four clusters (NGC 4833,NGC 5986, NGC 6543, and NGC 6638) we present for the first time a CMDfrom CCD data. The typical CMD span from the 22nd V magnitudeto the tip of the red giant branch. Based on a large number of standardstars, the absolute photometric calibration is reliable to the ~ 0.02mag level in both filters. This catalog, because of its homogeneity, isexpected to represent a useful data base for the measurement of the mainabsolute and relative parameters characterizing the CMD of GGCs. Basedon data collected at the European Southern Observatory, La Silla, Chile.
| Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way
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| Standard Giant Branches in the Washington Photometric System
We have obtained CCD photometry in the Washington system C, T_1 filtersfor some 850,000 objects associated with 10 Galactic globular clustersand two old open clusters. These clusters have well-known metalabundances, spanning a metallicity range of 2.5 dex from [Fe/H]~-2.25 to+0.25 at a spacing of ~0.2 dex. Two independent observations wereobtained for each cluster, and internal checks, as well as externalcomparisons with existing photoelectric photometry, indicate that thefinal colors and magnitudes have overall uncertainties of <~0.03 mag.Analogous to the method employed by Da Costa & Armandroff for V, Iphotometry, we then proceed to construct standard [M_T_1, (C-T_1)_0]giant branches for these clusters adopting the Lee et al. distancescale, using some 350 stars per globular cluster to define the giantbranch. We then determine the metallicity sensitivity of the (C-T_1)_0color at a given M_T_1 value. The Washington system technique is foundto have 3 times the metallicity sensitivity of the V, I technique. AtM_T_1=-2 (about a magnitude below the tip of the giant branch, roughlyequivalent to M_I=-3), the giant branches of 47 Tuc and M15 areseparated by 1.16 mag in (C-T_1)_0 and only 0.38 mag in (V-I)_0. Thus,for a given photometric accuracy, metallicities can be determined 3times more precisely with the Washington technique. We find a linearrelationship between (C-T_1)_0 (at M_T_1=-2) and metallicity (on theZinn scale) exists over the full metallicity range, with an rms of only0.04 dex. We also derive metallicity calibrations for M_T_1=-2.5 and-1.5, as well as for two other metallicity scales. The Washingtontechnique retains almost the same metallicity sensitivity at faintmagnitudes, and indeed the standard giant branches are still wellseparated even below the horizontal branch. The photometry is used toset upper limits in the range 0.03-0.09 dex for any intrinsicmetallicity dispersion in the calibrating clusters. The calibrations areapplicable to objects with ages >~5 Gyr-any age effects are small ornegligible for such objects. This new technique is found to have manyadvantages over the previous two-color diagram technique for derivingmetallicities from Washington photometry. In addition to requiring onlytwo filters instead of three or four, the new technique is generallymuch less sensitive to reddening and photometric errors, and themetallicity sensitivity is many times higher. The new technique isespecially advantageous for metal-poor objects. The five metal-poorclusters determined by Geisler et al., using the old technique, to bemuch more metal-poor than previous indications, yield metallicitiesusing the new technique that are in excellent agreement with the Zinnscale. The anomalously low metallicities derived previously areundoubtedly a result of the reduced metallicity sensitivity of the oldtechnique at low abundance. However, the old technique is stillcompetitive for metal-rich objects ([Fe/H]>~-1). We have extended themethod developed by Sarajedini to derive simultaneous reddening andmetallicity determinations from the shape of the red giant branch (RGB),the T_1 magnitude of the horizontal branch, and the apparent (C-T_1)color of the RGB at the level of the horizontal branch. This techniqueallows us to measure reddening to 0.025 magnitudes in E(B-V) andmetallicity to 0.15 dex. Reddenings can also be derived from the blueedge of the instability strip, with a similar error. We measure theapparent T_1 magnitude of the red giant branch bump in each of thecalibrating clusters and find that the difference in magnitude betweenthe bump and the horizontal branch is tightly and sensitively correlatedwith metallicity, with an rms dispersion of 0.1 dex. This feature cantherefore also be used to derive metallicity in suitable objects.Metallicity can be determined as well from the slope of the RGB, to asimilar accuracy. Our very populous color-magnitude diagrams reveal theasymptotic giant branch bump in several clusters. Although M_T_1 of theRGB tip is not as constant with metallicity and age as M_I, it is stillfound to be a useful distance indicator for objects with[Fe/H]<~-1.2. For the six standard clusters in this regime,M_T_1(TRGB) =-3.22+/-0.11(sigma), with only a small metallicitydependence. This result is found to be in very good agreement with thepredictions of the Bertelli et al. isochrones. We also note that theWashington system holds great potential for deriving accurate ages aswell as metallicities.
| Theoretical vs. semi-empirical relative ages of globular clusters
Theoretical relative ages of galactic globular clusters have recentlybeen challenged by a semi-empirical relation. It was used to point outthat tested sets of isochrones were unable to reproduce the relation andyield internally inconsistent relative ages. We find that differentialcluster ages derived with the isochrones by Salaris & Weiss (1998)are reliable and internally consistent. We also show that thisconsistency depends on using the lower absolute ages determined by SW98,which therefore receive more empirical support. Moreover, we discuss theeffect of the clusters absolute age on the evaluation of theirdifferential ages, and its connection with the question of their agedispersion.
| Metal-rich globular clusters in the galactic disk: new age determinations and the relation to halo clusters
New age determinations of the galactic disk globular clusters 47 Tuc, M71 and NGC 6352 have been performed with our up-to-date alpha -enhancedstellar models. We find that all three clusters are about 9.2 Gyr oldand therefore coeval with the oldest disk white dwarfs. Severalarguments are presented which indicate that the initial helium contentof the stars populating these clusters is close to the solar one. Wealso revisit a total of 28 halo clusters, for which we use an updated[Fe/H] scale. This new metallicity scale leads on average to an agereduction of around 0.8 Gyr relative to our previous results. We comparethe predicted cluster distances, which result from our dating method,with the most recent distances based on HIPPARCOS parallaxes of localsubdwarfs. We further demonstrate that for the most metal-rich clustersscaled-solar isochrones no longer can be used to replace alpha -enhancedones at the same total metallicity. The implications of the presentedage determinations are discussed in the context of the formation historyof the Galaxy.
| On the relative ages of galactic globular clusters. A new observable, a semi-empirical calibration and problems with the theoretical isochrones
A new procedure is described to derive homogeneous relative ages fromthe Color-Magnitude Diagrams (CMDs) of Galactic globular clusters(GGCs). It is based on the use of a new observable, Delta V(0.05) ,namely the difference in magnitude between an arbitrary point on theupper main sequence (V_{+0.05} -the V magnitude of the MS-ridge, 0.05mag redder than the Main Sequence (MS) Turn-off, (TO)) and thehorizontal branch (HB). The observational error associated to DeltaV(0.05) is substantially smaller than that of previous age-indicators,keeping the property of being strictly independent of distance andreddening and of being based on theoretical luminosities rather than onstill uncertain theoretical temperatures. As an additional bonus, thetheoretical models show that Delta V(0.05) has a low dependence onmetallicity. Moreover, the estimates of the relative age so obtained arealso sufficiently invariant (to within ~ +/- 1 Gyr) with varying adoptedmodels and transformations. Since the difference in the color differenceDelta (B-V)_{TO,RGB} (VandenBerg, Bolte and Stetson 1990 -VBS,Sarajedini and Demarque 1990 -SD) remains the most reliable technique toestimate relative cluster ages for clusters where the horizontal part ofthe HB is not adequately populated, we have used the differential agesobtained via the "vertical" Delta V(0.05) parameter for a selectedsample of clusters (with high quality CMDs, well populated HBs,trustworthy calibrations) to perform an empirical calibration of the"horizontal" observable in terms of [Fe/H] and age. A direct comparisonwith the corresponding calibration derived from the theoretical modelsreveals the existence of clear-cut discrepancies, which call intoquestion the model scaling with metallicity in the observational planes.Starting from the global sample of considered clusters, we have thusevaluated, within a homogeneous procedure, relative ages for 33 GGCshaving different metallicity, HB-morphologies, and galactocentricdistances. These new estimates have also been compared with previouslatest determinations (Chaboyer, Demarque and Sarajedini 1996, andRicher {et al. } 1996). The distribution of the cluster ages withvarying metallicity and galactocentric distance are briefly discussed:(a) there is no direct indication for any evident age-metallicityrelationship; (b) there is some spread in age (still partiallycompatible with the errors), and the largest dispersion is found forintermediate metal-poor clusters; (c) older clusters populate both theinner and the outer regions of the Milky Way, while the youngerglobulars are present only in the outer regions, but the sample is fartoo poor to yield conclusive evidences.
| 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.
| The stellar content of the Sagittarius Dwarf Galaxy
We present V,I deep CCD photometry for three fields of the dwarf galaxyin Sagittarius (Sgr), located at l=5.6(o) , b=-14.1(o) . One of thefields is centered on the globular cluster NGC 6715 (M54), which lies inone of the dense clumps of the Sgr galaxy. Comparing the CMD of Sgr withthose of globular clusters which are believed to be kinematicallyassociated with the dwarf galaxy (Da Costa & Armandroff 1995), weconclude that the stellar population of Sgr presents a spread inmetallicity of -0.71<=[Fe/H]<=-1.58, and that the dominantpopulation ( =~ 10 Gyr old) is extremely similar to the star content ofthe associated globular cluster Terzan 7. The estimated distance to Sgris d =~ 24.55 Kpc. Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile.
| On the Stringent Constraint on Massive Dark Clusters in the Galactic Halo
This Letter revises the recent claims of a dynamical constraint onmassive dark clusters from the Galactic globular cluster luminosityfunction (GCLF). We point out that this argument is invalid for tworeasons. The first point is that with a proper sample of globularclusters, the GCLF presents a significant trend with Galactocentricposition, and second, the disruption timescale of globular clusters byencounters with these massive objects depends strongly on many uncertainparameters. For the usual halo model of Cadwell & Ostriker, it maybe smaller by only a factor of 2 for distant globular clusters.Therefore, the existence of massive dark clusters in the halo could bepossible. This provides a scenario in which the formation of darkclusters and globular clusters may have a similar origin.
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