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Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge
We present near-IR color-magnitude diagrams and physical parameters fora sample of 24 Galactic globular clusters toward the bulge. In thispaper we discuss the properties of 12 new clusters (out of the 24) inaddition to those previously studied and published by our group. Thecompilation includes measurements of the cluster reddening, distance,photometric metallicity, horizontal branch red clump, and red giantbranch morphological (e.g., mean ridgelines) and evolutionary (e.g.,bump and tip) features. The compilation is available in electronic formon the World Wide Web, and it will be updated regularly.Based on data taken at the ESO New Technology Telescope, within theobserving programs 73.D-0313, 75.D-0372, and 77.D-0757.
| 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.
| The period-luminosity relation for type II Cepheids in globular clusters
We report the result of our near-infrared observations (JHKs)for type II Cepheids (including possible RV Tau stars) in galacticglobular clusters. We detected variations of 46 variables in 26 clusters(10 new discoveries in seven clusters) and present their light curves.Their periods range from 1.2 d to over 80 d. They show a well-definedperiod-luminosity relation at each wavelength. Two type II Cepheids inNGC 6441 also obey the relation if we assume the horizontal branch starsin NGC 6441 are as bright as those in metal-poor globular clusters inspite of the high metallicity of the cluster. This result supports thehigh luminosity which has been suggested for the RR Lyr variables inthis cluster. The period-luminosity relation can be reproduced using thepulsation equation assuming that all the stars have the same mass.Cluster RR Lyr variables were found to lie on an extrapolation of theperiod-luminosity relation. These results provide important constraintson the parameters of the variable stars.Using Two Micron All-Sky Survey (2MASS) data, we show that the type IICepheids in the Large Magellanic Cloud (LMC) fit our period-luminosityrelation within the expected scatter at the shorter periods. However, atlong periods (P > 40 d, i.e. in the RV Tau star range) the LMC fieldvariables are brighter by about one magnitude than those of similarperiods in galactic globular clusters. The long-period cluster starsalso differ from both these LMC stars and galactic field RV Tau stars ina colour-colour diagram. The reasons for these differences arediscussed.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| An Empirical Calibration of the Mixing-Length Parameter α
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| 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.
| 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.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| Near-infrared photometry of four metal-rich bulge globular clusters: NGC 6304, 6569, 6637 and 6638*
We present high-quality near-infrared (near-IR) photometry of four bulgemetal-rich globular clusters, namely NGC 6304, 6569, 6637 and 6638. Byusing the observed colour-magnitude diagrams we derived photometricestimates of the cluster reddening and distance. We performed a detailedanalysis of the red giant branch (RGB), presenting a completedescription of its morphologic parameters and evolutionary features(bump and tip). Photometric estimates of the cluster metallicity wereobtained using the updated data set (published by our group) linkingmetal abundance to a variety of near-IR indices measured along the RGB.The detection of the RGB bump and tip is also presented and brieflydiscussed.
| 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.
| 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.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| Infrared Echelle Spectroscopy of Palomar 6 and M71
We present high-resolution infrared echelle spectroscopy for theglobular clusters Palomar 6 and M71. Our mean heliocentric radialvelocity of Pal 6 is +180.6+/-3.2 km s-1 and is 20 kms-1 lower than that found by Minniti in 1995. Contrary to theprevious metallicity estimates using low-resolution spectroscopy, ourresults show that Pal 6 has an intermediate metallicity, with[Fe/H]=-1.0+/-0.1, and is slightly more metal poor than M71. Reasonablechanges in the surface temperature or the microturbulent velocity of themodel atmospheres do not affect [Fe/H] at more than +/-0.2 dex. In spiteof its high metallicity, on the basis of the spectrum of a singlecluster member the [Si/Fe] and [Ti/Fe] ratios of Pal 6 appear to beenhanced by 0.4 and 0.5 dex, respectively, suggesting that the Galacticinner halo may have experienced a very rapid chemical enrichmenthistory.Based on observations made with the Infrared Telescope Facility, whichis operated by the University of Hawaii under contract to the NationalAeronautics and Space Administration.
| The initial helium abundance of the Galactic globular cluster system
In this paper we estimate the initial He content in about 30% of theGalactic globular clusters (GGCs) from new star counts we have performedon the recently published HST snapshot database of Colour MagnitudeDiagrams (Piotto et al. \cite{Piotto02}). More specifically, we use theso-called R-parameter and estimate the He content from a theoreticalcalibration based on a recently updated set of stellar evolution models.We performed an accurate statistical analysis in order to assess whetherGGCs show a statistically significant spread in their initial Heabundances, and whether there is a correlation with the clustermetallicity. As in previous works on the subject, we do not find anysignificant dependence of the He abundance on the cluster metallicity;this provides an important constraint for models of Galaxy formation andevolution. Apart from GGCs with the bluest Horizontal Branch morphology,the observed spread in the individual helium abundances is statisticallycompatible with the individual errors. This means that either there isno intrinsic abundance spread among the GGCs, or that this is masked bythe errors. In the latter case we have estimated a firm 1σ upperlimit of 0.019 to the possible intrinsic spread. In case of the GGCswith the bluest Horizontal Branch morphology we detect a significantspread towards higher abundances inconsistent with the individualerrors; this can be fully explained by additional effects not accountedfor in our theoretical calibrations, which do not affect the abundancesestimated for the clusters with redder Horizontal Branch morphology. Inthe hypothesis that the intrinsic dispersion on the individual Heabundances is zero, taking into account the errors on the individualR-parameter estimates, as well as the uncertainties on the clustermetallicity scale and theoretical calibration, we have determined aninitial He abundance mass fraction YGGC=0.250±0.006.This value is in perfect agreement with current estimates based onCosmic Microwave Background radiation analyses and cosmologicalnucleosynthesis computations.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555, and on observations retrieved withthe ESO ST-ECF Archive.
| The Red Giant Branch luminosity function bump
We present observational estimates of the magnitude difference betweenthe luminosity function red giant branch bump and the horizontal branch(Delta F555WbumpHB), and of star counts in thebump region (Rbump), for a sample of 54 Galactic globularclusters observed by the HST. The large sample of stars resolved in eachcluster, and the high photometric accuracy of the data allowed us todetect the bump also in a number of metal poor clusters. To reduce thephotometric uncertainties, empirical values are compared withtheoretical predictions obtained from a set of updated canonical stellarevolution models which have been transformed directly into the HSTflight system. We found an overall qualitative agreement between theoryand observations. Quantitative estimates of the confidence level arehampered by current uncertainties on the globular cluster metallicityscale, and by the strong dependence of DeltaF555WbumpHB on the cluster metallicity. In case ofthe Rbump parameter, which is only weakly affected by themetallicity, we find a very good quantitative agreement betweentheoretical canonical models and observations. For our full clustersample the average difference between predicted and observedRbump values is practically negligible, and ranges from-0.002 to -0.028, depending on the employed metallicity scale. Theobserved dispersion around these values is entirely consistent with theobservational errors on Rbump. As a comparison, the value ofRbump predicted by theory in case of spurious bump detectionsdue to Poisson noise in the stellar counts would be ~ 0.10 smaller thanthe observed ones. We have also tested the influence on the predictedDelta F555WbumpHB and Rbump values ofan He-enriched component in the cluster stellar population, as recentlysuggested by D'Antona et al. (\cite{d02}). We find that, underreasonable assumptions concerning the size of this He-enrichedpopulation and the degree of enrichment, the predicted DeltaF555WbumpHB and Rbump values are onlymarginally affected.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555, and on observations retrieved withthe ESO ST-ECF Archive.
| 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.
| HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands
We present the complete photometric database and the color-magnitudediagrams for 74 Galactic globular clusters observed with the HST/WFPC2camera in the F439W and F555W bands. A detailed discussion of thevarious reduction steps is also presented, and of the procedures totransform instrumental magnitudes into both the HST F439W and F555Wflight system and the standard Johnson ( B ) and ( V ) systems. We alsodescribe the artificial star experiments which have been performed toderive the star count completeness in all the relevant branches of thecolor magnitude diagram. The entire photometric database and thecompleteness function will be made available on the Web immediatelyafter the publication of the present paper. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contractNAS5-26555, and on observations retrieved from the ESO ST-ECF Archive.
| 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.
| Colour Magnitude Diagrams of the moderately metal-rich globular clusters NGC 6569 and Palomar 11
For the first time Colour-Magnitude Diagrams are presented for NGC 6569and Palomar 11. NGC 6569 is a rather concentrated cluster whereasPalomar 11 is loose and sparsely populated. Cluster parameters arederived from V and I photometry. These are red Horizontal Branchglobular clusters, with metallicities comparable to that of 47 Tucanae.We derive a reddening E(B-V) ~ 0.53 and a distance from the Sundsun ~ 9.8 kpc for NGC 6569, and E(B-V) ~ 0.35 anddsun ~ 13.2 kpc for Palomar 11. NGC 6569 is located in thebulge, while Palomar 11, despite its rather high metallicity, is in theinner halo, a rare case similar to that of 47 Tucanae. Observationscollected at the European Southern Observatory - ESO, Chile, proposalno. 64L-0212(A).
| 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.
| 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.
| Globular clusters as microlensing targets
We investigate the possibility of using globular clusters as targets formicrolensing searches. Such searches will be challenging and requiremore powerful telescopes than now employed, but are feasible in the nearfuture. Although expected event rates are low, we show that the widevariety of lines of sight to globular clusters greatly enhances theability to distinguish between halo models using microlensingobservations as compared with LMC/SMC observations alone. In particular,the halo core radius and power-law exponent can be determined with goodaccuracy.
| CCD photometry of the metal-rich halo cluster NGC 6366.
We present new BV photometry for 6564 stars in the field of the globularcluster NGC 6366, using a wide field CCD camera at the prime focus ofthe Isaac Newton Telescope (INT 2.5m). The mean internal accuracy of thepresent V magnitudes and (B-V) colours ranges from 0.005mag for thestars at the top of the Red Giant Branch to 0.07mag for the faintestmeasured Main Sequence stars. From the resultant Colour-MagnitudeDiagram (CMD), and considering spectroscopic determinations of [Fe/H]from previous works, we derive the reddening (E(B-V)=0.70+/-0.04), thedistance modulus (m-M_o_=12.26+/-0.15) and the age (18_-3_^+2^Gyrs) ofthe cluster by comparing with the theoretical isochrones computed byStraniero & Chieffi (1991). The difference in V magnitude betweenthe Turn-Off and the Horizontal Branch ({DELTA}V^TO^_HB_) amounts3.51+/-0.15mag. We compare the CMD of NGC 6366 with the CMD of otherclusters with similar metallicity in order to estimate relative agedifferences. We extract two concluions from our analysis (1) NGC 6366seems to be coeval with the bulk of metal-poor halo clusters, and mucholder (4-6Gyr) than other metal-rich globular clusters (Pal 12, 47Tucand M71). (2) On the basis of the comparison between{DELTA}V^TO^_HB_-age and {DELTA}(B-V)_SGB_-age relations, there seems toexist a difference in the Helium abundance between the disk globularsystem (Y~0.28) and the (inner) halo globular system (Y~0.23).
| Destruction of the Galactic Globular Cluster System
We investigate the dynamical evolution of the Galactic globular clustersystem in considerably greater detail than has been done hitherto,finding that destruction rates are significantly larger than given byprevious estimates. The general scheme (but not the detailedimplementation) follows Aguilar, Hut, & Ostriker. For the evolutionof individual clusters, we use a Fokker-Planck code including the mostimportant physical processes governing the evolution: two-bodyrelaxation, tidal truncation of clusters, compressive gravitationalshocks while clusters pass through the Galactic disk, and tidal shocksdue to passage close to the bulge. Gravitational shocks are treatedcomprehensively, using a recent result by Kundic & Ostriker that the< Delta E2> shock-induced relaxation term, driving an additionaldispersion of energies, is generally more important than the usualenergy shift term < Delta E>. Various functional forms of thecorrection factor are adopted to allow for the adiabatic conservation ofstellar actions in a presence of transient gravitational perturbation.We use a recent compilation of the globular cluster positional andstructural parameters, and a collection of radial velocity measurements.Two transverse to the line-of-sight velocity components were assignedrandomly according to the two kinematic models for the cluster system(following the method of Aguilar, Hut, & Ostriker): one with anisotropic peculiar velocity distribution, corresponding to thepresent-day cluster population, and the other with the radiallypreferred peculiar velocities, similar to those of the stellar halo. Weuse the Ostriker & Caldwell and the Bahcall, Schmidt, & Soneiramodels for our Galaxy. For each cluster in our sample, we calculated itsorbits over a Hubble time, starting from the present observed positionsand assumed velocities. Medians of the resulting set of peri- andapogalactic distances and velocities are used then as an input for theFokker-Planck code. Evolution of the cluster is followed up to its totaldissolution due to a coherent action of all of the destructionmechanisms. The rate of destruction is then obtained as a median overall the cluster sample, in accord with Aguilar, Hut, & Ostriker. Wefind that the total destruction rate is much larger than that given byAguilar, Hut, & Ostriker with more than half of the present clusters(52%--58% for the Ostriker & Caldwell model, and 75%--86% for theBahcall, Schmidt, & Soneira model) destroyed in the next Hubbletime. Alternatively put, the typical time to destruction is comparableto the typical age, a result that would follow from (but is not requiredby) an initially power law distribution of destruction times. We discusssome implications for a past history of the globular cluster system andthe initial distribution of the destruction times, raising thepossibility that the current population is but a very small fraction ofthe initial population with the remnants of the destroyed clustersconstituting presently a large fraction of the spheroid (bulge + halo)stellar population.
| Some Integrated Properties of Galactic Globular Clusters
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....112.2634V
| A Catalog of Parameters for Globular Clusters in the Milky Way
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.1487H&db_key=AST
| A Search for Radio Pulsars in Globular Clusters, Supernova Remnants and Transient X-Ray Sources
We present the results of a targeted search for radio pulsars inglobular clusters, supernova remnants (SNRs) and transient X-raysources. 85 globular clusters, 29 SNRs and three transient X-ray sourceswere observed in the course of the search, during which four pulsarswere found. For each object searched, the sensitivity limit is given.The four new pulsars discovered were all in globular clusters, namelyPSR B1620-26 in M4, PSR B1718-19 in NGC6342, and PSR B1820-30A andB1820-30B in NGC6624. While PSR B1620-26 provides strong support for thebinary evolution hypothesis for the formation of millisecond pulsars,PSR B1718-19 and B1820-30A are somewhat anomalous as they are apparentlyrelatively young, the former pulsar and PSR B1820-30B have high magneticfields, and all are located in old globular clusters (ages ~10^10yr).Not surprisingly, the well-known pulsar PSR B0531+21 was detected in theCrab nebula SNR. The number of pulsars detected was probably limited bythe finite size of pulsar emission beams, as in any survey. Otherfactors such as large distance, low pulsar luminosity and orbital motionmay have inhibited the detection of other pulsars in globular clusters.Pulsars in SNRs and transient X-ray sources are probably less affectedby these factors. Our inability to detect the former is probably due tothe relatively high level of emission from the SNRs, the limited areathat we observed within each target and possibly the high birth velocityof pulsars which carries them away from the centres of their remnants.Processes concerning the accretion discs of transient X-ray sourcesprobably thwarted the detection of any potential radio pulsars in thesesystems.
| Microlensing effects in globular star clusters
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