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Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| 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.
| Deep near-IR photometry of eight metal-poor globular clusters in the Galactic bulge and halo
High quality J, H and K' images are used to investigate themorphological properties of the near-infrared color-magnitude diagramsfor five metal-poor bulge globular clusters and three halo clusters.Photometric parameters to describe the RGB shape, i.e., the colors atfixed magnitudes of M_K=M_H=(-5.5, -5, -4 and -3), the magnitudes atfixed colors of (J-K)o = (J-H)o = 0.70, and theRGB slope, have been measured from the fiducial normal points of theCMDs. We also measured the near-infrared magnitudes of the RGB bump andtip on the luminosity function of the RGB stars for each cluster. TheRGB parameters of the observed metal-poor bulge and halo clusters areconsistent with the previous empirical relationships between the RGBparameters and the cluster metallicity for metal-rich bulge clusters andhalo clusters. The near-infrared magnitudes of the RGB bump and tip arein good agreement with the theoretical prediction of the Yonsei-Yaleisochrone.
| Global fitting of globular cluster age indicators
Context: .Stellar models and the methods for the age determinations ofglobular clusters are still in need of improvement. Aims: .Weattempt to obtain a more objective method of age determination based oncluster diagrams, avoiding the introduction of biases due to thepreference of one single age indicator. Methods: .We compute newstellar evolutionary tracks and derive the dependence of age indicatingpoints along the tracks and isochrone - such as the turn-off or bumplocation - as a function of age and metallicity. The same criticalpoints are identified in the colour-magnitude diagrams of globularclusters from a homogeneous database. Several age indicators are thenfitted simultaneously, and the overall best-fitting isochrone isselected to determine the cluster age. We also determine thegoodness-of-fit for different sets of indicators to estimate theconfidence level of our results. Results: .We find that ourisochrones provide no acceptable fit for all age indicators. Inparticular, the location of the bump and the brightness of the tip ofthe red giant branch are problematic. On the other hand, the turn-offregion is very well reproduced, and restricting the method to indicatorsdepending on it results in trustworthy ages. Using an alternative set ofisochrones improves the situation, but neither leads to an acceptableglobal fit. Conclusions: .We conclude that evolutionary tracks oflow-mass metal-poor stars are far from reproducing all aspects ofglobular cluster colour-magnitude diagrams and that the determination ofcluster ages still depends on the favourite method or indicator chosen.
| 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.
| 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.
| SOAR BVI photometry of the metal-poor bulge globular cluster NGC 6642
We present BVI photometry of the globular cluster NGC 6642 using the SOIimager at the SOAR Telescope. The colour magnitude diagrams (CMD) reach≈1.5 mag in V below the main sequence turn-off. A comparison of theoverall sequences, and in particular the Red Giant Branch slope of NGC6642 with that of M 5, indicates that the two clusters must have asimilar metallicity of [Fe/H] ≈ -1.3. We also obtained a reddeningE(B-V)=0.42±0.03 for NGC 6642, and a distance from the Sun ofdȯ=7.2±0.5 kpc. Therefore NGC 6642 is amoderately metal-poor globular cluster that is spatially located in thebulge at a galactocentric distance of R_GC≈ 1.7 kpc. The comparisonof CMDs of NGC 6642 with those of M 5 shows that there is a very goodmatch of magnitude difference between turn-off and horizontal branch,suggesting comparable ages. The age of M 5 is typical of halo globulars,so NGC 6642 is coeval with the halo. It is a good candidate as one ofthe few genuine metal-poor and old bulge clusters, and might be one ofthe oldest fossils in the Galaxy.
| 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.
| 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.
| Near-Infrared Imaging of the Central Regions of Metal-Poor Inner Spheroid Globular Clusters
JHK images obtained with the Canada-France-Hawaii Telescope adaptiveoptics bonnette are used to investigate the near-infrared photometricproperties of red giant branch (RGB) and horizontal-branch (HB) stars ineight metal-poor globular clusters with RGC<=2 kpc. Theslope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature ofthese clusters, four of which (NGC 6287, 6293, 6333, and 6355) are foundto have metallicities that are comparable to M92. The luminosityfunctions of RGB stars in inner spheroid and outer halo clusters havesimilar slopes, although there is a tendency for core-collapsed clustersto have slightly flatter luminosity functions than noncollapsedclusters. The distribution of red HB stars on the (K, J-K) CMDs of innerspheroid clusters with [Fe/H]~-1.5 is very different from that ofclusters with [Fe/H]~-2.2, suggesting that metallicity is the mainparameter defining HB content among these objects. The RGB bump isdetected in four of the inner spheroid clusters, and this feature isused to compute distances to these objects. Finally, the specificfrequency of globular clusters in the inner Galaxy is discussed in thecontext of the early evolution of the bulge. Based on the ratio ofmetal-poor to metal-rich clusters in the inner Galaxy it is suggestedthat the metal-poor clusters formed during an early intense burst ofstar formation. It is also demonstrated that if the globular clusterformation efficiency for the inner Galaxy is similar to that measured inother spheroidal systems, then the main body of the bulge could haveformed from gas that was chemically enriched in situ; hence, materialfrom a separate pre-enriched reservoir, such as the disk or outer halo,may not be required to form the bulge.
| A census with ROSAT of low-luminosity X-ray sources in globular clusters
I analyze 101 observations from the ROSAT archive to search for X-raysources in or near 55 globular clusters. New sources are found in thecores of NGC 362 (a double source), NGC 6121 (marginally significant),NGC 6139, and NGC 6266; and outside the cores of NGC 6205, NGC 6352 andNGC 6388. More accurate positions are determined for the X-ray sourcesin some ten clusters. The improved position for the source in NGC 6341excludes the suggested ultraviolet counterpart. It is shown that one ofthe two sources reported near the core of NGC 6626 is spurious, as isthe detection of a pulsar period in the PSPC data of this cluster; thecentral source is resolved in three sources. One source reportedpreviously in NGC 6304 is demoted to an upper limit. For 20 clustercores better upper limits to the X-ray luminosity are obtained. From astatistical analysis I argue that several sources outside the clustercores may well belong to the clusters. All spectral energy distributionsobserved so far are relatively soft, with bremsstrahlung temperatures =~0.9 keV; there is evidence however that bremsstrahlung spectra do notcorrectly describe the spectra. The X-ray luminosity per unit mass forthe cluster as a whole does not depend on the concentration; theluminosity per unit mass for the core may increase with the clusterconcentration.
| 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.
| A Near-Infrared Photometric Survey of Metal-poor Inner Spheroid Globular Clusters and Nearby Bulge Fields
Images recorded through J, H, K, 2.2 μm continuum, and CO filtershave been obtained of a sample of metal-poor ([Fe/H]<=-1.3) globularclusters in the inner spheroid of the Galaxy. The shape and color of theupper giant branch on the (K, J-K) color-magnitude diagram (CMD),combined with the K brightness of the giant branch tip, are used toestimate the metallicity, reddening, and distance of each cluster. COindices are used to identify bulge stars, which will bias metallicityand distance estimates if not culled from the data. The distances andreddenings derived from these data are consistent with published values,although there are exceptions. The reddening-corrected distance modulusof the Galactic center, based on the Carney et al. horizontal-branch(HB) brightness calibration, is estimated to be 14.9+/-0.1. The meanupper giant branch CO index shows cluster-to-cluster scatter that (1) islarger than expected from the uncertainties in the photometriccalibration and (2) is consistent with a dispersion in CNO abundancescomparable to that measured among halo stars. The luminosity functions(LFs) of upper giant branch stars in the program clusters tend to besteeper than those in the halo clusters NGC 288, NGC 362, and NGC 7089.The majority of inner spheroid clusters fall along the integrated J-Kversus metallicity relation defined by halo clusters; however, many ofthe inner spheroid clusters do not follow the relation betweenintegrated CO index and metallicity measured for halo clusters, in thatthey have CO indices that are too small. Bulge fields were also observednear most clusters. The slope of the giant branch LF does not varysignificantly between most fields, although the LFs in Baade's windowand near NGC 6273 are significantly shallower than average.Metallicities estimated from the slope of the upper giant branch on the(K, J-K) CMDs of fields within 6° of the Galactic center areconsistent with previous studies. Finally, the data suggest that the HBcontent may not be uniform throughout the bulge, in the sense that alarger than average number of red HB stars may occur in fields closestto the Galactic center.
| 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.
| Gravitational Radiation from Globular Clusters
Space-based gravitational wave detectors will have the ability toobserve continuous low-frequency gravitational radiation from binarystar systems. They can determine the direction to continuous sourceswith an angular resolution approaching tens of arcminutes. Thisresolution should be sufficient to identify binary sources as members ofsome nearby globular clusters. Thus, gravitational radiation can be usedto determine the population of hard binaries in globular clusters. Forparticularly hard binaries, the orbital period may change as a result ofgravitational wave emission. If one of these binaries can be identifiedwith a globular cluster, then the distance to that cluster can bedetermined. Thus, gravitational radiation may providereddening-independent distance measurements to globular clusters and theRR Lyrae stars that inhabit them.
| 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.
| Kinematics of Bulge Giants in F588
Radial velocities for 331 stars in the F588 bulge field at 1, b = (80,70) are obtained. The mean Galactocentric velocity for 319 stars with R< 12.3 is V = 54.4 + 4.7 km S 1, and the velocity dispersion is Cr =84.4 + 3.3 km 5-1 These stars have metal abundances based on a set ofspectral indices calibrated against a grid of 400 standard stars.Dividing the sample according to metallicity, the halo and bulgecomponents are kinematically distinct. The 65 K giants with [Fe/H] <-1 (mostly halo giants) have = -7.0 + 13.6 km 1 and Cr = 109.4 + 9.6 kms 1 The 31 "pure" halo giants (with [Fe/ H] < -1.5) have V = -5.8 +20.4 km 5-1 and Cr = 113.5 + 14.4 km 5-1 The mean Galactocentricvelocity and velocity dispersion for 194 bulge K giants (stars with[Fe/H] > - 1.0) are = 65.9 + 5.1 km s 1 and q = 71.9 + 3.6 km 1 Thedifference between halo and bulge kinematics is of very high statisticalsignificance. The Galactic bulge and halo are distinct components,defined by consistency among [Fe/H], V , and Cr. Bulge giants are alsofound to have their kinematics correlated with their metallicities.After discussing other possible causes, including disk and halocontamination, it is argued that this effect is real. The kinematics of26 M giants in our sample are compared with those of other evolved bulgetracers (Mira variables, OH/IR stars, SiO masers, etc.). These M giantsobserved in the field have kinematics consistent with their being bulgemembers. The dependence of velocity dispersion on distance from theGalactic center is determined by combining the data with existing datafrom Baade's window. While the line-of-sight velocity dispersion of thebulge giants decreases steeply with distance from the Galactic center,that of halo giants decreases very slowly if at - all. The mass in theinner 1.5 kpc is determined to be M_{1 5 }= (1.38 + 0.26) x 1010 M_{0}and M_{1 5 }= (1.74 + 0.28) x 1010 M_{0 }for the cases of isotropic andanisotropic inner halo, respectively. On the basis of kinematics andmetallicities, we conclude that the bulge is a distinct galacticcomponent and not the inner extension of the halo, thin disk, or thickdisk.
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