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The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters without Previous Hubble Space Telescope Photometry
We present the first results of a large Advanced Camera for Surveys(ACS) survey of Galactic globular clusters. This Hubble Space Telescope(HST) Treasury project is designed to obtain photometry with S/N(signal-to-noise ratio) >~10 for main-sequence stars with masses>~0.2 Msolar in a sample of globulars using the ACS WideField Channel. Here we focus on clusters without previous HST imagingdata. These include NGC 5466, NGC 6779, NGC 5053, NGC 6144, Palomar 2,E3, Lyngå 7, Palomar 1, and NGC 6366. Our color-magnitude diagrams(CMDs) extend reliably from the horizontal branch to as much as 7 magfainter than the main-sequence turnoff and represent the deepest CMDspublished to date for these clusters. Using fiducial sequences for threestandard clusters (M92, NGC 6752, and 47 Tuc) with well-knownmetallicities and distances, we perform main-sequence fitting on thetarget clusters in order to obtain estimates of their distances andreddenings. These comparisons, along with fitting the cluster mainsequences to theoretical isochrones, yield ages for the target clusters.We find that the majority of the clusters have ages that are consistentwith the standard clusters at their metallicities. The exceptions areE3, which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could beas much as 8 Gyr younger than 47 Tuc.Based on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS5-26555, under program GO-10775 (PI:A. Sarajedini).
| The G-dwarf problem in the Galactic spheroid
This paper has two parts: one about observational constraints, and theother about chemical evolution models. In the first part, the empiricaldifferential metallicity distribution (EDMD) is deduced from twodifferent samples involving (i) 268 K-giant bulge stars [Sadler, E.M.,Rich, R.M., Terndrup, D.M., 1996. AJ 112, 171], and (ii) 149 globularclusters [Mackey, A.D., van den Bergh, S., 2005. MNRAS 360, 631], inaddition to previous results (Caimmi, R., 2001b, AN 322, 241 (C01))related to (iii) 372 solar neighbourhood halo subdwarfs [Ryan, S.G.,Norris, J.E., 1991. AJ 101, 1865]. Under the assumption that eachdistribution is typical for the corresponding subsystem, the EDMD of theGalactic spheroid is determined by weighting the mass. The empiricalage-metallicity relation (EAMR) involving absolute ages is deduced fromrecent results related to a homogeneous sample of globular clusters [DeAngeli, F., Piotto, G., Cassisi, S., et al., 2005. AJ 130, 116]. In thesecond part, models of chemical evolution for the Galactic halo andbulge are computed, assuming the instantaneous recycling approximation.The EDMD data are fitted, to an acceptable extent, by simple models ofchemical evolution implying both homogeneous and inhomogeneous starformation, provided that star formation is inhibited during haloformation and enhanced during bulge formation, with respect to the disksolar neighbourhood, taken to be representative of the whole disk. Theinitial mass function (IMF) is assumed to be a universal power law,which implies the same value of the true yield in different subsystems.The theoretical differential metallicity distribution (TDMD) is firstdetermined for the halo and the bulge separately, and then for theGalactic spheroid by weighting the mass. The EAMR cannot be fitted intothe Simple model that implies homogeneous star formation, but shows anon-monotonic trend characterized by large dispersion. On the otherhand, simple models involving inhomogeneous star formation yield atheoretical age-metallicity relation (TAMR) which reproduces the data toan acceptable extent. For gas ouflow from the proto-halo, acceptablemodels give rise to different predictions in different alternatives. Ifthe Galactic spheroid and disk underwent decoupled chemical evolution,i.e. no gas exchange between the related reservoirs, less than one thirdof the bulge mass outflowed from the proto-halo. If the Galacticspheroid and disk underwent coupled chemical evolution, i.e. some gasexchange between the related reservoirs, the existence of an unseenbaryonic halo (or equivalent amount of gas lost by the Galaxy) with masscomparable to bulge mass, is necessarily needed. In this view, theoutflowing proto-halo gas which remains bound to the Galaxy, producesboth the bulge and the disk.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| Dynamical Formation of Close Binaries in Globular Clusters: Cataclysmic Variables
We answer the long-standing question of which production mechanism isresponsible for the cataclysmic variables (CVs) in globular clusters.Arguments have been given that range from mostly primordial presence toa significant contribution of later dynamical formation in close stellarencounters. We conclude, based on a thorough analysis of a homogeneousChandra data set, that the majority of CVs in dense globular clustershave a dynamical origin.
| 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.
| Galactic Globular Cluster Relative Ages
We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the differencebetween the horizontal branch and the turnoff in two internallyphotometrically homogeneous databases. The mutual consistency of the twodata sets has been assessed by comparing the ages of 16 globularclusters in common between the two databases. We have also investigatedthe consistency of our relative age determination within the recentstellar model framework. All clusters with [Fe/H]<-1.7 are found tobe old and coeval, with the possible exception of two objects, which aremarginally younger. The age dispersion for the metal-poor clusters is0.6 Gyr (rms), consistent with a null age dispersion.Intermediate-metallicity clusters (-1.7<[Fe/H]<-0.8) are onaverage 1.5 Gyr younger than the metal-poor ones, with an age dispersionof 1.0 Gyr (rms) and a total age range of ~3 Gyr. About 15% of theintermediate-metallicity clusters are coeval with the oldest clusters.All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the mostmetal-poor ones, with a relatively small age dispersion, although themetal-rich sample is still too small to allow firmer conclusions. Thereis no correlation of the cluster age with the galactocentric distance.We briefly discuss the implication of these observational results forthe formation history of the Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555, and on observations made at the European SouthernObservatory, La Silla, Chile, and with the Isaac Newton GroupTelescopes.
| 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.
| Using X-rays to Probe the Compact Binary Content of Globular Clusters
Globular clusters (GCs) harbour a large number of close binaries whichare hard to identify optically due to high stellar densities. Observingthese GCs in X-rays, in which the compact binaries are bright,diminishes the over-crowding problem. Using the new generation of X-rayobservatories, it is possible to identify populations of neutron starlow mass X-ray binaries, cataclysmic variables and millisecond pulsarsas well as other types of binaries. We present the spectra of a varietyof binaries that we have identified in four GCs observed by XMM-Newton.We show that through population studies we can begin to understand theformation of individual classes of binaries in GCs and hence start tounfold the complex evolutionary paths of these systems.
| Investigating the Faint X-ray Sources in Globular Clusters with XMM-Newton
Globular clusters (GCs) harbour a large number of faint X-ray sourceswhose nature, until recently, was largely unknown. Using the new X-rayobservatories, it is possible to identify populations of low mass X-raybinaries, cataclysmic variables, millisecond pulsars, as well as othertypes of binaries belonging to the GCs, along with fore- and backgroundobjects. We present a variety of binaries, identified in four GCsobserved by XMM-Newton. We show that through population studies we canbegin to understand the formation of individual classes of binaries andhence start to unfold the complex evolutionary paths of such systems.
| Analysis of the Quiescent Low-Mass X-Ray Binary Population in Galactic Globular Clusters
Quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars havebeen identified in several globular clusters using Chandra or XMM X-rayobservations, via their distinctive soft thermal spectra. We report acomplete census of the qLMXB population in these clusters, identifyingthree additional probable qLMXBs in NGC 6440. We conduct severalanalyses of the qLMXB population and compare it with the harder,primarily cataclysmic variable (CV), population of low-luminosity X-raysources with1031ergss-1
| Dynamical Formation of Close Binary Systems in Globular Clusters
We know from observations that globular clusters are very efficientcatalysts in forming unusual short-period binary systems or theiroffspring, such as low-mass X-ray binaries (LMXBs; neutron starsaccreting matter from low-mass stellar companions), cataclysmicvariables (white dwarfs accreting matter from stellar companions), andmillisecond pulsars (rotating neutron stars with spin periods of a fewmilliseconds). Although there has been little direct evidence, theoverabundance of these objects in globular clusters has been attributedby numerous authors to the high densities in the cores, which leads toan increase in the formation rate of exotic binary systems through closestellar encounters. Many such close binary systems emit X-radiation atlow luminosities (LX<~1034 ergs s-1)and are being found in large numbers through observations with theChandra X-Ray Observatory. Here we present conclusive observationalevidence of a link between the number of close binaries observed inX-rays in a globular cluster and the stellar encounter rate of thecluster. We also make an estimate of the total number of LMXBs inglobular clusters in our Galaxy.
| Pixel lensing observations towards globular clusters
It has been suggested that a monitoring program employing the pixellensing method to search for microlensing events towards galacticglobular clusters may increase the statistics and discriminate amongdifferent halo models. Stimulated by this proposal, we evaluate an upperlimit to the pixel lensing event rate for such a survey. Four differentdark halo models have been considered changing both the flattening andthe slope of the mass density profile. The lens mass function has beenmodelled as a homogenous power - law for mu in (mul,muu) and both the mass limits and the slope of the massfunction have been varied to investigate their effect on the rate. Thetarget globular clusters have been selected in order to minimize thedisk contribution to the event rate. We find that a pixel lensing surveytowards globular clusters is unable to discriminate among different halomodels since the number of detectable events is too small to allow anyreliable statistical analysis.
| Discovery of a quiescent neutron star binary in the globular cluster M 13
We have discovered with XMM-Newton an X-ray source in the core of theglobular cluster M 13, whose X-ray spectral properties suggest that itis a quiescent neutron star X-ray binary. The spectrum can be wellfitted with a pure hydrogen atmosphere model, with Tinfty=76+/-3 eV, Rinfty =12.8+/-0.4 km and an X-rayluminosity of 7.3+/-0.6*E32 ergs s-1. In the lightof this result, we have discovered a strong correlation between thestellar encounter rate and the number of quiescent neutron stars foundin the ten globular clusters observed so far by either XMM-Newton orChandra. This result lends strong support to the idea that these systemsare primarily produced by stellar encounters in the core of globularclusters.
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
| A Globular Cluster Metallicity Scale Based on the Abundance of Fe II
Assuming that in the atmospheres of low-mass, metal-poor red giantstars, one-dimensional models based on local thermodynamic equilibriumaccurately predict the abundance of iron from Fe II, we derive aglobular cluster metallicity scale based on the equivalent widths of FeII lines measured from high-resolution spectra of giants in 16 keyclusters lying in the abundance range-2.4<[Fe/H]II<-0.7. We base the scale largely on theanalysis of spectra of 149 giant stars in 11 clusters by the Lick-Texasgroup supplemented by high-resolution studies of giants in five otherclusters. We also derive ab initio the true distance moduli for certainkey clusters (M5, M3, M13, M92, and M15) as a means of setting stellarsurface gravities. Allowances are made for changes in the abundancescale if one employs (1) Kurucz models with and without convectiveovershooting to represent giant star atmospheres in place of MARCSmodels and (2) the Houdashelt et al. color-temperature scale in place ofthe Alonso et al. scale.We find that [Fe/H]II is correlated linearly withW', the reduced strength of the near-infrared Ca II tripletdefined by Rutledge et al., although the actual correlation coefficientsdepend on the atmospheric model employed. The correlations, limited tothe range -2.4<[Fe/H]II<-0.7, are as follows:1.[Fe/H]II=0.531W'-3.279(MARCS),2.[Fe/H]II=0.537W'-3.225 (Kurucz withconvective overshooting),3.[Fe/H]II=0.562W'-3.329 (Kurucz withoutconvective overshooting).We also discuss how to estimate [X/Fe] ratios. We suggest that C, N, andO, as well as elements appearing in the spectrum in the singly ionizedstate, e.g., Ti, Sc, Ba, La, and Eu, should be normalized to theabundance of Fe II. Other elements, which appear mostly in the neutralstate, but for which the dominant species is nevertheless the ionizedstate, are probably best normalized to Fe I, but uncertainties remain.
| Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms
We present homogeneous age determinations for a large sample of 55Galactic globular clusters, which constitute about 30% of the totalGalactic population. A study of their age distribution reveals that allclusters from the most metal poor ones up to intermediate metallicitiesare coeval, whereas at higher [Fe/H] an age spread exists, together withan age-metallicity relationship. At the same time, all clusters within acertain galactocentric distance appear coeval, whereas an age spread ispresent further away from the Galactic centre, without any correlationwith distance. The precise value of [Fe/H] and galactocentric distancefor the onset of the age spread and the slope of the age-metallicityrelationship are strongly affected by the as yet uncertain [Fe/H] scale.We discuss how differences in the adopted [Fe/H] scale and clustersample size may explain discrepant results about the clusters agedistribution reached by different authors. Taking advantage of the largenumber of objects included in our sample, we also tested the possibilitythat age is the global second parameter which determines the HorizontalBranch morphology, and found indications that age could explain theglobal behaviour of the second parameter effect.
| Modified Newtonian Dynamics as an Alternative to Dark Matter
Modified Newtonian dynamics (MOND) is an empirically motivatedmodification of Newtonian gravity or inertia suggested by Milgrom as analternative to cosmic dark matter. The basic idea is that ataccelerations below ao ~ 10-8 cm/s2 ~cHo/6 the effective gravitational attraction approaches√(gnao), where gn is the usualNewtonian acceleration. This simple algorithm yields flat rotationcurves for spiral galaxies and a mass-rotation velocity relation of theform M ∝ V4 that forms the basis for the observedluminosity-rotation velocity relation-the Tully-Fisher law. We reviewthe phenomenological success of MOND on scales ranging from dwarfspheroidal galaxies to superclusters and demonstrate that the evidencefor dark matter can be equally well interpreted as evidence for MOND. Wediscuss the possible physical basis for an acceleration-basedmodification of Newtonian dynamics as well as the extention of MOND tocosmology and structure formation.
| Setting new constraints on the age of the Universe
There are three independent techniques for determining the age of theUniverse: via cosmochronology of long-lived radioactive nuclei, viastellar modelling and population synthesis of the oldest stellarpopulations, and, most recently, via the precision cosmology that hasbecome feasible with the mapping of the acoustic peaks in the cosmicmicrowave background. We demonstrate that all three methods givecompletely consistent results, and enable us to set rigorous bounds onthe maximum and minimum ages that are allowed for the Universe. Wepresent new constraints on the age of the Universe by performing amultiband colour analysis of bright cluster ellipticals over a largeredshift range (0.3
| 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.
| 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.
| Photometric catalog of nearby globular clusters. II. A large homogeneous (V,I) color-magnitude diagram data-base
In this paper we present the second and final part of a large andphotometrically homogeneous CCD color-magnitude diagram (CMD) data base,comprising 52 nearby Galactic globular clusters (GGC) imaged in the Vand I bands. The catalog has been collected using only two telescopes(one for each hemisphere). The observed clusters represent 75% of theknown Galactic globulars with (m-M)_V<= 16.15 mag, cover most of theglobular cluster metallicity range (-2.2 <= [Fe/H]<= -0.4), andspan Galactocentric distances from ~ 1.2 to ~ 18.5 kpc. In particular,here we present the CMDs for the 13 GGCs observed in the Northernhemisphere. The remaining 39 Southern hemisphere clusters of the cataloghave been presented in a companion paper (Rosenberg et al.\cite{rosenberg00}). We present the first CCD color magnitude diagramfor NGC 6779 (M 56). All the CMDs extend from at least ~ 2 magnitudesbelow the turn-off (i.e. V_lim>=22) to the tip of the red giantbranch. The calibration has been done using a large number of standardstars, and the absolute calibration is reliable to a ~ 0.02 mag level inboth filters. This catalog, because of its homogeneity, is expected torepresent a useful data base for the measurement of the main absoluteand relative parameters characterizing the CMD of GGCs. Based onobservations made with the 1~m Jacobus Kapteyn Telescope operated on theisland of La Palma by the ING in the Spanish Observatorio del Roque deLos Muchachos of the Instituto de Astrofìsica de Canarias.
| What Are These Blue Metal-Poor Stars?
The radial velocity behavior and chemical compositions of sixty-two bluemetal-poor (BMP) stars have been established from more than 1200 echellespectra obtained at Las Campanas Observatory from 1992 through 1999.Analysis of survey spectra provides abundances for this sample, which weuse to calibrate the K line versus B-V relation. Forty-four of the starshave [Fe/H]<-1, while eighteen lie on -1<[Fe/H]<0. One star,the SX Phe variable CS 22966-043, appears to be the most extreme exampleof a rare abundance class characterized by α-element deficiencies,high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and[Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities,while 42 are definite or probable members of binary systems. The binaryfraction of BMP stars appears to be independent of chemical composition.The high binary fraction fBMP~0.6 of BMP stars compared withthat found for the F- and G-type stars near the Sun, the systematicallylow mass functions of these binaries, and the paucity of double-linedbinaries among them lead us to suggest that at least half of the BMPbinaries are blue stragglers and that these blue stragglers are formedby McCrea mass transfer rather than by the various merger processes thatare currently believed to produce most blue stragglers in globularclusters. This conclusion is supported by the abnormally high proportionof BMP binaries with long periods and small orbital eccentricities,properties these binaries share with McClure's carbon star binaries. Thegreat majority of field blue stragglers (BSs) probably are created byRoche-lobe overflow during red giant branch evolution. Primaries of morewidely separated binaries that survive this phase of stellar evolutionmay engage in mass transfer during subsequent asymptotic giant branchevolution to form s-process abundance enhanced carbon stars. Our resultrequires a major downward revision of the fraction of BMP starsattributed to a captured intermediate-age population of metal-poor fieldstars. The high original estimate of the size of this component probablyarose from improper use of the globular cluster BS specific frequency,SBS=n(BS)/n(HB)~1, to estimate the halo BS space density. Weuse a simple model to calculate the specific frequency of BSs producedby McCrea mass transfer in an old metal-poor population with a givenprimordial binary fraction fB. Our model calculations returnvalues of SBS~5 for fB=0.15, much more like ourvalue for the field blue stragglers. We suggest that globular clusterseither destroy the primordial binaries that produce long period BSbinaries like those in the Galactic field reported here, or they neverpossessed them.
| Hubble Space Telescope Photometry of the Metal-rich Globular Clusters NGC 6624 and NGC 6637
We have observed the metal-rich globular clusters NGC 6624 and NGC 6637(M69) using the planetary camera of the WFPC2 on the Hubble SpaceTelescope (HST). Observations of the Ca II triplet lines in giant starsin these clusters show that NGC 6624 and NGC 6637 have metallicities onthe Zinn and West scale of [Fe/H]=-0.63+/-0.09 and -0.65+/-0.09, onlyslightly more metal rich than 47 Tuc [Fe/H]=-0.71+/-0.07. For clustersof identical (or nearly so) metallicity, one can make a directcomparison of the color-magnitude diagrams to derive the relative agesof the clusters. From the color-magnitude diagrams derived from the HSTphotometry, we find that NGC 6624 and NGC 6637 differ in age by lessthan 0.5 Gyr. Their color-magnitude diagrams are also compared withthose of 47 Tuc and NGC 6352, and while these latter diagrams are ofsomewhat lower quality, they are consistent with all of these clustershaving the same ages. Adopting an apparent distance modulus of 13.40 andreddening E(B-V)=0.04 for 47 Tuc, the new Yale isochrones yield an agefor the clusters of 14 Gyr. The positions of NGC 6624 and NGC 6637 inthe Galaxy suggest that they belong to the bulge population of globularclusters. The only other bulge clusters that have been dated so far arethe more metal rich clusters NGC 6528 and NGC 6553, which also appear tobe very old. Consequently, the age-metallicity relation of the bulge maybe very steep. The close similarity of the ages and metallicities of NGC6624 and NGC 6637 to the thick-disk globular clusters 47 Tuc and NGC6352 indicates that the age-metallicity relations of these populationsintersect. We briefly discuss the possibility that these populations hada common origin. Based on observations made with the Anglo-AustralianTelescope, Canada-France-Hawaii Telescope, and the NASA/ESA Hubble SpaceTelescope. The observations from the Hubble Space Telescope wereobtained from the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS5-26555.
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way
Not Available
| A catalogue of helium abundance indicators from globular cluster photometry
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
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