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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.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| The RR Lyrae period-K-luminosity relation for globular clusters: an observational approach
The period-metallicity-K-band luminosity (PLK) relation forRR Lyrae stars in 15 Galactic globular clusters and in the LargeMagellanic Cloud (LMC) globular cluster Reticulum has been derived. Itis based on accurate near-infrared (K) photometry combined withTwo-Micron All-Sky Survey (2MASS) and other literature data. ThePLK relation has been calibrated and compared with theprevious empirical and theoretical determinations in literature. Thezero point of the absolute calibration has been obtained from the Kmagnitude of RR Lyr whose distance modulus has been measured viatrigonometric parallax with Hubble Space Telescope (HST). Using thisrelation, we obtain a distance modulus to the LMC of (m - M)0= 18.54 +/- 0.15 mag, in good agreement with recent determinations basedon the analysis of Cepheid variable stars.Based on observations collected at the European Southern Observatorywithin the observing programs 49.5-0021, 51.5-0024, 59.E-0340,64.N-0038, 68.D-0287 and at the Telescopio Nazionale Galileo.E-mail: antonio.sollima@bo.astro.it (AS)
| 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.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| 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.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| 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.
| RR Lyrae variables in Galactic globular clusters. I. The observational scenario
In this paper we revisit observational data concerning RR Lyrae stars inGalactic globular clusters, presenting frequency histograms offundamentalized periods for the 32 clusters having more than 12pulsators with well recognized period and pulsation mode. One finds thatthe range of fundamentalized periods covered by the variables in a givencluster remains fairly constant in varying the cluster metallicity allover the metallicity range spanned by the cluster sample, with the onlytwo exceptions given by M 15 and NGC 6441. We conclude that the width intemperature of the RR Lyrae instability strip appears largelyindependent of the cluster metallicity. At the same time, it appearsthat the fundamentalized periods are not affected by the predictedvariation of pulsators luminosity with metal abundance, indicating theoccurrence of a correlated variation in the pulsator mass. We discussmean periods in a selected sample of statistically significant ``RRrich" clusters with no less than 10 RRab and 5 RRc variables. One findsa clear evidence for the well known Oosterhoff dichotomy in the meanperiod of ab-type variables, together with a similarlyclear evidence for a constancy of the mean fundamentalized period in passing from Oosterhoff type II to type I clusters. Onthis basis, the origin of the Oosterhoff dichotomy is discussed,presenting evidence against a strong dependence of the RR Lyraeluminosity on the metal content. On the contrary, i) the continuity ofthe mean fundamentalized period, ii) the period frequency histograms inthe two prototypes M 3 (type I) and M 15 (type II), iii) the relativeabundance of first overtone pulsators, and iv) the observed differencebetween mean fundamental and fundamentalized periods, all agree in suggesting the dominant occurrence of avariation in the pulsation mode in a middle region of the instabilitystrip (the ``OR" zone), where variables of Oosterhoff type I and type IIclusters are pulsating in the fundamental or first overtone mode,respectively.
| 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.
| Horizontal-Branch Morphology and the Photometric Evolution of Old Stellar Populations
Theoretical integrated broadband colors ranging from far-UV to near-IRhave been computed for old stellar systems from our evolutionarypopulation synthesis code. These models take into account, for the firsttime, the detailed systematic variation of horizontal-branch (HB)morphology with age and metallicity. Our models show that sometemperature-sensitive color indices are significantly affected by thepresence of blue HB stars. In particular, B-V does not becomemonotonically redder as metallicity increases at given ages, but becomesbluer by as much as ~0.15 mag because of the contribution from blue HBstars. Similar trends are also found in the Washington photometricsystem. In addition to appropriate age-sensitive spectrophotometricindices, the use of far-UV to optical colors is proposed as a powerfulage diagnostic for old stellar systems with differing HB morphologies.Our models are calibrated in the B-V, V-I, C-T1, andM-T1 versus [Fe/H] planes, using low-reddened Galacticglobular clusters (GCs) [E(B-V)<0.2], and the relative age differencebetween the older inner halo Galactic GCs and younger outer halocounterparts is well reproduced. Several empirical linearcolor-metallicity transformation relations are assessed with our models,and it is noted that they may not be safely used to estimate metallicityif there are sizable age differences among GCs within and betweengalaxies. M31 GCs are found to be fundamentally similar to those in theMilky Way, not only in the optical to near-IR range, but also in the UVrange. For globular cluster systems in two nearby giant ellipticals, M87and NGC 1399, the current available photometric data in the literaturedo not appear sufficient to provide robust age discrimination. It isanticipated, however, that the detailed population models presentedhere, coupled with further precise spectrophotometric observations ofglobular cluster systems in external galaxies from the largeground-based telescopes and space UV facilities, will enable us toaccurately estimate their ages and metallicities.
| 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.
| Space distribution and motional orbits of globular clusters in the galaxy
The 29 F globular clusters in the galaxy are selected as samples.According to the basic data, radial velocities and proper motions ofsample clusters, the initial positions and velocities of the samples arereduced using the galactic coordinates, and their orbits are integratedby numerical method for three different Galactic gravitational potentialmodels. The calculating results show: (1) most of samples are located in5 kpc---10 kpc from Galactic center. All of the sample clusters presenta spherical symmetrical distribution around the Galactic center, andtheir space velocities are presented a ellipsoidal distribution; (2)According to the metallicity and basic characters, the sample clustersare separated into HB subgroup and MP subgroup. The number of samplesare changed with metallicity [Fe/H], and there is a peak at [Fe/H]=-1.6; (3) The orbits of sample clusters show mostly limited, periodiccharacteristics, but the orbits are not closed completely, their maximalgalactocentric distance is less than 40 kpc. The differences in orbitalmorphologies due to different potentials is slighting, however, given acertain potential, for clusters that have perigalactic distance smallerthan 1 kpc, some orbits may exhibit a chaotic behavior. The correlationbetween the metallicity of samples and the orbital morphologies isunclearly; (4) It is found that the semi-major axis, apogalacticdistance and azimuth period of 29 example clusters are changed withtheir metallicity similarly, but a obvious correlation is seen betweenorbital eccentricity and metallicity. There is a fraction of 24% of thesample clusters with eccentricities lower than 0.4. The differentGalactic gravitational potential have not clear influence upon theperigalactic distance, eccentricity and uncertainty of orbitalparameters, but which is significant for other parameters, such as theapogalactic distance, semi-major axis, radial period and azimuth periodand so on.
| Global metallicity of globular cluster stars from colour-magnitude diagrams
We have developed an homogeneous evolutionary scenario for H- andHe-burning low-mass stars by computing updated stellar models for a widemetallicity and age range [0.0002<=Z<=0.004 and9<=t(Gyr)<=15, respectively] suitable to study globular clusters.This theoretical scenario allows us to provide self-consistentpredictions about the dependence of selected observational features ofthe colour-magnitude diagram, such as the brightness of the turn-off(TO), the zero-age horizontal branch (ZAHB) and the red giant branchbump (BUMP), on the cluster metallicity and age. Taking into accountthese predictions, we introduce a new observable based on the visualmagnitude difference between the TO and the ZAHB[ΔMV(TO-ZAHB)], and the TO and the BUMP[ΔMV(TO-BUMP)], given byA=ΔMV(TO-BUMP)-0.566ΔMV(TO-ZAHB). Weshow that the parameter A does not depend at all on the cluster age, butthat it does strongly depend on the cluster global metallicity. Thecalibration of the parameter A as a function of Z is then provided, asbased on our evolutionary models. We tested the reliability of thisresult by also considering stellar models computed by other authors,employing different input physics. Eventually, we present clear evidencethat the variation of ΔMV(TO-BUMP) withΔMV(TO-ZAHB) does supply a powerful probe of the globalmetal abundance, at least when homogeneous theoretical frameworks areadopted. Specifically, we show that the extensive set of models byVanden Berg et al. suggests a slightly different calibration of A versusZ calibration, which however provides global metallicities higher byonly 0.08+/-0.06dex with respect to the results from our computations.We provide an estimate of the global metallicity of 36 globular clustersin the Milky Way, based on our A-Z calibration, and a largeobservational data base of Galactic globular clusters. By consideringthe empirical [Fe/H] scales by both Zinn & West and Carretta &Gratton, we are able to provide an estimate of the α-elementenhancement for all clusters in our sample. We show that the trend of[α/Fe] with respect to the iron content significantly depends onthe adopted empirical [Fe/H] scale, with the Zinn & West onesuggesting α-element enhancements in fine agreement with currentspectroscopic measurements.
| Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms
We present homogeneous age determinations for a large sample of 55Galactic globular clusters, which constitute about 30% of the totalGalactic population. A study of their age distribution reveals that allclusters from the most metal poor ones up to intermediate metallicitiesare coeval, whereas at higher [Fe/H] an age spread exists, together withan age-metallicity relationship. At the same time, all clusters within acertain galactocentric distance appear coeval, whereas an age spread ispresent further away from the Galactic centre, without any correlationwith distance. The precise value of [Fe/H] and galactocentric distancefor the onset of the age spread and the slope of the age-metallicityrelationship are strongly affected by the as yet uncertain [Fe/H] scale.We discuss how differences in the adopted [Fe/H] scale and clustersample size may explain discrepant results about the clusters agedistribution reached by different authors. Taking advantage of the largenumber of objects included in our sample, we also tested the possibilitythat age is the global second parameter which determines the HorizontalBranch morphology, and found indications that age could explain theglobal behaviour of the second parameter effect.
| On the Distribution of Orbital Poles of Milky Way Satellites
In numerous studies of the outer Galactic halo some evidence foraccretion has been found. If the outer halo did form in part or whollythrough merger events, we might expect to find coherent streams of starsand globular clusters following orbits similar to those of their parentobjects, which are assumed to be present or former Milky Way dwarfsatellite galaxies. We present a study of this phenomenon by assessingthe likelihood of potential descendant ``dynamical families'' in theouter halo. We conduct two analyses: one that involves a statisticalanalysis of the spatial distribution of all known Galactic dwarfsatellite galaxies (DSGs) and globular clusters, and a second, morespecific analysis of those globular clusters and DSGs for which fullphase space dynamical data exist. In both cases our methodology isappropriate only to members of descendant dynamical families that retainnearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) isconsidered a paradigm for the type of merger/tidal interaction event forwhich we are searching, we also undertake a case study of the Sgr systemand identify several globular clusters that may be members of itsextended dynamical family. In our first analysis, the distribution ofpossible orbital poles for the entire sample of outer(Rgc>8 kpc) halo globular clusters is tested forstatistically significant associations among globular clusters and DSGs.Our methodology for identifying possible associations is similar to thatused by Lynden-Bell & Lynden-Bell, but we put the associations on amore statistical foundation. Moreover, we study the degree of possibledynamical clustering among various interesting ensembles of globularclusters and satellite galaxies. Among the ensembles studied, we findthe globular cluster subpopulation with the highest statisticallikelihood of association with one or more of the Galactic DSGs to bethe distant, outer halo (Rgc>25 kpc), second-parameterglobular clusters. The results of our orbital pole analysis aresupported by the great circle cell count methodology of Johnston,Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC6229, NGC 7006, and Pyxis are predicted to be among those most likely toshow the clusters to be following stream orbits, since these clustersare responsible for the majority of the statistical significance of theassociation between outer halo, second-parameter globular clusters andthe Milky Way DSGs. In our second analysis, we study the orbits of the41 globular clusters and six Milky Way-bound DSGs having measured propermotions to look for objects with both coplanar orbits and similarangular momenta. Unfortunately, the majority of globular clusters withmeasured proper motions are inner halo clusters that are less likely toretain memory of their original orbit. Although four potential globularcluster/DSG associations are found, we believe three of theseassociations involving inner halo clusters to be coincidental. While thepresent sample of objects with complete dynamical data is small and doesnot include many of the globular clusters that are more likely to havebeen captured by the Milky Way, the methodology we adopt will becomeincreasingly powerful as more proper motions are measured for distantGalactic satellites and globular clusters, and especially as resultsfrom the Space Interferometry Mission (SIM) become available.
| The Star Formation History of LGS 3
We have determined the distance and star formation history of the LocalGroup dwarf galaxy LGS 3 from deep Hubble Space Telescope WFPC2observations. LGS 3 is intriguing because ground-based observationsshowed that, while its stellar population is dominated by old,metal-poor stars, there is a handful of young, blue stars. Also, thepresence of H I gas makes this a possible ``transition object'' betweendwarf spheroidal and dwarf irregular galaxies. The HST data are deepenough to detect the horizontal branch and young main sequence for thefirst time. A new distance of D=620+/-20 kpc has been measured from thepositions of the tip of the red giant branch, the red clump, and thehorizontal branch. The mean metallicity of the stars older than 8 Gyr is[Fe/H]=-1.5+/-0.3. The most recent generation of stars has [Fe/H]~-1.For the first few Gyr the global star formation rate was several timeshigher than the historical average and has been fairly constant sincethen. However, we do see significant changes in stellar populations andstar formation history with radial position in the galaxy. Most of theyoung stars are found in the central 63 pc (21"), where the starformation rate has been relatively constant, while the outer parts havehad a declining star formation rate. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Variable Stars in Galactic Globular Clusters
Based on a search of the literature up to 2001 May, the number of knownvariable stars in Galactic globular clusters is approximately 3000. Ofthese, more than 2200 have known periods and the majority (approximately1800) are of the RR Lyrae type. In addition to the RR Lyrae population,there are approximately 100 eclipsing binaries, 120 SX Phoenicisvariables, 60 Cepheids (including Population II Cepheids, anomalousCepheids and RV Tauri), and 120 SR/red variables. The mean period of thefundamental mode RR Lyrae variables is 0.585 days, for the overtonevariables it is 0.342 days (0.349 days for the first-overtone pulsatorsand 0.296 days for the second-overtone pulsators) and approximately 30%are overtone pulsators. These numbers indicate that about 65% of RRLyrae variables in Galactic globular clusters belong to Oosterhoff typeI systems. The mean period of the RR Lyrae variables in the Oosterhofftype I clusters seems to be correlated with metal abundance in the sensethat the periods are longer in the more metal poor clusters. Such acorrelation does not exist for the Oosterhoff type II clusters. Most ofthe Cepheids are in clusters with blue horizontal branches.
| Ages and Metallicities of Fornax Dwarf Elliptical Galaxies
Narrowband photometry is presented on 27 dwarf ellipticals in the Fornaxcluster. Calibrated with Galactic globular cluster data andspectrophotometric population models, the colors indicated that dwarfellipticals have a mean [Fe/H] of -1.00+/-0.28 ranging from -1.6 to-0.4. The mean age of dwarf ellipticals, also determinedphotometrically, is estimated at 10+/-1 Gyr compared with 13 Gyr forbright Fornax ellipticals. Comparison of our metallicity color andMg2 indices demonstrates that the [Mg/Fe] ratio is lower indwarf ellipticals than their more massive cousins, which is consistentwith a longer duration of initial star formation to explain theiryounger ages. There is a increase in dwarf metallicity with distancefrom the Fornax cluster center, where core galaxies are on average 0.5dex more metal-poor than halo dwarfs. In addition, we find the halodwarfs are younger in mean age compared with core dwarfs. One possibleexplanation is that the intracluster medium ram pressure strips the gasfrom dwarf ellipticals, halting star formation (old age) and stoppingenrichment (low metallicity) as they enter the core.
| Globular Cluster Subsystems in the Galaxy
Data from the literature are used to construct a homogeneous catalog offundamental astrophysical parameters for 145 globular clusters of theMilky Way Galaxy. The catalog is used to analyze the relationshipsbetween chemical composition, horizontal-branch morphology, spatiallocation, orbital elements, age, and other physical parameters of theclusters. The overall globular-cluster population is divided by a gap inthe metallicity function at [Fe/H]=-1.0 into two discrete groups withwell-defined maxima at [Fe/H]=-1.60±0.03 and -0.60±0.04.The mean spatial-kinematic parameters and their dispersions changeabruptly when the metallicity crosses this boundary. Metal-poor clustersoccupy a more or less spherical region and are concentrated toward theGalactic center. Metal-rich clusters (the thick disk subsystem), whichare far fewer in number, are concentrated toward both the Galacticcenter and the Galactic plane. This subsystem rotates with an averagevelocity of V rot=165±28 km/s and has a very steep negativevertical metallicity gradient and a negligible radial gradient. It is,on average, the youngest group, and consists exclusively of clusterswith extremely red horizontal branches. The population ofspherical-subsystem clusters is also inhomogeneous and, in turn, breaksup into at least two groups according to horizontal-branch morphology.Clusters with extremely blue horizontal branches occupy a sphericalvolume of radius 9 kpc, have high rotational velocities (Vrot=77±33 km/s), have substantial and equal negative radial andvertical metallicity gradients, and are, on average, the oldest group(the old-halo subsystem). The vast majority of clusters withintermediate-type horizontal branches occupy a more or less sphericalvolume ≈18 kpc in radius, which is slightly flattened perpendicularto the Z direction and makes an angle of ≈30° to the X-axis. Onaverage, this population is somewhat younger than the old-halo clusters(the young-halo subsystem), and exhibits approximately the samemetallicity gradients as the old halo. As a result, since theirGalactocentric distance and distance from the Galactic plane are thesame, the young-halo clusters have metallicities that are, on average,Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. Theyoung-halo subsystem, which apparently consists of objects captured bythe Galaxy at various times, contains many clusters with retrogradeorbits, so that its rotational velocity is low and has large errors, Vrot=-23±54 km/s. Typical parameters are derived for all thesubsystems, and the mean characteristics of their member globularclusters are determined. The thick disk has a different nature than boththe old and young halos. A scenario for Galactic evolution is proposedbased on the assumption that only the thick-disk and old-halo subsystemsare genetically associated with the Galaxy. The age distributions ofthese two subsystems do not overlap. It is argued that heavy-elementenrichment and the collapse of the proto-Galactic medium occurred mainlyin the period between the formation of the old-halo and thick-disksubsystems.
| What Are These Blue Metal-Poor Stars?
The radial velocity behavior and chemical compositions of sixty-two bluemetal-poor (BMP) stars have been established from more than 1200 echellespectra obtained at Las Campanas Observatory from 1992 through 1999.Analysis of survey spectra provides abundances for this sample, which weuse to calibrate the K line versus B-V relation. Forty-four of the starshave [Fe/H]<-1, while eighteen lie on -1<[Fe/H]<0. One star,the SX Phe variable CS 22966-043, appears to be the most extreme exampleof a rare abundance class characterized by α-element deficiencies,high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and[Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities,while 42 are definite or probable members of binary systems. The binaryfraction of BMP stars appears to be independent of chemical composition.The high binary fraction fBMP~0.6 of BMP stars compared withthat found for the F- and G-type stars near the Sun, the systematicallylow mass functions of these binaries, and the paucity of double-linedbinaries among them lead us to suggest that at least half of the BMPbinaries are blue stragglers and that these blue stragglers are formedby McCrea mass transfer rather than by the various merger processes thatare currently believed to produce most blue stragglers in globularclusters. This conclusion is supported by the abnormally high proportionof BMP binaries with long periods and small orbital eccentricities,properties these binaries share with McClure's carbon star binaries. Thegreat majority of field blue stragglers (BSs) probably are created byRoche-lobe overflow during red giant branch evolution. Primaries of morewidely separated binaries that survive this phase of stellar evolutionmay engage in mass transfer during subsequent asymptotic giant branchevolution to form s-process abundance enhanced carbon stars. Our resultrequires a major downward revision of the fraction of BMP starsattributed to a captured intermediate-age population of metal-poor fieldstars. The high original estimate of the size of this component probablyarose from improper use of the globular cluster BS specific frequency,SBS=n(BS)/n(HB)~1, to estimate the halo BS space density. Weuse a simple model to calculate the specific frequency of BSs producedby McCrea mass transfer in an old metal-poor population with a givenprimordial binary fraction fB. Our model calculations returnvalues of SBS~5 for fB=0.15, much more like ourvalue for the field blue stragglers. We suggest that globular clusterseither destroy the primordial binaries that produce long period BSbinaries like those in the Galactic field reported here, or they neverpossessed them.
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| 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.
| The Giant, Horizontal, and Asymptotic Branches of Galactic Globular Clusters. I. The Catalog, Photometric Observables, and Features
A catalog including a set of the most recent color-magnitude diagrams(CMDs) is presented for a sample of 61 Galactic globular clusters(GGCs). We used this database to perform a homogeneous systematicanalysis of the evolved sequences (namely, the red giant branch [RGB],horizontal branch [HB], and asymptotic giant branch [AGB]). Based onthis analysis, we present (1) a new procedure to measure the level ofthe zero-age horizontal branch (V_ZAHB) and a homogeneous set ofdistance moduli obtained by adopting the HB as standard candle; (2) anindependent estimate for RGB metallicity indicators and new calibrationsof these parameters in terms of both spectroscopic ([Fe/H]_CG97) andglobal metallicity ([M/H], including also the α-elementenhancement), such that the set of equations presented can be used tosimultaneously derive a photometric estimate of the metal abundance andthe reddening from the morphology and the location of the RGB in the(V,B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and theAGB bump (in nine GGCs). The dependence of these features on metallicityis discussed. We find that by using the latest theoretical models andthe new metallicity scales, the earlier discrepancy between theory andobservations (~0.4 mag) completely disappears.
| RR Lyrae Luminosity Differences between Oosterhoff Group I and II Cluster Systems and the Origin of the Oosterhoff Dichotomy
We present a comparative study of the Oosterhoff II cluster M2 and theOosterhoff I cluster M3. Both have similar metallicities, [Fe/H]=-1.62for M2 and -1.66 for M3, but very different horizontal-branch (HB)morphologies (B-R)/(B+V+R)=0.92 for M2 and 0.08 for M3. A period shiftanalysis and main-sequence fitting show that RRab variables in M2 areabout 0.2 mag brighter than those in M3. Comparisons of the M2 periodshift with Oosterhoff I clusters NGC 3201 and NGC 7006 also yieldsimilar results, while a comparison between M2 and the Oosterhoff IIcluster NGC 5986 reveals that the RR Lyrae luminosities are verysimilar. The luminosity difference is thought to be due to theevolutionary effect described in 1990 by Lee, Demarque, & Zinn: theM2 RRab variables have evolved away from the zero-age horizontal branch(ZAHB), while most M3 RRab variables lie near the ZAHB. A comparison ofthe mean period change rates of two clusters supports this hypothesis.Our relative age estimation using the difference in color between thebase of giant branch and turn-off point shows that M2 is about 2 Gyrolder than M3. Our result strongly suggests that the Oosterhoffdichotomy is due to age differences between Oosterhoff group I and II.This is consistent with the idea that the global second parameter isage. We discuss the kinematic differences between Oosterhoff group I andII clusters. Our result shows that the Oosterhoff group I clusters havezero or retrograde rotation with =-68+/-56 km s^-1 andsigma_los=131+/-28 km s^-1, while the Oosterhoff group II clusters haveprograde rotation with =+94+/-47 km s^-1 andsigma_los=115+/-29 km s^-1, confirming a similar conclusion of van denBergh. The difference in kinematics and ages between Oosterhoff group Iand II clusters suggests that they may have different origins: TheOosterhoff II clusters were formed very early in the proto-Galaxy whilethe Oosterhoff I clusters were formed at different locations and at alater time, and were probably merger events. The period distributions ofan unbiased sample of field RRab variables with |Z|<=3 kpc and|Z|>=5 kpc indicate that they may belong to different populations,with peak periods of 0.65 and 0.55 days, respectively. If the hypothesisthat the Oosterhoff dichotomy is due to evolution is correct, then thisperiod shift among the field RR Lyrae variables suggests that the RRabpopulation with |Z|<=3 kpc is somewhat older than the RRab populationwith |Z|>=5 kpc. This also suggests different formation histories. Inan appendix, we discuss that the frequently used Gaussian HBmass-dispersion rate (i.e., the mass-loss rate at the red giant branch[RGB] tip) in synthetic HB model calculations cannot fully explain theextended blue HB population and the pulsational properties of RR Lyraevariables in M2. Comparisons with synthetic HB models strongly suggeststhat an enhanced mass loss is required that extends the HB toward lowerHB masses. We also discuss statistical effects on the metallicityestimate using P_0,min for field RRab variables reported by Castellani,Maceroni, & Tosi in 1983. Our calculations suggest that thestatistical effect is sufficient to explain the apparent gradient inP_0,min without introducing a metallicity gradient.
| The metal-rich bulge globular cluster NGC 6401
We present V and I photometry for the bulge globular cluster NGC 6401for the first time. The Colour-Magnitude Diagram reveals a redhorizontal branch, and the cluster is metal-rich ([Fe/H] ~ -0.7). NGC6401 is located at 5.3(deg) from the Galactic center, turning out to bean interesting target to trace the extent of the bulge. A reddeningE(B-V) = 0.53+/-0.15 and a distance from the Sun dsun ~12.0+/-1.0 kpc are derived. The cluster is slightly behind the bulk ofthe bulge population in that direction, but still within the bulgevolume. Since the number of clusters with Horizontal Branch informationhas increased enormously in the later years for the central 20(deg)x20(deg), we present a discussion on the distribution of red and bluehorizontal branch clusters and their possible relation to bulge and/orhalo. Observations collected at the European Southern Observatory --ESO, Chile, proposal no. 61.E-0335
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