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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.
| 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.
| The period-luminosity relation for type II Cepheids in globular clusters
We report the result of our near-infrared observations (JHKs)for type II Cepheids (including possible RV Tau stars) in galacticglobular clusters. We detected variations of 46 variables in 26 clusters(10 new discoveries in seven clusters) and present their light curves.Their periods range from 1.2 d to over 80 d. They show a well-definedperiod-luminosity relation at each wavelength. Two type II Cepheids inNGC 6441 also obey the relation if we assume the horizontal branch starsin NGC 6441 are as bright as those in metal-poor globular clusters inspite of the high metallicity of the cluster. This result supports thehigh luminosity which has been suggested for the RR Lyr variables inthis cluster. The period-luminosity relation can be reproduced using thepulsation equation assuming that all the stars have the same mass.Cluster RR Lyr variables were found to lie on an extrapolation of theperiod-luminosity relation. These results provide important constraintson the parameters of the variable stars.Using Two Micron All-Sky Survey (2MASS) data, we show that the type IICepheids in the Large Magellanic Cloud (LMC) fit our period-luminosityrelation within the expected scatter at the shorter periods. However, atlong periods (P > 40 d, i.e. in the RV Tau star range) the LMC fieldvariables are brighter by about one magnitude than those of similarperiods in galactic globular clusters. The long-period cluster starsalso differ from both these LMC stars and galactic field RV Tau stars ina colour-colour diagram. The reasons for these differences arediscussed.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| VLT spectroscopy of globular cluster systems. II. Spectroscopic ages, metallicities, and [ α/Fe] ratios of globular clusters in early-type galaxies
An analysis of ages, metallicities, and [ α/Fe] ratios of globularcluster systems in early-type galaxies is presented, based on Lick indexmeasurements summarized in Puzia et al. (2004, A&A, 415, 123, PaperI of this series). In the light of calibration and measurementuncertainties, age-metallicity degeneracy, and the relative dynamicrange of Lick indices, as well as systematics introduced by abundanceratio variations (in particular variations of [ α/Fe] ratios), wefind that the most reliable age indicator for our dataset is acombination of the Lick Balmer-line indices Hγ_A, Hβ, andHδ_A. [MgFe]´ is used as a spectroscopic metallicityindicator which is least affected by [ α/Fe] variations. Weintroduce an interpolation routine to simultaneously derive ages,metallicities, and [ α/Fe] ratios from diagnostic gridsconstructed from Lick indices. From a comparison of high-quality datawith SSP model predictions, we find that 2/3 of the globular clustersin early-type galaxies are older than 10 Gyr, up to 1/3 have ages in therange 5{-}10 Gyr, and only a few cluster are younger than 5 Gyr. Oursample of globular clusters covers metallicities from [Z/H] ≈ -1.3 upto 0.5 dex. We find that metal-rich globular clusters show on average asmaller mean age and a larger age scatter than their metal-poorcounterparts. [ α/Fe] diagnostic plots show that globular clustersystems in early-type galaxies have super-solar α/Fe abundanceratios with a mean [ α/Fe] = 0.47 ± 0.06 dex and adispersion of 0.3 dex. We find evidence for a correlation between [α/Fe] and metallicity, in the sense that more metal-rich clustersexhibit lower α-element enhancements. A discussion of systematicsrelated to the Lick index system shows that the method suffers to someextent from uncertainties due to unknown horizontal branch morphologiesat high metallicities. However, these systematics still allow us to makegood qualitative statements. A detailed investigation of indices as afunction of data quality reveals that the scatter in Balmer index valuesdecreases for higher-quality data. In particular, extremely low Balmerindex values that are lower than any SSP model prediction tend todisappear. Furthermore, we find that observed photometric colors are ingood agreement with computed SSP colors using ages and metallicities asderived from the spectroscopic line indices.
| 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.
| A robust method for the analysis of integrated spectra from globular clusters using Lick indices
We define a method for the analysis of the integrated spectra ofextragalactic globular clusters that provides more reliable measures ofthe age, metallicity and α-element abundance ratio than have sofar been achieved. The method involves the simultaneous fitting of up to25 Lick indices in a χ2 fitting technique that maximizesthe use of the available data. Here we compare three sets of singlestellar population (SSP) models of Lick indices to the highsignal-to-noise, integrated spectra of 20 Galactic globular clusters.The ages, [Fe/H] and α-element abundance ratios derived from theSSP models are compared with the results of resolved stellar populationstudies from the literature. We find good consistency with the publishedvalues, with an agreement of better than 0.1 dex in all three derivedparameters. The technique allows the identification of abundance ratioanomalies, such as the known nitrogen overabundance in Galactic globularclusters, and the presence of anomalous horizontal branch morphologies.It also minimizes the impact on the derived parameters of imperfectcalibration to the Lick system, and reduction errors in general. Themethod defined in this paper is therefore robust with respect to many ofthe difficulties that plague the application of SSP models in general.Consequently, it is well suited to the study of extragalactic globularcluster systems.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| 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.
| On the origin of red giant depletion through low-velocity collisions
We investigate a means of explaining the apparent paucity of red giantstars within post-core-collapse globular clusters. We propose thatcollisions between the red giants and binary systems can lead to thedestruction of some proportion of the red giant population, by eitherknocking out the core of the red giant or by forming a common envelopesystem which will lead to the dissipation of the red giant envelope.Treating the red giant as two point masses, one for the core and anotherfor the envelope (with an appropriate force law to take account of thedistribution of mass), and the components of the binary system alsotreated as point masses, we utilize a four-body code to calculate thetime-scales on which the collisions will occur. We then perform a seriesof smooth particle hydrodynamics runs to examine the details of masstransfer within the system. In addition, we show that collisions betweensingle stars and red giants lead to the formation of a common envelopesystem which will destroy the red giant star. We find that low-velocitycollision between binary systems and red giants can lead to thedestruction of up to 13 per cent of the red giant population. This couldhelp to explain the colour gradients observed in PCC globular clusters.We also find that there is the possibility that binary systems formedthrough both sorts of collision could eventually come into contactperhaps producing a population of cataclysmic variables.
| Star Clusters in the Galactic Anticenter Stellar Structure and the Origin of Outer Old Open Clusters
The Galactic anticenter stellar structure (GASS) has been identifiedwith excess surface densities of field stars in several large-area skysurveys and with an unusual, stringlike grouping of five globularclusters. At least two of these are diffuse, young ``transitional''clusters between open and globular types. Here we call attention to thefact that four younger open or transitional clusters extend thepreviously identified, stringlike cluster grouping, with at least onehaving a radial velocity consistent with the previously found GASSvelocity-longitude trend. All nine clusters lie close to a plane tipped17° to the Galactic plane. This planar orientation is used to foragefor additional potential cluster members in the inner Galaxy, and anumber are found along the same plane and stringlike sequence, includingalmost all 15 known outer, old open clusters. Tidal accretion of a dwarfsatellite galaxy on a low-inclination orbit-perhaps the GASSsystem-appears to be a plausible explanation for the origin of theouter, old open and transitional clusters of the Milky Way. We use theseclusters to explore the age-metallicity relation of the putativeaccreted GASS progenitor. Finally, we provide the first radial velocityof a star in the cluster BH 176 and discuss its implications.
| 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.
| Hubble Space Telescope Snapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441
We present the results of a Hubble Space Telescope snapshot program tosurvey the inner region of the metal-rich globular cluster NGC 6441 forits variable stars. A total of 57 variable stars were found, including38 RR Lyrae stars, six Population II Cepheids, and 12 long-periodvariables. Twenty-four of the RR Lyrae stars and all of the PopulationII Cepheids were previously undiscovered in ground-based surveys. Of theRR Lyrae stars observed in this survey, 26 are pulsating in thefundamental mode with a mean period of 0.753 days and 12 arefirst-overtone-mode pulsators with a mean period of 0.365 days. Thesevalues match up very well with those found in ground-based surveys.Combining all the available data for NGC 6441, we find mean periods of0.759 and 0.375 days for the RRab and RRc stars, respectively. We alsofind that the RR Lyrae stars in this survey are located in the sameregions of a period-amplitude diagram as those found in ground-basedsurveys. The overall ratio of RRc to total RR Lyrae stars is 0.33.Although NGC 6441 is a metal-rich globular cluster and would, on thatground, be expected either to have few RR Lyrae stars or to be anOosterhoff type I system, its RR Lyrae stars more closely resemble thosein Oosterhoff type II globular clusters. However, even compared withtypical Oosterhoff type II systems, the mean period of its RRab stars isunusually long. We also derived I-band period-luminosity relations forthe RR Lyrae stars. Of the six Population II Cepheids, five are of WVirginis type and one is a BL Herculis variable star. This makes NGC6441, along with NGC 6388, the most metal-rich globular cluster known tocontain these types of variable stars. Another variable, V118, may alsobe a Population II Cepheid, given its long period and its separation inmagnitude from the RR Lyrae stars. We examine the period-luminosityrelation for these Population II Cepheids and compare it with those inother globular clusters and in the Large Magellanic Cloud. We argue thatthere does not appear to be a change in the period-luminosity relationslope between the BL Herculis and W Virginis stars, but that a change ofslope does occur when the RV Tauri stars are added to theperiod-luminosity relation.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.
| 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.
| Dynamical evolution of star clusters in tidal fields
We report results of a large set of N-body calculations aimed to studythe evolution of multi-mass star clusters in external tidal fields. Ourclusters start with the same initial mass-functions, but varyingparticle numbers, orbital types and density profiles. Our main focus isto study how the stellar mass-function and other cluster parameterschange under the combined influence of stellar evolution, two-bodyrelaxation and the external tidal field. We find that the lifetimes ofstar clusters moving on similar orbits scale as T sim T_RH^x, where T_RHis the relaxation time, and the exponent x depends on the initialconcentration of the cluster and is around x approx 0.75. From theresults for the lifetimes, we predict that between 53% to 67% of allgalactic globular clusters will be destroyed within the next Hubbletime. Low-mass stars are preferentially lost and the depletion is strongenough to turn initially increasing mass-functions into mass-functionswhich decrease towards the low-mass end. The details of this depletionare insensitive to the starting condition of the cluster. Thepreferential depletion of low-mass stars from star clusters leads to adecrease of their mass-to-light ratios except for a short period closeto final dissolution. The fraction of compact remnants is increasingthroughout the evolution and they are more strongly concentrated towardsthe cluster cores than main-sequence stars. For a sample of galacticglobular clusters with well observed parameters, we find a correlationbetween the observed slope of the mass-function and the lifetimespredicted by us. It seems possible that galactic globular clustersstarted with a mass-function similar to what one observes for theaverage mass-function of the galactic disc and bulge. (Abridged)
| Stellar population models of Lick indices with variable element abundance ratios
We provide the whole set of Lick indices from CN1 toTiO2 in the wavelength range 4000 <~<~λ<~6500 Å of simple stellar population models with, for the firsttime, variable element abundance ratios, [α/Fe ]= 0.0, 0.3, 0.5,[α/Ca ]=-0.1, 0.0, 0.2, 0.5 and [α/N]=-0.5, 0.0. The modelscover ages between 1 and 15 Gyr, metallicities between 1/200 and 3.5solar. The impact from the element abundance changes on theabsorption-line indices is taken from Tripicco & Bell, using anextension of the method introduced by Trager et al. Our models are freefrom the intrinsic α/Fe bias that was imposed by the Milky Waytemplate stars up to now, hence they reflect well-defined α/Feratios at all metallicities. The models are calibrated with Milky Wayglobular clusters for which metallicities and α/Fe ratios areknown from independent spectroscopy of individual stars. Themetallicities that we derive from the Lick indices Mgb and Fe5270 are inexcellent agreement with the metallicity scale by Zinn & West, andwe show that the latter provides total metallicity rather than ironabundance. We can reproduce the relatively strong CN-absorption featuresCN1 and CN2 of galactic globular clusters withmodels in which nitrogen is enhanced by a factor of 3. An enhancement ofcarbon, instead, would lead to serious inconsistencies with the indicesMg1 and C24668. The calcium sensitive index Ca4227of globular clusters is well matched by our models with [Ca/Fe]= 0.3,including the metal-rich bulge clusters NGC 6528 and 6553. From ourα/Fe-enhanced models we infer that the index [MgFe] defined byGonzález is quite independent of α/Fe but still slightlydecreases with increasing α/Fe. We find that the index , instead,is completely independent of α/Fe and serves best as a tracer oftotal metallicity. Searching for blue indices that give similarinformation as Mg b and , we find that CN1 andFe4383 may be best suited to estimating α/Fe ratios of objects atredshifts z~ 1.
| Integrated spectroscopy of bulge globular clusters and fields. II. Implications for population synthesis models and elliptical galaxies
An empirical calibration is presented for the synthetic Lick indices(e.g. Mg2, , Hβ, etc.) of Simple StellarPopulation (SSP) models that for the first time extends up to solarmetallicity. This is accomplished by means of a sample of Milky Wayglobular clusters (GCs) whose metallicities range from ~Zsun/30 to Z ~ Zsun, thanks to the inclusion ofseveral metal rich clusters belonging to the Galactic bulge (e.g., NGC6553 and NGC 6528). This metallicity range approaches the regime that isrelevant for the interpretation of the integrated spectra of ellipticalgalaxies. It is shown that the spectra of both the globular clusters andthe Galactic bulge follow the same correlation between magnesium andiron indices that extends to elliptical galaxies, showing weaker ironindices at given magnesium indices with respect to the predictions ofmodels that assume solar-scaled abundances. This similarity providesrobust empirical evidence for enhanced [α/Fe] ratios in thestellar populations of elliptical galaxies, since the globular clustersare independently known to have enhanced [α/Fe] ratios fromspectroscopy of individual stars. We check the uniqueness of this alpha-overabundance solution by exploring the whole range of modelingredients and parameters, i.e. fitting functions, stellar tracks, andthe initial mass function (IMF). We argue that the standard models(meant for solar abundance ratios) succeed in reproducing the Mg-Fecorrelation at low metallicities ([Z/H]lapprox-0.7) because the stellartemplates used in the synthesis are Galactic halo stars that actuallyare alpha -enhanced. The same models, however, fail to predict theobserved Mg-Fe pattern at higher metallicities ([Z/H] >~-0.7) (i.e.,for bulge clusters and ellipticals alike) because the high-metallicitytemplates are disk stars that are not alpha -enhanced. We show that thenew set of SSP models which incorporates the dependence on the[α/Fe] ratio (Thomas et al. \cite{Thomas02b}) is able toreproduce the Mg and Fe indices of GCs at all metallicities, with analpha -enhancement [α/Fe]=+0.3, in agreement with the availablespectroscopic determinations. The Hβ index and the higher-orderBalmer indices are well calibrated, provided the appropriate morphologyof the Horizontal Branch is taken into account. In particular, theBalmer line indices of the two metal rich clusters NGC 6388 and NGC6441, which are known to exhibit a tail of warm Horizontal Branch stars,are well reproduced. Finally, we note that the Mg indices of verymetal-poor ([Z/H] <~-1.8) populations are dominated by thecontribution of the lower Main Sequence, hence are strongly affected bythe present-day mass function of individual globular clusters, which isknown to vary from cluster to cluster due to dynamical effects.
| Integrated spectroscopy of bulge globular clusters and fields. I. The data base and comparison of individual Lick indices in clusters and bulge
We present a comprehensive spectroscopic study of the integrated lightof metal-rich Galactic globular clusters and the stellar population inthe Galactic bulge. We measure line indices which are defined by theLick standard system and compare index strengths of the clusters andGalactic bulge. Both metal-rich globular clusters and the bulge aresimilar in most of the indices, except for the CN index. We find asignificant enhancement in the CN/ index ratio in metal-richglobular clusters compared with the Galactic bulge. The mean iron index of the two metal-rich globular clusters NGC 6528 and NGC 6553is comparable with the mean iron index of the bulge. Index ratios suchas Mgb/, Mg2/, Ca 4227/, andTiO/, are comparable in both stellar population indicatingsimilar enhancements in individual elements which are traced by theindices. From the globular cluster data we fully empirically calibrateseveral metallicity-sensitive indices as a function of [Fe/H] and findtightest correlations for the Mg2 index and the composite[MgFe] index. We find that all indices show a similar behavior withgalactocentric radius, except for the Balmer series, which show a largescatter at all radii. However, the scatter is entirely consistent withthe cluster-to-cluster variations in the horizontal branch morphology.Appendices B-D are only available in electronic form athttp://www.edpsciences.org
| Does the mixing length parameter depend on metallicity?. Further tests of evolutionary sequences using homogeneous databases
This paper is a further step in the investigation of the morphology ofthe color-magnitude diagram of Galactic globular clusters, and thefine-tuning of theoretical models, made possible by the recentobservational efforts to build homogeneous photometric databases. Inparticular, we examine here the calibration of the morphologicalparameter WHB vs. metallicity, originally proposed by Brocatoet al. (\cite{brocatoEtal98}; B98), which essentially measures the colorposition of the red-giant branch. We show that the parameter can be usedto have a first-order estimate of the cluster metallicity, since thedispersion around the mean trend with [Fe/H] is compatible with themeasurement errors. The tight WHB-[Fe/H] relation is thenused to show that variations in helium content or age do not affect theparameter, whereas it is strongly influenced by the mixing-lengthparameter alpha (as expected). This fact allows us, for the first time,to state that there is no trend of alpha with the metal content of acluster. A thorough examination of the interrelated questions of thealpha -elements enhancement and the color-Tefftransformations, highlights that there is an urgent need for anindependent assessment of which of the two presently acceptedmetallicity scales is the true indicator of a cluster's iron content.Whatever scenario is adopted, it also appears that a deep revision ofthe V-I-temperature relations is needed.
| 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.
| Red giant branch stars as probes of stellar populations. I. 2MASS calibration and application to 2MASS GC01
The near-infrared behavior of the red giant branch (RGB hereafter) as afunction of abundance is examined with an unprecedented large sample of27 Galactic globular clusters with Two Micron All Sky Survey photometry.We propose a new simplified analysis, involving the zero point of theRGB slope fit, and derive calibrations for the RGB slope, zero point,and tip. The weak metallicity sensitivity of the zero point leads to a``fan''-like diagram to obtain the abundance distributions in resolvedstellar systems, and reddening estimates. Finally, we apply the newcalibrations to the recently discovered Galactic globular cluster 2MASSGC01, to derive [Fe/H]H96=-1.19+/-0.38 mag. The uncertaintyis dominated by the severe foreground contamination. We estimate anextinction of AV=21.07+/-2.20 mag toward the cluster.
| 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.
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