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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.
| Exploring Halo Substructure with Giant Stars: The Dynamics and Metallicity of the Dwarf Spheroidal in Boötes
We report the results of a spectroscopic study of the Boötes (Boo)dwarf spheroidal (dSph) galaxy carried out with the WIYN telescope andthe Hydra multifiber spectrograph. Radial velocities have been measuredfor 58 Boo candidate stars selected to have magnitudes and colorsconsistent with its red and asymptotic giant branches. Within the 13'half-light radius, seven members of Boo yield a systemic velocity ofVr=95.6+/-3.4 km s-1 and a velocity dispersion ofσ0=6.6+/-2.3 km s-1. This implies a mass onthe order of 1×107 Msolar, similar to theinferred masses of other Galactic dSphs. Adopting a total Boo luminosityof L=1.8×104 to 8.6×104Lsolar implies M/L~610-130, making Boo, the most distortedknown Milky Way dwarf galaxy, potentially also the darkest. From thespectra of Boo member stars we estimate its metallicity to be [Fe/H]~-2.5, which would make it the most metal-poor dSph known to date.
| 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 Sagittarius dSph globular cluster system: variable stars
We have applied the image subtraction technique to images of fourglobular clusters which have been suggested to be associated with theSagittarius dSph galaxy: Arp, NGC634, Palomar2 and Terzan. As a resultwe have a found a sizeable population of variable stars incluiding RRLyrae and SX Phoenicis stars in the globular clusters Arp, NGC634 andTerzan. We do not confirm the presence of variable stars that has beenpreviously claimed in Palomar2. In NGC634 we have found 19 RR Lyraestars (six previously known), and 1 SXstar. Although the real amplitudesof all the variables are not completely determined, the Bailey diagramfor the RRab stars, together with the mean of its periods = 0.639 d, confirm the Oosterhoff II classification for this cluster. InArp 2 we have found 9 RRL stars (four previously known) and 3 SXPhestars. If we only consider the mean period of the RRab stars, = 0.593 d, we can classify this cluster asOosterhoff-intermediate. However, the Bailey diagram does not supportthis classification, because only one out of eight RRab stars arelocated in the intermediate region. The RRL specific frequency for thiscluster is S_{RR} = 68.8, the highest of all Sagittarius-relatedglobulars, which could be a hint that some of the studied RRL are notArp members but rather Sagittarius field stars.
| 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.
| The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars
We discuss a 175 deg2 spectroscopic survey for bluehorizontal branch (BHB) stars in the Galactic halo. We use the TwoMicron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) toselect BHB candidates, and we find that the 2MASS and SDSS colorselection is 38% and 50% efficient, respectively, for BHB stars. Oursamples include one likely runaway B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent withmembership in the halo population: the median metallicity is[Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and themean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is-4+/-30 km s-1. We discuss the theoretical basis of thePreston, Shectman, and Beers MV-color relation for BHB starsand conclude that the intrinsic shape of the BHB MV-colorrelation results from the physics of stars on the horizontal branch. Wecalculate the luminosity function for the field BHB star samples usingthe maximum likelihood method of Efstathiou and coworkers, which isunbiased by density variations. The field BHB luminosity functionexhibits a steep rise at bright luminosities, a peak between0.8
| Newly Discovered Variable Stars in the Globular Clusters NGC 5634, Arp 2 and Terzan 8
We report the discovery of a sizeable, previously unknown variable starpopulation in the Sagittarius dwarf spheroidal globular clusters NGC5634, Arp 2 and Terzan 8. Location, preliminary pulsation periods andB-band light curves in relative flux units are provided for all thesestars.
| 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.
| 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.
| Chemical Abundances for Seven Giant Stars in M68 (NGC 4590): A Globular Cluster with Abnormal Silicon and Titanium Abundances
We present a detailed chemical abundance study of seven giant stars inM68, including six red giants and one post-asymptotic giant branch (AGB)star. We find significant differences in the gravities determined usingphotometry and those obtained from ionization balance, which suggeststhat non-LTE (NLTE) affects are important for these low-gravity,metal-poor stars. We adopt an iron abundance using photometric gravitiesand Fe II lines to minimize those effects, finding [Fe/H]=-2.16+/-0.02(σ=0.04). For element-to-iron ratios, we rely on neutral linesversus Fe I and ionized lines versus Fe II (except for [O/Fe]) to alsominimize NLTE effects. We find variations in the abundances of sodiumamong the program stars. However, there is no correlation (oranticorrelation) with the oxygen abundances. Furthermore, the post-AGBstar has a normal (low) abundance of sodium. Both of these facts addfurther support to the idea that the variations seen among some lightelements within individual globular clusters arise from primordialvariations and not from deep mixing. M68, like M15, shows elevatedabundances of silicon compared with other globular clusters andcomparable-metallicity field stars. But M68 deviates even more inshowing a relative underabundance of titanium. We speculate that in M68titanium is behaving like an iron-peak element rather than its morecommonly observed adherence to enhancements seen in the so-calledα-elements such as magnesium, silicon, and calcium. We interpretthis result as implying that the chemical enrichment seen in M68 mayhave arisen from contributions from supernovae with somewhat moremassive progenitors than those that contribute to abundances normallyseen in other globular clusters. The neutron capture elements barium andeuropium vary among the stars in M15, according to earlier work bySneden et al., but [Ba/Eu] is relatively constant, suggesting that bothelements arise in the same nucleosynthesis events. M68 shares the same[Ba/Eu] ratio as the stars in M15, but the average abundance ratio ofthese elements, as well as lanthanum, are lower in M68 relative to ironthan in M15, implying a slightly weaker contribution of r-processnucleosynthesis in M68.
| 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.
| RR Lyrae Variables in Galactic Globular Clusters
RR Lyrae variables are the cornerstone of the Population II distancescale, and yet our knowledge of the RR Lyrae variable star content inGalactic globular clusters is now known to be surprisingly incomplete.In the present paper, we present our new results in this area.Highlights of our work includes: i) The discovery of a vast number ofvariable stars in M62 (NGC 6266), making it one of the three most RRLyrae-rich globular clusters known, and also placing it as Oosterhofftype I in spite of a blue horizontal branch morphology; ii) Thedetermination of light curves and Oosterhoff types for globular clustersassociated with the Sagittarius dSph galaxy, including NGC 5634, Arp 2,and Terzan 8; iii) A reassessment of the variable star content in themoderately metal-rich globular clusters M69 and NGC 6304; iv) The firsttheoretical calibration of the RR Lyrae period-luminosity-metallicityrelation in I, J, and H, as well as an updated calibration of the K-bandrelation---along with comparisons against the empirical data,particularly in I.This project was supported in part by Proyecto Fondecyt Regular 1030954.
| Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
| 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.
| A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms
We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxymapped by M-giant star tracers detected in the complete Two Micron AllSky Survey (2MASS). Near-infrared photometry of Sgr's prominent M-giantpopulation permits an unprecedentedly clear view of the center of Sgr.The main body is fitted with a King profile of limiting major-axisradius 30°-substantially larger than previously found orassumed-beyond which is a prominent break in the density profile fromstars in the Sgr tidal tails; thus the Sgr radial profile resembles thatof Galactic dwarf speroidal (dSph) satellites. Adopting traditionalmethods for analyzing dSph light profiles, we determine the brightnessof the main body of Sgr to be MV=-13.27 (the brightest of theknown Galactic dSph galaxies) and the total Sgr mass-to-light ratio tobe 25 in solar units. However, we regard the latter result withsuspicion and argue that much of the observed structure beyond theKing-fit core radius (224') may be outside the actual Sgr tidal radiusas the former dwarf spiral/irregular satellite undergoes catastrophicdisruption during its last orbits. The M-giant distribution of Sgrexhibits a central density cusp at the same location as, but not due to,the old stars constituting the globular cluster M54. A striking trailingtidal tail is found to extend from the Sgr center and arc across thesouth Galactic hemisphere with approximately constant density and meandistance varying from ~20 to 40 kpc. A prominent leading debris armextends from the Sgr center northward of the Galactic plane to anapogalacticon ~45 kpc from the Sun and then turns toward the northGalactic cap (NGC), from where it descends back toward the Galacticplane, becomes foreshortened, and, at brighter magnitudes, covers theNGC. The leading and trailing Sgr tails lie along a well-defined orbitalplane about the Galactic center. The Sun lies within a kiloparsec ofthat plane and near the path of leading Sgr debris; thus, it is possiblethat former Sgr stars are near or in the solar neighborhood. We discussthe implications of this new view of the Sgr galaxy and its entrails forthe character of the Sgr orbit, mass, mass-loss rate, and contributionof stars to the Milky Way halo. The minimal precession displayed by theSgr tidal debris along its inclined orbit supports the notion of anearly spherical Galactic potential. The number of M giants in the Sgrtails is at least 15% that contained within the King limiting radius ofthe main Sgr body. The fact that M giants, presumably formed within thepast few gigayears in the Sgr nucleus, are nevertheless so widespreadalong the Sgr tidal arms not only places limits on the dynamical age ofthese arms but also poses a timing problem that bears on the recentbinding energy of the Sgr core and that is most naturally explained byrecent and catastrophic mass loss. Sgr appears to contribute more than75% of the high-latitude, halo M giants, despite substantial reservoirsof M giants in the Magellanic Clouds. No evidence of extended M-gianttidal debris from the Magellanic Clouds is found. Generally goodcorrespondence is found between the M-giant, all-sky map of the Sgrsystem and all previously published detections of potential Sgr debris,with the exception of Sgr carbon stars, which must be subluminouscompared with counterparts in other Galactic satellites in order toresolve the discrepancy.
| 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.
| Tracing the Sgr Stream with 2MASS. Detection of Stream stars around Outer Halo globular clusters
We use infrared Color Magnitude Diagrams from the 2-Micron All-SkySurvey (2MASS) to search for stars belonging to the tidal stream of theSagittarius dwarf spheroidal galaxy (Sgr dSph) around selected Galacticglobular clusters. Statistically significant detections are presentedfor the cases of Pal 12 and NGC 4147, strongly supporting the idea thatthese clusters are associated with the Sgr Stream and that they wereprevious members of the Sgr dSph galaxy.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| 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.
| Building Up the Globular Cluster System of the Milky Way: The Contribution of the Sagittarius Galaxy
We demonstrate that there is a clear statistical correlation between the(X,Y,Z,Vr) phase-space distribution of the outer haloGalactic globular clusters (having 10 kpc<=RGC<=40 kpc)and the orbital path of the Sagittarius dwarf spheroidal galaxy (SgrdSph), as derived by Ibata & Lewis. At least four of the sample of35 globular clusters in this distance range were formerly members of theSagittarius galaxy (at the 95% confidence level) and are now distributedalong the Sagittarius Stream, a giant tidal structure that surrounds theMilky Way. This is the first instance that a statistically significantstructure associated with the Sgr dSph has been detected in the globularcluster population of the Galactic halo. Together with the fourwell-known globular clusters that are located near the center of thistidally disrupting dwarf galaxy, these clusters constitute >~20% ofthe population of outer halo (RGC>=10 kpc) clusters. TheSgr dSph was therefore not only an important contributor to the halofield-star population, but it also had a significant role in building upthe globular cluster system of the Milky Way.
| 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.
| The Stellar Population of NGC 5634: A Globular Cluster in the Sagittarius dSph Stream?
We present the first (V,V-I) color-magnitude diagram (CMD) for thepoorly studied globular cluster NGC 5634. The CMD shows a steep redgiant branch (RGB) and a predominantly blue horizontal branch (HB): boththese characteristics suggest a low metal content for this cluster. Fromthe position of the RGB in the CMD we estimate [Fe/H]=-1.94+/-0.10 andE(B-V)=0.06+/-0.01. The CMD presented here reaches V~=23, allowing us toobtain the first measure of the main-sequence turnoff (TO)VTO=21.22+/-0.15 for this cluster. By combining this figurewith the apparent luminosity of the zero-age HB (ZAHB),VZAHB=17.90+/-0.10 we obtainΔVHBTO=3.32+/-0.16, a value which is fullycompatible with that derived for the bulk of Galactic globulars. We alsoderive a true distance modulus of (m-M)0=17.17+/-0.12,corresponding to a distance of ~27.2 kpc. Most interestingly, thecluster is shown to have position and radial velocity fully compatiblewith the orbit of the Sagittarius dwarf spheroidal galaxy (dSph). Thesimilarity between the stellar populations of the cluster and theSagittarius globular Ter 8 and the metal-poor population of theSagittarius dSph also suggests that NGC 5634 was a former member of thisdisrupting galaxy, torn off by the Galactic tidal field and now lost inthe Sagittarius stream. Based on observations made with the ItalianTelescopio Nazionale Galileo, operated on the island of La Palma by theCentro Galileo Galilei of the Consorzio Nazionale per l'Astronomia el'Astrofisica at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| Radio Emission from Globular Clusters
Radio emission of globular clusters is studied by analyzing the VLAradio survey data of the NVSS and FIRST. We find that 13 clusters haveradio sources within their half-mass radii of clusters. Sources detectedpreviously in NGC 7078 and NGC 6440 are identified. Pulsars in NGC 6121,NGC 6440 and NGC 7078 cannot be detected because of the insufficientsurvey sensitivity and resolution. There may be a pulsar in the core ofTerzan 1. The nature of the extended radio source near the core of NGC6440 remains unclear. In the core of a globular cluster, there may bemany neutron stars or an intermediate mass black hole, but this cannotbe clarified with the current radio observations.
| 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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