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The Infrared Ca II Triplet as Metallicity Indicator
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
| Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample
We have measured the absolute proper motions of globular clusters NGC2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on theHipparcos system, and they are the first determinations ever made forthese low Galactic latitude clusters. The proper-motion uncertaintiesrange from 0.3 to 0.5 mas yr-1. The inferred orbits indicatethat (1) the single metal-rich cluster in our sample, NGC 5927,dynamically belongs to the thick disk; (2) the remaining metal-poorclusters have rather low-energy orbits of high eccentricity, and amongthese there appear to be two ``pairs'' of dynamically associatedclusters; (3) the most energetic cluster in our sample, NGC 3201, is ona highly retrograde orbit-which had already been surmised from radialvelocity alone-with an apocentric distance of 22 kpc; and (4) none ofthe metal-poor clusters appear to be associated with the recentlydetected SDSS streams or with the Monoceros structure. These are thefirst results of the Southern Proper Motion program where thesecond-epoch observations are taken with the recent CCD camera systeminstalled on the double astrograph at El Leoncito, Argentina.
| Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| The RR Lyrae period-K-luminosity relation for globular clusters: an observational approach
The period-metallicity-K-band luminosity (PLK) relation forRR Lyrae stars in 15 Galactic globular clusters and in the LargeMagellanic Cloud (LMC) globular cluster Reticulum has been derived. Itis based on accurate near-infrared (K) photometry combined withTwo-Micron All-Sky Survey (2MASS) and other literature data. ThePLK relation has been calibrated and compared with theprevious empirical and theoretical determinations in literature. Thezero point of the absolute calibration has been obtained from the Kmagnitude of RR Lyr whose distance modulus has been measured viatrigonometric parallax with Hubble Space Telescope (HST). Using thisrelation, we obtain a distance modulus to the LMC of (m - M)0= 18.54 +/- 0.15 mag, in good agreement with recent determinations basedon the analysis of Cepheid variable stars.Based on observations collected at the European Southern Observatorywithin the observing programs 49.5-0021, 51.5-0024, 59.E-0340,64.N-0038, 68.D-0287 and at the Telescopio Nazionale Galileo.E-mail: antonio.sollima@bo.astro.it (AS)
| Multicolour CCD photometry of the variable stars in globular cluster M3
We present time-series data on the variable stars of the galacticglobular cluster Messier 3 (M3). We give BVIC light curvesfor 226 RR Lyrae, 2 SX Phe and 1 W Vir type variables, along withestimated fundamental photometric parameters such as intensity andmagnitude-averaged brightness and pulsation periods. In some cases, theperiods we have found significantly differ from the previously publishedones. This is the first published light curve and period determinationfor variable V266. The I-band light curve has not been observedpreviously for numerous (76) variables. Three new RR Lyrae variableshave been discovered. Groups of RR Lyrae variables that belong todifferent evolutionary stages and have been separated previously on thebasis of V data were found here for all colours and colour indices bycluster analysis. The I-band period-luminosity relation is alsodiscussed. From the 66 modulated (Blazhko type) RR Lyrae stars weinvestigated, six are newly identified and two of them are firstovertone pulsators. In the case of 13 RR Lyrae, the period of Blazhkocycle has been estimated for the first time. V252 is identified as a newRRd variable. Amplitude ratios of RRd stars have been investigated tosearch possible mode content changes. In contrast to previouspublications no changes have been found. Problems with the sampling ofthe time-series of typical cluster variability surveys are demonstrated.
| 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 QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
| Manganese Abundances in Cluster and Field Stars
We have derived Mn abundances for more than 200 stars in 19 globularclusters. In addition, Mn abundance determinations have been made for acomparable number of halo field and disk stars possessing an overlappingrange of metallicities and stellar parameters. Our primary data set wascomprised of high-resolution spectra previously acquired at theMcDonald, Lick, and Keck Observatories. To enlarge our data pool, weacquired globular and open cluster spectra from several otherinvestigators. Data were analyzed using synthetic spectra of the 6000Å Mn I triplet. Hyperfine structure parameters were included inthe synthetic spectra computations. Our analysis shows that for themetallicity range -0.7>[Fe/H]>-2.7, stars of 19 globular clustershave a mean relative abundance of <[Mn/Fe]>=-0.37+/-0.01(σ=0.10), a value in agreement with that of the field stars,<[Mn/Fe]>=-0.36+/-0.01 (σ=0.08). Despite the 2 orders ofmagnitude span in metallicity, the <[Mn/Fe]> ratio remainsconstant in both stellar populations. Our Mn abundance data indicatethat there is no appreciable variation in the relative nucleosyntheticcontribution from massive stars that undergo core-collapse supernovaeand thus no significant change of the associated initial mass functionin the specified metallicity range.
| 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.
| Na-O anticorrelation and HB. I. The Na-O anticorrelation in NGC 2808
We derived the atmospheric parameters and elemental abundances of Fe, O,and Na for about 120 red giant stars in the Galactic globular clusterNGC 2808. Our results are based on the analysis of medium-highresolution (R=22 000{-}24 000) GIRAFFE spectra acquired with the FLAMESspectrograph at VLT-UT2 as a part of a project aimed at studying theNa-O anticorrelation as a function of physical parameters in globularclusters. We present the anticorrelation of Na and O abundances in NGC2808 here, and discuss the distribution function of stars along thisrelation. Besides a bulk of O-normal stars with the typical compositionof field halo stars, NGC 2808 seems to host two other groups of O-poorand super O-poor stars. In this regard, NGC 2808 is similar to M 13, thetemplate cluster for the Na-O anticorrelation. However, in contrast to M13, most stars in NGC 2808 are O-rich. This might be related to thehorizontal branch morphologies that are very different in these twoclusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10(rms = 0.065 dex, from 123 stars). We also found some evidence of asmall intrinsic spread in metallicity, but more definitive conclusionsare hampered by the presence of a small differential reddening.
| Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
| 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.
| Rubidium and Lead Abundances in Giant Stars of the Globular Clusters M13 and NGC 6752
We present measurements of the neutron-capture elements Rb and Pb infive giant stars of the globular cluster NGC 6752 and Pb measurements infour giants of the globular cluster M13. The abundances were derived bycomparing synthetic spectra with high-resolution, high signal-to-noiseratio spectra obtained using HDS on the Subaru telescope and MIKE on theMagellan telescope. The program stars span the range of the O-Alabundance variation. In NGC 6752, the mean abundances are[Rb/Fe]=-0.17+/-0.06 (σ=0.14), [Rb/Zr]=-0.12+/-0.06(σ=0.13), and [Pb/Fe]=-0.17+/-0.04 (σ=0.08). In M13 the meanabundance is [Pb/Fe]=-0.28+/-0.03 (σ=0.06). Within the measurementuncertainties, we find no evidence for star-to-star variation for eitherRb or Pb within these clusters. None of the abundance ratios [Rb/Fe],[Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 mayhave slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to thesmall sample of field stars at the same metallicity. For M13 and NGC6752 the Pb abundances are in accord with predictions from a Galacticchemical evolution model. If metal-poor intermediate-mass asymptoticgiant branch stars did produce the globular cluster abundance anomalies,then such stars do not synthesize significant quantities of Rb or Pb.Alternatively, if such stars do synthesize large amounts of Rb or Pb,then they are not responsible for the abundance anomalies seen inglobular clusters.Based in part on data collected at the Subaru Telescope, which isoperated by the National Astronomical Observatory of Japan, and onobservations made with the Magellan Clay Telescope at Las CampanasObservatory.
| Central energy equipartition in multimass models of globular clusters
In the construction of multimass King-Michie models of globularclusters, an approximated central energy equipartition between stars ofdifferent mass is usually imposed by scaling the velocity parameter ofeach mass class inversely with the stellar mass, as if the distributionfunction were isothermal. In this paper, this `isothermal approximation'has been checked and its consequences on the model parameters studied bya comparison with models including central energy equipartitioncorrectly. It is found that, under the isothermal approximation, the`temperatures' of a pair of components can differ to a non-negligibleamount for low concentration distributions. It is also found that, ingeneral, this approximation leads to a significantly reduced masssegregation in comparison with that given under the exact energyequipartition at the centre. As a representative example, an isotropicthree-component model fitting a given projected surface brightness andline-of-sight velocity dispersion profiles is discussed. In thisexample, the isothermal approximation gives a cluster envelope much moreconcentrated (central dimensionless potential W= 3.3) than under thetrue equipartition (W= 5.9 × 10-2), as well as a highermass function logarithmic slope. As a consequence, the inferred totalmass (and then the global mass-to-light ratio) is a factor of 1.4 timeslower than the correct value and the amount of mass in heavy darkremnants is 3.3 times smaller. Under energy equipartition, the fate ofstars having a mass below a certain limit is to escape from the system.This limit is derived as a function of the mass and W of the componentof giant and turn-off stars.
| 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.
| Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy
The Carina dwarf spheroidal galaxy is the only galaxy of this type thatshows clearly episodic star formation separated by long pauses. Here wepresent metallicities for 437 radial velocity members of this Galacticsatellite. The metallicities and radial velocities were measured as partof a Large Programme with the Very Large Telescope at the EuropeanSouthern Observatory, Chile. We obtained medium-resolution spectroscopywith the multiobject spectrograph FLAMES. Our target red giants coverthe entire projected surface area of Carina. Our spectra are centered atthe near-infrared Ca II triplet, which is a well-established metallicityindicator for old and intermediate-age red giants. The resulting datasample provides the largest collection of spectroscopically derivedmetallicities for a Local Group dwarf spheroidal galaxy to date. Four ofour likely radial velocity members of Carina lie outside this galaxy'snominal tidal radius, supporting earlier claims of the possibleexistence of such stars beyond the main body of Carina. We find a meanmetallicity of [Fe/H]~-1.7 dex on the 1997 metallicity scale of Carrettaand Gratton for Carina. The formal FWHM of the metallicity distributionfunction is 0.92 dex, while the full range of metallicities is found tospan approximately -3.0 dex<[Fe/H]<0.0 dex. The metallicitydistribution function might be indicative of several subpopulationsdistinct in metallicity. There appears to be a mild radial gradient suchthat more metal-rich populations are more centrally concentrated,matching a similar trend for an increasing fraction of intermediate-agestars (see the 2001 work of Harbeck and coworkers). This, as well as thephotometric colors of the more metal-rich red giants, suggests thatCarina exhibits an age-metallicity relation. Indeed, the age-metallicitydegeneracy seems to conspire to form a narrow red giant branch despitethe considerable spread in metallicity and wide range of ages. Themetallicity distribution function is not well matched by a simpleclosed-box model of chemical evolution. Qualitatively better matches areobtained by chemical models that also take into account infall andoutflows. A G dwarf problem remains for all these models.Based on observations collected at the European Southern Observatory atParanal, Chile; proposal 171.B-0520(A).
| Color functions of stellar systems
Model calculations of the photometric evolution of rather dense stellarsystems, such as globular clusters, are presented. On“luminosity-effective temperature” diagrams of thesesystems, low-mass stars are concentrated near the minimum and maximumtemperatures for a given luminosity and are deficient in theintermediate region. This sort of double-peaked distribution of thestars can be avoided in open models with ejection of excess metals intothe surrounding medium. The distributions of the stars with respect toeffective temperature on a “ luminosity-effectivetemperature” diagram are sensitive to the history of starformation in the system and to possible time variations in the initialmass function. In open systems with a single-peak distribution function,the asymmetry in the distribution varies over wide limits with the lowerbound for the initial mass function and this can be used to establishwhether the first generations of stars might have been more massive thanin the present epoch.
| XMM-Newton X-ray and optical observations of the globular clusters M 55 and NGC 3201
We have observed two low concentration Galactic globular clusters withthe X-ray observatory XMM-Newton. We detect 47 faint X-ray sources inthe direction of M 55 and 62 in the field of view ofNGC 3201. Using the statistical Log N - Log Srelationship of extragalactic sources derived from XMM-Newton LockmanHole observations, to estimate the background source population, weestimate that very few of the sources (1.5±1.0) in the field ofview of M 55 actually belong to the cluster. Thesesources are located in the centre of the cluster as we expect if thecluster has undergone mass segregation. NGC 3201 hasapproximately 15 related sources, which are centrally located but arenot constrained to lie within the half mass radius. The sourcesbelonging to this cluster can lie up to 5 core radii from the centre ofthe cluster which could imply that this cluster has been perturbed.Using X-ray (and optical, in the case of M 55)colours, spectral and timing analysis (where possible) and comparingthese observations to previous X-ray observations, we find evidence forsources in each cluster that could be cataclysmic variables, activebinaries, millisecond pulsars and possible evidence for a quiescent lowmass X-ray binary with a neutron star primary, even though we do notexpect any such objects in either of the clusters, due to their lowcentral concentrations. The majority of the other sources are backgroundsources, such as AGN.
| Rubidium and lead abundances in globular clusters .
We present a brief and biased summary of key star-to-star abundancevariations recently observed in globular clusters and some possibleexplanations for these variations. Measurements of the neutron-captureelements rubidium (Rb) and lead (Pb) in the globular clusters M 13 andNGC 6752 are then presented along with preliminary measurements in M 4and M 5. The abundance ratios [Rb/Zr] and [Pb/Fe] are used to test theglobular cluster AGB pollution scenario and to gain insight into AGBnucleosynthesis.
| High-amplitude delta Scuti stars in the Galactic Bulge from the OGLE-II and MACHO data .
Searching for main-sequence pulsators, we analyzed photometry of˜200,000 variable star candidates from the OGLE-II Galacticfields, finding 193 high-amplitude delta Scuti stars. This doubles thenumber of known stars of this type. The MACHO data, available for halfof stars, were also analyzed. In our sample of the HADS stars, we found50 multiperiodic objects, including 39 that have period ratios in therange of 0.76-0.80, an indication of the radial fundamental andfirst-overtone pulsation. We discuss the resulting Petersen diagram forthese stars in view of the period ratios predicted by models. Except forstars showing pulsations in the radial fundamental mode and firstovertone, we find the evidence for higher radial overtones andnon-radial modes in the analyzed sample of multiperiodic HADS stars.
| The RR Lyrae distance scale from near-infrared photometry: current results .
We present new observational results on the RR Lyrae K-bandPeriod-Luminosity relation (PLK). Data on the Galactic globular clustersNGC 3201 and NGC 4590 (M68), and on the Large Magellanic Cloud clusterReticulum are shown. We compare the observed slopes of the PLK relationsfor these three clusters with those predicted by pulsational andevolutionary models, finding a fair agreement. Trusting on this findingwe decided to adopt these theoretical calibrations to estimate thedistance to the target clusters, finding a good agreement withoptical-based RR Lyrae distances, but with a smaller formal scatter.
| 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 metal abundance distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy*
Low-resolution spectroscopy obtained with FORS2 at the Very LargeTelescope (VLT) has been used for the measurement of individual metalabundances ([Fe/H]) for 110 variable stars, including 107 RR Lyrae starsand one anomalous Cepheid, and to trace the metal distribution of theoldest stellar component in the Sculptor dwarf spheroidal galaxy. The RRLyrae stars are spread over a 15 × 15 arcmin2 areaaround the galaxy centre. Their metallicities have an average value of[Fe/H]=-1.83 +/- 0.03 (rms = 0.26 dex) and cover the metallicity range-2.40 < [Fe/H] < -0.85 (on the scale of Zinn & West), butthere is only one variable that has [Fe/H] > -1.3. The star-to-starscatter is larger than typical errors on individual metallicities(+/-0.15-0.16 dex), indicating a real spread in metal abundances. Theradial velocities measured from the RR Lyrae spectra have a dispersionof 12.9kms-1. This value is consistent with the dispersionderived by Tolstoy et al. for metal-poor red giants associated with theblue horizontal branch stars in Sculptor. Along with the metallicitydistribution these results suggest that most of the RR Lyrae stars inSculptor arise from the same burst of stellar formation that producedthe metal-poor component, originating the galaxy blue horizontal branch.The metal-rich red horizontal branch population found to be centrallyconcentrated only produced a few (if any) of the RR Lyrae stars in oursample. The spectroscopic metallicities were used along with theapparent luminosities to study the luminosity-metallicity relationfollowed by the RR Lyrae stars in Sculptor, for which we derive ashallow slope of 0.09magdex-1. This result can be due to ahigh level of evolution off the zero-age horizontal branch of the RRLyrae stars in this galaxy, again in agreement with their origin fromthe blue horizontal branch population.
| A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies
We have compiled a sample of globular clusters with high-quality stellarabundances from the literature to compare to the chemistries of stars inthe Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clustersexamined, 29 also have kinematic information. Most of the globularclusters belong to the Galactic halo; however, a significant number havedisk kinematics or belong to the bulge. Focusing on the [α/Fe] andlight r-process element ratios, we find that most globular cluster starsmimic field stars of similar metallicities, and neither clearlyresembles the currently available stellar abundances in dwarf galaxies(including globular clusters in the Large Magellanic Cloud). Theexceptions to these general elemental ratio comparisons are alreadyknown in the literature, e.g., ω Centauri, Palomar 12, and Terzan7 associated with the Sagittarius remnant and Ruprecht 106, which has ahigh radial velocity and low [α/Fe] ratio. A few other globularclusters show more marginal peculiarities. The most notable one is thehalo cluster M68, which has a high galactocentric rotational velocity, aslightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratiosdecrease with increasing [Fe/H] at intermediate metallicities, which isconsistent with very massive stars playing a larger role in the earlychemical evolution of the Galaxy. The chemical similarities betweenglobular clusters and field stars with [Fe/H]<=-1.0 suggests a sharedchemical history in a well-mixed early Galaxy. The differences in thepublished chemistries of stars in the dwarf spheroidal galaxies suggestthat neither the globular clusters, halo stars, nor thick disk stars hadtheir origins in small isolated systems like the present-day Milky Waydwarf satellites.
| CN Abundance Inhomogeneities in the Globular Cluster Messier 4 (NGC 6121): Results Based on Merged Data Sets from the Literature
In this paper, we investigate the behavior of the λ3883 CNabsorption bands in the spectra of red giants in the globular clusterMessier 4. Data from three literature sources, including the originalsurvey of Norris, have been combined to produce a picture of the CN bandstrength over an absolute magnitude range of-2.5~0)suggests the possibility of a pre-RGB component to the abundanceinhomogeneities. Accretion of material from a central reservoir ofstellar ejecta, or mass transfer within binary star systems, arediscussed as possible origins for the enrichment of the CN-strong stars.In the case of M4, these processes must have occurred at sufficientlyearly times that dynamical relaxation of the cluster has largely erasedany initial gradient in the relative space densities of CN-strong andCN-weak stars, at least within the volume of the current stellar samplethat extends to three half-mass radii from the cluster center.
| 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.
| Metal Abundances of RR Lyrae Stars in the Metal-rich Globular Cluster NGC 6441
Low-resolution spectra have been used to measure individual metalabundances of RR Lyrae stars in NGC 6441, a Galactic globular clusterknown to have very unusual horizontal-branch morphology and periods ofthe RR Lyrae stars for its high metallicity. We find an average metalabundance of [Fe/H]=-0.69+/-0.06 (rms=0.33 dex) and [Fe/H]=-0.41+/-0.06(rms=0.36 dex) on Zinn & West and Carretta & Gratton metallicityscales, respectively, consistent with the cluster metal abundancederived by Armandroff & Zinn. Most of the metallicities wereextrapolated from calibration relations defined for [Fe/H]<=-1however, they are clearly high and contrast with the rather long periodsof the NGC 6441 variables, thus confirming that the cluster does not fitin the general Oosterhoff classification scheme. The rms scatter of theaverage is larger than observational errors (0.15-0.16 dex), possiblyindicating some spread in metallicity. However, even the metal-poorvariables, if confirmed to be cluster members, are still more metal-richthan those commonly found in the Oosterhoff type II globular clusters.Based on data collected at the Very Large Telescope of the EuropeanSouthern Observatory, Paranal, Chile, program number 71.B-0621.
| Abundances in giant stars of the globular cluster NGC 6752
Recent theoretical yields and chemical evolution models demonstrate thatintermediate-mass AGB stars cannot reproduce the observed abundancedistributions of O, Na, Mg, and Al. As a further observational test ofthis finding, we present elemental abundance ratios [X/Fe] for 20elements in 38 bright giants of the globular cluster NGC 6752 based onhigh-resolution, high signal-to-noise spectra obtained with UVES on theVLT. This is the most complete spectroscopic analysis of this cluster interms of the number of elements considered and the number of stars inthe sample. The stars span more than 1000 K in effective temperature andmore than 3 visual magnitudes along the red giant branch. None of theabundance ratios [X/Fe] show a correlation with evolutionary status. ForSi and heavier elements, the small scatter in [X/Fe] may be attributableto the measurement uncertainties. Our mean abundance ratios [X/Fe] arein good agreement with previous studies of this cluster and are alsoconsistent with other globular clusters and field stars at the samemetallicity. The mean abundance ratios [Ba/Eu] and [La/Eu] exhibitvalues, in agreement with field stars at the same metallicity, that lieapproximately midway between the pure r-process and the solar (s-process+ r-process) mix, indicating that AGB stars have played a role in thechemical evolution of the proto-cluster gas. For the first time, we findpossible evidence for an abundance variation for elements heavier thanAl in this cluster. We find a correlation between [Si/Fe] and [Al/Fe]which is consistent with the abundance anomalies being synthesized viaproton captures at high temperatures. Leakage from the Mg-Al chain into28Si may explain the Si excess in stars with the highest[Al/Fe]. We identify correlations between [Y/Fe] and [Al/Fe], [Zr/Fe]and [Al/Fe], and [Ba/Fe] and [Al/Fe] suggesting that Y, Zr, and Baabundances may increase by about 0.1 dex as Al increases by about 1.3dex. While the correlations are statistically significant, theamplitudes of the variations are small. If the small variations in Y,Zr, and Ba are indeed real, then the synthesis of the Al anomalies musthave taken place within an unknown class of stars that also ran thes-process.
| 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.
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