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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.
| The Michigan/MIKE Fiber System Survey of Stellar Radial Velocities in Dwarf Spheroidal Galaxies: Acquisition and Reduction of Data
We introduce a stellar velocity survey of dwarf spheroidal galaxies,undertaken using the Michigan/MIKE Fiber System (MMFS) at theMagellan/Clay 6.5 m telescope at Las Campanas Observatory. As of 2006November we have used MMFS to collect 6415 high-resolution(R=20,000-25,000) spectra from 5180 stars in four dwarf spheroidalgalaxies: Carina, Fornax, Sculptor, and Sextans. Spectra sample therange 5140-5180 Å, which includes the prominent magnesium tripletabsorption feature. We measure radial velocity (RV) to a medianprecision of 2.0 km s-1 for stars as faint as V~20.5. Fromthe spectra we also are able to measure the strength of iron andmagnesium absorption features using spectral indices that correlate witheffective temperature, surface gravity, and chemical abundance.Measurement of line strength allows us to identify interlopingforeground stars independently of velocity and to examine themetallicity distribution among dSph members. Here we present detaileddescriptions of MMFS, our target selection and spectroscopicobservations, the data reduction procedure, and error analysis. Wecompare our RV results to previously published measurements forindividual stars. In some cases we find evidence for a mild,velocity-dependent offset between the RVs that we measure using themagnesium triplet and previously published RV measurements derived fromthe infrared calcium triplet. In companion papers we will present thecomplete data sets and kinematic analyses of these new observations.This paper includes data obtained with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| Near-infrared observations of the Fornax dwarf galaxy. I. The red giant branch
Aims.We present a study of the evolved stellar populations in the dwarfspheroidal galaxy Fornax based on wide-area near-infrared observations,aimed at obtaining new independent estimates of its distance andmetallicity distribution. Assessing the reliability of near-infraredmethods is most important in view of future space- and ground-based deepnear-infrared imaging of resolved stellar systems. Methods: We haveobtained JHK imaging photometry of the stellar populations in Fornax.The observations cover an 18.5×18.5 arcmin2 centralarea with a mosaic of SOFI images at the ESO NTT. Our data sample allthe red giant branch (RGB) for the whole area. Deeeper observationsreaching the red clump of helium-burning stars have also been obtainedfor a 4.5×4.5 arcmin2 region. Results: Near-infraredphotometry led to measurements of the distance to Fornax based on theK-band location of the RGB tip and the red clump. Once corrected for themean age of the stellar populations in the galaxy, the derived distancemodulus is (m-M)0 = 20.74±0.11, corresponding to adistance of 141 Kpc, in good agreement with estimates from optical data.We have obtained a photometric estimate of the mean metallicity of redgiant stars in Fornax from their (J-K) and (V-K) colors, using severalmethods. The effect of the age-metallicity degeneracy on the combinedoptical-infrared colors is shown to be less important than for opticalor infrared colors alone. By taking age effects into account, we havederived a distribution function of global metallicity [M/H] fromoptical-infrared colors of individual stars. Our photometric MetallicityDistribution Function covers the range -2.0 < [M/H] < -0.6, with amain peak at [M/H]≃ -0.9 and a long tail of metal-poor stars, andless metal-rich stars than derived by recent spectroscopy. Ifmetallicities from Ca II triplet lines are correct, this result confirmsa scenario of enhanced metal enrichment in the last 1-4 Gyr.Based on data collected at the European Southern Observatory, La Silla,Chile, Proposals No. 65.N-0167, 66.B-0247. Full Tables 2 and 3 are onlyavailable in electronic format at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/467/1025
| Different Characteristics of the Bright Branches of the Globular Clusters M15 and M92
We carried out relatively wide-field BVI CCD photometric observations ofthe globular clusters M15 (NGC 7078) and M92 (NGC 6341) using the 1.8 mtelescope of the Bohyun Optical Astronomy Observatory. We presentcolor-magnitude diagrams (V vs. B-V, V vs. V-I, and V vs. B-I) of M15and M92. We found asymptotic giant branch (AGB) bumps atVbumpAGB=15.20+/-0.05 mag andVbumpAGB=14.50+/-0.05 mag for M15 and M92,respectively. We identified the red giant branch (RGB) bumps of the twoclusters. We have estimated the population ratios R and R2for M15 and M92 in two cases: when only normal horizontal-branch (HB)stars are used and when all the HB stars are used. We have compared theobserved RGB luminosity functions of M15 and M92 with the theoreticalRGB luminosity functions of Bergbusch & VandenBerg and found nosignificant ``extra stars'' in the comparisons. This implies that the HBmorphology difference between M15 and M92 is not certain due to theresults of deep mixing in the RGB sequence.
| A Wide-Field Kinematic Survey for Tidal Tails around Five Globular Clusters
Using the AAOmega instrument of the Anglo-Australian Telescope, we haveobtained medium-resolution near-infrared spectra of 10,500 stars in2° fields centered on the galactic globular clusters 47 Tuc, NGC288, M12, M30, and M55. Radial velocities and equivalent widths of theinfrared Ca II triplet lines have been determined to constrain clustermembership, which in turn has been used to study the angular extent ofthe clusters. From the analysis of 140-1000 member stars in eachcluster, we do not find extended structures that go beyond the tidalradii. For three cluster we estimate a 1% upper limit of extratidal redgiant branch stars. We detect systemic rotation in 47 Tuc and M55.
| Tidal Tails around Globular Clusters: Are They a Good Tracer of Cluster Orbits?
We present the results of detailed N-body simulations of clusters movingin a realistic Milky Way (MW) potential. The strong interaction with thebulge and the disk of the Galaxy leads to the formation of tidal tails,emanating from opposite sides of the cluster. Some characteristicfeatures in the morphology and orientation of these streams arerecognized and interpreted. The tails have a complex morphology,particularly when the cluster approaches its apogalacticon, showingmultiple ``arms'' in remarkable similarity to the structures observedaround NGC 288 and Willman 1. Actually, the tails are generally goodtracers of the cluster path quite far from the cluster center (>7-8tidal radii), while on the smaller scale they are mainly pointing in thedirection of the Galaxy center. In particular, the orientation of theinner part of the tails is highly correlated with the cluster orbitalphase and the local orbital angular acceleration. This implies that, ingeneral, the orbital path cannot be estimated directly from theorientation of the tails, unless a sufficient large field around thecluster is available.
| Whiting 1: the youngest globular cluster associated with the Sagittarius dwarf spheroidal galaxy
Context: Recently, Carraro (2005) drew attention to the remarkable starcluster Whiting 1 by showing that it lies about 40 kpc from the Sun andis therefore unquestionably a member of the Galactic halo (b=-60.6 deg).Its Color Magnitude Diagram (CMD) indicated that Whiting 1 is very young(~5 Gyr) for a globular cluster. It is very likely that Whiting 1originated in a dwarf galaxy that has since been disrupted by the MilkyWay. Aims: The main goals of this investigation were to constrainbetter the age, metallicity, and distance of Whiting 1 and to assesswhether it belongs to a stellar stream from the Sagittarius dwarfspheroidal galaxy (Sgr dSph). Methods: Deep CCD photometry in the BVIpass-bands obtained with the VLT is used to improve the quality of theCMD and to determine the cluster's luminosity function and surfacedensity profile. High-resolution spectrograms obtained with Magellan areused to measure the cluster's radial velocity and to place limits on itspossible metallicity. The measurements of distance and radial velocityare used to test the cluster's membership in the stellar streams fromthe Sgr dSph. Results: From our CMD of Whiting 1, we derive newestimates for the cluster's age (6.5+1.0-0.5 Gyr),metallicity (Z=0.004±0.001, [Fe/H] = -0.65), and distance(29.4+1.8-2.0 kpc). From echelle spectrograms ofthree stars, we obtain -130.6±1.8 km s-1 for thecluster's radial velocity and show from measurements of two infra-redCaII lines that the [Fe/H] of the cluster probably lies in the range-1.1 to -0.4. Both the luminosity function and the surface densityprofile suggest that the cluster has undergone tidal stripping by theMilky Way. We demonstrate that the position of Whiting 1 on the sky, itsdistance from the Sun, and its radial velocity are identical to withinthe errors of both the theoretical predictions of the trailing stream ofstars from the Sgr dSph galaxy and the previous observations of the Mgiant stars that delineate the streams. Conclusions: With the additionof Whiting 1, there is now strong evidence that 6 globular clustersformed within the Sgr dSph. Whiting 1 is particularly interestingbecause it is the youngest and among the most metal rich. The relativelyyoung age of Whiting 1 demonstrates that this dwarf galaxy was able toform star clusters for a period of at least 6 Gyr, and the age andmetallicity of Whiting 1 are consistent with the age-metallicityrelationship in the main body of the Sgr dSph. The presence now ofWhiting 1 in the Galactic halo provides additional support for the viewthat the young halo clusters originated in dwarf galaxies that have beenaccreted by the Milky Way.Based on observations with the ESO VLT at the Paranal Observatory, underthe program 76.D-0128. This paper includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile. Tablesof the individual photometric measurements are available at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/181
| Why Haven't Loose Globular Clusters Collapsed Yet?
We report on the discovery of a surprising observed correlation betweenthe slope of the low-mass stellar global mass function (GMF) of globularclusters (GCs) and their central concentration parameterc=log(rt/rc), i.e., the logarithmic ratio of tidaland core radii. This result is based on the analysis of a sample of 20Galactic GCs with solid GMF measurements from deep HST or VLT data. Allthe high-concentration clusters in the sample have a steep GMF, mostlikely reflecting their initial mass function. Conversely,low-concentration clusters tend to have a flatter GMF, implying thatthey have lost many stars via evaporation or tidal stripping. No GCs arefound with a flat GMF and high central concentration. This findingappears counterintuitive, since the same two-body relaxation mechanismthat causes stars to evaporate and the cluster to eventually dissolveshould also lead to higher central density and possibly core collapse.Therefore, more concentrated clusters should have lost proportionatelymore stars and have a shallower GMF than low-concentration clusters,contrary to what is observed. It is possible that severely depleted GCshave also undergone core collapse and have already recovered a normalradial density profile. It is, however, more likely that GCs with a flatGMF have a much denser and smaller core than that suggested by theirsurface brightness profile and may well be undergoing collapse atpresent. In either case, we may have so far seriously underestimated thenumber of post-core collapse clusters, and many may be lurking in theMilky Way.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| Faint X-Ray Sources in the Globular Cluster Terzan 5
We report our analysis of a Chandra X-ray observation of the richglobular cluster Terzan 5, in which we detect 50 sources to a limiting1.0-6 keV X-ray luminosity of 3×1031 ergss-1 within the half-mass radius of the cluster. Thirty-threeof these have LX>1032 ergs s-1, thelargest number yet seen in any globular cluster. In addition to thequiescent low-mass X-ray binary (LMXB; identified by Wijnands et al.),another 12 relatively soft sources may be quiescent LMXBs. We comparethe X-ray colors of the harder sources in Terzan 5 to the Galacticcenter sources studied by Muno and collaborators and find the Galacticcenter sources to have harder X-ray colors, indicating a possibledifference in the populations. We cannot clearly identify a metallicitydependence in the production of low-luminosity X-ray binaries inGalactic globular clusters, but a metallicity dependence of the formsuggested by Jordán et al. for extragalactic LMXBs is consistentwith our data.
| 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.
| Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Using the FORS2 instrument on the Very Large Telescope, we have obtainednear-infrared spectra for more than 200 stars in 28 populous LMCclusters. This cluster sample spans a large range of ages (~1-13 Gyr)and metallicities (-0.3>~[Fe/H]>~-2.0) and has good areal coverageof the LMC disk. The strong absorption lines of the Ca II triplet areused to derive cluster radial velocities and abundances. We determinemean cluster velocities to typically 1.6 km s-1 and meanmetallicities to 0.04 dex (random error). For eight of these clusters,we report the first spectroscopically determined metallicities based onindividual cluster stars, and six of these eight have no publishedradial velocity measurements. Combining our data with archival HubbleSpace Telescope WFPC2 photometry, we find that the newly measuredcluster, NGC 1718, is one of the most metal-poor ([Fe/H]~-0.80)intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar towhat was found by previous authors, this cluster sample has radialvelocities consistent with that of a single rotating disk system, withno indication that the newly reported clusters exhibit halo kinematics.In addition, our findings confirm previous results that show that theLMC lacks the metallicity gradient typically seen in nonbarred spiralgalaxies, suggesting that the bar is driving the mixing of stellarpopulations in the LMC. However, in contrast to previous work, we findthat the higher metallicity clusters (>~-1.0 dex) in our sample showa very tight distribution (mean [Fe/H]=-0.48, σ=0.09), with notail toward solar metallicities. The cluster distribution is similar towhat has been found for red giant stars in the bar, which indicates thatthe bar and the intermediate-age clusters have similar star formationhistories. This is in good agreement with recent theoretical models thatsuggest the bar and intermediate-age clusters formed as a result of aclose encounter with the SMC ~4 Gyr ago.
| Line-of-sight velocity dispersions and a mass-distribution model of the Sa galaxy NGC 4594
In this paper, we develop an algorithm allowing to calculateline-of-sight velocity dispersions in an axisymmetric galaxy outside thegalactic plane. When constructing a self-consistent model, we take intoaccount the galactic surface brightness distribution, stellar rotationcurve and velocity dispersions. We assume that the velocity dispersionellipsoid is triaxial and lies under a certain angle with respect to thegalactic plane. This algorithm is applied to an Sa galaxy NGC 4594 = M104, for which there exist velocity dispersion measurements outside thegalactic major-axis. The mass-distribution model is constructed in twostages. In the first stage, we construct a luminosity-distributionmodel, where only galactic surface brightness distribution is taken intoaccount. Here, we assume the galaxy to consist of the nucleus, thebulge, the disc and the stellar metal-poor halo and determine structuralparameters of these components. Thereafter, in the second stage, wedevelop, based on the Jeans equations, a detailed mass-distributionmodel and calculate line-of-sight velocity dispersions and the stellarrotation curve. Here, a dark matter (DM) halo is added to visiblecomponents. Calculated dispersions are compared with observations alongdifferent slit positions perpendicular and parallel to the galacticmajor-axis. In the best-fitting model, velocity dispersion ellipsoidsare radially elongated withσθ/σR ~=0.9-0.4,σz/σR ~= 0.7-0.4, and lieunder the angles less than or equal to 30° with respect to thegalactic equatorial plane. Outside the galactic plane, velocitydispersion behaviour is more sensitive to the DM density distributionand allows to estimate dark halo parameters. For visible matter, thetotal M/LB = 4.5 +/- 1.2, M/LR = 3.1 +/- 0.7. Thecentral density of the DM halo is ρDM(0) =0.033Msolarpc-3.
| The Blue Straggler Population of the Globular Cluster M5: Comparison with M3
We have surveyed the blue straggler star population of the Galacticglobular cluster M5 using high-resolution images of the core along withwide-field ground-based images reaching to more than 19 core radii. Togauge M5's relative efficiency of producing stragglers, we compared oursample to five studies of other globular clusters. Using a ``bright''sample selected in the same way as that of Ferraro and coworkers, wefound a bimodal radial distribution similar to those found in threeother luminous clusters. When the radial distributions for differentclusters are scaled using the core radius, there is goodcluster-to-cluster agreement in the size of the core straggler sampleand the center of the ``zone of avoidance.'' However, M5 has thesmallest fraction of stragglers in the zone of avoidance of any of theclusters measured to date, and its zone of avoidance appears to be wider(in r/rc) than that of M3, which has a very similar surfacebrightness profile. Both of these facts indicate that M5's stragglerpopulation has dynamically evolved to a larger extent than M3's. Usingan ultraviolet sample from Hubble Space Telescope selected in the sameway as that of Ferraro and coworkers, we find that the frequency of bluestragglers in M5 is lower than in all but two of the clusters examined.We also identified seven bright blue stragglers that were previouslymisidentified as HB stars by Sandquist & Bolte. These brightstragglers are most likely the result of stellar collisions involvingbinary stars.
| Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process
We use high-resolution spectra obtained with the ESO Very LargeTelescope to measure surface abundance patterns of 43 blue stragglerstars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs showsa significant depletion of carbon and oxygen with respect to thedominant population. This evidence would suggest the presence of CNOburning products on the BSS surface coming from a deeply peeled parentstar, as expected in the case of a mass transfer process. This is thefirst detection of a chemical signature clearly pointing to a specificBSS formation process in a globular cluster.Based on observations collected at the ESO-VLT (Cerro Paranal, Chile)under program 072.D-0337.
| Chandra and Hubble Space Telescope Study of the Globular Cluster NGC 288
We report on the Chandra X-Ray Observatory observations of the globularcluster NGC 288. We detect four X-ray sources within the core radius andseven additional sources within the half-mass radius down to a limitingluminosity of LX=7×1030 ergs s-1(assuming cluster membership) in the 0.3-7 keV band. We also observedthe cluster with the Hubble Space Telescope Advanced Camera for Surveysand identify optical counterparts to seven X-ray sources out of the ninesources within the HST field of view. Based on the X-ray and opticalproperties, we find 2-5 candidates of cataclysmic variables (CVs) orchromospherically active binaries and 2-5 background galaxies inside thehalf-mass radius. Since the core density of NGC 288 is very low, thenumber of faint X-ray sources of NGC 288 found in the Chandra and HSTobservations is higher than the prediction on the basis of the collisionfrequency. We suggest that the CVs and chromospherically active binariesare primordial in origin, in agreement with the theoretical expectation.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated withprograms 10120 and 9959.
| Dynamical Formation of Close Binaries in Globular Clusters: Cataclysmic Variables
We answer the long-standing question of which production mechanism isresponsible for the cataclysmic variables (CVs) in globular clusters.Arguments have been given that range from mostly primordial presence toa significant contribution of later dynamical formation in close stellarencounters. We conclude, based on a thorough analysis of a homogeneousChandra data set, that the majority of CVs in dense globular clustershave a dynamical origin.
| 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.
| Abundance anomalies in hot horizontal branch stars of the Galactic globular cluster NGC 2808
Aims.We present metallicity measurements of 25 stars in the bluehorizontal branch of the Galactic globular cluster NGC 2808.Methods: . Our measurements are based on moderate-resolution spectrataken with the multi-object fiber facility FLAMES-UVES, mounted onKueyen at the Very Large Telescope. Results: . We confirm thatstars hotter than a threshold temperature have super-solar abundance,while the cooler ones respect the nominal metallicity of the cluster,i.e. [Fe/H]≃-1.1. The threshold temperature is estimated to beabout 12 000 K, corresponding to the so called u-jump, and coincideswith the sudden departure of the cluster horizontal branch from themodels. The metallicity increases with temperature for star hotter thanthe jump, confirming the hypothesis that the process responsible forthis abrupt metallic enhancement is the levitation due to the strongradiation field in absence of a significative convective envelope. Ametallicity dependence of the abundance enhancement is also suggested,with more metal poor clusters having a higher increase in metalcontent. Conclusions: .The slope in the temperature vs. abundancediagram is higher than the errors involved, and the metal content of thecluster plays possibly a role in determining the amplitude of the jump(more metal poor clusters show more enhancement after the jump),although other parameters, such as clusters' characteristics and eventhe atomic species involved, may also someway contribute.
| An Empirical Calibration of the Mixing-Length Parameter α
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| The Dynamical State and Blue Straggler Population of the Globular Cluster NGC 6266 (M62)
We have used a proper combination of multiband high-resolution HubbleSpace Telescope WFPC2 and wide-field ground-based observations to imagethe Galactic globular cluster NGC 6266 (M62). The extensive photometricdata set allows us to determine the center of gravity and to constructthe most extended radial profile ever published for this clusterincluding, for the first time, detailed star counts in the very innerregion. The star density profile is well reproduced by a standard Kingmodel with an extended core (~19") and a modest value of theconcentration parameter (c=1.5), indicating that the cluster has not yetexperienced core collapse. The millisecond pulsar population (whosemembers are all in binary systems) and the X-ray-emitting population(more than 50 sources within the cluster half-mass radius) suggest thatNGC 6266 is in a dynamical phase particularly active in generatingbinaries through dynamical encounters. UV observations of the centralregion have been used to probe the population of blue straggler stars,whose origin might be also affected by dynamical interactions. Thecomparison with other globular clusters observed with a similar strategyshows that the blue straggler content in NGC 6266 is relatively low,suggesting that the formation channel that produces binary systemshosting neutron stars or white dwarfs is not effective in significantlyincreasing the blue straggler population. Moreover, an anticorrelationbetween millisecond pulsar content and blue straggler specific frequencyin globular clusters seems to be emerging with increasing evidence.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 (AURA), Inc.,under NASA contract NAS5-26555. Also based on Wide Field Imagerobservations collected at the European Southern Observatory (ESO), LaSilla, Chile, within the observing programs 62.L-0354 and 64.L-0439.
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| 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.
| Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
| Galactic Globular Clusters with Luminous X-Ray Binaries
Luminous X-ray binaries (LMXBs; >1034 ergs s-1)have a neutron star or black hole primary, and in globular clusters,most of these close binaries are expected to have evolved from widerbinaries through dynamical interactions with other stars. We attempt tofind a predictor of this formation rate that is representative of theinitial properties of globular clusters rather than of the highlyevolved core quantities. Models indicate the half-light quantities bestreflect the initial conditions, so we examine whether the associateddynamical interaction rate, proportional toL1.5/r2.5h, is useful for understandingthe presence of luminous LMXBs in the Galactic globular cluster system.We find that while LMXB clusters with large values ofL1.5/r2.5h preferentially host LMXBs,the systems must also have half-mass relaxation times belowth,relax~109 yr. This relaxation time effectprobably occurs because several relaxation times are required to modifybinary separations, a timescale that must be shorter than cluster ages.The likelihood of finding an LMXB cluster is enhanced if the cluster ismetal-rich and if it is close to the bulge region. The dependence onmetallicity is most likely either due to differing initial massfunctions at the high-mass end or because bulge systems evolve morerapidly from tidal interactions with the bulge. This approach can beused to investigate globular cluster systems in external galaxies, wherecore properties are unresolved.
| Abundances in Red Giant Stars of NGC 2808 and Correlations between Chemical Anomalies and Global Parameters in Globular Clusters
We present the abundance analysis of stars from the tip of the red giantbranch (RGB) to below the RGB bump in the globular cluster NGC 2808based on high-resolution echelle spectra. We derive abundances of Al,α-process elements (Si I, Ca I, Ti I, and Ti II), and Fe-groupelements (Sc II, V I, Cr I, Cr II, Mn I, Co I, and Ni I). Apart from Mgbeing somewhat reduced, likely because it has been depleted at theexpense of Al in the MgAl cycle, the other α-element ratios showthe overabundance typical of halo stars of similar metallicity. Mn isunderabundant, whereas Fe-group elements have typical abundance ratiosnear the solar value. We detect star-to-star differences in Alabundances from the RGB tip down to the faintest star below the RGBbump, correlated with Na abundances at all luminosities. The slope ofthe Na-Al correlation is similar to the one found in M13 by Sneden etal., but it is different from those in other globular clusters ofsimilar metallicity. We find that the amount of chemical inhomogeneitiesalong the Na-O and Mg-Al anticorrelations in globular cluster red giantsis correlated with the present-day cluster mass and ellipticity.Moreover, we find for the first time a correlation between the spread inproton-capture elements and orbital parameters of clusters. The chemicalanomalies are more extended in clusters having large-sized orbits andlonger periods and those with larger inclination angles of the orbitwith respect to the Galactic plane.Based on data collected at the European Southern Observatory, Chile,during the FLAMES Science Verification program with the Ultraviolet andVisual Echelle Spectrograph at the VLT-UT2.
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