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A non-LTE abundance analysis of the post-AGB star ROA5701
An analysis of high-resolution Anglo-Australian Telescope(AAT)/University College London Échelle Spectrograph(UCLÉS) optical spectra for the ultraviolet (UV)-bright starROA5701 in the globular cluster ω Cen (NGC5139) is performed,using non-local thermodynamic equilibrium (non-LTE) model atmospheres toestimate stellar atmospheric parameters and chemical composition.Abundances are derived for C, N, O, Mg, Si and S, and compared withthose found previously by Moehler et al. We find a general metalunderabundance relative to young B-type stars, consistent with theaverage metallicity of the cluster. Our results indicate that ROA5701has not undergone a gas-dust separation scenario as previouslysuggested. However, its abundance pattern does imply that ROA5701 hasevolved off the asymptotic giant branch (AGB) prior to the onset of thethird dredge-up.
| Merging of Globular Clusters in Inner Galactic Regions. I. Do They Survive the Tidal Interaction?
The main topic of this paper is the investigation of the modes ofinteraction of globular clusters (GCs) moving in the inner part of agalaxy. This is tackled by means of high-resolution N-body simulations,whose first results are presented in this article. Our simulations dealtwith primordial, very massive (of the order of 107Msolar) GCs that were able to decay, because of dynamicalfriction, into the inner regions of triaxial galaxies on a timescalemuch shorter than their internal relaxation time. To check thedisruptive roles of both tidal forces and GC-GC collisions, theireffects were maximized by considering clusters on quasi-radial orbitsand choosing the initial conditions so as to give head-on collisions ateach passage through the center. The available CPU resources allowed usto simulate clusters with different structural parameters and to followthem on quasi-radial orbits during eight passages across the center. Themain findings are as follows: (1) clusters with an initial high-enoughKing concentration parameter (c>=1.2) preserve up to 50% of theirinitial mass, (2) the inner density distribution of the survivingclusters keeps a King model profile, (3) GC-GC collisions have anegligible effect compared with that caused by the passage through thegalactic center, (4) the orbital energy dissipation due to the tidalinteraction is of the same order as that caused by dynamical friction,(5) complex substructures like ``ripples'' and ``clumps'' are formed, asobserved around real clusters. These findings support the validity ofthe hypothesis of merging of GCs in the galactic central region, withmodes that deserve further careful investigation.
| 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.
| Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant Branch Bump
A Monte Carlo simulation exploring uncertainties in standard stellarevolution theory on the red giant branch of metal-poor globular clustershas been conducted. Confidence limits are derived on the absolute V-bandmagnitude of the bump in the red giant branch luminosity function(MV,b) and the excess number of stars in the bump,Rb. The analysis takes into account uncertainties in theprimordial helium abundance, abundance of α-capture elements,radiative and conductive opacities, nuclear reaction rates, neutrinoenergy losses, the treatments of diffusion and convection, the surfaceboundary conditions, and color transformations. The uncertainty intheoretical values for the red giant bump magnitude varies withmetallicity between +0.13 and -0.12 mag at [Fe/H]=-2.4 and between +0.23and -0.21 mag at [Fe/H]=-1.0. The dominant sources of uncertainty arethe abundance of the α-capture elements , the mixing length, andthe low-temperature opacities. The theoretical values of MV,bare in good agreement with observations. The uncertainty in thetheoretical value of Rb is +/-0.01 at all metallicitiesstudied. The dominant sources of uncertainty are the abundance of theα-capture elements, the mixing length, and the high-temperatureopacities. The median value of Rb varies from 0.44 at[Fe/H]=-2.4 to 0.50 at [Fe/H]=-1.0. These theoretical values forRb are in agreement with observations.
| 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.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| 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.
| 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.
| Broad-band photometric colors and effective temperature calibrations for late-type giants. II. Z < 0.02
We investigate the effects of metallicity on the broad-band photometriccolors of late-type giants, and make a comparison of synthetic colorswith observed photometric properties of late-type giants over a widerange of effective temperatures (T_eff=3500-4800 K) and gravities (logg=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on thesynthetic photometric colors is small at effective temperatures above 3800 K, but the effects grow larger at lower T_eff, due to the changingefficiency of molecule formation which reduces molecular opacities atlower [M/H]. To make a detailed comparison of the synthetic and observedphotometric colors of late type giants in the T_eff-color andcolor-color planes (which is done at two metallicities, [M/H]=-1.0 and-2.0), we derive a set of new T_eff-log g-color relations based onsynthetic photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. Theserelations are based on the T_eff-log g scales that we derive employingliterature data for 178 late-type giants in 10 Galactic globularclusters (with metallicities of the individual stars between [M/H]=-0.7and -2.5), and synthetic colors produced with the PHOENIX, MARCS andATLAS stellar atmosphere codes. Combined with the T_eff-log g-colorrelations at [M/H]=0.0 (Kučinskas et al. 2005), the set of newrelations covers metallicities [M/H]=0.0dots-2.0 (Δ[M/H]=0.5),effective temperatures T_eff=3500dots4800 K (Δ T_eff=100 K), andgravities log g=-0.5dots3.0. The new T_eff-log g-color relations are ingood agreement with published T_eff-color relations based on observedproperties of late-type giants, both at [M/H]=-1.0 and -2.0. Thedifferences in all T_eff-color planes are typically well within 100 K.We find, however, that effective temperatures predicted by the scalesbased on synthetic colors tend to be slightly higher than thoseresulting from the T_eff-color relations based on observations, with theoffsets up to 100 K. This is clearly seen both at [M/H]=-1.0 and -2.0,especially in the T_eff-(B-V) and T_eff-(V-K) planes. The consistencybetween T_eff-log g-color scales based on synthetic colors calculatedwith different stellar atmosphere codes is very good, with typicaldifferences being well within Δ T_eff 70 K at [M/H]=-1.0 andΔ T_eff 40 K at [M/H]=-2.0.
| 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.
| Hot stars in old stellar populations: a continuing need for intermediate ages
We investigate the effect of a low-level contamination of hot, old,metal-poor starlight on the inferred stellar populations of early-typegalaxies in the core of the Coma Cluster. We find that the requiredcorrection to the Balmer and metal absorption-line strengths for old,metal-poor stars does not significantly affect the inferred age of thestellar population when the Hβ strength is large. Intermediate-agedpopulations are therefore still needed to explain enhanced Balmer-linestrengths in early-type galaxies. This gives us increased confidence inour age estimates for these objects. For galaxies with weak Balmer-linestrengths corresponding to very old populations (t > 10 Gyr),however, a correction for hot stars may indeed alter the inferred age,as previously suggested. Finally, the inferred metallicity [Z/H] willalways be higher after any correction for old, metal-poor starlight thanif it were not taken into account, but the enhancement ratios [E/Fe]will strengthen only slightly.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| The evolution of binary fractions in globular clusters
We study the evolution of binary stars in globular clusters using a newMonte Carlo approach combining a population synthesis code (STARTRACK)and a simple treatment of dynamical interactions in the dense clustercore using a new tool for computing three- and four-body interactions(FEWBODY). We find that the combination of stellar evolution anddynamical interactions (binary-single and binary-binary) leads to arapid depletion of the binary population in the cluster core. Themaximum binary fraction today in the core of a typical dense clustersuch as 47 Tuc, assuming an initial binary fraction of 100 per cent, isonly ~5-10 per cent. We show that this is in good agreement with recentHubble Space Telescope observations of close binaries in the core of 47Tuc, provided that a realistic distribution of binary periods is used tointerpret the results. Our findings also have important consequences forthe dynamical modelling of globular clusters, suggesting that `realisticmodels' should incorporate much larger initial binary fractions than hasusually been the case in the past.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| The Unusual Luminosity Function of the Globular Cluster M10
We present the I-band luminosity function of the differentially reddenedglobular cluster M10. We combine photometric analysis derived fromwide-field (23'×23') images that include theouter regions of the cluster and high-resolution images of the clustercore. After making corrections for incompleteness and field starcontamination, we find that the relative numbers of stars on the lowergiant branch and near the main-sequence turnoff are in good agreementwith theoretical predictions. However, we detect significant (>6σ) excesses of red giant branch stars above and below the redgiant branch bump using a new statistic (a population ratio) for testingrelative evolutionary timescales of main-sequence and red giant stars.The statistic is insensitive to assumed cluster chemical composition,age, and main-sequence mass function. The excess number of red giantscannot be explained by reasonable systematic errors in our assumedcluster chemical composition, age, or main-sequence mass function.Moreover, M10 shows excesses when compared to the cluster M12, which hasnearly identical metallicity, age, and color-magnitude diagrammorphology. We discuss possible reasons for this anomaly, finding thatthe most likely cause is a mass function slope that shows significantvariations as a function of mass.
| 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.
| 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.
| Formation and Evolution of Clumpy Tidal Tails around Globular Clusters
We present some results of numerical simulations of a globular clusterorbiting in the central region of a triaxial galaxy on a set of ``loop''orbits. Tails start forming after about a quarter of the globularcluster orbital period and develop, in most cases, along the clusterorbit, showing clumpy substructures as observed, for example, in Palomar5. If completely detectable, clumps can contain about 7000Msolar each, i.e., about 10% of the cluster mass at thatepoch. The morphology of tails and clumps and the kinematical propertiesof stars in the tails are studied and compared with availableobservational data. Our finding is that the stellar velocity dispersiontends to level off at large radii, in agreement with that found for M15and ω Cen.
| A Comparison between Carbon and Nitrogen Abundances of Bright Giants in the Globular Clusters M13, M10, and NGC 7006
Carbon and nitrogen abundances have been derived for red giants in theglobular clusters M13, M10, and NGC 7006. These constitute a``second-parameter'' trio of clusters having very similar metallicities([Fe/H]~-1.55) but different horizontal-branch (HB) morphologies, withNGC 7006 having a much higher fraction of red HB stars than either M13or M10. The red giants observed have absolute magnitudes in the range-2.5
| Infrared Photometry of NGC 6791
We present deep JHK photometry of the old and metal-rich open clusterNGC 6791. The photometry reaches below the main-sequence turnoff toK~16.5 mag. We combine our photometry with that from Stetson et al. toprovide color-magnitude diagrams showing K versus J-K, K versus V-K, andV versus V-K. We study the slope of the red giant branch in the infraredbut find that it is not a useful metallicity indicator for the cluster,nor any metal-rich cluster that lacks a well-populated red giant branch,because it is not linear, as has often been assumed, in K versus J-K.The mean color of the red horizontal-branch/red clump stars provide anestimate of the cluster reddening, E(B-V)=0.14+/-0.04 mag for[Fe/H]=+0.4+/-0.1. The mean magnitudes of these stars also provide agood distance estimate, (m-M)0=13.07+/-0.04. Finally, we findthat the isochrones of Yi et al. provide optimal fits in V versus B-Vand V-K and K versus J-K and V-K for such values if [Fe/H] lies between+0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyr([Fe/H]=+0.3) and 7.5 Gyr ([Fe/H]=+0.5).Based on observations made with the Mayall 4 m Telescope of the NationalOptical Astronomy Observatory.
| 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.
| Eclipsing Binaries in the Young Large Magellanic Cloud Cluster NGC 1850
I present light curves for two detached eclipsing binary stars in theregion of the LMC cluster NGC 1850, which is possibly a young globularcluster still in formation. One, a likely O-type star, is a newlydetected eclipsing binary in the region of the very young subcluster NGC1850A. This binary is among a small number of highly massive O-typestars in binary systems found in LMC clusters. These two eclipsingbinaries are the first to be discovered in the well-studied NGC 1850,and the O-type star is the first eclipsing binary found in NGC 1850A.Light curves for two Cepheid variables in the NGC 1850 region are alsoshown. The discovery of two eclipsing binaries in the youngglobular-like cluster NGC 1850 is discussed in terms of the importanceof the binary fraction to globular cluster evolution.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| Red giant branch in near-infrared colour-magnitude diagrams - II. The luminosity of the bump and the tip
We present new empirical calibrations of the red giant branch (RGB) bumpand tip based on a homogeneous near-infrared data base of 24 Galacticglobular clusters. The luminosities of the RGB bump and tip in the J, Hand K bands and their dependence on the cluster metallicity have beenstudied, yielding empirical relationships. By using recenttransformations between the observational and theoretical planes, wealso derived similar calibrations in terms of bolometric luminosity.Direct comparisons between updated theoretical models and observationsshow an excellent agreement. The empirical calibration of the RGB tipluminosity in the near-infrared passbands presented here is afundamental tool to derive distances to galaxies far beyond the LocalGroup, in view of using the new ground-based adaptive optics facilitiesand, in the near future, the James Webb Space Telescope.
| Red giant branch in near-infrared colour-magnitude diagrams - I. Calibration of photometric indices
We present new high-quality near-infrared photometry of 10 Galacticglobular clusters spanning a wide metallicity range (-2.12<=[Fe/H]<=- 0.49): five clusters belong to the halo (NGC 288, 362,6752, M15 and M30) and five (NGC 6342, 6380, 6440, 6441 and 6624) to thebulge. By combining J, H and K observations with optical data, weconstructed colour-magnitude diagrams in various planes: (K, J-K), (K,V-K), (H, J-H) and (H, V-H). A set of photometric indices (colours,magnitudes and slopes) describing the location and the morphology of thered giant branch (RGB) have been measured. We have combined this newdata set with those collected by our group over the last 5 years, andhere we present an updated calibration of the various RGB indices in theTwo-Micron All-Sky Survey photometric system, in terms of the clustermetallicity.
| Abundance Variations Within Globular Clusters
Abundance variations within globular clusters (GCs), and of GC starswith respect to field stars, are important diagnostics of a variety ofphysical phenomena, related to the evolution of individual stars, masstransfer in binary systems, and chemical evolution in high densityenvironments. The broad astrophysical implications of GCs as buildingblocks of our knowledge of the Universe make a full understanding oftheir history and evolution basic in a variety of astrophysical fields.We review the current status of the research in this field, comparingthe abundances in GCs with those obtained for field stars, discussing indepth the evidence for H-burning at high temperatures in GC stars,describing the process of self-enrichment in GCs with particularreference to the case of the most massive Galactic GC ( Cen), anddiscussing various classes of cluster stars with abundance anomalies.Whereas the overall pattern might appear very complex at first sight,exciting new scenarios are opening where the interplay between GCdynamical and chemical properties are closely linked with each other.
| The Small Blue Straggler Star Population in the Dense Galactic Globular Cluster NGC 6752
We used high-resolution Hubble Space Telescope (HST)-WFPC2 andwide-field ground-based observations to construct a catalog of bluestraggler stars (BSSs) that spans the entire radial extent of theglobular cluster NGC 6752. The BSS sample is the most extensive everobtained for this cluster. Although NGC 6752 is a high-density clusterwith a large binary population, we found that its BSS content issurprisingly low: the specific number of BSSs is among the lowest evermeasured in a cluster. The BSS distribution is highly peaked in thecluster center, shows a rapid decrease at intermediate radii, and risesagain at larger distances. This distribution closely resembles thoseobserved in M3 and 47 Tuc by Ferraro and coworkers. To date, BSS surveyscovering the central regions with HST and the outer regions withwide-field CCD ground-based observations have been performed for onlythese three clusters. Despite the different dynamical properties, abimodal radial distribution has been found in each. A detailedcomparison of observed BSS luminosity and temperature distributions withtheoretical models reveals a population of luminous, hot BSSs that isnot easily interpreted.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, La Silla,Chile, within the observing program 62.L-0354 and 64.L-0439.
| M31's Undisturbed Thin Disk of Globular Clusters
We show that there is a subsystem of the M31 globular clusters withthin-disk kinematics. These clusters span the entire metallicity rangeof the M31 globular cluster system, in contrast to the (thick) diskglobular clusters in the Milky Way, which are predominantly metal-rich.Disk globular clusters are found across the entire disk of M31 and form~40% of the clusters projected on its disk. The existence of such a disksystem suggests that there was a relatively large thin disk in placevery early in M31's history. Accurate measures of the ages of theseclusters will constrain the epoch of disk formation in M31. There iscurrently no strong evidence for differences in age between Milky Wayand M31 globular clusters. While age differences are subtle for oldpopulations, it is unlikely that disk clusters with [Fe/H] around -2.0were formed after significant star formation began in the galaxy, as theprotocluster gas would be enriched by supernova ejecta. Thus it islikely that M31 had a rather large disk in place at early epochs. Thevery existence of such a cold disk means that M31 has suffered nomergers with an object of 10% or more of the disk mass since theclusters were formed. This makes the suggestion of Brown et al. that M31could have suffered an equal-mass merger 6-8 Gyr ago less viable.
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