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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.
| 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.
| 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.
| 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.
| 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.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| A 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.
| 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.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| 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 Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
| A Globular Cluster Metallicity Scale Based on the Abundance of Fe II
Assuming that in the atmospheres of low-mass, metal-poor red giantstars, one-dimensional models based on local thermodynamic equilibriumaccurately predict the abundance of iron from Fe II, we derive aglobular cluster metallicity scale based on the equivalent widths of FeII lines measured from high-resolution spectra of giants in 16 keyclusters lying in the abundance range-2.4<[Fe/H]II<-0.7. We base the scale largely on theanalysis of spectra of 149 giant stars in 11 clusters by the Lick-Texasgroup supplemented by high-resolution studies of giants in five otherclusters. We also derive ab initio the true distance moduli for certainkey clusters (M5, M3, M13, M92, and M15) as a means of setting stellarsurface gravities. Allowances are made for changes in the abundancescale if one employs (1) Kurucz models with and without convectiveovershooting to represent giant star atmospheres in place of MARCSmodels and (2) the Houdashelt et al. color-temperature scale in place ofthe Alonso et al. scale.We find that [Fe/H]II is correlated linearly withW', the reduced strength of the near-infrared Ca II tripletdefined by Rutledge et al., although the actual correlation coefficientsdepend on the atmospheric model employed. The correlations, limited tothe range -2.4<[Fe/H]II<-0.7, are as follows:1.[Fe/H]II=0.531W'-3.279(MARCS),2.[Fe/H]II=0.537W'-3.225 (Kurucz withconvective overshooting),3.[Fe/H]II=0.562W'-3.329 (Kurucz withoutconvective overshooting).We also discuss how to estimate [X/Fe] ratios. We suggest that C, N, andO, as well as elements appearing in the spectrum in the singly ionizedstate, e.g., Ti, Sc, Ba, La, and Eu, should be normalized to theabundance of Fe II. Other elements, which appear mostly in the neutralstate, but for which the dominant species is nevertheless the ionizedstate, are probably best normalized to Fe I, but uncertainties remain.
| HST observations of the metal rich globular clusters NGC 6496 and NGC 6352
Deep exposures of the metal-rich globular clusters NGC 6496 and NGC 6352were obtained with the WFPC2 camera on board the Hubble Space Telescope(HST) through the F606W and F814W filters. The resultingcolour-magnitude diagrams (CMD) reach down to absolute magnitudeM814 =~ 10-10.5, approximately 5 magnitudes below the mainsequence (MS) turn-off (TO). The MS of the two clusters are sharp andwell defined and their fiducial lines overlap almost exactly throughoutthis range. Their colour is, however, more than 0.1 mag redder than theMS fiducial line of the prototype metal-rich globular cluster 47 Tuc(NGC 104), after proper correction for the relative distances andreddening. This provides solid empirical evidence of a higher metalcontent, which is not surprising if these objects belong indeed to thebulge as their present location suggests. A good fit to the upper partof the MS of both clusters is obtained with a 10 Gyr-old theoreticalisochrone from Baraffe et al. (\cite{Baraffe98}) for a metallicity of[M/H]=-0.5, but at lower luminosities all models depart considerablyfrom the observations, probably because of a deficiency in the treatmentof the TiO opacity. The luminosity functions (LF) obtained from theobserved CMD are rather similar to one another and show a peak atM814 =~ 9. The present day mass functions (PDMF) of bothclusters are derived down to M814 =~ 10.5 or m =~ 0.2Msun and are consistent with power-law indices alpha =0.7 forNGC 6496 and alpha =0.6 for NGC 6352. The PDMF of NGC 104 is twice assteep in the same mass range (alpha =1.4). We investigate the origin ofthis discrepancy and show that it can be understood if the two clusterscontain a considerably higher fraction of primordial binaries amongsttheir MS population, similar to that expected in the bulge. We brieflydiscuss the implications of this finding on the process of star andbinary formation and on the universality of the IMF.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA forNASA under contract NAS5-26555.
| Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms
We present homogeneous age determinations for a large sample of 55Galactic globular clusters, which constitute about 30% of the totalGalactic population. A study of their age distribution reveals that allclusters from the most metal poor ones up to intermediate metallicitiesare coeval, whereas at higher [Fe/H] an age spread exists, together withan age-metallicity relationship. At the same time, all clusters within acertain galactocentric distance appear coeval, whereas an age spread ispresent further away from the Galactic centre, without any correlationwith distance. The precise value of [Fe/H] and galactocentric distancefor the onset of the age spread and the slope of the age-metallicityrelationship are strongly affected by the as yet uncertain [Fe/H] scale.We discuss how differences in the adopted [Fe/H] scale and clustersample size may explain discrepant results about the clusters agedistribution reached by different authors. Taking advantage of the largenumber of objects included in our sample, we also tested the possibilitythat age is the global second parameter which determines the HorizontalBranch morphology, and found indications that age could explain theglobal behaviour of the second parameter effect.
| Variable Stars in Galactic Globular Clusters
Based on a search of the literature up to 2001 May, the number of knownvariable stars in Galactic globular clusters is approximately 3000. Ofthese, more than 2200 have known periods and the majority (approximately1800) are of the RR Lyrae type. In addition to the RR Lyrae population,there are approximately 100 eclipsing binaries, 120 SX Phoenicisvariables, 60 Cepheids (including Population II Cepheids, anomalousCepheids and RV Tauri), and 120 SR/red variables. The mean period of thefundamental mode RR Lyrae variables is 0.585 days, for the overtonevariables it is 0.342 days (0.349 days for the first-overtone pulsatorsand 0.296 days for the second-overtone pulsators) and approximately 30%are overtone pulsators. These numbers indicate that about 65% of RRLyrae variables in Galactic globular clusters belong to Oosterhoff typeI systems. The mean period of the RR Lyrae variables in the Oosterhofftype I clusters seems to be correlated with metal abundance in the sensethat the periods are longer in the more metal poor clusters. Such acorrelation does not exist for the Oosterhoff type II clusters. Most ofthe Cepheids are in clusters with blue horizontal branches.
| Planets in 47 Tuc
We consider the survivability of planetary systems in the globularcluster 47 Tucanae. We compute the cross-sections for the breakup ofplanetary systems via encounters with single stars and binaries. We alsocompute the cross-sections to leave planets on eccentric orbits. We findthat wider planetary systems (d>~0.3au) are likely to be broken up inthe central regions of 47 Tucanae (within the half-mass radius of thecluster). However, tighter systems and those in less-dense regions maysurvive. Tight systems will certainly survive in less-dense clusterswhere subsequent surveys should be conducted.
| Ages and Metallicities of Fornax Dwarf Elliptical Galaxies
Narrowband photometry is presented on 27 dwarf ellipticals in the Fornaxcluster. Calibrated with Galactic globular cluster data andspectrophotometric population models, the colors indicated that dwarfellipticals have a mean [Fe/H] of -1.00+/-0.28 ranging from -1.6 to-0.4. The mean age of dwarf ellipticals, also determinedphotometrically, is estimated at 10+/-1 Gyr compared with 13 Gyr forbright Fornax ellipticals. Comparison of our metallicity color andMg2 indices demonstrates that the [Mg/Fe] ratio is lower indwarf ellipticals than their more massive cousins, which is consistentwith a longer duration of initial star formation to explain theiryounger ages. There is a increase in dwarf metallicity with distancefrom the Fornax cluster center, where core galaxies are on average 0.5dex more metal-poor than halo dwarfs. In addition, we find the halodwarfs are younger in mean age compared with core dwarfs. One possibleexplanation is that the intracluster medium ram pressure strips the gasfrom dwarf ellipticals, halting star formation (old age) and stoppingenrichment (low metallicity) as they enter the core.
| The Relative Age of the Thin and Thick Galactic Disks
We determine the relative ages of the open cluster NGC 188 and selectedHipparcos field stars by isochrone fitting and compare them to the ageof the thick-disk globular cluster 47 Tuc. The best-fit age for NGC 188was determined to be 6.5+/-1.0 Gyr. The solar-metallicity Hipparcosfield stars yielded a slightly older thin-disk age, 7.5+/-0.7 Gyr. Twoslightly metal-poor ([Fe/H]=-0.22) field stars whose kinematic andorbital parameters indicate that they are members of the thin disk werefound to have an age of 9.7+/-0.6 Gyr. The age for 47 Tuc was determinedto be 12.5+/-1.5 Gyr. All errors are internal errors due to theuncertainty in the values of metallicity and reddening. Thus, the oldeststars dated in the thin disk are found to be 2.8+/-1.6 Gyr younger than47 Tuc. Furthermore, as discussed by Chaboyer, Sarajedini, &Armandroff, 47 Tuc has a similar age to three globular clusters locatedin the inner part of the Galactic halo, implying that star formation inthe thin disk started within 2.8+/-1.6 Gyr of star formation in thehalo.
| Exploring Halo Substructure with Giant Stars. I. Survey Description and Calibration of the Photometric Search Technique
We have begun a survey of the structure of the Milky Way halo, as wellas the halos of other Local Group galaxies, as traced by theirconstituent giant stars. These giant stars are identified vialarge-area, CCD photometric campaigns. Here we present the basis for ourphotometric search method, which relies on the gravity sensitivity ofthe Mg I triplet+MgH features near 5150 Å in F-K stars, and whichis sensed by the flux in the intermediate-band DDO51 filter. Ourtechnique is a simplified variant of the combined Washington/DDO51four-filter technique described by Geisler, which we modify for thespecific purpose of efficiently identifying distant giant stars forfollow-up spectroscopic study: We show here that for most stars theWashington T1-T2 color is correlated monotonicallywith the Washington M-T2 color with relatively low scatter;for the purposes of our survey, this correlation obviates the need toimage in the T1 filter, as originally proposed by Geisler. Tocalibrate our (M-T2, M-DDO51) diagram as a means todiscriminate field giant stars from nearby dwarfs, we utilize newphotometry of the main sequences of the open clusters NGC 3680 and NGC2477 and the red giant branches of the clusters NGC 3680, Melotte 66,and ω Centauri, supplemented with data on field stars, globularclusters and open clusters by Doug Geisler and collaborators. Bycombining the data on stars from different clusters, and by takingadvantage of the wide abundance spread within ω Centauri, weverify the primary dependence of the M-DDO51 color on luminosity anddemonstrate the secondary sensitivity to metallicity among giant stars.Our empirical results are found to be generally consistent with thosefrom analysis of synthetic spectra by Paltoglou & Bell. Finally, weprovide conversion formulae from the (M, M-T2) system to the(V, V-I) system, corresponding reddening laws, as well as empirical redgiant branch curves from ω Centauri stars for use in derivingphotometric parallaxes for giant stars of various metallicities (butequivalent ages) to those of ω Centauri giants.
| Globular Cluster Subsystems in the Galaxy
Data from the literature are used to construct a homogeneous catalog offundamental astrophysical parameters for 145 globular clusters of theMilky Way Galaxy. The catalog is used to analyze the relationshipsbetween chemical composition, horizontal-branch morphology, spatiallocation, orbital elements, age, and other physical parameters of theclusters. The overall globular-cluster population is divided by a gap inthe metallicity function at [Fe/H]=-1.0 into two discrete groups withwell-defined maxima at [Fe/H]=-1.60±0.03 and -0.60±0.04.The mean spatial-kinematic parameters and their dispersions changeabruptly when the metallicity crosses this boundary. Metal-poor clustersoccupy a more or less spherical region and are concentrated toward theGalactic center. Metal-rich clusters (the thick disk subsystem), whichare far fewer in number, are concentrated toward both the Galacticcenter and the Galactic plane. This subsystem rotates with an averagevelocity of V rot=165±28 km/s and has a very steep negativevertical metallicity gradient and a negligible radial gradient. It is,on average, the youngest group, and consists exclusively of clusterswith extremely red horizontal branches. The population ofspherical-subsystem clusters is also inhomogeneous and, in turn, breaksup into at least two groups according to horizontal-branch morphology.Clusters with extremely blue horizontal branches occupy a sphericalvolume of radius 9 kpc, have high rotational velocities (Vrot=77±33 km/s), have substantial and equal negative radial andvertical metallicity gradients, and are, on average, the oldest group(the old-halo subsystem). The vast majority of clusters withintermediate-type horizontal branches occupy a more or less sphericalvolume ≈18 kpc in radius, which is slightly flattened perpendicularto the Z direction and makes an angle of ≈30° to the X-axis. Onaverage, this population is somewhat younger than the old-halo clusters(the young-halo subsystem), and exhibits approximately the samemetallicity gradients as the old halo. As a result, since theirGalactocentric distance and distance from the Galactic plane are thesame, the young-halo clusters have metallicities that are, on average,Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. Theyoung-halo subsystem, which apparently consists of objects captured bythe Galaxy at various times, contains many clusters with retrogradeorbits, so that its rotational velocity is low and has large errors, Vrot=-23±54 km/s. Typical parameters are derived for all thesubsystems, and the mean characteristics of their member globularclusters are determined. The thick disk has a different nature than boththe old and young halos. A scenario for Galactic evolution is proposedbased on the assumption that only the thick-disk and old-halo subsystemsare genetically associated with the Galaxy. The age distributions ofthese two subsystems do not overlap. It is argued that heavy-elementenrichment and the collapse of the proto-Galactic medium occurred mainlyin the period between the formation of the old-halo and thick-disksubsystems.
| Hubble Space Telescope Photometry of the Metal-rich Globular Clusters NGC 6624 and NGC 6637
We have observed the metal-rich globular clusters NGC 6624 and NGC 6637(M69) using the planetary camera of the WFPC2 on the Hubble SpaceTelescope (HST). Observations of the Ca II triplet lines in giant starsin these clusters show that NGC 6624 and NGC 6637 have metallicities onthe Zinn and West scale of [Fe/H]=-0.63+/-0.09 and -0.65+/-0.09, onlyslightly more metal rich than 47 Tuc [Fe/H]=-0.71+/-0.07. For clustersof identical (or nearly so) metallicity, one can make a directcomparison of the color-magnitude diagrams to derive the relative agesof the clusters. From the color-magnitude diagrams derived from the HSTphotometry, we find that NGC 6624 and NGC 6637 differ in age by lessthan 0.5 Gyr. Their color-magnitude diagrams are also compared withthose of 47 Tuc and NGC 6352, and while these latter diagrams are ofsomewhat lower quality, they are consistent with all of these clustershaving the same ages. Adopting an apparent distance modulus of 13.40 andreddening E(B-V)=0.04 for 47 Tuc, the new Yale isochrones yield an agefor the clusters of 14 Gyr. The positions of NGC 6624 and NGC 6637 inthe Galaxy suggest that they belong to the bulge population of globularclusters. The only other bulge clusters that have been dated so far arethe more metal rich clusters NGC 6528 and NGC 6553, which also appear tobe very old. Consequently, the age-metallicity relation of the bulge maybe very steep. The close similarity of the ages and metallicities of NGC6624 and NGC 6637 to the thick-disk globular clusters 47 Tuc and NGC6352 indicates that the age-metallicity relations of these populationsintersect. We briefly discuss the possibility that these populations hada common origin. Based on observations made with the Anglo-AustralianTelescope, Canada-France-Hawaii Telescope, and the NASA/ESA Hubble SpaceTelescope. The observations from the Hubble Space Telescope wereobtained from the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS5-26555.
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| Photometric catalog of nearby globular clusters. I. A large homogeneous (V,I) color-magnitude diagram data-base
We present the first part of the first large and homogeneous CCDcolor-magnitude diagram (CMD) data base, comprising 52 nearby Galacticglobular clusters (GGC) imaged in the V and I bands using only twotelescopes (one for each hemisphere). The observed clusters represent75% of the known Galactic globulars with (m-M)_V<= 16.15 mag, covermost of the globular cluster metallicity range (-2.2 <= [Fe/H]<=-0.4), and span Galactocentric distances from ~ 1.2 to ~ 18.5 kpc. Inthis paper, the CMDs for the 39 GGCs observed in the southern hemisphereare presented. The remaining 13 northern hemisphere clusters of thecatalog are presented in a companion paper. For four clusters (NGC 4833,NGC 5986, NGC 6543, and NGC 6638) we present for the first time a CMDfrom CCD data. The typical CMD span from the 22nd V magnitudeto the tip of the red giant branch. Based on a large number of standardstars, the absolute photometric calibration is reliable to the ~ 0.02mag level in both filters. This catalog, because of its homogeneity, isexpected to represent a useful data base for the measurement of the mainabsolute and relative parameters characterizing the CMD of GGCs. Basedon data collected at the European Southern Observatory, La Silla, Chile.
| Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way
Not Available
| The red giant branches of Galactic globular clusters in the [(V-I)0,MV] plane: metallicity indices and morphology
The purpose of this study is to carry out a thorough investigation ofthe changes in morphology of the red giant branch (RGB) of Galacticglobular clusters (GGC) as a function of metallicity, in the V,I bands.To this aim, two key points are developed in the course of the analysis.(a) Using our photometric V,I database for Galactic globular clusters(the largest homogeneous data sample to date; Rosenberg et al.\cite{rsp99}) we measure a complete set of metallicity indices, based onthe morphology and position of the red-giant branch. In particular, weprovide here the first calibration of the S, Delta V1.1 andDelta V1.4 indices in the (V-I,V) plane. We show that ourindices are internally consistent, and we calibrate each index in termsof metallicity, both on the Zinn & West (1984) and the Carretta& Gratton (1997) scales. Our new calibrations of the (V-I)_{0,g},Delta V1.2, (V-I)-3.0 and (V-I)-3.5indices are consistent with existing relations. (b) Using a grid ofselected RGB fiducial points, we define a function in the(V-I)0,MI,[Fe/H] space which is able to reproducethe whole set of GGC giant branches in terms of a single parameter (themetallicity). As a first test, we show that the function is able topredict the correct trend of our observed indices with metallicity. Theusage of this function will improve the current determinations ofmetallicity and distances within the Local Group, since it allows toeasily map (V-I)0,MI coordinates into[Fe/H],MI ones. To this aim the ``synthetic'' RGBdistribution is generated both for the currently used Lee et al. (1990)distance scale, and for the most recent results on the RR Lyr distancescale.
| The formation and early evolution of the Milky Way galaxy.
Not Available
| Galactic Globular Cluster Relative Ages
Based on a new large, homogeneous photometric database of 34 Galacticglobular clusters (GGCs plus Palomar 12), a set of distance- andreddening- independent relative age indicators has been measured. Theobserved δ(V-I)2.5 andΔVHBTO versus metallicity relations havebeen compared with the relations predicted by two recently updatedlibraries of isochrones. Using these models and two independent methods,we have found that self-consistent relative ages can be estimated forour GGC sample. In turn, this demonstrates that the models areinternally self-consistent. Based on the relative age versus metallicitydistribution, we conclude that (1) there is no evidence of an age spreadfor clusters with [Fe/H]<-1.2, all the clusters of our sample in thisrange being old and coeval; (2) for the intermediate-metallicity group(-1.2<=[Fe/H]<-0.9), there is a clear evidence of age dispersion,with clusters up to ~25% younger than the older members; and (3) theclusters within the metal-rich group ([Fe/H]>=-0.9) seem to be coevalwithin the uncertainties (except Pal 12) but younger (~17%) than thebulk of the GGCs. The latter result is totally model dependent. From theGalactocentric distribution of the GGC ages, we can divide the GGCs intwo groups: the old, coeval clusters and the young clusters. The secondgroup can be divided into two subgroups: the ``really young clusters''and the ``young, but model dependent,'' which are within theintermediate- and high-metallicity groups, respectively. From thisdistribution, we can present a possible scenario for the Milky Way'sformation: The globular cluster formation process started at the samezero age throughout the halo, at least out to ~20 kpc from the Galacticcenter. According to the present stellar evolution models, themetal-rich clusters are formed at a later time (~17% lower age).Finally, significantly younger halo GGCs are found at anyRGC>8 kpc. For these, a possible scenario associated withmergers of dwarf galaxies to the Milky Way is suggested.
| The Giant, Horizontal, and Asymptotic Branches of Galactic Globular Clusters. I. The Catalog, Photometric Observables, and Features
A catalog including a set of the most recent color-magnitude diagrams(CMDs) is presented for a sample of 61 Galactic globular clusters(GGCs). We used this database to perform a homogeneous systematicanalysis of the evolved sequences (namely, the red giant branch [RGB],horizontal branch [HB], and asymptotic giant branch [AGB]). Based onthis analysis, we present (1) a new procedure to measure the level ofthe zero-age horizontal branch (V_ZAHB) and a homogeneous set ofdistance moduli obtained by adopting the HB as standard candle; (2) anindependent estimate for RGB metallicity indicators and new calibrationsof these parameters in terms of both spectroscopic ([Fe/H]_CG97) andglobal metallicity ([M/H], including also the α-elementenhancement), such that the set of equations presented can be used tosimultaneously derive a photometric estimate of the metal abundance andthe reddening from the morphology and the location of the RGB in the(V,B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and theAGB bump (in nine GGCs). The dependence of these features on metallicityis discussed. We find that by using the latest theoretical models andthe new metallicity scales, the earlier discrepancy between theory andobservations (~0.4 mag) completely disappears.
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