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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.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| 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.
| A census with ROSAT of low-luminosity X-ray sources in globular clusters
I analyze 101 observations from the ROSAT archive to search for X-raysources in or near 55 globular clusters. New sources are found in thecores of NGC 362 (a double source), NGC 6121 (marginally significant),NGC 6139, and NGC 6266; and outside the cores of NGC 6205, NGC 6352 andNGC 6388. More accurate positions are determined for the X-ray sourcesin some ten clusters. The improved position for the source in NGC 6341excludes the suggested ultraviolet counterpart. It is shown that one ofthe two sources reported near the core of NGC 6626 is spurious, as isthe detection of a pulsar period in the PSPC data of this cluster; thecentral source is resolved in three sources. One source reportedpreviously in NGC 6304 is demoted to an upper limit. For 20 clustercores better upper limits to the X-ray luminosity are obtained. From astatistical analysis I argue that several sources outside the clustercores may well belong to the clusters. All spectral energy distributionsobserved so far are relatively soft, with bremsstrahlung temperatures =~0.9 keV; there is evidence however that bremsstrahlung spectra do notcorrectly describe the spectra. The X-ray luminosity per unit mass forthe cluster as a whole does not depend on the concentration; theluminosity per unit mass for the core may increase with the clusterconcentration.
| 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.
| Serendipitous 2MASS Discoveries near the Galactic Plane: A Spiral Galaxy and Two Globular Clusters
We present the basic properties of three objects near the GalacticPlane-a large galaxy and two candidate globular clusters-discovered inthe Two Micron All-Sky Survey (2MASS) data set. All were noted duringspot checks of the data during 2MASS quality assurance reviews. Thegalaxy is a late-type spiral galaxy (Sc-Sd), ~11 Mpc distant, atl=236.82d, b=-1.86d. From its observed angular extent of 6.3′ inthe near infrared, we estimate an extinction-corrected optical diameterof ~9.5′, making it larger than most Messier galaxies. Thecandidate globular clusters are ~2'-3' in extentand are hidden optically behind foreground extinctions ofAV~18-21 mag at l~10deg, b~0deg. Thesechance discoveries were not the result of any kind of systematic search,but they do hint at the wealth of obscured sources of all kinds, manypreviously unknown, that are in the 2MASS data set.
| 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.
| B and V photometry of the metal-rich bulge globular cluster NGC 6304
We present B and V photometry of the bulge globular cluster NGC 6304. Wederive a reddening E(B-V) ~ 0.5 and a distance from the Sundsun ~ 6 kpc. From the red giant branch morphology weestimate that the metallicity of NGC 6304 is intermediate between thoseof 47 Tuc and NGC 6528. The cluster is foreground to the bulk of thebulge population and the reddenings are comparable. Evidence of a hotstellar component belonging to the cluster is found. These hot starscould correspond to a blue extended horizontal branch and/or bluestragglers. Observations collected at the European Southern Observatory-- ESO, Chile, proposal no. 61.E-0335
| Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way
Not Available
| A catalogue of helium abundance indicators from globular cluster photometry
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
| The metal-rich bulge globular cluster NGC 6401
We present V and I photometry for the bulge globular cluster NGC 6401for the first time. The Colour-Magnitude Diagram reveals a redhorizontal branch, and the cluster is metal-rich ([Fe/H] ~ -0.7). NGC6401 is located at 5.3(deg) from the Galactic center, turning out to bean interesting target to trace the extent of the bulge. A reddeningE(B-V) = 0.53+/-0.15 and a distance from the Sun dsun ~12.0+/-1.0 kpc are derived. The cluster is slightly behind the bulk ofthe bulge population in that direction, but still within the bulgevolume. Since the number of clusters with Horizontal Branch informationhas increased enormously in the later years for the central 20(deg)x20(deg), we present a discussion on the distribution of red and bluehorizontal branch clusters and their possible relation to bulge and/orhalo. Observations collected at the European Southern Observatory --ESO, Chile, proposal no. 61.E-0335
| The metal-rich globular clusters of the Milky Way
We present new (V,V-I)-photometry of the metal-rich globular clustersNGC 5927, 6316, 6342, 6441 and 6760. The clustersshow differential reddening up to delta E_{V-I}=0.32 mag, for which theCMDs are corrected via extinction maps. There are hints of a variationin the extinction law. Two different ways to determine the parametersmetallicity, reddening and distance lead to consistent results. Themetallicities of the clusters range between -0.7 <= [M/H] <= 0.0dex and the absolute reddening between 0.43 <= E_{V-I} <= 0.76mag. Taking the differential reddening into account leads to slightlyincreased distances. From the resulting parameters we conclude that theusual halo-disk-distinction in the system of globular clusters seemsquestionable.
| VI CCD photometry of metal-rich globular clusters: NGC 6528
We present CCD photometry in V and I for the metal-rich globular clusterNGC 6528. A comparison with previous photometry reveals discrepancies ofthe order 0.1 to 0.2 mag in V, emphasizing the need for independentphotometry. As found previously, the giant branch (or the asymptoticgiant branch, which cannot be distinguished) of the cluster extends toV-I = 7. Population synthesis in V-I must take these red stars intoaccount in order to understand integrated red colours of bulgepopulations. Currently available theoretical isochrones do not reproducethe shape of the RGB/AGB. The derivation of reliable values forreddening, distance and metallicity of NGC 6528 is hampered byuncertainties in the extrapolation to high metallicity, which dominatethe absolute error rather than the photometric uncertainty does.Together with other properties (old age derived from HST data, highradial velocity), this object is more likely a member of the bulge thanof the disk population. Table 6 containing the photometry of individualstars is only available in electronic form via anonymous ftp130.79.128.5 or http: //cdsweb.u-strasbg.fr/Abstract.html Based onobservations obtained at the European Southern Observatory, La Silla,Chile.
| Galactic Globular Cluster Metallicity Scale from the Ca II Triplet II. Rankings, Comparisons, and Puzzles
We compare our compilation of the redew calcium index for 71 Galacticglobular clusters to the widely used markcite{zinn84}Zinn and West (1984ApJS, 55, 45) fe scale and to Carretta and Gratton's (1997 A&AS,121, 95) scale from high-dispersion spectra analyzed with Kurucz (1992,private communication) model atmospheres. We find our calcium ranking tobe tightly correlated with each comparison set, in a non-linear and alinear fashion, respectively. By combining our calcium index informationwith the Zinn and West ranking, we are able to rank the globularclusters in our sample with a typical precision of $pm 0.05$ dex for$feZW84 ≲ -0.5$; for clusters more metal rich than this,the ranking is less precise. The significant differences between thesemetallicity scales raise important questions about our understanding ofGalactic formation and chemical enrichment processes. Furthermore, inspite of the apparent improvement in metallicity ranking for theGalactic globular clusters that results from our addition of informationfrom the ca triplet lines to the potpourri of other metallicityindicators, caution -- perhaps considerable -- may be advisable whenusing redew as a surrogate for metallicity, especially for systems whereranges in age and metallicity are likel. (SECTION: Stellar Clusters andAssociations)
| Galactic Globular Cluster Metallicity Scale from the Ca II Triplet I. Catalog
We have obtained 2640 CCD spectra with resolution $\sim$4~\AA\ in theregion 7250--9000~\AA\ for 976 stars lying near the red giant branchesin color-magnitude diagrams of 52 Galactic globular clusters. Radialvelocities of $\sim$16 \kms\ accuracy per star determined from thespectra are combined with other criteria to assess quantitativemembership probabilities. Measurements of the equivalent widths of theinfrared calcium triplet lines yield a relative metal-abundance rankingwith a precision that compares favorably to other techniques.Regressions between our system and those of others are derived. Ourreduction procedures are discussed in detail, and the resultant catalogof derived velocities and equivalent widths is presented. The metalabundances derived from these data will be the subject of a future pape.(SECTION: Stellar Clusters and Associations)
| Destruction of the Galactic Globular Cluster System
We investigate the dynamical evolution of the Galactic globular clustersystem in considerably greater detail than has been done hitherto,finding that destruction rates are significantly larger than given byprevious estimates. The general scheme (but not the detailedimplementation) follows Aguilar, Hut, & Ostriker. For the evolutionof individual clusters, we use a Fokker-Planck code including the mostimportant physical processes governing the evolution: two-bodyrelaxation, tidal truncation of clusters, compressive gravitationalshocks while clusters pass through the Galactic disk, and tidal shocksdue to passage close to the bulge. Gravitational shocks are treatedcomprehensively, using a recent result by Kundic & Ostriker that the< Delta E2> shock-induced relaxation term, driving an additionaldispersion of energies, is generally more important than the usualenergy shift term < Delta E>. Various functional forms of thecorrection factor are adopted to allow for the adiabatic conservation ofstellar actions in a presence of transient gravitational perturbation.We use a recent compilation of the globular cluster positional andstructural parameters, and a collection of radial velocity measurements.Two transverse to the line-of-sight velocity components were assignedrandomly according to the two kinematic models for the cluster system(following the method of Aguilar, Hut, & Ostriker): one with anisotropic peculiar velocity distribution, corresponding to thepresent-day cluster population, and the other with the radiallypreferred peculiar velocities, similar to those of the stellar halo. Weuse the Ostriker & Caldwell and the Bahcall, Schmidt, & Soneiramodels for our Galaxy. For each cluster in our sample, we calculated itsorbits over a Hubble time, starting from the present observed positionsand assumed velocities. Medians of the resulting set of peri- andapogalactic distances and velocities are used then as an input for theFokker-Planck code. Evolution of the cluster is followed up to its totaldissolution due to a coherent action of all of the destructionmechanisms. The rate of destruction is then obtained as a median overall the cluster sample, in accord with Aguilar, Hut, & Ostriker. Wefind that the total destruction rate is much larger than that given byAguilar, Hut, & Ostriker with more than half of the present clusters(52%--58% for the Ostriker & Caldwell model, and 75%--86% for theBahcall, Schmidt, & Soneira model) destroyed in the next Hubbletime. Alternatively put, the typical time to destruction is comparableto the typical age, a result that would follow from (but is not requiredby) an initially power law distribution of destruction times. We discusssome implications for a past history of the globular cluster system andthe initial distribution of the destruction times, raising thepossibility that the current population is but a very small fraction ofthe initial population with the remnants of the destroyed clustersconstituting presently a large fraction of the spheroid (bulge + halo)stellar population.
| Some Integrated Properties of Galactic Globular Clusters
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....112.2634V
| A Catalog of Parameters for Globular Clusters in the Milky Way
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.1487H&db_key=AST
| Stellar Variability in the Central Populations of 47 Tucanae from WF/PC Observations with the Hubble Space Telescope. II. Binary Systems
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...468..241E
| The globular cluster population of low-luminosity X-ray sources.
More than thirty low-luminosity X-ray sources (withL_x_<~10^34.5^erg/s) have now been discovered in globular clusters.The nature of these sources is still unclear. One important question ishow the number of sources scales with cluster parameters such as centraldensity or cluster mass. Because the detected sample is biased towardsthe brightest sources, such an analysis must rely on knowledge of theunderlying luminosity function. We present here a statistical analysisof current observations of dim cluster X-ray sources. We determine theluminosity function to be dN/dL{prop.to}L^-1.5^, and find that thenumber of dim X-ray sources per cluster scales less steeply with coredensity than predicted by the two-body tidal capture model, just as docluster pulsars.
| ROSAT observations of ten globular clusters with large core radii.
We present X-ray observations of ten globular clusters observed with theRosat PSPC. The clusters were chosen to have large core radii and to benearby. Three clusters contain X-ray sources which are probablyassociated with the cluster. One is the previously discovered X-raytransient H1825-331 in NGC 6652. The other two, in NGC 6366 and NGC6809, are new members of the class of low-luminosity sources, withluminosities in the region of 10^32^erg/s. Upper limits can be placed onthe source temperatures of both sources, making them similar to sourcesfound in other globular clusters by Rosat .
| Sagittarius: the nearest dwarf galaxy
We have discovered a new Galactic satellite galaxy in the constellationof Sagittarius. The Sagittarius dwarf galaxy subtends an angle of ~10deg on the sky, lies at a distance of 24 kpc and is comparable in sizeand luminosity to the largest dwarf spheroidal, Fornax. The new galaxyhas many features in common with the other eight Galactic dwarfspheroidal systems, including an extended low-density spatial structure,a well-populated red horizontal branch with a blue extension, and asubstantial carbon star population. In terms of stellar populations itmost closely resembles the Fornax dwarf, having a strongintermediate-age stellar component and evidence of a metallicity spread.Sagittarius is the nearest galaxy known and currently lies only ~16 kpcfrom the centre of the Milky Way. Isodensity maps show it to be markedlyelongated along a direction pointing towards the Galactic Centre, andsuggest that it has been tidally distorted. The close proximity to theGalactic Centre, the morphological appearance and the radial velocity of140 km s^-1 indicate that this system must have undergone at most veryfew close orbital encounters with the Milky Way. It is currentlyundergoing strong tidal disruption prior to being integrated into theGalaxy. We find that at least some of the four globular clusters, M54,Arp 2, Ter 7 and Ter 8, are associated with the Sagittarius dwarfgalaxy, and will probably share the fate of their progenitor. TheSagittarius dwarf galaxy was found serendipitously using a combinationof UK Schmidt Telescope sky survey plates, the APM automatic platemeasuring facility and the Anglo-Australian Telescope multifibrespectrograph, AUTOFIB.
| C J0907-372 (Pyxis): A New Distant Galactic Globular Cluster
The prime focus camera of the Anglo-Australian Telescope has been usedto image the object listed by Weinberger (PASP, 107, 58 [1995]) as apossible distant star cluster or dwarf spheroidal galaxy. The CCD datareveal this object to be a distant globular cluster of low centralconcentration. In particular, the color-magnitude diagram for thecluster core shows a sparse giant branch, a prominent red horizontalbranch at R=18.75 and a main sequence turnoff near the limit of the dataat R=22.0. A population of possible blue stragglers is also evident.Based on comparisons with the giant branches of standard globularclusters, the reddening to this cluster, designated C J0907-372 (Pyxis),is estimated to be 0.25 <= E(B-V) <= 0.40. The cluster is thenapproximately 35 kpc from the Sun and 37 kpc from the galactic center.With a core radius of ~14 pc, an estimated absolute magnitude M_V=-5.7and a dominant red horizontal branch, C J0907-372 (Pyxis) has verysimilar properties to the other low luminosity globular clusters of theouter galactic halo. (SECTION: Stellar Clusters and Associations)
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