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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.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| 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.
| Galactic Globular Cluster Relative Ages
We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the differencebetween the horizontal branch and the turnoff in two internallyphotometrically homogeneous databases. The mutual consistency of the twodata sets has been assessed by comparing the ages of 16 globularclusters in common between the two databases. We have also investigatedthe consistency of our relative age determination within the recentstellar model framework. All clusters with [Fe/H]<-1.7 are found tobe old and coeval, with the possible exception of two objects, which aremarginally younger. The age dispersion for the metal-poor clusters is0.6 Gyr (rms), consistent with a null age dispersion.Intermediate-metallicity clusters (-1.7<[Fe/H]<-0.8) are onaverage 1.5 Gyr younger than the metal-poor ones, with an age dispersionof 1.0 Gyr (rms) and a total age range of ~3 Gyr. About 15% of theintermediate-metallicity clusters are coeval with the oldest clusters.All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the mostmetal-poor ones, with a relatively small age dispersion, although themetal-rich sample is still too small to allow firmer conclusions. Thereis no correlation of the cluster age with the galactocentric distance.We briefly discuss the implication of these observational results forthe formation history of the Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555, and on observations made at the European SouthernObservatory, La Silla, Chile, and with the Isaac Newton GroupTelescopes.
| Microarcsecond astrometry using the SKA
The sensitivity and versatility of SKA will provide microarcsecastrometric precision and high quality milliarcsec-resolution images bysimultaneously detecting calibrator sources near the target source. Toreach these goals, we suggest that the long-baseline component of SKAcontains at least 25% of the total collecting area in a region between1000 and 5000 km from the core SKA. We also suggest a minimum of 60elements in the long-baseline component of SKA to provide the necessary(u v) coverage. For simultaneous all-sky observations, which provideabsolute astrometric and geodetic parameters, we suggest using 10independent subarrays each composed of at least six long-baselineelements correlated with the core SKA. We discuss many anticipated SKAlong-baseline astrometric experiments: determination of distance, propermotion and orbital motion of thousands of stellar objects; planetarymotion detections; mass determination of degenerate stars using theirkinetics; calibration of the universal distance scale from 10 to107 pc; the core and inner-jet interactions of AGN. With anincrease by a factor of 10 in absolute astrometric accuracy usingsimultaneous all sky observations, the fundamental quasar referenceframe can be defined to <10 μas and tied to the solar-systemdynamic frame to this accuracy. Parameters associated with the earthrotation and orientation, nutation, and geophysical parameters, can beaccurately monitored. Tests of fundamental physics include: solar andJovian deflection experiments, the sky frame accuracy needed tointerpret the gravity wave/pulsar-timing experiment, accurate monitoringof spacecraft orbits that impact solar system dynamics.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| Radio emission as a test of the existence of intermediate-mass black holes in globular clusters and dwarf spheroidal galaxies
We take the established relation between black hole mass, X-rayluminosity and radio luminosity and show that intermediate-mass blackholes (IMBHs), such as those predicted to exist at the centres ofglobular clusters (GCs), will be easily identifiable objects in deepradio observations. We show that the radio observations will be far moresensitive than any possible X-ray observations. We also discuss thelikely optical photometric and spectroscopic appearance of such systemsin the event that radio detections are made.
| BVRI photometry of the galactic globular cluster NGC 6779
We present B, V, R and I photometry for NGC 6779 (M56), a metal-poorglobular cluster in the galactic halo. The observations were performedusing the 1.3-m telescope at Skinakas Observatory, in Crete. Thereddening of the cluster was found to be E(B-V) = 0.32 +/- 0.02 [E(V-I)= 0.43 +/- 0.02], significantly higher than previous estimates. Themetal abundance of the cluster was derived from various parametrizationsof red giant branch characteristics and it was found to be[Fe/H]ZW=-2.20 +/- 0.12 dex on the Zinn-West metallicityscale, or [Fe/H]CG=-2.00 +/- 0.21 dex on the Carretta-Grattonscale. The distance modulus of the cluster is estimated to be(m-M)V= 15.62 +/- 0.26 (or 14.62, if we correct for thereddening to the cluster). The horizontal branch of NGC 6779 shows aclear gap at (B-V)o= 0.0. Finally, the revised value for themetallicity of NGC 6779 led to a revision of its age to 13 Gyr, usingthe age-index calibrations of Salaris & Weiss.
| Discovery of Blue Hook Stars in the Massive Globular Cluster M54
We present BV photometry centered on the globular cluster M54 (NGC6715). The color-magnitude diagram clearly shows a blue horizontalbranch extending anomalously beyond the zero-age horizontal-branchtheoretical models. These kinds of horizontal-branch stars (also called``blue hook'' stars), which go beyond the lower limit of the envelopemass of canonical horizontal-branch hot stars, have so far been known toexist in only a few globular clusters: NGC 2808, ω Centauri (NGC5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among themost luminous in our Galaxy, indicating a possible correlation betweenthe existence of these types of horizontal-branch stars and the totalmass of the cluster. A gap in the observed horizontal branch of M54around Teff=27,000 K could be interpreted within the latehelium flash theoretical scenario, which is a possible explanation forthe origin of blue hook stars.
| Globular Cluster Formation in M82
We present high-resolution mid-infrared (mid-IR 11.7 and 17.65 μm)maps of the central 400 pc region of the starburst galaxy M82. Sevenstar-forming clusters are identified, which together provide ~15% of thetotal mid-IR luminosity of the galaxy. We find that these young stellarclusters have inferred masses and sizes comparable to globular clusters.At least 20% of the star formation in M82 is found to occur in superstar clusters.
| Infrared Echelle Spectroscopy of Palomar 6 and M71
We present high-resolution infrared echelle spectroscopy for theglobular clusters Palomar 6 and M71. Our mean heliocentric radialvelocity of Pal 6 is +180.6+/-3.2 km s-1 and is 20 kms-1 lower than that found by Minniti in 1995. Contrary to theprevious metallicity estimates using low-resolution spectroscopy, ourresults show that Pal 6 has an intermediate metallicity, with[Fe/H]=-1.0+/-0.1, and is slightly more metal poor than M71. Reasonablechanges in the surface temperature or the microturbulent velocity of themodel atmospheres do not affect [Fe/H] at more than +/-0.2 dex. In spiteof its high metallicity, on the basis of the spectrum of a singlecluster member the [Si/Fe] and [Ti/Fe] ratios of Pal 6 appear to beenhanced by 0.4 and 0.5 dex, respectively, suggesting that the Galacticinner halo may have experienced a very rapid chemical enrichmenthistory.Based on observations made with the Infrared Telescope Facility, whichis operated by the University of Hawaii under contract to the NationalAeronautics and Space Administration.
| The initial helium abundance of the Galactic globular cluster system
In this paper we estimate the initial He content in about 30% of theGalactic globular clusters (GGCs) from new star counts we have performedon the recently published HST snapshot database of Colour MagnitudeDiagrams (Piotto et al. \cite{Piotto02}). More specifically, we use theso-called R-parameter and estimate the He content from a theoreticalcalibration based on a recently updated set of stellar evolution models.We performed an accurate statistical analysis in order to assess whetherGGCs show a statistically significant spread in their initial Heabundances, and whether there is a correlation with the clustermetallicity. As in previous works on the subject, we do not find anysignificant dependence of the He abundance on the cluster metallicity;this provides an important constraint for models of Galaxy formation andevolution. Apart from GGCs with the bluest Horizontal Branch morphology,the observed spread in the individual helium abundances is statisticallycompatible with the individual errors. This means that either there isno intrinsic abundance spread among the GGCs, or that this is masked bythe errors. In the latter case we have estimated a firm 1σ upperlimit of 0.019 to the possible intrinsic spread. In case of the GGCswith the bluest Horizontal Branch morphology we detect a significantspread towards higher abundances inconsistent with the individualerrors; this can be fully explained by additional effects not accountedfor in our theoretical calibrations, which do not affect the abundancesestimated for the clusters with redder Horizontal Branch morphology. Inthe hypothesis that the intrinsic dispersion on the individual Heabundances is zero, taking into account the errors on the individualR-parameter estimates, as well as the uncertainties on the clustermetallicity scale and theoretical calibration, we have determined aninitial He abundance mass fraction YGGC=0.250±0.006.This value is in perfect agreement with current estimates based onCosmic Microwave Background radiation analyses and cosmologicalnucleosynthesis computations.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555, and on observations retrieved withthe ESO ST-ECF Archive.
| The ubiquitous nature of the horizontal branch second U-jump. A link with the Blue Hook scenario?
In a previous paper we reported on a discontinuity in the extremehorizontal branch (EHB) of the Galactic globular cluster NGC 6752, whichwe called the second U-jump. This feature was attributed to acombination of post zero-age horizontal branch evolution and diffusioneffects. In this follow-up study we analyze other EHB clusters and showthat the second U-jump is a common feature among EHB clusters reachingTeff≥23 000 K, and that its onset in different clustersconverges around Teff 21 000 ± 3000 K. We alsopresent near-ultraviolet diagrams of ωCen and NGC 2808, the onlytwo objects with spectroscopically confirmed ``blue hook'' stars(Teff≥35 000 K). We confirm predictions of a photometricdiscontinuity separating late from early-helium flashers. Moreover, wepresent empirical evidence that the second U-jump population might bemainly composed of early-helium flashers. Lastly, we revisit thediscussion on the ubiquitous nature of the gaps and jumps so faridentified in the blue HB tails, suggesting a possible discrete natureof the distribution in temperature of the HB stars.Based on observations with the ESO/MPI 2.2 m and ESO/NTT telescopes,located at La Silla Observatory (Chile) and on observations with theNASA/ESA Hubble Space Telescope.
| 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.
| 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.
| Hubble Space Telescope Snapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441
We present the results of a Hubble Space Telescope snapshot program tosurvey the inner region of the metal-rich globular cluster NGC 6441 forits variable stars. A total of 57 variable stars were found, including38 RR Lyrae stars, six Population II Cepheids, and 12 long-periodvariables. Twenty-four of the RR Lyrae stars and all of the PopulationII Cepheids were previously undiscovered in ground-based surveys. Of theRR Lyrae stars observed in this survey, 26 are pulsating in thefundamental mode with a mean period of 0.753 days and 12 arefirst-overtone-mode pulsators with a mean period of 0.365 days. Thesevalues match up very well with those found in ground-based surveys.Combining all the available data for NGC 6441, we find mean periods of0.759 and 0.375 days for the RRab and RRc stars, respectively. We alsofind that the RR Lyrae stars in this survey are located in the sameregions of a period-amplitude diagram as those found in ground-basedsurveys. The overall ratio of RRc to total RR Lyrae stars is 0.33.Although NGC 6441 is a metal-rich globular cluster and would, on thatground, be expected either to have few RR Lyrae stars or to be anOosterhoff type I system, its RR Lyrae stars more closely resemble thosein Oosterhoff type II globular clusters. However, even compared withtypical Oosterhoff type II systems, the mean period of its RRab stars isunusually long. We also derived I-band period-luminosity relations forthe RR Lyrae stars. Of the six Population II Cepheids, five are of WVirginis type and one is a BL Herculis variable star. This makes NGC6441, along with NGC 6388, the most metal-rich globular cluster known tocontain these types of variable stars. Another variable, V118, may alsobe a Population II Cepheid, given its long period and its separation inmagnitude from the RR Lyrae stars. We examine the period-luminosityrelation for these Population II Cepheids and compare it with those inother globular clusters and in the Large Magellanic Cloud. We argue thatthere does not appear to be a change in the period-luminosity relationslope between the BL Herculis and W Virginis stars, but that a change ofslope does occur when the RV Tauri stars are added to theperiod-luminosity relation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Urban Astronomy: Observing the Messier Objects from the City
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| The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda III
The Hubble Space Telescope Wide Field Planetary Camera 2 has been usedto image Andromeda III, a dwarf spheroidal (dSph) companion to M31. Theresulting color-magnitude diagrams (CMDs) reveal for the first time themorphology of the horizontal branch (HB) in this dwarf galaxy. We findthat like Andromeda I and Andromeda II, and like most of the GalacticdSph companions, the HB morphology of And III is predominantly red,redder indeed than that of both And I and And II, despite And III havinga lower mean metallicity. The And III HB morphology is also somewhatredder than that of the Galactic dSph Draco, which has a similar meanabundance to And III. We interpret this red HB morphology as indicatingthat the bulk of the And III population is ~3 Gyr younger than the ageof the majority of Galactic globular clusters. Nevertheless, the And IIICMD does reveal the presence of a few blue HB stars, and a number of RRLyrae variables are also evident in the data. This indicates that AndIII does contain an ``old'' population of age comparable to that of theGalactic globular clusters. There is no evidence, however, for any youngstars in And III despite a claimed association between this dSph and anH I cloud. As was the case for And II, but not And I, no radial gradientwas detected in the And III HB morphology. The mean V magnitude of theHB is 25.06+/-0.04, leading to (m-M)0=24.38+/-0.06 for thisdwarf. And III is then ~75 kpc from the center of M31, comparable to theGalactocentric distances of Sculptor and Draco. Comparison with standardglobular cluster red giant branches indicates a mean abundance for AndIII of <[Fe/H]>=-1.88+/-0.11, the lowest mean abundance of any ofM31's companions. This value, however, is consistent with the absolutemagnitude-mean abundance relation followed by dSph galaxies. The samecomparison yields an intrinsic abundance dispersion for And III ofσint([Fe/H])=0.12, a low value compared to And I andAnd II and to the Galactic dSph's of comparable luminosity to And III.If confirmed by future spectroscopic studies, this low value wouldsuggest that And III retained relatively little of the enrichmentproducts generated during its evolutionary history. The list ofcandidate variables reveals one definite and one probable anomalousCepheid variable star in And III. Such variables are common in GalacticdSph's, so their discovery in And III is not unexpected. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy (AURA), Inc., under NASAcontract NAS 5-26555.
| Does the mixing length parameter depend on metallicity?. Further tests of evolutionary sequences using homogeneous databases
This paper is a further step in the investigation of the morphology ofthe color-magnitude diagram of Galactic globular clusters, and thefine-tuning of theoretical models, made possible by the recentobservational efforts to build homogeneous photometric databases. Inparticular, we examine here the calibration of the morphologicalparameter WHB vs. metallicity, originally proposed by Brocatoet al. (\cite{brocatoEtal98}; B98), which essentially measures the colorposition of the red-giant branch. We show that the parameter can be usedto have a first-order estimate of the cluster metallicity, since thedispersion around the mean trend with [Fe/H] is compatible with themeasurement errors. The tight WHB-[Fe/H] relation is thenused to show that variations in helium content or age do not affect theparameter, whereas it is strongly influenced by the mixing-lengthparameter alpha (as expected). This fact allows us, for the first time,to state that there is no trend of alpha with the metal content of acluster. A thorough examination of the interrelated questions of thealpha -elements enhancement and the color-Tefftransformations, highlights that there is an urgent need for anindependent assessment of which of the two presently acceptedmetallicity scales is the true indicator of a cluster's iron content.Whatever scenario is adopted, it also appears that a deep revision ofthe V-I-temperature relations is needed.
| HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands
We present the complete photometric database and the color-magnitudediagrams for 74 Galactic globular clusters observed with the HST/WFPC2camera in the F439W and F555W bands. A detailed discussion of thevarious reduction steps is also presented, and of the procedures totransform instrumental magnitudes into both the HST F439W and F555Wflight system and the standard Johnson ( B ) and ( V ) systems. We alsodescribe the artificial star experiments which have been performed toderive the star count completeness in all the relevant branches of thecolor magnitude diagram. The entire photometric database and thecompleteness function will be made available on the Web immediatelyafter the publication of the present paper. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contractNAS5-26555, and on observations retrieved from the ESO ST-ECF Archive.
| Red giant branch stars as probes of stellar populations. I. 2MASS calibration and application to 2MASS GC01
The near-infrared behavior of the red giant branch (RGB hereafter) as afunction of abundance is examined with an unprecedented large sample of27 Galactic globular clusters with Two Micron All Sky Survey photometry.We propose a new simplified analysis, involving the zero point of theRGB slope fit, and derive calibrations for the RGB slope, zero point,and tip. The weak metallicity sensitivity of the zero point leads to a``fan''-like diagram to obtain the abundance distributions in resolvedstellar systems, and reddening estimates. Finally, we apply the newcalibrations to the recently discovered Galactic globular cluster 2MASSGC01, to derive [Fe/H]H96=-1.19+/-0.38 mag. The uncertaintyis dominated by the severe foreground contamination. We estimate anextinction of AV=21.07+/-2.20 mag toward the cluster.
| Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808
We present an ultraviolet color-magnitude diagram (CMD) spanning the hothorizontal branch (HB), blue straggler, and white dwarf populations ofthe globular cluster NGC 2808. These data were obtained with the far-UVand near-UV cameras on the Space Telescope Imaging Spectrograph (STIS).Although previous optical CMDs of NGC 2808 show a high-temperature gapwithin the hot HB population, no such gap is evident in our UV CMD.Instead, we find a population of hot subluminous HB stars, an anomalyonly previously reported for the globular cluster ω Cen. Ourtheoretical modeling indicates that the location of these subluminousstars in the UV CMD, as well as the high-temperature gap along the HB inoptical CMDs, can be explained if these stars underwent a latehelium-core flash while descending the white dwarf cooling curve. Weshow that the convection zone produced by such a late helium flash willpenetrate into the hydrogen envelope, thereby mixing hydrogen into thehot helium-burning interior, where it is rapidly consumed. Thisphenomenon is analogous to the ``born again'' scenario for producinghydrogen-deficient stars following a late helium-shell flash. The flashmixing of the envelope greatly enhances the envelope helium and carbonabundances, and leads, in turn, to a discontinuous increase in the HBeffective temperatures at the transition between canonical andflash-mixed stars. We argue that the hot HB gap is associated with thistheoretically predicted dichotomy in the HB properties. Moreover, thechanges in the emergent spectral energy distribution caused by theseabundance changes are primarily responsible for explaining the hotsubluminous HB stars. Although further evidence is needed to confirmthat a late helium-core flash can account for the subluminous HB starsand the hot HB gap, we demonstrate that an understanding of these starsrequires the use of appropriate theoretical models for their evolution,atmospheres, and spectra. Based on observations with the NASA/ESA HubbleSpace Telescope obtained at the Space Telescope Science Institute, whichis operated by AURA, Inc., under NASA contract NAS5-26555.
| Star Counts across the Red Giant Branch Bump and Below
We present a new observable-Rbump-which is the ratio betweenthe star counts across the red giant branch (RGB) bump and fainter RGBstars to investigate the occurrence of a deep-mixing phenomenon duringthese evolutionary phases. The comparison between predicted andempirical Rbump-values, based on a large and homogeneous setof Hubble Space Telescope data, brings out that evolutionary lifetimespredicted by canonical RGB models do account for the bulk of Galacticglobular clusters included in our sample (29). This evidence suggeststhat bump and fainter RGB stars do not show the occurrence of deepmixing, which significantly changes their chemical stratification. A fewpossible exceptions to this general rule are briefly discussed. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by AURA, Inc.,under NASA contract NAS5-26555, and on observations retrieved from theESO ST-ECF archive.
| 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.
| Near-Infrared Imaging of the Central Regions of Metal-Poor Inner Spheroid Globular Clusters
JHK images obtained with the Canada-France-Hawaii Telescope adaptiveoptics bonnette are used to investigate the near-infrared photometricproperties of red giant branch (RGB) and horizontal-branch (HB) stars ineight metal-poor globular clusters with RGC<=2 kpc. Theslope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature ofthese clusters, four of which (NGC 6287, 6293, 6333, and 6355) are foundto have metallicities that are comparable to M92. The luminosityfunctions of RGB stars in inner spheroid and outer halo clusters havesimilar slopes, although there is a tendency for core-collapsed clustersto have slightly flatter luminosity functions than noncollapsedclusters. The distribution of red HB stars on the (K, J-K) CMDs of innerspheroid clusters with [Fe/H]~-1.5 is very different from that ofclusters with [Fe/H]~-2.2, suggesting that metallicity is the mainparameter defining HB content among these objects. The RGB bump isdetected in four of the inner spheroid clusters, and this feature isused to compute distances to these objects. Finally, the specificfrequency of globular clusters in the inner Galaxy is discussed in thecontext of the early evolution of the bulge. Based on the ratio ofmetal-poor to metal-rich clusters in the inner Galaxy it is suggestedthat the metal-poor clusters formed during an early intense burst ofstar formation. It is also demonstrated that if the globular clusterformation efficiency for the inner Galaxy is similar to that measured inother spheroidal systems, then the main body of the bulge could haveformed from gas that was chemically enriched in situ; hence, materialfrom a separate pre-enriched reservoir, such as the disk or outer halo,may not be required to form the bulge.
| 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.
| 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.
| 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.
| On the Helium Content of Galactic Globular Clusters via the R-Parameter
We estimate the empirical R-parameter in 26 Galactic globular clusterscovering a wide metallicity range, imaged by Wide Field Planetary Camera2 on board the Hubble Space Telescope. The improved spatial resolutionpermits a large fraction of evolved stars to be measured and permitsaccurate assessment of radial population gradients and completenesscorrections. In order to evaluate both the He abundance and theHe-to-metal enrichment ratio, we construct a large set of evolutionarymodels by adopting similar metallicities and different He contents. Wefind an absolute He abundance that is lower than that estimated fromspectroscopic measurements in H II regions and from primordialnucleosynthesis models. This discrepancy could be removed by adopting a12C(α,γ)16O nuclear cross sectionabout a factor of 2 smaller than the canonical value, although differentassumptions for mixing processes also can introduce systematic effects.The trend in the R-parameter toward solar metallicity is consistent withan upper limit to the He-to-metal enrichment ratio of the order of 2.5.Detailed calculations of central He burning times as a function of thehorizontal-branch (HB) morphology suggest that He lifetimes for hot HBstars are on average ~20% longer than for RR Lyrae and red HB stars.Therefore, the increase in the empirical R-values of metal-poor clusterscharacterized by blue HB morphologies is due to an increase in the HBlifetime and not due to an increase in the He abundance. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by AURA, Inc.,under NASA contract NAS 5-26555, and on observations retrieved from theESO ST-ECF archive.
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