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The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters without Previous Hubble Space Telescope Photometry
We present the first results of a large Advanced Camera for Surveys(ACS) survey of Galactic globular clusters. This Hubble Space Telescope(HST) Treasury project is designed to obtain photometry with S/N(signal-to-noise ratio) >~10 for main-sequence stars with masses>~0.2 Msolar in a sample of globulars using the ACS WideField Channel. Here we focus on clusters without previous HST imagingdata. These include NGC 5466, NGC 6779, NGC 5053, NGC 6144, Palomar 2,E3, Lyngå 7, Palomar 1, and NGC 6366. Our color-magnitude diagrams(CMDs) extend reliably from the horizontal branch to as much as 7 magfainter than the main-sequence turnoff and represent the deepest CMDspublished to date for these clusters. Using fiducial sequences for threestandard clusters (M92, NGC 6752, and 47 Tuc) with well-knownmetallicities and distances, we perform main-sequence fitting on thetarget clusters in order to obtain estimates of their distances andreddenings. These comparisons, along with fitting the cluster mainsequences to theoretical isochrones, yield ages for the target clusters.We find that the majority of the clusters have ages that are consistentwith the standard clusters at their metallicities. The exceptions areE3, which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could beas much as 8 Gyr younger than 47 Tuc.Based on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS5-26555, under program GO-10775 (PI:A. Sarajedini).
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| The COMPLETE Survey of Star-Forming Regions: Phase I Data
We present an overview of data available for the Ophiuchus and Perseusmolecular clouds from Phase I of the COMPLETE Survey of Star-FormingRegions. This survey provides a range of data complementary to theSpitzer Legacy Program ``From Molecular Cores to Planet Forming Disks.''Phase I includes the following: extinction maps derived from the TwoMicron All Sky Survey (2MASS) near-infrared data using the NICERalgorithm; extinction and temperature maps derived from IRAS 60 and 100μm emission; H I maps of atomic gas; 12CO and13CO maps of molecular gas; and submillimeter continuumimages of emission from dust in dense cores. Not unexpectedly, themorphology of the regions appears quite different depending on thecolumn density tracer that is used, with IRAS tracing mainly warmer dustand CO being biased by chemical, excitation, and optical depth effects.Histograms of column density distribution are presented, showing thatextinction as derived from 2MASS NICER gives the closest match to alognormal distribution, as is predicted by numerical simulations. Allthe data presented in this paper, and links to more detailedpublications on their implications, are publicly available at theCOMPLETE Web site.
| 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.
| Galactic Orbits of Globular Clusters in a Barred Galaxy
We study the effect of a bar in the galactic orbits of forty-fiveglobular clusters whose absolute proper motions are known. The orbitalcharacteristics of the orbits are compared with those obtained for thecase of an axisymmetric galactic potential. Tidal radii are computed anddiscussed for both cases.
| Scorpius the Winter-Bug.
Not Available
| 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.
| 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.
| Models for the Gravitational Field of the Galactic Bar: An Application to Stellar Orbits in the Galactic Plane and Orbits of Some Globular Clusters
We built three models for the gravitational field of the Galactic bar.These models are an inhomogeneous ellipsoid, an inhomogeneous prolatespheroid, and a superposition of four inhomogeneous ellipsoids. Amongthe three models, the superposition provides our best approximation tothe observed boxy mass distribution of the Galactic bar. Adding the barcomponent to an axisymmetric Galactic model, we have calculated stellarmidplane orbits and orbits of some globular clusters with knownkinematical data. For all models we find a secular dispersion effect onthe orbital energy and angular momentum, as measured in the Galacticinertial frame. This effect might be relevant to explain the orbitalprograde-retrograde distribution of globular clusters. For the stellarkinematics, we study the connection between the sense of orbital motionin the midplane and the onset of chaos in the presence of the bar. Inthe inner region of the bar, chaos is induced by an axisymmetric centralcomponent (bulge), and it arises in orbits that change its orbital sensefrom prograde to retrograde and vice versa as seen from an inertialreference frame. Outside the bar region, chaos appears only in progradeorbits. Our results concerning such a connection are consistent andextend those obtained for midplane orbits in the presence of only aspiral pattern in the axisymmetric Galactic model.
| 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.
| Space distribution and motional orbits of globular clusters in the galaxy
The 29 F globular clusters in the galaxy are selected as samples.According to the basic data, radial velocities and proper motions ofsample clusters, the initial positions and velocities of the samples arereduced using the galactic coordinates, and their orbits are integratedby numerical method for three different Galactic gravitational potentialmodels. The calculating results show: (1) most of samples are located in5 kpc---10 kpc from Galactic center. All of the sample clusters presenta spherical symmetrical distribution around the Galactic center, andtheir space velocities are presented a ellipsoidal distribution; (2)According to the metallicity and basic characters, the sample clustersare separated into HB subgroup and MP subgroup. The number of samplesare changed with metallicity [Fe/H], and there is a peak at [Fe/H]=-1.6; (3) The orbits of sample clusters show mostly limited, periodiccharacteristics, but the orbits are not closed completely, their maximalgalactocentric distance is less than 40 kpc. The differences in orbitalmorphologies due to different potentials is slighting, however, given acertain potential, for clusters that have perigalactic distance smallerthan 1 kpc, some orbits may exhibit a chaotic behavior. The correlationbetween the metallicity of samples and the orbital morphologies isunclearly; (4) It is found that the semi-major axis, apogalacticdistance and azimuth period of 29 example clusters are changed withtheir metallicity similarly, but a obvious correlation is seen betweenorbital eccentricity and metallicity. There is a fraction of 24% of thesample clusters with eccentricities lower than 0.4. The differentGalactic gravitational potential have not clear influence upon theperigalactic distance, eccentricity and uncertainty of orbitalparameters, but which is significant for other parameters, such as theapogalactic distance, semi-major axis, radial period and azimuth periodand so on.
| 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.
| On the Distribution of Orbital Poles of Milky Way Satellites
In numerous studies of the outer Galactic halo some evidence foraccretion has been found. If the outer halo did form in part or whollythrough merger events, we might expect to find coherent streams of starsand globular clusters following orbits similar to those of their parentobjects, which are assumed to be present or former Milky Way dwarfsatellite galaxies. We present a study of this phenomenon by assessingthe likelihood of potential descendant ``dynamical families'' in theouter halo. We conduct two analyses: one that involves a statisticalanalysis of the spatial distribution of all known Galactic dwarfsatellite galaxies (DSGs) and globular clusters, and a second, morespecific analysis of those globular clusters and DSGs for which fullphase space dynamical data exist. In both cases our methodology isappropriate only to members of descendant dynamical families that retainnearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) isconsidered a paradigm for the type of merger/tidal interaction event forwhich we are searching, we also undertake a case study of the Sgr systemand identify several globular clusters that may be members of itsextended dynamical family. In our first analysis, the distribution ofpossible orbital poles for the entire sample of outer(Rgc>8 kpc) halo globular clusters is tested forstatistically significant associations among globular clusters and DSGs.Our methodology for identifying possible associations is similar to thatused by Lynden-Bell & Lynden-Bell, but we put the associations on amore statistical foundation. Moreover, we study the degree of possibledynamical clustering among various interesting ensembles of globularclusters and satellite galaxies. Among the ensembles studied, we findthe globular cluster subpopulation with the highest statisticallikelihood of association with one or more of the Galactic DSGs to bethe distant, outer halo (Rgc>25 kpc), second-parameterglobular clusters. The results of our orbital pole analysis aresupported by the great circle cell count methodology of Johnston,Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC6229, NGC 7006, and Pyxis are predicted to be among those most likely toshow the clusters to be following stream orbits, since these clustersare responsible for the majority of the statistical significance of theassociation between outer halo, second-parameter globular clusters andthe Milky Way DSGs. In our second analysis, we study the orbits of the41 globular clusters and six Milky Way-bound DSGs having measured propermotions to look for objects with both coplanar orbits and similarangular momenta. Unfortunately, the majority of globular clusters withmeasured proper motions are inner halo clusters that are less likely toretain memory of their original orbit. Although four potential globularcluster/DSG associations are found, we believe three of theseassociations involving inner halo clusters to be coincidental. While thepresent sample of objects with complete dynamical data is small and doesnot include many of the globular clusters that are more likely to havebeen captured by the Milky Way, the methodology we adopt will becomeincreasingly powerful as more proper motions are measured for distantGalactic satellites and globular clusters, and especially as resultsfrom the Space Interferometry Mission (SIM) become available.
| 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.
| The `second parameter': a memory from the globular cluster formation epoch
We study the correlations between the distribution of stars on thehorizontal branch (HB), the HB morphology, and some other properties ofglobular clusters (GCs) in a large sample of GCs. We strengthen previousresults that a general correlation exists only between HB morphology andmetallicity. Correlations with other properties, e.g. central densityand core radius, exist only for GCs within a narrow metallicity range.We conjecture that the lack of correlations with present properties ofGCs (besides metallicity) is because the variation of the HBmorphologies between GCs having similar metallicities is caused by aprocess, or processes, the effect of which was determined at theformation time of GCs. This process (or processes) is historicallytermed the `second parameter', metallicity being the `first parameter'.We then argue that the `planet second parameter' model, where thepresence of planets and to a lesser degree brown dwarfs and low-massmain-sequence stars is the `second parameter', fits this conjecture.This is because the processes that determine the presence of planets andtheir properties occur during the formation epoch of the star and itscircumstellar disc.
| CCD Photometry of the Globular Cluster NGC 5986 and Its Post-Asymptotic Giant Branch and RR Lyrae Stars
We have obtained new CCD BV photometry of the little-studied southernGalactic globular cluster NGC 5986, including light curves of five ofits RR Lyrae variables. The cluster's red giant branch bump is detectedfor the first time, at V=16.47+/-0.03. We derive a reddening and truedistance modulus of E(B-V)=0.29+/-0.02 and(m-M)0=15.15+/-0.10, respectively. The cluster'scolor-magnitude diagram reveals a mostly blue horizontal branch, likethat of M13 or M2, and quite unlike M3; yet all of these clusters havenearly identical metallicities ([Fe/H]CG97=-1.35). We showthat the RR Lyrae variables in NGC 5986 are about 0.2 mag brighter onaverage than those in M3, an important exception to the often-employed,universal MV(RR)-[Fe/H] relation. Finally, we note that NGC5986 contains two luminous stars with spectral types A-F, which arelikely to be post-asymptotic giant branch (PAGB) objects. The V-bandluminosity function of such yellow PAGB stars is a promising standardcandle. We suggest that the luminosity function is sharply peaked atMV(PAGB)=-3.28+/-0.07.
| 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.
| A Near-Infrared Photometric Survey of Metal-poor Inner Spheroid Globular Clusters and Nearby Bulge Fields
Images recorded through J, H, K, 2.2 μm continuum, and CO filtershave been obtained of a sample of metal-poor ([Fe/H]<=-1.3) globularclusters in the inner spheroid of the Galaxy. The shape and color of theupper giant branch on the (K, J-K) color-magnitude diagram (CMD),combined with the K brightness of the giant branch tip, are used toestimate the metallicity, reddening, and distance of each cluster. COindices are used to identify bulge stars, which will bias metallicityand distance estimates if not culled from the data. The distances andreddenings derived from these data are consistent with published values,although there are exceptions. The reddening-corrected distance modulusof the Galactic center, based on the Carney et al. horizontal-branch(HB) brightness calibration, is estimated to be 14.9+/-0.1. The meanupper giant branch CO index shows cluster-to-cluster scatter that (1) islarger than expected from the uncertainties in the photometriccalibration and (2) is consistent with a dispersion in CNO abundancescomparable to that measured among halo stars. The luminosity functions(LFs) of upper giant branch stars in the program clusters tend to besteeper than those in the halo clusters NGC 288, NGC 362, and NGC 7089.The majority of inner spheroid clusters fall along the integrated J-Kversus metallicity relation defined by halo clusters; however, many ofthe inner spheroid clusters do not follow the relation betweenintegrated CO index and metallicity measured for halo clusters, in thatthey have CO indices that are too small. Bulge fields were also observednear most clusters. The slope of the giant branch LF does not varysignificantly between most fields, although the LFs in Baade's windowand near NGC 6273 are significantly shallower than average.Metallicities estimated from the slope of the upper giant branch on the(K, J-K) CMDs of fields within 6° of the Galactic center areconsistent with previous studies. Finally, the data suggest that the HBcontent may not be uniform throughout the bulge, in the sense that alarger than average number of red HB stars may occur in fields closestto the Galactic center.
| 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.
| CCD Photometry of the Galactic Globular Cluster NGC 6144
We present CCD photometry in the BVI-filter passbands for the Galacticglobular cluster NGC 6144. This cluster is located in the direction ofthe ρ Ophiuchi dust cloud, approximately 30' northwest of the brightstar α Scorpii (Antares). Our color-magnitude diagrams (CMDs) ofNGC 6144 extend from above the tip of the first-ascent red giant branch(RGB) to ~3 mag below the horizontal branch (HB). The morphology of theHB is predominantly blueward of the RR Lyrae instability strip, whilethe RGB is relatively steep, signifying a low-to-intermediate metalabundance. Our CMD also reveals the presence of three candidatepost-asymptotic giant branch stars. We find VHB=16.30+/-0.07coupling the measured RGB color at the level of the HB with a polynomialdescribing the shape of the RGB, we have utilized the simultaneousreddening and metallicity method of Sarajedini to estimate a metallicityof [Fe/H]=-1.81+/-0.12 (on the Zinn & West scale) and a meanreddening of E(B-V)=0.41+/-0.02. In addition to this mean level, ourobservations reveal that the reddening has a spatially variablecomponent generally increasing from north to south consistent with theexpected density variations in the ρ Ophiuchi dust cloud.
| 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
| Gravitational Radiation from Globular Clusters
Space-based gravitational wave detectors will have the ability toobserve continuous low-frequency gravitational radiation from binarystar systems. They can determine the direction to continuous sourceswith an angular resolution approaching tens of arcminutes. Thisresolution should be sufficient to identify binary sources as members ofsome nearby globular clusters. Thus, gravitational radiation can be usedto determine the population of hard binaries in globular clusters. Forparticularly hard binaries, the orbital period may change as a result ofgravitational wave emission. If one of these binaries can be identifiedwith a globular cluster, then the distance to that cluster can bedetermined. Thus, gravitational radiation may providereddening-independent distance measurements to globular clusters and theRR Lyrae stars that inhabit them.
| Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure
We have compiled a catalog of absolute proper motions of globularclusters from various sources. The sample consists of 38 clusters, fromwhich most of the southern ones (15 clusters) were measured in ourprevious papers in this series. We have integrated orbits assuming twodifferent Galactic potential models adopted from the literature and havecalculated orbital parameters. The uncertainties associated with theorbital parameters were derived in a Monte Carlo approach, and weconclude that, overall, at the present level of measurement errors,orbital differences due to Galactic potential models are notsignificant. Three metal-poor clusters are found to have orbits similarto prototypical metal-rich disk clusters. These clusters are NGC 6254(M10), NGC 6626 (M28), and NGC 6752. We interpret this as a potentiallysignificant constraint on the formation of the disk. It is thus possiblethat part of the inner metal-poor halo is the low-metallicity tail ofthe thick disk. In this case, the ages of these clusters indicate thatthe formation of the disk partially overlapped with that of the halo.The clusters classified as ``young halo'' or ``red horizontal-branch''by Zinn show a radially anisotropic velocity distribution, their orbitsare of high total energy, with apocentric radii larger than 10 kpc andhighly eccentric. In this sense they may represent an accreted componentof our Galaxy. We also discuss omega Cen's orbit characteristics in theview of an accreted origin. We investigate the effect of the orbitalmotion on the internal dynamics of clusters. Adopting the formalism fromGnedin & Ostriker and their destruction rates due to two-bodyrelaxation, we find that, in most cases, this internal process is moreimportant than the destruction processes due to disk and bulge shocking.Hubble Space Telescope (HST) observations argue that NGC 6397'sluminosity function is depleted at the faint end, and this is blamed onits high total destruction rate. We propose a list of clusters withsimilar destruction rates that may also have depleted luminosityfunctions. We also note the bias toward deriving higher destructionrates in studies that statistically assign tangential velocities basedon a kinematic model of the globular cluster system, in contrast to therates derived from the measured tangential velocities. Clusters prone tosuch biases are those that have circular orbits (kinematicallythick-disk clusters) and some of those with orbits of high total energy.
| Instantaneous and average tidal radii of globular clusters
The main aim of this paper is an extension of the heuristic concept ofthe tidal radii of globular clusters to the case of very non-circularorbits, i.e. with large differences between the peri- and apogalacticdistances. We had found earlier that perigalactic tidal radii do clearlynot represent the observed limiting photometric radii. For 16 globularclusters with orbits based on absolute proper motions we derivedinstantaneous tidal radii along the orbit. We computed four kind ofaverages of the instantaneous values. The comparison between averagetidal radii and observed limiting radii relies on the ratios between thetwo, since the equations for the theoretical values may need theinclusion of an additional constant factor. In case that the said ratiosdo not depend on the shape of the orbits, we consider the underlyingmethod of averaging as satisfying. In a quantitative approximation, thisnon-dependence can be assumed for all methods within 3sigma-limits; butone method stands out in that it fulfils it within 1sigma. This methodor way consists in forming the reciprocal squares of the instantaneoustidal radii and their mean along the orbit.
| Space Velocities of Southern Globular Clusters. II. New Results for 10 Clusters
Absolute proper motions have been measured for 10 clusters, in additionto the sample of five clusters presented in an earlier paper. Thesenewly measured clusters are NGC 1904 (M79), NGC 2298, NGC 4590 (M68),NGC 5139 (omega Cen), NGC 5897, NGC 6093 (M80), NGC 6121 (M4), NGC 6144,NGC 6809 (M55), and NGC 7099 (M30). The correction to absolute propermotion is determined based on galaxies, except for one field with highextinction, for which we used the Hipparcos system. This lattercorrection proves to be very useful for fields at low Galactic latitude,provided magnitude-dependent systematics are well controlled. The errorsin absolute proper motion per cluster range between 0.4 and 1.0 masyr^-1. Space velocities are also determined, and the kinematics arebriefly discussed; a companion paper will present a detailed analysis oforbits in conjunction with other physical parameters of clusters.
| 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)
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