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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.
| Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.
| Cluster Ages Experiment (CASE): Detection of a dwarf nova in the globular cluster M55
We report the detection of a dwarf nova (DN) in the core region of theglobular cluster M55. Six outbursts were observed during eight observingseasons spanning the period 1997-2004. The variable has an X-raycounterpart detected on images taken with the ROSAT telescope. Althoughwe cannot offer proof of cluster membership, one can see that both theposition on the Hertzsprung-Russell diagram and the X-ray flux areconsistent with a bright DN at the cluster distance. According to ouroutburst statistics, no more than one similar DN could remain undetectedin our field of view, centred at the cluster core.
| 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.
| Caroline Herschel as observer
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.
| Discovery of More than 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue Horizontal Branch
We report on the discovery of a large number of RR Lyrae variable starsin the moderately metal-rich Galactic globular cluster M62 (NGC 6266),which places it among the top three most RR Lyrae-rich globular clustersknown. Likely members of the cluster in our studied field, from ourpreliminary number counts, include ~130 fundamental-mode (RRab)pulsators, with =0.548 days, and ~75first-overtone (RRc) pulsators, with =0.300 days.The average periods and the position of the RRab variables withwell-defined light curves in the Bailey diagram both suggest that thecluster is of Oosterhoff type I. However, the morphology of thecluster's horizontal branch (HB) is strikingly similar to that of theOosterhoff type II globular cluster M15 (NGC 7078), with a dominant blueHB component and a very extended blue tail. Since M15 and M62 differ inmetallicity by about 1 dex, we conclude that metallicity, at a fixed HBtype, is a key parameter determining the Oosterhoff status of a globularcluster and the position of its variables in the Bailey diagram.
| 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.
| An Erupting Classical Nova in a Globular Cluster of M87
Only one certain classical nova eruption has ever been detected inside aglobular cluster-nova T Sco (observed in A.D. 1860) in M80. During asurvey of M87 we have detected an erupting star coincident (to within0.08 pixels) with a globular cluster of that giant elliptical galaxy. Weare able to discount variables in the foreground or background of M87.The light curve and color of the erupting star match those expected fora nova at the distance of M87. The chance superposition of an M87 fieldnova on the globular cluster is very unlikely but cannot be completelyruled out. Our detection hints at a globular cluster nova frequencyf~0.004 nova per cluster per year, much higher than previousobservations have suggested.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| RR Lyrae variables in Galactic globular clusters. IV. Synthetic HB and RR Lyrae predictions
We present theoretical predictions concerning horizontal branch stars inglobular clusters, including RR Lyrae variables, as derived fromsynthetic procedures collating evolutionary and pulsational constraints.On this basis, we explore the predicted behavior of the pulsators as afunction of the horizontal branch morphology and over the metallicityrange Z= 0.0001 to 0.006, revealing an encouraging concordance with theobserved distribution of fundamentalised periods with metallicity.Theoretical relations connecting periods to K magnitudes and BV or VIWesenheit functions are presented, both appearing quite independent ofthe horizontal branch morphology only with Z≥ 0.001. Predictionsconcerning the parameter R are also discussed and compared under variousassumptions about the horizontal branch reference luminosity level.
| 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.
| RR Lyrae variables in Galactic globular clusters. I. The observational scenario
In this paper we revisit observational data concerning RR Lyrae stars inGalactic globular clusters, presenting frequency histograms offundamentalized periods for the 32 clusters having more than 12pulsators with well recognized period and pulsation mode. One finds thatthe range of fundamentalized periods covered by the variables in a givencluster remains fairly constant in varying the cluster metallicity allover the metallicity range spanned by the cluster sample, with the onlytwo exceptions given by M 15 and NGC 6441. We conclude that the width intemperature of the RR Lyrae instability strip appears largelyindependent of the cluster metallicity. At the same time, it appearsthat the fundamentalized periods are not affected by the predictedvariation of pulsators luminosity with metal abundance, indicating theoccurrence of a correlated variation in the pulsator mass. We discussmean periods in a selected sample of statistically significant ``RRrich" clusters with no less than 10 RRab and 5 RRc variables. One findsa clear evidence for the well known Oosterhoff dichotomy in the meanperiod of ab-type variables, together with a similarlyclear evidence for a constancy of the mean fundamentalized period in passing from Oosterhoff type II to type I clusters. Onthis basis, the origin of the Oosterhoff dichotomy is discussed,presenting evidence against a strong dependence of the RR Lyraeluminosity on the metal content. On the contrary, i) the continuity ofthe mean fundamentalized period, ii) the period frequency histograms inthe two prototypes M 3 (type I) and M 15 (type II), iii) the relativeabundance of first overtone pulsators, and iv) the observed differencebetween mean fundamental and fundamentalized periods, all agree in suggesting the dominant occurrence of avariation in the pulsation mode in a middle region of the instabilitystrip (the ``OR" zone), where variables of Oosterhoff type I and type IIclusters are pulsating in the fundamental or first overtone mode,respectively.
| The Red Giant Branch luminosity function bump
We present observational estimates of the magnitude difference betweenthe luminosity function red giant branch bump and the horizontal branch(Delta F555WbumpHB), and of star counts in thebump region (Rbump), for a sample of 54 Galactic globularclusters observed by the HST. The large sample of stars resolved in eachcluster, and the high photometric accuracy of the data allowed us todetect the bump also in a number of metal poor clusters. To reduce thephotometric uncertainties, empirical values are compared withtheoretical predictions obtained from a set of updated canonical stellarevolution models which have been transformed directly into the HSTflight system. We found an overall qualitative agreement between theoryand observations. Quantitative estimates of the confidence level arehampered by current uncertainties on the globular cluster metallicityscale, and by the strong dependence of DeltaF555WbumpHB on the cluster metallicity. In case ofthe Rbump parameter, which is only weakly affected by themetallicity, we find a very good quantitative agreement betweentheoretical canonical models and observations. For our full clustersample the average difference between predicted and observedRbump values is practically negligible, and ranges from-0.002 to -0.028, depending on the employed metallicity scale. Theobserved dispersion around these values is entirely consistent with theobservational errors on Rbump. As a comparison, the value ofRbump predicted by theory in case of spurious bump detectionsdue to Poisson noise in the stellar counts would be ~ 0.10 smaller thanthe observed ones. We have also tested the influence on the predictedDelta F555WbumpHB and Rbump values ofan He-enriched component in the cluster stellar population, as recentlysuggested by D'Antona et al. (\cite{d02}). We find that, underreasonable assumptions concerning the size of this He-enrichedpopulation and the degree of enrichment, the predicted DeltaF555WbumpHB and Rbump values are onlymarginally affected.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.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| The Cepheids of Population II and Related Stars
The Type II Cepheids include most intrinsic variables with periodsbetween 1 and about 50 days, except for the classical Cepheids and theshortest semiregular variables of type M. The Type II Cepheids may bedivided in groups by period, such that the stars with periods beween 1and 5 days (BL Her class), 10-20 days (W Vir class), and greater than 20days (RV Tau class) have differing evolutionary histories. The chemicalcomposition of Type II Cepheids reflects the material they were madefrom as modified by their internal nuclear evolution and mixing.Finally, RV Tau stars are affected by mass loss by dust and speciesattached to the dust. The populations to which the various classes ofType II Cepheids are assigned constitute important clues to the originand evolution of the halo of our Galaxy and the dwarf spheroidal systemsfrom which at least part of the halo seems to have been accreted.
| 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.
| The Nature of the Red Giant Branches in the Ursa Minor and Draco Dwarf Spheroidal Galaxies
Spectra for stars located redward of the fiducial red giant branches(RGBs) of the Ursa Minor and Draco dwarf spheroidal galaxies have beenobtained with the Hobby-Eberly telescope and the Marcario Low ResolutionSpectrometer. From a comparison of our radial velocities with thosereported in previous medium-resolution studies, we find an averagedifference of 10 km s-1 with a standard deviation of 11 kms-1. On the basis of these radial velocities, we confirm themembership of five stars in Ursa Minor and find two others to benonmembers. One of the confirmed members is a known carbon star thatlies redward of the RGB; three others are previously unidentified carbonstars. The fifth star is a red giant that was found previously byShetrone and coworkers to have [Fe/H]=-1.68+/-0.11 dex. In Draco, wefind eight nonmembers, confirm the membership of one known carbon star,and find two new members. One of these stars is a carbon star, while theother shows no evidence for C2 bands or strong atomic bands,although the signal-to-noise ratio of the spectrum is low. Thus, we findno evidence for a population of stars more metal-rich than [Fe/H]~=-1.45dex in either of these galaxies. Indeed, our spectroscopic surveysuggests that every candidate suspected of having a metallicity inexcess of this value based on its position in the color-magnitudediagram is, in actuality, a carbon star. Based on the census of 13 knowncarbon stars in these two galaxies, we estimate the carbon star specificfrequency to be ɛdSph~=2.4×10-5L-1V,solar, 25-100 times higher than that ofGalactic globular clusters. This work is based on observations obtainedwith the Hobby-Eberly Telescope, which is a joint project of theUniversity of Texas at Austin, Pennsylvania State University, StanfordUniversity, Ludwig-Maximillians-Universität München, andGeorg-August-Universität Göttingen.
| Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). I. New CFHT MOS Spectroscopy and the Composite Database
We present a comprehensive database of kinematic, photometric, andpositional information for 352 objects in the field of M87 (NGC 4486),the central giant elliptical galaxy in the Virgo Cluster; the majorityof the tracers are globular clusters associated with that galaxy. Newkinematic information comes from multislit observations with theMulti-Object Spectrograph (MOS) of the Canada-France-Hawaii Telescope(CFHT), an investigation that has added 96 new velocities to andconfirmed many of the earlier values in a preexisting data set of 256velocities published elsewhere. The photometry, consisting of magnitudesand colors in the Washington (T1, C-T1) system, isbased on CCD observations made at the Cerro Tololo Inter-AmericanObservatory (CTIO) and the Kitt Peak National Observatory (KPNO). Thecomposite database represents the largest compilation of pure PopulationII dynamical tracers yet identified in any external galaxy; moreover, itextends to larger spatial scales than have earlier investigations. Theinclusion of photometric information allows independent study of thedistinct red and blue subpopulations of the bimodal globular clustersystem of M87. In a companion paper we use this powerful data set toanalyze the present dynamical state of the M87 globular cluster systemand consider the question of its interaction and formation history.
| 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.
| 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.
| Predicted colours for simple stellar populations. II. The case of old stellar clusters
This paper presents theoretical integrated colours of old stellarpopulations as computed adopting an homogeneous set of stellar models,covering all the major evolutionary phases of globular cluster stars. Weshow that adopting Reimers parameterization of mass loss rates, thechoice eta = 0.4 gives synthetic CM diagrams of simple stellarpopulations in agreement with the typical dependence of globular clusterCM diagrams on metallicity. We present theoretical U-B, B-V, V-R and V-Iintegrated colours for cluster metallicity in the range Z=0.0001 to 0.02and for age between 8 and 15 Gyr. The stochastic occurrence of luminouspost-AGB stars is briefly discussed. We find that for an age t ~ 15 Gyrthe predicted integrated colours appear in good agreement with availabledata for both galactic and LMC old globulars. We discuss theuncertainties of integrated colours due to statistical fluctuations inthe number of luminous stars, giving for each colour the expecteduncertainty as a function of the cluster integrated V-magnitude.Comparison with Kurth et al. (1999) discloses that the still existingdifferences in the evolutionary results appear of minor relevance as faras integrated cluster colours is concerned. Finally, we show thatreasonable uncertainties either in the cluster age or in the efficiencyof mass loss have marginal effects on the predicted colours.
| 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.
| Stellar collisions and mergers in the cores of globular clusters.
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| Abundances of Red Giants in the Andromeda II Dwarf Spheroidal Galaxy
We have obtained spectra for 50 candidate red giants in Andromeda II, adwarf spheroidal companion of M31, using the Low Resolution ImagingSpectrometer on the Keck II Telescope. After eliminating backgroundgalaxies and Galactic foreground stars, we are left with a sample of 42red giants for which membership in Andromeda II can be establishedunambiguously from radial velocities. Line indexes measured on theLick/IDS system are combined with VI photometry obtained with the KeckII and Palomar 5 m telescopes to investigate the age and metallicitydistribution of these stars. Based on a comparison of the measured lineindexes with those of Lick/IDS standard stars in globular and openclusters, we derive a mean metallicity of <[Fe/H]>=-1.47+/-0.19dex. This confirms the earlier conclusion, based on Thuan-Gunn grphotometry, that Andromeda II obeys the familiar relation between meanstellar metallicity and galaxy luminosity. There is also evidence for adispersion in metallicity of σ([Fe/H])=0.35+/-0.10 dex, based onthe scatter in the measured Mg b line indexes and the observed width ofthe galaxy's giant branch. We note that, while existing observations ofLocal Group dwarf galaxies indicate that their mean metallicity dependsrather sensitively on total luminosity, the internal spread inmetallicity appears to be relatively independent of galaxy luminosity.Our spectroscopic sample contains one carbon star. We measure M_I~=-3.8for this star, which places it below the tip of the red giant branch andsuggests a common origin with the CH stars found in the Galactic halo.Although this carbon star alone does not provide evidence of anintermediate-age component in Andromeda II, two other stars in ourspectroscopic sample have M_I~=-4.7 and -4.5. Membership in Andromeda IIis unambiguous in both cases, indicating that these stars fall along anextended asymptotic giant branch and pointing to the presence of amodest intermediate-age population in this galaxy.
| RR Lyrae Luminosity Differences between Oosterhoff Group I and II Cluster Systems and the Origin of the Oosterhoff Dichotomy
We present a comparative study of the Oosterhoff II cluster M2 and theOosterhoff I cluster M3. Both have similar metallicities, [Fe/H]=-1.62for M2 and -1.66 for M3, but very different horizontal-branch (HB)morphologies (B-R)/(B+V+R)=0.92 for M2 and 0.08 for M3. A period shiftanalysis and main-sequence fitting show that RRab variables in M2 areabout 0.2 mag brighter than those in M3. Comparisons of the M2 periodshift with Oosterhoff I clusters NGC 3201 and NGC 7006 also yieldsimilar results, while a comparison between M2 and the Oosterhoff IIcluster NGC 5986 reveals that the RR Lyrae luminosities are verysimilar. The luminosity difference is thought to be due to theevolutionary effect described in 1990 by Lee, Demarque, & Zinn: theM2 RRab variables have evolved away from the zero-age horizontal branch(ZAHB), while most M3 RRab variables lie near the ZAHB. A comparison ofthe mean period change rates of two clusters supports this hypothesis.Our relative age estimation using the difference in color between thebase of giant branch and turn-off point shows that M2 is about 2 Gyrolder than M3. Our result strongly suggests that the Oosterhoffdichotomy is due to age differences between Oosterhoff group I and II.This is consistent with the idea that the global second parameter isage. We discuss the kinematic differences between Oosterhoff group I andII clusters. Our result shows that the Oosterhoff group I clusters havezero or retrograde rotation with =-68+/-56 km s^-1 andsigma_los=131+/-28 km s^-1, while the Oosterhoff group II clusters haveprograde rotation with =+94+/-47 km s^-1 andsigma_los=115+/-29 km s^-1, confirming a similar conclusion of van denBergh. The difference in kinematics and ages between Oosterhoff group Iand II clusters suggests that they may have different origins: TheOosterhoff II clusters were formed very early in the proto-Galaxy whilethe Oosterhoff I clusters were formed at different locations and at alater time, and were probably merger events. The period distributions ofan unbiased sample of field RRab variables with |Z|<=3 kpc and|Z|>=5 kpc indicate that they may belong to different populations,with peak periods of 0.65 and 0.55 days, respectively. If the hypothesisthat the Oosterhoff dichotomy is due to evolution is correct, then thisperiod shift among the field RR Lyrae variables suggests that the RRabpopulation with |Z|<=3 kpc is somewhat older than the RRab populationwith |Z|>=5 kpc. This also suggests different formation histories. Inan appendix, we discuss that the frequently used Gaussian HBmass-dispersion rate (i.e., the mass-loss rate at the red giant branch[RGB] tip) in synthetic HB model calculations cannot fully explain theextended blue HB population and the pulsational properties of RR Lyraevariables in M2. Comparisons with synthetic HB models strongly suggeststhat an enhanced mass loss is required that extends the HB toward lowerHB masses. We also discuss statistical effects on the metallicityestimate using P_0,min for field RRab variables reported by Castellani,Maceroni, & Tosi in 1983. Our calculations suggest that thestatistical effect is sufficient to explain the apparent gradient inP_0,min without introducing a metallicity gradient.
| CN and CH Band Strengths of Bright Giants in M3
CN and CH band strengths for 10 bright red giants in M3 have beenmeasured from archival spectra obtained with the Multiple MirrorTelescope. A CN-CH band strength anticorrelation is confirmed for theprogram stars together with other stars for which there is publisheddata. This suggests an anticorrelation between carbon and nitrogenabundances with a constant total abundance of carbon plus nitrogen.However, stars that do not follow the CN-CH anticorrelation are alsofound. The star III-77, which is found to have both strong CN and CHbands, is the most peculiar among them. Three other stars, VZ 194, VZ352, and VZ 1420, which show both weak CN and CH band strengths, couldbe asymptotic giant branch stars.
| 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
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