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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.
| Dynamical Formation of Close Binaries in Globular Clusters: Cataclysmic Variables
We answer the long-standing question of which production mechanism isresponsible for the cataclysmic variables (CVs) in globular clusters.Arguments have been given that range from mostly primordial presence toa significant contribution of later dynamical formation in close stellarencounters. We conclude, based on a thorough analysis of a homogeneousChandra data set, that the majority of CVs in dense globular clustershave a dynamical origin.
| Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant Branch Bump
A Monte Carlo simulation exploring uncertainties in standard stellarevolution theory on the red giant branch of metal-poor globular clustershas been conducted. Confidence limits are derived on the absolute V-bandmagnitude of the bump in the red giant branch luminosity function(MV,b) and the excess number of stars in the bump,Rb. The analysis takes into account uncertainties in theprimordial helium abundance, abundance of α-capture elements,radiative and conductive opacities, nuclear reaction rates, neutrinoenergy losses, the treatments of diffusion and convection, the surfaceboundary conditions, and color transformations. The uncertainty intheoretical values for the red giant bump magnitude varies withmetallicity between +0.13 and -0.12 mag at [Fe/H]=-2.4 and between +0.23and -0.21 mag at [Fe/H]=-1.0. The dominant sources of uncertainty arethe abundance of the α-capture elements , the mixing length, andthe low-temperature opacities. The theoretical values of MV,bare in good agreement with observations. The uncertainty in thetheoretical value of Rb is +/-0.01 at all metallicitiesstudied. The dominant sources of uncertainty are the abundance of theα-capture elements, the mixing length, and the high-temperatureopacities. The median value of Rb varies from 0.44 at[Fe/H]=-2.4 to 0.50 at [Fe/H]=-1.0. These theoretical values forRb are in agreement with observations.
| 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.
| Why is the mass function of NGC 6218 flat?
We have used the FORS-1 camera on the VLT to study the main sequence(MS) of the globular cluster NGC 6218 in the V and R bands. Theobservations cover an area of 3.4 arcmin × 3.4 arcmin around thecluster centre and probe the stellar population out to the cluster'shalf-mass radius (rh ≃ 2.2 arcmin). Thecolour-magnitude diagram (CMD) that we derive in this way reveals anarrow and well defined MS extending down to the 5 σ detectionlimit at V ≃ 25, or about 6 magnitudes below the turn-off,corresponding to stars of 0.25 {M}_ȯ. The luminosity function (LF)obtained with these data shows a marked radial gradient, in that theratio of lower- and higher-mass stars increases monotonically withradius. The mass function (MF) measured at the half-mass radius, and assuch representative of the cluster's global properties, is surprisinglyflat. Over the range 0.4{-}0.8 {M}_ȯ, the number of stars per unitmass follows a power-law distribution of the type dN/dm ∝m0, where, for comparison, Salpeter's IMF would be dN/dm∝ m-2.35. We expect that such a flat MF does notrepresent the cluster's IMF but is the result of severe tidal strippingof the stars from the cluster due to its interaction with the Galaxy'sgravitational field. Our results cannot be reconciled with thepredictions of recent theoretical models that imply a relativelyinsignificant loss of stars from NGC 6218 as measured by its expectedvery long time to disruption. They are more consistent with the orbitalparameters based on the Hipparcos reference system that imply a muchhigher degree of interaction of this cluster with the Galaxy thanassumed by those models. Our results indicate that, if the orbit of acluster is known, the slope of its MF could be useful in discriminatingbetween the various models of the Galactic potential.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| 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.
| The evolution of binary fractions in globular clusters
We study the evolution of binary stars in globular clusters using a newMonte Carlo approach combining a population synthesis code (STARTRACK)and a simple treatment of dynamical interactions in the dense clustercore using a new tool for computing three- and four-body interactions(FEWBODY). We find that the combination of stellar evolution anddynamical interactions (binary-single and binary-binary) leads to arapid depletion of the binary population in the cluster core. Themaximum binary fraction today in the core of a typical dense clustersuch as 47 Tuc, assuming an initial binary fraction of 100 per cent, isonly ~5-10 per cent. We show that this is in good agreement with recentHubble Space Telescope observations of close binaries in the core of 47Tuc, provided that a realistic distribution of binary periods is used tointerpret the results. Our findings also have important consequences forthe dynamical modelling of globular clusters, suggesting that `realisticmodels' should incorporate much larger initial binary fractions than hasusually been the case in the past.
| Caroline Herschel as observer
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| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| The Unusual Luminosity Function of the Globular Cluster M10
We present the I-band luminosity function of the differentially reddenedglobular cluster M10. We combine photometric analysis derived fromwide-field (23'×23') images that include theouter regions of the cluster and high-resolution images of the clustercore. After making corrections for incompleteness and field starcontamination, we find that the relative numbers of stars on the lowergiant branch and near the main-sequence turnoff are in good agreementwith theoretical predictions. However, we detect significant (>6σ) excesses of red giant branch stars above and below the redgiant branch bump using a new statistic (a population ratio) for testingrelative evolutionary timescales of main-sequence and red giant stars.The statistic is insensitive to assumed cluster chemical composition,age, and main-sequence mass function. The excess number of red giantscannot be explained by reasonable systematic errors in our assumedcluster chemical composition, age, or main-sequence mass function.Moreover, M10 shows excesses when compared to the cluster M12, which hasnearly identical metallicity, age, and color-magnitude diagrammorphology. We discuss possible reasons for this anomaly, finding thatthe most likely cause is a mass function slope that shows significantvariations as a function of mass.
| A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies
We have compiled a sample of globular clusters with high-quality stellarabundances from the literature to compare to the chemistries of stars inthe Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clustersexamined, 29 also have kinematic information. Most of the globularclusters belong to the Galactic halo; however, a significant number havedisk kinematics or belong to the bulge. Focusing on the [α/Fe] andlight r-process element ratios, we find that most globular cluster starsmimic field stars of similar metallicities, and neither clearlyresembles the currently available stellar abundances in dwarf galaxies(including globular clusters in the Large Magellanic Cloud). Theexceptions to these general elemental ratio comparisons are alreadyknown in the literature, e.g., ω Centauri, Palomar 12, and Terzan7 associated with the Sagittarius remnant and Ruprecht 106, which has ahigh radial velocity and low [α/Fe] ratio. A few other globularclusters show more marginal peculiarities. The most notable one is thehalo cluster M68, which has a high galactocentric rotational velocity, aslightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratiosdecrease with increasing [Fe/H] at intermediate metallicities, which isconsistent with very massive stars playing a larger role in the earlychemical evolution of the Galaxy. The chemical similarities betweenglobular clusters and field stars with [Fe/H]<=-1.0 suggests a sharedchemical history in a well-mixed early Galaxy. The differences in thepublished chemistries of stars in the dwarf spheroidal galaxies suggestthat neither the globular clusters, halo stars, nor thick disk stars hadtheir origins in small isolated systems like the present-day Milky Waydwarf satellites.
| The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars
We discuss a 175 deg2 spectroscopic survey for bluehorizontal branch (BHB) stars in the Galactic halo. We use the TwoMicron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) toselect BHB candidates, and we find that the 2MASS and SDSS colorselection is 38% and 50% efficient, respectively, for BHB stars. Oursamples include one likely runaway B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent withmembership in the halo population: the median metallicity is[Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and themean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is-4+/-30 km s-1. We discuss the theoretical basis of thePreston, Shectman, and Beers MV-color relation for BHB starsand conclude that the intrinsic shape of the BHB MV-colorrelation results from the physics of stars on the horizontal branch. Wecalculate the luminosity function for the field BHB star samples usingthe maximum likelihood method of Efstathiou and coworkers, which isunbiased by density variations. The field BHB luminosity functionexhibits a steep rise at bright luminosities, a peak between0.8
| 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.
| Eclipsing Binaries in the Young Large Magellanic Cloud Cluster NGC 1850
I present light curves for two detached eclipsing binary stars in theregion of the LMC cluster NGC 1850, which is possibly a young globularcluster still in formation. One, a likely O-type star, is a newlydetected eclipsing binary in the region of the very young subcluster NGC1850A. This binary is among a small number of highly massive O-typestars in binary systems found in LMC clusters. These two eclipsingbinaries are the first to be discovered in the well-studied NGC 1850,and the O-type star is the first eclipsing binary found in NGC 1850A.Light curves for two Cepheid variables in the NGC 1850 region are alsoshown. The discovery of two eclipsing binaries in the youngglobular-like cluster NGC 1850 is discussed in terms of the importanceof the binary fraction to globular cluster evolution.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| A robust method for the analysis of integrated spectra from globular clusters using Lick indices
We define a method for the analysis of the integrated spectra ofextragalactic globular clusters that provides more reliable measures ofthe age, metallicity and α-element abundance ratio than have sofar been achieved. The method involves the simultaneous fitting of up to25 Lick indices in a χ2 fitting technique that maximizesthe use of the available data. Here we compare three sets of singlestellar population (SSP) models of Lick indices to the highsignal-to-noise, integrated spectra of 20 Galactic globular clusters.The ages, [Fe/H] and α-element abundance ratios derived from theSSP models are compared with the results of resolved stellar populationstudies from the literature. We find good consistency with the publishedvalues, with an agreement of better than 0.1 dex in all three derivedparameters. The technique allows the identification of abundance ratioanomalies, such as the known nitrogen overabundance in Galactic globularclusters, and the presence of anomalous horizontal branch morphologies.It also minimizes the impact on the derived parameters of imperfectcalibration to the Lick system, and reduction errors in general. Themethod defined in this paper is therefore robust with respect to many ofthe difficulties that plague the application of SSP models in general.Consequently, it is well suited to the study of extragalactic globularcluster systems.
| Integrated spectral energy distributions and absorption-feature indices of single stellar populations
Using evolutionary population synthesis, we present integrated spectralenergy distributions and absorption-line indices defined by the LickObservatory image dissector scanner (referred to as Lick/IDS) system,for an extensive set of instantaneous burst single stellar populations(SSPs). The ages of the SSPs are in the range 1 Gyr <=τ<= 19Gyr and the metallicities are in the range -2.3 <=[Fe/H]<=+0.2.Our models use the rapid single stellar evolution algorithm of Hurley,Pols and Tout for the stellar evolutionary tracks, the empirical andsemi-empirical calibrated BaSeL-2.0 model of Lejeune, Cuisinier andBuser for the library of stellar spectra and the empirical fittingfunctions of Worthey, Faber, Gonzalez and Burstein for the Lick/IDSspectral absorption-feature indices.Applying our synthetic Lick/IDS absorption-line indices to the meritfunction, we obtain the age and the metallicity of the central region ofM32, which can be well explained by an instantaneous SSP with an age of~6.5 Gyr and a metallicity similar to solar. Applying the derived ageand the metallicity from the merit function to a number of index-indexdiagrams, we find that the plots of Hβ-Fe5015 andHβ-Fe5782 are the best index-index diagrams from whichwe can directly obtain reasonable age and metallicity.
| 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.
| The Luminosity Function and Color-Magnitude Diagram of the Globular Cluster M12
In this paper we present the V and I luminosity functions andcolor-magnitude diagrams derived from wide-field(23'×23') BVI photometry of theintermediate-metallicity ([Fe/H]~-1.3) Galactic globular cluster M12.Using observed values (and ranges of values) for the clustermetallicity, reddening, distance modulus, and age, we compare these datawith recent α-enhanced stellar evolution models for low-massmetal-poor stars. We describe several methods of making comparisonsbetween theoretical and observed luminosity functions to isolate theevolutionary timescale information that the luminosity functionscontain. We find no significant evidence of excesses of stars on the redgiant branch, although the morphology of the subgiant branch in theobserved luminosity function does not match theoretical predictions in asatisfactory way. Current uncertainties in Teff-colortransformations (and possibly also in other physics inputs to themodels) make more detailed conclusions about the subgiant branchmorphology impossible. Given the recent constraints on cluster ages fromthe WMAP experiment, we find that good-fitting models that do notinclude He diffusion (both color-magnitude diagrams and luminosityfunctions) are too old (by ~1-2 Gyr) to adequately represent the clusterluminosity function. The inclusion of helium diffusion in the modelsprovides an age reduction (compared with nondiffusive models) that isconsistent with the age of the universe being 13.7+/-0.2 Gyr.
| 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.
| Measuring the Slope of the Dust Extinction Law and the Power Spectrum of Dust Clouds Using Differentially Reddened Globular Clusters
We present three methods for measuring the slope of the Galactic dustextinction law, RV, and a method for measuring the fine-scalestructure of dust clouds in the direction of differentially reddenedglobular clusters. We apply these techniques to BVI photometry of starsin the low-latitude Galactic globular cluster NGC 4833, which displaysspatially variable extinction/reddening about a mean~1. An extensive suite of Monte Carlo simulationsis used to characterize the efficacy of the methods. The essence of thefirst two methods is to determine, for an assumed value ofRV, the relative visual extinction δAV ofeach cluster horizontal-branch (HB) star with respect to an empirical HBlocus; the locus is derived from the color-magnitude diagram (CMD) of asubset of stars in a small region near the cluster center for whichdifferential extinction/reddening are relatively small. A star-by-starcomparison of δAV from the (B-V, V) CMD with that fromthe (V-I, V) CMD is used to find the optimal RV. In the thirdmethod RV is determined by minimizing the scatter in the HBin the (B-V, V) CMD after correcting the photometry for extinction andreddening using the dust maps of Schlegel, Finkbeiner, & Davis. Theweighted average of the results from the three methods givesRV=3.0+/-0.4 for the dust along the line of sight to NGC4833. The fine-scale structure of the dust is quantified via thedifference,(ΔAV)ij≡(δAV)i-(δAV)j,between pairs of cluster HB stars (i, j) as a function of their angularseparation rij. The variance (mean square scatter) of(ΔAV)ij is found to have a power-lawdependence on angular scale: var(r)~rβ, withβ=+0.9+/-0.1. This translates into an angular power spectrumP(κ)~κα, with the index α=-1.9+/-0.1for r~1'-5', where κ≡1/r. The dustangular power spectrum on small scales (from optical data) matchessmoothly onto the larger scale power spectrum derived from Schlegel etal.'s far-infrared map of the dust thermal emission.
| Chemical composition and evolutionary status of nine UV-bright stars in five globular clusters from VLT/UVES spectra
We have derived the chemical composition of nine UV-bright starsbelonging to five Galactic globular clusters of various metallicities([Fe/H] from -1.0 to -2.4 dex). The analyses are based on highresolution spectra obtained with the UV-Visual Echelle Spectrograph(UVES) at VLT-UT2. The evolutionary status of the stars is assessed fromthe chemical analysis and location in the H-R diagram. The star ID7 inNGC 5986 is confirmed as a bona fide post-asymptotic giant branch star(post-AGB) whereas the high-luminosity star ID6 has probably left theAGB before the third dredge-up. ZNG 1 in NGC 6712 shows an overabundanceof sodium, oxygen, and silicon similar to overabundances we find in theUV-bright star ID6 in NGC 5986; both stars could be in a post-early-AGB(PEAGB) phase of evolution. The UV-bright star ZNG 7 in NGC 6218 seemsto be an AGB star. The stars V-4 and ZNG 5 in NGC 6656 are in apost-horizontal-branch phase of evolution, with V-4 being significantlyoverabundant in heavy elements. The origin of these overabundances isdiscussed in the context of the evolutionary versus primordial scenario.The three UV-bright stars K 260, K 996 and K 1082 observed in the verymetal-deficient globular cluster NGC 7078 are post-horizontal-branchstars, one of them being slightly enriched in s-elements but with aluminosity too low for third dredge-up to have occured. The abundancepatterns of K 1082 in NGC 7078 seem to indicate the presence of milddiffusion and a radiative levitation process, already reported in theblue HB stars of M 13 (Behr et al. \cite{Behr1999}, ApJ, 517, L135) andNGC 6752 (Moehler et al. \cite{Moehler1999}, A&A, 339, 537).Based on data collected at Paranal Observatory (ESO, Chile), programidentifier ID 69.D-0081.Full Table \ref{abund} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/353Appendix is only available in electronic form athttp://www.edpsciences.org
| 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.
| A Search for Eclipsing Binaries in Galactic Globular Clusters
We report on the discovery and analysis of short-period (0.1 days $
| 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.
| Stellar population models of Lick indices with variable element abundance ratios
We provide the whole set of Lick indices from CN1 toTiO2 in the wavelength range 4000 <~<~λ<~6500 Å of simple stellar population models with, for the firsttime, variable element abundance ratios, [α/Fe ]= 0.0, 0.3, 0.5,[α/Ca ]=-0.1, 0.0, 0.2, 0.5 and [α/N]=-0.5, 0.0. The modelscover ages between 1 and 15 Gyr, metallicities between 1/200 and 3.5solar. The impact from the element abundance changes on theabsorption-line indices is taken from Tripicco & Bell, using anextension of the method introduced by Trager et al. Our models are freefrom the intrinsic α/Fe bias that was imposed by the Milky Waytemplate stars up to now, hence they reflect well-defined α/Feratios at all metallicities. The models are calibrated with Milky Wayglobular clusters for which metallicities and α/Fe ratios areknown from independent spectroscopy of individual stars. Themetallicities that we derive from the Lick indices Mgb and Fe5270 are inexcellent agreement with the metallicity scale by Zinn & West, andwe show that the latter provides total metallicity rather than ironabundance. We can reproduce the relatively strong CN-absorption featuresCN1 and CN2 of galactic globular clusters withmodels in which nitrogen is enhanced by a factor of 3. An enhancement ofcarbon, instead, would lead to serious inconsistencies with the indicesMg1 and C24668. The calcium sensitive index Ca4227of globular clusters is well matched by our models with [Ca/Fe]= 0.3,including the metal-rich bulge clusters NGC 6528 and 6553. From ourα/Fe-enhanced models we infer that the index [MgFe] defined byGonzález is quite independent of α/Fe but still slightlydecreases with increasing α/Fe. We find that the index , instead,is completely independent of α/Fe and serves best as a tracer oftotal metallicity. Searching for blue indices that give similarinformation as Mg b and , we find that CN1 andFe4383 may be best suited to estimating α/Fe ratios of objects atredshifts z~ 1.
| 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.
| The Chemical Composition of Stars in the Globular Clusters M 10, M 12, and M 71
The abundances of 19 chemical elements in the atmospheres of five starsbelonging to three globular clusters have been determined by applyingthe model-atmospheremethod to 430.0 790.0 nm spectra obtained with theéchelle spectrometer of the 6-m telescope of the SpecialAstrophysical Observatory. The abundances of silicon, calcium, iron-peakelements, copper, zinc, and neutron-capture elements follow theabundance patterns for halo stars. The abundance of sodium in M 10giants provides evidence that different mixing mechanisms operate inhalo and cluster stars or that light elements are enriched in differentways in the pre-stellar matter from which some globular clusters andhalo stars were formed.
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