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|CCD Photometry of the Globular Cluster M15: RR Lyrae Fourier Decomposition and Physical Parameters|
Results of CCD photometry using V and R filters are reported for 33 RRLyrae stars in M15. The periodicities of some variables have beenrevised and new ephemerides are given. The Blazhko effect, previouslyreported in V12, was not detected. Applying the approach of Fourierdecomposition of the light curves, the physical parameters of the typeRRab and RRc variables were estimated. The cluster is Oosterhoff type IIand the values for the iron content and distance are:[Fe/H]=-1.98+/-0.24 and d=8.67+/-0.41 kpc, respectively. The mean valuesof the physical parameters determined for the RR Lyrae stars place thecluster precisely into the sequences Oosterhoff type --metallicity andmetallicity-- effective temperature, valid for globular clusters.Evidences of evolution from the ZAHB are found for the RRc but not forthe RRab stars.
|CN Abundance Inhomogeneities in the Globular Cluster Messier 13 (NGC 6205): Results Based on Merged Data Sets from the Literature|
The globular cluster Messier 13 (NGC 6205) has been shown by a number ofdifferent studies to be chemically inhomogeneous with regard to elementsranging from C through Al. A database of λ3883 CN-band indices,carbon, oxygen, and sodium abundances for red giants in M13 has beencompiled and homogenized from a variety of literature sources. The dataare used to document the distribution of CN band strength among both redgiant branch (RGB) and asymptotic giant branch (AGB) stars brighter thanMV=+0.8, as well as the relationships between the CNinhomogeneities and the dispersions in other elements. The CNdistribution among the M13 RGB stars is bimodal, although a fewintermediate-CN red giants are present in the cluster. The AGB starsshow uniformly weak CN bands. The spread in CN band strength, onceempirically corrected for effective temperature and gravity differencesamong the red giants, is very well correlated with the sodium abundanceand is anticorrelated with oxygen. Above MV=+0.8, the carbonabundance of red giants decreases with increasing luminosity, as firstfound by N. Suntzeff. There is a rather modest anticorrelation betweenCN band strength and [C/Fe] among RGB stars of comparable magnitude. Theabundance patterns within M13 can be interpreted as a primordial (orpre-RGB phase) abundance spread, coupled with the later effects of deepmixing within the red giants. In the CN-weak giants and those CN-strongstars with [O/Fe]~-0.2+/-0.2 dex on the upper RGB, deep mixing seems tohave been limited largely to the dredge-up of C-->N-processedmaterial. By contrast, there are some M13 stars, most notably a group ofvery oxygen-deficient giants near the RGB tip, but perhaps also afraction of CN-strong giants with lesser oxygen depletions, in whichsome measure of O-->N-processed material also appears to have beenbrought to the surface.
|Central energy equipartition in multimass models of globular clusters|
In the construction of multimass King-Michie models of globularclusters, an approximated central energy equipartition between stars ofdifferent mass is usually imposed by scaling the velocity parameter ofeach mass class inversely with the stellar mass, as if the distributionfunction were isothermal. In this paper, this `isothermal approximation'has been checked and its consequences on the model parameters studied bya comparison with models including central energy equipartitioncorrectly. It is found that, under the isothermal approximation, the`temperatures' of a pair of components can differ to a non-negligibleamount for low concentration distributions. It is also found that, ingeneral, this approximation leads to a significantly reduced masssegregation in comparison with that given under the exact energyequipartition at the centre. As a representative example, an isotropicthree-component model fitting a given projected surface brightness andline-of-sight velocity dispersion profiles is discussed. In thisexample, the isothermal approximation gives a cluster envelope much moreconcentrated (central dimensionless potential W= 3.3) than under thetrue equipartition (W= 5.9 × 10-2), as well as a highermass function logarithmic slope. As a consequence, the inferred totalmass (and then the global mass-to-light ratio) is a factor of 1.4 timeslower than the correct value and the amount of mass in heavy darkremnants is 3.3 times smaller. Under energy equipartition, the fate ofstars having a mass below a certain limit is to escape from the system.This limit is derived as a function of the mass and W of the componentof giant and turn-off stars.
|Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process|
We use high-resolution spectra obtained with the ESO Very LargeTelescope to measure surface abundance patterns of 43 blue stragglerstars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs showsa significant depletion of carbon and oxygen with respect to thedominant population. This evidence would suggest the presence of CNOburning products on the BSS surface coming from a deeply peeled parentstar, as expected in the case of a mass transfer process. This is thefirst detection of a chemical signature clearly pointing to a specificBSS formation process in a globular cluster.Based on observations collected at the ESO-VLT (Cerro Paranal, Chile)under program 072.D-0337.
|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.
|An Empirical Calibration of the Mixing-Length Parameter α|
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|The Pure Noncollisional Blue Straggler Population in the Giant Stellar System ω Centauri|
We have used high spatial resolution data from the Hubble SpaceTelescope (HST) and wide-field ground-based observations to search forblue straggler stars (BSSs) over the entire radial extent of the largestellar system ω Centauri. We have detected the largest populationof BSSs ever observed in any stellar system. Even though the sample isrestricted to the brightest portion of the BSS sequence, more than 300candidates have been identified. BSSs are thought to be produced by theevolution of binary systems (formed either by stellar collisions or massexchange in binary stars). Since systems like Galactic globular clusters(GGCs) and ω Cen evolve dynamically on timescales significantlyshorter than their ages, binaries should have settled toward the center,showing a more concentrated radial distribution than the ordinary, lessmassive single stars. Indeed, in all GGCs that have been surveyed forBSSs, the BSS distribution is peaked at the center. Conversely, inω Cen we find that the BSSs share the same radial distribution asthe adopted reference populations. This is the cleanest evidence everfound that such a stellar system is not fully relaxed even in thecentral region. We further argue that the absence of centralconcentration in the BSS distribution rules out a collisional origin.Thus, the ω Cen BSSs are the purest and largest population ofnoncollisional BSSs ever observed. Our results allow the first empiricalquantitative estimate of the production rate of BSSs via this channel.BSSs in ω Cen may represent the best local template for modelingthe BSS populations in distant galaxies where they cannot beindividually observed.Based on observations with the NASA/ESA HST, obtained at the SpaceTelescope Science Institute, which is operated by AURA, Inc., under NASAcontract NAS5-26555. Also based on WFI observations collected at theEuropean Southern Observatory, La Silla, Chile, within the observingprograms 62.L-0354 and 64.L-0439.
|Discovery of an SX Phoenicis Type Pulsating Component in the Algol-Type Semidetached Eclipsing Binary QU Sagittae in M71|
We report the discovery of an SX Phoenicis type pulsating component inthe Algol-type semidetached eclipsing binary QU Sge, in the metal-richglobular cluster M71. QU Sge is only about 80" from the center of M71and is located in the blue straggler region in the color-magnitudediagram of M71. It is considered to be a probable member of M71, with amembership probability greater than 60% deduced from a proper-motionstudy in the literature. From time-series CCD photometry, we find thatQU Sge has an orbital period of 3.790818 days and a primary minimumdepth of ΔV=1.333 mag. The eclipsing light curve solution showsthat QU Sge has a semidetached binary configuration with the secondarycomponent fully filling its Roche lobe. After subtracting the eclipsesfrom the light curve, we discover an SX Phoenicis type pulsationfeature. It is found to have a short period of about 0.03 days and asmall amplitude of about 0.024 mag. This is the first eclipsing binarysystem in a globular cluster to exhibit a pulsating feature. This resultsupports the model in which the origin of some blue stragglers inglobular clusters is mass transfer between two components in theprimordial binary systems.
|Spectroscopy of QUEST RR Lyrae Variables: The New Virgo Stellar Stream|
Eighteen RR Lyrae variables (RRLs) that lie in the ``12.4h clump''identified by the Quasar Equatorial Survey Team (QUEST) have beenobserved spectroscopically to measure their radial velocities and metalabundances. Ten blue horizontal branch (BHB) stars identified by theSloan Digital Sky Survey (SDSS) were added to this sample. Six of thenine stars in the densest region of the clump have a mean radialvelocity in the Galactic rest frame (Vgsr) of 99.8 andσ=17.3 km s-1, which is slightly smaller than theaverage error of the measurements. The whole sample contains eight RRLsand five BHB stars that have values of Vgsr suggestingmembership in this stream. For seven of these RRLs, the measurements of[Fe/H], which have an internal precision of 0.08 dex, yield<[Fe/H]>=-1.86 and σ=0.40. These values suggest that thestream is a tidally disrupted dwarf spheroidal galaxy of low luminosity.Photometry from the database of the SDSS indicates that this streamcovers at least 106 deg2 of the sky in the constellationVirgo. The name Virgo stellar stream is suggested.
|Manganese Abundances in Cluster and Field Stars|
We have derived Mn abundances for more than 200 stars in 19 globularclusters. In addition, Mn abundance determinations have been made for acomparable number of halo field and disk stars possessing an overlappingrange of metallicities and stellar parameters. Our primary data set wascomprised of high-resolution spectra previously acquired at theMcDonald, Lick, and Keck Observatories. To enlarge our data pool, weacquired globular and open cluster spectra from several otherinvestigators. Data were analyzed using synthetic spectra of the 6000Å Mn I triplet. Hyperfine structure parameters were included inthe synthetic spectra computations. Our analysis shows that for themetallicity range -0.7>[Fe/H]>-2.7, stars of 19 globular clustershave a mean relative abundance of <[Mn/Fe]>=-0.37+/-0.01(σ=0.10), a value in agreement with that of the field stars,<[Mn/Fe]>=-0.36+/-0.01 (σ=0.08). Despite the 2 orders ofmagnitude span in metallicity, the <[Mn/Fe]> ratio remainsconstant in both stellar populations. Our Mn abundance data indicatethat there is no appreciable variation in the relative nucleosyntheticcontribution from massive stars that undergo core-collapse supernovaeand thus no significant change of the associated initial mass functionin the specified metallicity range.
|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.
|VLT/UVES spectroscopy of individual stars in three globular clusters in the Fornax dwarf spheroidal galaxy|
We present a high resolution (R ~ 43 000) abundance analysis of a totalof nine stars in three of the five globular clusters associated with thenearby Fornax dwarf spheroidal galaxy. These three clusters (1, 2 and 3)trace the oldest, most metal-poor stellar populations in Fornax. Wedetermine abundances of O, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, Nd andEu in most of these stars, and for some stars also Mn and La. Wedemonstrate that classical indirect methods (isochrone fitting andintegrated spectra) of metallicity determination lead to values of[Fe/H] which are 0.3 to 0.5 dex too high, and that this is primarily dueto the underlying reference calibration typically used by these studies.We show that Cluster 1, with [Fe /H] = -2.5, now holds the record forthe lowest metallicity globular cluster. We also measure anover-abundance of Eu in Cluster 3 stars that has only been previouslydetected in a subgroup of stars in M 15. We find that the Fornaxglobular cluster properties are a global match to what is found in theirGalactic counterparts; including deep mixing abundance patterns in twostars. We conclude that at the epoch of formation of globular clustersboth the Milky Way and the Fornax dwarf spheroidal galaxy shared thesame initial conditions, presumably pre-enriched by the same processes,with identical nucleosynthesis patterns.
|Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452|
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
|XMM-Newton X-ray and optical observations of the globular clusters M 55 and NGC 3201|
We have observed two low concentration Galactic globular clusters withthe X-ray observatory XMM-Newton. We detect 47 faint X-ray sources inthe direction of M 55 and 62 in the field of view ofNGC 3201. Using the statistical Log N - Log Srelationship of extragalactic sources derived from XMM-Newton LockmanHole observations, to estimate the background source population, weestimate that very few of the sources (1.5±1.0) in the field ofview of M 55 actually belong to the cluster. Thesesources are located in the centre of the cluster as we expect if thecluster has undergone mass segregation. NGC 3201 hasapproximately 15 related sources, which are centrally located but arenot constrained to lie within the half mass radius. The sourcesbelonging to this cluster can lie up to 5 core radii from the centre ofthe cluster which could imply that this cluster has been perturbed.Using X-ray (and optical, in the case of M 55)colours, spectral and timing analysis (where possible) and comparingthese observations to previous X-ray observations, we find evidence forsources in each cluster that could be cataclysmic variables, activebinaries, millisecond pulsars and possible evidence for a quiescent lowmass X-ray binary with a neutron star primary, even though we do notexpect any such objects in either of the clusters, due to their lowcentral concentrations. The majority of the other sources are backgroundsources, such as AGN.
|Homogeneous Photometry. V. The Globular Cluster NGC 4147|
New BVRI broadband photometry and astrometry are presented for theglobular cluster NGC 4147, based upon measurements derived from 524ground-based CCD images mostly either donated by colleagues or retrievedfrom public archives. We have also reanalyzed five exposures of thecluster obtained with WFPC2 on the Hubble Space Telescope in the F439Wand F555W (B and V) filters. We present calibrated color-magnitude andcolor-color diagrams. Analysis of the color-magnitude diagram revealsmorphological properties generally consistent with publishedmetal-abundance estimates for the cluster, and an age typical of otherGalactic globular clusters of similar metallicity. We have alsoredetermined the periods and mean magnitudes for the RR Lyrae variables,including a new c-type variable reported here for the first time. Ourdata do not show clear evidence for photometric variability in candidateV18, recently reported by Arellano Ferro et al. (2004, Rev. Mex.A&A, 40, 209). These observations also support the nonvariablestatus of candidates V5, V9, and V15. The union of our light-curve datawith those of Newburn (1957, AJ, 62, 197), Mannino (1957, Mem. Soc.Astron. Italiana, 28, 285), and Arellano Ferro et al. (op. cit.) permitsthe derivation of significantly improved periods. The mean periods andthe Bailey period-amplitude diagrams support the classification of thecluster as Oosterhoff I, despite its predominantly blue horizontalbranch. The number ratio of c- to ab-type RR Lyrae stars, on the otherhand, is unusually high for an Oosterhoff I cluster. The calibratedresults have been made available through the first author's Web site.Based in part on archival observations made with ESO telescopes at theLa Silla and Paranal Observatory under program ID 60.A-9050(A).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
|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.
|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|
|ASAS 081933-2358.2: RRc-Type Variable with Two Closely Spaced Frequencies|
Electronically available photometry of ASAS 081933-2358.2 allow us toclassify the variable as a RRc-type star with two closely spacedfrequencies - the first known field star of this kind in our Galaxy.
|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.
|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.
|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.
|Infrared Photometry of NGC 6791|
We present deep JHK photometry of the old and metal-rich open clusterNGC 6791. The photometry reaches below the main-sequence turnoff toK~16.5 mag. We combine our photometry with that from Stetson et al. toprovide color-magnitude diagrams showing K versus J-K, K versus V-K, andV versus V-K. We study the slope of the red giant branch in the infraredbut find that it is not a useful metallicity indicator for the cluster,nor any metal-rich cluster that lacks a well-populated red giant branch,because it is not linear, as has often been assumed, in K versus J-K.The mean color of the red horizontal-branch/red clump stars provide anestimate of the cluster reddening, E(B-V)=0.14+/-0.04 mag for[Fe/H]=+0.4+/-0.1. The mean magnitudes of these stars also provide agood distance estimate, (m-M)0=13.07+/-0.04. Finally, we findthat the isochrones of Yi et al. provide optimal fits in V versus B-Vand V-K and K versus J-K and V-K for such values if [Fe/H] lies between+0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyr([Fe/H]=+0.3) and 7.5 Gyr ([Fe/H]=+0.5).Based on observations made with the Mayall 4 m Telescope of the NationalOptical Astronomy Observatory.
|Metal-poor Field Blue Stragglers: More Evidence for Mass Transfer|
We report radial velocity studies of five candidate metal-poor fieldblue stragglers, all known to be deficient in lithium. Four of the fivestars are single-lined spectroscopic binaries, with periods ranging from302 to 840 days, and low orbital eccentricities, in agreement withsimilar behavior found for other blue straggler candidates by Preston& Sneden and Carney et al. The limited data available for lithiumabundances indicate that all blue straggler binaries have depletedlithium, but that constant velocity stars generally have normal lithiumabundances. This suggests that the ``lithium gap'' for hot metal-poormain-sequence stars may not exist or lies at higher temperatures thanfound in the Hyades. Our results and those of Preston & Sneden showhigher values of vrotsini for the binary stars than those ofcomparable temperature constant velocity stars. The orbital periods aretoo long for tidal effects to be important, implying that spin-up duringmass transfer when the orbital separations and periods were smaller isthe cause of the enhanced rotation. The mass function distribution issteeper for the blue straggler binary stars than that of lower masssingle-lined spectroscopic binaries, indicating a narrower range insecondary masses. We argue that if all secondaries are white dwarfs withthe same mass, it is probably around 0.55 Msolar. The modelsof Rappaport et al., applied to white dwarf secondaries, suggest thatthe orbital elements of all metal-poor binary blue stragglers areconsistent with stable mass transfer, with the possible exception ofG202-65.Some of the results presented here used observations made with theMultiple Mirror Telescope, a joint facility of the SmithsonianInstitution and the University of Arizona.
|On the Distribution of the Modulation Frequencies of RR Lyrae Stars|
For the first time connection between the pulsation and modulationproperties of RRLyr stars has been detected. Based on the available datait is found that the possible range of the modulation frequencies, ie.,the possible maximum value of the modulation frequency depends on thepulsation frequency. Short period variables (P<0.4 d) can havemodulation period as short as some days, while longer period variables(P>0.6 d) always exhibit modulation with P_mod>20 d. We interpretthis tendency with the equality of the modulation period with thesurface rotation period, because similar distribution of the rotationalperiods is expected if an upper limit of the total angular momentum ofstars leaving the RGB exists. The distribution of the projectedrotational velocities of red and blue horizontal branch stars atdifferent temperatures shows a similar behavior as v_rot derived for RRLyr stars from their modulation periods. This common behavior givesreason to identify the modulation period with the rotational period ofthe modulated RR Lyr stars.
|Accounting for the anisoplanatic point spread function in deep wide-field adaptive optics images|
In this paper we present the approach we have used to determine andaccount for the anisoplanatic point spread function (PSF) in deepadaptive optics (AO) images for the Survey of a Wide Area with NACO(SWAN) at the ESO VLT. The survey comprises adaptive optics observationsin the Ks band totaling ~30~arcmin^2, assembled from 42discrete fields centered on different bright stars suitable for AOguiding. We develop a parametric model of the PSF variations across thefield of view in order to build an accurate model PSF for every galaxydetected in each of the fields. We show that this approach isparticularly convenient, as it uses only easily available data and makesno uncertain assumptions about the stability of the isoplanatic angleduring any given night. The model was tested using simulated galaxyprofiles to check its performance in terms of recovering the correctmorphological parameters; we find that the results are reliable up toKs 20.5 (KAB22.3) in a typical SWANfield. Finally, the model obtained was used to derive the first resultsfrom five SWAN fields, and to obtain the AO morphology of 55 galaxiesbrighter than Ks = 20. These preliminary results demonstratethe unique power of AO observations to derive the details of faintgalaxy morphologies and to study galaxy evolution.
|Analysis of medium resolution spectra by automated methods - Application to M 55 and ω Centauri|
We have employed feedforward neural networks trained on syntheticspectra in the range 3800 to 5600 Å with resolutions of ~2-3Å to determine metallicities from spectra of about 1000main-sequence turn-off, subgiant and red giant stars in the globularclusters M 55 and ω Cen. The overall metallicity accuracies are ofthe order of 0.15 to 0.2 dex. In addition, we tested how well thestellar parameters log g and T_eff can be retrieved from such datawithout additional colour or photometric information. We find overalluncertainties of 0.3 to 0.4 dex for log g and 140 to 190 K for T_eff. Inorder to obtain some measure of uncertainty for the determined values of[Fe/H], log g and T_eff, we applied the bootstrap method for the firsttime to neural networks for this kind of parametrization problem. Thedistribution of metallicities for stars in ω Cen clearly shows alarge spread in agreement with the well known multiple stellarpopulations in this cluster.
|Full computation of massive AGB evolution. I. The large impact of convection on nucleosynthesis|
It is well appreciated that the description of overadiabatic convectionaffects the structure of the envelopes of luminous asymptotic giantbranch (AGB) stars in the phase of ``hot bottom burning'' (HBB). Westress that this important uncertainty in the modeling plays a rolewhich is much more dramatic than the role which can be ascribed, e.g.,to the uncertainty in the nuclear cross-sections. Due to the roletentatively attributed today to the HBB nucleosynthesis as the site ofself-enrichment of Globular Clusters stars, it is necessary to explorethe difference in nucleosynthesis obtained by different prescriptionsfor convection. We present results of detailed evolutionary calculationsof the evolution of stars of intermediate mass during the AGB phase forthe metallicity typical of the Globular Clusters that show the largestspread in CNO abundances (Z 10-3). We follow carefullythe nucleosynthesis at the base of the external convective region,showing that very different results can be obtained according to thepresciption adopted to find out the temperature gradient within theinstability regions. We discuss the uncertainties in the yields of thevarious chemical species and the role which these sources can play aspolluters of the interstellar medium.
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