Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies
The colors of globular clusters in most large elliptical galaxies arebimodal. This is generally taken as evidence for the presence of twocluster subpopulations that have different geneses. However, here wefind that, because of the nonlinear nature of the metallicity-to-colortransformation, a coeval group of old clusters with a unimodalmetallicity spread can exhibit color bimodality. The models of clustercolors indicate that horizontal-branch stars are the main drivers behindthe empirical nonlinearity. We show that the scenario gives simple andcohesive explanations for all the key observations and could simplifytheories of elliptical galaxy formation.
| Evolved Stars in the Galactic Globular Cluster M55 (NGC 6809)
We have compiled the asymptotic giant, horizontal, and upper red giantbranch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809).Using the star counts and the R-parameter we compute the initial heliumabundance Y=0.274+/-0.016. TheR2=NAGB/NHB ratio (0.156+/-0.023) isunusually high for a globular cluster, being almost 2 σ away fromthe predicted values, and theR1=NAGB/NRGB ratio (0.272+/-0.047) isthe highest recorded for a massive globular cluster. We argue that M55'sparticular HB morphology and metallicity have produced long-lived HBstars that are not too blue to avoid producing AGB stars. This resulthints that we are able to map evolutionary effects on the HB. Finally,although we find no evidence of variations in HB morphology withdistance from the center of the cluster, the red HB stars aresignificantly less concentrated than the majority of HB stars, and thebluest HB stars are more centrally concentrated.
| 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.
| Population Synthesis in the Blue. IV. Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
We present a new set of model predictions for 16 Lick absorption lineindices from Hδ through Fe5335 and UBV colors for single stellarpopulations with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from-1.3 to +0.3, and variable abundance ratios. The models are based onaccurate stellar parameters for the Jones library stars and a new set offitting functions describing the behavior of line indices as a functionof effective temperature, surface gravity, and iron abundance. Theabundances of several key elements in the library stars have beenobtained from the literature in order to characterize the abundancepattern of the stellar library, thus allowing us to produce modelpredictions for any set of abundance ratios desired. We develop a methodto estimate mean ages and abundances of iron, carbon, nitrogen,magnesium, and calcium that explores the sensitivity of the variousindices modeled to those parameters. The models are compared to high-S/Ndata for Galactic clusters spanning the range of ages, metallicities,and abundance patterns of interest. Essentially all line indices arematched when the known cluster parameters are adopted as input.Comparing the models to high-quality data for galaxies in the nearbyuniverse, we reproduce previous results regarding the enhancement oflight elements and the spread in the mean luminosity-weighted ages ofearly-type galaxies. When the results from the analysis of blue and redindices are contrasted, we find good consistency in the [Fe/H] that isinferred from different Fe indices. Applying our method to estimate meanages and abundances from stacked SDSS spectra of early-type galaxiesbrighter than L*, we find mean luminosity-weighed ages of theorder of ~8 Gyr and iron abundances slightly below solar. Abundanceratios, [X/Fe], tend to be higher than solar and are positivelycorrelated with galaxy luminosity. Of all elements, nitrogen is the morestrongly correlated with galaxy luminosity, which seems to indicatesecondary nitrogen enrichment. If that interpretation is correct, thisresult may impose a lower limit of 50-200 Myr to the timescale of starformation in early-type galaxies. Unlike clusters, galaxies show asystematic effect whereby higher order, bluer, Balmer lines yieldyounger ages than Hβ. This age discrepancy is stronger for lowerluminosity galaxies. We examine four possible scenarios to explain thistrend. Contamination of the bluer indices by a metal-poor stellarpopulation with a blue horizontal branch cannot account for the data.Blue stragglers and abundance-ratio effects cannot be ruled out, as theycan potentially satisfy the data, even though this can only be achievedby resorting to extreme conditions, such as extremely high [O/Fe] orspecific blue-straggler frequencies. The most likely explanation is thepresence of small amounts of a young/intermediate-age stellar populationcomponent. We simulate this effect by producing two-component models andshow that they provide a reasonably good match to the data when the massfraction of the young component is typically a few percent. Ifconfirmed, this result implies star formation has been extended inearly-type galaxies, and more so in less massive galaxies, which seemsto lend support to the ``downsizing'' scenario. Moreover, it impliesthat stellar population synthesis models are capable of constraining notonly the mean ages of stellar populations in galaxies, but also theirage spread.
| A Panchromatic Study of the Globular Cluster NGC 1904. I. The Blue Straggler Population
By combining high-resolution (HST/WFPC2) and wide-field ground-based(2.2 m ESO/WFI) and space (GALEX) observations, we have collected amultiwavelength photometric database (ranging from the far-UV to thenear infrared) of the galactic globular cluster NGC 1904 (M79). Thesample covers the entire cluster extension, from the very centralregions up to the tidal radius. In the present paper, such a data set isused to study the BSS population and its radial distribution. A totalnumber of 39 bright (m218<=19.5) BSSs have been detected,and they have been found to be highly segregated in the cluster core. Nosignificant upturn in the BSS frequency has been observed in theoutskirts of NGC 1904, in contrast to other clusters (M3, 47 Tuc, NGC6752, M5) studied with the same technique. Such evidence, coupled withthe large radius of avoidance estimated for NGC 1904(ravoid~30 core radii), indicates that the vast majority ofthe cluster heavy stars (binaries) has already sunk to the core.Accordingly, extensive dynamical simulations suggest that BSSs formed bymass transfer activity in primordial binaries evolving in isolation inthe cluster outskirts represent only a negligible (0%-10%) fraction ofthe overall population.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 GALEX observations (program GI-056)and WFI observations collected at the European Southern Observatory, LaSilla, Chile, within the observing programs 62.L-0354 and 64.L-0439.
| Probing the Sagittarius stream with blue horizontal branch stars
We present two-degree field spectroscopic observations of a sample of 96A-type stars selected from the Sloan Digital Sky Survey Data Release 3(SDSS DR3). Our aim is to identify blue horizontal branch (BHB) stars inorder to measure the kinematic properties of the tidal tails of theSagittarius dwarf spheroidal galaxy. We confine our attention to the 44classifiable stars with spectra of signal-to-noise ratio>15Å-1. Classification produces a sample of 29 BHBstars at distances of 5-47 kpc from the Sun. We split our sample intothree bins based on their distance. We find 10 of the 12 stars at 14-25kpc appear to have coherent, smoothly varying radial velocities whichare plausibly associated with old debris in the Sagittarius tidalstream. Further observations along the orbit and at greater distancesare required to trace the full extent of this structure on the sky.Three of our BHB stars in the direction of the globular cluster Palomar(Pal) 5 appear to be in an overdensity but are in the foreground of Pal5. More observations are required around this overdensity to establishany relation to Pal 5 and/or the Sgr stream. We emphasize observationsof BHB stars have unlimited potential for providing accurate velocityand distance information in old distant halo streams and globularclusters alike. The next-generation multi-object spectrographs providean excellent opportunity to accurately trace the full extent of suchstructures.
| Photometry of the Globular Cluster NGC 5466: Red Giants and Blue Stragglers
We present wide-field BVI photometry for about 11,500 stars in thelow-metallicity cluster NGC 5466. We have detected the red giant branchbump for the first time, although it is at least 0.2 mag fainter thanexpected relative to the turnoff. The number of red giants (relative tomain-sequence turnoff stars) is in excellent agreement with stellarmodels from the Yonsei-Yale and Teramo groups, and slightly highcompared to Victoria-Regina models. This adds to evidence that anabnormally large ratio of red giant to main-sequence stars is notcorrelated with cluster metallicity. We discuss theoretical predictionsfrom different research groups and find that the inclusion or exclusionof helium diffusion and strong limit Coulomb interactions may be partlyresponsible. We also examine indicators of dynamical history: the massfunction exponent and the blue straggler frequency. NGC 5466 has a veryshallow mass function, consistent with large mass loss and recentlydiscovered tidal tails. The blue straggler sample is significantly morecentrally concentrated than the HB or RGB stars. We see no evidence ofan upturn in the blue straggler frequency at large distances from thecenter. Dynamical friction timescales indicate that the stragglersshould be more concentrated if the cluster's present density structurehas existed for most of its history. NGC 5466 also has an unusually lowcentral density compared to clusters of similar luminosity. In spite ofthis, the specific frequency of blue stragglers puts it right on thefrequency-cluster MV relation observed for other clusters.
| RR Lyrae variables in GGCs: distribution of periods and synthetic models. .
We present some applications of our Synthetic Horizontal Branches (SHB)simulations, aimed to reproduce the peculiar period distributions of RRLyrae belonging to the Galactic Globular Clusters M3 and M5. We showsome evidence, supporting the importance of SHBs in obtaining parameterssuch as the mass distribution inside the instability strip.
| Low mass stars
Low-mass stars dominate the population of clusters and old galaxies andserve as important tools for understanding the history of galaxies. Iwill review some recent developments and open questions concerning ourunderstanding of these comparatively simple stellar objects.
| The Blue Straggler Population of the Globular Cluster M5
By combining high-resolution HST and wide-field ground-basedobservations, in ultraviolet and optical bands, we study the bluestraggler star (BSS) population of the galactic globular cluster M5 (NGC5904) from its very central regions up to its periphery. The BSSdistribution is highly peaked in the cluster center, decreases atintermediate radii and rises again outward. Such a bimodal distributionis similar to those previously observed in other globular clusters (M3,47 Tucanae, NGC 6752). As for these clusters, dynamical simulationssuggest that, while the majority of BSSs in M5 could be originated bystellar collisions, a significant fraction (20%-40%) of BSSs generatedby mass transfer processes in primordial binaries is required toreproduce the observed radial distribution. A candidate BSS has beendetected beyond the cluster tidal radius. If confirmed, this couldrepresent an interesting case of an ``evaporating'' BSS.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 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.
| Abundances of Baade's Window Giants from Keck HIRES Spectra. II. The Alpha and Light Odd Elements
We report detailed abundances of O, Na, Mg, Al, Si, Ca, and Ti-elementsproduced by massive stars-for 27 red giants toward the Galactic bulge inBaade's window. These species are overabundant in the bulge relative tothe disk, consistent with enhancement by Type II SN ejecta. [Mg/Fe]=+0.3dex over the full range of [Fe/H], while O, Si, Ca, and Ti are elevatedbut follow more disklike trends. We propose that the decline in [O/Fe]is due to metallicity-dependent oxygen yields from massive stars,perhaps connected with the Wolf-Rayet phenomenon. The elements Si, Ca,and Ti, believed to be produced during explosive nucleosynthesis,possess identical trends with [Fe/H]. We attribute the decline of theseelements to metallicity-dependent yields in Type II SNe. The trend of[Al/Fe] is found to vary strikingly with environment; the range from theSgr dwarf to the bulge is 0.7 dex. The disjoint composition of thethick/thin disk and bulge stars is inconsistent with models in which thebulge formed from the thickening of the disk, while the elevated alphaelements are consistent with a rapid bulge formation timescale. Thestarkly smaller scatter of [/Fe] with [Fe/H] in the bulgecompared with the halo is consistent with the expectation that the bulgeshould have efficiently mixed. The metal-poor bulge [/Fe]ratios are higher than ~80% of the halo; the bulge could not have formedfrom gas with the present-day halo composition.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation.
| BVI Photometry and the Luminosity Functions of the Globular Cluster M92
We present new BVI ground-based photometry and VI space-based photometryfor the globular cluster M92 (NGC 6341) and examine luminosity functionsin B, V, and I containing over 50,000 stars ranging from the tip of thered giant branch to several magnitudes below the main-sequence turnoff.Once corrected for completeness, the observed luminosity functions agreevery well with theoretical models and do not show stellar excesses inany region of the luminosity function. Using reduced-χ2fitting, the new M92 luminosity function is shown to be an excellentmatch to the previously published luminosity function for M30. Thesepoints combine to establish that the ``subgiant excess'' found inpreviously published luminosity functions of Galactic globular clustersis due to deficiencies in the stellar models used at that time. Usingup-to-date stellar models results in good agreement between observationsand theory. Several statistical methods are presented to best determinethe age of M92. These methods prove to be insensitive to the exactchoice of metallicity within the published range. Using [Fe/H]=-2.17 tomatch recent studies we find an age of 14.2+/-1.2 Gyr for the cluster.
| 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.
| A Chemical Abundance Study of One Red Giant Star in NGC 5694: A Globular Cluster with a Dwarf Spheroidal Chemical Signature?
We report an abundance analysis of one red giant branch star in themetal-poor outer halo globular cluster NGC 5694. We obtain [Fe/H]=-1.93based on the ionized lines, and our metallicity measurement is in goodagreement with previous estimates. We find that [Ca+Ti/2Fe] and [Cu/Fe]in NGC 5694 are about 0.3-0.4 dex lower than other globular clusterswith similar metallicities, but similar to some LMC clusters and starsin some dwarf spheroidal galaxies. Differences persist, however, in theabundances of neutron-capture elements. The unique chemical abundancepattern and the large Galactocentric distance (30 kpc) and radialvelocity (-138.6 +/-1.0 km s-1) indicate that NGC 5694 had anextragalactic origin.This paper includes data gathered with the 6.5 m Magellan telescopeslocated at Las Campanas Observatory, Chile.
| N-body models of rotating globular clusters
In this paper we examine the dynamical evolution of rotating globularclusters with direct N-body models. Our initial models are rotating Kingmodels, and we obtain results both for equal-mass systems and forsystems composed of two mass components. Previous investigations using aFokker-Planck solver have shown that rotation has a noticeable influenceon stellar systems such as globular clusters that evolve by two-bodyrelaxation. In particular, it accelerates their dynamical evolutionthrough the gravogyro instability. We have validated the occurrence ofthe gravogyro instability with direct N-body models. In the case ofsystems composed of two mass components, mass segregation takes place, aprocess that competes with the rotation in the acceleration of the corecollapse. The `accelerating' effect of rotation was detected in ourisolated two-mass N-body models. Finally, we look at rotating N-bodymodels in a tidal field within the tidal approximation. It turns outthat rotation increases the escape rate significantly. A differencebetween retrograde- and prograde-rotating stellar clusters, with respectto the orbit of the cluster around the Galaxy, occurs. This differenceis the result of the presence of a `third integral' and chaoticscattering, respectively.
| AL 3 (BH 261): A New Globular Cluster in the Galaxy
AL 3 (BH 261), previously classified as a faint open cluster candidate,is shown to be a new globular cluster in the Milky Way, by means of B,V, and I color-magnitude diagrams. The main feature of AL 3 is aprominent blue extended horizontal branch. Its color-magnitude diagramsmatch those of the intermediate-metallicity cluster M5. The cluster isprojected in a rich bulge field, also contaminated by the disk mainsequence. The globular cluster is located in the Galactic bulge at adistance from the Sun dsolar=6.0+/-0.5 kpc. The reddening isE(B-V)=0.36+/-0.03, and the metallicity is estimated to be[Fe/H]~-1.3+/-0.25. AL 3 is probably one of the least massive globularclusters of the Galaxy.The observations were carried out at the European Southern Observatory,La Silla, Chile [proposal 64L-0212(A)].
| Origin of abundance inhomogeneity in globular clusters
We numerically investigate abundance properties of the Galactic globularclusters (GCs) by adopting a new `external pollution' scenario. In thisframework, GCs are assumed to originate in forming low-mass dwarfsembedded in dark matter subhaloes at very high redshifts (z) and thus bechemically influenced by field asymptotic giant branch (AGB) stars ofthe dwarfs during early GC formation processes. GCs within a dwarfgalaxy therefore can be formed from the mixture of (i) gas ejected fromthe field AGB stars formed earlier in the dwarf and (ii) theinterstellar gas infalling to the central region of the dwarf. In thisexternal pollution scenario, the ratio of the total mass of infallinggas to that of AGB ejecta during GC formation in a dwarf (s) and thetime-scale of gas infall (σI) are the most importantkey parameters that can determine abundance properties of GCs. We mainlyinvestigate the abundance inhomogeneity among light elements (e.g. C, N,O, Na and Al) of stars in GCs by using the latest stellar yield modelsof metal-poor AGB stars with and without third dredge-up. Our principalresults for the models with no third dredge-up, which are moreconsistent with observations, are as follows. (i) Both [N/Fe] and[C/Fe] can be diverse among stars within a GC owing to chemicalpollution from field AGB stars. [N/Fe] distributions in some GCs canclearly show bimodality, whereas [C/Fe] is monomodal in most models.[N/Fe] distributions depend on s such that models with smaller s (i.e.larger mass fraction of AGB ejecta used for GC formation) show the[N/Fe] bimodality more clearly. (ii) N-rich, C-poor stars in GCs alsohave higher He abundances owing to pollution from massive AGB stars withHe-rich ejecta. The number fraction of He-rich stars (Y > 0.30) ishigher for the models with smaller s and shorter σI for3 <= s <= 24 and 105 <= σI <=107 yr. He abundances of stars correlate with [N/Fe] and[Al/Fe] and anticorrelate with [C/Fe], [O/Fe] and [Na/Fe] within GCs inour models. (iii) Although our model can much better explain theobserved C-N and Mg-Al anticorrelations than previous theoreticalmodels, it is in strong disagreement with the observed O-Naanticorrelation. (iv) This model naturally provides an explanation forthe large fraction of CN-strong stars without recourse to an implausibleinitial mass function. Based on these results for the above externalpollution scenario, we discuss the long-standing problem of theCN-bimodality prevalent in the Galactic GCs, the possible heliumabundance inhomogeneity in these systems and their horizontal branchmorphologies.
| Kinematic Decoupling of Globular Clusters with the Extended Horizontal Branch
About 25% of the Milky Way globular clusters (GCs) exhibit unusuallyextended color distribution of stars in the core helium-burninghorizontal-branch (HB) phase. This phenomenon is now best understood asdue to the presence of helium-enhanced second-generation subpopulations,which has raised the possibility that these peculiar GCs might have aunique origin. Here we show that these GCs with extended HB are clearlydistinct from other normal GCs in kinematics and mass. The GCs withextended HB are more massive than normal GCs and are dominated by randommotion with no correlation between kinematics and metallicity.Surprisingly, however, when they are excluded, most normal GCs in theinner halo show clear signs of dissipational collapse that apparentlyled to the formation of the disk. Normal GCs in the outer halo sharetheir kinematic properties with the extended HB GCs, which is consistentwith the accretion origin. Our result further suggests heterogeneousorigins of GCs, and we anticipate this to be a starting point for moredetailed investigations of Milky Way formation, including early mergers,collapse, and later accretion.
| Various Modes of Helium Mixing in Globular Cluster Giants and Their Possible Effects on the Horizontal Branch Morphology
It has been known for a long time that some red giants in globularclusters exhibit large star-to-star variations in the abundances oflight elements that are not exhibited by field giants. This fact can betaken as evidence that the extra mixing mechanism(s) that operate inglobular cluster giants may be consequences of star-star interactions inthe dense stellar environment. In order to constrain the extra mixingmechanism(s), we study the influence of helium enrichment along the redgiant branch on the evolution of stars through the horizontal branch(HB). Three possible modes of helium enrichment are considered,associated with close encounters of stars in the globular clusters. Weshow that as a consequence of the variations in the core mass, as wellas in the total mass due to mass loss, the colors of horizontal branchmodels are distributed over almost the entire range of the horizontalbranch. The results are discussed in relation to a scenario for theorigin of the abundance anomalies and for the effects on the morphologyof the horizontal branch. We argue that the star-star interactions cannot only explain the source of the angular momentum of rapid rotationbut also provide a mechanism for the bimodal distribution of rotationrates in some globular clusters. We also propose the time elapsed fromthe latest core-collapse phase during the gravothermal oscillations asthe second parameter to explain the variations in HB morphology amongthe globular clusters.
| Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| 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.
| Na-O anticorrelation and horizontal branches. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218
We used the multifiber spectrograph FLAMES on the ESO Very LargeTelescope UT2 to derive atmospheric parameters, metallicities andabundances of O and Na for 79 red giant stars in the Galactic globularcluster NGC 6218 (M 12). We analyzed stars in the magnitude range fromabout 1 mag below the bump to the tip of the Red Giant Branch. Theaverage metallicity we derive is [Fe/H]=-1.31± 0.004±0.028 dex (random and systematic errors, respectively), with a verysmall star-to-star scatter (rms=0.033 dex), from moderatelyhigh-resolution Giraffe spectra. This is the first extensivespectroscopic abundance analysis in this cluster. Our results indicatethat NGC 6218 is very homogeneous as far as heavy elements areconcerned. On the other hand, light elements involved in the well knownproton-capture reactions of H-burning at high temperature, such as O andNa, show large variations, anticorrelated with each other, at allluminosities along the red giant branch. The conclusion is that the Na-Oanticorrelation must be established in early times at the clusterformation. We interpret the variation of Na found near the RGB-bump asthe effect of two distinct populations having different bumpluminosities, as predicted for different He content. To our knowledge,NGC 6218 is the first GC where such a signature has beenspectroscopically detected, when combined with consistent andhomogeneous data obtained for NGC 6752 to gain in statisticalsignificance.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables 2, 3, and 5 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/939
| 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.
| [Fe/H] relations for c-type RR Lyrae variables based upon Fourier coefficients
[Fe/H]-φ31-P relations are found for c-type RR Lyraestars in globular clusters. The relations are analogous to that found byJurcsik & Kovács for field ab-type RR Lyrae stars, where alonger period correlates with lower metallicity values for similarvalues of the Fourier coefficient φ31. The relationsobtained here are used to determine the metallicity of field c-type RRLyrae stars, those within ωCen, the Large Magellanic Cloud andtoward the galactic bulge. The results are found to compare favourablyto metallicity values obtained elsewhere.
| Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| 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.
| A Robust Test of Evolution near the Tip of the Red Giant Branch and Missing Giants in NGC 2808
We describe a new method for robustly testing theoretical predictions ofred giant evolution near the tip of the giant branch. When theoreticalcumulative luminosity functions are shifted to align the tip in the Iband and normalized at a luminosity level slightly brighter than the redgiant bump, virtually all dependence on age and composition (heavyelements and helium abundance) is eliminated. While significantcomparisons with observations require large samples of giant stars, suchsamples are available for some of the most massive Milky Way globularclusters. We present comparisons with the clusters NGC 2808 and M5 andfind that NGC 2808 has a deficiency of bright giants (with a probabilityof less than about 3% that a more extreme distribution of giant starswould have happened by chance). We discuss the possibilities thatunderestimated neutrino losses or strong mass loss could be responsiblefor the deficit of giants. While we cannot rule out the neutrinohypothesis, it cannot explain the apparent agreement between the M5observations and models. On the other hand, strong mass loss provides apotential link between the giant star observations and NGC 2808'sunusually blue horizontal branch. If the mass loss hypothesis is true,there is likely a significant population of He white dwarfs that couldbe uncovered with slightly deeper UV observations of the cluster.Based on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| Abundances in Red Giant Stars of NGC 2808 and Correlations between Chemical Anomalies and Global Parameters in Globular Clusters
We present the abundance analysis of stars from the tip of the red giantbranch (RGB) to below the RGB bump in the globular cluster NGC 2808based on high-resolution echelle spectra. We derive abundances of Al,α-process elements (Si I, Ca I, Ti I, and Ti II), and Fe-groupelements (Sc II, V I, Cr I, Cr II, Mn I, Co I, and Ni I). Apart from Mgbeing somewhat reduced, likely because it has been depleted at theexpense of Al in the MgAl cycle, the other α-element ratios showthe overabundance typical of halo stars of similar metallicity. Mn isunderabundant, whereas Fe-group elements have typical abundance ratiosnear the solar value. We detect star-to-star differences in Alabundances from the RGB tip down to the faintest star below the RGBbump, correlated with Na abundances at all luminosities. The slope ofthe Na-Al correlation is similar to the one found in M13 by Sneden etal., but it is different from those in other globular clusters ofsimilar metallicity. We find that the amount of chemical inhomogeneitiesalong the Na-O and Mg-Al anticorrelations in globular cluster red giantsis correlated with the present-day cluster mass and ellipticity.Moreover, we find for the first time a correlation between the spread inproton-capture elements and orbital parameters of clusters. The chemicalanomalies are more extended in clusters having large-sized orbits andlonger periods and those with larger inclination angles of the orbitwith respect to the Galactic plane.Based on data collected at the European Southern Observatory, Chile,during the FLAMES Science Verification program with the Ultraviolet andVisual Echelle Spectrograph at the VLT-UT2.
| UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
| 2D Fokker-Planck models of rotating clusters
Globular clusters rotate significantly, and with the increasing amountof detailed morphological and kinematical data obtained in recent yearson galactic globular clusters many interesting features show up. We showhow our theoretical evolutionary models of rotating clusters can be usedto obtain fits, which at least properly model the overall rotation andits implied kinematics in full 2D detail (dispersions, rotationvelocities). Our simplified equal mass axisymmetric rotating modelprovides detailed two-dimensional kinematical and morphological data forstar clusters. The degree of rotation is not dominant in energy, butalso non-negligible for the phase-space distribution function, shape andkinematics of clusters. Therefore, the models are well applicable forgalactic globular clusters. Since previously published papers on thatmatter by us made it difficult to do detailed comparisons withobservations, we provide a much more comprehensive and easy-to-use setof data here, which uses as entries dynamical age and flattening ofobserved cluster and then offers a limited range of applicable models infull detail. The method, data structure and some exemplary comparisonwith observations are presented. Future work will improve modelling anddata base to take a central black hole, a mass spectrum and stellarevolution into account.
| Discovery of a Faint Old Stellar System at 150 kpc
We report the detection of a faint old stellar system at (α,δ)=(194.29d, 34.32d) (SDSS J1257+3419), based on the spatialdistribution of bright red giant branch stars in the Sloan Digital SkySurvey Data Release 4. SDSS J1257+3419 has a half-light radius of38+/-12 pc and an absolute integrated V magnitude ofMV=-4.8+1.4-1.0 mag at a heliocentricdistance of 150+/-15 kpc. A comparison between SDSS J1257+3419 and knownGalactic halo objects suggests that SDSS J1257+3419 is either (1) afaint and small dwarf galaxy or (2) a faint and widely extended globularcluster. In the former case, SDSS J1257+3419 could represent an entityof a postulated subhalo of the Milky Way. Further photometric anddynamical study of this stellar system is vital to distinguish thesepossibilities.
| Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy
The Carina dwarf spheroidal galaxy is the only galaxy of this type thatshows clearly episodic star formation separated by long pauses. Here wepresent metallicities for 437 radial velocity members of this Galacticsatellite. The metallicities and radial velocities were measured as partof a Large Programme with the Very Large Telescope at the EuropeanSouthern Observatory, Chile. We obtained medium-resolution spectroscopywith the multiobject spectrograph FLAMES. Our target red giants coverthe entire projected surface area of Carina. Our spectra are centered atthe near-infrared Ca II triplet, which is a well-established metallicityindicator for old and intermediate-age red giants. The resulting datasample provides the largest collection of spectroscopically derivedmetallicities for a Local Group dwarf spheroidal galaxy to date. Four ofour likely radial velocity members of Carina lie outside this galaxy'snominal tidal radius, supporting earlier claims of the possibleexistence of such stars beyond the main body of Carina. We find a meanmetallicity of [Fe/H]~-1.7 dex on the 1997 metallicity scale of Carrettaand Gratton for Carina. The formal FWHM of the metallicity distributionfunction is 0.92 dex, while the full range of metallicities is found tospan approximately -3.0 dex<[Fe/H]<0.0 dex. The metallicitydistribution function might be indicative of several subpopulationsdistinct in metallicity. There appears to be a mild radial gradient suchthat more metal-rich populations are more centrally concentrated,matching a similar trend for an increasing fraction of intermediate-agestars (see the 2001 work of Harbeck and coworkers). This, as well as thephotometric colors of the more metal-rich red giants, suggests thatCarina exhibits an age-metallicity relation. Indeed, the age-metallicitydegeneracy seems to conspire to form a narrow red giant branch despitethe considerable spread in metallicity and wide range of ages. Themetallicity distribution function is not well matched by a simpleclosed-box model of chemical evolution. Qualitatively better matches areobtained by chemical models that also take into account infall andoutflows. A G dwarf problem remains for all these models.Based on observations collected at the European Southern Observatory atParanal, Chile; proposal 171.B-0520(A).
| The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario
We investigate the suggestion that there are stellar populations in someglobular clusters with enhanced helium (Y~0.28-0.40) compared to theprimordial value. We assume that a previous generation of massiveasymptotic giant branch (AGB) stars have polluted the cluster. Twoindependent sets of AGB yields are used to follow the evolution ofhelium and CNO using a Salpeter initial mass function (IMF) and twotop-heavy IMFs. In no case are we able to produce the postulated largeY~0.35 without violating the observational constraint that the CNOcontent is nearly constant.
| 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.
| Broad-band photometric colors and effective temperature calibrations for late-type giants. II. Z < 0.02
We investigate the effects of metallicity on the broad-band photometriccolors of late-type giants, and make a comparison of synthetic colorswith observed photometric properties of late-type giants over a widerange of effective temperatures (T_eff=3500-4800 K) and gravities (logg=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on thesynthetic photometric colors is small at effective temperatures above 3800 K, but the effects grow larger at lower T_eff, due to the changingefficiency of molecule formation which reduces molecular opacities atlower [M/H]. To make a detailed comparison of the synthetic and observedphotometric colors of late type giants in the T_eff-color andcolor-color planes (which is done at two metallicities, [M/H]=-1.0 and-2.0), we derive a set of new T_eff-log g-color relations based onsynthetic photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. Theserelations are based on the T_eff-log g scales that we derive employingliterature data for 178 late-type giants in 10 Galactic globularclusters (with metallicities of the individual stars between [M/H]=-0.7and -2.5), and synthetic colors produced with the PHOENIX, MARCS andATLAS stellar atmosphere codes. Combined with the T_eff-log g-colorrelations at [M/H]=0.0 (Kučinskas et al. 2005), the set of newrelations covers metallicities [M/H]=0.0dots-2.0 (Δ[M/H]=0.5),effective temperatures T_eff=3500dots4800 K (Δ T_eff=100 K), andgravities log g=-0.5dots3.0. The new T_eff-log g-color relations are ingood agreement with published T_eff-color relations based on observedproperties of late-type giants, both at [M/H]=-1.0 and -2.0. Thedifferences in all T_eff-color planes are typically well within 100 K.We find, however, that effective temperatures predicted by the scalesbased on synthetic colors tend to be slightly higher than thoseresulting from the T_eff-color relations based on observations, with theoffsets up to 100 K. This is clearly seen both at [M/H]=-1.0 and -2.0,especially in the T_eff-(B-V) and T_eff-(V-K) planes. The consistencybetween T_eff-log g-color scales based on synthetic colors calculatedwith different stellar atmosphere codes is very good, with typicaldifferences being well within Δ T_eff 70 K at [M/H]=-1.0 andΔ T_eff 40 K at [M/H]=-2.0.
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| 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.
| Empirical color transformations between SDSS photometry and other photometric systems
Aims.We present empirical color transformations between the SloanDigital Sky Survey (SDSS) ugriz photometry and the Johnson-Cousins UBVRIsystem and Becker's RGU system, respectively. Owing to the magnitude ofdata that is becoming available in the SDSS photometric system it isparticularly important to be able to convert between this new system andtraditional photometric systems. Unlike earlier publishedtransformations we based our calculations on stars actually measured bythe SDSS with the SDSS 2.5-m telescope. The photometric database of theSDSS provides in a sense a single-epoch set of "tertiary standards"covering more than one quarter of the sky. Our transformations shouldfacilitate their use to easily and reliably derive the correspondingapproximate Johnson-Cousins or RGU magnitudes. Methods: .The SDSSsurvey covers a number of areas that were previously established asstandard fields in the Johnson-Cousins system, in particular, fieldsestablished by Landolt and by Stetson. We used these overlapping fieldsto create well-photometered star samples on which our calculatedtransformations are based. For the RGU photometry we used fieldsobserved in the framework of the new Basel high-latitude field starsurvey. Results: .We calculated empirical color transformationsbetween SDSS photometry and Johnson-Cousins UBVRI and Becker's RGUsystem. For all transformations we found linear relations to besufficient. Furthermore we showed that the transformations between theJohnson-Cousins and the SDSS system have a slight dependence onmetallicity.
| The RR Lyrae period-K-luminosity relation for globular clusters: an observational approach
The period-metallicity-K-band luminosity (PLK) relation forRR Lyrae stars in 15 Galactic globular clusters and in the LargeMagellanic Cloud (LMC) globular cluster Reticulum has been derived. Itis based on accurate near-infrared (K) photometry combined withTwo-Micron All-Sky Survey (2MASS) and other literature data. ThePLK relation has been calibrated and compared with theprevious empirical and theoretical determinations in literature. Thezero point of the absolute calibration has been obtained from the Kmagnitude of RR Lyr whose distance modulus has been measured viatrigonometric parallax with Hubble Space Telescope (HST). Using thisrelation, we obtain a distance modulus to the LMC of (m - M)0= 18.54 +/- 0.15 mag, in good agreement with recent determinations basedon the analysis of Cepheid variable stars.Based on observations collected at the European Southern Observatorywithin the observing programs 49.5-0021, 51.5-0024, 59.E-0340,64.N-0038, 68.D-0287 and at the Telescopio Nazionale Galileo.E-mail: antonio.sollima@bo.astro.it (AS)
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| Multicolour CCD photometry of the variable stars in globular cluster M3
We present time-series data on the variable stars of the galacticglobular cluster Messier 3 (M3). We give BVIC light curvesfor 226 RR Lyrae, 2 SX Phe and 1 W Vir type variables, along withestimated fundamental photometric parameters such as intensity andmagnitude-averaged brightness and pulsation periods. In some cases, theperiods we have found significantly differ from the previously publishedones. This is the first published light curve and period determinationfor variable V266. The I-band light curve has not been observedpreviously for numerous (76) variables. Three new RR Lyrae variableshave been discovered. Groups of RR Lyrae variables that belong todifferent evolutionary stages and have been separated previously on thebasis of V data were found here for all colours and colour indices bycluster analysis. The I-band period-luminosity relation is alsodiscussed. From the 66 modulated (Blazhko type) RR Lyrae stars weinvestigated, six are newly identified and two of them are firstovertone pulsators. In the case of 13 RR Lyrae, the period of Blazhkocycle has been estimated for the first time. V252 is identified as a newRRd variable. Amplitude ratios of RRd stars have been investigated tosearch possible mode content changes. In contrast to previouspublications no changes have been found. Problems with the sampling ofthe time-series of typical cluster variability surveys are demonstrated.
| Na-O anticorrelation and HB. I. The Na-O anticorrelation in NGC 2808
We derived the atmospheric parameters and elemental abundances of Fe, O,and Na for about 120 red giant stars in the Galactic globular clusterNGC 2808. Our results are based on the analysis of medium-highresolution (R=22 000{-}24 000) GIRAFFE spectra acquired with the FLAMESspectrograph at VLT-UT2 as a part of a project aimed at studying theNa-O anticorrelation as a function of physical parameters in globularclusters. We present the anticorrelation of Na and O abundances in NGC2808 here, and discuss the distribution function of stars along thisrelation. Besides a bulk of O-normal stars with the typical compositionof field halo stars, NGC 2808 seems to host two other groups of O-poorand super O-poor stars. In this regard, NGC 2808 is similar to M 13, thetemplate cluster for the Na-O anticorrelation. However, in contrast to M13, most stars in NGC 2808 are O-rich. This might be related to thehorizontal branch morphologies that are very different in these twoclusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10(rms = 0.065 dex, from 123 stars). We also found some evidence of asmall intrinsic spread in metallicity, but more definitive conclusionsare hampered by the presence of a small differential reddening.
| Formation and evolution of compact binaries in globular clusters - I. Binaries with white dwarfs
In this paper, the first of a series, we study the stellar dynamical andevolutionary processes leading to the formation of compact binariescontaining white dwarfs (WDs) in dense globular clusters (GCs). Weexamine the processes leading to the creation of X-ray binaries such ascataclysmic variables (CVs) and AM CVn systems. Using numericalsimulations, we identify the dominant formation channels and we predictthe expected numbers and characteristics of detectable systems,emphasizing how the cluster sources differ from the field population. Weexplore the dependence of formation rates on cluster properties and weexplain in particular why the distribution of CVs has only a weakdependence on cluster density. We also discuss the frequency of dwarfnova outbursts in GCs and their connection with moderately strong WDmagnetic fields. We examine the rates of Type Ia supernovae (SNe Ia) viaboth single and double degenerate channels in clusters and we argue thatthose rates may contribute to the total SN Ia rate in ellipticalgalaxies. Considering coalescing WD binaries, we discuss possibleconstraints on the common envelope evolution of their progenitors and wederive theoretical expectations for gravitational wave detection byLaser Interferometer Space Antenna (LISA).
| SOAR BVI photometry of the metal-poor bulge globular cluster NGC 6642
We present BVI photometry of the globular cluster NGC 6642 using the SOIimager at the SOAR Telescope. The colour magnitude diagrams (CMD) reach≈1.5 mag in V below the main sequence turn-off. A comparison of theoverall sequences, and in particular the Red Giant Branch slope of NGC6642 with that of M 5, indicates that the two clusters must have asimilar metallicity of [Fe/H] ≈ -1.3. We also obtained a reddeningE(B-V)=0.42±0.03 for NGC 6642, and a distance from the Sun ofdȯ=7.2±0.5 kpc. Therefore NGC 6642 is amoderately metal-poor globular cluster that is spatially located in thebulge at a galactocentric distance of R_GC≈ 1.7 kpc. The comparisonof CMDs of NGC 6642 with those of M 5 shows that there is a very goodmatch of magnitude difference between turn-off and horizontal branch,suggesting comparable ages. The age of M 5 is typical of halo globulars,so NGC 6642 is coeval with the halo. It is a good candidate as one ofthe few genuine metal-poor and old bulge clusters, and might be one ofthe oldest fossils in the Galaxy.
| CCD photometry of the globular cluster M2: RR Lyrae physical parameters and new variables
We report the results of CCD V and R photometry of the RR Lyrae stars inM2. The periodicities of most variables are revised and new ephemeridesare calculated. Light-curve decomposition of the RR Lyrae stars wascarried out and the corresponding mean physical parameters [Fe/H] =-1.47, Teff = 6276K, logL = 1.63Lsolar andMV = 0.71 from nine RRab and [Fe/H] = -1.61, M =0.54Msolar, Teff = 7215K, logL =1.74Lsolar and MV = 0.71 from two RRc stars werecalculated. A comparison of the radii obtained from the above luminosityand temperature with predicted radii from non-linear convective modelsis discussed. The estimated mean distance to the cluster is 10.49 +/-0.15kpc. These results place M2 correctly in the general globularcluster sequences for Oosterhoff type, mass, luminosity and temperature,all as a function of the metallicity. Mean relationships for M,logL/Lsolar, Teff and MV as a functionof [Fe/H] for a family of globular clusters are offered. These trendsare consistent with evolutionary and structural notions on thehorizontal branch. Eight new variables are reported.
| VLT-UVES analysis of two giants in the bulge metal-poor globular cluster HP-1. Analysis of two giants in HP-1
Context: .Metal-poor globular clusters in the bulge are importanttracers of early chemical evolution. HP-1 is among the six metal-poorclusters within 5° of the Galactic center, and could be the oneclosest to the centerAims. The main purpose of this study is thedetermination of metallicity and elemental ratios. Methods.Highresolution spectra of two giants of the bulge globular cluster HP-1 wereobtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph.This is the second metal-poor globular cluster in the bulge for which adetailed abundance analysis is presented. MultibandV,I,J,H,Ks photometry was used to derive effectivetemperatures.Results. The present analysis provides a metallicity [Fe/H]= -1.00±0.2. The α-elements oxygen and silicon show [α/Fe] ≈ +0.3, whereas magnesium, calcium and titanium showsolar ratios. A proper motion analysis indicates that the two stars arecluster members. Conclusions.The metallicity is unexpected for a blueHorizontal Branch (BHB) cluster. HP-1 is the first known cluster withsuch a high metallicity combined with a BHB and a steep Red Giant Branch(RGB). Together with NGC 6388 and NGC 6441 of [Fe/H]-0.6 it wouldbe third with such characteristics, but it differs from them, sincethese two other clusters have also a populous Red HB, and a normal slopeof the RGB for their metallicity, which is not the case of HP-1.
| Does the oxygen-sodium anticorrelation in globular clusters require a lowering of the 23Na(p,α)20Ne reaction rate?
Context: .The chemical content of the ejecta of Asymptotic Giant Branchstars (AGBs) is important to understand whether intermediate mass starsplayed a role in the "self-enrichment scenario", to explain the chemicalanomalies observed among Globular Clusters stars. One difficulty of thismodel is that it is not fully consistent with the observed oxygen-sodiumanticorrelation. Aims: .We look for the combinations of theinput-physics parameters by which the yields of massive AGBs aresodium-rich and oxygen-poor. Methods: .Many evolutions from theearly evolutionary phases until the mass of the envelope drops below~0.5~Mȯ are calculated for a 5~MȯZ=0.001 model for various assumptions concerning the extra mixing fromthe bottom of the envelope and the relevant cross sections involvingsodium. Results: .A modest amount of extra mixing from the bottomof the surface convective zone leads to ejecta that are moderatelysodium rich and oxygen depleted. A different but appealing modelizationallows us to reproduce the observations of stars with a lower oxygencontent: extra mixing is not included, but the cross sections of thereaction 23Na(p, α)20Ne must be a factor of~4 lower than the recommended values. If the initial neon content of thestar is not solar-scaled but enhanced as the other α elements by afactor ~2, the afore mentioned cross section must be lessened by only afactor ~2.
| CN Abundance Inhomogeneities in the Globular Cluster Messier 4 (NGC 6121): Results Based on Merged Data Sets from the Literature
In this paper, we investigate the behavior of the λ3883 CNabsorption bands in the spectra of red giants in the globular clusterMessier 4. Data from three literature sources, including the originalsurvey of Norris, have been combined to produce a picture of the CN bandstrength over an absolute magnitude range of-2.5~0)suggests the possibility of a pre-RGB component to the abundanceinhomogeneities. Accretion of material from a central reservoir ofstellar ejecta, or mass transfer within binary star systems, arediscussed as possible origins for the enrichment of the CN-strong stars.In the case of M4, these processes must have occurred at sufficientlyearly times that dynamical relaxation of the cluster has largely erasedany initial gradient in the relative space densities of CN-strong andCN-weak stars, at least within the volume of the current stellar samplethat extends to three half-mass radii from the cluster center.
| Spatial and velocity clumping in a Sloan Digital Sky Survey blue horizontal branch star catalogue
We present evidence for eight new clumps of blue horizontal branch starsdiscovered in a catalogue of these stars compiled from the Sloan DigitalSky Survey by Sirko et al. and published in 2004. Clumps are identifiedby selecting pairs of stars separated by distances <=2 kpc and withdifferences in galactocentric radial velocities <25 kms-1. Each clump contains four or more stars. Four of theclumps have supporting evidence: two of them also contain overdensitiesof RR Lyrae stars which makes their reality very likely. At least one ofthe clumps is likely to be associated with the tidal debris of theSagittarius dwarf spheroidal galaxy. We emphasize that more accurateobservations of the radial velocities or proper motions of the stars inthe clumps, as well as the identification of other halo stars in theseregions, are required to establish the reality of the remaining clumps.
| 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.
| The mass of helium in white dwarf stars and the formation and evolution of hydrogen-deficient post-AGB stars
We use two sets of `cradle-to-grave' evolutionary calculations toinvestigate how the mass of the helium buffer layer between the CO coreand the hydrogen-burning shell in thermally pulsing asymptotic giantbranch (AGB) stars depends on the initial stellar mass and heavy elementabundance. Cool star mass loss is included by augmenting the Reimers'formula with fits to mass-loss rates observed for Galactic Miravariables, obscured Large Magellanic Cloud (LMC) AGB variables andGalactic OH/IR sources. Resulting white dwarf masses are in goodagreement with the semi-empirical final mass-initial mass relation. Wederive lower and upper limits on the mass of helium in white dwarfs asfunctions of their mass and initial heavy element abundance. We findthat stars that experience a very late thermal pulse (VLTP) have finalhelium masses that are about 25 per cent below the lower limit for starsthat do not experience a VLTP. We have derived a modified form of Iben'scriterion for a star to experience a post-AGB helium shell flash, andsubcriteria for discriminating between the occurrence of late and VLTPs.We find that a post-AGB flash does not necessarily result in formationof a non-DA white dwarf. Furthermore, we find that the relative amountof time spent burning helium or hydrogen depends on how the mass-lossrate varies with stellar surface parameters. These two considerationscomplicate the relationship between the probability that a starexperiences a post-AGB flash and the relative formation rates of DA andnon-DA white dwarfs. However, our calculations do lead to a non-DA/DAratio that is consistent with the observed ratio.We find that our predicted abundances for PG1159 stars agree within theerror bars with the observed abundances, without the need of convectiveovershoot. We also find that nitrogen is produced during VLTPs but notin the other evolutionary paths to hydrogen-deficient objects. Hence, wepropose this as the reason why nitrogen is observed in some PG1159 starsbut not all. Our VLTP models also produce surface abundances close tothose of the low-gravity hybrid PG1159 stars.An inconsistency between asteroseismological and our evolutionary modeldeterminations of helium layer mass remains.
| Distant field blue horizontal branch stars - III. Identification of a probable outer halo stream at Galactocentric distance r= 70 kpc
We present FOcal Reducer/low dispersion Spectrograph-1 spectra (from theEuropean Southern Observatory's Very Large Telescope) of a sample of 34faint 20.0 < g* < 21.1 A-type stars selected from the SloanDigital Sky Survey Early Data Release, with the goal of measuring thevelocity dispersion of blue horizontal branch (BHB) stars in the remoteGalactic halo, R~ 80kpc. We show that colour selection with 1.08 15 Å-1. Classification produces a sample ofeight BHB stars at distances 65-102kpc from the Sun (mean 80kpc), whichrepresents the most distant sample of Galactic stars with measuredradial velocities. The dispersion of the measured radial component ofthe velocity with respect to the centre of the Galaxy is 58 +/-15kms-1. This value is anomalously low in comparison withmeasured values for stars at smaller distances, as well as forsatellites at similar distances. Seeking an explanation for the lowmeasured velocity dispersion, further analysis reveals that six of theeight remote BHB stars are plausibly associated with a single orbit.Three previously known outer halo carbon stars also appear to belong tothis stream. The velocity dispersion of all nine stars relative to theorbit is only 15 +/- 4kms-1. Further observations along theorbit are required to trace the full extent of this structure on thesky.
| The Blue Straggler Population of the Globular Cluster M5: Comparison with M3
We have surveyed the blue straggler star population of the Galacticglobular cluster M5 using high-resolution images of the core along withwide-field ground-based images reaching to more than 19 core radii. Togauge M5's relative efficiency of producing stragglers, we compared oursample to five studies of other globular clusters. Using a ``bright''sample selected in the same way as that of Ferraro and coworkers, wefound a bimodal radial distribution similar to those found in threeother luminous clusters. When the radial distributions for differentclusters are scaled using the core radius, there is goodcluster-to-cluster agreement in the size of the core straggler sampleand the center of the ``zone of avoidance.'' However, M5 has thesmallest fraction of stragglers in the zone of avoidance of any of theclusters measured to date, and its zone of avoidance appears to be wider(in r/rc) than that of M3, which has a very similar surfacebrightness profile. Both of these facts indicate that M5's stragglerpopulation has dynamically evolved to a larger extent than M3's. Usingan ultraviolet sample from Hubble Space Telescope selected in the sameway as that of Ferraro and coworkers, we find that the frequency of bluestragglers in M5 is lower than in all but two of the clusters examined.We also identified seven bright blue stragglers that were previouslymisidentified as HB stars by Sandquist & Bolte. These brightstragglers are most likely the result of stellar collisions involvingbinary stars.
| 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.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
INHABITED SKY
