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Ýlgili Makaleler
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.
| 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.
| The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models
The ACS Survey of Galactic Globular Clusters, an HST Treasury Project,will deliver high-quality, homogeneous photometry of 65 globularclusters. This paper introduces a new collection of stellar evolutiontracks and isochrones suitable for analyzing the ACS survey data.Stellar evolution models were computed at [Fe/H]=-2.5, -2.0, -1.5, -1.0,-0.5, and 0; [α/Fe]=-0.2, 0, 0.2, 0.4, 0.6, and 0.8; and threeinitial He abundances for masses from 0.1 to 1.8 Msolar andages from 2 to 15 Gyr. Each isochrone spans a wide range in luminosity,from MV~14 up to the tip of the red giant branch. These arecomplemented by a set of He-burning tracks that extend from the zero-agehorizontal branch to the onset of thermal pulsations on the asymptoticgiant branch. In addition, a set of computer programs are provided thatmake it possible to interpolate the isochrones in [Fe/H], generateluminosity functions from the isochrones, and create synthetichorizontal-branch models. The tracks and isochrones have been convertedto the observational plane with two different color-Tefftransformations, one synthetic and one semiempirical, in ground-based B,V, and I, and F606W and F814W for both ACS WFC and WFPC2 systems. Allmodels and programs presented in this paper are available at theDartmouth Stellar Evolution Database and the Multimission Archive at theSpace Telescope Science Institute.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555, under program GO-10775 (PI: A.Sarajedini).
| Photometric observations of Supernovae 2000E, 2001B, 2001V, and 2001X
CCD BVRI photometry is presented for two type Ia supernovae 2000E and2001V, for SN Ib 2001B and SN II-P 2001X. The parameters of light curvesand absolute magnitudes at maximum light are estimated. It is shown thatall four supernovae are typical for their classes considering the shapeof their light curves and maximum luminosity.
| SN 2003du: 480 days in the life of a normal type Ia supernova
Aims.We present a study of the optical and near-infrared (NIR)properties of the Type Ia Supernova (SN Ia) 2003du. Methods: Anextensive set of optical and NIR photometry and low-resolution long-slitspectra was obtained using a number of facilities. The observationsstarted 13 days before B-band maximum light and continued for 480 dayswith exceptionally good time sampling. The optical photometry wascalibrated through the S-correction technique. Results: The {UBVRIJHK}light curves and the color indices of SN 2003duclosely resemble those of normal SNe Ia. SN 2003dureached a B-band maximum of 13.49 ± 0.02 mag on JD2 452 766.38± 0.5. We derive a B-band stretch parameter of 0.988±0.003, which corresponds to Δ m15=1.02±0.05, indicative of a SN Ia of standard luminosity. Thereddening in the host galaxy was estimated by three methods, and wasconsistently found to be negligible. Using an updated calibration of theV and {JHK} absolute magnitudes of SNe Ia, we find a distance modulusμ=32.79±0.15 mag to the host galaxy, UGC9391. We measure a peak {uvoir} bolometric luminosity of1.35(±0.20)×1043 erg s-1 and Arnett'srule implies that M56Ni≃0.68 ±0.14Mȯ of 56Ni was synthesized during theexplosion. Modeling of the {uvoir} bolometric light curve also indicatesM56Ni in the range 0.6-0.8 Mȯ. Thespectral evolution of SN 2003du at both optical andNIR wavelengths also closely resembles normal SNe Ia. In particular, theSi II ratio at maximum R(Si II) = 0.22 ±0.02 and the timeevolution of the blueshift velocities of the absorption line minima aretypical. The pre-maximum spectra of SN 2003du showedconspicuous high-velocity features in the Ca II H&K doublet andinfrared triplet, and possibly in Si II λ6355, lines. We comparethe time evolution of the profiles of these lines with otherwell-observed SNe Ia and we suggest that the peculiar pre-maximumevolution of Si II λ6355 line in many SNe Ia is due to thepresence of two blended absorption components.Table 3 and Appendix are only available in electronic form athttp://www.aanda.org
| 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.
| Gas Accretion by Globular Clusters and Nucleated Dwarf Galaxies and the Formation of the Arches and Quintuplet Clusters
We consider here the collective accretion of gas by globular clustersand dwarf galaxies moving through the interstellar medium. In the limitof high velocity and/or sound speed of the ISM, the collective potentialof the cluster is insufficient to accrete significant amounts of gas,and stars within the systems accrete gas individually. We show, however,that when the sound speed or the relative velocity of the ambient mediumis less than the central velocity dispersion of the cluster, it isaccreted into the collective potential of the cluster prior to beingaccreted onto the individual stars within the cluster. The collectiverate is strongly enhanced relative to the individual rates. This effectmay potentially modify the white dwarf cooling sequence in globularclusters with low-inclination and low-eccentricity Galactic orbits andlead to the rejuvenation of some marginally surviving cores of globularclusters and nucleated dwarf galaxies near the Galactic center. Sucheffects will only occur rarely, but may explain the existence ofclusters of young, massive stars near the Galactic center.
| A probable stellar solution to the cosmological lithium discrepancy
The measurement of the cosmic microwave background has stronglyconstrained the cosmological parameters of the Universe. When themeasured density of baryons (ordinary matter) is combined with standardBig Bang nucleosynthesis calculations, the amounts of hydrogen, heliumand lithium produced shortly after the Big Bang can be predicted withunprecedented precision. The predicted primordial lithium abundance is afactor of two to three higher than the value measured in the atmospheresof old stars. With estimated errors of 10 to 25%, this cosmologicallithium discrepancy seriously challenges our understanding of stellarphysics, Big Bang nucleosynthesis or both. Certain modifications tonucleosynthesis have been proposed, but found experimentally not to beviable. Diffusion theory, however, predicts atmospheric abundances ofstars to vary with time, which offers a possible explanation of thediscrepancy. Here we report spectroscopic observations of stars in themetal-poor globular cluster NGC6397 that reveal trends of atmosphericabundance with evolutionary stage for various elements. Theseelement-specific trends are reproduced by stellar-evolution models withdiffusion and turbulent mixing. We thus conclude that diffusion ispredominantly responsible for the low apparent stellar lithium abundancein the atmospheres of old stars by transporting the lithium deep intothe star.
| 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.
| Primordial pollution of globular clusters within their host dwarfs embedded in dark matter haloes at high redshifts
Recent observational studies have revealed star-to-star abundanceinhomogeneity among light elements (e.g. C, N, O, Na and Al) of stars onthe main sequence in the Galactic globular clusters (GCs). One of thepromising interpretations for this result is that the observed abundanceinhomogeneity is due to the second generation of stars formed fromejecta of the first generation of evolved stars (e.g. asymptotic giantbranch stars) within GCs. However, it remains unclear whether and howthis primordial pollution can occur within GCs. We here propose a newscenario in which primordial pollution of GCs is highly likely to occurif GCs are located in the central regions of high-redshift dark mattersubhaloes that can host low-mass dwarf galaxies. In this scenario, gasejected from the first generation of stars of GCs can be effectivelytrapped in the deep gravitational potential of their host haloes, andconsequently can be consumed for the formation of the second generationof stars without losing a significant amount of gas by ram pressurestripping of interstellar and intergalactic medium. During merging ofthese haloes with the proto-Galaxy, the haloes are completely destroyedowing to the strong tidal field of the Galaxy. The self-polluted GCslocated initially in the central regions of the haloes can survive tidaldestruction owing to their compactness and finally become the Galactichalo GCs. In this scenario, ejecta of field stars surrounding thecentral GCs can be also converted into stars within their host dwarfsand finally become the second generation of stars of GCs. We alsodiscuss the origin of the difference in the degree of abundanceinhomogeneity between different GCs, such as ω Centauri and NGC6752, in terms of the difference in physical properties between hosthaloes from which GCs originate.
| Different Characteristics of the Bright Branches of the Globular Clusters M15 and M92
We carried out relatively wide-field BVI CCD photometric observations ofthe globular clusters M15 (NGC 7078) and M92 (NGC 6341) using the 1.8 mtelescope of the Bohyun Optical Astronomy Observatory. We presentcolor-magnitude diagrams (V vs. B-V, V vs. V-I, and V vs. B-I) of M15and M92. We found asymptotic giant branch (AGB) bumps atVbumpAGB=15.20+/-0.05 mag andVbumpAGB=14.50+/-0.05 mag for M15 and M92,respectively. We identified the red giant branch (RGB) bumps of the twoclusters. We have estimated the population ratios R and R2for M15 and M92 in two cases: when only normal horizontal-branch (HB)stars are used and when all the HB stars are used. We have compared theobserved RGB luminosity functions of M15 and M92 with the theoreticalRGB luminosity functions of Bergbusch & VandenBerg and found nosignificant ``extra stars'' in the comparisons. This implies that the HBmorphology difference between M15 and M92 is not certain due to theresults of deep mixing in the RGB sequence.
| Effects of external tidal field on the evolution of the outer regions of multi-mass star clusters
We present N-body simulations (including an initial mass function) ofglobular clusters in the Galaxy in order to study effects of the tidalfield systematically on the properties of the outer parts of globularclusters. Using NBODY6, which correctly takes into account the two-bodyrelaxation, we investigate the development of tidal tails of globularclusters in the Galactic tidal field. For simplicity, we have employedonly the spherical components (bulge and halo) of the Galaxy, andignored the effects of stellar evolution which could have been importantin the very early phase of the cluster evolution. The total number ofstars in our simulations is about 20000, which is much smaller than therealistic number of stars. All simulations had been done for severalorbital periods in order to understand the development of the tidaltails. In our scaled-down models, the relaxation time is sufficientlyshort to show the mass segregation effect, but we did not go far enoughto see the core collapse, and the fraction of stars lost from thecluster at the end of the simulations is only ~10 per cent. The radialdistribution of extra-tidal stars can be described by a power law with aslope around -3 in surface density. The directions of tidal tails aredetermined by the orbits and locations of the clusters. We find that thelength of tidal tails increases towards the apogalacticon and decreasestowards the perigalacticon. This is an anti-correlation with thestrength of the tidal field, caused by the fact that the time-scale forthe stars to respond to the potential is similar to the orbitaltime-scale of the cluster. The escape of stars in the tidal tailstowards the pericentre could be another reason for the decrease of thelength of tidal tails. We find that the rotational angular velocity oftidally induced clusters shows quite different behaviour from that ofinitially rotating clusters.
| Tidal Tails around Globular Clusters: Are They a Good Tracer of Cluster Orbits?
We present the results of detailed N-body simulations of clusters movingin a realistic Milky Way (MW) potential. The strong interaction with thebulge and the disk of the Galaxy leads to the formation of tidal tails,emanating from opposite sides of the cluster. Some characteristicfeatures in the morphology and orientation of these streams arerecognized and interpreted. The tails have a complex morphology,particularly when the cluster approaches its apogalacticon, showingmultiple ``arms'' in remarkable similarity to the structures observedaround NGC 288 and Willman 1. Actually, the tails are generally goodtracers of the cluster path quite far from the cluster center (>7-8tidal radii), while on the smaller scale they are mainly pointing in thedirection of the Galaxy center. In particular, the orientation of theinner part of the tails is highly correlated with the cluster orbitalphase and the local orbital angular acceleration. This implies that, ingeneral, the orbital path cannot be estimated directly from theorientation of the tails, unless a sufficient large field around thecluster is available.
| Variable stars in the globular cluster NGC 2419 .
We have used DOLORES at the TNG to obtain B,V time series photometry ofNGC 2419, one of the most distant and bright clusters in the Galactichalo. These data will be used to study its variable star population inorder to check whether the cluster could be the relic of anextragalactic system accreted by the Milky Way. Using the ImageSubtraction technique we have identified about 300 candidate variables,many of which are in the cluster central regions. Several of thevariables appear to be RR Lyrae stars, but we detected variability alsoaround the tip of the red giant branch, and in other regions of thecolour-magnitude diagram. To improve the light curve sampling and toresolve variables in the cluster inner regions, the TNG data werecombined with HST archive data. Preliminary results are presented on thelight curves from the combined data set.
| The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters without Previous Hubble Space Telescope Photometry
We present the first results of a large Advanced Camera for Surveys(ACS) survey of Galactic globular clusters. This Hubble Space Telescope(HST) Treasury project is designed to obtain photometry with S/N(signal-to-noise ratio) >~10 for main-sequence stars with masses>~0.2 Msolar in a sample of globulars using the ACS WideField Channel. Here we focus on clusters without previous HST imagingdata. These include NGC 5466, NGC 6779, NGC 5053, NGC 6144, Palomar 2,E3, Lyngå 7, Palomar 1, and NGC 6366. Our color-magnitude diagrams(CMDs) extend reliably from the horizontal branch to as much as 7 magfainter than the main-sequence turnoff and represent the deepest CMDspublished to date for these clusters. Using fiducial sequences for threestandard clusters (M92, NGC 6752, and 47 Tuc) with well-knownmetallicities and distances, we perform main-sequence fitting on thetarget clusters in order to obtain estimates of their distances andreddenings. These comparisons, along with fitting the cluster mainsequences to theoretical isochrones, yield ages for the target clusters.We find that the majority of the clusters have ages that are consistentwith the standard clusters at their metallicities. The exceptions areE3, which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could beas much as 8 Gyr younger than 47 Tuc.Based on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS5-26555, under program GO-10775 (PI:A. Sarajedini).
| 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.
| Is Ursa Major II the progenitor of the Orphan Stream?
Prominent in the `Field of Streams' - the Sloan Digital Sky Survey mapof substructure in the Galactic halo - is an `Orphan Stream' withoutobvious progenitor. In this numerical study, we show a possibleconnection between the newly found dwarf satellite Ursa Major II (UMaII) and the Orphan Stream. We provide numerical simulations of thedisruption of UMa II that match the observational data on the position,distance and morphology of the Orphan Stream. We predict the radialvelocity of UMa II as -100kms-1, as well as the existence ofstrong velocity gradients along the Orphan Stream. The velocitydispersion of UMa II is expected to be high, though this can be causedboth by a high dark matter content or by the presence of unbound starsin a disrupted remnant. However, the existence of a gradient in the meanradial velocity across UMa II provides a clear-cut distinction betweenthese possibilities. The simulations support the idea that some of theanomalous, young halo globular clusters like Palomar 1 or Arp 2 orRuprecht 106 may be physically associated with the Orphan Stream.
| 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.
| 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.
| An Orphan in the ``Field of Streams''
We use Sloan Digital Sky Survey Data Release 5 photometry andspectroscopy to study a tidal stream that extends over ~50° in thenorth Galactic cap. From the analysis of the path of the stream and thecolors and magnitudes of its stars, the stream is~20+7-5 kpc away at its nearest detection (thecelestial equator). We detect a distance gradient: the stream is fartheraway from us at higher declination. The contents of the stream are madeup from a predominantly old and metal-poor population that is similar tothe globular clusters M13 and M92. The integrated absolute magnitude ofthe stream stars is estimated to be Mr~-7.5. There istentative evidence for a velocity signature, with the stream moving at~-40 km s-1 at low declinations and ~+100 km s-1at high declinations. The stream lies on the same great circle asComplex A, a roughly linear association of H I high-velocity cloudsstretching over ~30° on the sky, and as Ursa Major II, a recentlydiscovered dwarf spheroidal galaxy. Lying close to the same great circleare a number of anomalous, young, and metal-poor globular clusters,including Palomar 1 and Ruprecht 106.
| A Large Stellar Evolution Database for Population Synthesis Studies. II. Stellar Models and Isochrones for an α-enhanced Metal Distribution
We present a large, new set of stellar evolution models and isochronesfor an α-enhanced metal distribution typical of Galactic halo andbulge stars; it represents a homogeneous extension of our stellar modellibrary for a scaled-solar metal distribution already presented byPietrinferni et al. The effect of the α-element enhancement hasbeen properly taken into account in the nuclear network, opacity,equation of state, and for the first time in the bolometric correctionsand color transformations. This allows us to avoid the inconsistentuse-common to all α-enhanced model libraries currentlyavailable-of scaled-solar bolometric corrections and colortransformations for α-enhanced models and isochrones. We show howbolometric corrections to magnitudes obtained for the U, B portion ofstellar spectra for Teff<=6500 K are significantlyaffected by the metal mixture, especially at the higher metallicities.Our models cover both an extended mass range (between 0.5 and 10Msolar, with a fine mass spacing) and a broad metallicityrange, including 11 values of the metal mass fraction Z, correspondingto the range -2.6<=[Fe/H]<=0.05. The initial He mass fraction isY=0.245 for the most metal-poor models, and it increases with Z,according to ΔY/ΔZ=1.4. Models with and without theinclusion of overshoot from the convective cores during the centralH-burning phase are provided, as well as models with different mass lossefficiencies. We also provide complete sets of evolutionary models forlow-mass, He-burning stellar structures cover the whole metallicityrange. This database, used in combination with our scaled-solar modellibrary, is a valuable tool for investigating both Galactic andextragalactic simple and composite stellar populations, using stellarpopulation synthesis techniques.
| The Hercules-Aquila Cloud
We present evidence for a substantial overdensity of stars in thedirection of the constellations of Hercules and Aquila. The cloud iscentered at a Galactic longitude of l~40deg and extends aboveand below the Galactic plane by at least 50°. Given itsoff-centeredness and height, it is unlikely that the Hercules-Aquilacloud is related to the bulge or thick disk. More likely, this is a newstructural component of the Galaxy that passes through the disk. Thecloud stretches ~80° in longitude. Its heliocentric distance liesbetween 10 and 20 kpc so that the extent of the cloud in projection is~20 kpc by ~15 kpc. It has an absolute magnitude of Mv=-13,and its stellar population appears to be comparable to, but somewhatmore metal-rich than, M92.
| 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 Stellar Populations in the Outer Regions of M33. II. Deep ACS Imaging
Studying the stellar populations in the outskirts of spiral galaxies canprovide important constraints on their structure, formation, andevolution. To that end, we present VI photometry obtained with theAdvanced Camera for Surveys for three fields located ~20'-30' inprojected distance southeast of M33's nucleus (corresponding toapproximately four to six visual scale lengths or ~9-13 kpc indeprojected radius). The color-magnitude diagrams reveal a mixed stellarpopulation whose youngest constituents have ages no greater than ~100Myr and whose oldest members have ages of at least several gigayears.The presence of stars as massive as 3-5 Msolar is consistentwith global star formation thresholds in disk galaxies but could arguefor a threshold in M33 that is on the low end of observational andtheoretical expectations. The metallicity gradient as inferred bycomparing the observed red giant branch (RGB) to the Galactic globularclusters is consistent with M33's inner disk gradient traced by severalother studies. The surface density of the RGB stars drops offexponentially with a radial scale length of4.7'+/-0.1'. The scale length increases with agein a manner similar to the vertical scale height of several nearbylate-type spirals. Based on the metallicity gradient, density gradient,and mixed nature of the stellar populations, we conclude that thesefields are dominated by a disk population, although we cannot rule outthe presence of a small halo component.Based on observations made 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 NAS5-26555. These observations are associated with program9479.
| New Metallicities of RR Lyrae Stars in ω Centauri: Evidence for a Non-He-enhanced Metal-intermediate Population
We present new spectroscopic metal abundances for 74 RR Lyrae (RRL)stars in ω Cen obtained with FLAMES. The well-known metallicityspread is visible among the RRL variables. The metal-intermediate (MI)RRL stars ([Fe/H]~-1.2) are fainter than the bulk of the dominantmetal-poor population ([Fe/H]~-1.7), in good agreement with thecorresponding zero-age horizontal-branch models with cosmological heliumabundance Y=0.246. This result conflicts with the hypothesis that theprogenitors of the MI RRL stars correspond to the anomalous bluemain-sequence stars, which share a similar metallicity but whoseproperties are currently explained by assuming for them a large heliumenhancement. Therefore, in this scenario, the coexistence within thecluster of two different populations with similar metallicities([Fe/H]~-1.2) and different helium abundances has to be considered.Based on FLAMES/GIRAFFE observations collected with the Very LargeTelescope at the European Southern Observatory, Cerro Paranal, Chile,within the observing programs 74.B-0170(A).
| The Global Kinematics of the Globular Cluster M92
We report the determination of high-accuracy radial velocities for 306members of the globular cluster M92 using the Hydra multiobjectspectrograph on the WIYN telescope. We have concentrated on starsoutside of the central region of the cluster, located up to 14.4' fromthe cluster center. Candidate members were selected for spectroscopybased on a photometric metallicity index determined from three-bandWashington photometry, also obtained with the WIYN telescope. The medianerror in the velocities is 0.35 km s-1. We find theheliocentric radial velocity of the cluster to be -121.2+/-0.3 kms-1. We have used an improved Bayesian analysis to determinethe velocity dispersion profile of M92. The most probable profile is acored power law with a scale radius of 2', a velocity dispersion at 1'of 6.3 km s-1, and an outer power law with a slope of -0.6.We have also reanalyzed the M15 radial velocities of Drukier et al. andfind that a pure power law with a 1' velocity dispersion of 8 kms-1 and a slope of -0.5 and the combination of a power lawwith a slope of -0.4 and a scale of 7.5 km s-1 inside 9' anda dispersion of 4 km s-1 outside are equally likely. In bothclusters there is evidence that the samples include escaping stars. Wepresent results from a GRAPE-based N-body simulation of an isolatedcluster that demonstrates this effect. We suggest additional tests todetermine the relative importance of tidal heating and stellar ejectionfor establishing the velocity field in globular cluster halos.
| Na-O anticorrelation and horizontal branches. VI. The chemical composition of the peculiar bulge globular cluster NGC 6388
We present the LTE abundance analysis of high resolution spectra for redgiant stars in the peculiar bulge globular cluster NGC 6388. Spectra ofseven members were taken using the UVES spectrograph at the ESO VLT2 andthe multiobject FLAMES facility. We exclude any intrinsic metallicityspread in this cluster: on average, [Fe/H]=-0.44±0.01±0.03dex on the scale of the present series of papers, where the first errorbar refers to individual star-to-star errors and the second issystematic, relative to the cluster. Elements involved in H-burning athigh temperatures show large spreads, exceeding the estimated errors inthe analysis. In particular, the pairs Na and O, Al and Mg areanticorrelated and Na and Al are correlated among the giants in NGC6388, the typical pattern observed in all galactic globular clustersstudied so far. Stars in NGC 6388 shows an excess of α-processelements, similar to the one found in the twin bulge cluster NGC 6441.Mn is found underabundant in NGC 6388, in agreement with the averageabundance ratio shown by clusters of any metallicity. Abundances ofneutron-capture elements are homogeneously distributed within NGC 6388;the [Eu/Fe] ratio stands above the value found in field stars of similarmetallicity.Based on observations collected at ESO telescopes under programme073.D-0211. Full Table 3 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/967
| 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.
| Why Haven't Loose Globular Clusters Collapsed Yet?
We report on the discovery of a surprising observed correlation betweenthe slope of the low-mass stellar global mass function (GMF) of globularclusters (GCs) and their central concentration parameterc=log(rt/rc), i.e., the logarithmic ratio of tidaland core radii. This result is based on the analysis of a sample of 20Galactic GCs with solid GMF measurements from deep HST or VLT data. Allthe high-concentration clusters in the sample have a steep GMF, mostlikely reflecting their initial mass function. Conversely,low-concentration clusters tend to have a flatter GMF, implying thatthey have lost many stars via evaporation or tidal stripping. No GCs arefound with a flat GMF and high central concentration. This findingappears counterintuitive, since the same two-body relaxation mechanismthat causes stars to evaporate and the cluster to eventually dissolveshould also lead to higher central density and possibly core collapse.Therefore, more concentrated clusters should have lost proportionatelymore stars and have a shallower GMF than low-concentration clusters,contrary to what is observed. It is possible that severely depleted GCshave also undergone core collapse and have already recovered a normalradial density profile. It is, however, more likely that GCs with a flatGMF have a much denser and smaller core than that suggested by theirsurface brightness profile and may well be undergoing collapse atpresent. In either case, we may have so far seriously underestimated thenumber of post-core collapse clusters, and many may be lurking in theMilky Way.
| 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.
| Galactic Interstellar Gas Cloud Mass Functions: A Simple Quantitative Approach
We present here a simple approach to understanding the gas cloud massdistribution function by simulating formation and destruction of gasclouds and gas clumps in the ISM. We include as relevant processescoagulation to form bigger clouds, as well as disruption by collisionsand the removal of gas by collapse to form stars. We evolve initial setsof preexisting gas clumps with a range of initial distribution functions(flat, Gaussian, fractal) for their physical parameters and withdifferent geometrical forms (spherical or elongated) for the individualclouds, and constrain them within an imaginary box representinggravitational bounding, applying the kinematic laws of nonelasticcollisions. The results agree well with observations of the massdistribution function of Galactic giant gas clouds if we choose aGaussian for the initial distribution function, and initial gas cloudswhich are quasi-spherical.
| Atmospheres, Chemical Compositions, and Evolutionary Histories of Very Metal-Poor Red Horizontal-Branch Stars in the Galactic Field and in NGC 7078 (M15)
We have conducted spectrum analyses of 24 field metal-poor([Fe/H]<-2) red horizontal-branch (RHB) stars identified in the HKobjective-prism survey and 6 such stars in the globular cluster M15,based on high-quality spectra (R~40,000, S/N~100) obtained with theMagellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 mtelescope at Las Campanas Observatory. The atmospheric parameters of theRHB stars provide interesting bridges between turnoff stars of similartemperature and red giant branch (RGB) stars of similar gravity, andthey permit investigations of abundance trends [X/Fe] versus [Fe/H] in arelatively unexplored region of the temperature-gravity plane. We findthat the Teff, logg, vt, and [Fe/H] valuesdetermined from our spectra are consistent with expectations fromliterature spectroscopic studies of other evolved metal-poor stellarclasses. We show that the RHB stars have abundance distributions thatare consistent with typical halo stars of similar metallicities. Thephotometric and spectroscopic gravities of the M15 stars differ byamounts that grow with declining temperature. We use a regressionderived from these differences to calculate photometric gravities forthe field RHB stars. Then we use the locations of the field RHB starsamong the evolutionary tracks of Cassisi et al. in the logg versuslogTeff plane to estimate their masses and lifetimes as RHBstars. We use these lifetimes to estimate the size of the metal-poor HBpopulation from which they arise. Then, using counts of HB and RGB starsin metal-poor globular clusters, we conclude that the number ofmetal-poor RGB stars at high latitudes (|b|>30deg)brighter than V=15 exceeds those identified in extant objective-prismsurveys by more than an order of magnitude. Finally, we deduce theeffective temperature of the fundamental red edge of the metal-poor RRLyrae instability strip, logTeff(FRE)=3.80+/-0.01, from theinterface between the temperature distributions of metal-poor field RHBstars and the RR Lyrae stars of similar [Fe/H] in five metal-poorglobular clusters.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. This paper includes data gatheredwith the 6.5 m Magellan telescopes located at Las Campanas Observatory,Chile.
| ACS Photometry of Newly Discovered Globular Clusters in the Outer Halo of M31
We report the first results from deep ACS imaging of 10 classicalglobular clusters in the far outer regions (15kpc<~Rp<~100 kpc) of M31. Eight of the clusters,including two of the most remote M31 globular clusters presently known,are described for the first time. Our F606W, F814W color-magnitudediagrams extend ~3 mag below the horizontal branch and clearlydemonstrate that the majority of these objects are old (>~10 Gyr),metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional fourhave -1.9 <~ [Fe/H] <~ -1.5. The remaining object is moremetal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit thesecond-parameter effect. Using aperture photometry, we estimateintegrated luminosities and structural parameters for all clusters.Many, including all four clusters with projected radii greater than 45kpc, are compact and very luminous, with -8.9 <~ MV <~-8.3. These four outermost clusters are thus quite unlike their MilkyWay counterparts, which are typically diffuse, subluminous (-6.0 <~MV <~ -4.7), and more metal-rich (-1.8 <~ [Fe/H] <~-1.3).Based on observations made 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 NAS5-26555. These observations are associated with program10394.
| Integrated colours of Milky Way globular clusters and horizontal branch morphology
Broadband colours are often used as metallicity proxies in the study ofextragalactic globular clusters. A common concern is the effect ofvariations in horizontal branch (HB) morphology - the second-parametereffect - on such colours. We have used U BV I, Washington, and DDOphotometry for a compilation of over 80 Milky Way globular clusters toaddress this question. Our method is to fit linear relations betweencolour and [Fe/H], and study the correlations between the residualsabout these fits and two quantitative measures of HB morphology. Whilethere is a significant HB effect seen in U-B, for the commonly usedcolours B-V, V-I, and C-T_1, the deviations from the baselinecolour-[Fe/H] relations are less strongly related to HB morphology.There may be weak signatures in B-V and C-T_1, but these are at thelimit of observational uncertainties. The results may favour the use ofB-I in studies of extragalactic globular clusters, especially when itshigh [Fe/H]-sensitivity is considered.
| 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.
| The Dynamical State and Blue Straggler Population of the Globular Cluster NGC 6266 (M62)
We have used a proper combination of multiband high-resolution HubbleSpace Telescope WFPC2 and wide-field ground-based observations to imagethe Galactic globular cluster NGC 6266 (M62). The extensive photometricdata set allows us to determine the center of gravity and to constructthe most extended radial profile ever published for this clusterincluding, for the first time, detailed star counts in the very innerregion. The star density profile is well reproduced by a standard Kingmodel with an extended core (~19") and a modest value of theconcentration parameter (c=1.5), indicating that the cluster has not yetexperienced core collapse. The millisecond pulsar population (whosemembers are all in binary systems) and the X-ray-emitting population(more than 50 sources within the cluster half-mass radius) suggest thatNGC 6266 is in a dynamical phase particularly active in generatingbinaries through dynamical encounters. UV observations of the centralregion have been used to probe the population of blue straggler stars,whose origin might be also affected by dynamical interactions. Thecomparison with other globular clusters observed with a similar strategyshows that the blue straggler content in NGC 6266 is relatively low,suggesting that the formation channel that produces binary systemshosting neutron stars or white dwarfs is not effective in significantlyincreasing the blue straggler population. Moreover, an anticorrelationbetween millisecond pulsar content and blue straggler specific frequencyin globular clusters seems to be emerging with increasing evidence.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 (AURA), Inc.,under NASA contract NAS5-26555. Also based on Wide Field Imagerobservations collected at the European Southern Observatory (ESO), LaSilla, Chile, within the observing programs 62.L-0354 and 64.L-0439.
| 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.
| Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions
We present five new satellites of the Milky Way discovered in SloanDigital Sky Survey (SDSS) imaging data, four of which were followed upwith either the Subaru or the Isaac Newton Telescopes. They include fourprobable new dwarf galaxies-one each in the constellations of ComaBerenices, Canes Venatici, Leo, and Hercules-together with one unusuallyextended globular cluster, Segue 1. We provide distances, absolutemagnitudes, half-light radii, and color-magnitude diagrams for all fivesatellites. The morphological features of the color-magnitude diagramsare generally well described by the ridge line of the old, metal-poorglobular cluster M92. In the past two years, a total of 10 new Milky Waysatellites with effective surface brightness μv>~28 magarcsec-2 have been discovered in SDSS data. They are lessluminous, more irregular, and apparently more metal-poor than thepreviously known nine Milky Way dwarf spheroidals. The relationshipbetween these objects and other populations is discussed. We note thatthere is a paucity of objects with half-light radii between ~40 and ~100pc. We conjecture that this may represent the division between starclusters and dwarf galaxies.Based in part on data collected at Subaru Telescope, which is operatedby the National Astronomical Observatory of Japan.
| On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity
The purpose of this paper is to predict the temperature at thefundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL)variables from the pulsation equation that relates temperature toperiod, luminosity, and mass. Modern data for the correlations betweenperiod, luminosity, and metallicity at the FBE for field and cluster RRLstars are used for the temperature calculation. The predictedtemperatures are changed to B-V colors using an adopted colortransformation. The predicted temperatures at the FBE become hotter as[Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicitydecreases to -2.5. The temperature range over this interval ofmetallicity is ΔlogTe=0.04, or 640 K at 6900 K. Thepredicted color variation is at the level of 0.03 mag in B-V over mostof this range. The predictions are compared with the observed RRL colorsat the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)0, which, however,is the uncertainty of the reddening corrections. The focus of theproblem is then reversed by fitting a better envelope to the observedFBE relation between color and metallicity for metallicities smallerthan -1.8, which, when inserted in the pulsation equation, gives anonlinear calibration of the absolute magnitude of the average evolvedlevel of the horizontal branch (HB) ofMV=1.109+0.600([Fe/H])+0.140([Fe/H])2, where thezero point has been set by the observed RR Lyrae stars in the LMC at0=19.064 by Clementini et al. for [Fe/H]=-1.5, andusing an adopted LMC modulus of (m-M)0=18.54 from Tammann etal. that is independent of the LMC Cepheids. This equation givesMV=0.52 at [Fe/H]=-1.5.However, the calibration fails for the extreme second-parameter clustersNGC 6388 and NGC 6441. Proof that the MV absolute magnitudesfor their RR Lyrae variables are ~0.4 mag brighter than the calibrationequation predicts is from the unusually long periods for givenamplitudes at their high metallicities of [Fe/H]~-0.5. Allsecond-parameter clusters are believed to violate the equation, but lessseverely than these two extreme examples. An additional complication inusing RRL stars as distance indicators at the 0.1 mag level is shown bythe difference of ΔlogP=0.029+/-0.007 in the position of theenvelope locus at the shortest periods for the observedperiod-metallicity correlation between the field and cluster variables.The field variables have shorter periods than cluster variables at theenvelope. This requires the cluster RRL stars to be 0.09 mag brighterthan the field variables at the same temperature and mass, or to have atemperature difference of ΔlogTe=0.008 at fixedluminosity and mass. The field and cluster variables also differ in thenear absence of cluster RR Lyrae stars in the -1.7>[Fe/H]>-2.0metallicity interval, whereas the field variables show no such gap. Atest is proposed for different origins for the field and the clustervariables by comparing the morphology of the HBs in the local dwarfspheroidal galaxies with that in the Galactic globular clusters in theinner halo and by relating the differences with the relevantsecond-parameter indicators.
| 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.
| The Discovery of Cepheids and a Distance to NGC 5128
We discuss a new distance to NGC 5128 (Centaurus A) based on Cepheidvariables observed with the Hubble Space Telescope. Twelve F555W (V) andsix F814W (I) epochs of cosmic-ray-split WFPC2 observations wereobtained. A total of 56 bona fide Cepheids were discovered, with periodsranging from 5 to ~50 days; five of these are likely Population IICepheids of the W Virginis class, associated with the bulge or halo ofNGC 5128. Based on the period and V- and I-band luminosities of asubsample of 42 classical (Population I) Cepheids, and adopting an LMCdistance modulus and extinction of 18.50+/-0.10 mag and E(B-V)=0.10 mag,respectively, the true reddening-corrected distance modulus to NGC 5128is μ0=27.67+/-0.12 (random)+/-0.16 (systematic) mag,corresponding to a distance of 3.42+/-0.18 (random)+/-0.25 (systematic)Mpc. The random uncertainty in the distance is dominated by the error onthe assumed value for the ratio of total to selective absorption,RV, in NGC 5128, and by the possible metallicity dependenceof the Cepheid period-luminosity relation at V and I. This representsthe first determination of a Cepheid distance to an early-type 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.
| 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.
| 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.
| Monte Carlo simulations of metal-poor star clusters
Context: Metal-poor globular clusters (GCs) can provide a probe of theearliest epoch of star formation in the Universe, being the oldestobservable stellar systems. In addition, young and intermediate-agelow-metallicity GCs are present in external galaxies. Nevertheless,inferring their evolutionary status by using integrated properties maysuffer from large intrinsic uncertainty caused by the discrete nature ofstars in stellar systems, especially in the case of faint objects. Aims: In this paper, we evaluate the intrinsic uncertainty (due tostatistical effects) affecting the integrated colours and mass-to-lightratios as a function of the cluster's integrated visual magnitude(M_V^tot), which represents a directly measured quantity. We investigatethe case of metal-poor, single-burst stellar populations with age from afew million years to a likely upper value for the Galactic globularcluster ages (~15 Gyr). Methods: Our approach is based on Monte Carlotechniques for randomly generating stars distributed according to thecluster's mass function. Results: Integrated colours and mass-to-lightratios in different photometric bands are checked for good agreementwith the observational values of low-metallicity Galactic clusters; theeffect of different assumptions on the horizontal branch (HB) morphologyis shown to be irrelevant, at least for the photometric bands exploredhere. We present integrated colours and mass-to-light ratios as afunction of age for different assumptions on the cluster total Vmagnitude. We find that the intrinsic uncertainty cannot be neglected.In particular, in models with M_V^tot=-4 the broad-band colours show anintrinsic uncertainty high enough to prevent the precise age of thecluster from being evaluated. The effects of different assumptions onthe initial mass function and on the minimum mass for which carbonburning is ignited for both integrated colours and mass-to-light ratiosare also analysed. Finally, the present predictions are compared withrecent results available in the literature, showing non-negligibledifferences in some cases.Full Tables 2, 3 and 6 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/107 All the figuresare available as coloured figures in the electronic edition of theJournal.
| 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.
| Rotational mixing in low-mass stars. II. Self-consistent models of Pop II RGB stars
Aims.In this paper we study the effects of rotation in low-mass,low-metallicity RGB stars. Methods: .We present the firstevolutionary models taking into account self-consistently the latestprescriptions for the transport of angular momentum by meridionalcirculation and shear turbulence in stellar interiors as well as theassociated mixing processes for chemicals computed from the ZAMS to theupper RGB. We discuss the uncertainties associated with the physicaldescription of the rotational mixing in detail and carefully study theireffects on the rotation profile, diffusion coefficients, structuralevolution, lifetimes, and chemical signatures at the stellar surface. Wefocus in particular on the various assumptions concerning the rotationlaw in the convective envelope, the initial rotation velocitydistribution, the presence of μ-gradients, and the treatment of thehorizontal and vertical turbulence. Results: .This explorationleads to two main conclusions. (1) After completion of the firstdredge-up, the degree of differential rotation (and hence mixing) ismaximised in the case of a differentially rotating convective envelope(i.e., jCE(r) = const.), as anticipated in previous studies.(2) Even with this assumption, and contrary to some previous claims, thepresent treatment for the evolution of the rotation profile andassociated meridional circulation and shear turbulence does not lead toenough mixing of chemicals to explain the abundance anomalies inlow-metallicity field and globular cluster RGB stars observed around thebump luminosity. Conclusions: .This study raises questions thatneed to be addressed in the near future. These include, for example, theinteraction between rotation and convection and the trigger ofadditional hydrodynamical instabilities.
| ACS Photometry of Extended, Luminous Globular Clusters in the Outskirts of M31
A new population of extended, luminous globular clusters has recentlybeen discovered in the outskirts of M31. These objects have luminositiestypical of classical globular clusters, but much larger half-lightradii. We report the first results from deep ACS imaging of four suchclusters, one of which is a newly discovered example lying at aprojected distance of ~60 kpc from M31. Our F606W, F814W color-magnitudediagrams extend ~3 mag below the horizontal branch level, and clearlydemonstrate, for the first time, that all four clusters are composed of>~10 Gyr old, metal-poor stellar populations. No evidence formultiple populations is observed. From a comparison with Galacticglobular cluster fiducials we estimate metallicities in the range-2.2<~[Fe/H]<~-1.8. The observed horizontal branch morphologiesshow a clear second parameter effect between the clusters. Preliminaryradial luminosity profiles suggest integrated magnitudes in the range-7.7<~MV<~-6.6, near the median value of the globularcluster luminosity function. Our results confirm that these four objectsare bona fide old, metal-poor globular clusters, albeit with combinedstructures and luminosities unlike those observed for any other globularclusters in the Local Group or beyond.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith program 10394.
| 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.
| 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.
| The Hamburg/ESO R-process enhanced star survey (HERES). III. HE 0338-3945 and the formation of the r + s stars
We have derived abundances of 33 elements and upper limits for 6additional elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE0338-3945 from high-quality VLT-UVES spectra. The star is heavilyenriched, by about a factor of 100 relative to iron and the Sun, in theheavy s-elements (Ba, La, ...). It is also heavily enriched in Eu, whichis generally considered an r-element, and in other similar elements. Itis less enriched, by about a factor of 10, in the lighter s-elements(Sr, Y and Zr). C is also strongly enhanced and, to a somewhat lesserdegree, N and O. These abundance estimates are subject to severeuncertainties due to NLTE and thermal inhomogeneities which are nottaken into detailed consideration. However, an interesting result, whichis most probably robust in spite of these uncertainties, emerges: theabundances derived for this star are very similar to those of otherstars with an overall enhancement of all elements beyond the iron peak.We have defined criteria for this class of stars, r+s stars, and discussnine different scenarios to explain their origin. None of theseexplanations is found to be entirely convincing. The most plausiblehypotheses involve a binary system in which the primary component goesthrough its giant branch and asymptotic giant branch phases and producesCNO and s-elements which are dumped onto the observed star. Whether ther-element Eu is produced by supernovae before the star was formed(perhaps triggering the formation of a low-mass binary), by a companionas it explodes as a supernova (possibly triggered by mass transfer), orwhether it is possibly produced in a high-neutron-density version of thes-process is still unclear. Several suggestions are made on how toclarify this situation.
| 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.
| Observations and analysis of two type IIP supernovae: the intrinsically faint object SN 2005cs and the ambiguous object SN 2005ay
Aims. To derive observational properties and physical parameters of theprogenitor stars of type IIP supernovae SN 2005ay and SN 2005cs fromtheir U,B,V,R,I CCD photometry, and to define their velocitybehaviour. Methods: .Light curves are analysed, and the velocitiesand spectral characteristics of SN 2005cs are obtained using syntheticspectra modeling. Results: .Both supernovae are found to befainter than the average SN IIP, with SN 2005cs being more subluminousand showing slight brightening in the second half of plateau stage inthe V,R,I bands and a low expansion velocity. The effects of twodifferent plausible distance moduli on the derived physical parametersof SN 2005ay are considered. Two approaches are used to recover theamounts of the ejected 56Ni, indicating masses of the orderof ~0.02 M_ȯ, although late luminosities might indicate a higheramount for SN 2005ay, especially for the large distance case.Constraints on the progenitor properties are also presented, based onempirical analytical models. Two approaches are used to estimate theexpansion velocities at the middle of the plateau phase. SN 2005csrepresents an example of where all 3 physical parameters, velocity,energy and 56Ni mass are lower than average, a correlationnot always observed in SNe IIP. SN 2005ay may belong to the same classif the shorter distance possibility is adopted. Furthermore, theestimated mass range for SN 2005cs is in agreement with limitsestablished by using pre-supernova imaging.
| 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.
| An evolutionary disc model of the edge-on galaxy NGC 5907
Context: .We present an evolutionary disc model of the edge-on galaxyNGC 5907 based on a continuous star formation history and a continuousdynamical heating of the stellar subpopulations. Aims: .This modelexplains the disparate two observational facts: 1) the exponentialvertical disc structure in the optical and NIR of the non-obscured partof the stellar disc and 2) the FIR/submm luminosity enhanced by about afactor of four near the obscured mid-plane, which requires additionaldust and also stellar light to heat the dust component. Methods:.We use multi-band photometry in U, B, V, R, and I-band combined withradiative transfer through a dust component to simultaneously fit thevertical surface-brightness and colour index profiles in all bandsadopting a reasonable star formation history and dynamical heatingfunction. The vertical distribution of the stellar subpopulations arecalculated self-consistently in dynamical equilibrium and the intrinsicstellar emissivity is calculated by stellar population synthesis.Results: .The final disc model reproduces the surface-brightnessprofiles in all bands with a moderately declining star formation rateand a slowly starting heating function for young stars. The total dustmass is 5.7× 107 {M}ȯ as required fromthe FIR/submm measurements. Without a recent star burst we find in themidplane an excess of 5.2-, 4.0-, and 3.0-times more stellar light inthe U-, B-, and V-band, respectively. The corresponding stellarmass-to-light ratios are 0.91 in V- and 1.0 in R-band. The centralface-on optical depth in V-band is τ_V^f=0.81 and the radial scalelength of the dust is 40% larger than that of the stellar disc.Conclusions: .Evolutionary disc models are a powerful method tounderstand the vertical structure of edge-on galaxies. Insights into thestar formation history and the dynamical evolution of stellar discs canbe gained. FIR/submm observations are necessary to restrict theparameter space for the models.
| 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.
| 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)
| CCD Photometry of M92
We present Johnson B and V photometry for the galactic globular clusterM92 (NGC 6341). Photometric results for a total of 30,000 starsare obtained and are plotted on a V versus (B-V) diagram. We fittheoretical isochrones to this diagram in order to get an estimate forthe age of M92. The age which we find is 16×109years with the following values for the metallicity and He-abundance:[Fe/H] = -2.03, Y = 0.235. The distance modulus to this cluster turnsout to be m - M = 14.6 in accordance with that obtained by Stetson &Harris (1988). We also perform stellar counts in order to produce aluminosity function which is successfully fitted by the same theoreticalmodels fitted to the colour-magnitude diagram.
| The changing place of red giant stars in the evolutionary process
Not Available
| 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.
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