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The Michigan/MIKE Fiber System Survey of Stellar Radial Velocities in Dwarf Spheroidal Galaxies: Acquisition and Reduction of Data
We introduce a stellar velocity survey of dwarf spheroidal galaxies,undertaken using the Michigan/MIKE Fiber System (MMFS) at theMagellan/Clay 6.5 m telescope at Las Campanas Observatory. As of 2006November we have used MMFS to collect 6415 high-resolution(R=20,000-25,000) spectra from 5180 stars in four dwarf spheroidalgalaxies: Carina, Fornax, Sculptor, and Sextans. Spectra sample therange 5140-5180 Å, which includes the prominent magnesium tripletabsorption feature. We measure radial velocity (RV) to a medianprecision of 2.0 km s-1 for stars as faint as V~20.5. Fromthe spectra we also are able to measure the strength of iron andmagnesium absorption features using spectral indices that correlate witheffective temperature, surface gravity, and chemical abundance.Measurement of line strength allows us to identify interlopingforeground stars independently of velocity and to examine themetallicity distribution among dSph members. Here we present detaileddescriptions of MMFS, our target selection and spectroscopicobservations, the data reduction procedure, and error analysis. Wecompare our RV results to previously published measurements forindividual stars. In some cases we find evidence for a mild,velocity-dependent offset between the RVs that we measure using themagnesium triplet and previously published RV measurements derived fromthe infrared calcium triplet. In companion papers we will present thecomplete data sets and kinematic analyses of these new observations.This paper includes data obtained with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| Galactic Orbits of Globular Clusters in a Barred Galaxy
We study the effect of a bar in the galactic orbits of forty-fiveglobular clusters whose absolute proper motions are known. The orbitalcharacteristics of the orbits are compared with those obtained for thecase of an axisymmetric galactic potential. Tidal radii are computed anddiscussed for both cases.
| The Core Binary Fractions of Star Clusters from Realistic Simulations
We investigate the evolution of binary fractions in star clusters usingN-body models of up to 100,000 stars. Primordial binary frequencies inthese models range from 5% to 50%. Simulations are performed with theNBODY4 code and include a full mass spectrum of stars, stellarevolution, binary evolution, and the tidal field of the Galaxy. We findthat the overall binary fraction of a cluster almost always remainsclose to the primordial value, except at late times when a cluster isnear dissolution. A critical exception occurs in the central regions,where we observe a marked increase in binary fraction with time-asimulation starting with 100,000 stars and 5% binaries reached a corebinary frequency as high as 40% at the end of the core-collapse phase(occurring at 16 Gyr with ~20,000 stars remaining). Binaries aredestroyed in the core by a variety of processes as a cluster evolves,but the combination of mass segregation and creation of new binaries inexchange interactions produces the observed increase in relative number.We also find that binaries are cycled into and out of cluster cores in amanner that is analogous to convection in stars. For models of 100,000stars we show that the evolution of the core radius up to the end of theinitial phase of core collapse is not affected by the exact value of theprimordial binary frequency (for frequencies of 10% or less). We discussthe ramifications of our results for the likely primordial binarycontent of globular clusters.
| 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.
| X-Ray Emission from the Planet Pulsar B1257+12
We report the detection of the millisecond pulsar B1257+12 with theChandra X-ray Observatory. In a 20 ks exposure we detected 25 photonsfrom the pulsar, with energies between 0.4 and 2.0 keV, corresponding tothe flux FX=(4.4+/-0.9)×10-15 ergss-1 cm-2 in this energy range. The X-ray spectrumcan be described by a power-law model with photon index Γ~2.8 andluminosity LX~2.5×1029 ergs s-1in the 0.3-8 keV band, for a plausible distance of 500 pc and hydrogencolumn density NH=3×1020 cm-2.Alternatively, the spectrum can be fitted by a blackbody model withkT~0.22 keV and projected emitting area ~2000 m2. If thethermal X-rays are emitted from two symmetric polar caps, the bolometricluminosity of the two caps is 2Lbol~3×1029ergs s-1. We compared our results with the data on other 30millisecond pulsars observed in X-rays and found that the apparent X-rayefficiency of PSR B1257+12, LX/E˙~3×10-5for d=500 pc, is lower than those of most millisecond pulsars. Thismight be explained by an unfavorable orientation of the X-ray pulsarbeam if the radiation is magnetospheric, or by strong asymmetry of polarcaps if the radiation is thermal (e.g., one of the polar caps is muchbrighter than the other and remains invisible for most part of thepulsar period). Alternatively, it could be attributed to absorption ofX-rays in circumpulsar matter, such as a flaring debris disk left overafter formation of the planetary system around the pulsar.
| Scorpius the Winter-Bug.
Not Available
| A new search for planet transits in NGC 6791
Context: Searching for planets in open clusters allows us to study theeffects of dynamical environment on planet formation and evolution. Aims: Considering the strong dependence of planet frequency on stellarmetallicity, we studied the metal rich old open cluster NGC6791 and searched for close-in planets using the transittechnique. Methods: A ten-night observational campaign was performedusing the Canada-France-Hawaii Telescope (3.6 m), the San PedroMártir telescope (2.1 m), and the Loiano telescope (1.5 m). Toincrease the transit detection probability we also made use of theBruntt et al. (2003, A&A, 410, 323) eight-nights observationalcampaign. Adequate photometric precision for the detection of planetarytransits was achieved. Results: Should the frequency and properties ofclose-in planets in NGC 6791 be similar to thoseorbiting field stars of similar metallicity, then detailed simulationsforesee the presence of 2-3 transiting planets. Instead, we do notconfirm the transit candidates proposed by Bruntt et al. (2003, A&A,410, 323). The probability that the null detection is simply due tochance coincidence is estimated to be 3%-10%, depending on themetallicity assumed for the cluster. Conclusions: Possible explanationsof the null-detection of transits include: (i) a lower frequency ofclose-in planets in star clusters; (ii) a smaller planetary radius forplanets orbiting super metal rich stars; or (iii) limitations in thebasic assumptions. More extensive photometry with 3-4 m class telescopesis required to allow conclusive inferences about the frequency ofplanets in NGC 6791.Based on observation obtained at theCanada-France-Hawaii Telescope (CFHT) which isoperated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the CentreNational de la Recherche Scientifique of France, and theUniversity of Hawaii and on observations obtained at San PedroMártir 2.1 m telescope (Mexico),and Loiano 1.5 m telescope (Italy).
| NGC 6121-V46: a Low-Mass Double Degenerate Ellipsoidal Variable in a Globular Cluster
The variable sdB known as V46 in the globular cluster M 4 has remainedenigmatic since its discovery almost 10 years ago. We present hereradial velocity measurements obtained from medium-resolution VLT/FORS2spectra that show variations at twice the period of the luminositychanges. This implies that the system is an ellipsoidal variable. Unlikethe other sdB binaries of this nature, the fundamental parameters ofthis star we derive suggest that it lies below the Zero-Age ExtremeHorizontal Branch. From the cluster distance and the gravity wedetermine the mass of V46 to be 0.19 Mȯ. This istoo low to sustain core helium burning. From the mass function we derivea lower limit for the companion of only 0.26 Mȯ. Wediscuss the star's origin in the context of close binary evolution inthe field and globular clusters.
| Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample
We have measured the absolute proper motions of globular clusters NGC2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on theHipparcos system, and they are the first determinations ever made forthese low Galactic latitude clusters. The proper-motion uncertaintiesrange from 0.3 to 0.5 mas yr-1. The inferred orbits indicatethat (1) the single metal-rich cluster in our sample, NGC 5927,dynamically belongs to the thick disk; (2) the remaining metal-poorclusters have rather low-energy orbits of high eccentricity, and amongthese there appear to be two ``pairs'' of dynamically associatedclusters; (3) the most energetic cluster in our sample, NGC 3201, is ona highly retrograde orbit-which had already been surmised from radialvelocity alone-with an apocentric distance of 22 kpc; and (4) none ofthe metal-poor clusters appear to be associated with the recentlydetected SDSS streams or with the Monoceros structure. These are thefirst results of the Southern Proper Motion program where thesecond-epoch observations are taken with the recent CCD camera systeminstalled on the double astrograph at El Leoncito, Argentina.
| 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.
| 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.
| Chandra X-Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae
We present spectral and long-timescale variability analyses of ChandraX-Ray Observatory ACIS-S observations of the 19 millisecond pulsars(MSPs) with precisely known positions in the globular cluster 47Tucanae. The X-ray emission of the majority of these MSPs is welldescribed by a thermal (blackbody or neutron star hydrogen atmosphere)spectrum with a temperature Teff~(1-3)×106K, emission radius Reff~0.1-3 km, and luminosityLX~1030-1031 ergs s-1. Forseveral MSPs, there are indications that a second thermal component isrequired, similar to what is seen in some nearby field MSPs. Theobserved radiation most likely originates from the heated magnetic polarcaps of the MSPs. The small apparent scatter in LX isconsistent with thermal emission from the polar caps of a global dipolefield, although the small emission areas may imply either a more complexsmall-scale magnetic field configuration near the neutron star surfaceor nonuniform polar cap heating. The radio eclipsing binary MSPs 47 TucJ, O, and W show a significant nonthermal (power-law) component, withspectral photon index Γ~1-1.5, which most likely originates in anintrabinary shock formed due to interaction between the relativisticpulsar wind and matter from the stellar companion. We reexamine theX-ray-spin-down luminosity relation (LX-E˙ relation) andfind that for the MSPs with thermal spectraLX~E˙β, where β~0.2+/-1.1. Due tothe large uncertainties in both parameters, the result is consistentwith both the linear LX-E˙ relation and the flatterLX~E˙0.5 predicted by polar cap heatingmodels. In terms of X-ray properties, we find no clear systematicdifferences between MSPs in globular clusters and in the field of theGalaxy. We discuss the implications of these results on the presentunderstanding of the X-ray emission properties of MSPs.
| The Clusters Ages Experiment (CASE). I. V209 ω Cen: An Eclipsing Post-Common-Envelope Binary in the Globular Cluster ω Cen
We use photometric and spectroscopic observations of the detachedeclipsing binary V209 ω Cen to derive the masses, radii, andluminosities of the component stars. The system exhibits total eclipsesand, based on the measured systemic velocity and the derived distance,is a member of the globular cluster ω Cen. We obtain 0.945+/-0.043Msolar, 0.983+/-0.015 Rsolar, and 6.68+/-0.88Lsolar for the cooler but larger and more luminous primarycomponent. The secondary component has 0.144+/-0.008 Msolar,0.425+/-0.008 Rsolar, and 2.26+/-0.28 Lsolar. Theeffective temperatures are estimated at 9370 K for the primary and at10,866 K for the secondary. On the color-magnitude diagram of thecluster, the primary component occupies a position between the tip ofthe blue straggler region and the extended horizontal branch, while thesecondary component is located close to the red border of the areaoccupied by hot subdwarfs. However, its radius is too large and itseffective temperature is too low for it to be an sdB star. We propose ascenario leading to the formation of a system with such unusualproperties, with the primary component ``reborn'' from a former whitedwarf that accreted a new envelope through mass transfer from itscompanion. The secondary star has lost most of its envelope whilestarting its ascent onto the subgiant branch. It failed to ignite heliumin its core and is currently powered by a hydrogen-burning shell.This paper uses data obtained with the 6.5 m Magellan Telescopes locatedat Las Campanas Observatory, Chile.
| Inverting Color-Magnitude Diagrams to Access Precise Star Cluster Parameters: A Bayesian Approach
We demonstrate a new Bayesian technique to invert color-magnitudediagrams of main-sequence and white dwarf stars to reveal the underlyingcluster properties of age, distance, metallicity, and line-of-sightabsorption, as well as individual stellar masses. The advantages ourtechnique has over traditional analyses of color-magnitude diagrams areobjectivity, precision, and explicit dependence on prior knowledge ofcluster parameters. Within the confines of a given set of often-usedmodels of stellar evolution, a single mapping of initial to finalmasses, and white dwarf cooling, and assuming photometric errors thatone could reasonably achieve with the Hubble Space Telescope, ourtechnique yields exceptional precision for even modest numbers ofcluster stars. For clusters with 50-400 members and one to a few dozenwhite dwarfs, we find typical internal errors ofσ([Fe/H])<=0.03 dex, σ(m-MV)<=0.02 mag, andσ(AV)<=0.01 mag. We derive cluster white dwarf ageswith internal errors of typically only 10% for clusters with only threewhite dwarfs and almost always <=5% with 10 white dwarfs. Theseexceptional precisions will allow us to test white dwarf cooling modelsand standard stellar evolution models through observations of whitedwarfs in open and globular clusters.WIYN Open Cluster Study XXIII. The WIYN Observatory is a joint facilityof the University of Wisconsin-Madison, Indiana University, YaleUniversity, and the National Optical Astronomy Observatory.
| Near-infrared observations of the Fornax dwarf galaxy. I. The red giant branch
Aims.We present a study of the evolved stellar populations in the dwarfspheroidal galaxy Fornax based on wide-area near-infrared observations,aimed at obtaining new independent estimates of its distance andmetallicity distribution. Assessing the reliability of near-infraredmethods is most important in view of future space- and ground-based deepnear-infrared imaging of resolved stellar systems. Methods: We haveobtained JHK imaging photometry of the stellar populations in Fornax.The observations cover an 18.5×18.5 arcmin2 centralarea with a mosaic of SOFI images at the ESO NTT. Our data sample allthe red giant branch (RGB) for the whole area. Deeeper observationsreaching the red clump of helium-burning stars have also been obtainedfor a 4.5×4.5 arcmin2 region. Results: Near-infraredphotometry led to measurements of the distance to Fornax based on theK-band location of the RGB tip and the red clump. Once corrected for themean age of the stellar populations in the galaxy, the derived distancemodulus is (m-M)0 = 20.74±0.11, corresponding to adistance of 141 Kpc, in good agreement with estimates from optical data.We have obtained a photometric estimate of the mean metallicity of redgiant stars in Fornax from their (J-K) and (V-K) colors, using severalmethods. The effect of the age-metallicity degeneracy on the combinedoptical-infrared colors is shown to be less important than for opticalor infrared colors alone. By taking age effects into account, we havederived a distribution function of global metallicity [M/H] fromoptical-infrared colors of individual stars. Our photometric MetallicityDistribution Function covers the range -2.0 < [M/H] < -0.6, with amain peak at [M/H]≃ -0.9 and a long tail of metal-poor stars, andless metal-rich stars than derived by recent spectroscopy. Ifmetallicities from Ca II triplet lines are correct, this result confirmsa scenario of enhanced metal enrichment in the last 1-4 Gyr.Based on data collected at the European Southern Observatory, La Silla,Chile, Proposals No. 65.N-0167, 66.B-0247. Full Tables 2 and 3 are onlyavailable in electronic format 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/467/1025
| The Mystery Deepens: Spitzer Observations of Cool White Dwarfs
We present 4.5 and 8 μm photometric observations of 18 cool whitedwarfs obtained with the Spitzer Space Telescope. Our observationsdemonstrate that four white dwarfs with Teff<6000 K showslightly depressed mid-infrared fluxes relative to white dwarf models.In addition, another white dwarf with a peculiar optical andnear-infrared spectral energy distribution (LHS 1126) is found todisplay significant flux deficits in Spitzer observations. Thesemid-infrared flux deficits are not predicted by the current white dwarfmodels including collision-induced absorption due to molecular hydrogen.We postulate that either the collision-induced absorption calculationsare incomplete or there are other unrecognized physical processesoccurring in cool white dwarf atmospheres. The spectral energydistribution of LHS 1126 surprisingly fits a Rayleigh-Jeans spectrum inthe infrared, mimicking a hot white dwarf with effective temperaturewell in excess of 105 K. This implies that the source of thisflux deficit is probably not molecular absorption but some otherprocess.
| Full ionization in binary-binary encounters of equal masses with a small positive total energy
Interactions between pairs of binary stars play an important role in thedynamics of dense stellar systems. One possible outcome of abinary-binary encounter is full ionization: the destruction of bothinteracting binaries resulting in four separating single stars. Thisprocess is only possible for positive total energy, and thecross-section for full ionization falls to zero at zero total energy. Inthe context of a stellar system, full ionization is most likely toinvolve the interaction of two soft binaries with low binding energies.We consider the case where the four stars have a similar mass and derivean asymptotic power law relating the cross-section for full ionizationwith the total energy when this latter quantity is small. Our new theorymakes use of the fact that the most dramatic changes in scale andenergies of a few-body system occur when its components pass near to acentral configuration. Numerical simulations are used to verify theresult.
| 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.
| Dynamical Interactions of Planetary Systems in Dense Stellar Environments
We study dynamical interactions of star-planet binaries with othersingle stars. We derive analytical cross sections for all possibleoutcomes and confirm them with numerical scattering experiments. We findthat a wide mass ratio in the binary introduces a region in parameterspace that is inaccessible to comparable-mass systems, in which thenature of the dynamical interaction is fundamentally different from whathas traditionally been considered in the literature on binaryscattering. We study the properties of the planetary systems that resultfrom the scattering interactions for all regions of parameter space,paying particular attention to the location of the ``hard-soft''boundary. The structure of the parameter space turns out to besignificantly richer than a simple statement of the location of thehard-soft boundary would imply. We consider the implications of ourfindings, calculating characteristic lifetimes for planetary systems indense stellar environments and applying the results to previousanalytical studies, as well as past and future observations. Since werecognized that the system PSR B1620-26 in the globular cluster M4 liesin the ``new'' region of parameter space, we performed a detailedanalysis quantifying the likelihood of different scenarios in formingthe system we see today.
| Variability of 19 Millisecond Pulsars in 47 Tucanae with Chandra HRC-S
We present results from our 830 ks observation of the globular cluster47 Tucanae with the Chandra X-ray Observatory's High ResolutionCamera-S. We limit our analysis here to the 19 previously known,localized millisecond pulsars (MSPs) in the cluster. This work more thandoubles the sample of X-ray-detected MSPs observed with sensitivity torotational variability; it is also the first survey of a large group ofradio-discovered MSPs for which no previous X-ray pulsations have beendetected and is therefore an unbiased survey of the X-ray properties ofradio-discovered MSPs. We find that only 47 Tuc D, O, and R showsignificant pulsations at the >~4 σ level, but there isstatistical evidence for rotational variability in five additional MSPs.Furthermore, we constrain the pulsed magnetospheric emission of sevenmore MSPs using Monte Carlo simulations. The result is that the majorityof the 47 Tuc MSPs are characterized by low pulsed fractions, <~50%.In cases where larger pulsed fractions are measured, the folded pulseprofiles show relatively large duty cycles. When considered withprevious spectroscopic studies, this suggests that the X-ray emissionarises from the neutron star's heated polar caps and, in some cases,from intrabinary shocks, but generally not directly from the star'smagnetosphere. We discuss the impact of these results on ourunderstanding of high-energy emission from MSPs.
| 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.
| 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).
| Galactic Globular Clusters with Luminous X-Ray Binaries
Luminous X-ray binaries (LMXBs; >1034 ergs s-1)have a neutron star or black hole primary, and in globular clusters,most of these close binaries are expected to have evolved from widerbinaries through dynamical interactions with other stars. We attempt tofind a predictor of this formation rate that is representative of theinitial properties of globular clusters rather than of the highlyevolved core quantities. Models indicate the half-light quantities bestreflect the initial conditions, so we examine whether the associateddynamical interaction rate, proportional toL1.5/r2.5h, is useful for understandingthe presence of luminous LMXBs in the Galactic globular cluster system.We find that while LMXB clusters with large values ofL1.5/r2.5h preferentially host LMXBs,the systems must also have half-mass relaxation times belowth,relax~109 yr. This relaxation time effectprobably occurs because several relaxation times are required to modifybinary separations, a timescale that must be shorter than cluster ages.The likelihood of finding an LMXB cluster is enhanced if the cluster ismetal-rich and if it is close to the bulge region. The dependence onmetallicity is most likely either due to differing initial massfunctions at the high-mass end or because bulge systems evolve morerapidly from tidal interactions with the bulge. This approach can beused to investigate globular cluster systems in external galaxies, wherecore properties are unresolved.
| NGC 6388: Chemical Composition of Its Eight Cool Giants
Eight cool giants in the unusual globular cluster NGC 6388 have beeninvestigated in order to derive their elemental abundances. Effectivetemperatures from 3500 to 3850 K were derived using the method ofline-depth ratios. Surface gravities were derived in two ways.Spectroscopic gravities, derived by the requirement that the abundanceof iron be the same from Fe I and Fe II lines, were rather low, rangingfrom -0.3 to 0.0. Photometric gravities, derived from the assumedstellar mass of 0.7 Msolar and the luminosity andTeff of the stars, fell between +0.25 and +0.70. Mean [Fe/H]values were -0.8 for spectroscopic gravities and -0.6 for photometricgravities. A test using spectra of the same resolution of the twocoolest red giants in the globular cluster M4 obtained at the ApachePoint Observatory were analyzed for comparison with the definitiveanalysis of Ivans et al. (1999). The very cool metal-poor red giant HD232078 was also analyzed for comparison with the analysis of Gonzalez& Wallerstein (1998). Both comparisons showed that our methods yieldthe same abundance scale as previous works. We have compared thecomposition of stars in NGC 6388 with those of K giants with similar[Fe/H] in 47 Tuc and the Galactic bulge. The observed value of [O/Fe] isnear zero, which is less than in 47 Tuc and bulge stars of similarmetallicity. The α-elements behave similarly to oxygen and showonly small excesses at about the same level as do the α-elementsin the globular clusters associated with the Sgr system. It is unclearwhether these differences are responsible for the unusualcolor-magnitude diagram of NGC 6388.
| Rubidium and Lead Abundances in Giant Stars of the Globular Clusters M13 and NGC 6752
We present measurements of the neutron-capture elements Rb and Pb infive giant stars of the globular cluster NGC 6752 and Pb measurements infour giants of the globular cluster M13. The abundances were derived bycomparing synthetic spectra with high-resolution, high signal-to-noiseratio spectra obtained using HDS on the Subaru telescope and MIKE on theMagellan telescope. The program stars span the range of the O-Alabundance variation. In NGC 6752, the mean abundances are[Rb/Fe]=-0.17+/-0.06 (σ=0.14), [Rb/Zr]=-0.12+/-0.06(σ=0.13), and [Pb/Fe]=-0.17+/-0.04 (σ=0.08). In M13 the meanabundance is [Pb/Fe]=-0.28+/-0.03 (σ=0.06). Within the measurementuncertainties, we find no evidence for star-to-star variation for eitherRb or Pb within these clusters. None of the abundance ratios [Rb/Fe],[Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 mayhave slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to thesmall sample of field stars at the same metallicity. For M13 and NGC6752 the Pb abundances are in accord with predictions from a Galacticchemical evolution model. If metal-poor intermediate-mass asymptoticgiant branch stars did produce the globular cluster abundance anomalies,then such stars do not synthesize significant quantities of Rb or Pb.Alternatively, if such stars do synthesize large amounts of Rb or Pb,then they are not responsible for the abundance anomalies seen inglobular clusters.Based in part on data collected at the Subaru Telescope, which isoperated by the National Astronomical Observatory of Japan, and onobservations made with the Magellan Clay Telescope at Las CampanasObservatory.
| Draco, a flawless dwarf galaxy
The Draco dwarf spheroidal galaxy (dSph), with its apparent immensemass-to-light ratio and compact size, holds many clues to the nature ofthe enigmatic dark matter. Here we present deep photometric studies ofthis dwarf galaxy, undertaken with the MegaCam Camera at theCanada-France-Hawaii Telescope, the Wide Field Camera at the IsaacNewton Telescope and the Wide Field and Planetary Camera onboard theHubble Space Telescope (HST). The new photometric data cover theentirety of the galaxy, and reach i' = 24.5 at 50 per cent completeness,significantly deeper than previous panoramic studies, allowing searchesfor tidal disturbances of much lower surface brightness than has beenpossible before. With these improved statistics, we find no evidence forasymmetric disturbances or tidal tails that possess more than 3 per centof the stars found within the centre of the galaxy. We find that thecentral stellar density, as probed by the HST data, rises into thecentral 0.5 arcmin. Uncertainties in the position of the centroid of thegalaxy do not allow us to determine whether the apparent flattening ofthe profile interior to 0.5 arcmin is reliable or not. Draco istherefore a flawless dwarf galaxy, featureless and apparently unaffectedby Galactic tides.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT)which is operated by the National Research Council (NRC) of Canada, theInstitute National des Sciences de l'Univers of the Centre National dela Recherche Scientifique of France, and the University of Hawaii. Alsobased on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Institute. STScIis operated by the association of Universities for Research inAstronomy, Inc. under the NASA contract NAS 5-26555.E-mail: ibata@astro.u-strasbg.fr
| 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.
| Chandra X-Ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)
We report the detection of six discrete, low-luminosity(LX<1033 ergs s-1) X-ray sources,located within 12" of the center of the collapsed-core globular clusterM30 (NGC 7099), and a total of 13 sources within the half-mass radius,from a 50 ks Chandra ACIS-S exposure. Three sources lie within the verysmall upper limit of 1.9" on the core radius. The brightest of the threecore sources has a luminosity of LX(0.5-6keV)~6×1032 ergs s-1 and a blackbody-likesoft X-ray spectrum, which are both consistent with it being a quiescentlow-mass X-ray binary (qLMXB). We have identified optical counterpartsto four of the six central sources and a number of the outlying sources,using deep Hubble Space Telescope and ground-based imaging. While thetwo proposed counterparts that lie within the core may represent chancesuperpositions, the two identified central sources that lie outside ofthe core have X-ray and optical properties consistent with beingcataclysmic variables (CVs). Two additional sources outside of the corehave possible active binary counterparts. We discuss the X-ray sourcepopulation of M30 in light of its collapsed-core status.
| 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.
| 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 COMPLETE Survey of Star-Forming Regions: Phase I Data
We present an overview of data available for the Ophiuchus and Perseusmolecular clouds from Phase I of the COMPLETE Survey of Star-FormingRegions. This survey provides a range of data complementary to theSpitzer Legacy Program ``From Molecular Cores to Planet Forming Disks.''Phase I includes the following: extinction maps derived from the TwoMicron All Sky Survey (2MASS) near-infrared data using the NICERalgorithm; extinction and temperature maps derived from IRAS 60 and 100μm emission; H I maps of atomic gas; 12CO and13CO maps of molecular gas; and submillimeter continuumimages of emission from dust in dense cores. Not unexpectedly, themorphology of the regions appears quite different depending on thecolumn density tracer that is used, with IRAS tracing mainly warmer dustand CO being biased by chemical, excitation, and optical depth effects.Histograms of column density distribution are presented, showing thatextinction as derived from 2MASS NICER gives the closest match to alognormal distribution, as is predicted by numerical simulations. Allthe data presented in this paper, and links to more detailedpublications on their implications, are publicly available at theCOMPLETE Web site.
| 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.
| 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.
| 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.
| WLM-1: An old, Nonrotating, Gravitationally Unperturbed, Highly Elliptical Extragalactic Globular Cluster
Globular clusters have long been known for presenting (at times)significant deviations from spherical symmetry. While rotation has beenthe main proposed explanation, other complicating factors such as theirconstant interaction with the strong gravitational potential of theirhost galaxy have made it difficult for a consensus to be reached. Toaddress this question we have obtained high-resolution spectra of WLM-1,the lone, old globular cluster associated with the isolated, low-massdwarf irregular galaxy WLM. Using archival HST WFPC2 data, we measurethe radial ellipticity profile of WLM-1, finding it to be highlyelliptical, with a mean value of 0.17 in the region 0.5"-5", which iscomparable to what is found in our Galaxy for the most ellipticalglobular clusters. There is no evidence of isophote twisting, except forthe innermost regions of the cluster (r<0.5"). To investigate whetherthe observed flattening can be ascribed to rotation, we have obtainedlong-slit high-resolution Very Large Telescope UVES spectra of thiscluster along and perpendicular to the axis of flattening. Usingcross-correlation we find that the velocity profile of the cluster isconsistent with zero rotation along either axis. Thus, neither clusterrotation nor galactic tides can be responsible for the flattenedmorphology of WLM-1. We argue that the required velocity dispersionanisotropy between the semimajor and semiminor axes that would berequired to account for the observed flattening is relatively small, oforder 1 km s-1. Even though our errors preclude us fromconclusively establishing that such a difference indeed exists, velocityanisotropy remains at present the most plausible explanation for theshape of this cluster.
| 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.
| 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 extra-mixing efficiency in very low metallicity RGB stars
Aims.After the first dredge-up, low-mass Red Giant Branch (RGB) starsexperience an extra-mixing episode that strongly affects the chemicalabundances on their surface. This mixing occurs at the bump in theluminosity function. In this Letter we describe the efficiency of theextra-mixing in RGB stars found in very metal-poor globular clusters(GC). Methods: The VLT/ISAAC spectra of twenty stars located betweenthe bump and the tip of the RGB in four GCs with metallicities between[Fe/H]=-1.2 and -2.5 dex were collected. The carbon isotopic ratios ontheir surface were derived from the second overtone (Δ v=2) bandsof the CO molecule at 2.3 μm with the spectral synthesis method. Results: It is found that the carbon isotopic ratios of very metal-poorGC stars always reach the equilibrium value of the CNO cycle almostimmediately above the bump in the luminosity function. No additionalmixing episode at brighter luminosities and no variations with theclusters' metallicity were detected. The extra-mixing is therefore foundto be very efficient in metal-poor low-mass RGB stars, in very goodagreement with theoretical expectations.
| Veraenderliche in M 4. V46, V47 und V54.
Not Available
| 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.
| Analysing globular cluster observations. Models and analysis tools for Lick/IDS indices
We have extended our evolutionary synthesis code, galev, to includeLick/IDS absorption-line indices for both simple and composite stellarpopulation models (star clusters and galaxies), using polynomial fittingfunctions. We present a mathematically advanced Lick index analysis tool(LINO) for the determination of the ages and metallicities of globularclusters (CGs). An extensive grid of galev models for the evolution ofstar clusters at various metallicities over a Hubble time is compared toobserved sets of Lick indices of varying completeness and precision. Adedicated χ^2-minimisation procedure selects the best modelincluding 1 σ uncertainties on age and metallicity. We discuss theage and metallicity sensitivities of individual indices and show thatthese sensitivities themselves depend on age and metallicity; thus, weextend Worthey's (1994) concept of a "metallicity sensitivity parameter"for an old stellar population at solar metallicity to younger clustersof different metallicities. We find that indices at low metallicity aregenerally more age sensitive than at high metallicity. Our aim is toprovide a robust and reliable tool for the interpretation ofstar-cluster spectra becoming available from 10 m class telescopes in alarge variety of galaxies - metal-rich & metal-poor, starburst,post-burst, and dynamically young. We test our analysis tool usingobservations from various authors for Galactic and M 31 GCs, for whichreliable age and metallicity determinations are available in theliterature, and discuss to what extent the observational availability ofvarious subsets of Lick indices affects the results. For M 31 GCs, wediscuss the influence of non-solar abundance ratios on our results. Allmodels are accessible from our website,http://www.astro.physik.uni-goettingen.de/~galev/
| The MODEST questions: Challenges and future directions in stellar cluster research
We present a review of some of the current major challenges in stellarcluster research, including young clusters, globular clusters, andgalactic nuclei. Topics considered include: primordial mass segregationand runaway mergers, expulsion of gas from clusters, the production ofstellar exotica seen in some clusters (e.g., blue stragglers and extremehorizontal-branch stars), binary populations within clusters, theblack-hole population within stellar clusters, the final parsec problem,stellar dynamics around a massive black hole, and stellar collisions.The Modest Questions posed here are the outcome of discussions whichtook place at the Modest-6A workshop held in Lund, Sweden, in December,2005. Modest-6A was organised as part of the activities of the ModestCollaboration (see www.manybody.org for further details).
| Multisite campaign on the open cluster M67 - I. Observations and photometric reductions
We report on an ambitious multisite campaign aimed at detecting stellarvariability, particularly solar-like oscillations, in the red giantstars in the open cluster M67 (NGC 2682). During the six-week observingrun, which comprised 164 telescope nights, we used nine 0.6-m to 2.1-mclass telescopes located around the world to obtain uninterrupted timeseries photometry. We outline here the data acquisition and reduction,with emphasis on the optimization of the signal-to-noise ratio of thelow-amplitude (50-500μmag) solar-like oscillations. This includes anew and efficient method for obtaining the linearity profile of the CCDresponse at ultrahigh precision (~10 parts per million). The noise inthe final time series is 0.50 mmag per minute integration for the bestsite, while the noise in the Fourier spectrum of all sites combined is20μmag. In addition to the red giant stars, this data set proves tobe very valuable for studying high-amplitude variable stars such aseclipsing binaries, W UMa systems and δ Scuti stars.
| Ground-based CCD astrometry with wide field imagers. I. Observations just a few years apart allow decontamination of field objects from members in two globular clusters
This paper is the first of a series of papers in which we will apply themethods we have developed for high-precision astrometry (and photometry)with the Hubble Space Telescope (HST) to the case of wide-fieldground-based images. In particular, we adapt the software originallydeveloped for WFPC2 to ground-based, wide field images from the WFI atthe ESO 2.2 m telescope. In this paper, we describe in details the newsoftware, we characterize the WFI geometric distortion, discuss theadopted local transformation approach for proper-motion measurements,and apply the new technique to two-epoch archive data of the two closestGalactic globular clusters: NGC 6121 (M 4) and NGC 6397. The results ofthis exercise are more than encouraging. We find that we can achieve aprecision of ~7 mas (in each coordinate) in a single exposure for awell-exposed star, which allows a very good cluster-field separation inboth M 4, and NGC 6397, with a temporal baseline of only 2.8, and 3.1years, respectively.
| 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.
| 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.
| 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.
| Timing of Millisecond Pulsars in NGC 6752. II. Proper Motions of the Pulsars in the Cluster Outskirts
Exploiting a 5 year span of data, we present improved timing solutionsfor the five millisecond pulsars known in the globular cluster NGC 6752.These include proper-motion determinations for the two outermost pulsarsin the cluster, PSR J1910-5959A and PSR J1910-5959C. The values of theproper motions are in agreement with each other within currentuncertainties, but they do not match (at the 4 σ and 2 σlevels, respectively) the value of the proper motion of the entireglobular cluster derived in the optical band. The implications of theseresults for the cluster membership of the two pulsars are investigated.Prospects for the detection of the Shapiro delay in the binary systemJ1910-5959A are also discussed.
| 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 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.
| 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.
| A Moderate Sample Size, Multielement Analysis of the Globular Cluster M12 (NGC 6218)
We present chemical abundances of several proton-capture, α-,Fe-peak, and neutron-capture elements and radial velocities for 21 redgiant branch (RGB) and asymptotic giant branch members of the Galacticglobular cluster M12. Abundances are based on equivalent widthmeasurements and synthetic spectral analyses of moderate-resolutionspectra (R~15,000) obtained with the 3.5 m WIYN telescope and Hydramultifiber spectrograph. The stars observed range from the RGB tip(M0v=-2.47) down to about 0.50 mag above the levelof the horizontal branch (M0v=+0.11). Ourspectroscopic analysis suggests that M12 is a moderately metal-poorcluster with [Fe/H]=-1.54 (σ=0.09). While the Na abundancesexhibit a range of 0.90 dex, Mg and Al abundances are enhanced by 0.37and 0.54 dex and are nearly constant at all RGB luminosities, incontrast to the blue horizontal-branch cluster M13. The α- andFe-peak elements indicate that M12 has undergone a similar chemicalenrichment history to that of globular clusters and field stars ofcomparable metallicity, with <[α/Fe]>=+0.33 (σ=0.11).M12 also appears to be slightly r-process-rich, with<[Eu/Ba,La]>=+0.22 (σ=0.18).
| 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)
| Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
| 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).
| Chandra observations of the globular cluster M 54
We have carried out a Chandra observation of the globular cluster M 54.We detected 7 sources located within the half-mass radius of M 54, at aflux limit of 1.5 × 10-15 erg s-1cm-2 in the 0.3-8 keV energy band. The spatial distributionand the colour/spectral properties of the 7 sources suggest that theyare likely to be cataclysmic variables or LMXBs in the globular cluster.M 54 shows the largest number of X-ray sources with luminosities greaterthan 1032 erg s-1 compared to other globularclusters observed using Chandra and XMM-Newton. We searched for acorrelation between the number of sources above this luminosity levelwith globular cluster parameters. We found evidence that the number ofsources peaks at a King concentration parameter c ~ 1.7-1.9, withglobular clusters which are core-collapsed or have low-c values having asmaller number of sources. We speculate on possible reasons for this.
| The changing place of red giant stars in the evolutionary process
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| The dynamical distance and intrinsic structure of the globular cluster ω Centauri
We determine the dynamical distance D, inclination i, mass-to-lightratio M/L and the intrinsic orbital structure of the globular clusterω Cen, by fitting axisymmetric dynamical models to theground-based proper motions of van Leeuwen et al. and line-of-sightvelocities from four independent data-sets. We bring the kinematicmeasurements onto a common coordinate system, and select on clustermembership and on measurement error. This provides a homogeneousdata-set of 2295 stars with proper motions accurate to 0.20 masyr-1 and 2163 stars with line-of-sight velocities accurate to2 km s-1, covering a radial range out to about half the tidalradius. We correct the observed velocities for perspective rotationcaused by the space motion of the cluster, and show that the residualsolid-body rotation component in the proper motions (caused by relativerotation of the photographic plates from which they were derived) can betaken out without any modelling other than assuming axisymmetry. Thisalso provides a tight constraint on D tan i. The corrected mean velocityfields are consistent with regular rotation, and the velocity dispersionfields display significant deviations from isotropy. We model ωCen with an axisymmetric implementation of Schwarzschild's orbitsuperposition method, which accurately fits the surface brightnessdistribution, makes no assumptions about the degree of velocityanisotropy in the cluster, and allows for radial variations in M/L. Webin the individual measurements on the plane of the sky to searchefficiently through the parameter space of the models. Tests on ananalytic model demonstrate that this approach is capable of measuringthe cluster distance to an accuracy of about 6 per cent. Application toω Cen reveals no dynamical evidence for a significant radialdependence of M/L, in harmony with the relatively long relaxation timeof the cluster. The best-fit dynamical model has a stellar V-bandmass-to-light ratio M/L_V=2.5±0.1 M_ȯ/L_ȯ and aninclination i=50°±4°, which corresponds to an averageintrinsic axial ratio of 0.78±0.03. The best-fit dynamicaldistance D=4.8±0.3 kpc (distance modulus 13.75±0.13 mag)is significantly larger than obtained by means of simple spherical orconstant-anisotropy axisymmetric dynamical models, and is consistentwith the canonical value 5.0±0.2 kpc obtained by photometricmethods. The total mass of the cluster is(2.5±0.3)×106 M_ȯ. The best-fit model isclose to isotropic inside a radius of about 10 arcmin and becomesincreasingly tangentially anisotropic in the outer region, whichdisplays significant mean rotation. This phase-space structure may wellbe caused by the effects of the tidal field of the Milky Way. Thecluster contains a separate disk-like component in the radial rangebetween 1 and 3 arcmin, contributing about 4% to the total mass.
| Faint X-Ray Sources in the Globular Cluster Terzan 5
We report our analysis of a Chandra X-ray observation of the richglobular cluster Terzan 5, in which we detect 50 sources to a limiting1.0-6 keV X-ray luminosity of 3×1031 ergss-1 within the half-mass radius of the cluster. Thirty-threeof these have LX>1032 ergs s-1, thelargest number yet seen in any globular cluster. In addition to thequiescent low-mass X-ray binary (LMXB; identified by Wijnands et al.),another 12 relatively soft sources may be quiescent LMXBs. We comparethe X-ray colors of the harder sources in Terzan 5 to the Galacticcenter sources studied by Muno and collaborators and find the Galacticcenter sources to have harder X-ray colors, indicating a possibledifference in the populations. We cannot clearly identify a metallicitydependence in the production of low-luminosity X-ray binaries inGalactic globular clusters, but a metallicity dependence of the formsuggested by Jordán et al. for extragalactic LMXBs is consistentwith our data.
| 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.
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