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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.
| 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.
| 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.
| Wide-Field Survey around Local Group Dwarf Spheroidal Galaxy Leo II: Spatial Distribution of Stellar Content
We carried out a wide-field V, I imaging survey of the Local Group dwarfspheroidal galaxy Leo II using the Subaru Prime Focus Camera on the 8.2m Subaru Telescope. The survey covered an area of 26.67×26.67arcmin2, far beyond the tidal radius of Leo II (8.63'), downto the limiting magnitude of V~=26, which is roughly 1 mag deeper thanthe turnoff point of the main-sequence stars of Leo II. Radial numberdensity profiles of bright and faint red giant branch (RGB) stars werefound to change their slopes at around the tidal radius, and extendbeyond the tidal radius with shallower slopes. A smoothed surfacebrightness map of Leo II suggests the existence of a small substructure(4×2.5 arcmin2, 270×170 pc 2 inphysical size) of globular cluster luminosity beyond the tidal radius.We investigated the properties of the stellar population by means of acolor-magnitude diagram. The horizontal branch (HB) morphology indexshows a radial gradient in which red HB stars are more concentrated thanblue HB stars, which is common to many Local Group dwarf spheroidalgalaxies. The color distribution of RGB stars around the mean RGBsequence shows a larger dispersion at the center than in the outskirts,indicating a mixture of stellar populations at the center and a morehomogeneous population in the outskirts. Based on the age estimationusing subgiant branch stars, we found that although the major starformation took place ~8 Gyr ago, a considerable stellar populationyounger than 8 Gyr is found at the center; such a younger population isinsignificant in the outskirts. The following star formation history issuggested for Leo II. Star-forming activity occurred more than ~8 Gyrago throughout the galaxy at a modest star formation rate. Thestar-forming region gradually shrank from the outside toward the center,and star-forming activity finally dropped to ~0 by ~4 Gyr ago, exceptfor the center, where a small population younger than 4 Gyr is present.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Star Counts in the Globular Cluster ω Centauri. I. Bright Stellar Components
We present a photometric investigation on HB, RGB, and MSTO stars inω Cen=NGC 5139. The center of the cluster was covered with amosaic of F435W, F625W, and F658N band data collected with HST ACS. Theouter reaches were covered with a mosaic of U-, B-, V-, and I-band datacollected with the 2.2 m ESO/MPI telescope. The final catalog includes~1.7 million stars. We identified more than 3200 likely HB stars, thelargest sample ever collected in a globular cluster. We found that theHB morphology changes with the radial distance from the cluster center.The relative number of extreme HB stars decreases from ~30% to ~21% whenmoving from the center toward the outer reaches of the cluster, whilethe fraction of less hot HB stars increases from ~62% to ~72%. Thecomparison between theory and observations indicates that the empiricalstar counts of HB stars are on average larger (30%-40%) than predictedby canonical evolutionary models. Moreover, the rate of HB stars is ~43%larger than the MSTO rate. We also compared theory and observations byassuming a mix of stellar populations made with 70% of canonical He(Y=0.23) stars and 30% of He-enhanced (Y=0.33, 0.42) stars. We foundthat the observed RG/MSTO ratio agrees with the predicted lifetimes ofHe-mixed stellar populations. The discrepancy between theory andobservations decreases by a factor of 2 when compared with ratespredicted by canonical He content models, but still 15%-25% (Y=0.42) and15%-20% (Y=0.33) higher than observed. Furthermore, the ratios betweenHB and MSTO star counts are ~24% (Y=0.42) and 30% (Y=0.33) larger thanpredicted lifetime ratios.During the revision of this manuscript, Vittorio Castellani passed awayon 2006 May 19. His suggestions, ideas, and personality will be greatlymissed.Based on data obtained from the ESO Science Archive Facility and theHubble Space Telescope Archive Facility.
| 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.
| 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).
| Washington Photometry of the Red Giant Branch in NGC 6388
We present Washington photometry of the giant branches of the relativelymetal-rich globular cluster NGC 6388 and compare it with 47 Tuc. Byexamining the spatial distribution of stellar colors across the redgiant branch and using spectroscopic data on several bright giants, wefound that NGC 6388's center is affected by differential reddening of atleast ΔE(B-V)~0.1. Most of the width of the red giant branch iscaused by that differential reddening. The southeastern outer regions ofthe cluster seem to be less affected by variable dust extinction, and wefind that NGC 6388 is slightly more metal-rich than 47 Tuc, with adistinct asymptotic giant branch. We have put an upper limit on possiblevariations in [Fe/H] for NGC 6388, finding that it must be less than~0.2 dex. It is unlikely that such a small range in metallicity could beresponsible for the blue horizontal branch and blue tail stars in acluster with a metallicity similar to the more massive 47 Tuc, whichonly exhibits a red horizontal branch.
| Timing the Eccentric Binary Millisecond Pulsar in NGC 1851
We have used the Green Bank Telescope to observe the millisecond pulsarPSR J0514-4002A on 43 occasions spread over 2 years. This 5 ms pulsar islocated in the globular cluster NGC 1851; it belongs to a binary systemand has a highly eccentric (e=0.888) orbit. We have obtained aphase-coherent timing solution for this object, including very preciseposition, spin, and orbital parameters. The pulsar is located 4.6"(about 1.3 core radii) from the center of the cluster and is likely tolie on its more distant half. The nondetection of eclipses at superiorconjunction can be used, given the peculiar geometry of this system, torule out the possibility of an extended companion. We have measured therate of advance of periastron for this binary system to beω˙=0.01289(4)deg yr-1, which if duecompletely to general relativity implies a total system mass of2.453(14) Msolar. Given the known mass function, the pulsarmass has to be <1.5 Msolar, and the mass of the companionhas to be >0.96 Msolar, implying that it is a heavy whitedwarf. The 350 MHz flux density of this pulsar varies between 0.2 and1.4 mJy; the origin of these variations is not known.
| The Unusual Binary Pulsar PSR J1744-3922: Radio Flux Variability, Near-Infrared Observation, and Evolution
PSR J1744-3922 is a binary pulsar exhibiting highly variable pulsedradio emission. We report on a statistical multifrequency study of thepulsed radio flux variability which suggests that this phenomenon isextrinsic to the pulsar and possibly tied to the companion, although notstrongly correlated with orbital phase. The pulsar has an unusualcombination of characteristics compared to typical recycled pulsars: along spin period (172 ms); a relatively high magnetic field strength(1.7×1010 G); a very circular, compact orbit of 4.6 hr;and a low-mass companion (0.08 Msolar). These spin andorbital properties are likely inconsistent with standard evolutionarymodels. We find similarities between the properties of the PSRJ1744-3922 system and those of several other known binary pulsarsystems, motivating the identification of a new class of binary pulsars.We suggest that this new class could result from: a standard accretionscenario of a magnetar or a high magnetic field pulsar; common envelopeevolution with a low-mass star and a neutron star, similar to what isexpected for ultracompact X-ray binaries; or accretion induced collapseof a white dwarf. We also report the detection of a possibleK'=19.30(15) infrared counterpart at the position of thepulsar, which is relatively bright if the companion is a helium whitedwarf at the nominal distance, and discuss its implications for thepulsar's companion and evolutionary history.
| Origin of abundance inhomogeneity in globular clusters
We numerically investigate abundance properties of the Galactic globularclusters (GCs) by adopting a new `external pollution' scenario. In thisframework, GCs are assumed to originate in forming low-mass dwarfsembedded in dark matter subhaloes at very high redshifts (z) and thus bechemically influenced by field asymptotic giant branch (AGB) stars ofthe dwarfs during early GC formation processes. GCs within a dwarfgalaxy therefore can be formed from the mixture of (i) gas ejected fromthe field AGB stars formed earlier in the dwarf and (ii) theinterstellar gas infalling to the central region of the dwarf. In thisexternal pollution scenario, the ratio of the total mass of infallinggas to that of AGB ejecta during GC formation in a dwarf (s) and thetime-scale of gas infall (σI) are the most importantkey parameters that can determine abundance properties of GCs. We mainlyinvestigate the abundance inhomogeneity among light elements (e.g. C, N,O, Na and Al) of stars in GCs by using the latest stellar yield modelsof metal-poor AGB stars with and without third dredge-up. Our principalresults for the models with no third dredge-up, which are moreconsistent with observations, are as follows. (i) Both [N/Fe] and[C/Fe] can be diverse among stars within a GC owing to chemicalpollution from field AGB stars. [N/Fe] distributions in some GCs canclearly show bimodality, whereas [C/Fe] is monomodal in most models.[N/Fe] distributions depend on s such that models with smaller s (i.e.larger mass fraction of AGB ejecta used for GC formation) show the[N/Fe] bimodality more clearly. (ii) N-rich, C-poor stars in GCs alsohave higher He abundances owing to pollution from massive AGB stars withHe-rich ejecta. The number fraction of He-rich stars (Y > 0.30) ishigher for the models with smaller s and shorter σI for3 <= s <= 24 and 105 <= σI <=107 yr. He abundances of stars correlate with [N/Fe] and[Al/Fe] and anticorrelate with [C/Fe], [O/Fe] and [Na/Fe] within GCs inour models. (iii) Although our model can much better explain theobserved C-N and Mg-Al anticorrelations than previous theoreticalmodels, it is in strong disagreement with the observed O-Naanticorrelation. (iv) This model naturally provides an explanation forthe large fraction of CN-strong stars without recourse to an implausibleinitial mass function. Based on these results for the above externalpollution scenario, we discuss the long-standing problem of theCN-bimodality prevalent in the Galactic GCs, the possible heliumabundance inhomogeneity in these systems and their horizontal branchmorphologies.
| Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| 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.
| NGC 2298: a globular cluster on its way to disruption
We have studied the stellar main sequence (MS) of the globular clusterNGC 2298 using deep HST/ACS observations in the F606W and F814W bandscovering an area of 3.4 arcmin × 3.4 arcmin around the clustercentre or about twice the cluster's half-mass radius. Thecolour-magnitude diagram that we derive in this way reveals a narrow andwell defined MS extending down to the 10σ detection limit at m_606≃ 26.5, m_814 ≃ 25, corresponding to stars of 0.2 {M}_ȯ.The luminosity function (LF) obtained with these data, once correctedfor the limited effects of photometric incompleteness, reveals aremarkable deficiency of low-mass stars as well as a radial gradient, inthat the LF becomes progressively steeper with radius. Using themass-luminosity relation appropriate for the metallicity of NGC 2298, wederive the cluster's global mass function (GMF) by using a multi-massMichie-King model. Over the range 0.8-0.2 {M}_ȯ, the number ofstars per unit mass decreases following a power-law distribution of thetype dN/dm ∝ m0.5, where, for comparison, typical haloclusters have dN/dm ∝ m-1.5. If the IMF of NGC 2298 wassimilar to that of other metal poor halo clusters, like e.g. NGC 6397,the present GMF that we obtain implies that this object must have lostof the order of 85% of its original mass, at a rate much higher thanthat suggested by current models based on the available cluster orbit.The latter may, therefore, need revision.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555.
| Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal
This paper presents a study of the Cetus dwarf, an isolated dwarf galaxywithin the Local Group. A matched-filter analysis of the INT/WFC imagingof this system reveals no evidence for significant tidal debris thatcould have been torn off the galaxy, bolstering the hypothesis thatCetus has never significantly interacted with either the Milky Way orM31. Additionally, Keck/Deimos spectroscopic observations identify thisgalaxy as a distinct kinematic population possessing a systematicvelocity of -87 +/- 2kms-1 and with a velocity dispersion of17 +/- 2kms-1 while tentative, these data also suggest thatCetus possesses a moderate rotational velocity of ~8kms-1.The population is confirmed to be relatively metal-poor, consistent with[Fe/H] ~ -1.9, and, assuming virial equilibrium, implies that the Cetusdwarf galaxy possesses a mass-to-light ratio of ~70. It appears,therefore, that Cetus may represent a primordial dwarf galaxy, retainingthe kinematic and structural properties lost by other members of thedwarf population of the Local Group in their interactions with the largegalaxies. An analysis of Cetus' orbit through the Local Group indicatesthat it is at apocentre; taken in conjunction with the general dwarfpopulation, this shows the mass of the Local Group to be >~2 ×1012Msolar.
| Big Bang Nucleosynthesis in Scalar Tensor Gravity: The Key Problem of the Primordial 7Li Abundance
Combined with other cosmic microwave background experiments, theWilkinson Microwave Anisotropy Probe (WMAP) survey provides an accurateestimate of the baryon density of the universe. In the framework of thestandard big bang nucleosynthesis (BBN), such a baryon density leads topredictions for the primordial abundances of 4He and D ingood agreement with observations. However, it also leads to asignificant discrepancy between the predicted and observed primordialabundance of 7Li. Such a discrepancy is often termed ``thelithium problem.'' In this paper we analyze this problem in theframework of scalar tensor theories of gravity. It is shown that anexpansion of the universe slightly slower than in general relativitybefore BBN, but faster during BBN, solves the lithium problem and leadsto 4He and D primordial abundances consistent with theobservational constraints. This kind of behavior is obtained in numerousscalar tensor models, both with and without a self-interaction potentialfor the scalar field. In models with a self-interacting scalar field,the convergence toward general relativity is ensured without anycondition, thanks to an attraction mechanism that starts to work duringthe radiation-dominated epoch.
| The Space Motion of the Globular Cluster NGC 6397
As a by-product of high-precision, ultradeep stellar photometry in theGalactic globular cluster NGC 6397 with the Hubble Space Telescope, weare able to measure a large population of background galaxies whoseimages are nearly pointlike. These provide an extragalactic referenceframe of unprecedented accuracy, relative to which we measure the mostaccurate absolute proper motion ever determined for a globular cluster.We find μαcosδ=3.56+/-0.04 mas yr-1and μδ=-17.34+/-0.04 mas yr-1. We notethat the formal statistical errors quoted for the proper motion of NGC6397 do not include possible unavoidable sources of systematic errors,such as cluster rotation. These are very unlikely to exceed a fewpercent. We use this new proper motion to calculate NGC 6397's UVW spacevelocity and its orbit around the Milky Way and find that the clusterhas made frequent passages through the Galactic disk.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 NAS5-26555. These observations are associated with proposalGO-10424.
| 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.
| 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
| Na-O anticorrelation and horizontal branches. II. The Na-O anticorrelation in the globular cluster NGC 6752
We are studying the Na-O anticorrelation in several globular clusters ofdifferent Horizontal Branch (HB) morphology in order to derive apossible relation between (primordial) chemical inhomogeneities andmorphological parameters of the cluster population. We used themultifiber spectrograph FLAMES on the ESO Very Large Telescope UT2 andderived atmospheric parameters and elemental abundances of Fe, O and Nafor about 150 red giant stars in the Galactic globular cluster NGC 6752.The average metallicity we derive is [Fe/H] = -1.56, in agreement withother results from red giants, but lower than obtained for dwarfs orearly subgiants. In NGC 6752 there is not much space for an intrinsicspread in metallicity: on average, the rms scatter in [Fe/H] is 0.037± 0.003 dex, while the scatter expected on the basis of the majorerror sources is 0.039 ± 0.003 dex. The distribution of starsalong the Na-O anticorrelation is different to what was found in thefirst paper of this series for the globular cluster NGC 2808: in NGC6752 it is skewed toward more Na-poor stars, and it resembles more theone in M 13. Detailed modeling is required to clarify whether thisdifference may explain the very different distributions of stars alongthe HB.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables [see full textsee full textsee full text], [seefull textsee full textsee full text] and [see full textsee full textseefull text] are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/927
| Beryllium abundance in turn-off stars of NGC 6752
Aims.To measure the beryllium abundance in two TO stars of the GlobularCluster NGC 6752, one oxygen rich and sodium poor, the other presumablyoxygen poor and sodium rich. Be abundances in these stars are used toput on firmer grounds the hypothesis of Be as cosmochronometer and toinvestigate the formation of Globular Clusters. Methods: We presentnear UV spectra with resolution R˜ 45 000 obtained with the UVESspectrograph on the 8.2 m VLT Kueyen telescope, analysed with spectrumsynthesis based on plane parallel LTE model atmospheres. Results: Be isdetected in the O rich star with log(Be/H) = -12.04 ±0.15, whileBe is not detected in the other star for which we obtain the upper limitlog(Be/H) < -12.2. A large difference in nitrogen abundance (1.6 dex)is found between the two stars. Conclusions: .The Be measurement iscompatible with what found in field stars with the same [Fe/H] and[O/H]. The "Be age" of the cluster is found to be 13.3 Gyr, in excellentagreement with the results from main sequence fitting and stellarevolution. The presence of Be confirms the results previously obtainedfor the cluster NGC 6397 and supports the hypothesis that Be can be usedas a clock for the early formation of the Galaxy. Since only an upperlimit is found for the star with low oxygen abundance, we cannot decidebetween competing scenarios of Globular Cluster formation, but we canexclude that "polluted" stars are substantially younger than"unpolluted" ones. We stress that the Be test might be the onlymeasurement capable of distinguishing between these scenarios.Based on observations collected at the ESO VLT, Paranal Observatory,Chile, program 075.D-0807(A).
| 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.
| 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.
| First stars VII - Lithium in extremely metal poor dwarfs
Context: .The primordial lithium abundance is a key prediction of modelsof big bang nucleosynthesis, and its abundance in metal-poor dwarfs (theSpite plateau) is an important, independent observational constraint onsuch models. Aims: .This study aims to determine the level andconstancy of the Spite plateau as definitively as possible fromhomogeneous high-quality VLT-UVES spectra of 19 of the most metal-poordwarf stars known. Methods: .Our high-resolution (R 43 000),high S/N spectra are analysed with OSMARCS 1D LTE model atmospheres andturbospectrum synthetic spectra to determine effective temperatures,surface gravities, and metallicities, as well as Li abundances for ourstars. Results: .Eliminating a cool subgiant and a spectroscopicbinary, we find 8 stars to have -3.5 < [Fe/H] < -3.0 and 9 starswith -3.0 < [Fe/H] < -2.5. Our best value for the mean level ofthe plateau is A(Li) =2.10± 0.09. The scatter around the mean isentirely explained by our estimate of the observational error and doesnot allow for any intrinsic scatter in the Li abundances. In addition,we conclude that a systematic error of the order of 200 K in any of thecurrent temperature scales remains possible. The iron excitationequilibria in our stars support our adopted temperature scale, which isbased on a fit to wings of the Hα line, and disfavour hotterscales, which would lead to a higher Li abundance, but fail to achieveexcitation equilibrium for iron. Conclusions: .We confirm thepreviously noted discrepancy between the Li abundance measured inextremely metal-poor turnoff stars and the primordial Li abundancepredicted by standard Big-Bang nucleosynthesis models adopting thebaryonic density inferred from WMAP. We discuss recent work explainingthe discrepancy in terms of diffusion and find that uncertaintemperature scales remain a major question.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Large Programme "First Stars", ID 165.N-0276(A);P.I. R. Cayrel). Tables 4-8 and Appendix A are only available inelectronic form at http://www.aanda.org
| 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.
| 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.
| 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).
| 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.
| 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).
| 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.
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