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|The Infrared Ca II Triplet as Metallicity Indicator|
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
|CCD Photometry of the Globular Cluster M15: RR Lyrae Fourier Decomposition and Physical Parameters|
Results of CCD photometry using V and R filters are reported for 33 RRLyrae stars in M15. The periodicities of some variables have beenrevised and new ephemerides are given. The Blazhko effect, previouslyreported in V12, was not detected. Applying the approach of Fourierdecomposition of the light curves, the physical parameters of the typeRRab and RRc variables were estimated. The cluster is Oosterhoff type IIand the values for the iron content and distance are:[Fe/H]=-1.98+/-0.24 and d=8.67+/-0.41 kpc, respectively. The mean valuesof the physical parameters determined for the RR Lyrae stars place thecluster precisely into the sequences Oosterhoff type --metallicity andmetallicity-- effective temperature, valid for globular clusters.Evidences of evolution from the ZAHB are found for the RRc but not forthe RRab stars.
|CN Abundance Inhomogeneities in the Globular Cluster Messier 13 (NGC 6205): Results Based on Merged Data Sets from the Literature|
The globular cluster Messier 13 (NGC 6205) has been shown by a number ofdifferent studies to be chemically inhomogeneous with regard to elementsranging from C through Al. A database of λ3883 CN-band indices,carbon, oxygen, and sodium abundances for red giants in M13 has beencompiled and homogenized from a variety of literature sources. The dataare used to document the distribution of CN band strength among both redgiant branch (RGB) and asymptotic giant branch (AGB) stars brighter thanMV=+0.8, as well as the relationships between the CNinhomogeneities and the dispersions in other elements. The CNdistribution among the M13 RGB stars is bimodal, although a fewintermediate-CN red giants are present in the cluster. The AGB starsshow uniformly weak CN bands. The spread in CN band strength, onceempirically corrected for effective temperature and gravity differencesamong the red giants, is very well correlated with the sodium abundanceand is anticorrelated with oxygen. Above MV=+0.8, the carbonabundance of red giants decreases with increasing luminosity, as firstfound by N. Suntzeff. There is a rather modest anticorrelation betweenCN band strength and [C/Fe] among RGB stars of comparable magnitude. Theabundance patterns within M13 can be interpreted as a primordial (orpre-RGB phase) abundance spread, coupled with the later effects of deepmixing within the red giants. In the CN-weak giants and those CN-strongstars with [O/Fe]~-0.2+/-0.2 dex on the upper RGB, deep mixing seems tohave been limited largely to the dredge-up of C-->N-processedmaterial. By contrast, there are some M13 stars, most notably a group ofvery oxygen-deficient giants near the RGB tip, but perhaps also afraction of CN-strong giants with lesser oxygen depletions, in whichsome measure of O-->N-processed material also appears to have beenbrought to the surface.
|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.
|DDO 44 and UGC 4998: Distances, Metallicities, and Star Formation Histories|
We have obtained deep HST imaging of two candidate dwarf systems in thenearby M81 Group, DDO 44 and UGC 4998. Both are isolated, low surfacebrightness systems, but with likely very different star formationhistories based on their mean colors and color fluctuations. Thecolor-magnitude diagrams of these galaxies have been used to estimatetheir distances using the tip of the red giant branch (RGB), and theirmean metallicities from the colors of the RGB. For DDO 44 we find adistance of D=3.01+/-0.18 Mpc-confirming it to be a member of the M81Group-and a mean metallicity of [Fe/H]=-1.54+/-0.14. We also find fromthe properties and numbers of a population of luminous asymptotic giantbranch (AGB) stars that about 20% of the luminous population of DDO 44consists of ``intermediate-age'' stars (with ages between about 2 and 8Gyr) that give rise to the observed AGB. There is no difference in thespatial distribution of the RGB and AGB stars in this galaxy. For UGC4998 we derive a metallicity of [Fe/H]=-1.58+/-0.21 and a distance ofD=8.24+/-0.43 Mpc. The latter puts UGC 4998 well behind the M81 Group,confirming results from previous measurements. We find this galaxy tocontain a population of young (<50 Myr old) stars, and there isevidence of older RGB stars. The young stars are considerably morecentrally concentrated than the older stars. The current star formationrate in UGC 4998 is ~9×10-4 Msolaryr-1 over the entire galaxy, or ~5×10-4Msolar yr-1 kpc-2. Given its H I mass(1×107 Msolar), this galaxy can continue toform stars at this rate for another ~11 Gyr.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with proposalGO-8137.
|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.
|X-Ray Localization of the Globular Cluster G1 with XMM-Newton|
We present an accurate X-ray position of the massive globular cluster G1obtained using XMM-Newton and the Hubble Space Telescope (HST). TheX-ray emission of G1 has been detected recently with XMM-Newton. Thereare two possibilities for the origin of the X-ray emission: it could bedue to either accretion of the central intermediate-mass black hole, orordinary low-mass X-ray binaries. The precise location of the X-rayemission might distinguish between these two scenarios. By refining theastrometry of the XMM-Newton and HST data, we reduced the XMM-Newtonerror circle to 1.5". Despite the smaller error circle, the precision isnot sufficient to distinguish an intermediate-mass black hole orluminous low-mass X-ray binaries. This result, however, suggests thatfuture Chandra observations may reveal the origin of the X-ray emission.
|A Double-Mode RR Lyrae Star with a Strong Fundamental-Mode Component|
NSVS 5222076, a 13th magnitude star in the Northern Sky VariabilitySurvey, was identified by L. Oaster as a possible new double-mode RRLyrae star. We confirm the double-mode nature of NSVS 5222076,supplementing the survey data with new V-band photometry. NSVS 5222076has a fundamental-mode period (P0) of 0.4940 days and afirst-overtone period (P1) of about 0.3668 days, giving aperiod ratio of P1/P0=0.743. In most double-modeRR Lyrae stars, the amplitude of the first-overtone mode's pulsation isgreater than that of the fundamental-mode pulsation. That is not truefor this star. Its fundamental-mode light curve has an amplitude twiceas large as that of the first-overtone mode, a ratio very rarely seeneven among the double-mode RR Lyrae stars that have relatively strongfundamental-mode pulsation. Data from the literature are used to discussthe location in the Petersen diagram of double-mode RR Lyrae starshaving strong fundamental-mode pulsation. Such stars tend to occurtoward the short-period end of the Petersen diagram, and NSVS 5222076 isno exception to this rule.
|Upper limits on the central black hole masses of 47Tuc and NGC 6397 from radio continuum emission|
We present upper limits on the masses of the putative centralintermediate-mass black holes in two nearby Galactic globular clusters:47Tuc (NGC 104), the second brightest Galactic globular cluster, and NGC6397, a core-collapse globular cluster and, with a distance of 2.7 kpc,quite possibly the nearest globular cluster. These upper limits areobtained using a technique suggested by T. Maccarone. These massestimates have been derived from 3σ upper limits on the radiocontinuum flux at 1.4 GHz, assuming that the putative central black holeaccretes the surrounding matter at a rate of between 0.1 and 1 per centof the Bondi accretion rate. For 47Tuc, we find a 3σ upper limitof 2060-670 Msolar, depending on the actual accretion rate ofthe black hole and the distance to 47Tuc. For NGC 6397, which is closerto us, we derive a 3σ upper limit of 1290-390 Msolar.While estimating mass upper limits based on radio continuum observationsrequires making assumptions about the gas density and the accretion rateof the black hole, their derivation does not require complex andtime-consuming dynamical modelling. Thus, this method offers anindependent way of estimating black hole masses in nearby globularclusters. If, generally, central black holes in stellar systems accretematter faster than 0.1 per cent of the Bondi accretion rate, then theseresults indicate the absence of black holes in these globular clusterswith masses as predicted by the extrapolatedM•-σc relation.
|Tip of the Red Giant Branch Distances. II. Zero-Point Calibration|
The luminosity of the tip of the red giant branch (TRGB) provides anexcellent measure of galaxy distances and is easily determined in theresolved images of nearby galaxies observed with Hubble Space Telescope.There is now a large amount of archival data that are relevant to theTRGB methodology and offer comparisons with other distance estimators.Zero-point issues related to the TRGB distance scale are reviewed inthis paper. Consideration is given to the metallicity dependence of theTRGB, the transformations between HST flight systems and Johnson-Cousinsphotometry, the absolute magnitude scale based on horizontal branchmeasurements, and the effects of reddening. The zero point of the TRGBis established with a statistical accuracy of 1%, modulo the uncertaintyin the magnitude of the horizontal branch, with a typical rmsuncertainty of 3% in individual galaxy distances at high Galacticlatitude. The zero point is consistent with the Cepheidperiod-luminosity relation scale but invites reconsideration of theclaimed metallicity dependence with that method. The maser distance toNGC 4258 is consistent with TRGB but presently has lower accuracy.
|Gas Accretion by Globular Clusters and Nucleated Dwarf Galaxies and the Formation of the Arches and Quintuplet Clusters|
We consider here the collective accretion of gas by globular clustersand dwarf galaxies moving through the interstellar medium. In the limitof high velocity and/or sound speed of the ISM, the collective potentialof the cluster is insufficient to accrete significant amounts of gas,and stars within the systems accrete gas individually. We show, however,that when the sound speed or the relative velocity of the ambient mediumis less than the central velocity dispersion of the cluster, it isaccreted into the collective potential of the cluster prior to beingaccreted onto the individual stars within the cluster. The collectiverate is strongly enhanced relative to the individual rates. This effectmay potentially modify the white dwarf cooling sequence in globularclusters with low-inclination and low-eccentricity Galactic orbits andlead to the rejuvenation of some marginally surviving cores of globularclusters and nucleated dwarf galaxies near the Galactic center. Sucheffects will only occur rarely, but may explain the existence ofclusters of young, massive stars near the Galactic center.
|A non-LTE abundance analysis of the post-AGB star ROA5701|
An analysis of high-resolution Anglo-Australian Telescope(AAT)/University College London Échelle Spectrograph(UCLÉS) optical spectra for the ultraviolet (UV)-bright starROA5701 in the globular cluster ω Cen (NGC5139) is performed,using non-local thermodynamic equilibrium (non-LTE) model atmospheres toestimate stellar atmospheric parameters and chemical composition.Abundances are derived for C, N, O, Mg, Si and S, and compared withthose found previously by Moehler et al. We find a general metalunderabundance relative to young B-type stars, consistent with theaverage metallicity of the cluster. Our results indicate that ROA5701has not undergone a gas-dust separation scenario as previouslysuggested. However, its abundance pattern does imply that ROA5701 hasevolved off the asymptotic giant branch (AGB) prior to the onset of thethird dredge-up.
|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 (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/467/1025
|Primordial pollution of globular clusters within their host dwarfs embedded in dark matter haloes at high redshifts|
Recent observational studies have revealed star-to-star abundanceinhomogeneity among light elements (e.g. C, N, O, Na and Al) of stars onthe main sequence in the Galactic globular clusters (GCs). One of thepromising interpretations for this result is that the observed abundanceinhomogeneity is due to the second generation of stars formed fromejecta of the first generation of evolved stars (e.g. asymptotic giantbranch stars) within GCs. However, it remains unclear whether and howthis primordial pollution can occur within GCs. We here propose a newscenario in which primordial pollution of GCs is highly likely to occurif GCs are located in the central regions of high-redshift dark mattersubhaloes that can host low-mass dwarf galaxies. In this scenario, gasejected from the first generation of stars of GCs can be effectivelytrapped in the deep gravitational potential of their host haloes, andconsequently can be consumed for the formation of the second generationof stars without losing a significant amount of gas by ram pressurestripping of interstellar and intergalactic medium. During merging ofthese haloes with the proto-Galaxy, the haloes are completely destroyedowing to the strong tidal field of the Galaxy. The self-polluted GCslocated initially in the central regions of the haloes can survive tidaldestruction owing to their compactness and finally become the Galactichalo GCs. In this scenario, ejecta of field stars surrounding thecentral GCs can be also converted into stars within their host dwarfsand finally become the second generation of stars of GCs. We alsodiscuss the origin of the difference in the degree of abundanceinhomogeneity between different GCs, such as ω Centauri and NGC6752, in terms of the difference in physical properties between hosthaloes from which GCs originate.
|Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for HI and OH|
We present the results of a survey, the deepest to date, for HI emissionat 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720MHz) in the direction towards the Galactic globular clusters M15, M2,NGC6934, NGC7006 and Pal13. The aim is to measure the amount of hydrogenin the intracluster medium, and to find OH masers in the circumstellarenvelopes of globular cluster red giants. We present a tentativedetection of 0.3Msolar of neutral hydrogen in M15 andpossible detections of neutral hydrogen in M2 and Pal13. We derive upperlimits to the neutral hydrogen content of NGC6934 and 7006. No OHemission is detected. We also present deep HI data of the northern tipof the Magellanic Stream behind Pal13.
|High-energy γ-rays from globular clusters|
It is expected that specific globular clusters (GCs) can contain up to ahundred of millisecond pulsars. These pulsars can accelerate leptons atthe shock waves originated in collisions of the pulsar winds and/orinside the pulsar magnetospheres. Energetic leptons diffuse graduallythrough the GC Comptonizing stellar and microwave background radiation.We calculate the GeV-TeV γ-ray spectra for different models ofinjection of leptons and parameters of the GCs assuming reasonable, ofthe order of 1 per cent, efficiency of energy conversion from the pulsarwinds into the relativistic leptons. It is concluded that leptonsaccelerated in the GC cores should produce well localized γ-raysources which are concentric with these GCs. The results are shown forfour specific GCs (47 Tuc, Ter 5, M13 and M15), in which significantpopulation of millisecond pulsars have been already discovered. We arguethat the best candidates, which might be potentially detected by thepresent Cherenkov telescopes and the planned satellite telescopes(AGILE, GLAST), are 47 Tuc on the Southern hemisphere, and M13 on theNorthern hemisphere. We conclude that detection (or non-detection) ofGeV-TeV γ-ray emission from GCs by these instruments put importantconstraints on the models of acceleration of leptons by millisecondpulsars.
|N-body models of rotating globular clusters|
In this paper we examine the dynamical evolution of rotating globularclusters with direct N-body models. Our initial models are rotating Kingmodels, and we obtain results both for equal-mass systems and forsystems composed of two mass components. Previous investigations using aFokker-Planck solver have shown that rotation has a noticeable influenceon stellar systems such as globular clusters that evolve by two-bodyrelaxation. In particular, it accelerates their dynamical evolutionthrough the gravogyro instability. We have validated the occurrence ofthe gravogyro instability with direct N-body models. In the case ofsystems composed of two mass components, mass segregation takes place, aprocess that competes with the rotation in the acceleration of the corecollapse. The `accelerating' effect of rotation was detected in ourisolated two-mass N-body models. Finally, we look at rotating N-bodymodels in a tidal field within the tidal approximation. It turns outthat rotation increases the escape rate significantly. A differencebetween retrograde- and prograde-rotating stellar clusters, with respectto the orbit of the cluster around the Galaxy, occurs. This differenceis the result of the presence of a `third integral' and chaoticscattering, respectively.
|Blazhko effect and double-mode RR Lyrae stars .|
There are two kinds of multi-periodic RR Lyrae stars. The Blazhko effectis a modulation of the primary light cycle that takes place on a typicaltimescale of tens of days. Double-mode RR Lyrae stars pulsatesimultaneously in the fundamental and first overtone radial modes. Thestate of our knowledge of these multi-periodic variables is reviewed.
|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.
|Variable stars in the globular cluster M15|
We present new BV CCD photometry for variables in the globular clusterM15. The photometry used was obtained with the image subtraction packageISIS. The data were acquired on an observing run in 2001 and range over11 observing nights. For 32 previously known variables, we obtainedperiod for the first time. Improved periods were obtained for 28previously known variables, and 13 new variables were found. For some ofthe variables intensity- and magnitude-averaged B and V magnitudes,magnitude-averaged colors, and pulsation amplitudes were obtained usingDAOPHOT to calibrate the ISIS light curves.
|Do EC 14026 Stars Exist in Clusters?|
We have began a program to search for pulsating sdB stars in open andglobular clusters. Detection of pulsators in clusters would aid theunderstanding of their evolution, since the masses and metal content ofthe main-sequence progenitors would be known. Here we describe thebeginning of our search during which we plan to observe about 100 sdBstars. A failure to detect any pulsators would suggest differencesbetween sdB stars in clusters and in the field of the Milky Way (e.g.,metallicity, binarity) which would need to be taken into account by anypulsation/evolution scenario.
|Kinematic Decoupling of Globular Clusters with the Extended Horizontal Branch|
About 25% of the Milky Way globular clusters (GCs) exhibit unusuallyextended color distribution of stars in the core helium-burninghorizontal-branch (HB) phase. This phenomenon is now best understood asdue to the presence of helium-enhanced second-generation subpopulations,which has raised the possibility that these peculiar GCs might have aunique origin. Here we show that these GCs with extended HB are clearlydistinct from other normal GCs in kinematics and mass. The GCs withextended HB are more massive than normal GCs and are dominated by randommotion with no correlation between kinematics and metallicity.Surprisingly, however, when they are excluded, most normal GCs in theinner halo show clear signs of dissipational collapse that apparentlyled to the formation of the disk. Normal GCs in the outer halo sharetheir kinematic properties with the extended HB GCs, which is consistentwith the accretion origin. Our result further suggests heterogeneousorigins of GCs, and we anticipate this to be a starting point for moredetailed investigations of Milky Way formation, including early mergers,collapse, and later accretion.
|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.
|A Systematic Search for Periodicities in RXTE ASM Data|
We present the results of a systematic search in 8.5 yr of Rossi X-RayTiming Explorer All-Sky Monitor data for evidence of periodicities. Thesearch was conducted by application of the Lomb-Scargle periodogram tothe light curves of each of 458 actually or potentially detected sourcesin each of four energy bands (1.5-3, 3-5, 5-12, and 1.5-12 keV). Awhitening technique was applied to the periodograms before evaluation ofthe statistical significance of the powers. We discuss individualdetections with a focus on relatively new findings.
|The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting|
Seventy-two grids of stellar evolutionary tracks, along with the meansto generate isochrones and luminosity/color functions from them, arepresented in this investigation. Sixty of them extend (and encompass)the sets of models reported by VandenBerg et al. for 17 [Fe/H] valuesfrom -2.31 to -0.30 and α-element abundances corresponding to[α/Fe]=0.0, 0.3, and 0.6 (at each iron abundance) to the solarmetallicity and to sufficiently high masses (up to ~2.2Msolar) that isochrones may be computed for ages as low as 1Gyr. The remaining grids contain tracks for masses from 0.4 to 4.0Msolar and 12 [Fe/H] values between -0.60 and +0.49 (assumingsolar metal-to-hydrogen number abundance ratios): in this case,isochrones may be calculated down to ~0.2 Gyr. The extent of convectivecore overshooting has been modeled using a parameterized version of theRoxburgh criterion, in which the value of the free parameter at a givenmass and its dependence on mass have been determined from analyses ofbinary star data and the observed color-magnitude diagrams for severalopen clusters. Because the calculations reported herein satisfy manyempirical constraints, they should provide useful probes into theproperties of both simple and complex stellar populations.All of the model grids may be obtained from the Canadian Astronomy DataCenter(http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cvo/community/VictoriaReginaModels/).Included in this archive are (1) the interpolation software (FORTRAN 77)to produce isochrones, isochrone probability functions, luminosityfunctions, and color functions, along with instructions on how toimplement and use the software, (2) BVRI (VandenBerg & Clem 2003)and uvby (Clem et al. 2004) color-temperature relations, and (3)zero-age horizontal branch loci for all of the chemical compositionsconsidered.
|The Low-Mass X-Ray Binary and Globular Cluster Connection in Virgo Cluster Early-Type Galaxies: Optical Properties|
LMXBs form efficiently in GCs. By combining Chandra and HST ACSobservations of 11 massive early-type galaxies in the Virgo Cluster, weuse the most accurate identification of LMXBs and GCs to date to explorethe optical properties of 270 GCs with LMXBs and 6488 GCs withoutdetectable X-ray emission. More massive, redder, and more compact GCsare more likely to contain LMXBs. Unlike Galactic GCs, a large number ofGCs with LMXBs have half-mass relaxation times >2.5 Gyr; GCs need notsurvive for more than five relaxation timescales to produce LMXBs. Byfitting the dependence of the expected number of LMXBs per GC,λt, on the GC mass M, color (g-z), and half-massradius rh,cor, we find thatλt~M1.24+/-0.08×100.9+0.2-0.1(g-z)r-2.2+/-0.2h,cor.This rules out that the number of LMXBs per GC is linearly proportionalto GC mass (99.89% confidence limit) and leads us to predict that mostGCs with high X-ray luminosities contain a single LMXB. The detaileddependence of λt on GC properties appears mainly dueto a dependence on the encounter rate Γh and themetallicity Z,λt~Γ0.82+/-0.05hZ0.39+/-0.07.Our analysis provides strong evidence that dynamical formation andmetallicity play the primary roles in determining the presence of anLMXB in extragalactic GCs. The shallower than linear encounter ratedependence requires an explanation by theories of dynamical binaryformation. A metallicity-dependent variation in the number of neutronstars and black holes per unit GC mass, effects from irradiation-inducedwinds, or suppression of magnetic braking in metal-poor stars may all beconsistent with our abundance dependence; all three scenarios requirefurther development.
|Observing IMBH-IMBH Binary Coalescences via Gravitational Radiation|
Recent numerical simulations have suggested the possibility of formingdouble intermediate-mass black holes (IMBHs) via the collisional runawayscenario in young dense star clusters. The two IMBHs that formed wouldexchange into a common binary shortly after their birth and quicklyinspiral and merge. Since space-borne gravitational wave (GW)observatories such as LISA will be able to see the late phases of theirinspiral out to several gigaparsecs, and LIGO will be able to see themerger and ringdown out to similar distances, they represent potentiallysignificant GW sources. In this Letter we estimate the rate at whichLISA and LIGO will see their inspiral and merger in young star clusters,and we discuss the information that can be extracted from theobservations. We find that LISA will likely see tens of IMBH-IMBHinspirals per year, while advanced LIGO could see ~10 merger andringdown events per year, with both rates strongly dependent on thedistribution of cluster masses and densities.
|Different Characteristics of the Bright Branches of the Globular Clusters M15 and M92|
We carried out relatively wide-field BVI CCD photometric observations ofthe globular clusters M15 (NGC 7078) and M92 (NGC 6341) using the 1.8 mtelescope of the Bohyun Optical Astronomy Observatory. We presentcolor-magnitude diagrams (V vs. B-V, V vs. V-I, and V vs. B-I) of M15and M92. We found asymptotic giant branch (AGB) bumps atVbumpAGB=15.20+/-0.05 mag andVbumpAGB=14.50+/-0.05 mag for M15 and M92,respectively. We identified the red giant branch (RGB) bumps of the twoclusters. We have estimated the population ratios R and R2for M15 and M92 in two cases: when only normal horizontal-branch (HB)stars are used and when all the HB stars are used. We have compared theobserved RGB luminosity functions of M15 and M92 with the theoreticalRGB luminosity functions of Bergbusch & VandenBerg and found nosignificant ``extra stars'' in the comparisons. This implies that the HBmorphology difference between M15 and M92 is not certain due to theresults of deep mixing in the RGB sequence.
|Detection of a 60°-long Dwarf Galaxy Debris Stream|
We report on a 60°-long stream of stars, extending from Ursa Majorto Sextans, in the Sloan Digital Sky Survey. The stream is approximately2° wide and is clearly distinct from the northern tidal arm of theSagittarius dwarf galaxy. The apparent width of the stream indicates aprogenitor with a size and mass similar to that of a dwarf galaxy. Thestream is about 21 kpc distant and appears to be oriented almostperpendicular to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, although we cannot rule outthat the stream may form the inner part of a known dwarf galaxy's orbit.The most likely explanation is that the stream constitutes the remainsof a dwarf galaxy that has been completely disrupted at some point inthe past. We also briefly report on the discovery of a diminutiveGalactic satellite that lies near the projected path of the new streambut is unlikely to be related to it.
|Globular Cluster Metallicity Subpopulations in NGC 4472|
Bimodality is a common feature of globular cluster (GC) colordistributions in galaxies. Although it is well known that the GC systemof the Milky Way is bimodal in metallicity, this has yet to be directlydemonstrated for an elliptical galaxy. We use Lick index measurementsfrom the literature to derive metallicities for 47 GCs in the giantVirgo elliptical galaxy NGC 4472. The resulting distribution shows clearevidence for two metallicity subpopulations of GCs.
|Short-hard gamma-ray bursts|
Electronic Article Available from Elsevier Science.
|Near-Ultraviolet Observations of HD 221170: New Insights into the Nature of r-Process-rich Stars|
Employing high-resolution spectra obtained with the near-UV-sensitivedetector on the Keck I HIRES, supplemented by data obtained with theMcDonald Observatory 2d-coudé, we have performed a comprehensivechemical composition analysis of the bright r-process-rich metal-poorred giant star HD 221170. Analysis of 57 individual neutral and ionizedspecies yielded abundances for a total of 46 elements and significantupper limits for an additional five. Model stellar atmosphere parameterswere derived with the aid of ~200 Fe peak transitions. From more than350 transitions of 35 neutron-capture (Z>30) species, abundances for30 neutron-capture elements and upper limits for three others werederived. Utilizing 36 transitions of La, 16 of Eu, and seven of Th, wederive ratios of logε(Th/La)=-0.73 (σ=0.06) andlogε(Th/Eu)=-0.60 (σ=0.05), values in excellent agreementwith those previously derived for other r-process-rich metal-poor starssuch as CS 22892-052, BD +17 3248, and HD 115444. Based on the Th/Euchronometer, the inferred age is 11.7+/-2.8 Gyr. The abundancedistribution of the heavier neutron-capture elements (Z>=56) isfitted well by the predicted scaled solar system r-process abundances,as also seen in other r-process-rich stars. Unlike other r-process-richstars, however, we find that the abundances of the lighterneutron-capture elements (37
|Measurement of Orbital Decay in the Double Neutron Star Binary PSR B2127+11C|
We report the direct measurement of orbital period decay in the doubleneutron star pulsar system PSR B2127+11C in the globular cluster M15 atthe rate of (-3.95+/-0.13)×10-12, consistent with theprediction of general relativity at the ~3% level. We find the pulsarmass to be mp=1.358+/-0.010 Msolar and thecompanion mass mc=1.354+/-0.010 Msolar. We alsoreport long-term pulse timing results for the pulsars PSR B2127+11A andPSR B2127+11B, including confirmation of the cluster proper motion.
|Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge|
We present near-IR color-magnitude diagrams and physical parameters fora sample of 24 Galactic globular clusters toward the bulge. In thispaper we discuss the properties of 12 new clusters (out of the 24) inaddition to those previously studied and published by our group. Thecompilation includes measurements of the cluster reddening, distance,photometric metallicity, horizontal branch red clump, and red giantbranch morphological (e.g., mean ridgelines) and evolutionary (e.g.,bump and tip) features. The compilation is available in electronic formon the World Wide Web, and it will be updated regularly.Based on data taken at the ESO New Technology Telescope, within theobserving programs 73.D-0313, 75.D-0372, and 77.D-0757.
|X-Rays from the Globular Cluster G1: Intermediate-Mass Black Hole or Low-Mass X-Ray Binary?|
The globular cluster G1 (Mayall II) in M31 is the most massive(~107 Msolar) stellar cluster in the Local Group,and it has the highest central velocity dispersion (~28 kms-1). It has been claimed to host a central ~20,000Msolar black hole, but these claims have been controversial.We investigate here the XMM-Newton detection of X-ray emission from G1at a level of LX~2×1036 ergs s-1.This emission could be the result of Bondi-Hoyle accretion of ionizedcluster gas by a central black hole, or it could be produced by aconventional low-mass X-ray binary. A precise localization of the X-rayemission, which is not possible with the current XMM data, coulddistinguish between these possibilities. While such a measurement may bedifficult, it is of sufficient potential importance to pursue.
|Whiting 1: the youngest globular cluster associated with the Sagittarius dwarf spheroidal galaxy|
Context: Recently, Carraro (2005) drew attention to the remarkable starcluster Whiting 1 by showing that it lies about 40 kpc from the Sun andis therefore unquestionably a member of the Galactic halo (b=-60.6 deg).Its Color Magnitude Diagram (CMD) indicated that Whiting 1 is very young(~5 Gyr) for a globular cluster. It is very likely that Whiting 1originated in a dwarf galaxy that has since been disrupted by the MilkyWay. Aims: The main goals of this investigation were to constrainbetter the age, metallicity, and distance of Whiting 1 and to assesswhether it belongs to a stellar stream from the Sagittarius dwarfspheroidal galaxy (Sgr dSph). Methods: Deep CCD photometry in the BVIpass-bands obtained with the VLT is used to improve the quality of theCMD and to determine the cluster's luminosity function and surfacedensity profile. High-resolution spectrograms obtained with Magellan areused to measure the cluster's radial velocity and to place limits on itspossible metallicity. The measurements of distance and radial velocityare used to test the cluster's membership in the stellar streams fromthe Sgr dSph. Results: From our CMD of Whiting 1, we derive newestimates for the cluster's age (6.5+1.0-0.5 Gyr),metallicity (Z=0.004±0.001, [Fe/H] = -0.65), and distance(29.4+1.8-2.0 kpc). From echelle spectrograms ofthree stars, we obtain -130.6±1.8 km s-1 for thecluster's radial velocity and show from measurements of two infra-redCaII lines that the [Fe/H] of the cluster probably lies in the range-1.1 to -0.4. Both the luminosity function and the surface densityprofile suggest that the cluster has undergone tidal stripping by theMilky Way. We demonstrate that the position of Whiting 1 on the sky, itsdistance from the Sun, and its radial velocity are identical to withinthe errors of both the theoretical predictions of the trailing stream ofstars from the Sgr dSph galaxy and the previous observations of the Mgiant stars that delineate the streams. Conclusions: With the additionof Whiting 1, there is now strong evidence that 6 globular clustersformed within the Sgr dSph. Whiting 1 is particularly interestingbecause it is the youngest and among the most metal rich. The relativelyyoung age of Whiting 1 demonstrates that this dwarf galaxy was able toform star clusters for a period of at least 6 Gyr, and the age andmetallicity of Whiting 1 are consistent with the age-metallicityrelationship in the main body of the Sgr dSph. The presence now ofWhiting 1 in the Galactic halo provides additional support for the viewthat the young halo clusters originated in dwarf galaxies that have beenaccreted by the Milky Way.Based on observations with the ESO VLT at the Paranal Observatory, underthe program 76.D-0128. This paper includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile. Tablesof the individual photometric measurements are available at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/181
|Six new candidate ultracompact X-ray binaries|
Ultracompact X-ray binaries (UCXBs) appear able to sustain accretiononto the compact accretor at rates lower than in wider X-ray binaries.This may be understood by the smaller accretion disks in UCXBs: a lowerX-ray luminosity suffices to keep a disk completely ionized throughirradiation and, thus, keep the viscosity at a sufficiently high levelto allow effective transport of matter to the compact object. We employthis distinguishing factor on data from RXTE and BeppoSAX to identifysix new candidate UCXBs, thus increasing the population by one quarter.The candidates are drawn from the population of persistently accretingand type-I X-ray bursting low-mass X-ray binaries. The X-ray burstsestablish the low-mass X-ray binary nature and provide a handle on theaccretion rate. We find that the low accretion rates are supported bythe long burst recurrence times and the hard X-ray spectra of thepersistent emission as derived from the 2nd INTEGRAL catalog of softγ-ray sources. We discuss the peculiar light curves of some newUCXB candidates.
|From Supermassive Black Holes to Dwarf Elliptical Nuclei: A Mass Continuum|
Considerable evidence suggests that supermassive black holes reside atthe centers of massive galactic bulges. At a lower galactic mass range,many dwarf galaxies contain extremely compact nuclei that structurallyresemble massive globular clusters. We show that both these types ofcentral massive objects (CMOs) define a single unbroken relation betweenCMO mass and the luminosity of their host galaxy spheroid. Equivalently,MCMO is directly proportional to the host spheroid mass over4 orders of magnitude. We therefore suggest that the dE,N nuclei may bethe low-mass analogs of supermassive black holes and that these twotypes of CMOs may have both developed starting from similar initialformation processes. The overlap mass interval between the two types ofCMOs is small and suggests that for MCMO>107Msolar, the formation of a black hole was strongly favored,perhaps because the initial gas infall to the center was too rapid andviolent for star formation to occur efficiently.
|The Global Kinematics of the Globular Cluster M92|
We report the determination of high-accuracy radial velocities for 306members of the globular cluster M92 using the Hydra multiobjectspectrograph on the WIYN telescope. We have concentrated on starsoutside of the central region of the cluster, located up to 14.4' fromthe cluster center. Candidate members were selected for spectroscopybased on a photometric metallicity index determined from three-bandWashington photometry, also obtained with the WIYN telescope. The medianerror in the velocities is 0.35 km s-1. We find theheliocentric radial velocity of the cluster to be -121.2+/-0.3 kms-1. We have used an improved Bayesian analysis to determinethe velocity dispersion profile of M92. The most probable profile is acored power law with a scale radius of 2', a velocity dispersion at 1'of 6.3 km s-1, and an outer power law with a slope of -0.6.We have also reanalyzed the M15 radial velocities of Drukier et al. andfind that a pure power law with a 1' velocity dispersion of 8 kms-1 and a slope of -0.5 and the combination of a power lawwith a slope of -0.4 and a scale of 7.5 km s-1 inside 9' anda dispersion of 4 km s-1 outside are equally likely. In bothclusters there is evidence that the samples include escaping stars. Wepresent results from a GRAPE-based N-body simulation of an isolatedcluster that demonstrates this effect. We suggest additional tests todetermine the relative importance of tidal heating and stellar ejectionfor establishing the velocity field in globular cluster halos.
|Detection of a 63° Cold Stellar Stream in the Sloan Digital Sky Survey|
We report on the detection in Sloan Digital Sky Survey data of a63°-long tidal stream of stars, extending from Ursa Major to Cancer.The stream has no obvious association with the orbit of any knowncluster or galaxy. The contrast of the detected stream is greatest whenusing a star count filter that is matched to the color-magnitudedistribution of stars in M13, which suggests that the stars making upthe stream are old and metal-poor. The visible portion of the stream isvery narrow and about 8.5 kpc above the Galactic disk, suggesting thatthe progenitor is or was a globular cluster. While the surface densityof the stream varies considerably along its length, its path on the skyis very smooth and uniform, showing no evidence of perturbations bylarge mass concentrations in the nearby halo. While definitiveconstraints cannot be established without radial velocity information,the stream's projected path and estimates of its distance suggest thatwe are observing the stream near the perigalacticon of its orbit.
|Various Modes of Helium Mixing in Globular Cluster Giants and Their Possible Effects on the Horizontal Branch Morphology|
It has been known for a long time that some red giants in globularclusters exhibit large star-to-star variations in the abundances oflight elements that are not exhibited by field giants. This fact can betaken as evidence that the extra mixing mechanism(s) that operate inglobular cluster giants may be consequences of star-star interactions inthe dense stellar environment. In order to constrain the extra mixingmechanism(s), we study the influence of helium enrichment along the redgiant branch on the evolution of stars through the horizontal branch(HB). Three possible modes of helium enrichment are considered,associated with close encounters of stars in the globular clusters. Weshow that as a consequence of the variations in the core mass, as wellas in the total mass due to mass loss, the colors of horizontal branchmodels are distributed over almost the entire range of the horizontalbranch. The results are discussed in relation to a scenario for theorigin of the abundance anomalies and for the effects on the morphologyof the horizontal branch. We argue that the star-star interactions cannot only explain the source of the angular momentum of rapid rotationbut also provide a mechanism for the bimodal distribution of rotationrates in some globular clusters. We also propose the time elapsed fromthe latest core-collapse phase during the gravothermal oscillations asthe second parameter to explain the variations in HB morphology amongthe globular clusters.
|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 (18.104.22.168) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/967
|Multicolour CCD measurements of visual double and multiple stars. III|
Context: Recent CCD observations were performed in the period 1998-2004for a large sample of visual double and multiple stars selected from theHipparcos Catalogue and/or from the Gliese Catalogue of Nearby Stars. Aims: Accurate astrometric and photometric data allowing us tocharacterise the individual components are provided. These data arecompared to Hipparcos data or to data from an older epoch to assess thenature of the observed systems. Methods: We simultaneously apply aMoffat-Lorentz profile with a similar shape to all detected componentsand adjust the profile parameters from which we obtain the relativeastrometric position (epoch, position angle, angular separation) as wellas differential multi-colour photometry (filters (B)VRI). Results: Wethus acquired recent data for 71 visual systems of which 6 are orbitalbinaries, 27 are nearby, and 30 are multiple systems. In three of thesecases, the systems remained unresolved. 23 new components were detectedand measured. Two new visual double stars of intermediate separationwere also found. The estimated accuracies in relative position are0.04° and 0.01 arcsec respectively, while those in differentialphotometry are of the order of 0.01-0.02 mag in general. Conclusions:.The nature of the association of 55 systems is evaluated. New basicbinary properties are derived for 20 bound systems. Component coloursand masses are provided for two orbital binaries.Based on observations collected at the National AstronomicalObservatory, Rozhen, and the Astronomical Observatory, Belogradchik,both operated by the Institute of Astronomy, Bulgarian Academy ofSciences. Also based on data obtained by the Hipparcos astrometrysatellite. Appendix A is only available in electronic form athttp://www.aanda.org Tables 4-6 are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/641
|An Empirical Calibration of the Mixing-Length Parameter α|
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|The Puzzling Properties of the Helium White Dwarf Orbiting the Millisecond Pulsar PSR J1911-5958A in NGC 6752|
We have used phase-resolved high-resolution images and low-resolutionspectra taken at the ESO Very Large Telescope to study the properties ofthe low-mass helium white dwarf companion to the millisecond pulsar PSRJ1911-5958A (COM J1911-5958A), in the halo of the Galactic globularcluster NGC 6752. The radial velocity curve confirms that COMJ1911-5958A is orbiting the pulsar and allows us to derive a systemicvelocity of the binary system nicely in agreement with that of NGC 6752.This strongly indicates that the system is a member of the cluster,despite its very offset position (~74 core radii) with respect to thecore. Constraints on the orbital inclination (>~70°) and pulsarmass (1.2-1.5 Msolar) are derived from the mass ratioMPSR/MCOM=7.49+/-0.64 and photometric propertiesof COM J1911-5958A. The light curve in the B band shows two phases ofunequal brightening (Δmag~0.3 and 0.2, respectively) located closeto quadratures and superimposed on an almost steady baseline emission:this feature is quite surprising and needs to be further investigated.Based on observations collected at the ESO Very Large Telescope (CerroParanal, Chile), programs 071.D-0232A and 073.D-0067A, and on dataretrieved from the ESO Science Archive Facility.
|[Fe/H] relations for c-type RR Lyrae variables based upon Fourier coefficients|
[Fe/H]-φ31-P relations are found for c-type RR Lyraestars in globular clusters. The relations are analogous to that found byJurcsik & Kovács for field ab-type RR Lyrae stars, where alonger period correlates with lower metallicity values for similarvalues of the Fourier coefficient φ31. The relationsobtained here are used to determine the metallicity of field c-type RRLyrae stars, those within ωCen, the Large Magellanic Cloud andtoward the galactic bulge. The results are found to compare favourablyto metallicity values obtained elsewhere.
|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.
|Neutron-Capture Elements in the Metal-poor Globular Cluster M15|
We report on observations of six giants in the globular cluster M15 (NGC7078) using the Subaru Telescope to measure neutron-capture elementalabundances. Our abundance analyses, based on high-quality blue spectra,confirm the star-to-star scatter in the abundances of heavyneutron-capture elements (e.g., Eu), and we found no significants-process contribution to them, as was found in previous studies. Wehave found that, for the first time, there are anticorrelations betweenthe abundance ratios of light to heavy neutron-capture elements ([Y/Eu]and [Zr/Eu]) and the heavy neutron-capture elements (e.g., Eu). Thisindicates that the light neutron-capture elements in these stars cannotbe explained by only a single r-process. Another process thatcontributed significantly to the light neutron-capture elements isrequired in M15. Our results suggest a complicated enrichment historyfor M15 and its progenitor.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
|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.
|The Dynamical Mass-to-Light Ratio Profile and Distance of the Globular Cluster M15|
We construct orbit-based axisymmetric dynamical models for the globularcluster M15 that fit ground-based line-of-sight velocities and HubbleSpace Telescope line-of-sight velocities and proper motions. This allowsus to constrain the variation of the mass-to-light ratio M/L as afunction of radius in the cluster and to measure the distance andinclination of the cluster. We obtain a best-fitting inclination of60deg+/-15deg, a dynamical distance of 10.3+/-0.4kpc, and an M/L profile with a central peak. The inferred mass in thecentral 0.05 pc is 3400 Msolar, implying a central density ofat least 7.4×106 Msolar pc-3. Wecannot distinguish the nature of the central mass concentration. Itcould be an intermediate mass black hole, or it could be a large numberof compact objects, or it could be a combination. The central 4" of M15appears to contain a rapidly spinning core, and we speculate on itsorigin.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|Massive Black Hole Binaries from Collisional Runaways|
Recent theoretical work has solidified the viability of the collisionalrunaway scenario in young dense star clusters for the formation of verymassive stars (VMSs), which may be precursors to intermediate-mass blackholes (IMBHs). We present first results from a numerical study of thecollisional runaway process in dense star clusters containing primordialbinaries. Stellar collisions during binary scattering encounters providean alternate channel for runaway growth, somewhat independent of directcollisions between single stars. We find that clusters with binaryfractions >~10% yield two VMSs via collisional runaways, presentingthe exotic possibility of forming IMBH-IMBH binaries in star clusters.We discuss the implications for gravitational wave observations and theimpact on cluster structure.
|Star clusters with primordial binaries - III. Dynamical interaction between binaries and an intermediate-mass black hole|
We present the first study of the dynamical evolution of an isolatedstar cluster that combines a significant population of primordialbinaries with the presence of a central black hole. We use equal-massdirect N-body simulations, with N ranging from 4096 to 16384 and aprimordial binary ratio of 0-10 per cent; the black hole mass is about 1per cent of the total mass of the cluster. The evolution of the binarypopulation is strongly influenced by the presence of the black hole,which gives the cluster a large core with a central density cusp.Starting from a variety of initial conditions (Plummer and King models),we first encounter a phase, that last approximately 10 half-massrelaxation times, in which binaries are disrupted faster compared toanalogous simulations without a black hole. Subsequently, however,binary disruption slows down significantly due to the large core size.The dynamical interplay between the primordial binaries and the blackhole thus introduces new features with respect to the scenariosinvestigated so far, where the influence of the black hole and of thebinaries have been considered separately. A large core to half-massradius ratio appears to be a promising indirect evidence for thepresence of an intermediate-mass black hole in old globular clusters.
|Green Bank Telescope Studies of Giant Pulses from Millisecond Pulsars|
We have conducted a search for giant pulses from four millisecondpulsars using the 100 m Green Bank Telescope. Coherently dedispersedtime series from PSR J0218+4232 were found to contain giant pulses ofvery short intrinsic duration whose energies follow power-lawstatistics. The giant pulses are in phase with the two minima of theradio integrated pulse profile but are phase-aligned with the peaks ofthe X-ray profile. Historically, individual pulses more than 10-20 timesthe mean pulse energy have been deemed to be ``giant pulses.'' As only 4of the 155 pulses had energies greater than 10 times the mean pulseenergy, we argue the emission mechanism responsible for giant pulsesshould instead be defined through: (1) intrinsic timescales ofmicrosecond or nanosecond duration; (2) power-law energy statistics; and(3) emission occurring in narrow phase windows coincident with the phasewindows of nonthermal X-ray emission. Four short-duration pulses withgiant-pulse characteristics were also observed from PSR B1957+20. As theinferred magnetic fields at the light cylinders of the millisecondpulsars that emit giant pulses are all very high, this parameter haspreviously been considered to be an indicator of giant-pulse emissivity.However, the frequency of giant-pulse emission from PSR B1957+20 issignificantly lower than for other millisecond pulsars that have similarmagnetic fields at their light cylinders. This suggests that theinferred magnetic field at the light cylinder is a poor indicator of therate of emission of giant pulses.
|Using globular clusters to test gravity in the weak acceleration regime: NGC 7099|
Aims.A test of Newton's law of gravity in the low acceleration regimeusing globular clusters is presented and new results for the corecollapsed globular cluster NGC 7099 given. Methods: The run of thegravitational potential as a function of distance is probed by studyingthe velocity dispersion profile of the cluster, as derived from a set of125 radial velocities with accuracy better than 1 km s-1. Thevelocity dispersion profile is traced up to ~18 pc from the clustercenter. Results: The dispersion is found to be maximal at the center,then decrease until 10 ± 2 pc from the center, well inside thecluster tidal radius of 42 pc. After that the dispersion remainsbasically constant with an average value of 2.2 ± 0.3 kms-1. Assuming a total V mag of M(V)=-7.43 mag for NGC 7099,the acceleration at 10± 2 pc from the center is1.1+0.4-0.3τ × 10-8 cms-2, where τ is the mass-to-light ratio. Thus, for τ⪉ 2 typical of globular clusters, the flattening of the velocitydispersion profile occurs for a value of the internal acceleration ofgravity that is fully consistent with a_0=1.2× 10-8 cms-2 observed in galaxies. Conclusions: .This new result forNGC 7099 brings to 4 the clusters with velocity dispersion profileprobing acceleration below a_0. All four have been found to have a flatdispersion profile at large radii where the acceleration is below a_0,thereby mimicking elliptical galaxies qualitatively and quantitatively.Whether this indicates a failure of Newtonian dynamics in the lowacceleration limit or some more conventional dynamical effect (e.g.,tidal heating) is still unclear. However, the similarities emergingbetween very different globular clusters, as well as between globularclusters and elliptical galaxies, seem to favor the first of these twopossibilities.Based on observations collected at the European Southern Observatory,Chile (ESO Nos. 71.D-0311 and 075.D-0043). The radial velocities used inthis work are only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (188.8.131.52) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/L9
|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.
|Monte Carlo simulations of metal-poor star clusters|
Context: Metal-poor globular clusters (GCs) can provide a probe of theearliest epoch of star formation in the Universe, being the oldestobservable stellar systems. In addition, young and intermediate-agelow-metallicity GCs are present in external galaxies. Nevertheless,inferring their evolutionary status by using integrated properties maysuffer from large intrinsic uncertainty caused by the discrete nature ofstars in stellar systems, especially in the case of faint objects. Aims: In this paper, we evaluate the intrinsic uncertainty (due tostatistical effects) affecting the integrated colours and mass-to-lightratios as a function of the cluster's integrated visual magnitude(M_V^tot), which represents a directly measured quantity. We investigatethe case of metal-poor, single-burst stellar populations with age from afew million years to a likely upper value for the Galactic globularcluster ages (~15 Gyr). Methods: Our approach is based on Monte Carlotechniques for randomly generating stars distributed according to thecluster's mass function. Results: Integrated colours and mass-to-lightratios in different photometric bands are checked for good agreementwith the observational values of low-metallicity Galactic clusters; theeffect of different assumptions on the horizontal branch (HB) morphologyis shown to be irrelevant, at least for the photometric bands exploredhere. We present integrated colours and mass-to-light ratios as afunction of age for different assumptions on the cluster total Vmagnitude. We find that the intrinsic uncertainty cannot be neglected.In particular, in models with M_V^tot=-4 the broad-band colours show anintrinsic uncertainty high enough to prevent the precise age of thecluster from being evaluated. The effects of different assumptions onthe initial mass function and on the minimum mass for which carbonburning is ignited for both integrated colours and mass-to-light ratiosare also analysed. Finally, the present predictions are compared withrecent results available in the literature, showing non-negligibledifferences in some cases.Full Tables 2, 3 and 6 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/107 All the figuresare available as coloured figures in the electronic edition of theJournal.
|All Quiet in Globular Clusters|
Cataclysmic variables (CVs) should be present in large numbers inglobular clusters (GCs). Numerous low-luminosity X-ray sourcesidentified over the past few years as candidate CVs in GCs support thisnotion. Yet, very few ``cataclysms,'' the characteristic feature of thisclass of objects in the field, have been observed in GCs. We addressthis discrepancy here, within the framework of the standard diskinstability model for CV outbursts. We argue that the paucity ofoutbursts in GCs is probably not a direct consequence of the donors' lowmetallicities. We present diagnostics based on outburst propertiesallowing tests of the hypothesis that rare cataclysms are entirely dueto lower mass transfer rates in GCs relative to the field, and we argueagainst this explanation. Instead, we propose that a combination of lowmass transfer rates (>~1014-1015 gs-1) and moderately strong white dwarf magnetic moments(>~1030 G cm3) stabilize CV disks in GCs andthus prevent most of them from experiencing frequent outbursts. If it isso, rare cataclysms in GCs would signal important evolutionarydifferences between field and cluster CVs.
|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.
|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.
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