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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.
| 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.
| 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.
| Na-O anticorrelation and horizontal branches. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218
We used the multifiber spectrograph FLAMES on the ESO Very LargeTelescope UT2 to derive atmospheric parameters, metallicities andabundances of O and Na for 79 red giant stars in the Galactic globularcluster NGC 6218 (M 12). We analyzed stars in the magnitude range fromabout 1 mag below the bump to the tip of the Red Giant Branch. Theaverage metallicity we derive is [Fe/H]=-1.31± 0.004±0.028 dex (random and systematic errors, respectively), with a verysmall star-to-star scatter (rms=0.033 dex), from moderatelyhigh-resolution Giraffe spectra. This is the first extensivespectroscopic abundance analysis in this cluster. Our results indicatethat NGC 6218 is very homogeneous as far as heavy elements areconcerned. On the other hand, light elements involved in the well knownproton-capture reactions of H-burning at high temperature, such as O andNa, show large variations, anticorrelated with each other, at allluminosities along the red giant branch. The conclusion is that the Na-Oanticorrelation must be established in early times at the clusterformation. We interpret the variation of Na found near the RGB-bump asthe effect of two distinct populations having different bumpluminosities, as predicted for different He content. To our knowledge,NGC 6218 is the first GC where such a signature has beenspectroscopically detected, when combined with consistent andhomogeneous data obtained for NGC 6752 to gain in statisticalsignificance.Based on observations collected at ESO telescopes under programme073.D-0211. Full Tables 2, 3, and 5 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/939
| 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.
| 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.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| 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 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.
| The ultra-compact binary 4U 1850-087 observed with INTEGRAL: hard X-ray emission from an X-ray burster
Context: .The X-ray burster 4U 1850-087, located in the Galacticglobular cluster NGC 6712, is an ultracompact binary (orbital period ~21min), likely harbouring a degenerate companion. Aims: .The sourcehas been observed at soft γ-rays several times with the INTEGRALsatellite, during the monitoring of the Galactic plane, with anunprecedented exposure time. We analysed all available INTEGRALobservations, with the main aim of studying the long-term behaviour ofthis Galactic bulge X-ray burster. Methods: .The spectral resultsare based on the systematic analysis of all INTEGRAL observationscovering the source position performed between March 2003 and November2005. Results: .The source X-ray emission is hard and is observed,for the first time, up to 100 keV. A broad-band spectrum obtainedcombining the INTEGRAL spectrum together with a quasi-simultaneousXMM-Newton observation performed in September 2003 is well modeled witha disk-blackbody emission (with an inner disk temperature of ~0.8 keV)together with a power-law (with a photon index, Γ, of 2.1). The2-100 keV luminosity is 1.5×1036 erg s-1(assuming a distance of 6.8 kpc). Conclusions: .INTEGRALobservations reveal for the first time that this X-ray burster displaysa very hard X-ray spectrum, with a cut-off at energies higher than 100keV, and that the source spends most of the time in this low luminosityand hard state. Indeed, a previous BeppoSAX observation in April 1997observed high energy emission from 4U 1850-087 only up to 50 keV.
| Global fitting of globular cluster age indicators
Context: .Stellar models and the methods for the age determinations ofglobular clusters are still in need of improvement. Aims: .Weattempt to obtain a more objective method of age determination based oncluster diagrams, avoiding the introduction of biases due to thepreference of one single age indicator. Methods: .We compute newstellar evolutionary tracks and derive the dependence of age indicatingpoints along the tracks and isochrone - such as the turn-off or bumplocation - as a function of age and metallicity. The same criticalpoints are identified in the colour-magnitude diagrams of globularclusters from a homogeneous database. Several age indicators are thenfitted simultaneously, and the overall best-fitting isochrone isselected to determine the cluster age. We also determine thegoodness-of-fit for different sets of indicators to estimate theconfidence level of our results. Results: .We find that ourisochrones provide no acceptable fit for all age indicators. Inparticular, the location of the bump and the brightness of the tip ofthe red giant branch are problematic. On the other hand, the turn-offregion is very well reproduced, and restricting the method to indicatorsdepending on it results in trustworthy ages. Using an alternative set ofisochrones improves the situation, but neither leads to an acceptableglobal fit. Conclusions: .We conclude that evolutionary tracks oflow-mass metal-poor stars are far from reproducing all aspects ofglobular cluster colour-magnitude diagrams and that the determination ofcluster ages still depends on the favourite method or indicator chosen.
| The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
| Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.
| Globular cluster system and Milky Way properties revisited
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
| Why is the mass function of NGC 6218 flat?
We have used the FORS-1 camera on the VLT to study the main sequence(MS) of the globular cluster NGC 6218 in the V and R bands. Theobservations cover an area of 3.4 arcmin × 3.4 arcmin around thecluster centre and probe the stellar population out to the cluster'shalf-mass radius (rh ≃ 2.2 arcmin). Thecolour-magnitude diagram (CMD) that we derive in this way reveals anarrow and well defined MS extending down to the 5 σ detectionlimit at V ≃ 25, or about 6 magnitudes below the turn-off,corresponding to stars of 0.25 {M}_ȯ. The luminosity function (LF)obtained with these data shows a marked radial gradient, in that theratio of lower- and higher-mass stars increases monotonically withradius. The mass function (MF) measured at the half-mass radius, and assuch representative of the cluster's global properties, is surprisinglyflat. Over the range 0.4{-}0.8 {M}_ȯ, the number of stars per unitmass follows a power-law distribution of the type dN/dm ∝m0, where, for comparison, Salpeter's IMF would be dN/dm∝ m-2.35. We expect that such a flat MF does notrepresent the cluster's IMF but is the result of severe tidal strippingof the stars from the cluster due to its interaction with the Galaxy'sgravitational field. Our results cannot be reconciled with thepredictions of recent theoretical models that imply a relativelyinsignificant loss of stars from NGC 6218 as measured by its expectedvery long time to disruption. They are more consistent with the orbitalparameters based on the Hipparcos reference system that imply a muchhigher degree of interaction of this cluster with the Galaxy thanassumed by those models. Our results indicate that, if the orbit of acluster is known, the slope of its MF could be useful in discriminatingbetween the various models of the Galactic potential.
| 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.
| UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| An Ultracompact X-Ray Binary in the Globular Cluster M15 (NGC 7078)
We have used the Advanced Camera for Surveys on board the Hubble SpaceTelescope to image the core of the globular cluster M15 in thefar-ultraviolet (FUV) wave band. Based on these observations, weidentify the FUV counterpart of the recently discovered low-mass X-raybinary M15 X-2. Our time-resolved FUV photometry shows a modulation with0.062+/-0.004 mag semiamplitude, and we clearly detect a period of22.5806+/-0.0002 minutes. We have carried out extensive Monte Carlosimulations, which show that the signal is consistent with beingcoherent over the entire observational time range of more than 3000cycles. This strongly suggests that it represents the orbital period ofthe binary system. M15 X-2 is FUV-bright (FUV~=17 mag) and ischaracterized by an extremely blue spectral energy distribution(Fλ~λ-2.0). We also find evidencefor an excess of flux between 1500 and 1600 Å and probably between1600 and 2000 Å that might be due to C IV 1550 Å and He II1640 Å emission lines. We also show that M15 X-2's X-rayluminosity can be powered by accretion at the rate expected for masstransfer driven by gravitational waves at this binary period. Theobserved FUV emission appears to be dominated by an irradiated accretiondisk around the neutron star primary, and the variability can beexplained by irradiation of the low-mass white dwarf donor if theinclination of the system is ~34°. We conclude that allobservational characteristics of M15 X-2 are consistent with its beingan ultracompact X-ray binary, only the third confirmed such object in aglobular cluster.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 NAS 5-26555.
| Low-energy absorption towards the ultra-compact X-ray binary 4U 1850-087 located in the globular cluster NGC 6712
We report the results of two XMM-Newton observations of theultra-compact low-mass X-ray binary 4U 1850-087 located inthe galactic globular cluster NGC 6712. A broad emission featureat 0.7 keV was detected in an earlier ASCA observation and explained asthe result of an unusual Ne/O abundance ratio in the absorbing materiallocal to the source. We find no evidence for this feature and deriveNe/O ratios in the range 0.14-0.21, consistent with that of theinterstellar medium. During the second observation, when the source was~10% more luminous, there is some evidence for a slightly higher Ne/Oratio and additional absorption. Changes in the Ne/O abundance ratiohave been detected from another ultra-compact binary,4U 1543-624. We propose that these changes result from anX-ray-induced wind which is evaporated from an O and Ne rich degeneratedonor. As the source X-ray intensity increases so does the amount ofevaporation and hence the column densities and abundance ratio of Ne andO.
| X-Ray Spectroscopy of Candidate Ultracompact X-Ray Binaries
We present high-resolution spectroscopy of the neutron star/low-massX-ray binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of ourcontinuing study of known and candidate ultracompact binaries. The LMXB4U 1850-087 is one of four systems in which we had previously inferredan unusual Ne/O ratio in the absorption along the line of sight, mostlikely from material local to the binaries. However, our recent ChandraX-Ray Observatory LETGS spectrum of 4U 1850-087 finds a Ne/O ratio bynumber of 0.22+/-0.05, smaller than previously measured and consistentwith the expected interstellar value. We propose that variations in theNe/O ratio due to source variability, as previously observed in thesesources, can explain the difference between the low- and high-resolutionspectral results for 4U 1850-087. Our XMM-Newton RGS observation of 4U0513-40 also shows no unusual abundance ratios in the absorption alongthe line of sight. We also present spectral results from a thirdcandidate ultracompact binary, 4U 1822-000, whose spectrum is well fitby an absorbed power-law + blackbody model with absorption consistentwith the expected interstellar value. Finally, we present thenondetection of a fourth candidate ultracompact binary, 4U 1905+000,with an upper limit on the source luminosity of<1×1032 ergs s-1. Using archival data, weshow that the source has entered an extended quiescent state.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| 4U 1715-390 and NGC 6440 in Outburst
RXTE Proportional Counter Array scans of the galactic bulge region havebeen continuing. The burster in the globular cluster NGC 6440 (MXB1746-20) appears to be entering outburst. The previous outburst was inOctober 2001. The 2-10 keV fluxes on May 12.6, 15.6 and 16.1 were 5.4+/- 1.0, 7 +/- 3 and 21 +/- 1 mCrab, respectively. We appreciateinquiries by J. in 't Zand and R. Wijnands about this source.
| Formation of Ultracompact X-Ray Binaries in Dense Star Clusters
Bright, ultracompact X-ray binaries observed in dense star clusters,such as Galactic globular clusters, must have formed relativelyrecently, since their lifetimes as persistent bright sources are short(e.g., ~108 yr above 1036 ergs s-1 fora 1.4 Msolar neutron star accreting from a degenerate heliumcompanion with an initial mass of ~0.2 Msolar). Therefore, wecan use the present conditions in a cluster core to study possibledynamical formation processes for these sources. Here we show thatdirect physical collisions between neutron stars and red giants canprovide a sufficient formation rate to explain the observed numbers ofbright sources. These collisions produce tight, eccentric neutronstar-white dwarf binaries that decay to contact by gravitationalradiation on timescales ~106-1010 yr, usuallyshorter and often much shorter than the cluster age.
| Infrared Photometry of NGC 6791
We present deep JHK photometry of the old and metal-rich open clusterNGC 6791. The photometry reaches below the main-sequence turnoff toK~16.5 mag. We combine our photometry with that from Stetson et al. toprovide color-magnitude diagrams showing K versus J-K, K versus V-K, andV versus V-K. We study the slope of the red giant branch in the infraredbut find that it is not a useful metallicity indicator for the cluster,nor any metal-rich cluster that lacks a well-populated red giant branch,because it is not linear, as has often been assumed, in K versus J-K.The mean color of the red horizontal-branch/red clump stars provide anestimate of the cluster reddening, E(B-V)=0.14+/-0.04 mag for[Fe/H]=+0.4+/-0.1. The mean magnitudes of these stars also provide agood distance estimate, (m-M)0=13.07+/-0.04. Finally, we findthat the isochrones of Yi et al. provide optimal fits in V versus B-Vand V-K and K versus J-K and V-K for such values if [Fe/H] lies between+0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyr([Fe/H]=+0.3) and 7.5 Gyr ([Fe/H]=+0.5).Based on observations made with the Mayall 4 m Telescope of the NationalOptical Astronomy Observatory.
| Creating ultra-compact binaries in globular clusters through stable mass transfer
A binary in which a slightly evolved star starts mass transfer to aneutron star can evolve towards ultra-short orbital periods under theinfluence of magnetic braking. This is called magnetic capture. Weinvestigate in detail for which initial orbital periods and initialdonor masses binaries evolve to periods less than 30-40 min within theHubble time. We show that only small ranges of initial periods andmasses lead to ultra-short periods, and that for those only a small timeinterval is spent at ultra-short periods. Consequently, only a verysmall fraction of any population of X-ray binaries is expected to beobserved at ultra-short period at any time. If 2 to 6 of the 13 brightX-ray sources in globular clusters have an ultra-short period, assuggested by recent observations, their formation cannot be explained bythe magnetic capture model.
| Chemical composition and evolutionary status of nine UV-bright stars in five globular clusters from VLT/UVES spectra
We have derived the chemical composition of nine UV-bright starsbelonging to five Galactic globular clusters of various metallicities([Fe/H] from -1.0 to -2.4 dex). The analyses are based on highresolution spectra obtained with the UV-Visual Echelle Spectrograph(UVES) at VLT-UT2. The evolutionary status of the stars is assessed fromthe chemical analysis and location in the H-R diagram. The star ID7 inNGC 5986 is confirmed as a bona fide post-asymptotic giant branch star(post-AGB) whereas the high-luminosity star ID6 has probably left theAGB before the third dredge-up. ZNG 1 in NGC 6712 shows an overabundanceof sodium, oxygen, and silicon similar to overabundances we find in theUV-bright star ID6 in NGC 5986; both stars could be in a post-early-AGB(PEAGB) phase of evolution. The UV-bright star ZNG 7 in NGC 6218 seemsto be an AGB star. The stars V-4 and ZNG 5 in NGC 6656 are in apost-horizontal-branch phase of evolution, with V-4 being significantlyoverabundant in heavy elements. The origin of these overabundances isdiscussed in the context of the evolutionary versus primordial scenario.The three UV-bright stars K 260, K 996 and K 1082 observed in the verymetal-deficient globular cluster NGC 7078 are post-horizontal-branchstars, one of them being slightly enriched in s-elements but with aluminosity too low for third dredge-up to have occured. The abundancepatterns of K 1082 in NGC 7078 seem to indicate the presence of milddiffusion and a radiative levitation process, already reported in theblue HB stars of M 13 (Behr et al. \cite{Behr1999}, ApJ, 517, L135) andNGC 6752 (Moehler et al. \cite{Moehler1999}, A&A, 339, 537).Based on data collected at Paranal Observatory (ESO, Chile), programidentifier ID 69.D-0081.Full Table \ref{abund} 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/423/353Appendix is only available in electronic form athttp://www.edpsciences.org
| Models for the Gravitational Field of the Galactic Bar: An Application to Stellar Orbits in the Galactic Plane and Orbits of Some Globular Clusters
We built three models for the gravitational field of the Galactic bar.These models are an inhomogeneous ellipsoid, an inhomogeneous prolatespheroid, and a superposition of four inhomogeneous ellipsoids. Amongthe three models, the superposition provides our best approximation tothe observed boxy mass distribution of the Galactic bar. Adding the barcomponent to an axisymmetric Galactic model, we have calculated stellarmidplane orbits and orbits of some globular clusters with knownkinematical data. For all models we find a secular dispersion effect onthe orbital energy and angular momentum, as measured in the Galacticinertial frame. This effect might be relevant to explain the orbitalprograde-retrograde distribution of globular clusters. For the stellarkinematics, we study the connection between the sense of orbital motionin the midplane and the onset of chaos in the presence of the bar. Inthe inner region of the bar, chaos is induced by an axisymmetric centralcomponent (bulge), and it arises in orbits that change its orbital sensefrom prograde to retrograde and vice versa as seen from an inertialreference frame. Outside the bar region, chaos appears only in progradeorbits. Our results concerning such a connection are consistent andextend those obtained for midplane orbits in the presence of only aspiral pattern in the axisymmetric Galactic model.
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