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Galactic Orbits of Globular Clusters in a Barred Galaxy
We study the effect of a bar in the galactic orbits of forty-fiveglobular clusters whose absolute proper motions are known. The orbitalcharacteristics of the orbits are compared with those obtained for thecase of an axisymmetric galactic potential. Tidal radii are computed anddiscussed for both cases.
| Scorpius the Winter-Bug.
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| Spectroscopic Search for Binaries among Ehb Stars in Globular Clusters
We performed a spectroscopic search for binaries among hot horizontalbranch stars in globular clusters. We present final results for a sampleof 51 stars in NGC 6752 and preliminary results for the first 15 starsanalyzed in M 80. The observed stars are distributed along all the HBsin the range 8000 ≤ Teff ≤ 32 000 K, and have beenobserved during four nights. Radial velocity variations were measuredwith the cross-correlation technique. We analyzed the statistical andsystematic errors associated with the measurements in order to evaluatethe statistical significance of the observed variations. No close binarysystem has been detected, neither among cooler stars nor among thesample of hot EHB stars (18 stars with Teff≥ 22 000 K inNGC 6752). The data corrected for instrumental effects indicate that theradial velocity variations are always below the 3σ level of 15 km s-1. These results are in sharp contrast with thosefound for field hot subdwarfs, and open new questions about theformation of EHB stars in globular clusters and possibly of the fieldsubdwarfs.
| Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process
We use high-resolution spectra obtained with the ESO Very LargeTelescope to measure surface abundance patterns of 43 blue stragglerstars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs showsa significant depletion of carbon and oxygen with respect to thedominant population. This evidence would suggest the presence of CNOburning products on the BSS surface coming from a deeply peeled parentstar, as expected in the case of a mass transfer process. This is thefirst detection of a chemical signature clearly pointing to a specificBSS formation process in a globular cluster.Based on observations collected at the ESO-VLT (Cerro Paranal, Chile)under program 072.D-0337.
| Dynamical Formation of Close Binaries in Globular Clusters: Cataclysmic Variables
We answer the long-standing question of which production mechanism isresponsible for the cataclysmic variables (CVs) in globular clusters.Arguments have been given that range from mostly primordial presence toa significant contribution of later dynamical formation in close stellarencounters. We conclude, based on a thorough analysis of a homogeneousChandra data set, that the majority of CVs in dense globular clustershave a dynamical origin.
| The Pure Noncollisional Blue Straggler Population in the Giant Stellar System ω Centauri
We have used high spatial resolution data from the Hubble SpaceTelescope (HST) and wide-field ground-based observations to search forblue straggler stars (BSSs) over the entire radial extent of the largestellar system ω Centauri. We have detected the largest populationof BSSs ever observed in any stellar system. Even though the sample isrestricted to the brightest portion of the BSS sequence, more than 300candidates have been identified. BSSs are thought to be produced by theevolution of binary systems (formed either by stellar collisions or massexchange in binary stars). Since systems like Galactic globular clusters(GGCs) and ω Cen evolve dynamically on timescales significantlyshorter than their ages, binaries should have settled toward the center,showing a more concentrated radial distribution than the ordinary, lessmassive single stars. Indeed, in all GGCs that have been surveyed forBSSs, the BSS distribution is peaked at the center. Conversely, inω Cen we find that the BSSs share the same radial distribution asthe adopted reference populations. This is the cleanest evidence everfound that such a stellar system is not fully relaxed even in thecentral region. We further argue that the absence of centralconcentration in the BSS distribution rules out a collisional origin.Thus, the ω Cen BSSs are the purest and largest population ofnoncollisional BSSs ever observed. Our results allow the first empiricalquantitative estimate of the production rate of BSSs via this channel.BSSs in ω Cen may represent the best local template for modelingthe BSS populations in distant galaxies where they cannot beindividually observed.Based on observations with the NASA/ESA HST, obtained at the SpaceTelescope Science Institute, which is operated by AURA, Inc., under NASAcontract NAS5-26555. Also based on WFI observations collected at theEuropean Southern Observatory, La Silla, Chile, within the observingprograms 62.L-0354 and 64.L-0439.
| 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.
| Tramp Classical Novae as Tracers of Intergalactic Stars
Simulations predict that collisions between galaxies must liberate starsinto intergalactic space. The stripping of a galaxy's stars by thepotential of a cluster in which it resides must also occur. Thisprediction is verified by the detections of classical novae, red giants,and planetary nebulae between the galaxies of the Virgo and FornaxClusters. These tracers suggest a tramp stellar component of 10%-40% ofthe cluster baryonic mass. I point out that classical novae can usefullyextend these results to the 250,000 Mpc3 of intergalacticspace outside of galaxy clusters surrounding the Local Group. This isbecause individual novae are well-understood standard candles, withlight curves and spectra that are distinct from all other astrophysicalphenomena. In addition, the frequency of nova outbursts in any givengalaxy is measured to be directly proportional to that galaxy's K-bandluminosity (and independent of its Hubble type). Thus, intergalacticnovae should be excellent tracers of the fraction of stars liberatedfrom galaxies over the past 13 Gyr. Pan-STARRS, the Large SynopticSurvey Telescope (LSST), and other large-area synoptic survey telescopeswill begin to regularly discover tramp classical novae out to 20-40 Mpcin the coming decade. I estimate the expected discovery rates withLSST-like surveys to be hundreds of intergalactic tramp novae per year,and suggest survey parameters to optimize detections of these tramps.
| The COMPLETE Survey of Star-Forming Regions: Phase I Data
We present an overview of data available for the Ophiuchus and Perseusmolecular clouds from Phase I of the COMPLETE Survey of Star-FormingRegions. This survey provides a range of data complementary to theSpitzer Legacy Program ``From Molecular Cores to Planet Forming Disks.''Phase I includes the following: extinction maps derived from the TwoMicron All Sky Survey (2MASS) near-infrared data using the NICERalgorithm; extinction and temperature maps derived from IRAS 60 and 100μm emission; H I maps of atomic gas; 12CO and13CO maps of molecular gas; and submillimeter continuumimages of emission from dust in dense cores. Not unexpectedly, themorphology of the regions appears quite different depending on thecolumn density tracer that is used, with IRAS tracing mainly warmer dustand CO being biased by chemical, excitation, and optical depth effects.Histograms of column density distribution are presented, showing thatextinction as derived from 2MASS NICER gives the closest match to alognormal distribution, as is predicted by numerical simulations. Allthe data presented in this paper, and links to more detailedpublications on their implications, are publicly available at theCOMPLETE Web site.
| The Dynamical State and Blue Straggler Population of the Globular Cluster NGC 6266 (M62)
We have used a proper combination of multiband high-resolution HubbleSpace Telescope WFPC2 and wide-field ground-based observations to imagethe Galactic globular cluster NGC 6266 (M62). The extensive photometricdata set allows us to determine the center of gravity and to constructthe most extended radial profile ever published for this clusterincluding, for the first time, detailed star counts in the very innerregion. The star density profile is well reproduced by a standard Kingmodel with an extended core (~19") and a modest value of theconcentration parameter (c=1.5), indicating that the cluster has not yetexperienced core collapse. The millisecond pulsar population (whosemembers are all in binary systems) and the X-ray-emitting population(more than 50 sources within the cluster half-mass radius) suggest thatNGC 6266 is in a dynamical phase particularly active in generatingbinaries through dynamical encounters. UV observations of the centralregion have been used to probe the population of blue straggler stars,whose origin might be also affected by dynamical interactions. Thecomparison with other globular clusters observed with a similar strategyshows that the blue straggler content in NGC 6266 is relatively low,suggesting that the formation channel that produces binary systemshosting neutron stars or white dwarfs is not effective in significantlyincreasing the blue straggler population. Moreover, an anticorrelationbetween millisecond pulsar content and blue straggler specific frequencyin globular clusters seems to be emerging with increasing evidence.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555. Also based on Wide Field Imagerobservations collected at the European Southern Observatory (ESO), LaSilla, Chile, within the observing programs 62.L-0354 and 64.L-0439.
| Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
| Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| Chandra observations of the globular cluster M 54
We have carried out a Chandra observation of the globular cluster M 54.We detected 7 sources located within the half-mass radius of M 54, at aflux limit of 1.5 × 10-15 erg s-1cm-2 in the 0.3-8 keV energy band. The spatial distributionand the colour/spectral properties of the 7 sources suggest that theyare likely to be cataclysmic variables or LMXBs in the globular cluster.M 54 shows the largest number of X-ray sources with luminosities greaterthan 1032 erg s-1 compared to other globularclusters observed using Chandra and XMM-Newton. We searched for acorrelation between the number of sources above this luminosity levelwith globular cluster parameters. We found evidence that the number ofsources peaks at a King concentration parameter c ~ 1.7-1.9, withglobular clusters which are core-collapsed or have low-c values having asmaller number of sources. We speculate on possible reasons for this.
| XMM-Newton X-ray and optical observations of the globular clusters M 55 and NGC 3201
We have observed two low concentration Galactic globular clusters withthe X-ray observatory XMM-Newton. We detect 47 faint X-ray sources inthe direction of M 55 and 62 in the field of view ofNGC 3201. Using the statistical Log N - Log Srelationship of extragalactic sources derived from XMM-Newton LockmanHole observations, to estimate the background source population, weestimate that very few of the sources (1.5±1.0) in the field ofview of M 55 actually belong to the cluster. Thesesources are located in the centre of the cluster as we expect if thecluster has undergone mass segregation. NGC 3201 hasapproximately 15 related sources, which are centrally located but arenot constrained to lie within the half mass radius. The sourcesbelonging to this cluster can lie up to 5 core radii from the centre ofthe cluster which could imply that this cluster has been perturbed.Using X-ray (and optical, in the case of M 55)colours, spectral and timing analysis (where possible) and comparingthese observations to previous X-ray observations, we find evidence forsources in each cluster that could be cataclysmic variables, activebinaries, millisecond pulsars and possible evidence for a quiescent lowmass X-ray binary with a neutron star primary, even though we do notexpect any such objects in either of the clusters, due to their lowcentral concentrations. The majority of the other sources are backgroundsources, such as AGN.
| Cluster Ages Experiment (CASE): Detection of a dwarf nova in the globular cluster M55
We report the detection of a dwarf nova (DN) in the core region of theglobular cluster M55. Six outbursts were observed during eight observingseasons spanning the period 1997-2004. The variable has an X-raycounterpart detected on images taken with the ROSAT telescope. Althoughwe cannot offer proof of cluster membership, one can see that both theposition on the Hertzsprung-Russell diagram and the X-ray flux areconsistent with a bright DN at the cluster distance. According to ouroutburst statistics, no more than one similar DN could remain undetectedin our field of view, centred at the cluster core.
| 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.
| 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.
| Cataclysmic and Close Binaries in Star Clusters. V. Erupting Dwarf Novae, Faint Blue Stars, X-Ray Sources, and the Classical Nova in the Core of M80
Large populations of cataclysmic variables (CVs) in globular clustershave long been predicted, but the number of absolutely certain clusterCVs known in globular clusters is still less than 10. HST and Chandraobservers have recently found dozens of CV candidates in severalpopulous globular clusters. Confirmation and characterization of thesecandidates are extremely difficult; thus, identification of unambiguousCVs remains important. We have searched all archival HST images of thedense globular cluster M80 for erupting dwarf novae (DNe) and to checkthe outburst behaviors of two very blue objects first identified adecade ago. Two new erupting DNe were found in eight searched epochs,making M80 a record holder for erupting DNe. The quiescent classicalnova in M80 varies by no more than a few tenths of a magnitude ontimescales of minutes to years, and a similar faint, blue object variesby a similar amount. Simulations and completeness tests indicate thatthere are at most three erupting DNe like SS Cyg and at most nine UGem-like DNe in M80. Either this very dense cluster contains about anorder of magnitude fewer CVs than theory predicts, or most M80 CVs areextremely faint and/or erupt very infrequently like WZ Sge. We havedetected a sequence of 54 objects running parallel to the main sequenceand several tenths of a magnitude blueward of it. These blue objects aresignificantly more centrally concentrated than the main-sequence stars,but not as centrally concentrated as the blue stragglers. We suggestthat these objects are white dwarf-red dwarf binaries and that some arethe faint CV population of M80.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedfrom the Data Archive at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555. These observations are associatedwith program 6460.
| Which Globular Clusters Contain Intermediate-Mass Black Holes?
It has been assumed that intermediate-mass black holes (IMBHs) inglobular clusters can only reside in the most centrally concentratedclusters, with a so-called core-collapsed density profile. While thiswould be a natural guess, it is in fact wrong. We have followed theevolution of star clusters containing IMBHs with masses between125<=MBH<=1000 Msolar through detailedN-body simulations, and we find that a cluster with an IMBH, inprojection, appears to have a relatively large ``core'' with surfacebrightness only slightly rising toward the center. This makes it highlyunlikely that any of the ``core-collapsed'' clusters will harbor anIMBH. On the contrary, the places to look for an IMBH are those clustersthat can be fitted well by medium-concentration King models. Thevelocity dispersion of the visible stars in a globular cluster with anIMBH is nearly constant well inside the apparent core radius. For acluster of mass MC containing an IMBH of mass MBH,the influence of the IMBH becomes significant only at a fraction2.5MBH/MC of the half-mass radius, deep within thecore, where it will affect only a small number of stars. In conclusion,observational detection of an IMBH may be possible, but will bechallenging.
| A Study of Far-Ultraviolet Extinction in the Upper Scorpius Cloud Using the SPINR Sounding Rocket Experiment
In this study, six new interstellar extinction curves in thefar-ultraviolet are presented using data from a sounding rocketexperiment. The sounding rocket data were combined with IUE data for sixlines of sight in the Upper Scorpius group to cover the wavelength rangeof 912-3030 Å. The extinction curves were produced using the paircomparison method with B stars of similar spectral types.Parameterizations from Fitzpatrick & Massa, Cardelli et al., andFitzpatrick were then fitted to the derived extinction curves. From thederived extinction curves, their corresponding fits, and the dust modelof Weingartner & Draine, it is concluded that the dust population inthe Upper Scorpius region exhibits a larger than average grainpopulation with a depletion of smaller grains.
| A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies
We have compiled a sample of globular clusters with high-quality stellarabundances from the literature to compare to the chemistries of stars inthe Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clustersexamined, 29 also have kinematic information. Most of the globularclusters belong to the Galactic halo; however, a significant number havedisk kinematics or belong to the bulge. Focusing on the [α/Fe] andlight r-process element ratios, we find that most globular cluster starsmimic field stars of similar metallicities, and neither clearlyresembles the currently available stellar abundances in dwarf galaxies(including globular clusters in the Large Magellanic Cloud). Theexceptions to these general elemental ratio comparisons are alreadyknown in the literature, e.g., ω Centauri, Palomar 12, and Terzan7 associated with the Sagittarius remnant and Ruprecht 106, which has ahigh radial velocity and low [α/Fe] ratio. A few other globularclusters show more marginal peculiarities. The most notable one is thehalo cluster M68, which has a high galactocentric rotational velocity, aslightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratiosdecrease with increasing [Fe/H] at intermediate metallicities, which isconsistent with very massive stars playing a larger role in the earlychemical evolution of the Galaxy. The chemical similarities betweenglobular clusters and field stars with [Fe/H]<=-1.0 suggests a sharedchemical history in a well-mixed early Galaxy. The differences in thepublished chemistries of stars in the dwarf spheroidal galaxies suggestthat neither the globular clusters, halo stars, nor thick disk stars hadtheir origins in small isolated systems like the present-day Milky Waydwarf satellites.
| Galactic Globular Cluster Relative Ages
We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the differencebetween the horizontal branch and the turnoff in two internallyphotometrically homogeneous databases. The mutual consistency of the twodata sets has been assessed by comparing the ages of 16 globularclusters in common between the two databases. We have also investigatedthe consistency of our relative age determination within the recentstellar model framework. All clusters with [Fe/H]<-1.7 are found tobe old and coeval, with the possible exception of two objects, which aremarginally younger. The age dispersion for the metal-poor clusters is0.6 Gyr (rms), consistent with a null age dispersion.Intermediate-metallicity clusters (-1.7<[Fe/H]<-0.8) are onaverage 1.5 Gyr younger than the metal-poor ones, with an age dispersionof 1.0 Gyr (rms) and a total age range of ~3 Gyr. About 15% of theintermediate-metallicity clusters are coeval with the oldest clusters.All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the mostmetal-poor ones, with a relatively small age dispersion, although themetal-rich sample is still too small to allow firmer conclusions. Thereis no correlation of the cluster age with the galactocentric distance.We briefly discuss the implication of these observational results forthe formation history of the Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555, and on observations made at the European SouthernObservatory, La Silla, Chile, and with the Isaac Newton GroupTelescopes.
| Helium self-enrichment in globular clusters and the second parameter problem in M 3 and M 13
Inspection of the CM diagrams of globular clusters having similar heavyelement content shows that the luminosity of the red giant bump relativeto the turnoff (Δ V_TO^bump) differs by more than 0.1 mag betweenclusters with different horizontal branch morphology. Unfortunately,careful consideration of the data leaves us with only one pair (M 3 andM 13) of clusters suitable for a quantitative discussion. For this pairwe consider differences in age and helium content as possible causes forthe difference in Δ V_TO^bump, and find more convincing supportfor the latter. A larger helium content in M 13 stars (Y 0.28 vs.Y 0.24) accounts for various CM diagram features, such as thedifference in the luminosity level of RR Lyr variables and of the redgiant bump with respect to the turnoff luminosity and the horizontalbranch morphology. This enhanced helium can be tentatively understood inthe framework of self-enrichment by massive asymptotic giant branchstars in the first ~100 Myr of the cluster life. A modestself-enrichment can be present also in M 3 and can be the reason for thestill unexplained presence of a not negligible number of luminous,Oosterhoff II type RR Lyr variables. The hypothesis that a larger heliumcontent is the second parameter for clusters with very blue horizontalbranch morphology could be checked by an accurate set of data for moreclusters giving turnoff, RR Lyrs and bump magnitudes within a uniquephotometry.
| Using X-rays to Probe the Compact Binary Content of Globular Clusters
Globular clusters (GCs) harbour a large number of close binaries whichare hard to identify optically due to high stellar densities. Observingthese GCs in X-rays, in which the compact binaries are bright,diminishes the over-crowding problem. Using the new generation of X-rayobservatories, it is possible to identify populations of neutron starlow mass X-ray binaries, cataclysmic variables and millisecond pulsarsas well as other types of binaries. We present the spectra of a varietyof binaries that we have identified in four GCs observed by XMM-Newton.We show that through population studies we can begin to understand theformation of individual classes of binaries in GCs and hence start tounfold the complex evolutionary paths of these systems.
| Optical and X-ray observations of the neutron star soft X-ray transient XTE J1709-267
In this paper we report on the discovery of the optical counterpart tothe neutron star soft X-ray transient (SXT) XTE J1709-267 at an R-bandmagnitude of R= 20.5 +/- 0.1 and 22.24 +/- 0.03, in outburst andquiescence, respectively. We further report the detection of type IX-ray bursts in RXTE data obtained during an outburst of the source in2002. These bursts show a precursor before the onset of the main burstevent, reminiscent of photospheric radius expansion bursts. Siftingthrough the archival RXTE data for the burster 4U 1636-53, we found anearly identical burst with precursor in 4U 1636-53. A comparison ofthis burst to true photospheric radius expansion bursts in 4U 1636-53leads us to conclude that these bursts-with-precursor do not reach theEddington limit. Nevertheless, from the burst properties we can derivethat the distance to XTE J1709-267 is consistent with the distance ofthe Globular Cluster NGC 6293. We further report on the analysis of a22.7 ks observation of XTE J1709-267 obtained with the Chandra satellitewhen the source was in quiescence. We found that the source has a softquiescent spectrum which can be fit well by an absorbed black body orneutron star atmosphere model. A power law contributes less than ~20 percent to the 0.5-10 keV unabsorbed flux of (1.0 +/- 0.3) ×10-13 erg cm-2 s-1. This flux is onlyslightly lower than the flux measured right after the outburst in 2002.This is in contrast to the recent findings for MXB 1659-29, where thequiescent source flux decreased gradually by a factor of ~7-9 over aperiod of 18 months. Finally, we compared the fractional power-lawcontribution to the unabsorbed 0.5-10 keV luminosity for neutron starSXTs in quiescence for which the distance is well-known. We find thatthe power-law contribution is low only when the source quiescentluminosity is close to ~1-2 × 1033 erg s-1.Both at higher and lower values the power-law contribution to the 0.5-10keV luminosity increases. We discuss how models for the quiescent X-rayemission can explain these trends.
| Stellar collisions during binary-binary and binary-single star interactions
Physical collisions between stars occur frequently in dense starclusters, either via close encounters between two single stars, orduring strong dynamical interactions involving binary stars. Here westudy stellar collisions that occur during binary-single andbinary-binary interactions, by performing numerical scatteringexperiments. Our results include cross-sections, branching ratios andsample distributions of parameters for various outcomes. Forinteractions of hard binaries containing main-sequence stars, we findthat the normalized cross-section for at least one collision to occur(between any two of the four stars involved) is essentially unity, andthat the probability of collisions involving more than two stars issignificant. Hydrodynamic calculations have shown that the effectiveradius of a collision product can be 2-30 times larger than the normalmain-sequence radius for a star of the same total mass. We study theeffect of this expansion, and find that it increases the probability offurther collisions considerably. We discuss these results in the contextof recent observations of blue stragglers in globular clusters withmasses exceeding twice the main-sequence turn-off mass. We also presentFEWBODY, a new, freely available numerical toolkit for simulatingsmall-N gravitational dynamics that is particularly suited to performingscattering experiments.
| On the origin of red giant depletion through low-velocity collisions
We investigate a means of explaining the apparent paucity of red giantstars within post-core-collapse globular clusters. We propose thatcollisions between the red giants and binary systems can lead to thedestruction of some proportion of the red giant population, by eitherknocking out the core of the red giant or by forming a common envelopesystem which will lead to the dissipation of the red giant envelope.Treating the red giant as two point masses, one for the core and anotherfor the envelope (with an appropriate force law to take account of thedistribution of mass), and the components of the binary system alsotreated as point masses, we utilize a four-body code to calculate thetime-scales on which the collisions will occur. We then perform a seriesof smooth particle hydrodynamics runs to examine the details of masstransfer within the system. In addition, we show that collisions betweensingle stars and red giants lead to the formation of a common envelopesystem which will destroy the red giant star. We find that low-velocitycollision between binary systems and red giants can lead to thedestruction of up to 13 per cent of the red giant population. This couldhelp to explain the colour gradients observed in PCC globular clusters.We also find that there is the possibility that binary systems formedthrough both sorts of collision could eventually come into contactperhaps producing a population of cataclysmic variables.
| Investigating the Faint X-ray Sources in Globular Clusters with XMM-Newton
Globular clusters (GCs) harbour a large number of faint X-ray sourceswhose nature, until recently, was largely unknown. Using the new X-rayobservatories, it is possible to identify populations of low mass X-raybinaries, cataclysmic variables, millisecond pulsars, as well as othertypes of binaries belonging to the GCs, along with fore- and backgroundobjects. We present a variety of binaries, identified in four GCsobserved by XMM-Newton. We show that through population studies we canbegin to understand the formation of individual classes of binaries andhence start to unfold the complex evolutionary paths of such systems.
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