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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.
|Caroline Herschel's catalogue of nebulae|
|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.
|Chandra X-Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae|
We present spectral and long-timescale variability analyses of ChandraX-Ray Observatory ACIS-S observations of the 19 millisecond pulsars(MSPs) with precisely known positions in the globular cluster 47Tucanae. The X-ray emission of the majority of these MSPs is welldescribed by a thermal (blackbody or neutron star hydrogen atmosphere)spectrum with a temperature Teff~(1-3)×106K, emission radius Reff~0.1-3 km, and luminosityLX~1030-1031 ergs s-1. Forseveral MSPs, there are indications that a second thermal component isrequired, similar to what is seen in some nearby field MSPs. Theobserved radiation most likely originates from the heated magnetic polarcaps of the MSPs. The small apparent scatter in LX isconsistent with thermal emission from the polar caps of a global dipolefield, although the small emission areas may imply either a more complexsmall-scale magnetic field configuration near the neutron star surfaceor nonuniform polar cap heating. The radio eclipsing binary MSPs 47 TucJ, O, and W show a significant nonthermal (power-law) component, withspectral photon index Γ~1-1.5, which most likely originates in anintrabinary shock formed due to interaction between the relativisticpulsar wind and matter from the stellar companion. We reexamine theX-ray-spin-down luminosity relation (LX-E˙ relation) andfind that for the MSPs with thermal spectraLX~E˙β, where β~0.2+/-1.1. Due tothe large uncertainties in both parameters, the result is consistentwith both the linear LX-E˙ relation and the flatterLX~E˙0.5 predicted by polar cap heatingmodels. In terms of X-ray properties, we find no clear systematicdifferences between MSPs in globular clusters and in the field of theGalaxy. We discuss the implications of these results on the presentunderstanding of the X-ray emission properties of MSPs.
|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.
|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.
|Atmospheres, Chemical Compositions, and Evolutionary Histories of Very Metal-Poor Red Horizontal-Branch Stars in the Galactic Field and in NGC 7078 (M15)|
We have conducted spectrum analyses of 24 field metal-poor([Fe/H]<-2) red horizontal-branch (RHB) stars identified in the HKobjective-prism survey and 6 such stars in the globular cluster M15,based on high-quality spectra (R~40,000, S/N~100) obtained with theMagellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 mtelescope at Las Campanas Observatory. The atmospheric parameters of theRHB stars provide interesting bridges between turnoff stars of similartemperature and red giant branch (RGB) stars of similar gravity, andthey permit investigations of abundance trends [X/Fe] versus [Fe/H] in arelatively unexplored region of the temperature-gravity plane. We findthat the Teff, logg, vt, and [Fe/H] valuesdetermined from our spectra are consistent with expectations fromliterature spectroscopic studies of other evolved metal-poor stellarclasses. We show that the RHB stars have abundance distributions thatare consistent with typical halo stars of similar metallicities. Thephotometric and spectroscopic gravities of the M15 stars differ byamounts that grow with declining temperature. We use a regressionderived from these differences to calculate photometric gravities forthe field RHB stars. Then we use the locations of the field RHB starsamong the evolutionary tracks of Cassisi et al. in the logg versuslogTeff plane to estimate their masses and lifetimes as RHBstars. We use these lifetimes to estimate the size of the metal-poor HBpopulation from which they arise. Then, using counts of HB and RGB starsin metal-poor globular clusters, we conclude that the number ofmetal-poor RGB stars at high latitudes (|b|>30deg)brighter than V=15 exceeds those identified in extant objective-prismsurveys by more than an order of magnitude. Finally, we deduce theeffective temperature of the fundamental red edge of the metal-poor RRLyrae instability strip, logTeff(FRE)=3.80+/-0.01, from theinterface between the temperature distributions of metal-poor field RHBstars and the RR Lyrae stars of similar [Fe/H] in five metal-poorglobular clusters.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation. This paper includes data gatheredwith the 6.5 m Magellan telescopes located at Las Campanas Observatory,Chile.
|Manganese Abundances in Cluster and Field Stars|
We have derived Mn abundances for more than 200 stars in 19 globularclusters. In addition, Mn abundance determinations have been made for acomparable number of halo field and disk stars possessing an overlappingrange of metallicities and stellar parameters. Our primary data set wascomprised of high-resolution spectra previously acquired at theMcDonald, Lick, and Keck Observatories. To enlarge our data pool, weacquired globular and open cluster spectra from several otherinvestigators. Data were analyzed using synthetic spectra of the 6000Å Mn I triplet. Hyperfine structure parameters were included inthe synthetic spectra computations. Our analysis shows that for themetallicity range -0.7>[Fe/H]>-2.7, stars of 19 globular clustershave a mean relative abundance of <[Mn/Fe]>=-0.37+/-0.01(σ=0.10), a value in agreement with that of the field stars,<[Mn/Fe]>=-0.36+/-0.01 (σ=0.08). Despite the 2 orders ofmagnitude span in metallicity, the <[Mn/Fe]> ratio remainsconstant in both stellar populations. Our Mn abundance data indicatethat there is no appreciable variation in the relative nucleosyntheticcontribution from massive stars that undergo core-collapse supernovaeand thus no significant change of the associated initial mass functionin the specified metallicity range.
|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.
|VLT/UVES spectroscopy of individual stars in three globular clusters in the Fornax dwarf spheroidal galaxy|
We present a high resolution (R ~ 43 000) abundance analysis of a totalof nine stars in three of the five globular clusters associated with thenearby Fornax dwarf spheroidal galaxy. These three clusters (1, 2 and 3)trace the oldest, most metal-poor stellar populations in Fornax. Wedetermine abundances of O, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, Nd andEu in most of these stars, and for some stars also Mn and La. Wedemonstrate that classical indirect methods (isochrone fitting andintegrated spectra) of metallicity determination lead to values of[Fe/H] which are 0.3 to 0.5 dex too high, and that this is primarily dueto the underlying reference calibration typically used by these studies.We show that Cluster 1, with [Fe /H] = -2.5, now holds the record forthe lowest metallicity globular cluster. We also measure anover-abundance of Eu in Cluster 3 stars that has only been previouslydetected in a subgroup of stars in M 15. We find that the Fornaxglobular cluster properties are a global match to what is found in theirGalactic counterparts; including deep mixing abundance patterns in twostars. We conclude that at the epoch of formation of globular clustersboth the Milky Way and the Fornax dwarf spheroidal galaxy shared thesame initial conditions, presumably pre-enriched by the same processes,with identical nucleosynthesis patterns.
|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.
|Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452|
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
|The formation of helium white dwarfs in close binary systems - II|
We present a set of calculations of the evolution of low-mass, solarcomposition stars in close binary systems together with a canonical1.4-Msolar, neutron star (NS). We restrict the initial massand period values to those that give rise to the formation ofultracompact systems or low-mass helium white dwarf (He WD) stars.Specifically, we computed the evolution of 40 systems for which theinitial masses of the normal (donor) stars were of 1.00, 1.25, 1.50,1.75, 2.00, 2.50, 3.00 and 3.50 Msolar, while the range ofinitial periods covered in this work was from 0.5 to 12 d.Calculations were performed employing the binary hydro code developed bythe present authors, which handles the mass transfer rate in a fullyimplicit way together with state-of-the-art physical ingredients anddiffusion processes. In this work we have assumed the standard schemefor the orbital evolution of the binary, considering the usual processesthat produce angular momentum losses: mass loss from the system,magnetic braking and gravitational radiation. In the main part of thiswork we assume that the NS is able to retain only half of the mattercoming from the donor star. The range of final masses has been from0.01961 to 0.34351 Msolar and periods from 39 min to 187 d.26 out of the 40 considered systems give rise to the formation of a HeWD as a compact remnant.In performing a comparison of our results with observations, we haveemployed three WD-NS systems, which are among the best known ones. Theseare PSR J0437-4715, PSR J1012+5307 and PSR B1855+09. In order to obtaingood agreement between models and observations we have had to assumethat the NS is able to retain only ~10 per cent of the material releasedby the donor star. Otherwise, for the cases of PSR J0437-4715 and PSRB1855+09 we would be in conflict with the observed NS masses. For thesetwo objects we have been able to fit WD and NS masses, the orbitalperiod and also the time-scale for cooling of the WD with the evolutionof one binary system. For the case of PSR J1012+5307, the problem ismore delicate because the object has a mass near the threshold for theoccurrence of thermonuclear flashes and the initial period is close tobifurcation point.
|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.
|The Unusual Luminosity Function of the Globular Cluster M10|
We present the I-band luminosity function of the differentially reddenedglobular cluster M10. We combine photometric analysis derived fromwide-field (23'×23') images that include theouter regions of the cluster and high-resolution images of the clustercore. After making corrections for incompleteness and field starcontamination, we find that the relative numbers of stars on the lowergiant branch and near the main-sequence turnoff are in good agreementwith theoretical predictions. However, we detect significant (>6σ) excesses of red giant branch stars above and below the redgiant branch bump using a new statistic (a population ratio) for testingrelative evolutionary timescales of main-sequence and red giant stars.The statistic is insensitive to assumed cluster chemical composition,age, and main-sequence mass function. The excess number of red giantscannot be explained by reasonable systematic errors in our assumedcluster chemical composition, age, or main-sequence mass function.Moreover, M10 shows excesses when compared to the cluster M12, which hasnearly identical metallicity, age, and color-magnitude diagrammorphology. We discuss possible reasons for this anomaly, finding thatthe most likely cause is a mass function slope that shows significantvariations as a function of mass.
|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.
|The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars|
We discuss a 175 deg2 spectroscopic survey for bluehorizontal branch (BHB) stars in the Galactic halo. We use the TwoMicron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) toselect BHB candidates, and we find that the 2MASS and SDSS colorselection is 38% and 50% efficient, respectively, for BHB stars. Oursamples include one likely runaway B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent withmembership in the halo population: the median metallicity is[Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and themean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is-4+/-30 km s-1. We discuss the theoretical basis of thePreston, Shectman, and Beers MV-color relation for BHB starsand conclude that the intrinsic shape of the BHB MV-colorrelation results from the physics of stars on the horizontal branch. Wecalculate the luminosity function for the field BHB star samples usingthe maximum likelihood method of Efstathiou and coworkers, which isunbiased by density variations. The field BHB luminosity functionexhibits a steep rise at bright luminosities, a peak between0.8
|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.
|The Australia Telescope National Facility Pulsar Catalogue|
We have compiled a new and complete catalog of the main properties ofthe 1509 pulsars for which published information currently exists. Thecatalog includes all spin-powered pulsars, as well as anomalous X-raypulsars and soft gamma-ray repeaters showing coherent pulsed emission,but excludes accretion-powered systems. References are given for alldata listed. We have also developed a new World Wide Web interface foraccessing and displaying either tabular or plotted data with the optionof selecting pulsars to be displayed via logical conditions on parameterexpressions. The Web interface has an ``expert'' mode giving access to awider range of parameters and allowing the use of custom databases. Forusers with locally installed software and database on Unix or Linuxsystems, the catalog may be accessed from a command-line interface.C-language functions to access specified parameters are also available.The catalog is updated from time to time to include new information.
|The Chemical Properties of Milky Way and M31 Globular Clusters. II. Stellar Population Model Predictions|
We derive ages, metallicities, and abundance ratios ([α/Fe]) fromthe integrated spectra of 23 globular clusters in M31 by employingmultivariate fits to two different stellar population models. We alsoperform a parallel analysis on 21 Galactic globular clusters as aconsistency check and in order to facilitate a differential analysis.Our analysis shows that the M31 globular clusters separate into threedistinct components in age and metallicity; we identify an old,metal-poor group (seven clusters), an old, metal-rich group (10clusters), and an intermediate-age (3-6 Gyr), intermediate-metallicity([Z/H]~-1) group (six clusters). This third group is not identified inthe Galactic globular cluster sample. We also see evidence that the old,metal-rich Galactic globular clusters are 1-2 Gyr older than theircounterparts in M31. The majority of globular clusters in both samplesappear to be enhanced in α-elements, but the degree of enhancementis rather model-dependent. The intermediate-age globular clusters appearto be the most enhanced, with [α/Fe]~0.4. These clusters areclearly depressed in CN with respect to the models and the bulk of theM31 and Milky Way sample. Compared with the bulge of M31, M32, and NGC205, these clusters most resemble the stellar populations in NGC 205 interms of age, metallicity, and CN abundance. We infer horizontal branchmorphologies for the M31 clusters using the Rose Ca II index anddemonstrate that blue horizontal branches are not leading to erroneousage estimates in our analysis. We discuss and reject as unlikely thehypothesis that these objects are in fact foreground stars contaminatingthe optical catalogs. The intermediate-age clusters have generallyhigher velocities than the bulk of the M31 cluster population.Spatially, three of these clusters are projected onto the bulge region,and the remaining three are distributed at large radii. We discuss theseobjects within the context of the build-up of the M31 halo and suggestthat these clusters possibly originated in a gas-rich dwarf galaxy,which may or may not be presently observable in M31.
|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.
|Full computation of massive AGB evolution. I. The large impact of convection on nucleosynthesis|
It is well appreciated that the description of overadiabatic convectionaffects the structure of the envelopes of luminous asymptotic giantbranch (AGB) stars in the phase of ``hot bottom burning'' (HBB). Westress that this important uncertainty in the modeling plays a rolewhich is much more dramatic than the role which can be ascribed, e.g.,to the uncertainty in the nuclear cross-sections. Due to the roletentatively attributed today to the HBB nucleosynthesis as the site ofself-enrichment of Globular Clusters stars, it is necessary to explorethe difference in nucleosynthesis obtained by different prescriptionsfor convection. We present results of detailed evolutionary calculationsof the evolution of stars of intermediate mass during the AGB phase forthe metallicity typical of the Globular Clusters that show the largestspread in CNO abundances (Z 10-3). We follow carefullythe nucleosynthesis at the base of the external convective region,showing that very different results can be obtained according to thepresciption adopted to find out the temperature gradient within theinstability regions. We discuss the uncertainties in the yields of thevarious chemical species and the role which these sources can play aspolluters of the interstellar medium.
|Pulsars in Binary Systems: Probing Binary Stellar Evolution and General Relativity|
Radio pulsars in binary orbits often have short millisecond spin periodsas a result of mass transfer from their companion stars. They thereforeact as very precise, stable, moving clocks that allow us to investigatea large set of otherwise inaccessible astrophysical problems. Theorbital parameters derived from high-precision binary pulsar timingprovide constraints on binary evolution, characteristics of the binarypulsar population, and the masses of neutron stars with differentmass-transfer histories. These binary systems also test gravitationaltheories, setting strong limits on deviations from general relativity.Surveys for new pulsars yield new binary systems that increase ourunderstanding of all these fields and may open up whole new areas ofphysics, as most spectacularly evidenced by the recent discovery of anextremely relativistic double-pulsar system.
|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.
|Red giant branch in near-infrared colour-magnitude diagrams - II. The luminosity of the bump and the tip|
We present new empirical calibrations of the red giant branch (RGB) bumpand tip based on a homogeneous near-infrared data base of 24 Galacticglobular clusters. The luminosities of the RGB bump and tip in the J, Hand K bands and their dependence on the cluster metallicity have beenstudied, yielding empirical relationships. By using recenttransformations between the observational and theoretical planes, wealso derived similar calibrations in terms of bolometric luminosity.Direct comparisons between updated theoretical models and observationsshow an excellent agreement. The empirical calibration of the RGB tipluminosity in the near-infrared passbands presented here is afundamental tool to derive distances to galaxies far beyond the LocalGroup, in view of using the new ground-based adaptive optics facilitiesand, in the near future, the James Webb Space Telescope.
|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.
|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.
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