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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.
| 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.
| 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.
| Homogeneous Photometry. V. The Globular Cluster NGC 4147
New BVRI broadband photometry and astrometry are presented for theglobular cluster NGC 4147, based upon measurements derived from 524ground-based CCD images mostly either donated by colleagues or retrievedfrom public archives. We have also reanalyzed five exposures of thecluster obtained with WFPC2 on the Hubble Space Telescope in the F439Wand F555W (B and V) filters. We present calibrated color-magnitude andcolor-color diagrams. Analysis of the color-magnitude diagram revealsmorphological properties generally consistent with publishedmetal-abundance estimates for the cluster, and an age typical of otherGalactic globular clusters of similar metallicity. We have alsoredetermined the periods and mean magnitudes for the RR Lyrae variables,including a new c-type variable reported here for the first time. Ourdata do not show clear evidence for photometric variability in candidateV18, recently reported by Arellano Ferro et al. (2004, Rev. Mex.A&A, 40, 209). These observations also support the nonvariablestatus of candidates V5, V9, and V15. The union of our light-curve datawith those of Newburn (1957, AJ, 62, 197), Mannino (1957, Mem. Soc.Astron. Italiana, 28, 285), and Arellano Ferro et al. (op. cit.) permitsthe derivation of significantly improved periods. The mean periods andthe Bailey period-amplitude diagrams support the classification of thecluster as Oosterhoff I, despite its predominantly blue horizontalbranch. The number ratio of c- to ab-type RR Lyrae stars, on the otherhand, is unusually high for an Oosterhoff I cluster. The calibratedresults have been made available through the first author's Web site.Based in part on archival observations made with ESO telescopes at theLa Silla and Paranal Observatory under program ID 60.A-9050(A).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.
| 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.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| Pulsar Timing and the Detection of Black Hole Binary Systems in Globular Clusters
The possible existence of intermediate-mass binary black holes (IMBBHs)in globular clusters (GCs) offers us a unique geometry in which todetect spacetime oscillations. For certain pulsar-IMBBH configurationspossible within a GC, the usual far-field plane wave approximation forthe IMBBH metric perturbation severely underestimates the induced pulsetime-of-arrival (TOA) fluctuations. In this Letter, the expected TOAfluctuations induced by an IMBBH lying close to the line of sightbetween a pulsar and the Earth are calculated for the first time. For anIMBBH consisting of 10 and 103 Msolar components,a 10 yr orbital period, and located 0.1 lt-yr from the Earth-pulsar lineof sight, the induced TOA fluctuations will be of order 5-500 ns.
| Discovery of More than 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue Horizontal Branch
We report on the discovery of a large number of RR Lyrae variable starsin the moderately metal-rich Galactic globular cluster M62 (NGC 6266),which places it among the top three most RR Lyrae-rich globular clustersknown. Likely members of the cluster in our studied field, from ourpreliminary number counts, include ~130 fundamental-mode (RRab)pulsators, with =0.548 days, and ~75first-overtone (RRc) pulsators, with =0.300 days.The average periods and the position of the RRab variables withwell-defined light curves in the Bailey diagram both suggest that thecluster is of Oosterhoff type I. However, the morphology of thecluster's horizontal branch (HB) is strikingly similar to that of theOosterhoff type II globular cluster M15 (NGC 7078), with a dominant blueHB component and a very extended blue tail. Since M15 and M62 differ inmetallicity by about 1 dex, we conclude that metallicity, at a fixed HBtype, is a key parameter determining the Oosterhoff status of a globularcluster and the position of its variables in the Bailey diagram.
| 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.
| 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.
| RR Lyrae variables in Galactic globular clusters. V. The case of M 3 pulsators
We use our synthetic Horizontal Branch (HB) procedure to approach theoften debated problem of how adequate canonical HB stellar models are toaccount for the observed peaked distribution of RR Lyrae fundamentalisedperiods in the globular cluster M 3. We find that by assuming a suitablebimodal mass distribution, canonical models do account for the observedperiod distribution. In particular, the best fit model, out of ninerandom extractions, reaches a 99.9% Kolmogorov-Smirnov (KS) probability.We also attempt to predict the relative distribution of variables infundamental and first overtone pulsators, reaching a satisfactoryagreement. However, one finds that canonical models outnumber theobserved number of red HB stars by roughly a factor of two. Possiblesolutions for this discrepancy are outlined. Alternative evolutionaryscenarios are also briefly discussed.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| 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.
| Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
| Radio emission as a test of the existence of intermediate-mass black holes in globular clusters and dwarf spheroidal galaxies
We take the established relation between black hole mass, X-rayluminosity and radio luminosity and show that intermediate-mass blackholes (IMBHs), such as those predicted to exist at the centres ofglobular clusters (GCs), will be easily identifiable objects in deepradio observations. We show that the radio observations will be far moresensitive than any possible X-ray observations. We also discuss thelikely optical photometric and spectroscopic appearance of such systemsin the event that radio detections are made.
| 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.
| Green Bank Telescope Discovery of Two Binary Millisecond Pulsars in the Globular Cluster M30
We report the discovery of two binary millisecond pulsars in thecore-collapsed globular cluster M30 using the Green Bank Telescope (GBT)at 20 cm. PSR J2140-2310A (M30A) is an eclipsing 11 ms pulsar in a 4 hrcircular orbit, and PSR J2140-23B (M30B) is a 13 ms pulsar in an as yetundetermined but most likely highly eccentric (e>0.5) andrelativistic orbit. Timing observations of M30A with a 20 month baselinehave provided precise determinations of the pulsar's position (within 4"of the optical centroid of the cluster) and spin and orbital parameters,which constrain the mass of the companion star to bem2>~0.1Msolar. The position of M30A iscoincident with a possible thermal X-ray point source found in archivalChandra data, which is most likely caused by emission from hot polarcaps on the neutron star. In addition, there is a faint(V555~23.8) star visible in archival Hubble Space Telescope(HST) F555W data that may be the companion to the pulsar. Eclipses ofthe pulsed radio emission from M30A by the ionized wind from the compactcompanion star show a frequency-dependent duration(~ν-α with α~0.4-0.5) and delay the pulsearrival times near eclipse ingress and egress by up to 2-3 ms. Futureobservations of M30 may allow both the measurement of post-Keplerianorbital parameters from M30B and the detection of new pulsars throughthe effects of strong diffractive scintillation.
| Infrared Echelle Spectroscopy of Palomar 6 and M71
We present high-resolution infrared echelle spectroscopy for theglobular clusters Palomar 6 and M71. Our mean heliocentric radialvelocity of Pal 6 is +180.6+/-3.2 km s-1 and is 20 kms-1 lower than that found by Minniti in 1995. Contrary to theprevious metallicity estimates using low-resolution spectroscopy, ourresults show that Pal 6 has an intermediate metallicity, with[Fe/H]=-1.0+/-0.1, and is slightly more metal poor than M71. Reasonablechanges in the surface temperature or the microturbulent velocity of themodel atmospheres do not affect [Fe/H] at more than +/-0.2 dex. In spiteof its high metallicity, on the basis of the spectrum of a singlecluster member the [Si/Fe] and [Ti/Fe] ratios of Pal 6 appear to beenhanced by 0.4 and 0.5 dex, respectively, suggesting that the Galacticinner halo may have experienced a very rapid chemical enrichmenthistory.Based on observations made with the Infrared Telescope Facility, whichis operated by the University of Hawaii under contract to the NationalAeronautics and Space Administration.
| RR Lyrae Variables in Galactic Globular Clusters
RR Lyrae variables are the cornerstone of the Population II distancescale, and yet our knowledge of the RR Lyrae variable star content inGalactic globular clusters is now known to be surprisingly incomplete.In the present paper, we present our new results in this area.Highlights of our work includes: i) The discovery of a vast number ofvariable stars in M62 (NGC 6266), making it one of the three most RRLyrae-rich globular clusters known, and also placing it as Oosterhofftype I in spite of a blue horizontal branch morphology; ii) Thedetermination of light curves and Oosterhoff types for globular clustersassociated with the Sagittarius dSph galaxy, including NGC 5634, Arp 2,and Terzan 8; iii) A reassessment of the variable star content in themoderately metal-rich globular clusters M69 and NGC 6304; iv) The firsttheoretical calibration of the RR Lyrae period-luminosity-metallicityrelation in I, J, and H, as well as an updated calibration of the K-bandrelation---along with comparisons against the empirical data,particularly in I.This project was supported in part by Proyecto Fondecyt Regular 1030954.
| RR Lyrae variables in Galactic globular clusters. IV. Synthetic HB and RR Lyrae predictions
We present theoretical predictions concerning horizontal branch stars inglobular clusters, including RR Lyrae variables, as derived fromsynthetic procedures collating evolutionary and pulsational constraints.On this basis, we explore the predicted behavior of the pulsators as afunction of the horizontal branch morphology and over the metallicityrange Z= 0.0001 to 0.006, revealing an encouraging concordance with theobserved distribution of fundamentalised periods with metallicity.Theoretical relations connecting periods to K magnitudes and BV or VIWesenheit functions are presented, both appearing quite independent ofthe horizontal branch morphology only with Z≥ 0.001. Predictionsconcerning the parameter R are also discussed and compared under variousassumptions about the horizontal branch reference luminosity level.
| The metal content of the bulge globular cluster NGC 6528
High resolution spectra of five stars in the bulge globular cluster NGC6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVESspectrograph. Out of the five stars, two of them showed evidence ofbinarity. The target stars belong to the horizontal and red giant branchstages, at 4000 < Tefflt; 4800 K. Multiband V, I, J, H,Ks photometry was used to derive initial effectivetemperatures and gravities. The main purpose of this study is thedetermination of metallicity and elemental ratios for this templatebulge cluster, as a basis for the fundamental calibration of metal-richpopulations. The present analysis provides a metallicity [Fe/H] =-0.1±0.2 and the α-elements O, Mg and Si, show [α/Fe]≈ +0.1, whereas Ca and Ti are around the solar value or below,resulting in an overall metallicity Z ≈ Zȯ.Observations collected both at the European Southern Observatory,Paranal and La Silla, Chile (ESO programme 65.L-0340) and with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, operated by AURA Inc. under contract to NASA.Tables \ref{targets}, \ref{logobs}, \ref{tablines} and Fig. \ref{chart}are only available in electronic form at http://www.edpsciences.org
| The initial helium abundance of the Galactic globular cluster system
In this paper we estimate the initial He content in about 30% of theGalactic globular clusters (GGCs) from new star counts we have performedon the recently published HST snapshot database of Colour MagnitudeDiagrams (Piotto et al. \cite{Piotto02}). More specifically, we use theso-called R-parameter and estimate the He content from a theoreticalcalibration based on a recently updated set of stellar evolution models.We performed an accurate statistical analysis in order to assess whetherGGCs show a statistically significant spread in their initial Heabundances, and whether there is a correlation with the clustermetallicity. As in previous works on the subject, we do not find anysignificant dependence of the He abundance on the cluster metallicity;this provides an important constraint for models of Galaxy formation andevolution. Apart from GGCs with the bluest Horizontal Branch morphology,the observed spread in the individual helium abundances is statisticallycompatible with the individual errors. This means that either there isno intrinsic abundance spread among the GGCs, or that this is masked bythe errors. In the latter case we have estimated a firm 1σ upperlimit of 0.019 to the possible intrinsic spread. In case of the GGCswith the bluest Horizontal Branch morphology we detect a significantspread towards higher abundances inconsistent with the individualerrors; this can be fully explained by additional effects not accountedfor in our theoretical calibrations, which do not affect the abundancesestimated for the clusters with redder Horizontal Branch morphology. Inthe hypothesis that the intrinsic dispersion on the individual Heabundances is zero, taking into account the errors on the individualR-parameter estimates, as well as the uncertainties on the clustermetallicity scale and theoretical calibration, we have determined aninitial He abundance mass fraction YGGC=0.250±0.006.This value is in perfect agreement with current estimates based onCosmic Microwave Background radiation analyses and cosmologicalnucleosynthesis computations.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, and on observations retrieved withthe ESO ST-ECF Archive.
| A Globular Cluster Metallicity Scale Based on the Abundance of Fe II
Assuming that in the atmospheres of low-mass, metal-poor red giantstars, one-dimensional models based on local thermodynamic equilibriumaccurately predict the abundance of iron from Fe II, we derive aglobular cluster metallicity scale based on the equivalent widths of FeII lines measured from high-resolution spectra of giants in 16 keyclusters lying in the abundance range-2.4<[Fe/H]II<-0.7. We base the scale largely on theanalysis of spectra of 149 giant stars in 11 clusters by the Lick-Texasgroup supplemented by high-resolution studies of giants in five otherclusters. We also derive ab initio the true distance moduli for certainkey clusters (M5, M3, M13, M92, and M15) as a means of setting stellarsurface gravities. Allowances are made for changes in the abundancescale if one employs (1) Kurucz models with and without convectiveovershooting to represent giant star atmospheres in place of MARCSmodels and (2) the Houdashelt et al. color-temperature scale in place ofthe Alonso et al. scale.We find that [Fe/H]II is correlated linearly withW', the reduced strength of the near-infrared Ca II tripletdefined by Rutledge et al., although the actual correlation coefficientsdepend on the atmospheric model employed. The correlations, limited tothe range -2.4<[Fe/H]II<-0.7, are as follows:1.[Fe/H]II=0.531W'-3.279(MARCS),2.[Fe/H]II=0.537W'-3.225 (Kurucz withconvective overshooting),3.[Fe/H]II=0.562W'-3.329 (Kurucz withoutconvective overshooting).We also discuss how to estimate [X/Fe] ratios. We suggest that C, N, andO, as well as elements appearing in the spectrum in the singly ionizedstate, e.g., Ti, Sc, Ba, La, and Eu, should be normalized to theabundance of Fe II. Other elements, which appear mostly in the neutralstate, but for which the dominant species is nevertheless the ionizedstate, are probably best normalized to Fe I, but uncertainties remain.
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
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