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Primordial pollution of globular clusters within their host dwarfs embedded in dark matter haloes at high redshifts
Recent observational studies have revealed star-to-star abundanceinhomogeneity among light elements (e.g. C, N, O, Na and Al) of stars onthe main sequence in the Galactic globular clusters (GCs). One of thepromising interpretations for this result is that the observed abundanceinhomogeneity is due to the second generation of stars formed fromejecta of the first generation of evolved stars (e.g. asymptotic giantbranch stars) within GCs. However, it remains unclear whether and howthis primordial pollution can occur within GCs. We here propose a newscenario in which primordial pollution of GCs is highly likely to occurif GCs are located in the central regions of high-redshift dark mattersubhaloes that can host low-mass dwarf galaxies. In this scenario, gasejected from the first generation of stars of GCs can be effectivelytrapped in the deep gravitational potential of their host haloes, andconsequently can be consumed for the formation of the second generationof stars without losing a significant amount of gas by ram pressurestripping of interstellar and intergalactic medium. During merging ofthese haloes with the proto-Galaxy, the haloes are completely destroyedowing to the strong tidal field of the Galaxy. The self-polluted GCslocated initially in the central regions of the haloes can survive tidaldestruction owing to their compactness and finally become the Galactichalo GCs. In this scenario, ejecta of field stars surrounding thecentral GCs can be also converted into stars within their host dwarfsand finally become the second generation of stars of GCs. We alsodiscuss the origin of the difference in the degree of abundanceinhomogeneity between different GCs, such as ω Centauri and NGC6752, in terms of the difference in physical properties between hosthaloes from which GCs originate.
| Beyond the big Galaxy: the structure of the stellar system 1900 - 1952
Not Available
| 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.
| Neutron-Capture Elements in the Metal-poor Globular Cluster M15
We report on observations of six giants in the globular cluster M15 (NGC7078) using the Subaru Telescope to measure neutron-capture elementalabundances. Our abundance analyses, based on high-quality blue spectra,confirm the star-to-star scatter in the abundances of heavyneutron-capture elements (e.g., Eu), and we found no significants-process contribution to them, as was found in previous studies. Wehave found that, for the first time, there are anticorrelations betweenthe abundance ratios of light to heavy neutron-capture elements ([Y/Eu]and [Zr/Eu]) and the heavy neutron-capture elements (e.g., Eu). Thisindicates that the light neutron-capture elements in these stars cannotbe explained by only a single r-process. Another process thatcontributed significantly to the light neutron-capture elements isrequired in M15. Our results suggest a complicated enrichment historyfor M15 and its progenitor.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| A Blue Straggler Binary with Three Progenitors in the Core of a Globular Cluster?
We show that the X-ray source W31 in the core of the globular cluster 47Tucanae is physically associated with the bright blue straggler BSS 7.The two sources are astrometrically matched to 0.061", with a chancecoincidence probability of less than 1%. We then analyze opticaltime-series photometry obtained with the Hubble Space Telescope (HST)and find that BSS 7 displays a 1.56 day periodic signal in the I band.We also construct a broadband (far-ultraviolet through far-red) spectralenergy distribution for BSS 7 and fit this with single and binarymodels. The binary model is a better fit to the data, and we derive thecorresponding stellar parameters. All of our findings are consistentwith BSS 7 being a detached binary consisting of a blue stragglerprimary with an X-ray-active, upper-main-sequence companion. Theformation of such a system would necessarily involve at least threestars, which is consistent with recent N-body models in which bluestragglers often form via multiple encounters that can involve bothsingle and binary stars. However, we cannot yet entirely rule out thepossibility that BSS 7 descended directly from a binary system via masstransfer. The system parameters needed to distinguish definitivelybetween these scenarios may be obtainable from time-resolvedspectroscopy.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 NAS5-26555.
| 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.
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| Chandra observations of the globular cluster M 54
We have carried out a Chandra observation of the globular cluster M 54.We detected 7 sources located within the half-mass radius of M 54, at aflux limit of 1.5 × 10-15 erg s-1cm-2 in the 0.3-8 keV energy band. The spatial distributionand the colour/spectral properties of the 7 sources suggest that theyare likely to be cataclysmic variables or LMXBs in the globular cluster.M 54 shows the largest number of X-ray sources with luminosities greaterthan 1032 erg s-1 compared to other globularclusters observed using Chandra and XMM-Newton. We searched for acorrelation between the number of sources above this luminosity levelwith globular cluster parameters. We found evidence that the number ofsources peaks at a King concentration parameter c ~ 1.7-1.9, withglobular clusters which are core-collapsed or have low-c values having asmaller number of sources. We speculate on possible reasons for this.
| The dynamical distance and intrinsic structure of the globular cluster ω Centauri
We determine the dynamical distance D, inclination i, mass-to-lightratio M/L and the intrinsic orbital structure of the globular clusterω Cen, by fitting axisymmetric dynamical models to theground-based proper motions of van Leeuwen et al. and line-of-sightvelocities from four independent data-sets. We bring the kinematicmeasurements onto a common coordinate system, and select on clustermembership and on measurement error. This provides a homogeneousdata-set of 2295 stars with proper motions accurate to 0.20 masyr-1 and 2163 stars with line-of-sight velocities accurate to2 km s-1, covering a radial range out to about half the tidalradius. We correct the observed velocities for perspective rotationcaused by the space motion of the cluster, and show that the residualsolid-body rotation component in the proper motions (caused by relativerotation of the photographic plates from which they were derived) can betaken out without any modelling other than assuming axisymmetry. Thisalso provides a tight constraint on D tan i. The corrected mean velocityfields are consistent with regular rotation, and the velocity dispersionfields display significant deviations from isotropy. We model ωCen with an axisymmetric implementation of Schwarzschild's orbitsuperposition method, which accurately fits the surface brightnessdistribution, makes no assumptions about the degree of velocityanisotropy in the cluster, and allows for radial variations in M/L. Webin the individual measurements on the plane of the sky to searchefficiently through the parameter space of the models. Tests on ananalytic model demonstrate that this approach is capable of measuringthe cluster distance to an accuracy of about 6 per cent. Application toω Cen reveals no dynamical evidence for a significant radialdependence of M/L, in harmony with the relatively long relaxation timeof the cluster. The best-fit dynamical model has a stellar V-bandmass-to-light ratio M/L_V=2.5±0.1 M_ȯ/L_ȯ and aninclination i=50°±4°, which corresponds to an averageintrinsic axial ratio of 0.78±0.03. The best-fit dynamicaldistance D=4.8±0.3 kpc (distance modulus 13.75±0.13 mag)is significantly larger than obtained by means of simple spherical orconstant-anisotropy axisymmetric dynamical models, and is consistentwith the canonical value 5.0±0.2 kpc obtained by photometricmethods. The total mass of the cluster is(2.5±0.3)×106 M_ȯ. The best-fit model isclose to isotropic inside a radius of about 10 arcmin and becomesincreasingly tangentially anisotropic in the outer region, whichdisplays significant mean rotation. This phase-space structure may wellbe caused by the effects of the tidal field of the Milky Way. Thecluster contains a separate disk-like component in the radial rangebetween 1 and 3 arcmin, contributing about 4% to the total mass.
| Age and Metallicity Estimation of Globular Clusters from Strömgren Photometry
We present a new technique for the determination of age and metallicityin composite stellar populations using Strömgren filters. Usingprincipal component (PC) analysis on multicolor models, we isolate therange of values necessary to uniquely determine age and metallicityeffects. The technique presented here can only be applied to old(τ>3 Gyr) stellar systems composed of simple stellar populations,such as globular clusters and elliptical galaxies. Calibration using newphotometry of 40 globular clusters with spectroscopic [Fe/H] values andmain-sequence-fitted ages links the PC values to the Strömgrencolors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.
| SiO Maser Sources toward Globular Clusters
We report on the detection of SiO masers in Asymptotic Giant Branchvariables toward bulge/disk globular clusters. In five out of six cases,the radial velocities are compatible with the optically measured radialvelocities of globular clusters in the assessed uncertainty. Twosources, toward Terzan 5 and Terzan 12, lie very close to the clustercenters. The objects toward Pal 6 and Terzan 12 have luminositiesappropriate to the AGB tip in globular clusters, while those toward NGC6171, Pal 10, and Terzan 5 are brighter than expected. It is suggestedthat the latter three may have evolved from merged binaries, offering atest for binary-evolution scenarios in globular clusters, if themembership is approved.
| Deepsky delights.
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| Cluster Ages Experiment (CASE): Detection of a dwarf nova in the globular cluster M55
We report the detection of a dwarf nova (DN) in the core region of theglobular cluster M55. Six outbursts were observed during eight observingseasons spanning the period 1997-2004. The variable has an X-raycounterpart detected on images taken with the ROSAT telescope. Althoughwe cannot offer proof of cluster membership, one can see that both theposition on the Hertzsprung-Russell diagram and the X-ray flux areconsistent with a bright DN at the cluster distance. According to ouroutburst statistics, no more than one similar DN could remain undetectedin our field of view, centred at the cluster core.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| The X-ray source population of the globular cluster M15: Chandra high-resolution imaging
The globular cluster M15 was observed on three occasions with the HighResolution Camera on-board Chandra in 2001 in order to investigate theX-ray source population in the cluster centre. After subtraction of thetwo bright central sources, four faint sources were identified within 50arcsec of the core. One of these sources is probably the planetarynebula K648, making this the first positive detection of X-rays from aplanetary nebula inside a globular cluster. Another two are identifiedwith UV variables (one previously known), which we suggest arecataclysmic variables (CVs). The nature of the fourth source is moredifficult to ascertain, and we discuss whether it is possibly aquiescent soft X-ray transient or also a CV.
| Washington Photometry of Red Giants in Five Galactic Globular Clusters
Surface metal abundances and effective temperatures of 165 stars in theGalactic globular clusters NGC 288, NGC 6397, NGC 6656, NGC 7078 and NGC7089 are determined from photoelectric photometry in the Washingtonsystem. Mean cluster metal abundances are determined by the two-colordiagram (TCD) technique described by Geisler et al. (1991). Theresulting metallicities are compared with those derived by the standardgiant branches (SGBs) method developed by Geisler and Sarajedini (1999).Although both methods provide metallicities in reasonably goodagreement, we find that the SGBs technique yields metallicities in abetter agreement with the existing metallicity scales. This is due tothe higher metallicity sensitivity of the SGBs method as well as to itslower sensitivity to reddening and photometric errors.
| 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.
| Reddening Distribution across the Center of the Globular Cluster ω Centauri
We present new medium-band uvby Strömgren and broadband VIphotometry for the central regions of the globular cluster ω Cen.From this photometry we have obtained differential reddening estimatesrelative to two other globular clusters (M13 and NGC 288) using ametallicity-independent, reddening-free temperature index,[c]≡(u-v)-(v-b)-0.2(b-y), for hot horizontal-branch (HB) stars(Teff>=8500 K). We estimate color excesses of these hot HBstars using optical and near-infrared colors, and we find clumpyextinction variations of almost a factor of 2 within the area of thecluster core. In particular, the greatest density of more highlyreddened objects appears to be shifted along the right ascension axiswhen compared with less reddened ones. These findings complicatephotometric efforts to investigate the star formation history of ωCen.Based in part on observations collected with the Danish and NewTechnology Telescopes, operated at ESO La Silla.
| Abundance Anomalies and Rotational Evolution of Low-Mass Red Giants: A Maximal Mixing Approach
We use a fully self-consistent evolutionary code to follow therotational evolution of low-mass red giants, making a comprehensiveattempt to assess the role of rotationally induced mixing in thedevelopment of abundance anomalies in giants with a range of masses andmetallicities in stellar clusters and the field. We adopt a maximalmixing approach with reasonable initial conditions of angular momentumdistribution and main-sequence rotation rates as a function of stellartype. Unlike most previous work, we do not focus on the determination ofcombinations of mixing rate and depth that reproduce the data on aparticular stellar type. Instead, we concentrate on the more fundamentalproblem of the simultaneous reproduction of the patterns of CNO surfaceabundances in both Population I and Population II giants using the samephysics and models. We follow and discuss the essential physics ofrotational mixing in terms of the structural and angular momentumevolution along the red giant branch (RGB).A general result of all our models is that rotational mixing, althoughpresent in small amounts, is inefficient on the lower RGB independentlyof any inhibiting effect of μ-barriers. Therefore, the lack ofwell-mixed stars before the luminosity of the RGB bump in globularclusters and the field does not constitute unquestionable evidence forthe inhibition of mixing by μ-barriers. Instead, we argue that therapid disappearance of the RGB bump as soon as mixing is allowed topenetrate μ-barriers is what actually constitutes the first solidevidence of such inhibition.Maximal mixing models with differentially rotating envelopes are able toreproduce the 12C/13C data on M67 giants withinitial rotation rates adequate to their progenitors but fail to do sofor open clusters of larger turnoff mass and for metal-poor giants inthe field and globular clusters. Possible solutions are discussed. Ourfavored scenario is one in which the overall strength of canonical extramixing has been underestimated by existent derivations, but whichadditionally needs to be coupled with a much lower efficiency forrotational mixing among the rapidly rotating open cluster giants than inthe slowly rotating ones in the field and globular clusters. Wehypothesize that this last requirement is provided by the interactionbetween convection and rotation in the envelopes of giants, in the sensethat rapidly rotating stars would develop much shallower angularvelocity profiles in their envelopes than do slowly rotating stars.
| 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.
| Multiple Outbursts of a Cataclysmic Variable in the Globular Cluster M22
We present a 4 yr light curve of a cataclysmic variable in M22, based onan analysis of accumulated data from the Microlensing Observations inAstrophysics (MOA) microlensing survey. The position of the starcoincides with that of a transient event observed by the Hubble SpaceTelescope (HST) in 1999, originally attributed to microlensing but latersuspected to be a dwarf nova outburst. Two outburst episodes, one in2002 and one in 2003, with ΔI~3 are seen in the MOA data, thusconfirming that the HST event was a dwarf nova outburst. The MOA and HSTdata show that this dwarf nova underwent at least three outburstepisodes during 1999-2004. Further close monitoring of this event isencouraged, as future outburst episodes are expected.
| 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.
| Cluster AgeS Experiment (CASE): Dwarf Novae and a Probable Microlensing Event in the Globular Cluster M22
We report the identification of a new cataclysmic variable (denoted asCV2) and a probable microlensing event in the field of the globularcluster M22. Two outbursts were observed for CV2. During one of themsuperhumps with P_sh=0.0887 d were present in the light curve. CV2 hasan X-ray counterpart detected by XMM-Newton. A very likely microlensingevent at a radius of 2.3 arcmin from the cluster center was detected. Ithad an amplitude of Delta V=0.75 mag and a characteristic time of 15.9days. Based on model considerations we show that the most likelyconfiguration has the source in the Galactic bulge with the lens in thecluster. Two outbursts were observed for the already known dwarf novaCV1.
| POINT-AGAPE pixel lensing survey of M 31. Evidence for a MACHO contribution to galactic halos
The POINT-AGAPE collaboration is carrying out a search for gravitationalmicrolensing toward M 31 to reveal galactic dark matter in the form ofMACHOs (Massive Astrophysical Compact Halo Objects) in the halos of theMilky Way and M 31. A high-threshold analysis of 3 years of data yields6 bright, short-duration microlensing events, which are confronted to asimulation of the observations and the analysis. The observed signal ismuch larger than expected from self lensing alone and we conclude, atthe 95% confidence level, that at least 20% of the halo mass in thedirection of M 31 must be in the form of MACHOs if their average masslies in the range 0.5-1 M_ȯ. This lower bound drops to 8% forMACHOs with masses ~0.01 M_ȯ. In addition, we discuss a likelybinary microlensing candidate with caustic crossing. Its location, some32' away from the centre of M 31, supports our conclusion that we aredetecting a MACHO signal in the direction of M 31.
| The all-sky distribution of 511 keV electron-positron annihilation emission
We present a map of 511 keV electron-positron annihilation emission,based on data accumulated with the SPI spectrometer aboard ESA'sINTEGRAL gamma-ray observatory, that covers approximately ~95% of thecelestial sphere. Within the exposed sky area, 511 keV line emission issignificantly detected towards the galactic bulge region and, at a verylow level, from the galactic disk. The bulge emission is highlysymmetric and is centred on the galactic centre with an extension of ~8° (FWHM). The emission is equally well described by models thatrepresent the stellar bulge or halo populations. The detectionsignificance of the bulge emission is ~ 50σ, that of the galacticdisk is ~ 4σ. The disk morphology is only weakly constrained bythe present data, being compatible with both the distribution of youngand old stellar populations. The 511 keV line flux from the bulge anddisk components is (1.05 ± 0.06) × 10-3 phcm-2 s-1 and (0.7 ± 0.4) ×10-3 ph cm-2 s-1 respectively,corresponding to a bulge-to-disk flux ratio in the range 1-3. Assuming apositronium fraction of f_p=0.93 this translates into annihilation ratesof (1.5 ± 0.1) × 1043 s-1and (0.3± 0.2) × 1043 s-1, respectively. Theratio of the bulge luminosity to that of the disk is in the range 3-9.We find no evidence for a point-like source in addition to the diffuseemission, down to a typical flux limit of ~10-4 phcm-2 s-1. We also find no evidence for thepositive latitude enhancement that has been reported from OSSEmeasurements; our 3σ upper flux limit for this feature is 1.5× 10-4 ph cm-2 s-1. The diskemission can be attributed to the β^+-decay of the radioactivespecies 26 Al and 44Ti. The bulge emission arisesfrom a different source which has only a weak or no disk component. Wesuggest that Type Ia supernovae and/or low-mass X-ray binaries are theprime candidates for the source of the galactic bulge positrons. Lightdark matter annihilation could also explain the observed 511 keV bulgeemission characteristics.
| Self-enrichment in globular clusters. I. An analytic approach
By means of analytical calculations, we explore the self-enrichmentscenario for Globular Cluster formation. According to this scenario, aninitial burst of star formation occurs inside the core radius of theinitial gaseous distribution. The outward-propagating shock wave sweepsup a shell in which gravitational instabilities may arise, leading tothe formation of a second, metal-enriched, population of stars. We finda minimum mass of the proto-globular cluster of the order of106 M_ȯ. We also find that the observed spread in themagnitude-metallicity relation can be explained assumingcluster-to-cluster variations of some parameters like the thermalizationefficiency, the mixing efficiency and the Initial Mass Function, as wellas variations of the external pressure.
| Binary formation within globular clusters: X-ray clues
We have investigated the effect of the number of primordial binaries onthe relationship between the total number of detected binaries withinglobular cluster and its collision rate. We have used simulatedpopulations of binary stars in globular clusters: primordial binariesand binaries formed through gravitational interactions. We show that theinitial number of primordial binaries influences the relationshipbetween the number of detected sources and the collision rate, which wefind to be a power law. We also show that observing an incomplete sampleprovides the same results as those obtained with a complete sample. Weuse observations made by XMM-Newton and Chandra to constrain theformation mechanism of sources with X-ray luminosities larger than1031 erg s-1, and show that some of thecataclysmic variables within globular clusters should be primordialobjects. We point out a possibly hidden population of neutron starswithin high mass globular clusters with a low collision rate.
| 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.
| The astrophysics of cool white dwarfs
Electronic Article Available from Elsevier Science.
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