Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
The Unusual Binary Pulsar PSR J1744-3922: Radio Flux Variability, Near-Infrared Observation, and Evolution
PSR J1744-3922 is a binary pulsar exhibiting highly variable pulsedradio emission. We report on a statistical multifrequency study of thepulsed radio flux variability which suggests that this phenomenon isextrinsic to the pulsar and possibly tied to the companion, although notstrongly correlated with orbital phase. The pulsar has an unusualcombination of characteristics compared to typical recycled pulsars: along spin period (172 ms); a relatively high magnetic field strength(1.7×1010 G); a very circular, compact orbit of 4.6 hr;and a low-mass companion (0.08 Msolar). These spin andorbital properties are likely inconsistent with standard evolutionarymodels. We find similarities between the properties of the PSRJ1744-3922 system and those of several other known binary pulsarsystems, motivating the identification of a new class of binary pulsars.We suggest that this new class could result from: a standard accretionscenario of a magnetar or a high magnetic field pulsar; common envelopeevolution with a low-mass star and a neutron star, similar to what isexpected for ultracompact X-ray binaries; or accretion induced collapseof a white dwarf. We also report the detection of a possibleK'=19.30(15) infrared counterpart at the position of thepulsar, which is relatively bright if the companion is a helium whitedwarf at the nominal distance, and discuss its implications for thepulsar's companion and evolutionary history.
| Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| Formation and evolution of the Galactic bulge: constraints from stellar abundances
Aims.We compute the chemical evolution of the Galactic bulge in thecontext of an inside-out model for the formation of the Milky Way. Themodel contains updated stellar yields from massive stars. The mainpurpose of the paper is to compare the predictions of this model withnew observations of chemical abundance ratios and metallicitydistributions in order to put constraints on the formation and evolutionof the bulge. Methods: We computed the evolution of severalα-elements and Fe and performed several tests by varying differentparameters such as star formation efficiency, slope of the initial massfunction and infall timescale. We also tested the effect of adopting aprimary nitrogen contribution from massive stars. Results: The [α/Fe] abundance ratios in the Bulge are predicted to be supersolarfor a very large range in [Fe/H], each element having a different slope.These predictions are in very good agreement with most recent accurateabundance determinations. We also find a good fit of the most recentBulge stellar metallicity distributions. Conclusions: We conclude thatthe Bulge formed on a very short timescale (even though timescales muchshorter than ~0.1 Gyr are excluded) with a quite high star formationefficiency of ν ≃ 20 Gyr-1 and with an initial massfunction more skewed toward high masses (i.e. x ≤ 0.95) than thesolar neighbourhood and rest of the disk. The results obtained here aremore robust than previous ones since they are based on very accurateabundance measurements.
| Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge
We present near-IR color-magnitude diagrams and physical parameters fora sample of 24 Galactic globular clusters toward the bulge. In thispaper we discuss the properties of 12 new clusters (out of the 24) inaddition to those previously studied and published by our group. Thecompilation includes measurements of the cluster reddening, distance,photometric metallicity, horizontal branch red clump, and red giantbranch morphological (e.g., mean ridgelines) and evolutionary (e.g.,bump and tip) features. The compilation is available in electronic formon the World Wide Web, and it will be updated regularly.Based on data taken at the ESO New Technology Telescope, within theobserving programs 73.D-0313, 75.D-0372, and 77.D-0757.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near-Infrared Color-Magnitude Diagrams
We present an empirical method to derive photometric metallicity,reddening, and distance to old stellar populations by using a few majorfeatures of the red giant branch (RGB) in near-IR color-magnitudediagrams. We combine the observed RGB features with a set of equationslinking the global metallicity [M/H] to suitable RGB parameters (colors,magnitudes, and slope), as calibrated from a homogeneous sample ofGalactic globular clusters with different metallicities. This techniquecan be applied to efficiently derive the main population parameters ofold stellar systems, with the goal of using ground-based adaptive opticsand space facilities to probe the stellar content of remote galaxies.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 La Palma bythe Fundacion Galileo Galilei of INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| 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.
| 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.
| 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.
| 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.
| The bimodal metallicity distribution function of the globular clusters in the Galaxy: halo disc complementarity
Aims.Our aim in this paper is to present an explanatory scenario for theformation of the observed relatively metal rich globular clustersassociated with the thick disc of the Galaxy, distinct from the mode offormation of the lower metallicity halo clusters.Methods.Theobservations to be accounted for here are the two peaks in themetallicity distribution of the thick disc globular clusters, at [ Fe/H] -0.7 and at [ Fe/H] -0.4. The first step is to verify thestatistical significance of these peaks, and the insignificance of amuch smaller peak at [ Fe/H] -0.2. The basic model assumption isthat these globular clusters were formed as the most massive long termsurvivors of a much larger set of open clusters whose epochs offormation coincided with the main epochs of star formation in the thindisc. These latter are identified using established data sets giving thelocal stellar frequency distribution in time based on stellar activityindices.Results.Our simple stellar accretion model accounts reasonablyfor the presence of the observed peaks in the cluster metallicitydistribution, and the long time constant for the accretion as a massivecluster moves through the stellar environment explains qualitatively whythe most recent peak in the local star formation rate has not yet givenrise to a corresponding peak in the globular cluster distribution. Italso explains in broad terms how a uniform process of cluster formationoriginating both open clusters and disc globular clusters can yield theobserved high numbers of open clusters and the few surviving globulars.
| RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| Near-infrared photometry and spectroscopy of NGC 6539 and UKS 1: two intermediate metallicity bulge globular clusters*
Using the `son of ISAAC' (Infrared Spectrometer And Array Camera) imagerat the European Southern Observatory New Technology Telescope and thenear-infrared spectrographs on Keck II, we have obtained J, K images andechelle spectra covering the range 1.5-1.8μm for the intermediatemetallicity bulge globular clusters NGC 6539 and UKS 1. We find[Fe/H]=-0.76 and -0.78, respectively, and an average α-enhancementof ~+0.44 and ~+0.31 dex, consistent with previous measurements ofmetal-rich bulge clusters, and favouring the scenario of rapid chemicalenrichment. We also measure very low 12C/13C ~ 4.5+/- 1 isotopic ratios in both clusters, suggesting that extra-mixingmechanisms due to cool bottom processing are at work during evolutionalong the red giant branch. Finally, we measure accurate radialvelocities of =+31 +/- 4 and=+57 +/- 6 km s-1 and velocitydispersions of ~8 and ~11 km s-1 for NGC 6539 and UKS 1,respectively,
| 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.
| 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.
| Observations of the companion to the pulsar PSR B1718-19. The role of tidal circularisation
We present optical and infrared observations taken with the Very LargeTelescope of the eclipsing binary pulsar system PSRB1718-19. The candidate companion of the pulsar, identifiedearlier in Hubble Space Telescope observations, has been detected in allthree bands, R, I, and J. These detections allowed us to deriveconstraints on temperature, radius, and mass, pointing to a companionthat has expanded to a radius between one of a main sequence star andone at the Roche-limit. We focus on the role of tidal circularisation inthe system, which will have transformed the initially eccentric orbitexpected from formation scenarios into the nearly circular orbitpresently observed. Based on simple energy balance arguments, we areable to draw a picture of the companion's evolution resulting from theenergy deposition in the star due to circularisation. In this picture,our measurement of the companion's parameters is consistent with theexpected initial eccentricity. However, with the present understandingof tidal dissipation, it remains difficult to account for the short timein which the system was circularised.
| 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.
| High-resolution infrared spectra of NGC 6342 and 6528: two moderately reddened bulge globular clusters
Using the Near Infrared Spectrometer (NIRSPEC) spectrograph at Keck II,we have obtained infrared (IR) echelle spectra covering the range 1.5-1.8 μm for the moderately reddened bulge globular clusters NGC 6342and 6528, finding [Fe/H]=-0.60 and -0.17 dex, respectively. We measurean average α-enhancement of ~+0.33 dex in both clusters,consistent with previous measurements on other metal-rich bulgeclusters, and favouring the scenario of a rapid bulge formation andchemical enrichment. We also measure very low12C/13C isotopic ratios (~5 in NGC 6342 and ~8 inNGC 6528), suggesting that extra-mixing mechanisms resulting from coolbottom processing are at work during evolution along the red giantbranch (RGB).
| 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.
| The planet in M4: implications for planet formation in globular clusters
We consider the formation and evolution of the planetary system PSRB1620-26 in the globular cluster M4. We propose that as M4 is a verylow-metallicity environment the standard model of planet formationaround main-sequence stars through the accretion of gas on to metallicrocky cores should not be applied. Consequently the previously suggestedmethods for formation are unlikely. We propose that the planet formedthrough the interaction of a passing star with a circumbinary discduring the common-envelope phase of the evolution of the inner binary.This formation route is favoured by dense stellar systems such asglobular clusters.
| 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.
| Long-term timing observations of 374 pulsars
We present pulsar timing solutions for 374 pulsars. Each ephemeris wasobtained by analysing archival data stored at Jodrell Bank Observatory.This data archive contains over 5600 yr of pulsar rotational historywith individual data-spans of up to 34 yr. A new method has beendeveloped to mitigate the effects of timing noise by whitening thepulsar timing residuals. This whitening is applied before standardfitting procedures are followed to measure the astrometric anddispersion measure (DM) parameters of a pulsar. We show that the valuesobtained using this new technique are consistent with other methods, andthat the new timing solutions are, in general, significantly moreprecise than those in earlier publications. We consider the secondderivative of the frequency ν of pulsars, , and the DM gradient,d(DM)/dt, in detail. The values are obtained by fitting to timingresiduals that have not been whitened and are found to be orders ofmagnitude larger than those expected from magnetic dipole radiation; themeasured values are dominated by the effects of timing noise, andtherefore lead to braking indices that are not consistent with magneticdipole radiation. We find a dependence between |d(DM)/dt| and DM of pcyr-1, which allows DM variations to be estimated for anyradio pulsar.
| Red giant branch in near-infrared colour-magnitude diagrams - I. Calibration of photometric indices
We present new high-quality near-infrared photometry of 10 Galacticglobular clusters spanning a wide metallicity range (-2.12<=[Fe/H]<=- 0.49): five clusters belong to the halo (NGC 288, 362,6752, M15 and M30) and five (NGC 6342, 6380, 6440, 6441 and 6624) to thebulge. By combining J, H and K observations with optical data, weconstructed colour-magnitude diagrams in various planes: (K, J-K), (K,V-K), (H, J-H) and (H, V-H). A set of photometric indices (colours,magnitudes and slopes) describing the location and the morphology of thered giant branch (RGB) have been measured. We have combined this newdata set with those collected by our group over the last 5 years, andhere we present an updated calibration of the various RGB indices in theTwo-Micron All-Sky Survey photometric system, in terms of the clustermetallicity.
| 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.
| Giant Metrewave Radio Telescope Discovery of a Millisecond Pulsar in a Very Eccentric Binary System
We report the discovery of the binary millisecond pulsar PSRJ0514-4002A, which is the first known pulsar in the globular cluster NGC1851 and the first pulsar discovered using the Giant Metrewave RadioTelescope. The pulsar has a rotational period of 4.99 ms, an orbitalperiod of 18.8 days, and the most eccentric pulsar orbit yet measured(e=0.89). The companion has a minimum mass of 0.9 Msolar, andits nature is currently unclear. After accreting matter from a low-masscompanion star that spun it up to a (few) millisecond spin period, thepulsar eventually exchanged the low-mass star for its more massivepresent companion. This is exactly the same process that could form asystem containing a millisecond pulsar and a black hole; the discoveryof NGC 1851A demonstrates that such systems might exist in the universe,provided that stellar mass black holes exist in globular clusters.
| 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.
| 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.
| The Red Giant Branch luminosity function bump
We present observational estimates of the magnitude difference betweenthe luminosity function red giant branch bump and the horizontal branch(Delta F555WbumpHB), and of star counts in thebump region (Rbump), for a sample of 54 Galactic globularclusters observed by the HST. The large sample of stars resolved in eachcluster, and the high photometric accuracy of the data allowed us todetect the bump also in a number of metal poor clusters. To reduce thephotometric uncertainties, empirical values are compared withtheoretical predictions obtained from a set of updated canonical stellarevolution models which have been transformed directly into the HSTflight system. We found an overall qualitative agreement between theoryand observations. Quantitative estimates of the confidence level arehampered by current uncertainties on the globular cluster metallicityscale, and by the strong dependence of DeltaF555WbumpHB on the cluster metallicity. In case ofthe Rbump parameter, which is only weakly affected by themetallicity, we find a very good quantitative agreement betweentheoretical canonical models and observations. For our full clustersample the average difference between predicted and observedRbump values is practically negligible, and ranges from-0.002 to -0.028, depending on the employed metallicity scale. Theobserved dispersion around these values is entirely consistent with theobservational errors on Rbump. As a comparison, the value ofRbump predicted by theory in case of spurious bump detectionsdue to Poisson noise in the stellar counts would be ~ 0.10 smaller thanthe observed ones. We have also tested the influence on the predictedDelta F555WbumpHB and Rbump values ofan He-enriched component in the cluster stellar population, as recentlysuggested by D'Antona et al. (\cite{d02}). We find that, underreasonable assumptions concerning the size of this He-enrichedpopulation and the degree of enrichment, the predicted DeltaF555WbumpHB and Rbump values are onlymarginally affected.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.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
Каталог и designations:
|
