Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Rings around Lyra.
Not Available
| Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for HI and OH
We present the results of a survey, the deepest to date, for HI emissionat 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720MHz) in the direction towards the Galactic globular clusters M15, M2,NGC6934, NGC7006 and Pal13. The aim is to measure the amount of hydrogenin the intracluster medium, and to find OH masers in the circumstellarenvelopes of globular cluster red giants. We present a tentativedetection of 0.3Msolar of neutral hydrogen in M15 andpossible detections of neutral hydrogen in M2 and Pal13. We derive upperlimits to the neutral hydrogen content of NGC6934 and 7006. No OHemission is detected. We also present deep HI data of the northern tipof the Magellanic Stream behind Pal13.
| 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.
| 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.
| Caroline Herschel as observer
Not Available
| 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 Comprehensive Model for the Monoceros Tidal Stream
We have compiled an extensive data set on potential parts of theMonoceros tidal stream and performed an exhaustive survey of dwarfgalaxy semianalytic orbits in order to constrain its orbital properties.The best-fit orbits are subsequently realized as self-consistent N-bodysimulations in order to reproduce the spatial and velocity distributionof satellite debris. We find that all kinematic and geometricconstraints can be fit by a single stream allowing for multiple wraps.The orbital eccentricity and inclination of the progenitor are stronglyconstrained to be e=0.10+/-0.05 andi=25deg+/-5deg. Ten new estimates of propermotions from the Sloan Digital Sky Survey clearly exclude all retrogradeorbits. Particles lost by the satellite populate two nearly concentricrings, naturally explaining the detection of stream stars at both 6-8kpc (Ibata et al.; Newberg et al.) and 12-18 kpc (the Tri/And stream;Rocha-Pinto et al.) from the Sun. We have attempted to predict thepresent location of the Monoceros stream progenitor using differentinformation: (1) the kinematical and spatial distribution of detections,and (2) the different mean metallicity in the inner and the outer rings.Because of the lack of observational data in the whole range of Galacticlatitudes, the geometrical/kinematical constraints lead to a wide rangeof possible locations. By associating older parts of the model streamwith lower metallicity parts of the observed data, we argue in favor ofa current location of l~245deg, b~-18deg, with adistance to the Sun rs~=15 kpc. The mass of the progenitorhas been poorly constrained because of the slow orbital decay. Similarfits have been obtained for masses(3-9)×108Msolar. We have analyzed thepossible common origin of the Canis Major dwarf and the Monocerosstream. The Canis Major dwarf moves on a prograde, nearly circular orbit(e~=0.16) in the Milky Way disk (i~=4+14-4 deg).This orbital inclination is too low to account for the large verticaldispersion of stream stars. However, the bimodal distribution of radialvelocities in the central region found by Martin et al. probablyindicates that their selection criteria for identifying dwarf stars leadto a contamination of background stars. In that case, the kinematicaldata outlined above might result in an underestimate of the orbitalinclination. Finally, the distance estimation to Canis Major dwarf isaround a factor of 2 smaller than that obtained from our model.Unfortunately, the possible identification of the Monoceros streamprogenitor in Canis Major remains unclear.
| BVRI photometry of the galactic globular cluster NGC 6779
We present B, V, R and I photometry for NGC 6779 (M56), a metal-poorglobular cluster in the galactic halo. The observations were performedusing the 1.3-m telescope at Skinakas Observatory, in Crete. Thereddening of the cluster was found to be E(B-V) = 0.32 +/- 0.02 [E(V-I)= 0.43 +/- 0.02], significantly higher than previous estimates. Themetal abundance of the cluster was derived from various parametrizationsof red giant branch characteristics and it was found to be[Fe/H]ZW=-2.20 +/- 0.12 dex on the Zinn-West metallicityscale, or [Fe/H]CG=-2.00 +/- 0.21 dex on the Carretta-Grattonscale. The distance modulus of the cluster is estimated to be(m-M)V= 15.62 +/- 0.26 (or 14.62, if we correct for thereddening to the cluster). The horizontal branch of NGC 6779 shows aclear gap at (B-V)o= 0.0. Finally, the revised value for themetallicity of NGC 6779 led to a revision of its age to 13 Gyr, usingthe age-index calibrations of Salaris & Weiss.
| A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way
We present an analysis of the asymmetries in the population of GalacticM-giant stars present in the 2MASS All Sky catalogue. Severallarge-scale asymmetries are detected, the most significant of which is astrong elliptical-shaped stellar overdensity, close to the Galacticplane at (l= 240°, b=-8°), in the constellation of Canis Major.A small grouping of globular clusters (NGC 1851, 1904, 2298 and 2808),coincident in position and radial velocity, surround this structure, asdo a number of open clusters. The population of M-giant stars in thisoverdensity is similar in number to that in the core of the Sagittariusdwarf galaxy. We argue that this object is the likely dwarf galaxyprogenitor of the ring-like structure that has recently been found atthe edge of the Galactic disc. A numerical study of the tidal disruptionof an accreted dwarf galaxy is presented. The simulated debris fits theextant position, distance and velocity information on the Galactic`Ring', as well as that of the M-giant overdensities, suggesting thatall these structures are the consequence of a single accretion event.The disrupted dwarf galaxy stream orbits close to the Galactic plane,with a pericentre at approximately the solar circle, an orbitaleccentricity similar to that of stars in the Galactic thick disc, aswell as a vertical scaleheight similar to that of the thick disc. Thisfinding strongly suggests that the Canis Major dwarf galaxy is abuilding block of the Galactic thick disc, that the thick disc iscontinually growing, even up to the present time, and that thick discglobular clusters were accreted on to the Milky Way from dwarf galaxiesin co-planar orbits.
| 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.
| A global and a local criterion in defining the tidal radius
The basic theory of potential-energy tensors related to heterogeneousspheres is reviewed, and the special case of truncated, singular,isothermal spheres is examined in detail. Special effort is devoted toa system made of two isothermal spheres, one completely lying within theother, the mass and the radius of the embedded sphere being negligiblewith respect to the mass and radius of the embedding sphere, and theradius of the embedded sphere being negligible with respect to thedistance between the centres. The potential-energy tensors related tothe potential induced by the embedding sphere on the mass distributionof the embedded sphere, are expressed as the sum of two contributions:one, coming from the embedded sphere after collapse towards its centre,and one other, related to the actual mass distribution of the embeddedsphere. Using the latter, both a global and a local criterion indefining the tidal radius of the embedded sphere, are formulated inconnection with either the binding-energy tensor or the virial-energytensor. In doing this, the tensor components along the axis joining thecentre of the embedding and the embedded sphere, are considered. Theglobal criterion is related to the whole, embedded sphere, while thelocal criterion is related to an infinitesimal mass element placed atthe boundary of the embedded sphere, where the distance from the centreof the embedding sphere attains a maximum. The virial theorem intensor form is splitted into two distinct expressions, related toorbital and intrinsic motions of the embedded sphere. Alternativecriterions in defining the tidal radius of the embedded sphere, areformulated taking into consideration the centrifugal tensor potentialand the tensor potential induced by orbital motions. With regard to aselected criterion, the tidal radius calculated with and without theinclusion of the centrifugal potential, exhibits a maximum variation bya factor of about two, related to circular orbits. An application ismade, where the embedding and the embedded sphere are taken asrepresentative of the Galaxy and a globular cluster, respectively. Itis found that a stability region exists for both the global and thelocal criterion and, in addition, the instability first occurs at theperigalacticon, as expected in connection with instantaneous tidalradius. A power-law dependence of tidal radius from cluster mass andgalactocentric distance, aC* ∝MC1/3R02/3, is shown to beconsistent with data from a sample of 16 objects investigated byte{bra99} (1999). No significant correlation is found between theratio of cluster radius to tidal radius and the orbital ratio ofapogalacticon to perigalacticon, similar to averaged tidal radii definedby te{bra99} (1999). An additional object, Pal 5, which is experiencingprogressive disruption via tidal shocks during disk passages, is shownto be among the less bound (or more unbound) clusters, within theframework of the model. If the representation of globular clusters asisothermal spheres introduces only systematic errors in the ratio ofcluster radius to tidal radius,aC/aC*=γ/η, then at leastone other cluster, NGC 5466 (which has the highest value between thesample objects), is inferred to undergo tidal disruption, in the modelinterpretation.
| Hubble Space Telescope Snapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441
We present the results of a Hubble Space Telescope snapshot program tosurvey the inner region of the metal-rich globular cluster NGC 6441 forits variable stars. A total of 57 variable stars were found, including38 RR Lyrae stars, six Population II Cepheids, and 12 long-periodvariables. Twenty-four of the RR Lyrae stars and all of the PopulationII Cepheids were previously undiscovered in ground-based surveys. Of theRR Lyrae stars observed in this survey, 26 are pulsating in thefundamental mode with a mean period of 0.753 days and 12 arefirst-overtone-mode pulsators with a mean period of 0.365 days. Thesevalues match up very well with those found in ground-based surveys.Combining all the available data for NGC 6441, we find mean periods of0.759 and 0.375 days for the RRab and RRc stars, respectively. We alsofind that the RR Lyrae stars in this survey are located in the sameregions of a period-amplitude diagram as those found in ground-basedsurveys. The overall ratio of RRc to total RR Lyrae stars is 0.33.Although NGC 6441 is a metal-rich globular cluster and would, on thatground, be expected either to have few RR Lyrae stars or to be anOosterhoff type I system, its RR Lyrae stars more closely resemble thosein Oosterhoff type II globular clusters. However, even compared withtypical Oosterhoff type II systems, the mean period of its RRab stars isunusually long. We also derived I-band period-luminosity relations forthe RR Lyrae stars. Of the six Population II Cepheids, five are of WVirginis type and one is a BL Herculis variable star. This makes NGC6441, along with NGC 6388, the most metal-rich globular cluster known tocontain these types of variable stars. Another variable, V118, may alsobe a Population II Cepheid, given its long period and its separation inmagnitude from the RR Lyrae stars. We examine the period-luminosityrelation for these Population II Cepheids and compare it with those inother globular clusters and in the Large Magellanic Cloud. We argue thatthere does not appear to be a change in the period-luminosity relationslope between the BL Herculis and W Virginis stars, but that a change ofslope does occur when the RV Tauri stars are added to theperiod-luminosity relation.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.
| Urban Astronomy: Observing the Messier Objects from the City
Not Available
| The Cepheids of Population II and Related Stars
The Type II Cepheids include most intrinsic variables with periodsbetween 1 and about 50 days, except for the classical Cepheids and theshortest semiregular variables of type M. The Type II Cepheids may bedivided in groups by period, such that the stars with periods beween 1and 5 days (BL Her class), 10-20 days (W Vir class), and greater than 20days (RV Tau class) have differing evolutionary histories. The chemicalcomposition of Type II Cepheids reflects the material they were madefrom as modified by their internal nuclear evolution and mixing.Finally, RV Tau stars are affected by mass loss by dust and speciesattached to the dust. The populations to which the various classes ofType II Cepheids are assigned constitute important clues to the originand evolution of the halo of our Galaxy and the dwarf spheroidal systemsfrom which at least part of the halo seems to have been accreted.
| Logarithmic Density Range and the Percentage of Stars in the Cores of Various Subsystems of the Globular Clusters M56, M12, NGC 6535, NGC 6171, NGC 5466, and M92
Lower limits for the percentages of stars with various luminosities inthe cores of six globular clusters are derived using stellar spatialdensity distributions f(r) to deep limiting B magnitudes obtainedearlier. For NGC 6535 and NGC 5466, the logarithmic density range andKholopov parameters D f and D r are also determined. These twoparameters are correlated with the mean masses of stars of varioussubsystems and the total mass (number) of stars in the cluster.
| Spatial Structure of the Old Open Cluster NGC 2420
The spatial structure of the intermediate and outer regions of the oldopen cluster NGC 2420 is analyzed using data from the catalog ofPaparó. The differential and integrated distributions of theprojected [ΔF(r) and F(r)] and spatial [Δf(r) and f(r)]stellar densities are obtained for various subsystems of cluster starsusing Kholopov’s star-count method. Analysis of these curves showsthat: (1) the cluster has at least three distinct spatial zones withdifferent stellar-density gradients, (2) the each cluster subsystem hasa layered structure, and (3) the spatial structure of the clusterchanges systematically in the transition to subsystems containingfainter main-sequence stars. Empirical relations describing thestructure of the cluster are also derived. Similar structural featurescan also be found in other globular and open clusters.
| Spatial Structure of the Globular Cluster M15
The spatial structure of the metal-poor globular cluster M15 is studiedas a function of magnitude interval ΔB and the limiting Bmagnitude of star counts. Astrometric and photometric measurements oftwo plates obtained with the 2-m reflector of the National Academy ofSciences of Bulgaria were used. Analysis of the differential andintegrated apparent (ΔF(r) and F(r)) and spatial (Δf(r) andf(r)) stellar density distributions in different ΔB intervals andto different limiting magnitudes reaching B=21.5m indicates that thecluster structure changes systematically as we consider fainter stars,beginning with the transition region between the subgiant branch and themain-sequence turnoff in the (V, B-V) diagram. This variation ismanifest in two ways: a homological growth in the radii of spatial zonesof the cluster and of the cluster radius in accordance with a singlelaw, and variations in the stellar density, with the rate of increase inthe number of faint stars growing toward the outer zones of the cluster.Empirical relations describing these variations and parametersdetermining the cluster structure are obtained.
| On the Distribution of Orbital Poles of Milky Way Satellites
In numerous studies of the outer Galactic halo some evidence foraccretion has been found. If the outer halo did form in part or whollythrough merger events, we might expect to find coherent streams of starsand globular clusters following orbits similar to those of their parentobjects, which are assumed to be present or former Milky Way dwarfsatellite galaxies. We present a study of this phenomenon by assessingthe likelihood of potential descendant ``dynamical families'' in theouter halo. We conduct two analyses: one that involves a statisticalanalysis of the spatial distribution of all known Galactic dwarfsatellite galaxies (DSGs) and globular clusters, and a second, morespecific analysis of those globular clusters and DSGs for which fullphase space dynamical data exist. In both cases our methodology isappropriate only to members of descendant dynamical families that retainnearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) isconsidered a paradigm for the type of merger/tidal interaction event forwhich we are searching, we also undertake a case study of the Sgr systemand identify several globular clusters that may be members of itsextended dynamical family. In our first analysis, the distribution ofpossible orbital poles for the entire sample of outer(Rgc>8 kpc) halo globular clusters is tested forstatistically significant associations among globular clusters and DSGs.Our methodology for identifying possible associations is similar to thatused by Lynden-Bell & Lynden-Bell, but we put the associations on amore statistical foundation. Moreover, we study the degree of possibledynamical clustering among various interesting ensembles of globularclusters and satellite galaxies. Among the ensembles studied, we findthe globular cluster subpopulation with the highest statisticallikelihood of association with one or more of the Galactic DSGs to bethe distant, outer halo (Rgc>25 kpc), second-parameterglobular clusters. The results of our orbital pole analysis aresupported by the great circle cell count methodology of Johnston,Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC6229, NGC 7006, and Pyxis are predicted to be among those most likely toshow the clusters to be following stream orbits, since these clustersare responsible for the majority of the statistical significance of theassociation between outer halo, second-parameter globular clusters andthe Milky Way DSGs. In our second analysis, we study the orbits of the41 globular clusters and six Milky Way-bound DSGs having measured propermotions to look for objects with both coplanar orbits and similarangular momenta. Unfortunately, the majority of globular clusters withmeasured proper motions are inner halo clusters that are less likely toretain memory of their original orbit. Although four potential globularcluster/DSG associations are found, we believe three of theseassociations involving inner halo clusters to be coincidental. While thepresent sample of objects with complete dynamical data is small and doesnot include many of the globular clusters that are more likely to havebeen captured by the Milky Way, the methodology we adopt will becomeincreasingly powerful as more proper motions are measured for distantGalactic satellites and globular clusters, and especially as resultsfrom the Space Interferometry Mission (SIM) become available.
| Space distribution and motional orbits of globular clusters in the galaxy
The 29 F globular clusters in the galaxy are selected as samples.According to the basic data, radial velocities and proper motions ofsample clusters, the initial positions and velocities of the samples arereduced using the galactic coordinates, and their orbits are integratedby numerical method for three different Galactic gravitational potentialmodels. The calculating results show: (1) most of samples are located in5 kpc---10 kpc from Galactic center. All of the sample clusters presenta spherical symmetrical distribution around the Galactic center, andtheir space velocities are presented a ellipsoidal distribution; (2)According to the metallicity and basic characters, the sample clustersare separated into HB subgroup and MP subgroup. The number of samplesare changed with metallicity [Fe/H], and there is a peak at [Fe/H]=-1.6; (3) The orbits of sample clusters show mostly limited, periodiccharacteristics, but the orbits are not closed completely, their maximalgalactocentric distance is less than 40 kpc. The differences in orbitalmorphologies due to different potentials is slighting, however, given acertain potential, for clusters that have perigalactic distance smallerthan 1 kpc, some orbits may exhibit a chaotic behavior. The correlationbetween the metallicity of samples and the orbital morphologies isunclearly; (4) It is found that the semi-major axis, apogalacticdistance and azimuth period of 29 example clusters are changed withtheir metallicity similarly, but a obvious correlation is seen betweenorbital eccentricity and metallicity. There is a fraction of 24% of thesample clusters with eccentricities lower than 0.4. The differentGalactic gravitational potential have not clear influence upon theperigalactic distance, eccentricity and uncertainty of orbitalparameters, but which is significant for other parameters, such as theapogalactic distance, semi-major axis, radial period and azimuth periodand so on.
| Does the mixing length parameter depend on metallicity?. Further tests of evolutionary sequences using homogeneous databases
This paper is a further step in the investigation of the morphology ofthe color-magnitude diagram of Galactic globular clusters, and thefine-tuning of theoretical models, made possible by the recentobservational efforts to build homogeneous photometric databases. Inparticular, we examine here the calibration of the morphologicalparameter WHB vs. metallicity, originally proposed by Brocatoet al. (\cite{brocatoEtal98}; B98), which essentially measures the colorposition of the red-giant branch. We show that the parameter can be usedto have a first-order estimate of the cluster metallicity, since thedispersion around the mean trend with [Fe/H] is compatible with themeasurement errors. The tight WHB-[Fe/H] relation is thenused to show that variations in helium content or age do not affect theparameter, whereas it is strongly influenced by the mixing-lengthparameter alpha (as expected). This fact allows us, for the first time,to state that there is no trend of alpha with the metal content of acluster. A thorough examination of the interrelated questions of thealpha -elements enhancement and the color-Tefftransformations, highlights that there is an urgent need for anindependent assessment of which of the two presently acceptedmetallicity scales is the true indicator of a cluster's iron content.Whatever scenario is adopted, it also appears that a deep revision ofthe V-I-temperature relations is needed.
| Red giant branch stars as probes of stellar populations. I. 2MASS calibration and application to 2MASS GC01
The near-infrared behavior of the red giant branch (RGB hereafter) as afunction of abundance is examined with an unprecedented large sample of27 Galactic globular clusters with Two Micron All Sky Survey photometry.We propose a new simplified analysis, involving the zero point of theRGB slope fit, and derive calibrations for the RGB slope, zero point,and tip. The weak metallicity sensitivity of the zero point leads to a``fan''-like diagram to obtain the abundance distributions in resolvedstellar systems, and reddening estimates. Finally, we apply the newcalibrations to the recently discovered Galactic globular cluster 2MASSGC01, to derive [Fe/H]H96=-1.19+/-0.38 mag. The uncertaintyis dominated by the severe foreground contamination. We estimate anextinction of AV=21.07+/-2.20 mag toward the cluster.
| Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms
We present homogeneous age determinations for a large sample of 55Galactic globular clusters, which constitute about 30% of the totalGalactic population. A study of their age distribution reveals that allclusters from the most metal poor ones up to intermediate metallicitiesare coeval, whereas at higher [Fe/H] an age spread exists, together withan age-metallicity relationship. At the same time, all clusters within acertain galactocentric distance appear coeval, whereas an age spread ispresent further away from the Galactic centre, without any correlationwith distance. The precise value of [Fe/H] and galactocentric distancefor the onset of the age spread and the slope of the age-metallicityrelationship are strongly affected by the as yet uncertain [Fe/H] scale.We discuss how differences in the adopted [Fe/H] scale and clustersample size may explain discrepant results about the clusters agedistribution reached by different authors. Taking advantage of the largenumber of objects included in our sample, we also tested the possibilitythat age is the global second parameter which determines the HorizontalBranch morphology, and found indications that age could explain theglobal behaviour of the second parameter effect.
| CCD photometry in the Vilnius system in the vicinity of the globular cluster M 56
CCD observations of field stars in an area north-east of the globularcluster M 56 were obtained in six bands of the Vilnius photometricsystem. The data sets cover a field of 9.9 m x 9.5 m. An outline of thedata pathway from observations to reduction via IRAF is outlined in thistext. Photometry of 366 stars brighter than 18th V magnitude has beenobtained and the analysis of the observational errors is discussed.
| Detection of Ionized Gas in the Globular Cluster 47 Tucanae
We report the detection of ionized intracluster gas in the globularcluster 47 Tucanae. Pulsars in this cluster with a negative periodderivative, which must lie in the distant half of the cluster, havesignificantly higher measured integrated electron column densities thanthe pulsars with a positive period derivative. We derive the plasmadensity within the central few parsecs of the cluster using twodifferent methods that yield consistent values. Our best estimate ofne=0.067+/-0.015 cm-3 is about 100 times the freeelectron density of the interstellar medium in the vicinity of 47Tucanae, and the ionized gas is probably the dominant component of theintracluster medium.
| Age as the Second Parameter in NGC 288/NGC 362? II. The Horizontal Branch Revisited
We revisit the ``second-parameter'' pair of globular clusters NGC288/362 on the basis of theoretical models for red giant branch (RGB)and horizontal branch (HB) stars. The results of the most extensive setof RGB/HB simulations computed so far for these clusters are presentedfor two different metallicities. Using several different analyticalmass-loss formulae for RGB stars, we derive relative ``HB morphologyages.'' We compare them with the relative main-sequence turnoff agesderived by application of the ``bridge test'' in Paper I, where it wasfound that NGC 288 is 2+/-1 Gyr older than NGC 362. We find thatadoption of a higher metallicity ([Fe/H]~-1.2), as favored by theCarretta & Gratton metallicity scale, makes age a much moreplausible second-parameter candidate for this pair than is the case whena lower metallicity ([Fe/H]~-1.5), closer to the Zinn & West scale,is adopted. However, while the different HB morphology of these twoclusters can be reproduced by canonical HB models with [Fe/H]~-1.2 andan age difference of 2 Gyr, this explanation is not without difficulty.In particular, we confirm previous suggestions that canonical models areunable to reproduce the detailed HB morphology of NGC 288 at its redend, for as yet unknown reasons. Finally, we show that the massdispersion on the HB of NGC 362 is substantially larger than for NGC 288and suggest that there is a correlation between the mass dispersion onthe HB phase and the central density of globular clusters. This ispresumably related to the way environmental effects affect RGB massloss-another major second-parameter candidate. We argue that, ifconfirmed, this central density-HB mass dispersion correlation will haveto be taken into account in order to conclusively determine whether agemay be considered the (sole) second parameter of HB morphology for this(and other) second-parameter pair(s).
| Variable Stars in Galactic Globular Clusters
Based on a search of the literature up to 2001 May, the number of knownvariable stars in Galactic globular clusters is approximately 3000. Ofthese, more than 2200 have known periods and the majority (approximately1800) are of the RR Lyrae type. In addition to the RR Lyrae population,there are approximately 100 eclipsing binaries, 120 SX Phoenicisvariables, 60 Cepheids (including Population II Cepheids, anomalousCepheids and RV Tauri), and 120 SR/red variables. The mean period of thefundamental mode RR Lyrae variables is 0.585 days, for the overtonevariables it is 0.342 days (0.349 days for the first-overtone pulsatorsand 0.296 days for the second-overtone pulsators) and approximately 30%are overtone pulsators. These numbers indicate that about 65% of RRLyrae variables in Galactic globular clusters belong to Oosterhoff typeI systems. The mean period of the RR Lyrae variables in the Oosterhofftype I clusters seems to be correlated with metal abundance in the sensethat the periods are longer in the more metal poor clusters. Such acorrelation does not exist for the Oosterhoff type II clusters. Most ofthe Cepheids are in clusters with blue horizontal branches.
| K 413, a star near the AGB in the globular cluster M 12
CCD spectra obtained with the echelle spectrometer of the 6-metertelescope were used to determine, by the model atmospheres method, thefundamental parameters ( Teff = 4800 K, log g = 0.7) anddetailed chemical abundances for the star K 413, a member of theglobular cluster M 12. The resulting value, [Fe/H] = -1.38, is the firstmetallicity determination for M 12 using high-resolution spectra. Themain characteristic feature of the star's atmospheric chemical abundancepattern is a large oxygen excess, [O/Fe] ~ +2. The s-process elementsare probably slightly depleted compared to metallicity: [X/Fe] = -0.04for yttrium and zirconium, [Ba/Fe] = -0.12 for barium. Abundances of theheavier elements: La, Ce, Nd, and Pr, do not differ, relative to iron,from the solar ones: [heavy/Fe] = 0.0. The europium excess, [Eu/Fe] =+0.48, is typical of members of low-metallicity globular clusters. Thespectrum of K 413 shows, for the Hα line, an variable absorptionand emission profile. From its high luminosity and chemical abundanceanomalies, we can suppose that K 413 is in an evolutionary stage afterthe AGB. In the spectrum, we find absorption details that can beidentified with diffuse interstellar bands displaced by 16 kms-1 to longer wavelengths relative to the star's velocity.
| A census with ROSAT of low-luminosity X-ray sources in globular clusters
I analyze 101 observations from the ROSAT archive to search for X-raysources in or near 55 globular clusters. New sources are found in thecores of NGC 362 (a double source), NGC 6121 (marginally significant),NGC 6139, and NGC 6266; and outside the cores of NGC 6205, NGC 6352 andNGC 6388. More accurate positions are determined for the X-ray sourcesin some ten clusters. The improved position for the source in NGC 6341excludes the suggested ultraviolet counterpart. It is shown that one ofthe two sources reported near the core of NGC 6626 is spurious, as isthe detection of a pulsar period in the PSPC data of this cluster; thecentral source is resolved in three sources. One source reportedpreviously in NGC 6304 is demoted to an upper limit. For 20 clustercores better upper limits to the X-ray luminosity are obtained. From astatistical analysis I argue that several sources outside the clustercores may well belong to the clusters. All spectral energy distributionsobserved so far are relatively soft, with bremsstrahlung temperatures =~0.9 keV; there is evidence however that bremsstrahlung spectra do notcorrectly describe the spectra. The X-ray luminosity per unit mass forthe cluster as a whole does not depend on the concentration; theluminosity per unit mass for the core may increase with the clusterconcentration.
| Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way
Not Available
| A possible detection of diffuse extended X-ray emission in the environment of the globular cluster NGC 6779
We report the possible detection of diffuse X-ray emission in theenvironment of NGC 6779, and find the emission to be well aligned withthe proper motion of the cluster. The position of the emission suggestswe are observing heated ISM in the wake of the cluster that could be theresult of an interaction between the intracluster medium and the halogas surrounding it.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
INHABITED SKY
