Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

M 53



Upload your image

DSS Images   Other Images

Related articles

Discovery of an SX Phoenicis Type Pulsating Component in the Algol-Type Semidetached Eclipsing Binary QU Sagittae in M71
We report the discovery of an SX Phoenicis type pulsating component inthe Algol-type semidetached eclipsing binary QU Sge, in the metal-richglobular cluster M71. QU Sge is only about 80" from the center of M71and is located in the blue straggler region in the color-magnitudediagram of M71. It is considered to be a probable member of M71, with amembership probability greater than 60% deduced from a proper-motionstudy in the literature. From time-series CCD photometry, we find thatQU Sge has an orbital period of 3.790818 days and a primary minimumdepth of ΔV=1.333 mag. The eclipsing light curve solution showsthat QU Sge has a semidetached binary configuration with the secondarycomponent fully filling its Roche lobe. After subtracting the eclipsesfrom the light curve, we discover an SX Phoenicis type pulsationfeature. It is found to have a short period of about 0.03 days and asmall amplitude of about 0.024 mag. This is the first eclipsing binarysystem in a globular cluster to exhibit a pulsating feature. This resultsupports the model in which the origin of some blue stragglers inglobular clusters is mass transfer between two components in theprimordial binary systems.

Nearby Spiral Globular Cluster Systems. I. Luminosity Functions
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.

Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.

Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.

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.

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.

Highlights from the Observatories
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.

The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars
We discuss a 175 deg2 spectroscopic survey for bluehorizontal branch (BHB) stars in the Galactic halo. We use the TwoMicron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) toselect BHB candidates, and we find that the 2MASS and SDSS colorselection is 38% and 50% efficient, respectively, for BHB stars. Oursamples include one likely runaway B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent withmembership in the halo population: the median metallicity is[Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and themean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is-4+/-30 km s-1. We discuss the theoretical basis of thePreston, Shectman, and Beers MV-color relation for BHB starsand conclude that the intrinsic shape of the BHB MV-colorrelation results from the physics of stars on the horizontal branch. Wecalculate the luminosity function for the field BHB star samples usingthe maximum likelihood method of Efstathiou and coworkers, which isunbiased by density variations. The field BHB luminosity functionexhibits a steep rise at bright luminosities, a peak between0.8

Galactic Globular Cluster Relative Ages
We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the differencebetween the horizontal branch and the turnoff in two internallyphotometrically homogeneous databases. The mutual consistency of the twodata sets has been assessed by comparing the ages of 16 globularclusters in common between the two databases. We have also investigatedthe consistency of our relative age determination within the recentstellar model framework. All clusters with [Fe/H]<-1.7 are found tobe old and coeval, with the possible exception of two objects, which aremarginally younger. The age dispersion for the metal-poor clusters is0.6 Gyr (rms), consistent with a null age dispersion.Intermediate-metallicity clusters (-1.7<[Fe/H]<-0.8) are onaverage 1.5 Gyr younger than the metal-poor ones, with an age dispersionof 1.0 Gyr (rms) and a total age range of ~3 Gyr. About 15% of theintermediate-metallicity clusters are coeval with the oldest clusters.All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the mostmetal-poor ones, with a relatively small age dispersion, although themetal-rich sample is still too small to allow firmer conclusions. Thereis no correlation of the cluster age with the galactocentric distance.We briefly discuss the implication of these observational results forthe formation history of the Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555, and on observations made at the European SouthernObservatory, La Silla, Chile, and with the Isaac Newton GroupTelescopes.

The Australia Telescope National Facility Pulsar Catalogue
We have compiled a new and complete catalog of the main properties ofthe 1509 pulsars for which published information currently exists. Thecatalog includes all spin-powered pulsars, as well as anomalous X-raypulsars and soft gamma-ray repeaters showing coherent pulsed emission,but excludes accretion-powered systems. References are given for alldata listed. We have also developed a new World Wide Web interface foraccessing and displaying either tabular or plotted data with the optionof selecting pulsars to be displayed via logical conditions on parameterexpressions. The Web interface has an ``expert'' mode giving access to awider range of parameters and allowing the use of custom databases. Forusers with locally installed software and database on Unix or Linuxsystems, the catalog may be accessed from a command-line interface.C-language functions to access specified parameters are also available.The catalog is updated from time to time to include new information.

Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.

Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.

Integrated spectral energy distributions and absorption-feature indices of single stellar populations
Using evolutionary population synthesis, we present integrated spectralenergy distributions and absorption-line indices defined by the LickObservatory image dissector scanner (referred to as Lick/IDS) system,for an extensive set of instantaneous burst single stellar populations(SSPs). The ages of the SSPs are in the range 1 Gyr <=τ<= 19Gyr and the metallicities are in the range -2.3 <=[Fe/H]<=+0.2.Our models use the rapid single stellar evolution algorithm of Hurley,Pols and Tout for the stellar evolutionary tracks, the empirical andsemi-empirical calibrated BaSeL-2.0 model of Lejeune, Cuisinier andBuser for the library of stellar spectra and the empirical fittingfunctions of Worthey, Faber, Gonzalez and Burstein for the Lick/IDSspectral absorption-feature indices.Applying our synthetic Lick/IDS absorption-line indices to the meritfunction, we obtain the age and the metallicity of the central region ofM32, which can be well explained by an instantaneous SSP with an age of~6.5 Gyr and a metallicity similar to solar. Applying the derived ageand the metallicity from the merit function to a number of index-indexdiagrams, we find that the plots of Hβ-Fe5015 andHβ-Fe5782 are the best index-index diagrams from whichwe can directly obtain reasonable age and metallicity.

Globular Cluster and Galaxy Formation: M31, the Milky Way, and Implications for Globular Cluster Systems of Spiral Galaxies
We find that the globular cluster (GC) systems of the Milky Way and ofour neighboring spiral galaxy, M31, comprise two distinct entities,differing in three respects. First, M31 has a set of young GCs, rangingin age from a few times 102 Myr to 5 Gyr old, as well as oldGCs. No such very young GCs are known in the Milky Way. Second, weconfirm that the oldest M31 GCs have much higher nitrogen abundancesthan do Galactic GCs at equivalent metallicities, while carbonabundances appear normal for the GCs in both galaxies. Third, Morrisonand coworkers have shown that M31 has a subcomponent of GCs that followclosely the disk rotation curve of that galaxy. Such a GC system in ourown Galaxy has yet to be found. The only plausible scenario for theexistence of the young M31 GC comes from thehierarchical-clustering-merging (HCM) paradigm for galaxy formation. Weinfer that M31 has absorbed more of its contingent of dwarf systems inthe recent past than has the Milky Way. This inference has threeimplications: First, all spiral galaxies could differ in their GCproperties, depending on how many companions each galaxy has and whenthe parent galaxy absorbs them. In this spectrum of possibilities,apparently the Milky Way ties down one end, in which almost all of itsGCs were absorbed 10-12 Gyr ago. Second, it suggests that young GCs arepreferentially formed in the dwarf companions of parent galaxies andthen absorbed by the parent galaxy during mergers. Third, the young GCsseen in tidally interacting galaxies might come from the dwarfcompanions of these galaxies, rather than be made anew in the tidalinteraction. However, there is no ready explanation for the markeddifference in nitrogen abundance for the stars in the old M31 GCsrelative to those in the oldest Galactic GCs, especially the mostmetal-poor GCs in both galaxies. The predictions made by Li &Burstein regarding the origin of nitrogen abundance in GCs areconsistent with what is found for the old M31 GCs compared to that forthe two 5 Gyr old M31 GCs.Observations reported here were obtained at the MMT Observatory, a jointfacility of the Smithsonian Institution and the University of Arizona.

Exploring the Upper Red Giant and Asymptotic Giant Branches: The Globular Cluster M5
We have tabulated lists of upper red giant, horizontal-, and asymptoticgiant branch (RGB, HB, and AGB) stars in the globular cluster M5 thatare complete to over 10' from the core for the RGB and AGB samples, and8' for the HB sample. The large samples give us the most precise valueof R2=NAGB/NHB to date for a singleglobular cluster (0.176+/-0.018). This is incompatible with theoreticalcalculations using the most recent physical inputs. The discrepancy canprobably be attributed to the dependence of observed R2values on HB morphology. We identify the cluster M55 as another possibleexample of this effect. Samples of HB and AGB stars in populous clustersmay provide a means of calibrating the masses of HB stars in globularclusters. The cumulative luminosity function of the upper RGB shows anapparent deficit of observed stars near the tip of the branch. Thisfeature has less than a 2% chance of being due to statisticalfluctuations. The slope of the cumulative luminosity function for AGBstars is consistent with the theoretically predicted value from stellarmodels when measurement bias is taken into account. We also introduce anew diagnostic Rclump that reflects the fraction of the AGBlifetime that a star spends in the AGB clump. For M5 we findRclump=0.42+/-0.05, in marginal disagreement with theoreticalpredictions. Finally, we note that the blue half of M5's instabilitystrip (where first-overtone RR Lyrae stars reside) is underpopulated,based on the large numbers of fundamental mode RR Lyrae stars and on thenonvariable stars at the blue end of the instability strip. This factmay imply that the evolutionary tracks (and particularly the colors) ofstars in the instability strip are affected by pulsations.

The Evolutionary Status of M3 RR Lyrae Variable Stars: Breakdown of the Canonical Framework?
In order to test the prevailing paradigm of horizontal-branch (HB)stellar evolution, we use the large databases of measured RR Lyraeparameters for the globular cluster M3 (NGC 5272) recently provided byBakos et al. and Corwin & Carney. We compare the observeddistribution of fundamentalized periods against the predictions ofsynthetic HBs. The observed distribution shows a sharp peak atPf~0.55 days, which is primarily due to the RRab variables,whereas the model predictions instead indicate that the distributionshould be more uniform in Pf, with a buildup of variableswith shorter periods (Pf<0.5 days). Detailed statisticaltests show, for the first time, that the observed and predicteddistributions are incompatible with one another at a high significancelevel. This indicates either that canonical HB models are inappropriate,or that M3 is a pathological case that cannot be consideredrepresentative of the Oosterhoff type I (OoI) class. In this sense, weshow that the OoI cluster with the next largest number of RR Lyraevariables, M5 (NGC 5904), presents a similar, although less dramatic,challenge to the models. We show that the sharp peak in the M3 perioddistribution receives a significant contribution from the Blazhkovariables in the cluster. We also show that M15 (NGC 7078) and M68 (NGC4590) show similar peaks in their Pf distributions, which inspite of being located at a Pf value similar to that of M3,can, however, be primarily ascribed to the RRc variables. Again similarto M3, a demise of RRc variables toward the blue edge of the instabilitystrip is also identified in these two globulars. This is again in sharpcontrast to the evolutionary scenario, which also foresees a strongbuildup of RRc variables with short periods in OoII globulars. Wespeculate that in OoI systems RRab variables may somehow get ``trapped''close to the transition line between RRab and RRc pulsators as theyevolve to the blue in the H-R diagram, whereas in OoII systems it is theRRc variables that may get similarly trapped instead, as they evolve tothe red, before changing their pulsation mode to RRab. Such a scenariois supported by the available CMDs and Bailey diagrams for M3, M15, andM68.

SX Phoenicis Stars in the Globular Cluster NGC 5466
Through time-series CCD photometry of the globular cluster NGC 5466, wehave detected nine SX Phoenicis stars, including three new ones. All theSX Phoenicis stars are located in the blue straggler region in thecolor-magnitude diagram of NGC 5466. Five of them clearly showdouble-radial mode features, the periods of which are consistent withthe theoretical ratio of the first-overtone mode to the fundamental mode(P1H/PF). Normally, it has not been easy to securea P-L relation for the SX Phoenicis stars because their pulsational modehas been difficult to determine. The existence of five SX Phoenicisstars in NGC 5466 with double-radial modes allows us to reliably derivea P-L relation for the fundamental mode of SX Phoenicis stars. Usingseven SX Phoenicis stars, including five stars with double-radial modes,we derive a P-L relation for the fundamental mode in NGC 5466,=-3.25(+/-0.46)logP+14.70(+/-0.06),(σ=+/-0.04),corresponding to =-3.25(+/-0.46)logP-1.30(+/-0.06)for an adopted distance modulus of (m-M)0=16.00 and zeroreddening.

A CN Band Survey of Red Giants in the Globular Cluster M53
We investigate the star-to-star variations in λ 3883 CNbandstrength among red giant stars in the low-metallicity globularcluster M53 ([Fe/H] = --2.0). Our data were taken with the Kastspectrograph on the 3-meter Shane telescope at Lick Observatory in April2001. Star-to-star variations in CN bandstrength are common inintermediate- and high-metallicity globular clusters ([Fe/H] ≥--1.6). Our data were obtained to test whether that variation will alsobe present in a low-metallicity globular cluster, or whether it will besuppressed by the overall lack of metals in the stars. Our preliminaryresult is that the λ 3883 CN band is weak in our program stars,which span the brightest magnitude of the red giant branch. On visualinspection, the M53 giants appear to be similar in their CN bandstrengthto the four CN-weak giants in NGC 6752 whose average spectrum is plottedin Fig. 4 of Norris et al. (1981, ApJ, 244, 205). This work is plannedto form part of a larger study of the metallicity dependence of CNbandstrength and carbon abundance behavior on the upper giant branch ofglobular clusters. This work is supported by NSF grant AST 00-98453 andby an award from the ARCS foundation, Northern California Chapter.

Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.

RR Lyrae variables in Galactic globular clusters. IV. Synthetic HB and RR Lyrae predictions
We present theoretical predictions concerning horizontal branch stars inglobular clusters, including RR Lyrae variables, as derived fromsynthetic procedures collating evolutionary and pulsational constraints.On this basis, we explore the predicted behavior of the pulsators as afunction of the horizontal branch morphology and over the metallicityrange Z= 0.0001 to 0.006, revealing an encouraging concordance with theobserved distribution of fundamentalised periods with metallicity.Theoretical relations connecting periods to K magnitudes and BV or VIWesenheit functions are presented, both appearing quite independent ofthe horizontal branch morphology only with Z≥ 0.001. Predictionsconcerning the parameter R are also discussed and compared under variousassumptions about the horizontal branch reference luminosity level.

The 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.

Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.

Abundances in globular cluster dwarfs
Not Available

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.

Nitrogen Overabundance: Globular Cluster and Halo Formation
Halo globular clusters pose four succinct issues that must be solved inany scenario of their formation: single-age, single-metallicity stellarpopulations; a lower limit ([Fe/H]~-2.5) to their average metallicity;comprising only 1% of the stellar halo mass; and being among the oldeststars in our Galaxy. New spectra are presented of Galactic stars andintegrated spectra of Galactic globular clusters that extend to 3250Å. These spectra show that the most metal-poor and among thebest-studied Galactic globular clusters show strong NH 3360 Åabsorption, even though their spectral energy distributions in thenear-UV are dominated by blue horizontal-branch AF-type stars. Thesestrong NH features must be coming from the main-sequence stars in theseclusters. These new data are combined with existing data on the widerange of carbon and nitrogen abundance in very metal-poor ([Fe/H] <-3.5) halo giant and dwarf stars, together with recent models ofzero-metal star formation, to make a strawman scenario for globularcluster formation that can reproduce three of the above four issues, aswell as two of the three related issues pertaining to nitrogenoverabundance. This strawman proposal makes observational andtheoretical predictions that are testable, needing specific help fromthe modelers to understand all of the elemental constraints on globularcluster and halo formation.

Galaxy Formation: Cold Dark Matter, Feedback, and the Hubble Sequence
TreeSPH simulations of galaxy formation in a standard Λ cold darkmatter cosmology, including star formation and the effects of energeticstellar feedback processes and of a metagalactic UV field, have beenperformed, resulting in a mix of realistic disk, lenticular, andelliptical galaxies at redshift z=0. The disk galaxies are deficient inangular momentum by only about a factor of 2 compared with observed diskgalaxies for simulations with fairly strong starbursts in early,protogalactic clouds, leading to ``blow-away'' of the remaining gas inthe clouds. In this respect the present scenario is hence doing almostas well as the warm dark matter (WDM) scenarios discussed bySommer-Larsen & Dolgov. The surface density profiles of the stellardisks are approximately exponential, and those of the bulges range fromexponential to r1/4, as observed. The bulge-to-disk ratios ofthe disk galaxies are consistent with observations, as are theirintegrated B-V colors, which have been calculated using stellarpopulation synthesis techniques. Furthermore, the observed I-bandTully-Fisher relation can be matched, provided that the stellarmass-to-light ratio of disk galaxies is M/LI~0.8, similar towhat was found by Sommer-Larsen & Dolgov from their WDM simulationsand in fair agreement with several recent observational determinationsof M/LI for disk galaxies. The elliptical and lenticulargalaxies have approximately r1/4 stellar surface densityprofiles, are dominated by nondisklike kinematics, and are flattened asa result of nonisotropic stellar velocity distributions, againconsistent with observations. Hot halo gas is predicted to cool out andbe accreted onto the Galactic disk at a rate of 0.5-1 Msolaryr-1 at z=0, consistent with upper limits deduced from FarUltraviolet Spectroscopic Explorer observations of O VI. We haveanalyzed in detail the formation history of two disk galaxies withcircular speeds comparable to that of the Milky Way and find gasaccretion rates, and hence bolometric X-ray luminosities of the halos,6-7 times larger at z~1 than at z=0 for these disk galaxies. Moregenerally, it is found that gas infall rates onto these disks are nearlyexponentially declining with time, both for the total disk and for the``solar cylinder.'' This theoretical result hence supports theexponentially declining gas infall approximation often used in chemicalevolution models. The infall timescales deduced are ~5-6 Gyr, comparableto what is adopted in current chemical evolution models to solve the ``Gdwarf problem.'' The disk of one of the two galaxies forms``inside-out,'' the other ``outside-in,'' but in both cases the meanages of the stars in the outskirts of the disks are >~6-8 Gyr, fairlyconsistent with the findings of Ferguson & Johnson for the disk ofM31. The amount of hot gas in disk galaxy halos is consistent withobservational upper limits. The globular cluster M53 and the LMC are``inserted'' in the halos of the two Milky Way-like disk galaxies, anddispersion measures to these objects are calculated. The results areconsistent with upper limits from observed dispersion measures topulsars in these systems. Finally, the results of the simulationsindicate that the observed peak in the cosmic star formation rate atredshift z~2 can be reproduced. Depending on the star formation andfeedback scenarios, one predicts either a cosmic star formation ratethat decreases monotonically with redshift beyond these redshifts or asecond peak at z~6-8, corresponding to the putative Population III andinterestingly similar to recent estimates of the redshift at which theuniverse was reionized. These various scenarios should hence beobservationally constrainable with upcoming instruments such as theJames Webb Space Telescope and the Atacama Large Millimeter Array.

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.

Wide-Field Stellar Distributions around the Remote Young Galactic Globular Clusters Palomar 3 and Palomar 4
In a search for tidal extension features and/or streams of the probableparent satellite galaxies around the remote young globular clusters Pal3 and Pal 4, we used wide-field VI photometry of an areaa~1.3d×1.3d around Pal 3 and an area a~1.3d×0.9d around Pal4, obtained with the CFH12K mosaic CCD. Applying the CMD-mask algorithmto stars in the vicinity of the clusters, we selected member starcandidates that were used to examine the characteristics of the spatialdistribution of stars around Pal 3 and Pal 4. The isodensity contours inthe Gaussian-smoothed spatial stellar density maps around the clusters,and the Kolmogorov-Smirnov test applied to the corresponding luminosityfunctions, indicate tidal halos around Pal 3 at distances of up to~4rt, and around Pal 4 of up to ~6rt. The stellardistribution around Pal 3 suggests north-south elongations along thedirections of the Galactic center and anticenter, and a northeastextension in the direction of the cluster's proper motion. In thevicinity of Pal 4 an extension of a tail around the cluster in theopposite direction to the Galactic center and a possible extension ofstars toward the Galactic center have also been detected in theisodensity maps and in the angular luminosity functions. We discuss therelevance of the spatial distribution of stars in possible streamsaround Pal 3 and Pal 4 to these spatial correlations and to the spaceorbits of the possible parent satellite galaxies.This is a part of the Searching for the Galactic Halo project using theCFH12K mosaic CCD, organized by the Korea Astronomical Observatory.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Constellation:Coma Berenices
Right ascension:13h12m54.00s
Apparent magnitude:7.7

Catalogs and designations:
Proper Names   (Edit)
MessierM 53
NGC 2000.0NGC 5024

→ Request more catalogs and designations from VizieR