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New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| The RR Lyrae period-K-luminosity relation for globular clusters: an observational approach
The period-metallicity-K-band luminosity (PLK) relation forRR Lyrae stars in 15 Galactic globular clusters and in the LargeMagellanic Cloud (LMC) globular cluster Reticulum has been derived. Itis based on accurate near-infrared (K) photometry combined withTwo-Micron All-Sky Survey (2MASS) and other literature data. ThePLK relation has been calibrated and compared with theprevious empirical and theoretical determinations in literature. Thezero point of the absolute calibration has been obtained from the Kmagnitude of RR Lyr whose distance modulus has been measured viatrigonometric parallax with Hubble Space Telescope (HST). Using thisrelation, we obtain a distance modulus to the LMC of (m - M)0= 18.54 +/- 0.15 mag, in good agreement with recent determinations basedon the analysis of Cepheid variable stars.Based on observations collected at the European Southern Observatorywithin the observing programs 49.5-0021, 51.5-0024, 59.E-0340,64.N-0038, 68.D-0287 and at the Telescopio Nazionale Galileo.E-mail: antonio.sollima@bo.astro.it (AS)
| Multicolour CCD photometry of the variable stars in globular cluster M3
We present time-series data on the variable stars of the galacticglobular cluster Messier 3 (M3). We give BVIC light curvesfor 226 RR Lyrae, 2 SX Phe and 1 W Vir type variables, along withestimated fundamental photometric parameters such as intensity andmagnitude-averaged brightness and pulsation periods. In some cases, theperiods we have found significantly differ from the previously publishedones. This is the first published light curve and period determinationfor variable V266. The I-band light curve has not been observedpreviously for numerous (76) variables. Three new RR Lyrae variableshave been discovered. Groups of RR Lyrae variables that belong todifferent evolutionary stages and have been separated previously on thebasis of V data were found here for all colours and colour indices bycluster analysis. The I-band period-luminosity relation is alsodiscussed. From the 66 modulated (Blazhko type) RR Lyrae stars weinvestigated, six are newly identified and two of them are firstovertone pulsators. In the case of 13 RR Lyrae, the period of Blazhkocycle has been estimated for the first time. V252 is identified as a newRRd variable. Amplitude ratios of RRd stars have been investigated tosearch possible mode content changes. In contrast to previouspublications no changes have been found. Problems with the sampling ofthe time-series of typical cluster variability surveys are demonstrated.
| The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog
The first catalog of the RR Lyrae stars (RRLSs) in the Galactic halo bythe Quasar Equatorial Survey Team (QUEST) has been searched forsignificant overdensities that may be debris from disrupted dwarfgalaxies or globular clusters. These RRLSs are contained in a band ~2.3dwide in declination that spans ~165° in right ascension and lie ~4to ~60 kpc from the Sun. Away from the major overdensities, thedistribution of these stars is adequately fitted by a smooth halo model,in which the flattening of the halo decreases with increasinggalactocentric distance (as reported by Preston et al.). This model wasused to estimate the ``background'' of RRLSs on which the halooverdensities are overlaid. A procedure was developed for recognizinggroups of stars that constitute significant overdensities with respectto this background. To test this procedure, a Monte Carlo routine wasused to make artificial RRLS surveys that follow the smooth halo modelbut with Poisson-distributed noise in the numbers of RRLSs and, withinlimits, random variations in the positions and magnitudes of theartificial stars. The 104 artificial surveys created by thisroutine were examined for significant groups in exactly the same way asthe QUEST survey. These calculations provided estimates of thefrequencies with which random fluctuations produce significant groups.In the QUEST survey there are six significant overdensities that containsix or more stars and several smaller ones. The small ones and possiblyone or two of the larger ones may be artifacts of statisticalfluctuations, and they need to be confirmed by measurements of radialvelocity and/or proper motion. The most prominent groups are thenorthern stream from the Sagittarius dwarf spheroidal galaxy and a largegroup in Virgo, formerly known as the ``12.4 hr clump,'' which Duffauand coworkers have recently shown to contain a stellar stream (the Virgostellar stream). Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Anothergroup is related to the globular cluster Palomar 5.
| 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.
| 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.
| Homogeneous Photometry. V. The Globular Cluster NGC 4147
New BVRI broadband photometry and astrometry are presented for theglobular cluster NGC 4147, based upon measurements derived from 524ground-based CCD images mostly either donated by colleagues or retrievedfrom public archives. We have also reanalyzed five exposures of thecluster obtained with WFPC2 on the Hubble Space Telescope in the F439Wand F555W (B and V) filters. We present calibrated color-magnitude andcolor-color diagrams. Analysis of the color-magnitude diagram revealsmorphological properties generally consistent with publishedmetal-abundance estimates for the cluster, and an age typical of otherGalactic globular clusters of similar metallicity. We have alsoredetermined the periods and mean magnitudes for the RR Lyrae variables,including a new c-type variable reported here for the first time. Ourdata do not show clear evidence for photometric variability in candidateV18, recently reported by Arellano Ferro et al. (2004, Rev. Mex.A&A, 40, 209). These observations also support the nonvariablestatus of candidates V5, V9, and V15. The union of our light-curve datawith those of Newburn (1957, AJ, 62, 197), Mannino (1957, Mem. Soc.Astron. Italiana, 28, 285), and Arellano Ferro et al. (op. cit.) permitsthe derivation of significantly improved periods. The mean periods andthe Bailey period-amplitude diagrams support the classification of thecluster as Oosterhoff I, despite its predominantly blue horizontalbranch. The number ratio of c- to ab-type RR Lyrae stars, on the otherhand, is unusually high for an Oosterhoff I cluster. The calibratedresults have been made available through the first author's Web site.Based in part on archival observations made with ESO telescopes at theLa Silla and Paranal Observatory under program ID 60.A-9050(A).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| 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.
| CCD Photometry of Candidate RR Lyrae Stars in the Sloan Digital Sky Survey
We present unfiltered CCD photometry of a sample of 71 candidate RRLyrae stars during 2003 and 2004, out of 148 candidate stars that wereselected from Sloan Digital Sky Survey (SDSS) commissioning data byIvezić and coworkers. We obtain light curves of 69 candidatestars and present a catalog of their positions, amplitudes, periods,types, and mean magnitudes. We confirm that the 69 stars are true RRLyrae variables based on periods and light curves, for the first timefor 41 of them. We identify 57 type ab, 11 type c, and 1 type d(double-mode) in our sample. The double-mode RR Lyrae star showsproperties similar to globular cluster stars and are unlike field starsof the Galactic halo in the Petersen diagram. We also find one RR Lyraestar that is related to Pal 5 and recover another. In our sample, onlytwo candidate stars do not have a reasonable light curve. We concludethat the ratio of contamination in our sample is less than ~8%, whichshows that the criteria used by Ivezić and coworkers to select RRLyrae stars from the SDSS two-epoch data are reliable.
| 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.
| 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.
| Near-Infrared Observations of RR Lyrae Variables in Galactic Globular Clusters. I. The Case of M92
We present near-infrared (NIR) J-, H-, and K-band time seriesobservations of the Galactic globular cluster (GGC) M92. On the basis ofthese data, we derive well-sampled light curves for 11 out of the 17cluster RR Lyrae variables and, in turn, accurate mean NIR magnitudes.The comparison between the predicted and empirical slopes of the NIRperiod-luminosity (PL) relations indicates a very good agreement.Cluster distance determinations based on independent theoretical NIR PLrelations present uncertainties smaller than 5% and agree quite wellwith recent distance estimates based on different distance indicators.We also obtain accurate and deep NIR color-magnitude diagrams rangingfrom the tip of the red giant branch (RGB) down to the main-sequenceturnoff. We detect the RGB bump, and the NIR luminosities of thisevolutionary feature are, within theoretical and empiricaluncertainties, in good agreement with each other.
| Globular Clusters with Dark Matter Halos. I. Initial Relaxation
In a series of two papers, we test the primordial scenario of globularcluster formation using results of high-resolution N-body simulations.In this first paper we study the initial relaxation of a stellar coreinside a live dark matter minihalo in the early universe. Our darkmatter-dominated globular clusters show features that are usuallyattributed to the action of the tidal field of the host galaxy. Amongthem are the presence of an apparent cutoff (tidal radius) or a break inthe outer parts of the radial surface brightness profile and a flatline-of-sight velocity dispersion profile in the outskirts of thecluster. The apparent mass-to-light ratios of our hybrid (stars + darkmatter) globular clusters are very close to those of purely stellarclusters. We suggest that additional observational evidence, such as thepresence of obvious tidal tails, is required to rule out the presence ofsignificant amounts of dark matter in present-day globular clusters.
| 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.
| 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.
| Automated nonlinear stellar pulsation calculations: Applications to RR Lyrae stars. The slope of the fundamental blue edge and the first RRd model survey
We describe a methodology that allows us to follow the pulsationalbehavior of an RR Lyrae model consistently and automatically along itsevolutionary track throughout the whole instability strip. It is basedon the powerful amplitude equation formalism, and resorts to a judiciouscombination of numerical hydrodynamical simulations, the analyticalsignal time-series analysis, and amplitude equations. A large-scalesurvey of the nonlinear pulsations in RR Lyr instability strip is thenpresented, and the mode selection mechanism is delineated throughout therelevant regions of parameter space. We obtain and examine two regionswith hysteresis, where the pulsational state depends on the direction ofthe evolutionary tracks, namely a region with either fundamental (RRab)or first overtone (RRc) pulsations and a region with either fundamental(RRab) or double-mode (RRd) pulsations. The regions where stabledouble-mode (DM, or RRd) pulsations occur are very narrow and hard tofind in astrophysical parameter (L, M, Teff, X, Z) space withhydrodynamic simulations, but our systematic and efficient methodologyallows us to investigate them with unprecedented detail. It is shownthat by simultaneously considering the effects of mode selection and ofhorizontal branch evolution we can naturally solve one of the extantpuzzles involving the topologies of the theoretical and observedinstability strips, namely the slope of the fundamental blue edge. Theimportance of the interplay between mode selection and stellarevolutionary effects is also demonstrated for the properties ofdouble-mode RR Lyr. Finally, the Petersen diagram of double-mode RR Lyrmodels is discussed for the first time.
| RR Lyrae Stars in Galactic Globular Clusters. III. Pulsational Predictions for Metal Content Z=0.0001 to Z=0.006
The results of nonlinear, convective models of RR Lyrae pulsators withmetal content Z=0.0001-0.006 are discussed and several predictedrelations connecting pulsational (period and amplitude of pulsation) andevolutionary parameters (mass, absolute magnitude, and color of thepulsator) are derived. These relations, when linked with the averagemass of RR Lyrae stars, as suggested by horizontal-branch evolutionarymodels, provide a ``pulsational'' route to the determination of thedistance modulus, both apparent and intrinsic, of RR Lyrae-rich globularclusters. Based on a preliminary set of synthetic horizontal-branchsimulations, we compare the predicted relations with observed variablesin selected globular clusters (M2, M3, M5, M15, M55, M68, NGC 1851, NGC3201, NGC 5466, NGC 6362, NGC 6934, and IC 4499). We show that thedistance moduli inferred by the various theoretical relations aremutually consistent within the errors, provided that the value of themixing-length parameter slightly increases from the blue to the red edgeof the pulsation region. Moreover, we show that the relative``pulsational'' distance moduli fit well previous empirical results andthat the parallax of the prototype variable RR Lyr, as inferred by thepredicted period-Wesenheit relation, is in close agreement with theHubble Space Telescope astrometric measurement.
| 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.
| The Universality of Initial Conditions for Globular Cluster Formation
We investigate a simple model for globular cluster (GC) formation. Wesimulate the violent relaxation of initially homogeneous isothermalstellar spheres and show that it leads to the formation of clusters withradial density profiles that match the observed profiles of GCs. Thebest match is achieved for dynamically unevolved clusters. In thismodel, all the observed correlations between global GC parameters areaccurately reproduced if one assumes that all the clusters initially hadthe same value of the stellar density and the velocity dispersion. Thissuggests that the gas that formed GCs had the same values of density andtemperature throughout the universe.
| The distance to IC 4499 from K-band photometry of 32 RR Lyrae stars
Single epoch K-band photometry of 32 RR Lyrae stars in the younggalactic globular cluster IC 4499 is presented. Themean magnitudes have been estimated using the K-band templatelight-curves of Jones et al. (\cite{Jones96}). We derive an independentestimate of the distance and reddening for the cluster based on the new< MK>-\log P-[Fe/H] relation from Bono et al.(\cite{Bono03}) which has a zero-point in agreement with the canonicalLMC distance of (m-M)0 = 18.5. For an assumed metallicity of[Fe/H] =-1.65±0.1 we find (m-M)0 = 16.47 ±0.04±0.06 (random and systematic errors respectively) andE(B-V)=0.24±0.03 in good agreement with the findings of Walker& Nemec (\cite{WN96}) based on optical data. The distance estimateis very robust to uncertainties in the reddening estimate but naturallydepends directly on the adopted zero-point of the -log P-[Fe/H] relation.Based on data from the European Southern Observatory.
| Image-Subtraction Photometry of the Globular Cluster M3: Identification of New Double-Mode RR Lyrae Stars
We have applied the image subtraction method of Alard and Lupton to theextensive M3 data set previously analyzed by Corwin and Carney usingDAOPHOT and ALLSTAR. This new analysis has produced light curves andperiods for 15 variables not found in the previous study but alreadyknown to be variables, and it has also resulted in improved periods forseveral other variables. The additional variables recovered with theimage subtraction analysis are in the very central region of M3, wherecrowding is severe and the photometry was not of sufficient quality thatit could be put on the standard system. The present study brings to 222the total number of RR Lyrae variables in Corwin and Carney's M3 dataset for which light curves and periods are available. Among them we haveidentified three new candidate double-mode pulsating variables (V13,V200, and V251), reported here for the first time. This brings to eightthe total number of double-mode RR Lyrae (RRd's) identified in M3. Ofthe newly discovered RRd's V13 is unusual in that it has the fundamentalas the dominant pulsation mode. M3 is unique among the globular clustersin having RRd variables with a dominant fundamental mode. Two of the newcandidate RRd's (V13 and V200) have period ratios as low as 0.738-0.739.They lie well separated from all previously known double-mode variablestars in the Petersen diagram in positions implying a large spread inmass and/or, less likely, in heavy element mass fraction, among the M3horizontal-branch (HB) stars. We explore mass transfer and heliumenhancement as possible explanations for the apparent spread in HBmasses. We also note that the masses derived from the double-modeanalyses now favor little mass loss on the red giant branch. We findthat V200 has changed its dominant pulsation mode from fundamental tofirst overtone, while V251 has changed its dominant mode from firstovertone to fundamental in the interval 1992 to 1993. Together withM3-V166 this is the first time that double-mode variables are observedto switch their dominant pulsation modes while remaining RRd's. Thephenomenon is found to occur in a 1 yr time span, thus suggesting thatthese stars are undergoing a rapid evolutionary phase and that bothredward and blueward evolution may take place among the HB stars in theOosterhoff type I cluster M3. The unusual behavior of the M3 RRd's isdiscussed in detail and compared with that of the double-mode RR Lyraeidentified so far in globular clusters and in the field of our and otherLocal Group galaxies. We find a lack of correlation between the presenceof RRd variables and any of the cluster structural parameters.
| The Monoperiodic δ Scuti Star UY Camelopardalis: An Analog to SX Phoenicis and RR Lyrae Variables
We present the results of a 4 year photometric study of thehigh-amplitude δ Scuti (HADS) star UY Camelopardalis. Analysis ofthe available data from 1985 to 2003 shows that UY Cam is monoperiodic.Fourier solutions for individual data sets do not reveal period changesin the star. Although forced parabolic fits to the O-C residualsindicate a measurable period change, the distribution of the data pointsin the O-C diagram and the deviations between fits and observationssuggest that the existence of the period change still has not beenestablished. We demonstrate the presence of amplitude variations fromcycle to cycle and on longer timescales. The pulsation amplitude seemedto change from 1985 to the 2000s, but it remained constant in 2000-2003.UY Cam is located in the upper portion of the δ Scuti instabilityregion. Its photometric properties and estimated physical parametersreveal that UY Cam is an interesting object in terms of its poormetallicity, long period, high luminosity, low surface gravity, andlarge radius among the HADS stars. UY Cam could be a younger (0.7+/-0.1Gyr) Population I HADS star with low metal abundance (Z=0.004) evolvingon its post-main-sequence shell hydrogen burning evolutionary phase. UYCam intervenes among the Population I/II HADS and type c RR Lyraevariables. These characteristics suggest the star to be an analog ofHADS, SX Phoenicis, and RRc variables.
| RR Lyrae variables in Galactic globular clusters. I. The observational scenario
In this paper we revisit observational data concerning RR Lyrae stars inGalactic globular clusters, presenting frequency histograms offundamentalized periods for the 32 clusters having more than 12pulsators with well recognized period and pulsation mode. One finds thatthe range of fundamentalized periods covered by the variables in a givencluster remains fairly constant in varying the cluster metallicity allover the metallicity range spanned by the cluster sample, with the onlytwo exceptions given by M 15 and NGC 6441. We conclude that the width intemperature of the RR Lyrae instability strip appears largelyindependent of the cluster metallicity. At the same time, it appearsthat the fundamentalized periods are not affected by the predictedvariation of pulsators luminosity with metal abundance, indicating theoccurrence of a correlated variation in the pulsator mass. We discussmean periods in a selected sample of statistically significant ``RRrich" clusters with no less than 10 RRab and 5 RRc variables. One findsa clear evidence for the well known Oosterhoff dichotomy in the meanperiod of ab-type variables, together with a similarlyclear evidence for a constancy of the mean fundamentalized period in passing from Oosterhoff type II to type I clusters. Onthis basis, the origin of the Oosterhoff dichotomy is discussed,presenting evidence against a strong dependence of the RR Lyraeluminosity on the metal content. On the contrary, i) the continuity ofthe mean fundamentalized period, ii) the period frequency histograms inthe two prototypes M 3 (type I) and M 15 (type II), iii) the relativeabundance of first overtone pulsators, and iv) the observed differencebetween mean fundamental and fundamentalized periods, all agree in suggesting the dominant occurrence of avariation in the pulsation mode in a middle region of the instabilitystrip (the ``OR" zone), where variables of Oosterhoff type I and type IIclusters are pulsating in the fundamental or first overtone mode,respectively.
| 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.
| Consistent distances from Baade-Wesselink analyses of Cepheids and RR Lyraes
By using the same algorithm in the Baade-Wesselink analyses of GalacticRR Lyrae and Cepheid variables, it is shown that, within 0.03-mag1σ statistical error, they yield the same distance modulus for theLarge Magellanic Cloud. By fixing the zero-point of thecolour-temperature calibration to those of the current infrared fluxmethods and using updated period-luminosity-colour relations, we get anaverage value of 18.55 for the true distance modulus of the LMC.
| CCD Photometry of the Old Clusters ESO 093-SC08 and van den Bergh-Hagen 176
Faint-magnitude CCD photometry has been obtained for the clusters ESO093-SC08 and van den Bergh-Hagen 176. These data reveal ESO 093-SC08 tobe an old open cluster, with a derived age of 5.5+/-1.0 Gyr, aheliocentric distance of 14.00+/-0.75 kpc, an inferred metallicity inthe range -0.60<=[Fe/H]<=-0.20 dex, and Galactocentric positionsRGC=13.0+/-0.7 kpc and Z=-1000+/-50 pc. For van denBergh-Hagen 176, a derived age of 7.0+/-1.5 Gyr, a heliocentric distanceof 18.00+/-1.00 kpc, an inferred metallicity in the range-0.20<=[Fe/H]<=0.20 dex, and Galactocentric positionsRGC=12.0+/-0.9 kpc and Z=+1350+/-100 pc are found. Thecurrent results for ESO 093-SC08 are in good agreement with a study byBica et al. Van den Bergh-Hagen 176 appears to be a massive metal-richold cluster, similar to NGC 6791, or a young, metal-rich globularcluster.
| The Carina Project. I. Bright Variable Stars
We present new BV time series data of the Carina dwarf spheroidal galaxy(dSph). Current data cover an area of ~0.3 deg2 around thecenter of the galaxy and allow us to identify 92 variables. Among them75 are RR Lyrae stars, 15 are bona fide anomalous Cepheids, one might bea Galactic field RR Lyrae star, and one is located along the Carina redgiant branch. Expanding upon the seminal photographic investigation bySaha, Monet, & Seitzer we supply, for the first time, accurateestimates of their pulsation parameters (periods, amplitudes, meanmagnitudes, and colors) on the basis of CCD photometry. Approximately50% of both RR Lyrae stars and anomalous Cepheids are newidentifications. Among the RR Lyrae sample, six objects are newcandidate double-mode (RRd) variables. On the basis of their pulsationproperties we estimate that two variables (V158, V182) are about 50%more massive than typical RR Lyrae stars, while the bulk of theanomalous Cepheids are roughly a factor of 2 more massive thanfundamental-mode (RRab) RR Lyrae stars. This finding supports theevidence that these objects are intermediate-mass stars during centralHe-burning phases. We adopted three different approaches to estimate theCarina distance modulus, namely, the first-overtone blue edge method,the period-luminosity-amplitude relation, and theperiod-luminosity-color relation. We found DM=20.19+/-0.12, a resultthat agrees quite well with similar estimates based on differentdistance indicators. The data for Carina, together with data availablein the literature, strongly support the conclusion that dSph's canbarely be classified into the classical Oosterhoff dichotomy. The meanperiod of RRab's in Carina resembles that found for Oosterhoff type IIclusters, whereas the ratio between first-overtone (RRc) pulsators andthe total number of RR Lyrae stars is quite similar to that found inOosterhoff type I clusters.Based on observations collected at the European Southern Observatory, LaSilla, Chile, on Osservatorio Astronomico di Capodimonte guaranteedtime.
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
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