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|Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters|
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
|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.
|Atmospheres, Chemical Compositions, and Evolutionary Histories of Very Metal-Poor Red Horizontal-Branch Stars in the Galactic Field and in NGC 7078 (M15)|
We have conducted spectrum analyses of 24 field metal-poor([Fe/H]<-2) red horizontal-branch (RHB) stars identified in the HKobjective-prism survey and 6 such stars in the globular cluster M15,based on high-quality spectra (R~40,000, S/N~100) obtained with theMagellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 mtelescope at Las Campanas Observatory. The atmospheric parameters of theRHB stars provide interesting bridges between turnoff stars of similartemperature and red giant branch (RGB) stars of similar gravity, andthey permit investigations of abundance trends [X/Fe] versus [Fe/H] in arelatively unexplored region of the temperature-gravity plane. We findthat the Teff, logg, vt, and [Fe/H] valuesdetermined from our spectra are consistent with expectations fromliterature spectroscopic studies of other evolved metal-poor stellarclasses. We show that the RHB stars have abundance distributions thatare consistent with typical halo stars of similar metallicities. Thephotometric and spectroscopic gravities of the M15 stars differ byamounts that grow with declining temperature. We use a regressionderived from these differences to calculate photometric gravities forthe field RHB stars. Then we use the locations of the field RHB starsamong the evolutionary tracks of Cassisi et al. in the logg versuslogTeff plane to estimate their masses and lifetimes as RHBstars. We use these lifetimes to estimate the size of the metal-poor HBpopulation from which they arise. Then, using counts of HB and RGB starsin metal-poor globular clusters, we conclude that the number ofmetal-poor RGB stars at high latitudes (|b|>30deg)brighter than V=15 exceeds those identified in extant objective-prismsurveys by more than an order of magnitude. Finally, we deduce theeffective temperature of the fundamental red edge of the metal-poor RRLyrae instability strip, logTeff(FRE)=3.80+/-0.01, from theinterface between the temperature distributions of metal-poor field RHBstars and the RR Lyrae stars of similar [Fe/H] in five metal-poorglobular clusters.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. This paper includes data gatheredwith the 6.5 m Magellan telescopes located at Las Campanas Observatory,Chile.
|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.
|RR Lyrae-based calibration of the Globular Cluster Luminosity Function|
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
|Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters|
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
|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.
|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.
|Physical Characteristics of the RR Lyrae Stars in the Very Metal Poor Globular Cluster NGC 5053|
The physical characteristics of the 10 RR Lyrae stars in the verymetal-poor globular cluster NGC 5053 are derived from photometry of~1000 B and V CCD frames acquired from 1994 to 2002 with the DominionAstrophysical Observatory 1.8 m Plaskett Telescope. Revised pulsationperiods and light curves, mean magnitudes, colors, amplitudes, andFourier parameters are presented. Periods accurate to<~10-5 days are now known for all 10 RR Lyrae stars. Usingtimes of maximum light dating back to Baade's original 1923-1927observations, period change rates, dP/dt, accurate to <~0.07 daysMyr-1, have been derived for the 10 stars. Seven stars haveincreasing periods, and three have decreasing periods, with theestimated period change rates for V1, V2, V9, and V10 being very closeto zero. The mean dP/dt is equal to 0.04+/-0.04 days Myr-1and is consistent with Lee's evolutionary model predictions for acluster with horizontal-branch type ~0.5. Mean B - V colors range from0.20 to 0.40 and are more consistent with near-zero reddening thanalternative higher estimates. A reddening EB-V=0.018+/-0.003is derived from the 1998 SFD maps. Mean effective temperatures vary from6040 K (V10) to 7290 K (V6), with 2.6<=logg<=3.1. Visual andbolometric absolute magnitudes, bolometric corrections, and luminositiesare derived using Fourier methods and using intensity- andmagnitude-averaged mean magnitudes. Mean locations of the stars in theH-R diagram tend to progress from hotter, lower L stars to cooler,higher L stars and are consistent with theoretical blue and red edges ofthe instability strip. Masses estimated assuming zero reddening andDorman's oxygen-enhanced models range from 0.68 Msolar(V6) to 0.78 Msolar (V10) for the 10 stars. The meanmetal abundance for NGC 5053 derived using the Jurcsik-Kovácsmethod lies significantly higher than the range -2.3 to -2.6 dexdetermined using other, more well established methods. This findingsupports recent suggestions that metallicities derived fromFourier-based [Fe/H] calibrations need to be revised downward by atleast 0.3 dex for RR Lyrae stars with very low metal abundances.
|The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3|
We report new chemical abundances of 23 bright red giant members of theglobular cluster M3, based on high-resolution (R~45,000) spectraobtained with the Keck I telescope. The observations, which involve theuse of multislits in the HIRES Keck I spectrograph, are described indetail. Combining these data with a previously reported small sample ofM3 giants obtained with the Lick 3 m telescope, we compare metallicitiesand [X/Fe] ratios for 28 M3 giants with a 35-star sample in thesimilar-metallicity cluster M13, and with Galactic halo field starshaving [Fe/H]<-1. For elements having atomic number A>=A(Si), wederive little difference in [X/Fe] ratios in the M3, M13, or halo fieldsamples. All three groups exhibit C depletion with advancingevolutionary state beginning at the level of the red giant branch``bump,'' but the overall depletion of about 0.7-0.9 dex seen in theclusters is larger than that associated with the field stars. Thebehaviors of O, Na, Mg, and Al are distinctively different among thethree stellar samples. Field halo giants and subdwarfs have a positivecorrelation of Na with Mg, as predicted from explosive or hydrostaticcarbon burning in Type II supernova sites. Both M3 and M13 show evidenceof high-temperature proton-capture synthesis from the ON, NeNa, and MgAlcycles, while there is no evidence for such synthesis among halo fieldstars. But the degree of such extreme proton-capture synthesis in M3 issmaller than it is in M13: the M3 giants exhibit only modestdeficiencies of O and corresponding enhancements of Na, less extremeoverabundances of Al, fewer stars with low Mg and correspondingly highNa, and no indication that O depletions are a function of advancingevolutionary state, as has been claimed for M13. We have also consideredNGC 6752, for which Mg isotopic abundances have been reported by Yong etal. Giants in NGC 6752 and M13 satisfy the same anticorrelation of Oabundances with the ratio(25Mg+26Mg)/24Mg, which measures therelative contribution of rare to abundant isotopes of Mg. This points toa scenario in which these abundance ratios arose in the ejected materialof 3-6 Msolar cluster stars, material that was then used toform the atmospheres of the presently evolving low-mass cluster stars.It also suggests that the low oxygen abundance seen among the mostevolved M13 giants arose in hot bottom O-to-N processing in these sameintermediate-mass cluster stars. Thus, mixing is required by thedependence of some abundance ratios on luminosity, but an earliernucleosynthesis process in a hotter environment than giants ormain-sequence stars is required by the variations previously seen instars near the main sequence. The nature and the site of the earlierprocess is constrained but not pinpointed by the observed Mg isotopicratio.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation. The authorswish to recognize and acknowledge the very significant cultural role andreverence that the summit of Mauna Kea has always had within theindigenous Hawaiian community. We are most fortunate to have theopportunity to conduct observations from this mountain.
|BVRI photometry of the galactic globular cluster NGC 6779|
We present B, V, R and I photometry for NGC 6779 (M56), a metal-poorglobular cluster in the galactic halo. The observations were performedusing the 1.3-m telescope at Skinakas Observatory, in Crete. Thereddening of the cluster was found to be E(B-V) = 0.32 +/- 0.02 [E(V-I)= 0.43 +/- 0.02], significantly higher than previous estimates. Themetal abundance of the cluster was derived from various parametrizationsof red giant branch characteristics and it was found to be[Fe/H]ZW=-2.20 +/- 0.12 dex on the Zinn-West metallicityscale, or [Fe/H]CG=-2.00 +/- 0.21 dex on the Carretta-Grattonscale. The distance modulus of the cluster is estimated to be(m-M)V= 15.62 +/- 0.26 (or 14.62, if we correct for thereddening to the cluster). The horizontal branch of NGC 6779 shows aclear gap at (B-V)o= 0.0. Finally, the revised value for themetallicity of NGC 6779 led to a revision of its age to 13 Gyr, usingthe age-index calibrations of Salaris & Weiss.
|A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way|
We present an analysis of the asymmetries in the population of GalacticM-giant stars present in the 2MASS All Sky catalogue. Severallarge-scale asymmetries are detected, the most significant of which is astrong elliptical-shaped stellar overdensity, close to the Galacticplane at (l= 240°, b=-8°), in the constellation of Canis Major.A small grouping of globular clusters (NGC 1851, 1904, 2298 and 2808),coincident in position and radial velocity, surround this structure, asdo a number of open clusters. The population of M-giant stars in thisoverdensity is similar in number to that in the core of the Sagittariusdwarf galaxy. We argue that this object is the likely dwarf galaxyprogenitor of the ring-like structure that has recently been found atthe edge of the Galactic disc. A numerical study of the tidal disruptionof an accreted dwarf galaxy is presented. The simulated debris fits theextant position, distance and velocity information on the Galactic`Ring', as well as that of the M-giant overdensities, suggesting thatall these structures are the consequence of a single accretion event.The disrupted dwarf galaxy stream orbits close to the Galactic plane,with a pericentre at approximately the solar circle, an orbitaleccentricity similar to that of stars in the Galactic thick disc, aswell as a vertical scaleheight similar to that of the thick disc. Thisfinding strongly suggests that the Canis Major dwarf galaxy is abuilding block of the Galactic thick disc, that the thick disc iscontinually growing, even up to the present time, and that thick discglobular clusters were accreted on to the Milky Way from dwarf galaxiesin co-planar orbits.
|Star Clusters in the Galactic Anticenter Stellar Structure and the Origin of Outer Old Open Clusters|
The Galactic anticenter stellar structure (GASS) has been identifiedwith excess surface densities of field stars in several large-area skysurveys and with an unusual, stringlike grouping of five globularclusters. At least two of these are diffuse, young ``transitional''clusters between open and globular types. Here we call attention to thefact that four younger open or transitional clusters extend thepreviously identified, stringlike cluster grouping, with at least onehaving a radial velocity consistent with the previously found GASSvelocity-longitude trend. All nine clusters lie close to a plane tipped17° to the Galactic plane. This planar orientation is used to foragefor additional potential cluster members in the inner Galaxy, and anumber are found along the same plane and stringlike sequence, includingalmost all 15 known outer, old open clusters. Tidal accretion of a dwarfsatellite galaxy on a low-inclination orbit-perhaps the GASSsystem-appears to be a plausible explanation for the origin of theouter, old open and transitional clusters of the Milky Way. We use theseclusters to explore the age-metallicity relation of the putativeaccreted GASS progenitor. Finally, we provide the first radial velocityof a star in the cluster BH 176 and discuss its implications.
|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.
|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.
|Building Up the Globular Cluster System of the Milky Way: The Contribution of the Sagittarius Galaxy|
We demonstrate that there is a clear statistical correlation between the(X,Y,Z,Vr) phase-space distribution of the outer haloGalactic globular clusters (having 10 kpc<=RGC<=40 kpc)and the orbital path of the Sagittarius dwarf spheroidal galaxy (SgrdSph), as derived by Ibata & Lewis. At least four of the sample of35 globular clusters in this distance range were formerly members of theSagittarius galaxy (at the 95% confidence level) and are now distributedalong the Sagittarius Stream, a giant tidal structure that surrounds theMilky Way. This is the first instance that a statistically significantstructure associated with the Sgr dSph has been detected in the globularcluster population of the Galactic halo. Together with the fourwell-known globular clusters that are located near the center of thistidally disrupting dwarf galaxy, these clusters constitute >~20% ofthe population of outer halo (RGC>=10 kpc) clusters. TheSgr dSph was therefore not only an important contributor to the halofield-star population, but it also had a significant role in building upthe globular cluster system of the Milky Way.
|On the reliability of the semi-empirical RR Lyrae period-V-band luminosity-blue amplitude relation|
We investigate the accuracy and reliability of the semi-empiricalperiod-V-band luminosity-blue amplitude (PLA) relationship for ab-typeRR Lyrae stars originally obtained by Castellani and De Santis in themid-1990s. We infer that the zero-point of this relationship depends onthe metallicity, by studying a sample of both field and clustervariables. We also show that the use of this relationship can still beuseful for those stellar systems showing an intrinsic metallicityspread, since in this case the metallicity has a negligible effect onthe final distance modulus estimate. We compare the adoptedsemi-empirical relationship with the fully empirical one recentlyprovided by Kovács & Walker. When the zero-point of thelatter relation is fixed consistently with the former one, the twoequations are equivalent. By applying the semi-empirical period-V-bandluminosity-blue amplitude relation, as well as the technique proposedrecently by Cassisi, De Santis & Piersimoni, to the globular clusterω Cen, we show that the empirical slope of the relationshipbetween the mass of the fundamental RR Lyrae pulsators and theirmetallicity is in fair agreement with the one predicted by updatedevolutionary models for horizontal branch stars.
|V982 Oph is a Dwarf Nova|
V982 Oph, an SR: star in the GCVS, is actually a dwarf nova.
|An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way|
One of the long-standing problems in modern astronomy is the curiousdivision of Galactic globular clusters, the ``Oosterhoff dichotomy,''according to the properties of their RR Lyrae stars. Here, we find thatmost of the lowest metallicity ([Fe/H] 〈 -2.0) clusters, whichare essential to an understanding of this phenomenon, display a planaralignment in the outer halo. This alignment, combined with evidence fromkinematics and stellar population, indicates a captured origin from asatellite galaxy. We show that, together with the horizontal-branchevolutionary effect, the factor producing the dichotomy could be a smalltime gap between the cluster-formation epochs in the Milky Way and thesatellite. The results oppose the traditional view that themetal-poorest clusters represent the indigenous and oldest population ofthe Galaxy.
|Homogeneous age dating of 55 Galactic globular clusters. Clues to the Galaxy formation mechanisms|
We present homogeneous age determinations for a large sample of 55Galactic globular clusters, which constitute about 30% of the totalGalactic population. A study of their age distribution reveals that allclusters from the most metal poor ones up to intermediate metallicitiesare coeval, whereas at higher [Fe/H] an age spread exists, together withan age-metallicity relationship. At the same time, all clusters within acertain galactocentric distance appear coeval, whereas an age spread ispresent further away from the Galactic centre, without any correlationwith distance. The precise value of [Fe/H] and galactocentric distancefor the onset of the age spread and the slope of the age-metallicityrelationship are strongly affected by the as yet uncertain [Fe/H] scale.We discuss how differences in the adopted [Fe/H] scale and clustersample size may explain discrepant results about the clusters agedistribution reached by different authors. Taking advantage of the largenumber of objects included in our sample, we also tested the possibilitythat age is the global second parameter which determines the HorizontalBranch morphology, and found indications that age could explain theglobal behaviour of the second parameter effect.
|On the Distribution of Orbital Poles of Milky Way Satellites|
In numerous studies of the outer Galactic halo some evidence foraccretion has been found. If the outer halo did form in part or whollythrough merger events, we might expect to find coherent streams of starsand globular clusters following orbits similar to those of their parentobjects, which are assumed to be present or former Milky Way dwarfsatellite galaxies. We present a study of this phenomenon by assessingthe likelihood of potential descendant ``dynamical families'' in theouter halo. We conduct two analyses: one that involves a statisticalanalysis of the spatial distribution of all known Galactic dwarfsatellite galaxies (DSGs) and globular clusters, and a second, morespecific analysis of those globular clusters and DSGs for which fullphase space dynamical data exist. In both cases our methodology isappropriate only to members of descendant dynamical families that retainnearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) isconsidered a paradigm for the type of merger/tidal interaction event forwhich we are searching, we also undertake a case study of the Sgr systemand identify several globular clusters that may be members of itsextended dynamical family. In our first analysis, the distribution ofpossible orbital poles for the entire sample of outer(Rgc>8 kpc) halo globular clusters is tested forstatistically significant associations among globular clusters and DSGs.Our methodology for identifying possible associations is similar to thatused by Lynden-Bell & Lynden-Bell, but we put the associations on amore statistical foundation. Moreover, we study the degree of possibledynamical clustering among various interesting ensembles of globularclusters and satellite galaxies. Among the ensembles studied, we findthe globular cluster subpopulation with the highest statisticallikelihood of association with one or more of the Galactic DSGs to bethe distant, outer halo (Rgc>25 kpc), second-parameterglobular clusters. The results of our orbital pole analysis aresupported by the great circle cell count methodology of Johnston,Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC6229, NGC 7006, and Pyxis are predicted to be among those most likely toshow the clusters to be following stream orbits, since these clustersare responsible for the majority of the statistical significance of theassociation between outer halo, second-parameter globular clusters andthe Milky Way DSGs. In our second analysis, we study the orbits of the41 globular clusters and six Milky Way-bound DSGs having measured propermotions to look for objects with both coplanar orbits and similarangular momenta. Unfortunately, the majority of globular clusters withmeasured proper motions are inner halo clusters that are less likely toretain memory of their original orbit. Although four potential globularcluster/DSG associations are found, we believe three of theseassociations involving inner halo clusters to be coincidental. While thepresent sample of objects with complete dynamical data is small and doesnot include many of the globular clusters that are more likely to havebeen captured by the Milky Way, the methodology we adopt will becomeincreasingly powerful as more proper motions are measured for distantGalactic satellites and globular clusters, and especially as resultsfrom the Space Interferometry Mission (SIM) become available.
|Variable Stars in Galactic Globular Clusters|
Based on a search of the literature up to 2001 May, the number of knownvariable stars in Galactic globular clusters is approximately 3000. Ofthese, more than 2200 have known periods and the majority (approximately1800) are of the RR Lyrae type. In addition to the RR Lyrae population,there are approximately 100 eclipsing binaries, 120 SX Phoenicisvariables, 60 Cepheids (including Population II Cepheids, anomalousCepheids and RV Tauri), and 120 SR/red variables. The mean period of thefundamental mode RR Lyrae variables is 0.585 days, for the overtonevariables it is 0.342 days (0.349 days for the first-overtone pulsatorsand 0.296 days for the second-overtone pulsators) and approximately 30%are overtone pulsators. These numbers indicate that about 65% of RRLyrae variables in Galactic globular clusters belong to Oosterhoff typeI systems. The mean period of the RR Lyrae variables in the Oosterhofftype I clusters seems to be correlated with metal abundance in the sensethat the periods are longer in the more metal poor clusters. Such acorrelation does not exist for the Oosterhoff type II clusters. Most ofthe Cepheids are in clusters with blue horizontal branches.
|The distance to Galactic globular clusters through RR Lyrae pulsational properties|
By adopting the same approach outlined by De Santis & Cassisi, weevaluate the absolute bolometric magnitude of the zero-age horizontalbranch (ZAHB) at the level of the RR Lyrae variable instability strip inselected Galactic globular clusters. This allows us to estimate the ZAHBabsolute visual magnitude for these clusters and to investigate itsdependence on the cluster metallicity. The derivedMV(ZAHB)-[Fe/H] relation, corrected in order to account forthe luminosity difference between the ZAHB and the mean RR Lyraemagnitude, has been compared with some of the most recent empiricaldeterminations in this field, such as the one provided byBaade-Wesselink analyses, RR Lyrae periods, Hipparcos data for fieldvariables and main-sequence fitting based on Hipparcos parallaxes forfield subdwarfs. As a result, our relation provides a clear support tothe `long' distance scale. We discuss also another method for measuringthe distance to Galactic globular clusters. This method is quite similarto the one adopted for estimating the absolute bolometric magnitude ofthe ZAHB but it relies only on the pulsational properties of the Lyraevariables in each cluster. The reliability and accuracy of this methodhave been tested by applying it to a sample of globular clusters forwhich, owing to the morphology of their horizontal branch (HB), the useof the commonly adopted ZAHB fitting is a risky procedure. We noticethat the two approaches for deriving the cluster distance modulusprovide consistent results when applied to globular clusters, the RRLyrae instability strip is well populated. As the adopted method relieson theoretical predictions on both the fundamental pulsational equationand the allowed mass range for fundamental pulsators, we give anestimate of the error affecting present results, owing to systematicuncertainties in the adopted theoretical framework.
|Metallicities for Double-Mode RR Lyrae Stars in the Large Magellanic Cloud|
Metallicities for six double-mode RR Lyrae stars (RRd's) in the LargeMagellanic Cloud have been estimated using the ΔS method. Thederived [Fe/H] values are in the range [Fe/H]=-1.09 to -1.78 (or -0.95to -1.58, adopting a different calibration of [Fe/H] versus ΔS).Two stars in our sample are at the very metal-rich limit of all RRd'sfor which metal abundance has been estimated, either by direct measure(for field objects) or on the basis of the hosting system (for objectsin globular clusters or external galaxies). These metal abundances,coupled with mass determinations from pulsational models and thePetersen diagram, are used to compare the mass-metallicity distributionof field and cluster RR Lyrae variables. We find that field and clusterRRd's seem to follow the same mass-metallicity distribution, within theobservational errors, strengthening the case for uniformity ofproperties between field and cluster variables. At odds to what isusually assumed, we find no significant difference in mass for RR Lyraevariables in globular clusters of different metallicity and Oosterhofftypes, or there may even be a difference contrary to the commonlyaccepted one, depending on the metallicity scale adopted to derive themasses. This ``unusual'' result for the mass-metallicity relation isprobably due, at least in part, to the inclusion of updated opacitytables in the computation of metal-dependent pulsation models. Based onobservations collected at the European Southern Observatories, Chile.
|Globular Cluster Subsystems in the Galaxy|
Data from the literature are used to construct a homogeneous catalog offundamental astrophysical parameters for 145 globular clusters of theMilky Way Galaxy. The catalog is used to analyze the relationshipsbetween chemical composition, horizontal-branch morphology, spatiallocation, orbital elements, age, and other physical parameters of theclusters. The overall globular-cluster population is divided by a gap inthe metallicity function at [Fe/H]=-1.0 into two discrete groups withwell-defined maxima at [Fe/H]=-1.60±0.03 and -0.60±0.04.The mean spatial-kinematic parameters and their dispersions changeabruptly when the metallicity crosses this boundary. Metal-poor clustersoccupy a more or less spherical region and are concentrated toward theGalactic center. Metal-rich clusters (the thick disk subsystem), whichare far fewer in number, are concentrated toward both the Galacticcenter and the Galactic plane. This subsystem rotates with an averagevelocity of V rot=165±28 km/s and has a very steep negativevertical metallicity gradient and a negligible radial gradient. It is,on average, the youngest group, and consists exclusively of clusterswith extremely red horizontal branches. The population ofspherical-subsystem clusters is also inhomogeneous and, in turn, breaksup into at least two groups according to horizontal-branch morphology.Clusters with extremely blue horizontal branches occupy a sphericalvolume of radius 9 kpc, have high rotational velocities (Vrot=77±33 km/s), have substantial and equal negative radial andvertical metallicity gradients, and are, on average, the oldest group(the old-halo subsystem). The vast majority of clusters withintermediate-type horizontal branches occupy a more or less sphericalvolume ≈18 kpc in radius, which is slightly flattened perpendicularto the Z direction and makes an angle of ≈30° to the X-axis. Onaverage, this population is somewhat younger than the old-halo clusters(the young-halo subsystem), and exhibits approximately the samemetallicity gradients as the old halo. As a result, since theirGalactocentric distance and distance from the Galactic plane are thesame, the young-halo clusters have metallicities that are, on average,Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. Theyoung-halo subsystem, which apparently consists of objects captured bythe Galaxy at various times, contains many clusters with retrogradeorbits, so that its rotational velocity is low and has large errors, Vrot=-23±54 km/s. Typical parameters are derived for all thesubsystems, and the mean characteristics of their member globularclusters are determined. The thick disk has a different nature than boththe old and young halos. A scenario for Galactic evolution is proposedbased on the assumption that only the thick-disk and old-halo subsystemsare genetically associated with the Galaxy. The age distributions ofthese two subsystems do not overlap. It is argued that heavy-elementenrichment and the collapse of the proto-Galactic medium occurred mainlyin the period between the formation of the old-halo and thick-disksubsystems.
|Foreground and background dust in star cluster directions|
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
|A catalogue of helium abundance indicators from globular cluster photometry|
We present a survey of helium abundance indicators derived from acomprehensive study of globular cluster photometry in the literature.For each of the three indicators used, we conduct a thorough erroranalysis, and identify systematic errors in the computationalprocedures. For the population ratio RNHBNRGB, wefind that there is no evidence of a trend with metallicity, althoughthere appears to be real scatter in the values derived. Although thisindicator is the one best able to provide useful absolute heliumabundances, the mean value is Y~0.20, indicating the probable presenceof additional systematic error. For the magnitude difference from thehorizontal branch to the main sequence Δ and the RR Lyraemass-luminosity exponent A, it is only possible to determine relativehelium abundances reliably. This is due to continuing uncertainties inthe absolute metallicity scale for Δ, and uncertainty in the RRLyrae temperature scale for A. Both indicators imply that the heliumabundance is approximately constant as a function of [Fe/H]. Accordingto the A indicator, both Oosterhoff I and II group clusters haveconstant values independent of [Fe/H] and horizontal branch type. Inaddition, the two groups have slopes dlog/d[Fe/H]that are consistent with each other, but significantly smaller than theslope for the combined sample.
|The NGC 6426 RR Lyrae Variables and Horizontal-Branch Morphology|
We present BVRI CCD photometry for 12 RR Lyrae variables, includingthree newly discovered ones in the Oosterhoff type II globular clusterNGC 6426. New light curves and ephemerides are presented. The meanperiods of the RRab and RRc variables whose light curves are analyzed inthis work are =0.70+/-0.02 days and=0.34+/-0.03 days, respectively. The number ratioof the RRc type variables to the total number of RR Lyrae type variablesis n(c)/n(ab+c)=0.36. The period-amplitude relation for fundamental-modeRR Lyrae variables (RRab) in NGC 6426 supports the recent conclusion ofClement & Shelton that this relation is not a function of metalabundance. Fourier decomposition of the light curves has been used todetermine the mass, luminosity, and temperature for the RRc stars.Application of the formula of Jurcsik & Kovács, which relatesFourier parameters of RRab stars to [Fe/H], yielded the value[Fe/H]=-2.16+/-0.13 dex, which is compatible (given the errors) with thevalue -2.33+/-0.15 dex, derived from the red giant branch color indexdeveloped by Hatzidimitriou et al. From star counts along the horizontalbranch (HB), we obtain the Lee et al. HB color distribution index,(B-R)/(B+V+R)=0.58+/-0.18. The HB of NGC 6426 resembles that of NGC 5053and M68. We find almost no faint blue stars analogous to the onesconstituting the extended HB in M15.
|The Age of NGC 6426, a Metal-poor Globular Cluster in the Galactic Halo|
We present B, V, R, and I photometry of the metal-poor globular clusterin the galactic halo, NGC 6426. The observations were performed usingthe 1.3 m Telescope at Skinakas Observatory in Crete. We derived thereddening of the cluster to be E(B-V)=0.39+/-0.02 [E(V-I)=0.53+/-0.03].The metal abundance of the cluster was estimated from the shape of thered giant branch following the techniques by Sarajedini and by Da Costa& Armandroff. It was found to be [Fe/H]=-2.33+/-0.15 dex. The mean Vmagnitude of the RR Lyrae variables found in the cluster is 18.14+/-0.02(based on a paper in preparation). The resulting distance modulus of thecluster is 16.41+/-0.07. Finally, we derived the relative age of NGC6426. Using the methodology of Harris et al., we found that NGC 6426 ismarginally older than M92 (by ~=0.7 Gyr). Following the Chaboyer,Demarque, & Sarajedini method we found NGC 6426 to have the same ageas the mean metal-poor globular cluster of the galactic halo.
|Double-Mode RR Lyrae Variables in the Globular Cluster M3|
We present new B-band CCD photometry for five double-mode RR Lyrae (RRd)variables (V68, V79, V87, V99, and V166) in the globular cluster M3. Thepulsational behavior of the RRd variables is described. V68 and V87 havebeen known as RRd variables since 1982, V79 was recently discovered asan RRd (Clement et al.), and our data have identified V99 and V166 asRRd variables (Corwin et al.). Earlier studies of V79 and V166 do notshow double-mode behavior, which indicates that these stars have onlyrecently become RRd stars. V166 changed its dominant pulsation mode fromfundamental to first overtone in the interval 1992 to 1993. Thecandidate double-mode variables V28 and V126 do not exhibit clear RRdbehavior in the 1992-1993 data.
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