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Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment
The formation of blue stragglers is still not completely understood,particularly the relationship between formation environment andmechanism. We use a large, homogeneous sample of blue stragglers in thecores of 57 globular clusters to investigate the relationships betweenblue straggler populations and their environments. We use a consistentdefinition of ``blue straggler'' based on position in thecolor-magnitude diagram and normalize the population relative to thenumber of red giant branch stars in the core. We find that thepreviously determined anticorrelation between blue straggler frequencyand total cluster mass is present in the purely core population. We findsome weak anticorrelations with central velocity dispersion and withhalf-mass relaxation time. The blue straggler frequency does not showany trend with any other cluster parameter. Even though collisions maybe expected to be a dominant blue straggler formation process inglobular cluster cores, we find no correlation between the frequency ofblue stragglers and the collision rate in the core. We also investigatedthe blue straggler luminosity function shape and found no relationshipbetween any cluster parameter and the distribution of blue stragglers inthe color-magnitude diagram. Our results are inconsistent with somerecent models of blue straggler formation that include collisionalformation mechanisms and may suggest that almost all observed bluestragglers are formed in binary systems.
| [Fe/H] relations for c-type RR Lyrae variables based upon Fourier coefficients
[Fe/H]-φ31-P relations are found for c-type RR Lyraestars in globular clusters. The relations are analogous to that found byJurcsik & Kovács for field ab-type RR Lyrae stars, where alonger period correlates with lower metallicity values for similarvalues of the Fourier coefficient φ31. The relationsobtained here are used to determine the metallicity of field c-type RRLyrae stars, those within ωCen, the Large Magellanic Cloud andtoward the galactic bulge. The results are found to compare favourablyto metallicity values obtained elsewhere.
| 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.
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| CCD photometry of the globular cluster M2: RR Lyrae physical parameters and new variables
We report the results of CCD V and R photometry of the RR Lyrae stars inM2. The periodicities of most variables are revised and new ephemeridesare calculated. Light-curve decomposition of the RR Lyrae stars wascarried out and the corresponding mean physical parameters [Fe/H] =-1.47, Teff = 6276K, logL = 1.63Lsolar andMV = 0.71 from nine RRab and [Fe/H] = -1.61, M =0.54Msolar, Teff = 7215K, logL =1.74Lsolar and MV = 0.71 from two RRc stars werecalculated. A comparison of the radii obtained from the above luminosityand temperature with predicted radii from non-linear convective modelsis discussed. The estimated mean distance to the cluster is 10.49 +/-0.15kpc. These results place M2 correctly in the general globularcluster sequences for Oosterhoff type, mass, luminosity and temperature,all as a function of the metallicity. Mean relationships for M,logL/Lsolar, Teff and MV as a functionof [Fe/H] for a family of globular clusters are offered. These trendsare consistent with evolutionary and structural notions on thehorizontal branch. Eight new variables are reported.
| Global fitting of globular cluster age indicators
Context: .Stellar models and the methods for the age determinations ofglobular clusters are still in need of improvement. Aims: .Weattempt to obtain a more objective method of age determination based oncluster diagrams, avoiding the introduction of biases due to thepreference of one single age indicator. Methods: .We compute newstellar evolutionary tracks and derive the dependence of age indicatingpoints along the tracks and isochrone - such as the turn-off or bumplocation - as a function of age and metallicity. The same criticalpoints are identified in the colour-magnitude diagrams of globularclusters from a homogeneous database. Several age indicators are thenfitted simultaneously, and the overall best-fitting isochrone isselected to determine the cluster age. We also determine thegoodness-of-fit for different sets of indicators to estimate theconfidence level of our results. Results: .We find that ourisochrones provide no acceptable fit for all age indicators. Inparticular, the location of the bump and the brightness of the tip ofthe red giant branch are problematic. On the other hand, the turn-offregion is very well reproduced, and restricting the method to indicatorsdepending on it results in trustworthy ages. Using an alternative set ofisochrones improves the situation, but neither leads to an acceptableglobal fit. Conclusions: .We conclude that evolutionary tracks oflow-mass metal-poor stars are far from reproducing all aspects ofglobular cluster colour-magnitude diagrams and that the determination ofcluster ages still depends on the favourite method or indicator chosen.
| Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter
Aims.The interpretation of globular cluster horizontal branch (HB)morphology is a classical problem that can significantly blur ourunderstanding of stellar populations. Methods: .We present a newmultivariate analysis connecting the effective temperature extent of theHB with other cluster parameters. The work is based on Hubble SpaceTelescope photometry of 54 Galactic globular clusters. Results: .The present study reveals the important role of the total mass of theglobular cluster on its HB morphology. More massive clusters tend tohave HBs more extended to higher temperatures. For a set of three inputvariables including the temperature extension of the HB, [Fe/H] and M_V,the first two eigenvectors account for 90% of the total samplevariance. Conclusions: . Possible effects of clusterself-pollution on HB morphology, stronger in more massive clusters,could explain the results derived here.
| CCD Photometry of the Globular Cluster M15: RR Lyrae Fourier Decomposition and Physical Parameters
Results of CCD photometry using V and R filters are reported for 33 RRLyrae stars in M15. The periodicities of some variables have beenrevised and new ephemerides are given. The Blazhko effect, previouslyreported in V12, was not detected. Applying the approach of Fourierdecomposition of the light curves, the physical parameters of the typeRRab and RRc variables were estimated. The cluster is Oosterhoff type IIand the values for the iron content and distance are:[Fe/H]=-1.98+/-0.24 and d=8.67+/-0.41 kpc, respectively. The mean valuesof the physical parameters determined for the RR Lyrae stars place thecluster precisely into the sequences Oosterhoff type --metallicity andmetallicity-- effective temperature, valid for globular clusters.Evidences of evolution from the ZAHB are found for the RRc but not forthe RRab stars.
| 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.
| Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for HI and OH
We present the results of a survey, the deepest to date, for HI emissionat 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720MHz) in the direction towards the Galactic globular clusters M15, M2,NGC6934, NGC7006 and Pal13. The aim is to measure the amount of hydrogenin the intracluster medium, and to find OH masers in the circumstellarenvelopes of globular cluster red giants. We present a tentativedetection of 0.3Msolar of neutral hydrogen in M15 andpossible detections of neutral hydrogen in M2 and Pal13. We derive upperlimits to the neutral hydrogen content of NGC6934 and 7006. No OHemission is detected. We also present deep HI data of the northern tipof the Magellanic Stream behind Pal13.
| The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars
We discuss a 175 deg2 spectroscopic survey for bluehorizontal branch (BHB) stars in the Galactic halo. We use the TwoMicron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) toselect BHB candidates, and we find that the 2MASS and SDSS colorselection is 38% and 50% efficient, respectively, for BHB stars. Oursamples include one likely runaway B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent withmembership in the halo population: the median metallicity is[Fe/H]=-1.7, the velocity dispersion is 108 km s-1, and themean Galactic rotation of the BHB stars 3 kpc<|z|<15 kpc is-4+/-30 km s-1. We discuss the theoretical basis of thePreston, Shectman, and Beers MV-color relation for BHB starsand conclude that the intrinsic shape of the BHB MV-colorrelation results from the physics of stars on the horizontal branch. Wecalculate the luminosity function for the field BHB star samples usingthe maximum likelihood method of Efstathiou and coworkers, which isunbiased by density variations. The field BHB luminosity functionexhibits a steep rise at bright luminosities, a peak between0.8
| Galactic Globular Cluster Relative Ages
We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the differencebetween the horizontal branch and the turnoff in two internallyphotometrically homogeneous databases. The mutual consistency of the twodata sets has been assessed by comparing the ages of 16 globularclusters in common between the two databases. We have also investigatedthe consistency of our relative age determination within the recentstellar model framework. All clusters with [Fe/H]<-1.7 are found tobe old and coeval, with the possible exception of two objects, which aremarginally younger. The age dispersion for the metal-poor clusters is0.6 Gyr (rms), consistent with a null age dispersion.Intermediate-metallicity clusters (-1.7<[Fe/H]<-0.8) are onaverage 1.5 Gyr younger than the metal-poor ones, with an age dispersionof 1.0 Gyr (rms) and a total age range of ~3 Gyr. About 15% of theintermediate-metallicity clusters are coeval with the oldest clusters.All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the mostmetal-poor ones, with a relatively small age dispersion, although themetal-rich sample is still too small to allow firmer conclusions. Thereis no correlation of the cluster age with the galactocentric distance.We briefly discuss the implication of these observational results forthe formation history of the Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555, and on observations made at the European SouthernObservatory, La Silla, Chile, and with the Isaac Newton GroupTelescopes.
| Helium self-enrichment in globular clusters and the second parameter problem in M 3 and M 13
Inspection of the CM diagrams of globular clusters having similar heavyelement content shows that the luminosity of the red giant bump relativeto the turnoff (Δ V_TO^bump) differs by more than 0.1 mag betweenclusters with different horizontal branch morphology. Unfortunately,careful consideration of the data leaves us with only one pair (M 3 andM 13) of clusters suitable for a quantitative discussion. For this pairwe consider differences in age and helium content as possible causes forthe difference in Δ V_TO^bump, and find more convincing supportfor the latter. A larger helium content in M 13 stars (Y 0.28 vs.Y 0.24) accounts for various CM diagram features, such as thedifference in the luminosity level of RR Lyr variables and of the redgiant bump with respect to the turnoff luminosity and the horizontalbranch morphology. This enhanced helium can be tentatively understood inthe framework of self-enrichment by massive asymptotic giant branchstars in the first ~100 Myr of the cluster life. A modestself-enrichment can be present also in M 3 and can be the reason for thestill unexplained presence of a not negligible number of luminous,Oosterhoff II type RR Lyr variables. The hypothesis that a larger heliumcontent is the second parameter for clusters with very blue horizontalbranch morphology could be checked by an accurate set of data for moreclusters giving turnoff, RR Lyrs and bump magnitudes within a uniquephotometry.
| Eclipsing Binaries in the Young Large Magellanic Cloud Cluster NGC 1850
I present light curves for two detached eclipsing binary stars in theregion of the LMC cluster NGC 1850, which is possibly a young globularcluster still in formation. One, a likely O-type star, is a newlydetected eclipsing binary in the region of the very young subcluster NGC1850A. This binary is among a small number of highly massive O-typestars in binary systems found in LMC clusters. These two eclipsingbinaries are the first to be discovered in the well-studied NGC 1850,and the O-type star is the first eclipsing binary found in NGC 1850A.Light curves for two Cepheid variables in the NGC 1850 region are alsoshown. The discovery of two eclipsing binaries in the youngglobular-like cluster NGC 1850 is discussed in terms of the importanceof the binary fraction to globular cluster evolution.
| 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.
| 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.
| 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.
| 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.
| Comparing Deep Mixing in Globular Cluster and Halo Field Giants: Carbon Abundance Data from the Literature
The behavior of carbon abundance as a function of luminosity is used tocompare the rates of deep mixing within red giants of four globularclusters and the Galactic halo field population. Measurements of [C/Fe]for the clusters M92, NGC 6397, M3, and M13 have been compiled from theliterature, together with the Gratton et al. data for halo field stars.Plots of [C/Fe] versus absolute visual magnitude show that forMV<+1.6 the rate of decline of carbon abundance withincreasing luminosity on the red giant branch isd[C/Fe]/dMV~0.22+/-0.03 among the field stars, as well as inM92, NGC 6397, and M3. Among giants fainter than MV=+1.6 thevariation of [C/Fe] with absolute magnitude is much less. The dataindicate that the rate at which deep mixing introduces carbon-depletedmaterial into the convective envelopes of field halo stars during theupper red giant branch phase of evolution is similar to that of manyglobular cluster giants. The notable exception appears to be M13, inwhich stars exhibit deep mixing at a greater rate; this may account forthe high incidence of very low oxygen abundances among the most luminousgiants in M13 in comparison to M3.
| A global and a local criterion in defining the tidal radius
The basic theory of potential-energy tensors related to heterogeneousspheres is reviewed, and the special case of truncated, singular,isothermal spheres is examined in detail. Special effort is devoted toa system made of two isothermal spheres, one completely lying within theother, the mass and the radius of the embedded sphere being negligiblewith respect to the mass and radius of the embedding sphere, and theradius of the embedded sphere being negligible with respect to thedistance between the centres. The potential-energy tensors related tothe potential induced by the embedding sphere on the mass distributionof the embedded sphere, are expressed as the sum of two contributions:one, coming from the embedded sphere after collapse towards its centre,and one other, related to the actual mass distribution of the embeddedsphere. Using the latter, both a global and a local criterion indefining the tidal radius of the embedded sphere, are formulated inconnection with either the binding-energy tensor or the virial-energytensor. In doing this, the tensor components along the axis joining thecentre of the embedding and the embedded sphere, are considered. Theglobal criterion is related to the whole, embedded sphere, while thelocal criterion is related to an infinitesimal mass element placed atthe boundary of the embedded sphere, where the distance from the centreof the embedding sphere attains a maximum. The virial theorem intensor form is splitted into two distinct expressions, related toorbital and intrinsic motions of the embedded sphere. Alternativecriterions in defining the tidal radius of the embedded sphere, areformulated taking into consideration the centrifugal tensor potentialand the tensor potential induced by orbital motions. With regard to aselected criterion, the tidal radius calculated with and without theinclusion of the centrifugal potential, exhibits a maximum variation bya factor of about two, related to circular orbits. An application ismade, where the embedding and the embedded sphere are taken asrepresentative of the Galaxy and a globular cluster, respectively. Itis found that a stability region exists for both the global and thelocal criterion and, in addition, the instability first occurs at theperigalacticon, as expected in connection with instantaneous tidalradius. A power-law dependence of tidal radius from cluster mass andgalactocentric distance, aC* ∝MC1/3R02/3, is shown to beconsistent with data from a sample of 16 objects investigated byte{bra99} (1999). No significant correlation is found between theratio of cluster radius to tidal radius and the orbital ratio ofapogalacticon to perigalacticon, similar to averaged tidal radii definedby te{bra99} (1999). An additional object, Pal 5, which is experiencingprogressive disruption via tidal shocks during disk passages, is shownto be among the less bound (or more unbound) clusters, within theframework of the model. If the representation of globular clusters asisothermal spheres introduces only systematic errors in the ratio ofcluster radius to tidal radius,aC/aC*=γ/η, then at leastone other cluster, NGC 5466 (which has the highest value between thesample objects), is inferred to undergo tidal disruption, in the modelinterpretation.
| M75, A Globular Cluster with a Trimodal Horizontal Branch. II. BV photometry of the RR Lyrae Variables
We present new BV CCD photometry, light curves, and ephemerides for ninepreviously known, 29 newly detected RR Lyrae variables, and one newlydetected variable of an unknown type in the globular cluster M75. Thephotometry used for the detection of the additional variables wasobtained with the image subtraction package ISIS. The data were acquiredon an observing run in 1999 July and range over seven observing nights.Estimates of fundamental photometric parameters are presented includingintensity- and magnitude-averaged B and V magnitudes, magnitude-averagedcolors, pulsation periods, and pulsation amplitudes. The mean period ofthe RRab variables, =0.5868 days, and the numberfraction of RRc stars, Nc/NRR=0.342, are bothlarge for an Oosterhoff type I (OoI) globular cluster, suggesting thatM75 may be Oosterhoff-intermediate. Possible conflicts betweenOosterhoff-type determination based on the AV-logP andAB-logP diagrams are discussed. The physical parameters ofthe RRc and RRab variables, as obtained from Fourier decomposition oftheir light curves, do not show any clear deviation from normal OoIbehavior.
| Homogeneous Photometry. III. A Star Catalog for the Open Cluster NGC 6791
We present broadband BVI photometry for the open cluster NGC 6791, basedupon analysis of 1764 individual CCD images. We discuss in detail thetransformation of the instrumental magnitudes to the standardphotometric system of Landolt and then discuss methods for selecting asubset of cluster stars whose photometric indices are most likely to beof high reliability. Color-magnitude and color-color diagrams for thecluster are presented and discussed. A detailed comparison of thisphotometry with the results of previous observational studies ispresented. We make preliminary comparisons with theoretical isochronesprovided by D. A. VandenBerg and conclude that an excellent match can beachieved if the metal abundance and age of the cluster are both near theupper end of the range of recent estimates ([Fe/H]~+0.3, age~12 Gyr) andthe reddening and true distance modulus are both near the low end of therange of recent estimates [E(B-V)~0.09 mag and (m-M)0~12.79mag] if the isochrone predictions are reliable. A companion paper willdiscuss candidate variable stars and stars possibly showing planetarytransits. Data tables listing measured magnitudes and standard errors,image-quality indices, a variability index, and equinox J2000.0equatorial coordinates for 14,342 stars to V~24 have been made availableto the public through the services of the Canadian Astronomy DataCentre. Equatorial coordinates only have been provided for a further1916 stars, the photometry for which we were unable to calibrate becauseof lack of color information.Based in part on observations made with the Nordic Optical Telescope,operated on the island of La Palma jointly by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofísica de Canarias.
| 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.
| Does NGC 6934 Have a Tidal Tail?
The study of extended tidal tails in globular clusters can yieldimportant discoveries about the nature of the Galactic halo and thedisruption rates of the clusters. Tails can be used to contrain, theorbital parameters for a cluster, the evaporation rate of the cluster,and the percentage of former cluster stars now in the halo field. NGC6934 is one of numerous Galactic globular clusters which have beenshown, from statistical star counts, to have extended tidal tails thatreach far beyond the classical King tidal radius. NGC 6934 is anexceptional candidate for a kinematic survey to probe the existence ofsuch tails, because it has a line-of-sight velocity (Vlos = -412 km/s)which is far different from the average for the halo. We will bepresenting kinematic results for a sample of 303 tidal candidates whichextend out to one degree along the previously identified N-W tidalextension. The sample covers a range of magnitudes (14.0 < V <17.5) with a faint magnitude cutoff slightly below the horizontal branchof the cluster. The data was taken using Hydra/MOS on the WIYN 3.5mtelescope at KPNO. The medium resolution (1.4 Å) spectroscopy wassufficient to determine radial velocities to within +/- 10 km/s. Resultsshow no stars from this sample having cluster-like velocities beyond theKing tidal radius. New BVRI photometry taken from the 0.8m telescope atMcDonald Observatory has been used to determine distances and metalabundance for the sample stars, which include members of the thin disk,thick disk, halo and NGC 6934 cluster stars. We will also presentphotometry which extends the faint magnitude limit of tidal tail surveyto V ~ 19.5.
| The Incidence of CN-strong Giants in Globular Clusters
Using a database from the published literature, the fraction ofCN-enhanced red giants in globular clusters is correlated against anumber of cluster physical properties. Newer data verify the earlierresult of Norris that the fraction of CN-strong stars correlates withcluster ellipticity, suggesting that the production of CN enhancementsamong globular cluster stars is somehow influenced by the angularmomentum content of the parent cluster. This correlation withellipticity appears to be more pronounced than any dependence on clustermass or metallicity.
| CN Abundance Inhomogeneities in the Globular Cluster M3: Results Based on Merged Data Sets from the Literature
Measurements of the λ3883 CN band absorption strength in spectraof red giant members of the globular cluster M3 have been compiled fromseveral sources in the literature, including the pioneering survey of N.B. Suntzeff, and transformed onto a common system. The homogenizedindices give a picture of the behavior of the CN band strength over arange in stellar magnitude of ~3.6 mag, from the tip of the red giantbranch (RGB) to MV~+1.0. As with other well-studied globularclusters, the red giant branch stars of M3 exhibit a bimodaldistribution of CN band strengths that extends to the limit of theavailable survey. The asymptotic giant branch stars also show evidenceof an intrinsic spread in CN content. Anticorrelations exist between theCN band strength and both carbon and oxygen abundances, suggesting thatthe high nitrogen abundances of the CN-strong giants have derived from aconversion of C and/or O into nitrogen, either within the cluster starsthemselves or in some primordial site. Among both CN-strong and CN-weakstars, the [C/Fe] abundance decreases with increasing luminosity on theRGB. Stellar atmosphere models indicate that in the magnitude range-1.5
| 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.
| The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI
We have surveyed Andromeda VI, a dwarf spheroidal galaxy companion toM31, for variable stars by using F450W and F555W observations obtainedwith the Hubble Space Telescope. A total of 118 variables were found,including 111 RR Lyrae stars, six anomalous Cepheids, and one variablethat we were unable to classify. We find that the Andromeda VI anomalousCepheids have properties consistent with those of anomalous Cepheids inother dwarf spheroidal galaxies. We revise the existingperiod-luminosity relations for these variables. Further, using theseand other available data, we show that there is no clear differencebetween fundamental and first-overtone anomalous Cepheids in aperiod-amplitude diagram at shorter periods, unlike the RR Lyrae stars.For the Andromeda VI RR Lyrae stars, we find that they lie close to theOosterhoff type I Galactic globular clusters in the period-amplitudediagram, although the mean period of the RRab stars,=0.588 days, is slightly longer than that of thetypical Oosterhoff type I cluster. The mean V magnitude of the RR Lyraestars in Andromeda VI is 25.29+/-0.03, resulting in a distance 815+/-25kpc on the Lee, Demarque, & Zinn distance scale. This is consistentwith the distance derived from the I magnitude of the tip of the redgiant branch. Similarly, the properties of the RR Lyrae stars indicate amean abundance for Andromeda VI that is consistent with that derivedfrom the mean red giant branch color. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555.
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