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|ACS Photometry of Newly Discovered Globular Clusters in the Outer Halo of M31|
We report the first results from deep ACS imaging of 10 classicalglobular clusters in the far outer regions (15kpc<~Rp<~100 kpc) of M31. Eight of the clusters,including two of the most remote M31 globular clusters presently known,are described for the first time. Our F606W, F814W color-magnitudediagrams extend ~3 mag below the horizontal branch and clearlydemonstrate that the majority of these objects are old (>~10 Gyr),metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional fourhave -1.9 <~ [Fe/H] <~ -1.5. The remaining object is moremetal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit thesecond-parameter effect. Using aperture photometry, we estimateintegrated luminosities and structural parameters for all clusters.Many, including all four clusters with projected radii greater than 45kpc, are compact and very luminous, with -8.9 <~ MV <~-8.3. These four outermost clusters are thus quite unlike their MilkyWay counterparts, which are typically diffuse, subluminous (-6.0 <~MV <~ -4.7), and more metal-rich (-1.8 <~ [Fe/H] <~-1.3).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with program10394.
|Effects of external tidal field on the evolution of the outer regions of multi-mass star clusters|
We present N-body simulations (including an initial mass function) ofglobular clusters in the Galaxy in order to study effects of the tidalfield systematically on the properties of the outer parts of globularclusters. Using NBODY6, which correctly takes into account the two-bodyrelaxation, we investigate the development of tidal tails of globularclusters in the Galactic tidal field. For simplicity, we have employedonly the spherical components (bulge and halo) of the Galaxy, andignored the effects of stellar evolution which could have been importantin the very early phase of the cluster evolution. The total number ofstars in our simulations is about 20000, which is much smaller than therealistic number of stars. All simulations had been done for severalorbital periods in order to understand the development of the tidaltails. In our scaled-down models, the relaxation time is sufficientlyshort to show the mass segregation effect, but we did not go far enoughto see the core collapse, and the fraction of stars lost from thecluster at the end of the simulations is only ~10 per cent. The radialdistribution of extra-tidal stars can be described by a power law with aslope around -3 in surface density. The directions of tidal tails aredetermined by the orbits and locations of the clusters. We find that thelength of tidal tails increases towards the apogalacticon and decreasestowards the perigalacticon. This is an anti-correlation with thestrength of the tidal field, caused by the fact that the time-scale forthe stars to respond to the potential is similar to the orbitaltime-scale of the cluster. The escape of stars in the tidal tailstowards the pericentre could be another reason for the decrease of thelength of tidal tails. We find that the rotational angular velocity oftidally induced clusters shows quite different behaviour from that ofinitially rotating clusters.
|Globular cluster system and Milky Way properties revisited|
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
|Nearby Spiral Globular Cluster Systems. I. Luminosity Functions|
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
|RR Lyrae-based calibration of the Globular Cluster Luminosity Function|
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
|Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters|
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
|A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies|
We have compiled a sample of globular clusters with high-quality stellarabundances from the literature to compare to the chemistries of stars inthe Galaxy and in dwarf spheroidal galaxies. Of the 45 globular clustersexamined, 29 also have kinematic information. Most of the globularclusters belong to the Galactic halo; however, a significant number havedisk kinematics or belong to the bulge. Focusing on the [α/Fe] andlight r-process element ratios, we find that most globular cluster starsmimic field stars of similar metallicities, and neither clearlyresembles the currently available stellar abundances in dwarf galaxies(including globular clusters in the Large Magellanic Cloud). Theexceptions to these general elemental ratio comparisons are alreadyknown in the literature, e.g., ω Centauri, Palomar 12, and Terzan7 associated with the Sagittarius remnant and Ruprecht 106, which has ahigh radial velocity and low [α/Fe] ratio. A few other globularclusters show more marginal peculiarities. The most notable one is thehalo cluster M68, which has a high galactocentric rotational velocity, aslightly younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratiosdecrease with increasing [Fe/H] at intermediate metallicities, which isconsistent with very massive stars playing a larger role in the earlychemical evolution of the Galaxy. The chemical similarities betweenglobular clusters and field stars with [Fe/H]<=-1.0 suggests a sharedchemical history in a well-mixed early Galaxy. The differences in thepublished chemistries of stars in the dwarf spheroidal galaxies suggestthat neither the globular clusters, halo stars, nor thick disk stars hadtheir origins in small isolated systems like the present-day Milky Waydwarf satellites.
|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.
|Outer Versus Inner Halo Globular Clusters: NGC 7492 Abundances|
We have carried out a detailed abundance analysis for 21 elements in asample of four RGB stars in the outer halo globular cluster NGC 7492(RGC 25 kpc); we find [Fe/H]=-1.82 dex inferred from Fe Ilines (-1.79 from Fe II) using high-dispersion(R=λ/Δλ=35,000) spectra obtained with HIRES at theKeck Observatory. Most elements show no sign of star-to-star variationwithin our limited sample. We have, however, detected an anticorrelationbetween O and Na abundances similar to that seen in our previousanalyses of inner halo GCs as well as in studies of relatively nearbyGCs by others. We compare the abundance ratios in NGC 7492 with those wepreviously determined for the much closer old halo GCs M3 and M13. Aftermaking corrections for trends of abundance ratio with metallicitycharacteristic of halo stars, we find that for these three GCs, for eachof the elements in common we deduce identical abundance ratios withrespect to Fe to within the probable measurement uncertainties. Thus,the chemical history of the outer halo as exemplified by the metal-poorouter halo globular cluster NGC 7492 is indistinguishable from that ofthe inner halo, exemplified by M3 and M13, at least through the epoch offormation of these old globular clusters. This applies to the neutroncapture processes as well.Based in part on observations obtained at the W. M. Keck Observatory,which is operated jointly by the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration.
|Abundances in a Large Sample of Stars in M3 and M13|
We have carried out a detailed abundance analysis for 21 elements in asample of 25 stars with a wide range in luminosity from luminous giantsto stars near the main-sequence turnoff in the globular cluster M13([Fe/H]=-1.50 dex) and in a sample of 13 stars distributed from the tipto the base of the red giant branch (RGB) in the globular cluster M3([Fe/H]=-1.39 dex). The analyzed spectra, obtained with HIRES at theKeck Observatory, are of high dispersion(R=λ/Δλ=35,000). Most elements, including Fe, showno trend with Teff and low scatter around the mean betweenthe top of the RGB and near the main-sequence turnoff, suggesting thatat this metallicity, non-LTE effects and gravitationally inducedheavy-element diffusion are not important for this set of elements overthe range of stellar parameters spanned by our sample. We have detectedan anticorrelation between O and Na abundances, observed previouslyamong the most luminous RGB stars in both of these clusters, in both M3and in M13 over the full range of luminosity of our samples, i.e., inthe case of M13 to near the main-sequence turnoff. M13 shows a largerrange in both O and Na abundance than does M3 at all luminosities, inparticular having a few stars at its RGB tip with unusually stronglydepleted O. We detect a correlation between Mg abundance and O abundanceamong the stars in the M13 sample. We also find a decrease in the meanMg abundance as one moves toward lower luminosity, which we tentativelysuggest is due to our ignoring non-LTE effects in Mg. Although CNburning must be occurring in both M3 and in M13, and ON burning isrequired for M13, we combine our new O abundances with published C and Nabundances to confirm with quite high precision that the sum of C+N+O isconstant near the tip of the giant branch, and we extend this down tothe bump in the luminosity function. The same holds true for a smallersample in M3, with somewhat larger variance. Star I-5 in M13 has largeexcesses of Y and of Ba, with no strong enhancement of Eu, suggestingthat an s-process event contributed to its heavy-element abundances. Themean abundance ratios for M3 and for M13 are identical to within theerrors. They show the typical pattern for metal-poor globular clustersof scatter among the light elements, with the odd atomic number elementsappearing in the mean enhanced. The Fe-peak elements, where the oddatomic number elements are excessively depleted, do not show anydetectable star-to-star variations in either cluster. The abundanceratios for 13 Galactic globular clusters with recent detailed abundanceanalyses, obtained by combining our samples with published data, arecompared with those of published large surveys of metal-poor halo fieldstars. For most elements, the agreement is very good, suggesting acommon chemical history for the halo field and cluster stars.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. The authors wish to recognize and acknowledge the verysignificant cultural role and reverence that the summit of Mauna Kea hasalways had within the indigenous Hawaiian community. We are mostfortunate to have the opportunity to conduct observations from thismountain.
|The structure of our stellar system.|
|Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo|
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.
|Mass Segregation and Tidal Tails of the Globular Cluster NGC 7492|
We present a wide-field CCD photometric study of the Galactic globularcluster NGC 7492. The derived VR color-magnitude diagram (CMD) extendsdown to about 3.5 mag below the cluster main-sequence turnoff. The fieldcovers 42'×42', about 3 times larger thanthe known tidal radius of this cluster. The sample of cluster membercandidates obtained by the CMD-mask process has been used to constructluminosity (LFs) and mass functions (MFs) and a surface density map. NGC7492 has a very flat MF with very little variation in the slope withdistance from the cluster center. However, there is a clear evidence foran increase of the MF slope from inner to outer regions, indicating masssegregation of the cluster. The surface density map of NGC 7492 showsextensions toward the Galactic anticenter (northeast) and northwest fromthe cluster center. A comparison of the LF for stars in the tails withthat for stars within the tidal radius suggests that the extensionsshown in the surface density map could be a real feature. The overallshape of NGC 7492 is significantly flattened. If the flattened shape ofthe NGC 7492 is caused by its rotation, the Galactic tidal field musthave been an important influence, since the initial rotation would havebeen almost completely removed by dynamical relaxation.
|Globular clusters and the formation of the outer Galactic halo|
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
|A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream|
We have obtained moderate resolution (~6 km s-1) spectroscopyof several hundred M giant candidates selected from Two Micron All SkySurvey photometry. Radial velocities are presented for stars mainly inthe southern Galactic hemisphere, and the primary targets have Galacticpositions consistent with association to the tidal tail system of theSagittarius (Sgr) dwarf galaxy. M giant stars selected from the apparenttrailing debris arm of Sgr have velocities showing a clear trend withorbital longitude, as expected from models of the orbit and destructionof Sgr. A minimum 8 kpc width of the trailing stream about the Sgrorbital midplane is implied by verified radial velocity members. Thecoldness of this stream (σv~10 km s-1)provides upper limits on the combined contributions of stream heating bya lumpy Galactic halo and the intrinsic dispersion of released stars,which is a function of the Sgr core mass. We find that the Sgr trailingarm is consistent with a Galactic halo that contains one dominant,LMC-like lump; however, some lumpier halos are not ruled out. An upperlimit to the total mass-to-light ratio of the Sgr core is 21 in solarunits. Evidence for other velocity structures is found among the moredistant (>13 kpc) M giants. A second structure that roughly mimicsexpectations for wrapped, leading Sgr arm debris crosses the trailingarm in the southern hemisphere; however, this may also be an unrelatedtidal feature. Among the bright, nearby (<13 kpc) M giants toward thesouth Galactic pole are a number with large velocities that identifythem as halo stars; these too may trace halo substructure, perhaps partof the Sgr leading arm near the Sun. The positions and velocities ofsouthern hemisphere M giants are compared with those of southernhemisphere globular clusters potentially stripped from the Sgr system.Support for association of the globular clusters Pal 2 and Pal 12 withSgr debris is found, based on positional and radial velocity matches.Our discussion includes description of a masked-filteredcross-correlation methodology that achieves better than 1/20 of aresolution element velocities in moderate-resolution spectra. Theimproved velocity resolution achieved allows tighter constraints to beplaced on the coldness of the Sgr stream than previously established.
|Globular clusters as probes of galaxy evolution: NGC 5128|
We present the results of our analysis of the photometric data ofglobular clusters in the elliptical galaxy NGC 5128 (Cen A). We showthat the integrated colour U-B can be an effective metallicity indicatorfor simple stellar populations. This is because it is sensitive tometallicity via the opacity effect but relatively insensitive to theeffective main-sequence turn-off temperature of the population (and thusto age) when Teff~ 7000-12 000 K, that is, when the BalmerJump is strong. This flat U-B versus Teff relation is aresult of the fact that the blueing effect of the optical continuum withincreasing temperature is temporarily stopped by the Balmer Jump, whichbecomes greater with temperature in this range. In this study we use U-Band B-V as metallicity and age indicators, respectively. We first showthat the use of the U-B versus B-V two-colour diagram roughly yields themetallicities and ages of the Milky Way globular clusters independentlydetermined, and then apply the technique to the clusters in NGC 5128.There is a large range in U-B, which corresponds to [Fe/H] of from -2.0to over +0.3. The large uncertainties from the data and the modelscurrently prevent us from pinning down their ages and metallicities.Although a constant age for all these clusters cannot be ruled out,there is a hint of the metal-rich clusters being younger. Significanceof these results and caveats of the analysis are discussed.
|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.
|Hubble Space Telescope Snapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441|
We present the results of a Hubble Space Telescope snapshot program tosurvey the inner region of the metal-rich globular cluster NGC 6441 forits variable stars. A total of 57 variable stars were found, including38 RR Lyrae stars, six Population II Cepheids, and 12 long-periodvariables. Twenty-four of the RR Lyrae stars and all of the PopulationII Cepheids were previously undiscovered in ground-based surveys. Of theRR Lyrae stars observed in this survey, 26 are pulsating in thefundamental mode with a mean period of 0.753 days and 12 arefirst-overtone-mode pulsators with a mean period of 0.365 days. Thesevalues match up very well with those found in ground-based surveys.Combining all the available data for NGC 6441, we find mean periods of0.759 and 0.375 days for the RRab and RRc stars, respectively. We alsofind that the RR Lyrae stars in this survey are located in the sameregions of a period-amplitude diagram as those found in ground-basedsurveys. The overall ratio of RRc to total RR Lyrae stars is 0.33.Although NGC 6441 is a metal-rich globular cluster and would, on thatground, be expected either to have few RR Lyrae stars or to be anOosterhoff type I system, its RR Lyrae stars more closely resemble thosein Oosterhoff type II globular clusters. However, even compared withtypical Oosterhoff type II systems, the mean period of its RRab stars isunusually long. We also derived I-band period-luminosity relations forthe RR Lyrae stars. Of the six Population II Cepheids, five are of WVirginis type and one is a BL Herculis variable star. This makes NGC6441, along with NGC 6388, the most metal-rich globular cluster known tocontain these types of variable stars. Another variable, V118, may alsobe a Population II Cepheid, given its long period and its separation inmagnitude from the RR Lyrae stars. We examine the period-luminosityrelation for these Population II Cepheids and compare it with those inother globular clusters and in the Large Magellanic Cloud. We argue thatthere does not appear to be a change in the period-luminosity relationslope between the BL Herculis and W Virginis stars, but that a change ofslope does occur when the RV Tauri stars are added to theperiod-luminosity relation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations|
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
|A Globular Cluster Metallicity Scale Based on the Abundance of Fe II|
Assuming that in the atmospheres of low-mass, metal-poor red giantstars, one-dimensional models based on local thermodynamic equilibriumaccurately predict the abundance of iron from Fe II, we derive aglobular cluster metallicity scale based on the equivalent widths of FeII lines measured from high-resolution spectra of giants in 16 keyclusters lying in the abundance range-2.4<[Fe/H]II<-0.7. We base the scale largely on theanalysis of spectra of 149 giant stars in 11 clusters by the Lick-Texasgroup supplemented by high-resolution studies of giants in five otherclusters. We also derive ab initio the true distance moduli for certainkey clusters (M5, M3, M13, M92, and M15) as a means of setting stellarsurface gravities. Allowances are made for changes in the abundancescale if one employs (1) Kurucz models with and without convectiveovershooting to represent giant star atmospheres in place of MARCSmodels and (2) the Houdashelt et al. color-temperature scale in place ofthe Alonso et al. scale.We find that [Fe/H]II is correlated linearly withW', the reduced strength of the near-infrared Ca II tripletdefined by Rutledge et al., although the actual correlation coefficientsdepend on the atmospheric model employed. The correlations, limited tothe range -2.4<[Fe/H]II<-0.7, are as follows:1.[Fe/H]II=0.531W'-3.279(MARCS),2.[Fe/H]II=0.537W'-3.225 (Kurucz withconvective overshooting),3.[Fe/H]II=0.562W'-3.329 (Kurucz withoutconvective overshooting).We also discuss how to estimate [X/Fe] ratios. We suggest that C, N, andO, as well as elements appearing in the spectrum in the singly ionizedstate, e.g., Ti, Sc, Ba, La, and Eu, should be normalized to theabundance of Fe II. Other elements, which appear mostly in the neutralstate, but for which the dominant species is nevertheless the ionizedstate, are probably best normalized to Fe I, but uncertainties remain.
|Palomar 13: An Unusual Stellar System in the Galactic Halo|
We report the first results of a program to study the internalkinematics of globular clusters in the outer halo of the Milky Way.Using the Keck telescope and High Resolution Echelle Spectrometer, wehave measured precise radial velocities for 30 candidate red giants inthe direction of Palomar 13, an object traditionally cataloged as acompact, low-luminosity globular cluster. We have combined these radialvelocities with published proper motion membership probabilities and newCCD photometry from the Keck and Canada-France-Hawaii telescopes toisolate a sample of 21 probable members. We find a systemic velocity ofs=24.1+/-0.5 km s-1 and aprojected, intrinsic velocity dispersion ofσp=2.2+/-0.4 km s-1. Although modest, thisdispersion is nevertheless several times larger than that expected for aglobular cluster of this luminosity and central concentration. Taken atface value, it implies a mass-to-light ratio ofΥV=40+24-17 based on the best-fitKing-Michie model. The surface density profile of Palomar 13 alsoappears unusual compared to most Galactic globular clusters; dependingupon the details of background subtraction and model-fitting, Palomar 13either contains a substantial population of ``extratidal'' stars, or isconsiderably more spatially extended than previously suspected. The fullsurface density profile is equally well fitted by a King-Michie modelhaving a high concentration and large tidal radius, or by aNavarro-Frenk-White model. We examine-and tentatively reject-a number ofpossible origins for the observed characteristics of Palomar 13 (e.g.,velocity ``jitter'' among the red giant branch stars, spectroscopicbinary stars, nonstandard mass functions, modified Newtonian dynamics)and conclude that the two leading explanations are either catastrophicheating during a recent perigalacticon passage or the presence of a darkmatter halo. The available evidence therefore suggests that Palomar 13is either a globular cluster that is now in the process of dissolvinginto the Galactic halo or a faint, dark matter-dominated stellar system.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 NASA, and was madepossible by the generous financial support of the W. M. Keck Foundation.
|Global metallicity of globular cluster stars from colour-magnitude diagrams|
We have developed an homogeneous evolutionary scenario for H- andHe-burning low-mass stars by computing updated stellar models for a widemetallicity and age range [0.0002<=Z<=0.004 and9<=t(Gyr)<=15, respectively] suitable to study globular clusters.This theoretical scenario allows us to provide self-consistentpredictions about the dependence of selected observational features ofthe colour-magnitude diagram, such as the brightness of the turn-off(TO), the zero-age horizontal branch (ZAHB) and the red giant branchbump (BUMP), on the cluster metallicity and age. Taking into accountthese predictions, we introduce a new observable based on the visualmagnitude difference between the TO and the ZAHB[ΔMV(TO-ZAHB)], and the TO and the BUMP[ΔMV(TO-BUMP)], given byA=ΔMV(TO-BUMP)-0.566ΔMV(TO-ZAHB). Weshow that the parameter A does not depend at all on the cluster age, butthat it does strongly depend on the cluster global metallicity. Thecalibration of the parameter A as a function of Z is then provided, asbased on our evolutionary models. We tested the reliability of thisresult by also considering stellar models computed by other authors,employing different input physics. Eventually, we present clear evidencethat the variation of ΔMV(TO-BUMP) withΔMV(TO-ZAHB) does supply a powerful probe of the globalmetal abundance, at least when homogeneous theoretical frameworks areadopted. Specifically, we show that the extensive set of models byVanden Berg et al. suggests a slightly different calibration of A versusZ calibration, which however provides global metallicities higher byonly 0.08+/-0.06dex with respect to the results from our computations.We provide an estimate of the global metallicity of 36 globular clustersin the Milky Way, based on our A-Z calibration, and a largeobservational data base of Galactic globular clusters. By consideringthe empirical [Fe/H] scales by both Zinn & West and Carretta &Gratton, we are able to provide an estimate of the α-elementenhancement for all clusters in our sample. We show that the trend of[α/Fe] with respect to the iron content significantly depends onthe adopted empirical [Fe/H] scale, with the Zinn & West onesuggesting α-element enhancements in fine agreement with currentspectroscopic measurements.
|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 `second parameter': a memory from the globular cluster formation epoch|
We study the correlations between the distribution of stars on thehorizontal branch (HB), the HB morphology, and some other properties ofglobular clusters (GCs) in a large sample of GCs. We strengthen previousresults that a general correlation exists only between HB morphology andmetallicity. Correlations with other properties, e.g. central densityand core radius, exist only for GCs within a narrow metallicity range.We conjecture that the lack of correlations with present properties ofGCs (besides metallicity) is because the variation of the HBmorphologies between GCs having similar metallicities is caused by aprocess, or processes, the effect of which was determined at theformation time of GCs. This process (or processes) is historicallytermed the `second parameter', metallicity being the `first parameter'.We then argue that the `planet second parameter' model, where thepresence of planets and to a lesser degree brown dwarfs and low-massmain-sequence stars is the `second parameter', fits this conjecture.This is because the processes that determine the presence of planets andtheir properties occur during the formation epoch of the star and itscircumstellar disc.
|A census with ROSAT of low-luminosity X-ray sources in globular clusters|
I analyze 101 observations from the ROSAT archive to search for X-raysources in or near 55 globular clusters. New sources are found in thecores of NGC 362 (a double source), NGC 6121 (marginally significant),NGC 6139, and NGC 6266; and outside the cores of NGC 6205, NGC 6352 andNGC 6388. More accurate positions are determined for the X-ray sourcesin some ten clusters. The improved position for the source in NGC 6341excludes the suggested ultraviolet counterpart. It is shown that one ofthe two sources reported near the core of NGC 6626 is spurious, as isthe detection of a pulsar period in the PSPC data of this cluster; thecentral source is resolved in three sources. One source reportedpreviously in NGC 6304 is demoted to an upper limit. For 20 clustercores better upper limits to the X-ray luminosity are obtained. From astatistical analysis I argue that several sources outside the clustercores may well belong to the clusters. All spectral energy distributionsobserved so far are relatively soft, with bremsstrahlung temperatures =~0.9 keV; there is evidence however that bremsstrahlung spectra do notcorrectly describe the spectra. The X-ray luminosity per unit mass forthe cluster as a whole does not depend on the concentration; theluminosity per unit mass for the core may increase with the clusterconcentration.
|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.
|What Are These Blue Metal-Poor Stars?|
The radial velocity behavior and chemical compositions of sixty-two bluemetal-poor (BMP) stars have been established from more than 1200 echellespectra obtained at Las Campanas Observatory from 1992 through 1999.Analysis of survey spectra provides abundances for this sample, which weuse to calibrate the K line versus B-V relation. Forty-four of the starshave [Fe/H]<-1, while eighteen lie on -1<[Fe/H]<0. One star,the SX Phe variable CS 22966-043, appears to be the most extreme exampleof a rare abundance class characterized by α-element deficiencies,high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and[Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities,while 42 are definite or probable members of binary systems. The binaryfraction of BMP stars appears to be independent of chemical composition.The high binary fraction fBMP~0.6 of BMP stars compared withthat found for the F- and G-type stars near the Sun, the systematicallylow mass functions of these binaries, and the paucity of double-linedbinaries among them lead us to suggest that at least half of the BMPbinaries are blue stragglers and that these blue stragglers are formedby McCrea mass transfer rather than by the various merger processes thatare currently believed to produce most blue stragglers in globularclusters. This conclusion is supported by the abnormally high proportionof BMP binaries with long periods and small orbital eccentricities,properties these binaries share with McClure's carbon star binaries. Thegreat majority of field blue stragglers (BSs) probably are created byRoche-lobe overflow during red giant branch evolution. Primaries of morewidely separated binaries that survive this phase of stellar evolutionmay engage in mass transfer during subsequent asymptotic giant branchevolution to form s-process abundance enhanced carbon stars. Our resultrequires a major downward revision of the fraction of BMP starsattributed to a captured intermediate-age population of metal-poor fieldstars. The high original estimate of the size of this component probablyarose from improper use of the globular cluster BS specific frequency,SBS=n(BS)/n(HB)~1, to estimate the halo BS space density. Weuse a simple model to calculate the specific frequency of BSs producedby McCrea mass transfer in an old metal-poor population with a givenprimordial binary fraction fB. Our model calculations returnvalues of SBS~5 for fB=0.15, much more like ourvalue for the field blue stragglers. We suggest that globular clusterseither destroy the primordial binaries that produce long period BSbinaries like those in the Galactic field reported here, or they neverpossessed them.
|Models for Old, Metal-Poor Stars with Enhanced α-Element Abundances. II. Their Implications for the Ages of the Galaxy's Globular Clusters and Field Halo Stars|
The ages of globular clusters (GCs) and post-turnoff field stars in theGalactic halo are derived using new stellar evolutionary models thatexplicitly take into account the observed abundances of theα-elements. Whereas the distances of the field subgiantsconsidered in this study are based on Hipparcos parallax measurements,theoretical zero-age horizontal-branch (ZAHB) loci have been used to setthe GC distance scale. (As reported in Paper I, the latter imply RRLyrae luminosities that are within 0.1-0.15 mag, on the high side, ofthose found for field variables from Baade-Wesselink, trignometicparallax, and statistical parallax studies.) Both the field and clusterobservations indicate that the most metal-deficient stars are >~14Gyr old while those with [Fe/H] <~-1.3 are 2-3 Gyr younger. Unlessthe O/Fe (or, more generally, the α/Fe) number abundance ratiorises quite steeply with decreasing [Fe/H], it seems unlikely that asignificant age-metallicity relation can be avoided. Using what iseffectively the ΔVHBTO method of determiningrelative cluster ages, we find that the dispersion in age at any [Fe/H]less than ~-1.0 is small (<~10%-15%). Even Arp 2 and IC 4499, whichhad been previously categorized as ``young GCs,'' probably havenear-normal ages for their metallicities-though a more definitiveconclusion must await improved photometry for these systems. Ruprecht106 appears no more than 1-1.5 Gyr younger than M3, as opposed to the ~4Gyr age difference that others have found. However, Palomar 12 andTerzan 7 are undoubtedly young, and they provide strong evidence that,at [Fe/H] >~-1.0, the ages of GCs differ by as much as 4 Gyr (or~25%). (A much larger sample of metal-rich clusters must be studied toascertain whether the distribution of ages at higher metallicities isbroad or narrow: Pal 12 and Ter 7 could well be atypical and representthe tail of a distribution that is strongly peaked near 12 Gyr.)Importantly, for the best-observed systems, there is no obvious conflictbetween the relative age estimates based on theΔVHBTO method, on the one hand, and theΔ(B-V)TO,RGB technique, on the other. However, thereare some exceptions, like M68, M53, M13, and NGC 288 for which the twoapproaches give slightly different results, indicating that something inaddition to, or besides, age must be playing a role. From isochrone/ZAHBfits to the C-M diagrams (CMDs) of the outer-halo GCs Pal 3, Pal 4, andEridanus, we conclude that there is no more than a small dependence ofage on Galactocentric distance. Pal 4 seems to have very close to thesame age as NGC 362, NGC 1261, and NGC 1851, while Eridanus and Pal 3appear to be <~1 Gyr younger than most inner-halo systems of the samemetallicity. Unless our understanding of the HB phase of evolution isseriously in error, cluster-to-cluster age differences (at a givenmetallicity) are much too small for age to be the most important of thepossible second parameters in determining the morphology of thehorizontal branch (the first parameter being [Fe/H]). Finally, wesuggest that the GC distance scale as inferred from studies of nearby RRLyraes is not necessarily in conflict with that based on localsubdwarfs. A reconciliation of the two approaches is possible simply byadopting a particular metallicity scale for the globular clusters-onethat is much closer to the Zinn-West scale than to that recentlyproposed by Carretta & Gratton. Indeed, such a simple way ofachieving consistency between subdwarf-based distances and thoseinferred from RR Lyraes seems compelling. If this suggestion is correct,then the ``long'' distance scale and low estimates of GC ages are nottenable. In fact, the adoption of the ``short'' distance scale, implyingages >~15 Gyr for the most metal-poor GCs (and <~18.30 for thetrue distance modulus of the Large Magellanic Cloud), leads to muchimproved agreement between synthetic and observed CMDs in the vicinityof the turnoff. It does cause a mismatch between predicted and observedHB luminosities, in the sense that the models are too bright by~0.1-0.15 mag, but this may simply be an indication that currentconductive opacities or the assumed chemical abundance parameters (Y,[O/Fe], and/or [α/Fe]) are not quite right.
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