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The Infrared Ca II Triplet as Metallicity Indicator
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
| The Michigan/MIKE Fiber System Survey of Stellar Radial Velocities in Dwarf Spheroidal Galaxies: Acquisition and Reduction of Data
We introduce a stellar velocity survey of dwarf spheroidal galaxies,undertaken using the Michigan/MIKE Fiber System (MMFS) at theMagellan/Clay 6.5 m telescope at Las Campanas Observatory. As of 2006November we have used MMFS to collect 6415 high-resolution(R=20,000-25,000) spectra from 5180 stars in four dwarf spheroidalgalaxies: Carina, Fornax, Sculptor, and Sextans. Spectra sample therange 5140-5180 Å, which includes the prominent magnesium tripletabsorption feature. We measure radial velocity (RV) to a medianprecision of 2.0 km s-1 for stars as faint as V~20.5. Fromthe spectra we also are able to measure the strength of iron andmagnesium absorption features using spectral indices that correlate witheffective temperature, surface gravity, and chemical abundance.Measurement of line strength allows us to identify interlopingforeground stars independently of velocity and to examine themetallicity distribution among dSph members. Here we present detaileddescriptions of MMFS, our target selection and spectroscopicobservations, the data reduction procedure, and error analysis. Wecompare our RV results to previously published measurements forindividual stars. In some cases we find evidence for a mild,velocity-dependent offset between the RVs that we measure using themagnesium triplet and previously published RV measurements derived fromthe infrared calcium triplet. In companion papers we will present thecomplete data sets and kinematic analyses of these new observations.This paper includes data obtained with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample
We have measured the absolute proper motions of globular clusters NGC2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on theHipparcos system, and they are the first determinations ever made forthese low Galactic latitude clusters. The proper-motion uncertaintiesrange from 0.3 to 0.5 mas yr-1. The inferred orbits indicatethat (1) the single metal-rich cluster in our sample, NGC 5927,dynamically belongs to the thick disk; (2) the remaining metal-poorclusters have rather low-energy orbits of high eccentricity, and amongthese there appear to be two ``pairs'' of dynamically associatedclusters; (3) the most energetic cluster in our sample, NGC 3201, is ona highly retrograde orbit-which had already been surmised from radialvelocity alone-with an apocentric distance of 22 kpc; and (4) none ofthe metal-poor clusters appear to be associated with the recentlydetected SDSS streams or with the Monoceros structure. These are thefirst results of the Southern Proper Motion program where thesecond-epoch observations are taken with the recent CCD camera systeminstalled on the double astrograph at El Leoncito, Argentina.
| 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.
| 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.
| 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 Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy
Using HST ACS, we have obtained deep optical images reaching stars wellbelow the oldest main-sequence turnoff in the spheroid, tidal stream,and outer disk of Andromeda. We have reconstructed the star formationhistory in these fields by comparing their color-magnitude diagrams to agrid of isochrones calibrated to Galactic globular clusters observed inthe same bands. Each field exhibits an extended star formation history,with many stars younger than 10 Gyr but few younger than 4 Gyr.Considered together, the star counts, kinematics, and populationcharacteristics of the spheroid argue against some explanations for itsintermediate-age, metal-rich population, such as a significantcontribution from stars residing in the disk or a chance intersectionwith the stream's orbit. Instead, it is likely that this population isintrinsic to the inner spheroid, whose highly disturbed structure isclearly distinct from the pressure-supported metal-poor halo thatdominates farther from the galaxy's center. The stream and spheroidpopulations are similar, but not identical, with the stream's mean agebeing ~1 Gyr younger; this similarity suggests that the inner spheroidis largely polluted by material stripped from either the stream'sprogenitor or similar objects. The disk population is considerablyyounger and more metal-rich than the stream and spheroid populations,but not as young as the thin-disk population of the solar neighborhood;instead, the outer disk of Andromeda is dominated by stars of age 4-8Gyr, resembling the Milky Way's thick disk. The disk data areinconsistent with a population dominated by ages older than 10 Gyr andin fact do not require any stars older than 10 Gyr.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS 5-26555. These observations areassociated with proposals 9453 and 10265. Some of the data presentedherein were obtained at the W. M. Keck Observatory, which is operated asa scientific partnership among the California Institute of Technology,the University of California, and NASA. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.
| 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.
| A New Color-Magnitude Diagram for Palomar 11
We present new photometry for the Galactic thick-disk globular clusterPalomar 11 extending well past the main-sequence turnoff in the V and Ibands. This photometry shows noticeable, but depleted, red giant andsubgiant branches. The difference in magnitude between the redhorizontal branch (red clump) and the subgiant branch is used todetermine that Palomar 11 has an age of 10.4+/-0.5 Gyr. The red clump isused to derive a distance dsolar=14.3+/-0.4 kpc and a meancluster reddening of E(V-I)=0.40+/-0.03. There is differential reddeningacross the cluster, of order δE(V-I)~0.07. The color-magnitudediagram of Palomar 11 is virtually identical to that of the thick-diskglobular cluster NGC 5927, implying that these two clusters have asimilar age and metallicity. Palomar 11 has a slightly redder red giantbranch than 47 Tuc, implying that Palomar 11 is 0.15 dex moremetal-rich, or 1 Gyr older, than 47 Tuc. Ca II triplet observations,such as those of Rutledge and coworkers, favor the hypothesis thatPalomar 11 is the same age as 47 Tuc, but slightly more metal-rich.
| 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.
| UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
| 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.
| Stellar Cluster Fiducial Sequences with the Advanced Camera for Surveys
We present color-magnitude diagrams of five Galactic globular clustersand one Galactic open cluster spanning a wide range of metallicity(-2.1<~[Fe/H]<~+0.3), as observed in the F606W (broad V) and F814W(I) bands with the Advanced Camera for Surveys (ACS) on the Hubble SpaceTelescope. These clusters are part of two large ACS programs measuringthe star formation history in the Andromeda halo, tidal stream, andouter disk. In these programs, the clusters serve as empiricalisochrones and as calibrators for the transformation of theoreticalisochrones to the ACS bandpasses. To make these data more accessible tothe community, for each cluster we provide a ridgeline tracing the starson the main sequence, subgiant branch, and red giant branch, plus thelocus of stars on the horizontal branch. In addition, we provide thetransformation of the Victoria-Regina isochrones to the ACS bandpasses.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555. These observations areassociated with proposals 9453 and 10265.
| A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.
| Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch
Using the Hubble Space Telescope (HST) Wide Field Camera (WFC) of theAdvanced Camera for Surveys (ACS), we have obtained deep (V, I)photometry of an outer halo field in NGC 5128, to a limiting magnitudeof I~=29. Our photometry directly reveals the core helium burningstellar population (the ``red clump'' or horizontal branch) in a giantE/S0 galaxy for the first time. The color-magnitude diagram displays avery wide red giant branch (RGB), an asymptotic giant branch (AGB) bump,and the red clump; no noticeable population of blue horizontal branchstars is present, confirming previous suggestions that old, verymetal-poor population is not ubiquitous in the halo of this galaxy. Fromthe upper RGB we derive the metallicity distribution, which we find tobe very broad and moderately metal-rich, with average [M/H]=-0.64 anddispersion 0.49 dex. The metallicity distribution function is virtuallyidentical to that found in other halo fields observed previously withHST, but with an enhanced metal-rich population that was partiallymissed in the previous surveys due to V-band incompleteness for thesevery red stars. Combining the metallicity-sensitive colors of the RGBstars with the metallicity- and age-sensitive features of the AGB bumpand the red clump, we infer the average age of the halo stars to be~8+3-3.5 Gyr. As part of our study, we present anempirical calibration of the ACS F606W and F814W filters to the standardV and I bands, achieved with ground-based observations of the same fieldmade from the EMMI camera of the New Technology Telescope of the ESO LaSilla Observatory.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 NAS 5-26555. These observations are associated withprogram GO-9373. Also partially based on observations collected at theEuropean Southern Observatory, La Silla, Chile, in Observing Programme071.D-0560.
| A New Version of Reimers' Law of Mass Loss Based on a Physical Approach
We present a new semiempirical relation for the mass loss of coolstellar winds, which so far has frequently been described by ``Reimers'law.'' Originally, this relation was based solely on dimensional scalingarguments without any physical interpretation. In our approach, the windis assumed to result from the spillover of the extended chromosphere,possibly associated with the action of waves, especially Alfvénwaves, which are used as guidance in the derivation of the new formula.We obtain a relation akin to the original Reimers law, but whichincludes two new factors. They reflect how the chromospheric heightdepends on gravity and how the mechanical energy flux depends, mainly,on the effective temperature. The new relation is tested and sensitivelycalibrated by modeling the blue end of the horizontal branch of globularclusters. The most significant difference from mass-loss rates predictedby the Reimers relation is an increase by up to a factor of 3 forluminous late-type (super)giants, in good agreement with observations.
| 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.
| Transforming observational data and theoretical isochrones into the ACS/WFC Vega-mag system
We propose a zero-point photometric calibration of the data from theAdvanced Camera for Surveys (ACS) Wide Field Channel (WFC) on board theHubble Space Telescope, based on a spectrum of Vega and the mostup-to-date in-flight transmission curves of the camera. This calibrationis accurate at the level of a few hundredths of a magnitude. The mainpurpose of this effort is to transform the entire set of evolutionarymodels into a simple observational photometric system for ACS/WFC data,and to make them available to the astronomical community. We provide thezero-points for the most used ACS/WFC bands, and give basic recipes forcalibrating both the observed data and the models. We also present thecolour-magnitude diagram from ACS data of five Galactic globularclusters, spanning the metallicity range -2.2 <[Fe/H] < -0.04, andwe provide fiducial points representing their sequences from severalmagnitudes below the turn-off to the red giant branch tip. The observedsequences are compared with the models in the newly defined photometricsystem.
| Infrared Photometry of NGC 6791
We present deep JHK photometry of the old and metal-rich open clusterNGC 6791. The photometry reaches below the main-sequence turnoff toK~16.5 mag. We combine our photometry with that from Stetson et al. toprovide color-magnitude diagrams showing K versus J-K, K versus V-K, andV versus V-K. We study the slope of the red giant branch in the infraredbut find that it is not a useful metallicity indicator for the cluster,nor any metal-rich cluster that lacks a well-populated red giant branch,because it is not linear, as has often been assumed, in K versus J-K.The mean color of the red horizontal-branch/red clump stars provide anestimate of the cluster reddening, E(B-V)=0.14+/-0.04 mag for[Fe/H]=+0.4+/-0.1. The mean magnitudes of these stars also provide agood distance estimate, (m-M)0=13.07+/-0.04. Finally, we findthat the isochrones of Yi et al. provide optimal fits in V versus B-Vand V-K and K versus J-K and V-K for such values if [Fe/H] lies between+0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyr([Fe/H]=+0.3) and 7.5 Gyr ([Fe/H]=+0.5).Based on observations made with the Mayall 4 m Telescope of the NationalOptical Astronomy Observatory.
| A robust method for the analysis of integrated spectra from globular clusters using Lick indices
We define a method for the analysis of the integrated spectra ofextragalactic globular clusters that provides more reliable measures ofthe age, metallicity and α-element abundance ratio than have sofar been achieved. The method involves the simultaneous fitting of up to25 Lick indices in a χ2 fitting technique that maximizesthe use of the available data. Here we compare three sets of singlestellar population (SSP) models of Lick indices to the highsignal-to-noise, integrated spectra of 20 Galactic globular clusters.The ages, [Fe/H] and α-element abundance ratios derived from theSSP models are compared with the results of resolved stellar populationstudies from the literature. We find good consistency with the publishedvalues, with an agreement of better than 0.1 dex in all three derivedparameters. The technique allows the identification of abundance ratioanomalies, such as the known nitrogen overabundance in Galactic globularclusters, and the presence of anomalous horizontal branch morphologies.It also minimizes the impact on the derived parameters of imperfectcalibration to the Lick system, and reduction errors in general. Themethod defined in this paper is therefore robust with respect to many ofthe difficulties that plague the application of SSP models in general.Consequently, it is well suited to the study of extragalactic globularcluster systems.
| Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
| Age Constraints for an M31 Globular Cluster from Main-Sequence Photometry
We present a color-magnitude diagram (CMD) of the globular cluster SKHB312 in the Andromeda galaxy (M31), obtained with the Advanced Camera forSurveys on the Hubble Space Telescope. The cluster was included in deepobservations taken to measure the star formation history of the M31halo. Overcoming a very crowded field, our photometry of SKHB 312reaches mV~30.5 mag, more than 1 mag below the main-sequenceturnoff. These are the first observations to allow a direct age estimatefrom the turnoff in an old M31 cluster. We analyze its CMD andluminosity function using a finely spaced grid of isochrones that havebeen calibrated using observations of Galactic clusters taken with thesame camera and filters. The luminosity difference between the subgiantand horizontal branches is ~0.2 mag smaller in SKHB 312 than in theGalactic clusters 47 Tuc and NGC 5927, implying that SKHB 312 is 2-3 Gyryounger. A quantitative comparison to isochrones yields an age of10+2.5-1 Gyr.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 (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withproposal 9453.
| A search for late-type supergiants in the inner regions of the Milky Way
We present the results of a narrow-band infrared imaging survey of anarrow strip (12' wide) around the Galactic equator between6o and 21o of galactic longitude aimed atdetecting field stars with strong CO absorption, mainly late-type giantsand supergiants. Our observations include follow-up low resolutionspectroscopy (R = 980) of 191 selected candidates in the H and K bands.Most of these objects have photometric and spectroscopic characteristicsconsistent with their being red giants, and some display broad, strongabsorption wings due to water vapor absorption between the H and Kbands. We also identify in our sample 18 good supergiant candidatescharacterized by their lack of noticeable water absorption, strong CObands in the H and K windows, and HKS photometry suggestiveof high intrinsic luminosity and extinction reaching up to AV≃ 40 mag. Another 9 candidates share the same features except forweak H2O absorption, which is also observed among some Msupergiants in the solar neighbourhood. Interesting differences arenoticed when comparing our stars with a local sample of late-type giantsand supergiants, as well as with a sample of red giants in globularclusters of moderately subsolar metallicity and to a sample of bulgestars. A large fraction of the stars in our sample have CaI and NaIfeatures markedly stronger than those typical in the local referencesample (both giants and supergiants), whereas the equivalent widths ofthe CO bands are similar or weaker. In this regard, our stars in theinner Milky Way disk display differences very similar to thoseidentified by other authors between cool giants and supergiants near thegalactic center and their counterparts in the solar neighbourhood. Wepropose that the systematic spectroscopic differences of our innerGalaxy stars are due to their higher metallicities that cause deepermixing in their mantles, resulting in lower surface abundances of C andO and higher abundances of CN, which contribute to the strength of theCaI and NaI features at low resolution. Our results stress thelimitations of using local stars as templates for the study of compositecool stellar populations such as central starbursts in galaxies.Based on observations collected at the Centro AstronómicoHispano-Alemán, Calar Alto, Spain; and at the European SouthernObservatory, La Silla, Chile (programme 71.B-0274(A)).Full Table 1 and spectra in FITS format are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/425/489
| The initial helium abundance of the Galactic globular cluster system
In this paper we estimate the initial He content in about 30% of theGalactic globular clusters (GGCs) from new star counts we have performedon the recently published HST snapshot database of Colour MagnitudeDiagrams (Piotto et al. \cite{Piotto02}). More specifically, we use theso-called R-parameter and estimate the He content from a theoreticalcalibration based on a recently updated set of stellar evolution models.We performed an accurate statistical analysis in order to assess whetherGGCs show a statistically significant spread in their initial Heabundances, and whether there is a correlation with the clustermetallicity. As in previous works on the subject, we do not find anysignificant dependence of the He abundance on the cluster metallicity;this provides an important constraint for models of Galaxy formation andevolution. Apart from GGCs with the bluest Horizontal Branch morphology,the observed spread in the individual helium abundances is statisticallycompatible with the individual errors. This means that either there isno intrinsic abundance spread among the GGCs, or that this is masked bythe errors. In the latter case we have estimated a firm 1σ upperlimit of 0.019 to the possible intrinsic spread. In case of the GGCswith the bluest Horizontal Branch morphology we detect a significantspread towards higher abundances inconsistent with the individualerrors; this can be fully explained by additional effects not accountedfor in our theoretical calibrations, which do not affect the abundancesestimated for the clusters with redder Horizontal Branch morphology. Inthe hypothesis that the intrinsic dispersion on the individual Heabundances is zero, taking into account the errors on the individualR-parameter estimates, as well as the uncertainties on the clustermetallicity scale and theoretical calibration, we have determined aninitial He abundance mass fraction YGGC=0.250±0.006.This value is in perfect agreement with current estimates based onCosmic Microwave Background radiation analyses and cosmologicalnucleosynthesis computations.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555, and on observations retrieved withthe ESO ST-ECF Archive.
| The 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.
| 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.
| Stellar population models of Lick indices with variable element abundance ratios
We provide the whole set of Lick indices from CN1 toTiO2 in the wavelength range 4000 <~<~λ<~6500 Å of simple stellar population models with, for the firsttime, variable element abundance ratios, [α/Fe ]= 0.0, 0.3, 0.5,[α/Ca ]=-0.1, 0.0, 0.2, 0.5 and [α/N]=-0.5, 0.0. The modelscover ages between 1 and 15 Gyr, metallicities between 1/200 and 3.5solar. The impact from the element abundance changes on theabsorption-line indices is taken from Tripicco & Bell, using anextension of the method introduced by Trager et al. Our models are freefrom the intrinsic α/Fe bias that was imposed by the Milky Waytemplate stars up to now, hence they reflect well-defined α/Feratios at all metallicities. The models are calibrated with Milky Wayglobular clusters for which metallicities and α/Fe ratios areknown from independent spectroscopy of individual stars. Themetallicities that we derive from the Lick indices Mgb and Fe5270 are inexcellent agreement with the metallicity scale by Zinn & West, andwe show that the latter provides total metallicity rather than ironabundance. We can reproduce the relatively strong CN-absorption featuresCN1 and CN2 of galactic globular clusters withmodels in which nitrogen is enhanced by a factor of 3. An enhancement ofcarbon, instead, would lead to serious inconsistencies with the indicesMg1 and C24668. The calcium sensitive index Ca4227of globular clusters is well matched by our models with [Ca/Fe]= 0.3,including the metal-rich bulge clusters NGC 6528 and 6553. From ourα/Fe-enhanced models we infer that the index [MgFe] defined byGonzález is quite independent of α/Fe but still slightlydecreases with increasing α/Fe. We find that the index , instead,is completely independent of α/Fe and serves best as a tracer oftotal metallicity. Searching for blue indices that give similarinformation as Mg b and , we find that CN1 andFe4383 may be best suited to estimating α/Fe ratios of objects atredshifts z~ 1.
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