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Washington Photometry of the Red Giant Branch in NGC 6388
We present Washington photometry of the giant branches of the relativelymetal-rich globular cluster NGC 6388 and compare it with 47 Tuc. Byexamining the spatial distribution of stellar colors across the redgiant branch and using spectroscopic data on several bright giants, wefound that NGC 6388's center is affected by differential reddening of atleast ΔE(B-V)~0.1. Most of the width of the red giant branch iscaused by that differential reddening. The southeastern outer regions ofthe cluster seem to be less affected by variable dust extinction, and wefind that NGC 6388 is slightly more metal-rich than 47 Tuc, with adistinct asymptotic giant branch. We have put an upper limit on possiblevariations in [Fe/H] for NGC 6388, finding that it must be less than~0.2 dex. It is unlikely that such a small range in metallicity could beresponsible for the blue horizontal branch and blue tail stars in acluster with a metallicity similar to the more massive 47 Tuc, whichonly exhibits a red horizontal branch.
| Origin of abundance inhomogeneity in globular clusters
We numerically investigate abundance properties of the Galactic globularclusters (GCs) by adopting a new `external pollution' scenario. In thisframework, GCs are assumed to originate in forming low-mass dwarfsembedded in dark matter subhaloes at very high redshifts (z) and thus bechemically influenced by field asymptotic giant branch (AGB) stars ofthe dwarfs during early GC formation processes. GCs within a dwarfgalaxy therefore can be formed from the mixture of (i) gas ejected fromthe field AGB stars formed earlier in the dwarf and (ii) theinterstellar gas infalling to the central region of the dwarf. In thisexternal pollution scenario, the ratio of the total mass of infallinggas to that of AGB ejecta during GC formation in a dwarf (s) and thetime-scale of gas infall (σI) are the most importantkey parameters that can determine abundance properties of GCs. We mainlyinvestigate the abundance inhomogeneity among light elements (e.g. C, N,O, Na and Al) of stars in GCs by using the latest stellar yield modelsof metal-poor AGB stars with and without third dredge-up. Our principalresults for the models with no third dredge-up, which are moreconsistent with observations, are as follows. (i) Both [N/Fe] and[C/Fe] can be diverse among stars within a GC owing to chemicalpollution from field AGB stars. [N/Fe] distributions in some GCs canclearly show bimodality, whereas [C/Fe] is monomodal in most models.[N/Fe] distributions depend on s such that models with smaller s (i.e.larger mass fraction of AGB ejecta used for GC formation) show the[N/Fe] bimodality more clearly. (ii) N-rich, C-poor stars in GCs alsohave higher He abundances owing to pollution from massive AGB stars withHe-rich ejecta. The number fraction of He-rich stars (Y > 0.30) ishigher for the models with smaller s and shorter σI for3 <= s <= 24 and 105 <= σI <=107 yr. He abundances of stars correlate with [N/Fe] and[Al/Fe] and anticorrelate with [C/Fe], [O/Fe] and [Na/Fe] within GCs inour models. (iii) Although our model can much better explain theobserved C-N and Mg-Al anticorrelations than previous theoreticalmodels, it is in strong disagreement with the observed O-Naanticorrelation. (iv) This model naturally provides an explanation forthe large fraction of CN-strong stars without recourse to an implausibleinitial mass function. Based on these results for the above externalpollution scenario, we discuss the long-standing problem of theCN-bimodality prevalent in the Galactic GCs, the possible heliumabundance inhomogeneity in these systems and their horizontal branchmorphologies.
| Integrated colours of Milky Way globular clusters and horizontal branch morphology
Broadband colours are often used as metallicity proxies in the study ofextragalactic globular clusters. A common concern is the effect ofvariations in horizontal branch (HB) morphology - the second-parametereffect - on such colours. We have used U BV I, Washington, and DDOphotometry for a compilation of over 80 Milky Way globular clusters toaddress this question. Our method is to fit linear relations betweencolour and [Fe/H], and study the correlations between the residualsabout these fits and two quantitative measures of HB morphology. Whilethere is a significant HB effect seen in U-B, for the commonly usedcolours B-V, V-I, and C-T_1, the deviations from the baselinecolour-[Fe/H] relations are less strongly related to HB morphology.There may be weak signatures in B-V and C-T_1, but these are at thelimit of observational uncertainties. The results may favour the use ofB-I in studies of extragalactic globular clusters, especially when itshigh [Fe/H]-sensitivity is considered.
| Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| A Moderate Sample Size, Multielement Analysis of the Globular Cluster M12 (NGC 6218)
We present chemical abundances of several proton-capture, α-,Fe-peak, and neutron-capture elements and radial velocities for 21 redgiant branch (RGB) and asymptotic giant branch members of the Galacticglobular cluster M12. Abundances are based on equivalent widthmeasurements and synthetic spectral analyses of moderate-resolutionspectra (R~15,000) obtained with the 3.5 m WIYN telescope and Hydramultifiber spectrograph. The stars observed range from the RGB tip(M0v=-2.47) down to about 0.50 mag above the levelof the horizontal branch (M0v=+0.11). Ourspectroscopic analysis suggests that M12 is a moderately metal-poorcluster with [Fe/H]=-1.54 (σ=0.09). While the Na abundancesexhibit a range of 0.90 dex, Mg and Al abundances are enhanced by 0.37and 0.54 dex and are nearly constant at all RGB luminosities, incontrast to the blue horizontal-branch cluster M13. The α- andFe-peak elements indicate that M12 has undergone a similar chemicalenrichment history to that of globular clusters and field stars ofcomparable metallicity, with <[α/Fe]>=+0.33 (σ=0.11).M12 also appears to be slightly r-process-rich, with<[Eu/Ba,La]>=+0.22 (σ=0.18).
| Fluorine Abundance Variations as a Signature of Enhanced Extra Mixing in Red Giants of the Globular Cluster M4
We show that enhanced extra mixing in low-mass red giants can result ina fluorine abundance that is correlated with abundance variations ofother elements participating in H burning, such as C, N, O, and Na. Thisfinding is used to explain the fluorine abundance variations recentlyfound in bright red giants of the globular cluster M4.
| Manganese Abundances in Cluster and Field Stars
We have derived Mn abundances for more than 200 stars in 19 globularclusters. In addition, Mn abundance determinations have been made for acomparable number of halo field and disk stars possessing an overlappingrange of metallicities and stellar parameters. Our primary data set wascomprised of high-resolution spectra previously acquired at theMcDonald, Lick, and Keck Observatories. To enlarge our data pool, weacquired globular and open cluster spectra from several otherinvestigators. Data were analyzed using synthetic spectra of the 6000Å Mn I triplet. Hyperfine structure parameters were included inthe synthetic spectra computations. Our analysis shows that for themetallicity range -0.7>[Fe/H]>-2.7, stars of 19 globular clustershave a mean relative abundance of <[Mn/Fe]>=-0.37+/-0.01(σ=0.10), a value in agreement with that of the field stars,<[Mn/Fe]>=-0.36+/-0.01 (σ=0.08). Despite the 2 orders ofmagnitude span in metallicity, the <[Mn/Fe]> ratio remainsconstant in both stellar populations. Our Mn abundance data indicatethat there is no appreciable variation in the relative nucleosyntheticcontribution from massive stars that undergo core-collapse supernovaeand thus no significant change of the associated initial mass functionin the specified metallicity range.
| Broad-band photometric colors and effective temperature calibrations for late-type giants. II. Z < 0.02
We investigate the effects of metallicity on the broad-band photometriccolors of late-type giants, and make a comparison of synthetic colorswith observed photometric properties of late-type giants over a widerange of effective temperatures (T_eff=3500-4800 K) and gravities (logg=0.0-2.5), at [M/H]=-1.0 and -2.0. The influence of metallicity on thesynthetic photometric colors is small at effective temperatures above 3800 K, but the effects grow larger at lower T_eff, due to the changingefficiency of molecule formation which reduces molecular opacities atlower [M/H]. To make a detailed comparison of the synthetic and observedphotometric colors of late type giants in the T_eff-color andcolor-color planes (which is done at two metallicities, [M/H]=-1.0 and-2.0), we derive a set of new T_eff-log g-color relations based onsynthetic photometric colors, at [M/H]=-0.5, -1.0, -1.5, and -2.0. Theserelations are based on the T_eff-log g scales that we derive employingliterature data for 178 late-type giants in 10 Galactic globularclusters (with metallicities of the individual stars between [M/H]=-0.7and -2.5), and synthetic colors produced with the PHOENIX, MARCS andATLAS stellar atmosphere codes. Combined with the T_eff-log g-colorrelations at [M/H]=0.0 (Kučinskas et al. 2005), the set of newrelations covers metallicities [M/H]=0.0dots-2.0 (Δ[M/H]=0.5),effective temperatures T_eff=3500dots4800 K (Δ T_eff=100 K), andgravities log g=-0.5dots3.0. The new T_eff-log g-color relations are ingood agreement with published T_eff-color relations based on observedproperties of late-type giants, both at [M/H]=-1.0 and -2.0. Thedifferences in all T_eff-color planes are typically well within 100 K.We find, however, that effective temperatures predicted by the scalesbased on synthetic colors tend to be slightly higher than thoseresulting from the T_eff-color relations based on observations, with theoffsets up to 100 K. This is clearly seen both at [M/H]=-1.0 and -2.0,especially in the T_eff-(B-V) and T_eff-(V-K) planes. The consistencybetween T_eff-log g-color scales based on synthetic colors calculatedwith different stellar atmosphere codes is very good, with typicaldifferences being well within Δ T_eff 70 K at [M/H]=-1.0 andΔ T_eff 40 K at [M/H]=-2.0.
| Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.
| Na-O anticorrelation and HB. I. The Na-O anticorrelation in NGC 2808
We derived the atmospheric parameters and elemental abundances of Fe, O,and Na for about 120 red giant stars in the Galactic globular clusterNGC 2808. Our results are based on the analysis of medium-highresolution (R=22 000{-}24 000) GIRAFFE spectra acquired with the FLAMESspectrograph at VLT-UT2 as a part of a project aimed at studying theNa-O anticorrelation as a function of physical parameters in globularclusters. We present the anticorrelation of Na and O abundances in NGC2808 here, and discuss the distribution function of stars along thisrelation. Besides a bulk of O-normal stars with the typical compositionof field halo stars, NGC 2808 seems to host two other groups of O-poorand super O-poor stars. In this regard, NGC 2808 is similar to M 13, thetemplate cluster for the Na-O anticorrelation. However, in contrast to M13, most stars in NGC 2808 are O-rich. This might be related to thehorizontal branch morphologies that are very different in these twoclusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10(rms = 0.065 dex, from 123 stars). We also found some evidence of asmall intrinsic spread in metallicity, but more definitive conclusionsare hampered by the presence of a small differential reddening.
| Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
| 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.
| On the Predicted and Observed Color Boundaries of the RR Lyrae Instability Strip as a Function of Metallicity
The purpose of this paper is to predict the temperature at thefundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL)variables from the pulsation equation that relates temperature toperiod, luminosity, and mass. Modern data for the correlations betweenperiod, luminosity, and metallicity at the FBE for field and cluster RRLstars are used for the temperature calculation. The predictedtemperatures are changed to B-V colors using an adopted colortransformation. The predicted temperatures at the FBE become hotter as[Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicitydecreases to -2.5. The temperature range over this interval ofmetallicity is ΔlogTe=0.04, or 640 K at 6900 K. Thepredicted color variation is at the level of 0.03 mag in B-V over mostof this range. The predictions are compared with the observed RRL colorsat the FBE for both the field and cluster variables, showing generalagreement at the level of 0.02 mag in (B-V)0, which, however,is the uncertainty of the reddening corrections. The focus of theproblem is then reversed by fitting a better envelope to the observedFBE relation between color and metallicity for metallicities smallerthan -1.8, which, when inserted in the pulsation equation, gives anonlinear calibration of the absolute magnitude of the average evolvedlevel of the horizontal branch (HB) ofMV=1.109+0.600([Fe/H])+0.140([Fe/H])2, where thezero point has been set by the observed RR Lyrae stars in the LMC at0=19.064 by Clementini et al. for [Fe/H]=-1.5, andusing an adopted LMC modulus of (m-M)0=18.54 from Tammann etal. that is independent of the LMC Cepheids. This equation givesMV=0.52 at [Fe/H]=-1.5.However, the calibration fails for the extreme second-parameter clustersNGC 6388 and NGC 6441. Proof that the MV absolute magnitudesfor their RR Lyrae variables are ~0.4 mag brighter than the calibrationequation predicts is from the unusually long periods for givenamplitudes at their high metallicities of [Fe/H]~-0.5. Allsecond-parameter clusters are believed to violate the equation, but lessseverely than these two extreme examples. An additional complication inusing RRL stars as distance indicators at the 0.1 mag level is shown bythe difference of ΔlogP=0.029+/-0.007 in the position of theenvelope locus at the shortest periods for the observedperiod-metallicity correlation between the field and cluster variables.The field variables have shorter periods than cluster variables at theenvelope. This requires the cluster RRL stars to be 0.09 mag brighterthan the field variables at the same temperature and mass, or to have atemperature difference of ΔlogTe=0.008 at fixedluminosity and mass. The field and cluster variables also differ in thenear absence of cluster RR Lyrae stars in the -1.7>[Fe/H]>-2.0metallicity interval, whereas the field variables show no such gap. Atest is proposed for different origins for the field and the clustervariables by comparing the morphology of the HBs in the local dwarfspheroidal galaxies with that in the Galactic globular clusters in theinner halo and by relating the differences with the relevantsecond-parameter indicators.
| 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.
| Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for HI and OH
We present the results of a survey, the deepest to date, for HI emissionat 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720MHz) in the direction towards the Galactic globular clusters M15, M2,NGC6934, NGC7006 and Pal13. The aim is to measure the amount of hydrogenin the intracluster medium, and to find OH masers in the circumstellarenvelopes of globular cluster red giants. We present a tentativedetection of 0.3Msolar of neutral hydrogen in M15 andpossible detections of neutral hydrogen in M2 and Pal13. We derive upperlimits to the neutral hydrogen content of NGC6934 and 7006. No OHemission is detected. We also present deep HI data of the northern tipof the Magellanic Stream behind Pal13.
| Variable stars in the globular cluster NGC 7006
Our BVR CCD photometric study of the globular cluster NGC 7006 permittedus to find two new suspected RR Lyrae variables. A cluster chart ispresented with the position of known and suspected variables marked.
| Intragroup diffuse light in compact groups of galaxies: HCG 79, 88 and 95
Deep B and R images of three Hickson Compact Groups, HCG 79, 88 and 95,have been analysed using a new wavelet technique to measure possibleintragroup diffuse light present in these systems. The method used,OV_WAV, is a wavelet technique particularly suitable fordetecting low surface brightness extended structures, down to asignal-to-noise ratio (S/N) = 0.1 per pixel, which corresponds to a5σ detection level in wavelet space. The three groups studied arein different evolutionary stages, as can be judged by their verydifferent fractions of the total light contained in their intragrouphaloes: 46 +/- 11 per cent for HCG 79 and 11 +/- 26 per cent for HCG 95,in the B band, and HCG 88 had no component detected down to a limitingsurface brightness of 29.1B mag arcsec-2. For HCG 95, theintragroup light (IGL) is red, similar to the mean colours of the groupgalaxies themselves, suggesting that it is formed by an old populationwith no significant ongoing star formation. For HCG 79, however, theintragroup material has a significantly bluer colour than the meancolour of the group galaxies, suggesting that the diffuse light may, atleast in part, come from stripping of dwarf galaxies which dissolvedinto the group potential well.
| 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.
| 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.
| A Comparison between Carbon and Nitrogen Abundances of Bright Giants in the Globular Clusters M13, M10, and NGC 7006
Carbon and nitrogen abundances have been derived for red giants in theglobular clusters M13, M10, and NGC 7006. These constitute a``second-parameter'' trio of clusters having very similar metallicities([Fe/H]~-1.55) but different horizontal-branch (HB) morphologies, withNGC 7006 having a much higher fraction of red HB stars than either M13or M10. The red giants observed have absolute magnitudes in the range-2.5
| The structure of our stellar system.
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| 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.
| 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.
| Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
| RR Lyrae variables in Galactic globular clusters. IV. Synthetic HB and RR Lyrae predictions
We present theoretical predictions concerning horizontal branch stars inglobular clusters, including RR Lyrae variables, as derived fromsynthetic procedures collating evolutionary and pulsational constraints.On this basis, we explore the predicted behavior of the pulsators as afunction of the horizontal branch morphology and over the metallicityrange Z= 0.0001 to 0.006, revealing an encouraging concordance with theobserved distribution of fundamentalised periods with metallicity.Theoretical relations connecting periods to K magnitudes and BV or VIWesenheit functions are presented, both appearing quite independent ofthe horizontal branch morphology only with Z≥ 0.001. Predictionsconcerning the parameter R are also discussed and compared under variousassumptions about the horizontal branch reference luminosity level.
| A note on the second-parameter phenomenon in globular clusters
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| Abundance Variations Within Globular Clusters
Abundance variations within globular clusters (GCs), and of GC starswith respect to field stars, are important diagnostics of a variety ofphysical phenomena, related to the evolution of individual stars, masstransfer in binary systems, and chemical evolution in high densityenvironments. The broad astrophysical implications of GCs as buildingblocks of our knowledge of the Universe make a full understanding oftheir history and evolution basic in a variety of astrophysical fields.We review the current status of the research in this field, comparingthe abundances in GCs with those obtained for field stars, discussing indepth the evidence for H-burning at high temperatures in GC stars,describing the process of self-enrichment in GCs with particularreference to the case of the most massive Galactic GC ( Cen), anddiscussing various classes of cluster stars with abundance anomalies.Whereas the overall pattern might appear very complex at first sight,exciting new scenarios are opening where the interplay between GCdynamical and chemical properties are closely linked with each other.
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