WIKISKY.ORG
 Home Getting Started To Survive in the Universe News@Sky Astro Photo The Collection Forum Blog New! FAQ Press Login

# NGC 6522

Contents

### Images

DSS Images   Other Images

### Related articles

 Abundances of Baade's Window Giants from Keck HIRES Spectra. II. The Alpha and Light Odd ElementsWe report detailed abundances of O, Na, Mg, Al, Si, Ca, and Ti-elementsproduced by massive stars-for 27 red giants toward the Galactic bulge inBaade's window. These species are overabundant in the bulge relative tothe disk, consistent with enhancement by Type II SN ejecta. [Mg/Fe]=+0.3dex over the full range of [Fe/H], while O, Si, Ca, and Ti are elevatedbut follow more disklike trends. We propose that the decline in [O/Fe]is due to metallicity-dependent oxygen yields from massive stars,perhaps connected with the Wolf-Rayet phenomenon. The elements Si, Ca,and Ti, believed to be produced during explosive nucleosynthesis,possess identical trends with [Fe/H]. We attribute the decline of theseelements to metallicity-dependent yields in Type II SNe. The trend of[Al/Fe] is found to vary strikingly with environment; the range from theSgr dwarf to the bulge is 0.7 dex. The disjoint composition of thethick/thin disk and bulge stars is inconsistent with models in which thebulge formed from the thickening of the disk, while the elevated alphaelements are consistent with a rapid bulge formation timescale. Thestarkly smaller scatter of [/Fe] with [Fe/H] in the bulgecompared with the halo is consistent with the expectation that the bulgeshould have efficiently mixed. The metal-poor bulge [/Fe]ratios are higher than ~80% of the halo; the bulge could not have formedfrom gas with the present-day halo composition.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation. Kinematic Decoupling of Globular Clusters with the Extended Horizontal BranchAbout 25% of the Milky Way globular clusters (GCs) exhibit unusuallyextended color distribution of stars in the core helium-burninghorizontal-branch (HB) phase. This phenomenon is now best understood asdue to the presence of helium-enhanced second-generation subpopulations,which has raised the possibility that these peculiar GCs might have aunique origin. Here we show that these GCs with extended HB are clearlydistinct from other normal GCs in kinematics and mass. The GCs withextended HB are more massive than normal GCs and are dominated by randommotion with no correlation between kinematics and metallicity.Surprisingly, however, when they are excluded, most normal GCs in theinner halo show clear signs of dissipational collapse that apparentlyled to the formation of the disk. Normal GCs in the outer halo sharetheir kinematic properties with the extended HB GCs, which is consistentwith the accretion origin. Our result further suggests heterogeneousorigins of GCs, and we anticipate this to be a starting point for moredetailed investigations of Milky Way formation, including early mergers,collapse, and later accretion. Where the Blue Stragglers Roam: Searching for a Link between Formation and EnvironmentThe 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. Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular ClustersWe 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 GalaxyThe 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. ISO mid-infrared spectroscopy of Galactic Bulge AGB starsAims.To study the nature of Bulge AGB stars and in particular theircircumstellar dust, we have analysed mid-infrared spectra obtained withthe ISOCAM CVF spectrometer in three Bulge fields. Methods: .TheISOCAM 5-16.5 μm CVF spectra were obtained as part of the ISOGALinfrared survey of the inner Galaxy. A classification of the shape ofthe 10 μm dust feature was made for each case. The spectra of theindividual sources were modelled using a radiative transfer model.Different combinations of amorphous silicates and aluminium-oxide dustwere used in the modelling. Results: .Spectra were obtained for 29sources of which 26 are likely to be Bulge AGB stars. Our modellingshows that the stars suffer mass loss rates in the range of10-8-5 × 10-7~M_ȯ/yr, which is at thelow end of the mass-loss rates experienced on the thermally pulsing AGB.The luminosities range from 1700 to 7700 L_ȯ as expected for apopulation of AGB stars with M_init of 1.5-2 M_ȯ. In agreement withthe condensation sequence scenario, we find that the dust content isdominated by Al2O3 grains in this sample of low mass-loss rate stars. Global fitting of globular cluster age indicatorsContext: .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. AL 3 (BH 261): A New Globular Cluster in the GalaxyAL 3 (BH 261), previously classified as a faint open cluster candidate,is shown to be a new globular cluster in the Milky Way, by means of B,V, and I color-magnitude diagrams. The main feature of AL 3 is aprominent blue extended horizontal branch. Its color-magnitude diagramsmatch those of the intermediate-metallicity cluster M5. The cluster isprojected in a rich bulge field, also contaminated by the disk mainsequence. The globular cluster is located in the Galactic bulge at adistance from the Sun dsolar=6.0+/-0.5 kpc. The reddening isE(B-V)=0.36+/-0.03, and the metallicity is estimated to be[Fe/H]~-1.3+/-0.25. AL 3 is probably one of the least massive globularclusters of the Galaxy.The observations were carried out at the European Southern Observatory,La Silla, Chile [proposal 64L-0212(A)]. Ground-based CCD astrometry with wide field imagers. I. Observations just a few years apart allow decontamination of field objects from members in two globular clustersThis paper is the first of a series of papers in which we will apply themethods we have developed for high-precision astrometry (and photometry)with the Hubble Space Telescope (HST) to the case of wide-fieldground-based images. In particular, we adapt the software originallydeveloped for WFPC2 to ground-based, wide field images from the WFI atthe ESO 2.2 m telescope. In this paper, we describe in details the newsoftware, we characterize the WFI geometric distortion, discuss theadopted local transformation approach for proper-motion measurements,and apply the new technique to two-epoch archive data of the two closestGalactic globular clusters: NGC 6121 (M 4) and NGC 6397. The results ofthis exercise are more than encouraging. We find that we can achieve aprecision of ~7 mas (in each coordinate) in a single exposure for awell-exposed star, which allows a very good cluster-field separation inboth M 4, and NGC 6397, with a temporal baseline of only 2.8, and 3.1years, respectively. Globular cluster system and Milky Way properties revisitedAims.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. VLT-UVES analysis of two giants in the bulge metal-poor globular cluster HP-1. Analysis of two giants in HP-1Context: .Metal-poor globular clusters in the bulge are importanttracers of early chemical evolution. HP-1 is among the six metal-poorclusters within 5° of the Galactic center, and could be the oneclosest to the centerAims. The main purpose of this study is thedetermination of metallicity and elemental ratios. Methods.Highresolution spectra of two giants of the bulge globular cluster HP-1 wereobtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph.This is the second metal-poor globular cluster in the bulge for which adetailed abundance analysis is presented. MultibandV,I,J,H,Ks photometry was used to derive effectivetemperatures.Results. The present analysis provides a metallicity [Fe/H]= -1.00±0.2. The α-elements oxygen and silicon show [α/Fe] ≈ +0.3, whereas magnesium, calcium and titanium showsolar ratios. A proper motion analysis indicates that the two stars arecluster members. Conclusions.The metallicity is unexpected for a blueHorizontal Branch (BHB) cluster. HP-1 is the first known cluster withsuch a high metallicity combined with a BHB and a steep Red Giant Branch(RGB). Together with NGC 6388 and NGC 6441 of [Fe/H]-0.6 it wouldbe third with such characteristics, but it differs from them, sincethese two other clusters have also a populous Red HB, and a normal slopeof the RGB for their metallicity, which is not the case of HP-1. RR Lyrae-based calibration of the Globular Cluster Luminosity FunctionWe 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 ParametersWe 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. The First Detailed Abundances for M Giants in Baade's Window from Infrared SpectroscopyWe report the first abundance analysis of 14 M giant stars in theGalactic bulge, based on R=25,000 infrared spectroscopy (1.5-1.8 μm)using NIRSPEC at the Keck telescope. Because some of the bulge M giantsreach high luminosities and have very late spectral type, it has beensuggested that they are the progeny of only the most metal-rich bulgestars, or possibly members of a younger bulge population. We find thatthe iron abundance and composition of the M giants are similar to thoseof the K giants that have abundances determined from optical highresolution spectroscopy, <[Fe/H]>=-0.190+/-0.020, with a 1 σdispersion of 0.080+/-0.015. Comparing our bulge M giants to a controlsample of local disk M giants in the solar vicinity, we find that thebulge stars are enhanced in α-elements at the level of +0.3 dexrelative to the solar composition stars, consistent with other studiesof bulge globular clusters and field stars. This small sample shows nodependence of spectral type on metallicity, nor is there any indicationthat the M giants are the evolved members of a subset of the bulgepopulation endowed with special characteristics such as relative youthor high metallicity. We also find low12C/13C<=10, confirming the presence ofextramixing processes during the red giant phase of evolution.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation. Mira variables in the Galactic bulge with OGLE-II dataWe have extracted a total of 1968 Mira variables from the OpticalGravitational Lensing Experiment II (OGLE-II) data base in the Galacticbulge region. Among them, 1960 are associated with 2 Micron All-skySurvey (2MASS) sources, and 1541 are further identified with MidcourseSpace Exploration (MSX) point sources. Their photometric properties arecompared with those of Mira variables in the Large and Small MagellanicClouds. We have found that mass-losing stars with circumstellar matterare reddened such that the colour dependence of the absorptioncoefficient is similar to that of interstellar matter. We also discussthe structure of the bulge. The surface number density of the bulge Miravariables is well correlated with the 2.2-μm surface brightnessobtained by the Cosmic Background Explorer (COBE) satellite. Using thisrelation, the total number of Mira variables in the bulge is estimatedto be about 6×105. The logP-K relation of the Miravariables gives their space distribution which supports the well-knownasymmetry of the bar-like bulge. Mira variables in the OGLE bulge fieldsThe 222 000 I-band light curves of variable stars detected by theogle-ii survey in the direction of the Galactic Bulge have been fittedand also correlated with the denis and 2mass all-sky release databasesand with lists of known objects. Lightcurves and the results of thelightcurve fitting (periods and amplitudes) and denis and 2mass data arepresented for 2691 objects with I-band semi-amplitude larger than 0.45mag, corresponding to classical Mira variables. That the Mira perioddistribution of 6 fields at similar longitude but spanning latitudesfrom -1.2 to -5.8 are statistically indistinguishable indicates similarpopulations with initial masses of 1.5-2 Mȯ,corresponding to ages of 1-3 Gyr. A field at similar longitude at b =-0.05 from Glass et al. (2001, MNRAS, 321, 77; erratum: 2002, MNRAS,336, 1390) does show a significantly different period distribution,indicating the presence of a younger population of 2.5-3Mȯ and ages below 1 Gyr. The K-band period-luminosityrelation is presented for the whole sample and for sub-fields. The zeropoint depends on Galactic longitude. Simulations are carried out to showthat the observed dependence of the zero point with l, and the number ofstars per field are naturally explained using the model of disk andbulge stars of Binney et al. (1997, MNRAS, 288, 365), for a viewingangle (major-axis Bar - axis perpendicular to the line-of-sight to theGalactic Centre) of 43 ± 17 degrees. The simulations also showthat biases in the observed zero point are small, <0.02 mag. Acomparison is made with similar objects in the Magellanic Clouds. Theslope of the PL-relation in the Bulge and the MCs agree within theerrorbars. Assuming the zero point does not depend on metallicity, adistance modulus difference of 3.72 between Bulge and LMC is derived.This implies a LMC DM of 18.21 for an assumed distance to the GalacticCentre (GC) of 7.9 kpc, or, assuming a LMC DM of 18.50, a distance tothe GC of 9.0 kpc. From the results in Groenewegen (2004, A&A, 425,595) it is found for carbon-rich Miras that the PL-relation implies arelative SMC-LMC DM of 0.38, assuming no metallicity dependence. This issomewhat smaller than the often quoted value near 0.50. Followingtheoretical work by Wood (1990, in From Miras to Planetary Nebulae, ed.M. O. Mennessier, & A. Omont (Gif-sur-Yvette: ÉditionsFrontières), 67) a metallicity term of the form MK˜ β log Z is introduced. If a relative SMC-LMC DM of 0.50 isimposed, β = 0.4 is required, and for that value the distance tothe GC becomes 8.6 ± 0.7 kpc (for a LMC DM of 18.50), within theerrorbar of the geometric determination of 7.9 ± 0.4 kpc. Anindependent estimate leads to a distance estimate to the GC of 8.8± 0.4 kpc. A Library of Integrated Spectra of Galactic Globular ClustersWe 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. Pulsation properties of C stars in the Small Magellanic CloudA sample of carbon-rich stars (C-stars) in the Small Magellanic Cloud(SMC) was selected from the combined 2MASS and DENIS catalogues on thebasis of their J-Ks colour. This sample was extended toinclude confirmed C-stars from the Rebeirot et al. (1993) spectroscopicatlas. In this combined sample (N = 1149), a smaller number (N = 1079)were found to have MACHO observations. For this sub-sample, light curveswere determined and 919 stars were found to have high qualitylight-curves with amplitudes of at least 0.05 mag. Of these stars, only4% have a well-defined single period - most of these have multiplewell-defined periods, while 15% have highly irregular light-curves. Thedistribution of the logarithm of the period versus magnitude, colour,period ratio (if applicable), and amplitude was analyzed and comparedwith previous works. Variable C-stars are distributed in threesequences: B, C, and D from Wood et al. (1999), and do not populatesequences with periods shorter than log P ˜ 1.5. Stellar ages andmasses were estimated using stellar evolutionary models. On the origin of the radial mass density profile of the Galactic halo globular cluster systemWe investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin. Comparing the properties of local globular cluster systems: implications for the formation of the Galactic haloWe 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. Late-type giant variables in NGC 6522, LMC and SMC. How do they differ?Complete samples of 2MASS stars in three fields of differingmetallicities (and possibly differing age distributions) have beenextracted and cross-correlated with MACHO and ISO data to determinetheir variability and mass-loss properties. In each MKhistogram a fall-off in numbers is seen above the tip of the RGB. Theluminosity of the tip increases with metallicity as does the average(J-K)0 at a given MK_0. These quantities have beencompared with the data for galactic globular clusters obtained byFerraro et al. (\cite{Ferraro2000}). In the J-H, H-K colour-colourdiagrams, the increasing proportion of stars with high H-K values isconspicuous at lower metallicities. This is well-known to result fromthe increasing proportion of carbon stars. All three fields containsimilar types of variables, mainly short-period (usually tens of days),Mira like (long-period, large amplitude) and double-period (having bothlong and short periods). The proportion of stars that vary decreases atlower metallicities and the minimum period associated with a givenamplitude gets longer. Various trends are seen in the K, log P diagramsof each field. The Magellanic Cloud fields largely resemble each otherbut the Bulge field is noticeably different. The stars that follow theA'' K, log P relation in the Bulge hardly extend beyondMK,0Tip and the other groups also appear truncatedin MK relative to their Magellanic Cloud counterparts. In theClouds there are many stars with periods around 200-300 d that followthe `C'' or Mira relation but have relatively small amplitudes. Themid-IR sample detected by ISOCAM during the Magellanic Cloud Mini-Survey(MCMS) appears to be reliable and complete for sources withMK more luminous than -7 mag, i.e., for those close to thetop of the AGB. The various colour-colour and colour-magnitude diagramsreflect the increasing dominance of carbon stars at low metallicity.Mira magnitude vs. log P relations exist at least up to 7 \mum.Mass-loss from longer-period and double-period SRVs occurs at similarrates in each field, in spite of the metallicity differences. Infrared Echelle Spectroscopy of Palomar 6 and M71We present high-resolution infrared echelle spectroscopy for theglobular clusters Palomar 6 and M71. Our mean heliocentric radialvelocity of Pal 6 is +180.6+/-3.2 km s-1 and is 20 kms-1 lower than that found by Minniti in 1995. Contrary to theprevious metallicity estimates using low-resolution spectroscopy, ourresults show that Pal 6 has an intermediate metallicity, with[Fe/H]=-1.0+/-0.1, and is slightly more metal poor than M71. Reasonablechanges in the surface temperature or the microturbulent velocity of themodel atmospheres do not affect [Fe/H] at more than +/-0.2 dex. In spiteof its high metallicity, on the basis of the spectrum of a singlecluster member the [Si/Fe] and [Ti/Fe] ratios of Pal 6 appear to beenhanced by 0.4 and 0.5 dex, respectively, suggesting that the Galacticinner halo may have experienced a very rapid chemical enrichmenthistory.Based on observations made with the Infrared Telescope Facility, whichis operated by the University of Hawaii under contract to the NationalAeronautics and Space Administration. The metal content of the bulge globular cluster NGC 6528High resolution spectra of five stars in the bulge globular cluster NGC6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVESspectrograph. Out of the five stars, two of them showed evidence ofbinarity. The target stars belong to the horizontal and red giant branchstages, at 4000 < Tefflt; 4800 K. Multiband V, I, J, H,Ks photometry was used to derive initial effectivetemperatures and gravities. The main purpose of this study is thedetermination of metallicity and elemental ratios for this templatebulge cluster, as a basis for the fundamental calibration of metal-richpopulations. The present analysis provides a metallicity [Fe/H] =-0.1±0.2 and the α-elements O, Mg and Si, show [α/Fe]≈ +0.1, whereas Ca and Ti are around the solar value or below,resulting in an overall metallicity Z ≈ Zȯ.Observations collected both at the European Southern Observatory,Paranal and La Silla, Chile (ESO programme 65.L-0340) and with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, operated by AURA Inc. under contract to NASA.Tables \ref{targets}, \ref{logobs}, \ref{tablines} and Fig. \ref{chart}are only available in electronic form at http://www.edpsciences.org The initial helium abundance of the Galactic globular cluster systemIn 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. OGLE small-amplitude variables in the Galactic bar Among over 200 000 Galactic bulge variable stars in the public domainOptical Gravitational Lensing Experiment (OGLE) catalogue, we found over15 000 red giant variables following two well-defined period-amplituderelations. The periods are in the range 10 < P < 100 d, andamplitudes in the range 0.005 < A < 0.13 mag in the I band. Thevariables cover a broad range of reddening-corrected colours, 1 <(V-I)0 < 5, and a fairly narrow range ofextinction-corrected apparent magnitudes, 10.5 < I0 <13. A subset of variables (type A) has an rms scatter of only 0.44 mag.The average magnitudes for these stars are well correlated with theGalactic longitude, and vary from Ik,0= 11.82 for l=+ 8°to Ik,0= 12.07 for l=-5°, clearly indicating that theyare located in the Galactic bar. Most variables have several oscillationperiods. Chemical composition of UV-bright star ZNG 4 in the globular cluster M 13We present a detailed model-atmosphere analysis of ZNG 4, a UV-brightstar in the globular cluster M 13. From the analysis of a highresolution ( R ≈ 45 000 ) spectrum of the object, we derive theatmospheric parameters to be Teff = 8500± 250 K, log g= 2.5 ± 0.5 and [Fe/H]= -1.5. Except for magnesium, chromium andstrontium, all other even Z elements are enhanced with titanium andcalcium being overabundant by a factor of 0.8 dex. Sodium is enhanced bya factor of 0.2 dex. The luminosity of ZNG 4 and its position in thecolor-magnitude diagram of the cluster indicate that it is a SupraHorizontal Branch (SHB) (post-HB) star. The underabundance of He andoverabundances of Ca, Ti, Sc and Ba in the photosphere of ZNG 4 indicatethat diffusion and radiative levitation of elements may be in operationin M 13 post-HB stars even at Teff of 8500 K. Detailed andmore accurate abundance analysis of post-HB stars in several globularclusters is needed to further understand their abundance anomalies.Based on observations obtained with the Subaru 8.2 m Telescope, which isoperated by the National Astronomical Observatory of Japan.Table \ref{t3} is only available in electronic form athttp://www.edpsciences.org Red giant depletion in globular cluster coresWe investigate the observed depletion of red giants in the cores ofpost-core-collapse globular clusters. In particular, the evolutionaryscenario we consider is a binary consisting of two low-mass stars whichundergoes two common-envelope phases. The first common-envelope phaseoccurs when the primary is a red giant resulting in a helium white dwarfand main-sequence star in a detached binary. The second common-envelopephase occurs shortly after the secondary becomes a red giant. During thesecond common-envelope phase, the degenerate helium cores mergeresulting in a core mass greater than the helium burning limit and theformation of a horizontal branch star. We show that stellar encountersenhance this evolutionary route in post-core-collapse clusters. Theseencounters increase the population of binary secondaries which wouldhave evolved on to the red giant branch in the recent past. On the origin of red giant depletion through low-velocity collisionsWe investigate a means of explaining the apparent paucity of red giantstars within post-core-collapse globular clusters. We propose thatcollisions between the red giants and binary systems can lead to thedestruction of some proportion of the red giant population, by eitherknocking out the core of the red giant or by forming a common envelopesystem which will lead to the dissipation of the red giant envelope.Treating the red giant as two point masses, one for the core and anotherfor the envelope (with an appropriate force law to take account of thedistribution of mass), and the components of the binary system alsotreated as point masses, we utilize a four-body code to calculate thetime-scales on which the collisions will occur. We then perform a seriesof smooth particle hydrodynamics runs to examine the details of masstransfer within the system. In addition, we show that collisions betweensingle stars and red giants lead to the formation of a common envelopesystem which will destroy the red giant star. We find that low-velocitycollision between binary systems and red giants can lead to thedestruction of up to 13 per cent of the red giant population. This couldhelp to explain the colour gradients observed in PCC globular clusters.We also find that there is the possibility that binary systems formedthrough both sorts of collision could eventually come into contactperhaps producing a population of cataclysmic variables. The Red Giant Branch luminosity function bumpWe 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. Period-magnitude relations for M giants in Baade's Window NGC 6522A large and complete sample of stars with K < 9.75 in the NGC 6522Baade's Window towards the Galactic bulge is examined using light curvesextracted from MACHO and IJK photometry from DENIS.The improved statistics resulting from a sample of over 1000 variablesallow all four of the sequences A, B, C and D in the KS, logPdiagram of the Large Magellanic Cloud to be seen in the Galactic bulge.The bulge sequences, however, show some differences from those in theMagellanic Clouds, possibly due to the effects of higher metallicity.These sequences may have value as distance indicators.A new diagram of the frequency of late-type variables at a givenamplitude is derived and compared with the old one fromPayne-Gaposchkin. The catalogued semiregular variables of the solarneighbourhood are found to be a subset of the total of semiregulars,biased towards large amplitude.
Submit a new article

• - No Links Found -