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Why Haven't Loose Globular Clusters Collapsed Yet?
We report on the discovery of a surprising observed correlation betweenthe slope of the low-mass stellar global mass function (GMF) of globularclusters (GCs) and their central concentration parameterc=log(rt/rc), i.e., the logarithmic ratio of tidaland core radii. This result is based on the analysis of a sample of 20Galactic GCs with solid GMF measurements from deep HST or VLT data. Allthe high-concentration clusters in the sample have a steep GMF, mostlikely reflecting their initial mass function. Conversely,low-concentration clusters tend to have a flatter GMF, implying thatthey have lost many stars via evaporation or tidal stripping. No GCs arefound with a flat GMF and high central concentration. This findingappears counterintuitive, since the same two-body relaxation mechanismthat causes stars to evaporate and the cluster to eventually dissolveshould also lead to higher central density and possibly core collapse.Therefore, more concentrated clusters should have lost proportionatelymore stars and have a shallower GMF than low-concentration clusters,contrary to what is observed. It is possible that severely depleted GCshave also undergone core collapse and have already recovered a normalradial density profile. It is, however, more likely that GCs with a flatGMF have a much denser and smaller core than that suggested by theirsurface brightness profile and may well be undergoing collapse atpresent. In either case, we may have so far seriously underestimated thenumber of post-core collapse clusters, and many may be lurking in theMilky Way.
| 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.
| AL 3 (BH 261): A New Globular Cluster in the Galaxy
AL 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)].
| 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.
| VLT-UVES analysis of two giants in the bulge metal-poor globular cluster HP-1. Analysis of two giants in HP-1
Context: .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 Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
| On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.
| 86 GHz SiO maser survey of late-type stars in the inner Galaxy. III. Interstellar extinction and colours of the SiO targets
We have determined extinction corrections for a sample of 441 late-typestars in the inner Galaxy, which we previously searched for SiO maseremission, using the 2MASS near-infrared photometry of the surroundingstars. From this, the near-infrared extinction law is found to beapproximated by a power law A_λ ∝ λ^-1.9±0.1.Near- and mid-infrared colour-colour properties of known Mira stars arereviewed. From the distribution of the dereddened infrared colours ofthe SiO target stars we infer mass-loss rates between 10-7and 10-5~M_ȯ yr-1.
| 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.
| HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands
We present the complete photometric database and the color-magnitudediagrams for 74 Galactic globular clusters observed with the HST/WFPC2camera in the F439W and F555W bands. A detailed discussion of thevarious reduction steps is also presented, and of the procedures totransform instrumental magnitudes into both the HST F439W and F555Wflight system and the standard Johnson ( B ) and ( V ) systems. We alsodescribe the artificial star experiments which have been performed toderive the star count completeness in all the relevant branches of thecolor magnitude diagram. The entire photometric database and thecompleteness function will be made available on the Web immediatelyafter the publication of the present paper. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contractNAS5-26555, and on observations retrieved from the ESO ST-ECF Archive.
| Colour Magnitude Diagrams of the moderately metal-rich globular clusters NGC 6569 and Palomar 11
For the first time Colour-Magnitude Diagrams are presented for NGC 6569and Palomar 11. NGC 6569 is a rather concentrated cluster whereasPalomar 11 is loose and sparsely populated. Cluster parameters arederived from V and I photometry. These are red Horizontal Branchglobular clusters, with metallicities comparable to that of 47 Tucanae.We derive a reddening E(B-V) ~ 0.53 and a distance from the Sundsun ~ 9.8 kpc for NGC 6569, and E(B-V) ~ 0.35 anddsun ~ 13.2 kpc for Palomar 11. NGC 6569 is located in thebulge, while Palomar 11, despite its rather high metallicity, is in theinner halo, a rare case similar to that of 47 Tucanae. Observationscollected at the European Southern Observatory - ESO, Chile, proposalno. 64L-0212(A).
| A census with ROSAT of low-luminosity X-ray sources in globular clusters
I analyze 101 observations from the ROSAT archive to search for X-raysources in or near 55 globular clusters. New sources are found in thecores of NGC 362 (a double source), NGC 6121 (marginally significant),NGC 6139, and NGC 6266; and outside the cores of NGC 6205, NGC 6352 andNGC 6388. More accurate positions are determined for the X-ray sourcesin some ten clusters. The improved position for the source in NGC 6341excludes the suggested ultraviolet counterpart. It is shown that one ofthe two sources reported near the core of NGC 6626 is spurious, as isthe detection of a pulsar period in the PSPC data of this cluster; thecentral source is resolved in three sources. One source reportedpreviously in NGC 6304 is demoted to an upper limit. For 20 clustercores better upper limits to the X-ray luminosity are obtained. From astatistical analysis I argue that several sources outside the clustercores may well belong to the clusters. All spectral energy distributionsobserved so far are relatively soft, with bremsstrahlung temperatures =~0.9 keV; there is evidence however that bremsstrahlung spectra do notcorrectly describe the spectra. The X-ray luminosity per unit mass forthe cluster as a whole does not depend on the concentration; theluminosity per unit mass for the core may increase with the clusterconcentration.
| Globular Cluster Subsystems in the Galaxy
Data from the literature are used to construct a homogeneous catalog offundamental astrophysical parameters for 145 globular clusters of theMilky Way Galaxy. The catalog is used to analyze the relationshipsbetween chemical composition, horizontal-branch morphology, spatiallocation, orbital elements, age, and other physical parameters of theclusters. The overall globular-cluster population is divided by a gap inthe metallicity function at [Fe/H]=-1.0 into two discrete groups withwell-defined maxima at [Fe/H]=-1.60±0.03 and -0.60±0.04.The mean spatial-kinematic parameters and their dispersions changeabruptly when the metallicity crosses this boundary. Metal-poor clustersoccupy a more or less spherical region and are concentrated toward theGalactic center. Metal-rich clusters (the thick disk subsystem), whichare far fewer in number, are concentrated toward both the Galacticcenter and the Galactic plane. This subsystem rotates with an averagevelocity of V rot=165±28 km/s and has a very steep negativevertical metallicity gradient and a negligible radial gradient. It is,on average, the youngest group, and consists exclusively of clusterswith extremely red horizontal branches. The population ofspherical-subsystem clusters is also inhomogeneous and, in turn, breaksup into at least two groups according to horizontal-branch morphology.Clusters with extremely blue horizontal branches occupy a sphericalvolume of radius 9 kpc, have high rotational velocities (Vrot=77±33 km/s), have substantial and equal negative radial andvertical metallicity gradients, and are, on average, the oldest group(the old-halo subsystem). The vast majority of clusters withintermediate-type horizontal branches occupy a more or less sphericalvolume ≈18 kpc in radius, which is slightly flattened perpendicularto the Z direction and makes an angle of ≈30° to the X-axis. Onaverage, this population is somewhat younger than the old-halo clusters(the young-halo subsystem), and exhibits approximately the samemetallicity gradients as the old halo. As a result, since theirGalactocentric distance and distance from the Galactic plane are thesame, the young-halo clusters have metallicities that are, on average,Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. Theyoung-halo subsystem, which apparently consists of objects captured bythe Galaxy at various times, contains many clusters with retrogradeorbits, so that its rotational velocity is low and has large errors, Vrot=-23±54 km/s. Typical parameters are derived for all thesubsystems, and the mean characteristics of their member globularclusters are determined. The thick disk has a different nature than boththe old and young halos. A scenario for Galactic evolution is proposedbased on the assumption that only the thick-disk and old-halo subsystemsare genetically associated with the Galaxy. The age distributions ofthese two subsystems do not overlap. It is argued that heavy-elementenrichment and the collapse of the proto-Galactic medium occurred mainlyin the period between the formation of the old-halo and thick-disksubsystems.
| B and V photometry of the faint open cluster UKS 2 (BH 66)
Colour-magnitude diagrams are presented for the first time, of the faintsouthern star cluster UKS 2, which was for a long time reported as aglobular cluster candidate. The V vs. B-V diagrams indicate that UKS 2is an open cluster with age comparable to that of the Hyades. It has areddening of E(B-V) = 0.40 and is located at a distance from the Sun ofdsun ~ 7 kpc. The cluster is located outside the solar radiusat galactocentric distance projected on the plane r_GC ~ 11 kpc, and aheight z ~ -370 pc. Observations collected at the European SouthernObservatory - ESO, Chile; Tables 2 and 3 are available only inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| ESO 280-SC06: a new globular cluster in the Galaxy
By means of VI CCD photometry we identify a new globular cluster in theGalaxy: ESO 280-SC06. The images and Colour Magnitude Diagrams indicatethat we are dealing with a sparsely populated globular cluster, showinga compact core. The cluster is located in Ara, projected at only12.6o from the plane, and appears to be metal poor ([Fe/H] ~-1.8). We derive a reddening E(B-V) = 0.07 and a distance from the Sund_sun = 21.9 kpc. It is a halo globular cluster on the opposite side ofthe Galaxy. Observations collected at the European Southern Observatory-- ESO, Chile; ESO proposal no. 64L-0212(A)
| New star clusters projected close to the Galactic Centre
We carried out a systematic search for new star clusters in a field of5deg x5deg centred close to the Galactic Nucleususing the infrared JHK_s 2MASS Survey archive. In addition we searchedfor embedded clusters in the directions of HII regions and dark cloudsfor |l|<=4deg. As a result we present a list of 58 IR starclusters or candidates. We provide positions, sizes and reddeningestimated from 100 mu m dust emission. Their angular distributiontogether with previously catalogued objects in the region and possiblerelation with star forming complexes are also discussed.
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| Blue horizontal branch globular clusters towards the bulge: Terzan 9, NGC 6139 and NGC 6453
We present V and I photometry of the globular clusters Terzan 9 and NGC6453 for the first time, and also of NGC 6139. The three clusters areprojected towards the bulge. The Colour-Magnitude Diagram morphologiesreveal that the sample clusters have blue horizontal branches. Thischaracterizes an important fraction of the clusters projected on thebulge, with implications on the early Galaxy history. From thecolour-magnitude diagrams we derive reddenings of E(B-V) = 1.95, 0.77,0.70 and distances dsun~ 4.9, 9.4, 8.5 kpc respectively forTerzan 9, NGC 6139 and NGC 6453. We conclude that these metal-poorclusters are within the bulge volume. Observations collected at theEuropean Southern Observatory - ESO, Chile, proposal No. 61.E-0335.
| The metal-rich bulge globular cluster NGC 6401
We present V and I photometry for the bulge globular cluster NGC 6401for the first time. The Colour-Magnitude Diagram reveals a redhorizontal branch, and the cluster is metal-rich ([Fe/H] ~ -0.7). NGC6401 is located at 5.3(deg) from the Galactic center, turning out to bean interesting target to trace the extent of the bulge. A reddeningE(B-V) = 0.53+/-0.15 and a distance from the Sun dsun ~12.0+/-1.0 kpc are derived. The cluster is slightly behind the bulk ofthe bulge population in that direction, but still within the bulgevolume. Since the number of clusters with Horizontal Branch informationhas increased enormously in the later years for the central 20(deg)x20(deg), we present a discussion on the distribution of red and bluehorizontal branch clusters and their possible relation to bulge and/orhalo. Observations collected at the European Southern Observatory --ESO, Chile, proposal no. 61.E-0335
| Gravitational Radiation from Globular Clusters
Space-based gravitational wave detectors will have the ability toobserve continuous low-frequency gravitational radiation from binarystar systems. They can determine the direction to continuous sourceswith an angular resolution approaching tens of arcminutes. Thisresolution should be sufficient to identify binary sources as members ofsome nearby globular clusters. Thus, gravitational radiation can be usedto determine the population of hard binaries in globular clusters. Forparticularly hard binaries, the orbital period may change as a result ofgravitational wave emission. If one of these binaries can be identifiedwith a globular cluster, then the distance to that cluster can bedetermined. Thus, gravitational radiation may providereddening-independent distance measurements to globular clusters and theRR Lyrae stars that inhabit them.
| Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
The High-Resolution Echelle Spectrometer (HIRES) on the Keck I telescopehas been used to measure the first radial velocities for stars belongingto 11 heavily reddened globular clusters in the direction of the innerGalaxy. The sample consists of the clusters Terzan 3, NGC 6256, IC 1257,NGC 6380 (=Ton 1), Ton 2 (=Pismis 26), Djorg 1, NGC 6540 (=Djorg 3), IC1276 (=Pal 7), Terzan 12, NGC 6749, and Pal 10. Candidate clustermembers were selected from a combination of previously publishedcolor-magnitude diagrams (CMDs) and new instrumental CMDs obtained withthe Palomar 1.5 m telescope. The systemic velocities of Djorg 1 and Pal10 should be considered provisional, since velocities are available foronly two stars. For the remaining nine clusters, we have measured radialvelocities for three to nine member stars. Using our HIRES spectra, weestimate metallicities of [Fe/H]~=-0.75 for both Terzan 3 and IC 1276,two clusters lacking previous metallicity estimates. The question ofkinematic substructuring among the Galactic globular clusters isinvestigated using an updated catalog of globular cluster distances,metallicities, and velocities. It is found that the population ofmetal-rich globular clusters shows significant rotation at allGalactocentric radii. For the metal-rich clusters within 4 kpc of theGalactic center, the measured rotation velocity and line-of-sightvelocity dispersion are similar to those of bulge field stars. Weinvestigate claims that the metal-rich clusters are associated with thecentral Galactic bar by comparing the kinematics of the innermostclusters to that of the atomic hydrogen in the inner Galaxy. Thelongitude-velocity diagram of both metal-rich and metal-poor clustersbears a remarkable similarity to that of the gas, including the samenoncircular motions that have traditionally been interpreted as evidencefor a Galactic bar, or, alternatively, a nonaxisymmetric bulge. However,uncertainties in the existing three-dimensional Galactocentric positionsfor most of the clusters do not yet allow an unambiguous discriminationbetween the competing scenarios of membership in a rigidly rotating baror in a bulge that is an oblate isotropic rotator. We conclude that themajority of metal-rich clusters within the central ~4 kpc of the Galaxyare probably associated with the bulge/bar, and not the thick disk.
| Colour-magnitude diagrams of the post-core collapse globular clusters NGC 6256 and NGC 6717 (Palomar 9)
{ We present photometry of the globular clusters NGC 6256 in V, I, NGC6717, NGC 6256 in V, I, and NGC 6717 in B, V, obtaining clusterparameters. NGC 6256 has a post core-collapse structure, while suchfeature is suspected for NGC 6717. Both clusters are projected on thebulge. From the colour-magnitude diagrams for NGC 6256 we derivereliable cluster parameters which considerably revise its properties,with respect to previous literature. It has a blue-extended HorizontalBranch already observed in other post-core collapse clusters in thebulge, such as HP1. We derive a reddening of E(B-V) = 1.10, a distancedsun ~ 6.4 kpc, and an intermediate metallicity for NGC 6256.NGC 6717 presents a blue Horizontal Branch. We derive a reddening ofE(B-V) = 0.23 and a distance dsun ~ 7.1 kpc, locating thecluster in the bulge. The photometry is deep and we study the luminosityfunction, which presents some evidence of depletion of Main Sequencestars. This effect is not unexpected for such a poorly populated Palomarcluster in the bulge. We also estimate the cluster age from themagnitude difference between the horizontal branch and the turnoff. Weobtain Delta V() TO_HB = 3.7, which would place this intermediatemetallicity bulge globular as coeval with the halo.} Observationscollected at the European Southern Observatory - ESO, Chile.
| VI Photometry of the Post-Core-Collapse Globular Cluster NGC 6558 and the Adjacent Bulge Field Population
We present VI color-magnitude diagrams (CMDs) of the globular clusterNGC 6558 and its surrounding field, obtaining cluster parameters. Thecluster has a blue extended horizontal branch and a depleted red giantbranch, characteristics already observed in other post-core-collapseclusters in the bulge, such as NGC 6522, NGC 6540, and HP 1. Theseclusters do not belong to the metal-rich stellar population, and theymay define a distinct class in the bulge. We derive a reddening of E(B -V) = 0.50 and a distance d_ȯ ~ 6.3 kpc. The CMD morphology isconsistent with a metallicity range of -1.6 < [Fe/H] < -1.2. Wealso report photometry of the bulge field population (b = -6 deg on theminor axis) in which NGC 6558 is imbedded. As is the case for otherbulge fields, there is a prominent red clump and a strong descending redgiant branch, similar to that seen in lower latitude bulge fields; weestimate the metallicity of the bulge population to lie between that of47 Tuc and the nearly solar-metallicity cluster NGC 6553 (i.e.,approximately -0.3 dex). Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile.
| Gravitational microlensing by globular clusters
Stars in globular clusters can act either as sources for MACHOs (MassiveAstrophysical Compact Halo Objects) located along the line of sight oras lenses for more distant background stars. Although the expected rateof microlensing events is small, such observations can lead to veryuseful results. In particular, one could get information on the shape ofthe galactic halo along different lines of sight, allowing to betterconstrain its total dark matter content. Moreover, on can also infer thetotal dark matter content of globular clusters, which is presently notwell known. To this latter purpose, we analyse the microlensing eventstowards the galactic bulge, which lie close to the three globularclusters NGC 6522, NGC 6528 and NGC 6540. We find evidence that somemicrolensing events are indeed due to MACHOs located in the globularclusters, suggesting, therefore, that these clusters contain asignificant amount of dark matter.
| Globular clusters within 5(deg) of the Galactic center
In the recent years we have concentrated efforts to collectColour-Magnitude Diagrams of the globular clusters projected in thecentral parts of the Galaxy. So far we were able to gather photometricdata in the V, I and Gunn z bandpasses, for 16 out of the 17 knownclusters in the central 5(deg) radius, most of them severely reddened.Reddening, distance and metallicity are estimated from the horizontaland red giant branches. We study the resulting cluster spatialdistribution and conclude that essentially no cluster is detected beyondthe Galactic Center distance. The results favour a flattened bulgeextending from the Galactic Center to 4.5kpc from the Sun. The densitydistribution of the clusters follows the current models for bulge fieldstars. We estimate that missing clusters on the opposite side of theGalaxy bulge may amount ~ 15 clusters similar to those detected on ourside. The projected distribution of clusters is asymmetrical, withhigher absorption in the southern Galactic hemisphere. The metallicitydistribution of our sample clusters results similar to that of bulgefield stars. The present ground-based results coupled to main sequencedata of two genuine bulge clusters using the Hubble Space Telescope,point to a scenario of an old flat bulge with common origin for thestellar populations in both globular clusters and field. Observationscollected at the European Southern Observatory -- ESO, Chile
| Palomar 10 and NGC 6749: A Study in Contrasts
We present new photometry in $V$ and $I$ for Palomar 10 and NGC 6749,two heavily obscured and sparse globular clusters. The color-magnitudediagram (CMD) of Pal 10 exhibits marked differential reddening with amean reddening $E(B-V) = 1.66 \pm 0.1$. Its CMD has the purely redhorizontal branch characteristic of a high-metallicity cluster; weestimate [Fe/H] $\sim -0.1$ on the Zinn-West metallicity scale. NGC 6749(= Berkeley 42, listed in some other sources as an old open cluster) hasa similarly large reddening ($E(B-V) = 1.5 \pm 0.07$) but its CMD isthat of a typically metal-poor ([Fe/H] $\sim -1.6$) cluster with apurely blue HB. We estimate their distance moduli as $(m-M)_0$ = 13.85(d = 5.9 kpc) for Pal 10 and $(m-M)_0$ = 14.45 (7.8 kpc) for NGC 6749.King-model fits to their radial distributions yield very low centralconcentrations for both: c = 0.58 (Pal 10) and c = 0.83 (NGC 6749.(SECTION: Stellar Clusters and Associations)
| High quality seeing V, I and Gunn Z imaging of Terzan 4: a blue horizontal branch bulge globular cluster.
We study the globular cluster Terzan 4 projected close to the Galacticcenter, by means of V, I and Gunn z Colour-Magnitude Diagrams. Theimages were obtained under exceptional seeing conditions (0.35"-0.55")at the ESO New Technology Telescope equipped with SUSI. We derive areddening of E(B-V)=2.35 and a distance from the Sundsun_=8.3kpc for Terzan 4. From the similarity of the VIColour-Magnitude Diagrams of Terzan 4 and M30, in particular thepresence of a blue horizontal branch, it could be as metal-poor as[Fe/H]=~-2.0. It is therefore another blue horizontal branch globularcluster located close to the Galactic center.
| IC 1257:A New Globular Cluster in the Galactic Halo
New CCD photometry of the faint, compact star cluster IC 1257 (l=17(o,)b=+15(o) ), obtained with the Palomar 5m telescope, reveals that it is ahighly reddened globular cluster well beyond the Galactic center. Withan apparent distance modulus (m-M)_V = 19.2 and a foreground reddeningE(B-V) = 0.75, it is 24 kpc from the Sun and ~ 16 kpc beyond theGalactic center. The morphology of its color-magnitude diagram issimilar to that of M13, and suggests that it has moderately lowmetallicity ([Fe/H] =~ -1.7).The Palomar 5m telescope is operated in a joint agreement among theCalifornia Institute of Technology, the Jet Propulsion Laboratory, andCornell University.
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