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New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| The Bologna Open Cluster Chemical Evolution Project: Midterm Results from the Photometric Sample
We describe a long-term project aimed at deriving information on thechemical evolution of the Galactic disk from a large sample of openclusters. The main property of this project is that all clusters areanalyzed in a homogeneous way to guarantee the robustness of the rankingin age, distance, and metallicity. Special emphasis is devoted to theevolution of the earliest phases of the Galactic disk evolution, forwhich clusters have superior reliability with respect to other types ofevolution indicators. The project is twofold: on one hand we derive theage, distance, and reddening (and indicative metallicity) byinterpreting deep and accurate photometric data with stellar evolutionmodels, and on the other hand, we derive the chemical abundances fromhigh-resolution spectroscopy. Here we describe our overall goals andapproaches and report on the midterm project status of the photometricpart, with 16 clusters already studied, covering an age interval from0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. Theimportance of quantifying the theoretical uncertainties by deriving thecluster parameters with various sets of stellar models is emphasized.Stellar evolution models assuming overshooting from convective regionsappear to better reproduce the photometric properties of the clusterstars. The examined clusters show a clear metallicity dependence on thegalactocentric distance and no dependence on age. The tight relationbetween cluster age and magnitude difference between the main-sequenceturnoff and the red clump is confirmed.
| Musca - the heavenly fly.
Not Available
| Luminosity and mass functions of galactic open clusters. II. NGC 4852
We present wide-field deep {UBVI} photometry for the previouslyunstudied open cluster NGC 4852 down to a limiting magnitude I˜24,obtained from observations taken with the Wide Field Imager cameraon-board the MPG/ESO 2.2 m telescope at La Silla (ESO, Chile). Thesedata are used to obtain the first estimate of the cluster basicparameters, to study the cluster spatial extension by means of starcounts, and to derive the Luminosity (LF) and Mass Function (MF). Thecluster radius turns out to be 5.0±1.0 arcmin. The clusteremerges clearly from the field down to V=20 mag. At fainter magnitudes,it is completely confused with the general Galactic disk field. Thestars inside this region define a young open cluster (200 million yearsold) 1.1 kpc far from the Sun (m-M = 11.60, E(B-V) = 0.45). The PresentDay Mass Functions (PDMF) from the V photometry is one of the mostextended in mass obtained to date, and can be represented as a power-lawwith a slope α = 2.3±0.3 and (the Salpeter MF in thisnotation has a slope α = 2.35), in the mass range 3.2 ≤{m}/{mȯ} ≤ 0.6. Below this mass, the MF cannot beconsidered as representative of the cluster MF, as the cluster mergeswith the field and therefore the MF is the result of the combined effectof strong irregularities in the stellar background and interaction ofthe cluster with the dense Galactic field. The cluster total mass at thelimiting magnitude results to be 2570±210 Mȯ.
| On the recent star formation history of the Milky Way disk
We have derived the star formation history of the Milky Way disk overthe last 2 Gyr from the age distribution diagram of a large sample ofopen clusters comprising more than 580 objects. By interpreting the agedistribution diagram using numerical results from an extensive libraryof N-body calculations carried out during the last ten years, wereconstruct the recent star formation history of the Milky Way disk.Under the assumption that the disk has never been polluted by anyextragalactic stellar populations, our analysis suggests thatsuperimposed on a relatively small level of constant star formationactivity mainly in small-N star clusters, the star formation rate hasexperienced at least five episodes of enhanced star formation lastingabout 0.2 Gyr with production of larger clusters. This cyclic behaviourshows a period of 0.4+/-0.1 Gyr and could be the result of density wavesand/or interactions with satellite galaxies. On the other hand, the starformation rate history from a volume-limited sample of open clusters inthe solar neighbourhood appears to be consistent with the overall starformation history obtained from the entire sample. Pure continuous starformation both in the solar neighbourhood and the entire Galactic diskis strongly ruled out. Our results also indicate that, in the Milky Waydisk, about 90% of open clusters are born with N<=150 and the slopein the power-law frequency distribution of their masses is about -2.7when quiescent star formation takes place. If the above results arere-interpreted taking into consideration accretion events onto the MilkyWay, it is found that a fraction of the unusually high number of openclusters with ages older than 0.6 Gyr may have been formed in disruptedsatellites. Problems arising from the selection effects and the ageerrors in the open cluster sample used are discussed in detail.
| WX Cen (≡ WR 48c) - a possible Type Ia supernova progenitor
We confirm the orbital period of WX Cen ≡ WR 48c determined byDiaz & Steiner and refined its value to Porb= 0.4169615(+/-22) d. The light curve of this object has a peak-to-peak variationof approximately 0.32 mag. It is non-sinusoidal in the sense that it hasa V-shaped narrow minimum, similar to the ones seen in V Sge, V617 Sgrand in compact binary supersoft sources (CBSS).Most of the emission lines in the optical spectrum are due to Balmer,HeII, CIV, NV, OV and OVI. An analysis of the HeII Pickering seriesdecrement shows that the system has significant amount of hydrogen. Theemission lines of HeII 4686 Å became weaker between the 1991 and2000/2002 observations, indicating distinct levels of activity. Thespectra of WX Cen show variable absorption features in the Balmer lineswith V=-2900 km s-1 and in emission with V=+/-3500 kms-1. These highly variable features remind us of thesatellite emission lines found in the spectra of CBSS.We estimate the colour excess as E(B-V) = 0.63 on the basis of theobserved diffuse interstellar band at 5780 Å. Given thedistance-colour excess relation in the direction of WX Cen, this impliesa distance of 2.8 +/- 0.3 kpc. Interstellar absorption of the NaI Dlines show components at -4.1 km s-1, which corresponds tothe velocity of the Coalsack, and three other components at -23.9, -32.0and -39.0 km s-1. These components are also seen with similarstrengths in field stars that have distances between 1.8 and 2.7 kpc.The intrinsic colour of WX Cen is (B-V)0=-0.2 and theabsolute magnitude is MV=-0.5.Extended red wings in the strong emission lines are seen. A possibleexplanation is that the system has a spill-over stream similar to whatis seen in V617 Sgr. We predict that when observed in opposite phase,blue wings would be observed. A puzzling feature that remains to beexplained is the highly variable red wing (V~ 700 km s-1) ofthe OVI emission lines as well as of the red wings of the H and Helines.The velocity of the satellite-like feature is consistent with the ideathat the central star is a white dwarf with a mass of M~ 0.9Msolar. With the high accretion rate under consideration, thestar may become a Type Ia supernova (SN Ia) in a time-scale of 5 ×106 yr.
| The age of the oldest Open Clusters
We determine ages of 71 old Open Clusters by a two-step method: we usemain-squence fitting to 10 selected clusters, in order to obtain theirdistances, and derive their ages from comparison with our own isochronesused before for Globular Clusters. We then calibrate the morphologicalage indicator δ(V), which can be obtained for all remainingclusters, in terms of age and metallicity. Particular care is taken toensure consistency in the whole procedure. The resulting Open Clusterages connect well to our previous Globular Cluster results. From theOpen Cluster sample, as well as from the combined sample, questionsregarding the formation process of Galactic components are addressed.The age of the oldest open clusters (NGC 6791 and Be 17) is of the orderof 10 Gyr. We determine a delay by 2.0±1.5 Gyr between the startof the halo and thin disk formation, whereas thin and thick disk startedto form approximately at the same time. We do not find any significantage-metallicity relationship for the open cluster sample. The cumulativeage distribution of the whole open cluster sample shows a moderatelysignificant (˜ 2σ level) departure from the predictions foran exponentially declining dissolution rate with timescale of 2.5 Gyr.The cumulative age distribution does not show any trend withgalactocentric distance, but the clusters with larger height to theGalactic plane have an excess of objects between 2-4 and 6 Gyr withrespect to their counterpart closer to the plane of the Galaxy.
| Metallicity distribution on the galactic disk
Depending mainly on UBVCCD data, the metallicities of 91 open starclusters nearby the galactic disk have been estimated using Cameron's[A&A 147 (1985b) 39] method. The metallicity radial gradient alongthe galactic plane is found to be -0.09 dex/kpc; which is in a very goodagreement with Panagia and Tosi [A&A 96 (1981) 306] and Carraro etal. [MNRAS 296 (1998) 1045]. Vertically on the galactic disk, withinabout 800 pc, the metallicity gradient is found to be so trivial. Anaverage age-metallicity relation has been examined, which confirms theprevious suggestion that the metallicity of a cluster depending mainlyon its position on the galactic disk more than its age.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| A multicolour CCD photometric and mass function study of the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253
We derive cluster parameters and mass functions from new UBVRI CCDphotometric observations of ~3500 stars reaching down to V~20mag for thedistant southern open star clusters NGC 3105, NGC 3603, Melotte 105,Hogg 15, NGC 4815, Pismis 20 and NGC 6253. For NGC 3105 and Hogg 15, CCDdata are presented for the first time. The reddening is non-uniformacross the face of the young (age <300Myr) clusters NGC 3105, NGC3603, Melotte 105, Hogg 15 and Pismis 20, with average values ofE(B-V)=1.06, 1.44, 0.52, 1.15 and 1.20mag respectively, while it isuniform with average values of E(B-V)=0.72 and 0.20mag for the olderclusters NGC 4815 and 6253 respectively. The values of colour excessratios indicate the presence of normal interstellar reddening across thecluster regions studied here. Well-defined main sequences can be seen inall the clusters. However, main-sequence turn-off points and subgiantbranches are well defined only in the older clusters NGC 4815 and 6253.The distances to the clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15,NGC 4815, Pismis 20 and NGC 6253 are 9.5+/-1.5, 7.2+/-1.2, 2.3+/-0.2,3.0+/-0.3, 2.75+/-0.2, 3.55+/-0.35 and 1.8+/-0.12kpc respectively, whilethe corresponding ages derived using theoretical convective coreovershooting stellar evolutionary isochrones are 25+/-10, 3+/-2,250+/-30, 6+/-2, 400+/-50, 6+/-2 and 2500+/-600Myr respectively. In themass range ~1-75Msolar, the mass functions of all clustersexcept for NGC 6253 have been studied. The slopes of their mass spectraagree within errors with the Salpeter value (1.35). The slope of themass function for stars more massive than 10Msolar is almostthe same as for the lower mass stars. The mass function slopes ofclusters younger than 500Myr seem to have no dependence on Galacticlongitude, Galactocentric distance and cluster age. As the inherentuncertainties in the mass function determinations of young Galactic starclusters can produce internal scatter that is larger than the externalscatter, we conclude that, above 1Msolar, the initial massfunction is universal with a slope of Salpeter value.
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| Luminosity and mass function of galactic open clusters I. NGC 4815
We present deep V and I photometry for the open cluster NGC 4815 andfour surrounding Galactic fields down to a limiting magnitude V ~ 25.These data are used to study cluster spatial extension by means of starcounts, and to derive the luminosity (LF) and mass function (MF). Theradius turns out to be 3.6+/-0.3 arcmin at V=19.0, whereas the mass is880+/-230 msun down to V=20.8. From the color-magnitudediagram, we obtain the LFs in the V and I bands, using both the standardhistogram and an adaptive kernel. After correction for incompletenessand field star contamination, the LFs were transformed into the presentday mass functions (PDMF). The PDMFs from the V and I photometry can berepresented as a power-law with a slope alpha = 3.1+/-0.3 and alpha =2.9+/-0.3 (the (Salpeter \cite{salp55}) MF in this notation has a slopealpha = 2.35) respectively, in the mass range 2.5 <=(m)/(msun) <= 0.8. Below this mass, the MF cannot beconsidered as representative of the cluster IMF, as it is the result ofthe combined effect of strong irregularities in the stellar background,probable internal dynamical evolution of the cluster and/or interactionof the cluster with the dense Galactic field. Unresolved binaries andmass segregation can only flatten the apparent derived IMF, so we expectthat the real IMF must be steeper than the quoted slope by an unknownamount. Based on observations made at the European Southern Observatory,La Silla, Chile.
| Constraining galactic structure parameters from a new extinction model and four star count samples
We propose a new 3-dimensional extinction model based on the COBE/IRASall sky reddening map. Its application to globular and open cluster dataevidences that the COBE/IRAS reddening map has an accuracy of 18%, butoverestimates visual absorption by a factor of 1.16. This systematicerror does not change with galactic latitude and opacity significantly.The implementation of the new extinction model has optimized ourgalactic structure and kinematic model to low-galactic latitudes. Fourstar count samples distributed in different galactic directions havebeen compared with galactic model simulations. Numerical experimentsallow us to constrain the radial distribution of the galactic disk. Thedisk scale length is found to be 2250 +/- 50 pc and the displacement ofthe Sun from the galactic plane ZSun = 27.5 +/- 6.0 pc.
| Astrophysics in 1998
From Alpha (Orionis and the parameter in mixing-length theory) to Omega(Centauri and the density of the universe), the Greeks had a letter forit. In between, we look at the Sun and planets, some very distantgalaxies and nearby stars, neutrinos, gamma rays, and some of theanomalies that arise in a very large universe being studied by roughlyone astronomer per 10^7 Galactic stars.
| The spatial distribution and luminosity function of the open cluster NGC 4815
NGC 4815 is a distant and populous open cluster, which lies in thegalactic plane in a region of strong absorption. As a consequence, itsmembership, spatial distribution and luminosity function are not welldetermined. In this paper, we present an algorithm which uses bothpositional and photometric data simultaneously to search for opencluster members. The contribution of the field stars is estimated by ourGalaxy model software. The method has been developed for deep CCDphotometric observations, but it can also be used to investigate themembership in an n-dimensional space, including proper motions, radialvelocity and metallicity. A clean and well defined colour-magnitudediagram (CMD) for NGC 4815 has been obtained after decontaminating fromfield stars. We have investigated the spatial distribution of clustermembers and the luminosity function. We have shown that the projectedsurface distribution of stars is well-represented by an exponentiallydecreasing function with a scale length of 1.67 +/- 0.06 arcmin. We havefound evidence for mass segregation. The luminosity function for NGC4815 is similar to that of the Hyades cluster, and shows a gap at V = 15mag.
| A photometric study of the open cluster Haffner 6
We have obtained CCD BV photometry of 665 stars in the region of thesouthern open cluster Haffner 6. The photometry of a nearby field isalso reported. This poorly studied object is shown to be of intermediateage and to have a solar metallicity. Adopting a metal content Z=0.020,we found a colour excess E(B-V)=0.43, an apparent distance modulus(m-M)=13.90 and an age of about 1.0 Gyr. Consequently, we obtain adistance of about 3.4 kpc from the Sun. The main-sequence (MS)integrated luminosity function (ILF) is compatible with the standardSalpeter (chi=1.35) initial mass function (IMF).
| The galactic system of old star clusters: The development of the galactic disk
The vast majority of open clusters persist as clusters for no more thana few hundred million years, but the few which survive for much longerperiods constitute a unique sample for probing the evolution of thegalactic disk. In a charge coupled device (CCD) photometric survey forpossible old open clusters combined with previously publishedphotometry, we have developed a list of 72 clusters of the age of theHyades or older (Phelps (1994). Using our version of parameters based onthe luminosity difference between the main sequence turnoff and thehorizontal branch and on the color difference between the turnoff andthe giant branch, we have calculated a 'Morphological Age Index' (MAI)for the clusters in our list and for a sample of globular clusters. Wefind that the MAI is well correlated with the logarithm of cluster ages,as determined by fitting to theoretical isochrones. We conclude that theindex is a good measure of the relative ages of both globular and openclusters, although uncertainties in the models and residual metallicityeffects prevent the use of the MAI as a definitive calibration of actualcluster ages. The age distribution of the open clusters overlaps that ofthe globular clusters, indicating that the galactic disk began todevelop toward the end of the period of star formation in the galactichalo. The open cluster age distribution can be fit approximately with atwo-component exponential decay function; one component can beidentified as the tail of the dominant population of thin disk openclusters with an age scale factor of 200 Myr, and the other consists oflonger-lived clusters with an age scale of 4 Gyr. The young openclusters are distributed on the galactic plane almost symmetricallyabout the Sun with a scale height perpendicular to the galactic plane of55 pc. The old population consists of rich clusters found only in theouter disk, nore than RGC = 7.5 kpc from the galactic center;this population has a scale height of 375 pc. After accounting for thetwo exponential distributions of cluster ages, there are indications ofan excess of clusters in the age range of 5-7 Gyr; there may have beenlarge bursts of star formation in that period, or perhaps a largerproportion of the clusters forming at that time had advantageous orbitsfor survival. Either circumstance is consistent with the idea that thegalactic disk has been repeatedly disturbed, possibly in collisions orother interactions with external systems, resulting in the occasionalformation of clusters with relatively large velocities perpendicular tothe plane; these are the clusters that have survived until the present.Finally, the repeated accretion of low angular momentum material ontothe disk from the halo or beyond would also explain the observed radialcomposition gradient and the lack of a correlation between open clustermetallicity and age found by Friel & Janes (1993).
| PISMIS 3: an unstudied old open cluster
We have obtained CCD Bv photometry for the open cluster Pismis 3 and anearby field. This previously unstudied object appears to be rich, looseand strongly absorbed. Our analysis suggests that Pismis 3 is ofintermediate age and probably metal poor. Adopting the theoretical metalcontent Z = 0.008, which provides the best global fit, we obtain an ageof about 2 Gyr. Simultaneously a color excess EB-V = 1.35 andan apparent distance modulus (m - M) = 14.70 are derived. These resultsput Pismis 3 about 1.5 Kpc distance from the Sun. The relation of thecluster with the surrounding field is also investigated.
| Deep CCD BV photometry of the poorly studied open cluster NGC 4815.
We report BV Johnson CCD photometry of 2498 stars in the region of thepoorly studied open cluster NGC 4815. This object appears in the Janes(1988) list of possible old open clusters. NGC 4815 lies in the galacticplane, in a region of strong absorption. The color magnitude diagram(CMD) we derive shows that NGC 4815 has about the Hyades age (5x10^8^yr)and a probable lower than solar metal abundance. The color excess E_B-V_and the distance modulus (m-M) turn out to be 0.70 and 14.10,respectively. Accordingly a distance of 2.5 Kpc from the Sun is derived.The luminosity function (LF) we obtain for the main sequence (MS) starsis consistent with a Salpeter (x=1.55) initial mass function (IMF).
| CCD BV photometry of the intermediate age open cluster GC 1245
A new CCD color magnitude diagram (CMD) for the intermediate age opencluster NGC 1245 is presented. The cluster appears to be rich, butloose, irregular and very broad. Adopting the metallicity value of(Fe/H) = +0.14 from Lynga (1987) we obtain a color excessEB-V = 0.26, an apparent distance modulus (m - M) = 13.20,and an age of 8.0 x 108 yr. This puts NGC 1245 at a distanceof about 3.0 Kpc from the Sun. The analysis of the CMD shows thepresence of various field populations, and some evidences ofdifferential absorption.
| Development of the Galactic disk: A search for the oldest open clusters
In an extensive charge coupled devices (CCD) photometric survey ofpotential old open clusters, we have identified a number of systems thatare indeed old; some of them are among the oldest of the open clusters.Using our versions of two well-known morphological age indices, onebased on the luminosity difference between the main sequence turnoff andthe horizontal branch and the other on the color difference between theturnoff and the giant branch, we have ranked the open clusters inapproximate order of age. Our data together with previously publishedphotometry of other old open clusters, yields a catalogue of 72 clustersof the age of Hyades or older with 19 of the clusters as old or olderthan M67 (about 5 Gyr). Among the oldest open clusters are Be 17, Cr261, NGC 6791, Be 54, and AM 2. Be 17 and another old cluster, Lynga 7,are possibly as old as the youngest globulars. The data also suggestthat the formation rate of open clusters may have been higher early inthe history of the disk than at intermediate times since numerousclusters have survived from that time.
| Stellar photometric stability. II - Ages and distances for 13 open clusters with time series observations
Distances, reddenings, and ages have been determined for 13 southernopen clusters using data from CCD observations. It is shown that thephotometry will become an extremely efficient and accurate technique forcollecting data on clusters. The potential use of the photometry datafor studies of the H-R diagram, especially the structure of the mainsequence, is examined. Evidence is found for some gaps in the mainsequence, and it is argued that these gaps are statisticallysignificant. The mass functions for stars with masses of 1-6 solar aredetermined.
| Catalogue of UBV Photometry and MK Spectral Types in Open Clusters (Third Edition)
Not Available
| Open clusters and galactic structure
A total of 610 references to 434 clusters are employed in thecompilation of a catalog of open clusters with color-magnitude diagramson the UBV or RGU systems. Estimates of reddening, distance modulus, ageand number of cluster members are included. Although the sample isconsidered representative of the discoverable clusters in the galaxy,the observed distribution is nonuniform because of interstellarobscuration. Cluster distribution in the galactic plane is found to bedominated by the locations of dust clouds rather than by spiralstructure. The distributions of clusters as a function of age andrichness class show that the lifetimes of poor clusters are much shorterthan rich ones, and that clusters in the outer disk survive longer thanthose in the inner disk. An outer disk age which is only about 50% theage of the globular clusters is indicated by cluster statistics. Thethickening of the galactic disk with increasing galactocentric distancemay be due to either a younger dynamical age or a lower gravitationalpotential in the outer regions.
| Uniform survey of clusters in the southern Milky Way.
Abstract image available at:http://adsabs.harvard.edu/abs/1975AJ.....80...11V
| Southern open stars clusters. III. UBV-Hbeta photometry of 28 clusters between galactic longitudes 297d and 353d
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973A&AS...10..135M&db_key=AST
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