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K-band magnitude of the red clump as a distance indicator
We have investigated how the K-band magnitude of the red clump [M_K(RC)]depends on age and metallicity, using 2MASS infrared data for a sampleof 24 open clusters with known distances. We show that a constant valueof M_K(RC)=-1.57 ± 0.05 is a reasonable assumption to use indistance determinations for clusters with metallicity between -0.5 and+0.4 dex and age between 108.5 and 109.9 years.Figures 8 and 9 are available in electronic form at http://www.aanda.org
| Faint open clusters with 2MASS: BH 63, Lyngå 2, Lyngå 12 and King 20
Context: .Structural and dynamical parameters of faint open clusters areprobed with quality 2MASS-photometry and analytical procedures developedfor bright clusters. Aims: .We derive fundamental parameters ofthe faint open clusters Lyngå 2, BH 63, Lyngå 12 and King20, the last three of which have no prior determinations. We also focuson the structure and dynamical state of these clusters. Methods:.J, H and Ks 2MASS photometry with errors smaller than 0.2mag are used to build CMDs, radial density profiles, colour-colourdiagrams, luminosity and mass functions. Colour-magnitude filters areused to isolate probable member stars. Field-star decontamination isapplied to Lyngå 2, Lyngå 12 and King 20. Results:.Reddening values are in the range 0.22≤E(B-V)≤1.9, with BH 63 themost reddened object. Ages of Lyngå 2, King 20, Lyngå 12 andBH 63 are ≈90, ≈200, ≈560 and ≈700 Myr, respectively. Theradial density distributions of Lyngå 12 and King 20 arewell-represented by King profiles. Lyngå 2 and BH 63 are verysmall with core and limiting radii of ≈0.12 pc and ≈1.5 pc. Yet,they fit in the small-radii tail of the open cluster size distribution.Lyngå 12 and King 20 have R_core≈0.43 pc and R_lim≈3.9 pc.Lyngå 2 and Lyngå 12 are inside the Solar circle. Totalstellar masses (extrapolating the MFs to stars with 0.08 M_ȯ) rangefrom ≈340 M_ȯ (BH 63) to ≈2300 M_ȯ (Lyngå 12).Observed masses are ~1/4 of these values. In all clusters the core massfunction is flatter than the halo's. Conclusions: .Faint openclusters can be probed with 2MASS when associated with colour-magnitudefilters and field-star decontamination. BH 63 appears to be in anadvanced dynamical state, both in the core and halo. To a lesser degreethe same applies to King 20. Marginal evidence of dynamical evolution ispresent in the cores of Lyngå 2 and Lyngå 12.
| Old open clusters as key tracers of Galactic chemical evolution. I. Fe abundances in NGC 2660, NGC 3960, and Berkeley 32
Aims.We obtained high-resolution UVES/FLAMES observations of a sample ofnine old open clusters spanning a wide range of ages and Galactocentricradii. The goal of the project is to investigate the radial metallicitygradient in the disk, as well as the abundance of key elements (αand Fe-peak elements). In this paper we present the results for themetallicity of three clusters: NGC 2660 (age ~1 Gyr, Galactocentricdistance of 8.68 kpc), NGC 3960 (~1 Gyr, 7.80 kpc), and Be 32 (~6-7 Gyr,11.30 kpc). For Be 32 and NGC 2660, our study provides the firstmetallicity determination based on high-resolution spectra.Methods: .We performed equivalent width analysis with the spectral codeMOOG, which allows us to define a metallicity scale and build ahomogeneous sample. Results: .We find that NGC 3960 and NGC 2660have a metallicity that is very close to solar ([Fe/H] = +0.02 and+0.04, respectively), while the older Be 32 turns out to have [Fe/H] =-0.29.
| Multiband photometry towards the inner Galactic bulge: extinction, atmospheric parameters, and metallicities
Context: .We have obtained BVRI photometry for 21 stellar fields locatedin directions towards the central 3.5^circ of the Galaxy. Each field is10× 10 arcmin in size, and a total of 2 × 105stars were measured. Aims: .Previous analyses based on 2 MicronAll Sky Survey (2MASS) data have shown that these directions have arelatively low extinction (AK ≃ 0.25-0.30). Thecombined optical-infrared sample contains about 3.5 ×104 stars. This large dataset provides a more detailedextinction map within these surveyed areas than previously derived. Thelarge number of colour indexes available also yielded estimates ofeffective temperatures and metallicities for the inner bulge stars. Methods: .Extinction and effective temperature were estimated foreach star based on comparing the observed colours to model predictions.The extinction-corrected colour magnitude diagrams indicate the presenceof a broad red giant branch of inner bulge stars detached from thehelium-burning red clump of both bulge and disc stars. The red giantstars belonging to the bulge form a wide sequence mostly due to variablemetallicity. As previously done by other authors, we derived photometricmetallicities by comparing the position of each bulge red giant to a setof templates based on Galactic globular clusters. Results: .Weconfirm the existence of regions with AK < 0.30 in thesefields, as attested by analysis of the stellar colours. The resultingmetallicity distribution function is peaked at [Fe/H] ≃ -0.25 andextends to supersolar values up to [Fe/H] ≃ 0.3. This distributionis similar among the 4 regions, with relatively small field-to-fieldvariations within the central 1kpc. Our results are also in goodagreement with the metallicity distributions from other authors, bothphotometric and spectroscopic. Put together, this result strongly limitsthe existence of an abundance gradient in the inner bulge.
| Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960
Aims.We derive accurate parameters related to the CMD, structure anddynamical state of M 52 and NGC 3960, whose fields are affected bydifferential reddening. Previous works estimated their ages in theranges 35-135 Myr and 0.5-1.0 Gyr, respectively. Methods: .J, Hand Ks 2MASS photometry with errors <0.2 mag is used tobuild CMDs, radial density profiles, luminosity and mass functions, andcorrect for differential reddening. Field-star decontamination isapplied to uncover the cluster's intrinsic CMD morphology, andcolour-magnitude filters are used to isolate stars with high probabilityof being cluster members. Results: .The differential-reddeningcorrected radial density profile of M 52 follows King's law with coreand limiting radii of R_core =0.91 ± 0.14 pc and R_lim =8.0± 1.0 pc. NGC 3960 presents an excess of the stellar density overKing's profile (R_core = 0.62 ± 0.11 pc and R_lim =6.0 ±0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 areR_tidal=13.1 ± 2.2 pc and R_tidal=10.7 ± 3.7 pc. Clusterages of M 52 and NGC 3960 derived with Padova isochrones are constrainedto 60 ± 10 Myr and 1.1 ± 0.1 Gyr. In M 52 the core MF(χ_core=0.89 ± 0.12) is flatter than the halo's(χ_halo=1.65 ± 0.12). In NGC 3960 they are χ_core=-0.74± 0.35 and χ_halo=1.26 ± 0.26. The mass locked up inMS/evolved stars in M 52 is ~1200 M_ȯ, and the total mass(extrapolated to 0.08M_ȯ) is ~3800 M_ȯ. The total mass in NGC3960 is ~1300 M_ȯ. Conclusions: .Compared to open clusters indifferent dynamical states studied with similar methods, the core andoverall parameters of M 52 are consistent with an open cluster moremassive than 1000 M_ȯ and ~60 Myr old, with some mass segregationin the inner region. The core of NGC 3960 is in an advanced dynamicalstate with strong mass segregation in the core/halo region, while thesomewhat flat overall MF (χ≈ 1.07) suggests low-mass starevaporation. The excess stellar density in the core may suggestpost-core collapse. The dynamical evolution of NGC 3960 may have beenaccelerated by the tidal Galactic field, since it lies ≈0.5 kpcinside the Solar circle.
| uvbyCaHβ CCD Photometry of Clusters. VII. The Intermediate-Age Anticenter Cluster Melotte 71
CCD photometry on the intermediate-band uvbyCaHβ system ispresented for the anticenter, intermediate-age open cluster Melotte 71.Restricting the data to probable single members of the cluster using thecolor-magnitude diagram and the photometric indices alone generates asample of 48 F dwarfs on the unevolved main sequence. The averageE(b-y)=0.148+/-0.003 (standard error of the mean [s.e.m.]) orE(B-V)=0.202+/-0.004 (s.e.m.), where the errors refer to internal errorsalone. With this reddening, [Fe/H] is derived from both m1and hk, using Hβ and b-y as the temperature index, with excellentagreement among the four approaches and a final weighted average of[Fe/H]=-0.17+/-0.02 (s.e.m.) for the cluster, on a scale in which theHyades has [Fe/H]=+0.12. When adjusted for the higher reddeningestimate, the previous metallicity estimates from Washington photometryand from spectroscopy are now in agreement with the intermediate-bandresult. From comparisons to isochrones of appropriate metallicity, thecluster age and distance are determined as 0.9+/-0.1 Gyr and(m-M)=12.2+/-0.1 or (m-M)0=11.6+/-0.1. At this distance fromthe Sun, Mel 71 has a galactocentric distance of 10.0 kpc on a scale inwhich the Sun is 8.5 kpc from the Galactic center. Based on its age,distance, and elemental abundances, Mel 71 appears to be a less populousanalog to NGC 3960.
| Kinematics of the Open Cluster System in the Galaxy
Absolute proper motions and radial velocities of 202 open clusters inthe solar neighborhood, which can be used as tracers of the Galacticdisk, are used to investigate the kinematics of the Galaxy in the solarvicinity, including the mean heliocentric velocity components(u1,u2,u3) of the open cluster system,the characteristic velocity dispersions(σ1,σ2,σ3), Oortconstants (A,B) and the large-scale radial motion parameters (C,D) ofthe Galaxy. The results derived from the observational data of propermotions and radial velocities of a subgroup of 117 thin disk young openclusters by means of a maximum likelihood algorithm are:(u1,u2,u3) =(-16.1+/-1.0,-7.9+/-1.4,-10.4+/-1.5) km s-1,(σ1,σ2,σ3) =(17.0+/-0.7,12.2+/-0.9,8.0+/-1.3) km s-1,(A,B) =(14.8+/-1.0,-13.0+/-2.7) km s-1 kpc-1, and (C,D) =(1.5+/-0.7,-1.2+/-1.5) km s-1 k pc-1. A discussionon the results and comparisons with what was obtained by other authorsis given.
| Photometric and spectroscopic study of the intermediate-age open cluster NGC 3960
We present charge-coupled device UBVI photometry and high-resolutionspectroscopy of the intermediate-age open cluster NGC 3960. Thecolour-magnitude diagrams (CMDs) derived from the photometric data andinterpreted with the synthetic CMD method allow us to estimate thecluster parameters. We derive the following: age τ= 0.9 or 0.6Gyr(depending on whether or not overshooting from convective regions isincluded in the adopted stellar models); distance (m-M)0=11.6 +/- 0.1; reddening E(B-V) = 0.29 +/- 0.02; differential reddeningΔE(B-V) = 0.05; and approximate metallicity between solar andhalf-solar. We obtained high-resolution spectra of three clump stars,and derived an average [Fe/H]=-0.12 (rms 0.04dex), in very goodagreement with the photometric determination. We also obtained theabundances of α-elements, of Fe-peak elements and of Ba. Thereddenings towards individual stars derived from the spectroscopictemperatures and the calibrations of Alonso et al. give further supportto the existence of significant variations across the cluster.
| 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.
| Proper motion determination of open clusters based on the UCAC2 catalogue
We present the kinematics of hundreds of open clusters, based on theUCAC2 Catalogue positions and proper motions. Membership probabilitieswere obtained for the stars in the cluster fields by applying astatistical method uses stellar proper motions. All open clusters withknown distance were investigated, and for 75 clusters this is the firstdetermination of the mean proper motion. The results, including the DSSimages of the cluster's fields with the kinematic members marked, areincorporated in the Open Clusters Catalogue supported on line by ourgroup.
| FLAMES observations of old open clusters: Constraints on the evolution of the Galactic disc and mixing processes in stars.
Not Available
| New variable stars in open clusters. I. Methods and results for 20 open clusters
We present high precision CCD photometry of 1791 objects in 20 openclusters with an age of 10 Myr to 1 Gyr. These observations wereperformed within the Δ a photometric system which is primarilyused to detect chemically peculiar stars of the upper main sequence.Time bases range from 30 min up to 60 days with data from severalnights. We describe the time series analysis reaching a detection limitof down to 0.006 mag for apparent variability. In total, we havedetected 35 variable objects of which four are not members of theircorresponding clusters. The variables cover the entireHertzsprung-Russell diagram, hence they are interesting targets forfollow-up observations.Based on observations obtained at Complejo Astronómico elLeoncito (CASLEO), operated under the agreement between the ConsejoNacional de Investigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba y San Juan; ESO-La Silla and UTSO-Las Campanas.
| Parameter properties and stellar population of the old open cluster NGC 3960
We present a BV I photometric and astrometric catalogue of the opencluster NGC 3960, down to limiting magnitude V˜22, obtained fromobservations taken with the Wide Field Imager camera at the MPG/ESO 2.2m Telescope at La Silla. The photometry of all the stars detected in ourfield of view has been used to estimate a map of the strong differentialreddening affecting this area. Our results indicate that, within theregion where the cluster dominates, the E(V-I) values range from 0.21 upto 0.78, with E(V-I)=0.36 (E(B-V)=0.29) at the nominal cluster centroidposition; color excesses E(V-I) up to 1 mag have been measured in theexternal regions of the field of view where field stars dominate. Thereddening-corrected color-magnitude diagram (CMD) allows us to concludethat the cluster has an age between 0.9 and 1.4 Gyr and a distancemodulus of (V-M_V)_0=11.35.In order to minimize field star contamination, their number has beenstatistically subtracted based on the surface density map. The empiricalcluster main sequence has been recovered in the V vs. V-I and in the Jvs. J-K_S planes, using optical and infrared data, respectively. Fromthese empirical cluster main sequences, two samples of candidate clustermembers were derived in order to obtain the luminosity distributions asfunctions of the V and J magnitudes. The Luminosity Functions have beentransformed into the corresponding Mass Functions; for M>1 M_ȯ,the two distributions have been fitted with a power law of indexαV=2.95±0.53 andαJ=2.81±0.84 in V and in J, respectively, whilethe Salpeter Mass Function in this notation has index α=2.35.Based on observations made with the European Southern Observatorytelescopes obtained from the ESO/ST-ECF Science Archive Facility.Tables 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/417/945
| Intermediate-age Galactic open clusters: fundamental parameters of NGC 2627
Charge-coupled device (CCD) photometry in the Johnson V, Kron-Cousins Iand Washington CMT1 systems is presented in the field of thepoorly known open cluster NGC 2627. Four independent Washingtonabundance indices yield a mean cluster metallicity of [Fe/H]=-0.12 +/-0.08, which is compatible with the existence of a radial gradient in theGalactic disc. The resultant colour-magnitude diagrams indicate that thecluster is an intermediate-age object of 1.4 Gyr. Based on the best fitsof the Geneva group's isochrones to the (V, V-I) and (T1,C-T1) diagrams, we estimate E(V-I) = 0.25 +/- 0.05 andV-MV= 11.80 +/- 0.25 for logt= 9.15, and E(C-T1) =0.23 +/- 0.07 and T1-MT1= 11.85 +/-0.25 for logt= 9.10, respectively, assuming solar metal content. Thederived reddening value E(C-T1) implies E(B-V) = 0.12 +/-0.07 and a distance from the Sun of 2.0 +/- 0.4 kpc. Using the WEBDAdata base and the available literature, we re-examined the overallproperties of all the open clusters with ages between 0.6 and 2.5 Gyr.We identified peaks of cluster formation at 0.7-0.8, 1.0-1.1, 1.6-1.7and 2.0-2.1 Gyr, separated by relative quiescent epochs of ~0.2-0.3 Gyr.We also estimated a radial abundance gradient of -0.08 +/- 0.02, whichis consistent with the most recent determinations for the Galactic disc,but no clear evidence for a gradient perpendicular to the Galactic planeis found.
| Theoretical isochrones for the Delta a photometric system
We have calculated theoretical isochrones for the photometric Delta asystem to derive astrophysical parameters such as the age, reddening anddistance modulus for open clusters. The Delta a system samples the fluxdepression at 520 nm which is highly efficient to detect chemicallypeculiar (CP) objects of the upper main sequence. The evolutionarystatus of CP stars is still a matter of debate and very important totest, for example, the dynamo and diffusion theories. In fact, thedynamo or fossil origin of the magnetic fields present in this kind ofstar is still not clear. Using the stellar evolutionary models by Claret(\cite{Cla95}), a grid of isochrones with different initial chemicalcompositions for the Delta a system was generated. The published dataof 23 open clusters were used to fit these isochrones with astrophysicalparameters (age, reddening and distance modulus) from the literature. Asan additional test, isochrones with the same parameters for Johnson UBVdata of these open clusters were also considered. The fits show a goodagreement between the observations and the theoretical grid. We findthat the accuracy of fitting isochrones to Delta a data without theknowledge of the cluster parameters is between 5 and 15%.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| 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.
| Metallicities of Old Open Clusters
We present radial velocities and metallicities for a sample of 39 openclusters with ages greater than about 700 million years. For 24 clustersnew moderate-resolution spectroscopic data obtained with multiobjectspectrographs on the Kitt Peak National Observatory and the Cerro TololoInter-American Observatory 4 m telescopes are used to determine radialvelocities and mean cluster metallicities. These new results arecombined with data published previously by Friel & Janes to providea sample of 459 giants in 39 old open clusters, which are used toinvestigate radial abundance gradients in the Galactic disk. Based on anupdated abundance calibration of spectroscopic indices measuring Fe andFe-peak element blends, this larger sample yields an abundance gradientof -0.06+/-0.01 dex kpc-1 over a range in Galactocentricradius of 7 to 16 kpc. There is a slight suggestion of a steepening ofthe abundance gradient with increasing cluster age in this sample, butthe significance of the result is limited by the restricted distancerange for the youngest clusters. The clusters show no correlation ofmetallicity with age in the solar neighborhood. Consistent with theevidence for a steepening of the gradient with age, the clusters in theouter disk beyond 10 kpc show a suggestion at the 1.5 σ level of adependence of metallicity on age.
| CCD photometric search for peculiar stars in open clusters. III. NGC 2439, NGC 3960, NGC 6134, NGC 6192 and NGC 6451
In total, more than 550 objects within the fields of five open clusterswere investigated in order to find classical chemically peculiar stars.One bona-fide CP candidate was detected in each of the open clusters NGC3960, NGC 6192 and NGC 6451. All three objects seem to be members of thecorresponding cluster, taking the results from our photometry as well asfrom the literature. For NGC 6192 and NGC 6451 we found widely differentinterstellar reddening values and therefore distances and ages in theliterature. From an analysis of published Johnson UBVRI photometry, wewere able to rule out a rather old age for NGC 6451. This is alsosupported by the finding of one peculiar object in this cluster withspectral type of about A2. Five variable objects within the field of NGC6134 were detected. The variability exceeds 10 times (or typically 0.03mag) the standard deviation of the weighted individual measurements. Butsince our observations were obtained mostly within one hour (typicalintegration times of five minutes per filter) on several nights within45 days, no light curves could be derived. For this cluster, some Amcandidates were reported for which we were not able to detect asignificant positive Delta a-value. Based on observations at ESO-LaSilla and\ UTSO-Las Campanas.
| Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution
We compile a new sample of 89 open clusters with ages, distances andmetallicities available. We derive a radial iron gradient of about-0.099±0.008 dexkpc (unweighted) for the whole sample, which issomewhat greater than the most recent determination of oxygen gradientfrom nebulae and young stars. By dividing the clusters into age groups,we show that the iron gradient was steeper in the past and has evolvedslowly in time. Current data show a substantial scatter of the clustermetallicities indicating that the Galactic disk has undergone a veryrapid, inhomogeneous enrichment.Also, based on a simple, but quitesuccessful model of chemical evolution of the Milky Way disk, we make adetailed calculation of the iron abundance gradient and its timeevolution. The predicted current iron gradient is about -0.072 dexkpc.The model also predicts a steady flattening of the iron gradient withtime, which agrees with the result from our open cluster sample.
| Membership, Binarity and Stellar Evolution of Red Giants in the Open Clusters NGC 2324, NGC 2818, NGC 3960 and NGC 6259
Not Available
| Statistics and supermetallicity: The metallicity of NGC 6791
For the old galactic cluster NGC 6791, Peterson & Green(\cite{pg98a}) and Chaboyer et al. (\cite{cgl99}) have found that [Fe/H]~ +0.4 dex. A second look at that conclusion is taken in this paper.Zero-point problems are reviewed for a high-dispersion analysis done byPeterson & Green, and it is found that accidental errors have notbeen determined rigorously for the results of that analysis. It is alsonoted that in a color-magnitude analysis performed by Chaboyer et al.,the important metallicity range between 0.0 and +0.3 dex is not exploredand hence is not ruled out. Moreover, that analysis does not yieldstatistically rigorous results, and it appears that such results may notbe produced in color-magnitude analysis of clusters in general. Resultsin the two cited papers and elsewhere are re-evaluated statistically,with an allowance being made for uncertainty in the cluster reddening.Apparently the best that can be said at present is that the clustermetallicity lies in the range from +0.16 to +0.44 dex. This conclusionis stressed by reviewing the immaturity of the underlying data base. Thepremature conclusion for a high metallicity turns out to be due largelyto neglect of accidental errors, though a tendency to ascribe too muchweight to high derived metallicities may also play a role.
| Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259
We present accurate radial velocities and photoelectric UBV photometryfor 73 and 57 red-giant candidates, respectively, in theintermediate-age open clusters NGC 2324, 2818, 3960 and 6259. These dataconfirm the membership of 47 stars, 12 of which (26%) are spectroscopicbinaries; three preliminary orbits have been determined in NGC 3960.From Washington photometry of 8 red giant members, the metallicity ofNGC 6259 is found to be [Fe/H] = 0.06 +/- 0.08. At the age of theseclusters, most of the red giants are observed in the core-helium (clump)burning phase, the general morphology of which is well reproduced bytheoretical models with convective overshooting. However, a number ofbona fide cluster giant members are found significantly to the red ofthe isochrones fitting the rest of the CMD of these and a few otherclusters. Some of these stars are binaries, but others seem to besingle. In either case, their red colours and/or low luminosities remainunexplained by current stellar evolution theory. Based on observationscollected with the Danish 1.54-m telescope at the European SouthernObservatory, La Silla (Chile) and at Cerro Tololo Inter-AmericanObservatory, National Optical Astronomy Observatories, operated by theAssociation of the Universities for Research in Astronomy, Inc., undercontract with the National Science Foundation.
| 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.
| On the Galactic disc age-metallicity relation
A comparison is made between the age-metallicity relations obtained fromfour different types of studies: F and G stars in the solarneighbourhood, analysis of open clusters, galactic structure studieswith the stellar population synthesis technique and chemical evolutionmodels. Metallicities of open clusters are corrected for the effects ofthe radial gradient, which we find to be -0.09 dex kpc^-1 and mostlikely constant in time. We do not correct for the vertical gradient,because its existence and value are not firmly established. Stars andclusters trace a similar age-metallicity relation, showing an excess ofrather metal-rich objects in the age range 5-9 Gyr. Galactic structurestudies tend to give a more metal-poor relation than chemical evolutionmodels. Neither relation explains the presence of old, relativelymetal-rich stars and clusters. This might be caused by uncertainties inthe ages of the local stars, or pre-enrichment of the disc with materialfrom the bulge, possibly as a result of a merger event in the earlyphases of the formation of our Galaxy.
| HIPPARCOS absolute magnitudes for metal-rich K giants and the calibration of DDO photometry
Parallaxes for 581 bright K giants have been determined using theHipparcos satellite. We combine the trigonometric parallaxes withground-based photometric data to determine the K giant absolutemagnitudes. For all these giants, absolute magnitude estimates can alsobe made using the intermediate-band photometric David Dunlop Observatory(DDO) system. We compare the DDO absolute magnitudes with the veryaccurate Hipparcos absolute magnitudes, finding various systematicoffsets in the DDO system. These systematic effects can be corrected,and we provide a new calibration of the DDO system allowing absolutemagnitude to be determined with an accuracy of 0.35 mag in the Mv rangebetween 2 and -1. The new calibration performs well when tested on Kgiants with DDO photometry in a selection of low-reddening open clusterswith well-measured distance moduli.
| Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2556T&db_key=AST
| Chemical Evolution of the Galactic Disk: Evidence for a Gradient Perpendicular to the Galactic Plane
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.2813P&db_key=AST
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| The Old Open Clusters Of The Milky Way
The Galactic open clusters, in particular the oldest members, serve asexcellent probes of the structure and evolution of the Galactic disk.Individual clusters provide excellent tests of stellar and dynamicalevolution. Cluster spatial and age distributions provide insight intothe processes of cluster formation and destruction that have allowedsubstantial numbers of old open clusters to survive.Spectroscopic andphotometric data for the old clusters yield kinematic, abundance, andage information that clarifies the relationship between the old opencluster population and other Galactic populations. New samples of oldopen clusters, which span a large range in distance and age, are used todefine disk abundance gradients and the cluster age-metallicityrelationship, and they point to a complex history of chemical enrichmentand mixing in the disk.
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Miembro de los siguientes grupos:
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Datos observacionales y astrométricos
Catálogos y designaciones:
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