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Towards absolute scales for the radii and masses of open clusters
Aims. In this paper we derive tidal radii and masses of open clusters inthe nearest kiloparsecs around the Sun. Methods: For each cluster, themass is estimated from tidal radii determined from a fitting ofthree-parameter King profiles to the observed integrated densitydistribution. Different samples of members are investigated. Results:For 236 open clusters, all contained in the catalogue ASCC-2.5, weobtain core and tidal radii, as well as tidal masses. The distributionsof the core and tidal radii peak at about 1.5 pc and 7-10 pc,respectively. A typical relative error of the core radius lies between15% and 50%, whereas, for the majority of clusters, the tidal radius wasdetermined with a relative accuracy better than 20%. Most of theclusters have tidal masses between 50 and 1000 m_ȯ, and for abouthalf of the clusters, the masses were obtained with a relative errorbetter than 50%.Full Table 2 is 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?J/A+A/468/151
| 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
| The population in the background of open clusters: tracer of the Norma-Cygnus arm
We present colour-magnitude diagrams of open clusters, located in therange 112° < l < 252°, manifesting stellar populations inthe background of clusters. Some of the populations are found to belocated beyond the Perseus arm and may be the part of the Norma-Cygnus(outer) arm. The outer arm seems to be continued from l ~ 120° to~235°. The background populations follow the downward warp of theGalactic plane around l ~ 240°.
| On the current status of open-cluster parameters
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
| 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.
| Mergers of Close Primordial Binaries
We study the production of main-sequence mergers of tidally synchronizedprimordial short-period binaries. The principal ingredients of ourcalculation are the angular momentum loss rates inferred from thespin-down of open cluster stars and the distribution of binaryproperties in young open clusters. We compare our results with theexpected number of systems that experience mass transfer in thepost-main-sequence phases of evolution and compute the uncertainties inthe theoretical predictions. We estimate that main-sequence mergers canaccount for the observed number of single blue stragglers in M67.Applied to the blue straggler population, this implies that such mergersare responsible for about one-quarter of the population of halo bluemetal-poor stars and at least one-third of the blue stragglers in openclusters for systems older than 1 Gyr. The observed trends as a functionof age are consistent with a saturated angular momentum loss rate forrapidly rotating tidally synchronized systems. The predicted number ofblue stragglers from main-sequence mergers alone is comparable to thenumber observed in globular clusters, indicating that the net effect ofdynamical interactions in dense stellar environments is to reduce ratherthan increase the blue straggler population. A population of subturnoffmergers of order 3%-4% of the upper main sequence population is alsopredicted for stars older than 4 Gyr, which is roughly comparable to thesmall population of highly Li-depleted halo dwarfs. Other observationaltests are discussed.
| Ground-based CCD astrometry with wide field imagers. I. Observations just a few years apart allow decontamination of field objects from members in two globular clusters
This paper is the first of a series of papers in which we will apply themethods we have developed for high-precision astrometry (and photometry)with the Hubble Space Telescope (HST) to the case of wide-fieldground-based images. In particular, we adapt the software originallydeveloped for WFPC2 to ground-based, wide field images from the WFI atthe ESO 2.2 m telescope. In this paper, we describe in details the newsoftware, we characterize the WFI geometric distortion, discuss theadopted local transformation approach for proper-motion measurements,and apply the new technique to two-epoch archive data of the two closestGalactic globular clusters: NGC 6121 (M 4) and NGC 6397. The results ofthis exercise are more than encouraging. We find that we can achieve aprecision of ~7 mas (in each coordinate) in a single exposure for awell-exposed star, which allows a very good cluster-field separation inboth M 4, and NGC 6397, with a temporal baseline of only 2.8, and 3.1years, respectively.
| 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.
| Photometry of seven overlooked open clusters in the first and fourth Galactic quadrants
CCD BVI photometry is presented for seven previously unstudied starclusters projected towards the inner side of the Galaxy: Trumpler 23,Lynga 3, Collinder 307, Ruprecht 134, ESO552SC16, AL 5 and Kronberger 3.Colour-magnitude diagrams of the cluster regions allow us to concludethat Lynga 3 and ESO552SC16 are not clusters, but groups of bright starsprobably located in the Carina-Sagittarius spiral arm. AL 5 andKronberger 3 are so embedded in a dense stellar field that we cannotconfirm their nature. Trumpler 23 and Ruprecht 134 are twointermediate-age open clusters located well inside the solar ring whichdeserve further attention.Finally, Collinder 307 is an obscured younger cluster (250Myr) locatedin the Carina-Sagittarius spiral arm. Our results emphasize thedifficulty to search for open clusters in the inner regions of theGalaxy due to the richness of the field and the patchy nature of theinterstellar absorption, but at the same time significantly contributeto a better understanding of this complicated regions of the Milky Way.
| 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.
| Probing disk properties with open clusters
We use the open clusters (OCs) with known parameters available in theWEBDA database and in recently published papers to derive propertiesrelated to the disk structure such as the thin-disk scale height,displacement of the Sun above the Galactic plane, scale length and theOC age-distribution function. The sample totals 654 OCs, consistingbasically of Trumpler types I to III clusters whose spatial distributiontraces out the local geometry of the Galaxy. We find that the populationof OCs with ages younger than 200 Myr distributes in the disk followingan exponential-decay profile with a scale height ofzh=48±3 pc. For the clusters with ages in the range200 Myr to 1 Gyr we derive zh=150±27 pc. Clustersolder than 1 Gyr distribute nearly uniformly in height from the plane sothat no scale height can be derived from exponential fits. Consideringclusters of all ages we obtain an average scale height ofzh=57±3 pc. We confirm previous results thatzh increases with Galactocentric distance. The scale heightimplied by the OCs younger than 1 Gyr outside the Solar circle is afactor 1.4-2 larger than zh of those interior to theSolar circle. We derive the displacement of the Sun above the Galacticplane as zȯ=14.8±2.4 pc, which agrees withprevious determinations using stars. As a consequence of thecompleteness effects, the observed radial distribution of OCs withrespect to Galactocentric distance does not follow the expectedexponential profile, instead it falls off both for regions external tothe Solar circle and more sharply towards the Galactic center. Wesimulate the effects of completeness assuming that the observeddistribution of the number of OCs with a given number of stars above thebackground, measured in a restricted zone outside the Solar circle, isrepresentative of the intrinsic distribution of OCs throughout theGalaxy. Two simulation models are considered in which the intrinsicnumber of observable stars are distributed: (i) assuming the actualpositions of the OCs in the sample, and (ii) random selection of OCpositions. As a result we derive completeness-corrected radialdistributions which agree with exponential disks throughout the observedGalactocentric distance range 5-14 kpc, with scale lengths in the rangeRD=1.5-1.9 kpc, smaller than those inferred by means ofstars. In particular we retrieve the expected exponential-disk radialprofile for the highly depleted regions internal to the Solar circle.The smaller values of RD may reflect intrinsic differences inthe spatial distributions of OCs and stars. We derive a number-densityof Solar-neighbourhood (with distances from the Sundȯ≤1.3 kpc) OCs ofρȯ=795±70 kpc-3, which implies atotal number of (Trumpler types I to III) OCs of 730 of which47% would already have been observed. Extrapolation of thecompleteness-corrected radial distributions down to the Galactic centerindicates a total number of OCs in the range(1.8-3.7)×105. These estimates are upper-limits becausethey do not take into account depletion in the number of OCs bydynamical effects in the inner parts of the Galaxy. The observed andcompleteness-corrected age-distributions of the OCs can be fitted by acombination of two exponential-decay profiles which can be identifiedwith the young and old OC populations, characterized by age scales of100 Myr and 1.9 Gyr, respectively. This rules out evolutionaryscenarios based on constant star-formation and OC-disruption rates.Comparing the number of observed embedded clusters and candidates in theliterature with the expected fraction of very young OCs, derived fromthe observed age-distribution function, we estimate that 3.4-8% of theembedded clusters do actually emerge from the parent molecular clouds asOCs.
| Properties of five low-contrast open clusters in the third quadrant
We derive photometric, structural and dynamical evolution-relatedparameters of five as yet unstudied low-contrast open clusters locatedin the third quadrant using 2MASS data. The target clusters are Czernik31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply astatistical field-star decontamination procedure to infer on theintrinsic colour-magnitude diagram (CMD) morphology which is criticalfor such low-contrast objects. Consequently, it became possible toderive accurate reddening, age, distance from the Sun and Galactocentricdistance for the five clusters. In the structural andluminosity/mass-function analyses we apply a colour-magnitude filterwhich encompasses the cluster evolutionary CMD sequences and excludesstars with discrepant colours. Using this procedure we derive core andlimiting radii, mass function (MF) slope, total mass, mass density andrelaxation time. We derive ages in the range 140-1100 Myr,Galactocentric distances within 7.7-11.4 kpc, and total masses within360-2900 M_ȯ. Reflecting large-scale mass segregation, the MF slopein the core is significantly flatter than that in the halo of the fiveclusters. Although some of the present clusters are relatively youngerthan the Gyr-old clusters, they present evidence of advanced dynamicalevolution. This kind of study has become possible because of thephotometric uniformity and spatial coverage of 2MASS which allows aproper subtraction of the field-star contamination on the target CMDs.The present study indicates that low-contrast clusters can be studiedwith 2MASS, particularly after field-star subtraction, which isimportant since most of the unstudied open clusters belong to thisclass.
| Structure and stellar content analysis of the open cluster M 11 with 2MASS photometry
An overall analysis of the structure and stellar content of M 11 ispresented, thanks to the wide-angle 2MASS spatial coverage. We derivephotometric and structural parameters and discuss the spatial dependanceof the luminosity and mass functions. Photometric parameters basicallyagree with previous ones mostly based on the optical. We obtained a coreradius of 1.23 pc and a tidal radius of 29 pc. In particular, thecluster is populous enough so that the tidal radius could be obtained byfitting the three-parameter King profile to the radial distribution ofstars. We analyzed the spatial distribution of mass functions, findingthat the slope changes from -0.73 in the core to +2.88 in the outerhalo. The spatial distribution of mass function slopes derived from2MASS agrees with that derived from optical CCD data, which furtherconfirms the reliability of 2MASS data for future analyses of this kindat comparable observational limits. We detect mass segregation up todistances from the center of ≈20 arcmin. We emphasize that the massfunction slope in the core is flatter than anywhere else as aconsequence of mass segregation. The derived total cluster mass is≈11 000 M_ȯ.
| FLAMES observations of old open clusters: Constraints on the evolution of the Galactic disc and mixing processes in stars.
Not Available
| Detection of a Young Stellar Population in the Background of Open Clusters in the Third Galactic Quadrant
We report the detection of a young stellar population (<=100 Myr) inthe background of nine young open clusters belonging to a homogenoussample of 30 star clusters in the third Galactic quadrant (at217deg<=l<=260deg). Deep and accurate UBVRIphotometry allows us to measure model-independent age and distance forthe clusters and the background population with high confidence. Thispopulation is exactly the same population (the blue plume) recentlydetected in three intermediate-age open clusters and suggested to be a<=1-2 Gyr old population belonging to the Canis Major (CMa)overdensity (Bellazzini et al.; Martínez-Delgado et al.).However, we find that the young population in those three clusters andin six clusters of our sample follows the pattern of the Norma-Cygnusspiral arm as defined by CO clouds remarkably well, while in the otherthree program clusters it lies in the Perseus arm. We finally provideone example (out of 21) of a cluster that does not show any backgroundpopulation, demonstrating that this population is not ubiquitous towardCMa.
| Astrophysical parameters of Galactic open clusters
We present a catalogue of astrophysical data for 520 Galactic openclusters. These are the clusters for which at least three most probablemembers (18 on average) could be identified in the ASCC-2.5, a catalogueof stars based on the Tycho-2 observations from the Hipparcos mission.We applied homogeneous methods and algorithms to determine angular sizesof cluster cores and coronae, heliocentric distances, mean propermotions, mean radial velocities, and ages. For the first time we derivedistances for 200 clusters, radial velocities for 94 clusters, and agesof 196 clusters. This homogeneous new parameter set is compared withearlier determinations, where we find, in particular, that the angularsizes were systematically underestimated in the literature.
| Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane
We derive photometric, structural and dynamical evolution-relatedparameters of 11 nearby open clusters with ages in the range 70 Myr to 7Gyr and masses in the range ≈400 M_ȯ to ≈5300 M_ȯ. Theclusters are homogeneously analysed by means of J, H and KS2MASS photometry, which provides spatial coverage wide enough toproperly take into account the contamination of the cluster field byGalaxy stars. Structural parameters such as core and limiting radii arederived from the background-subtracted radial density profiles.Luminosity and mass functions (MFs) are built for stars later than theturnoff and brighter than the 2MASS PSC 99.9% completeness limit. Thetotal mass locked up in stars in the core and the whole cluster, as wellas the corresponding mass densities, are calculated by taking intoaccount the observed stars (evolved and main sequence) and extrapolatingthe MFs down to the H-burning mass limit, 0.08 M_ȯ. We illustratethe methods by analysing for the first time in the near-infrared thepopulous open clusters NGC 2477 and NGC 2516. For NGC 2477 we derive anage of 1.1 ± 0.1 Gyr, distance from the Sun d_ȯ=1.2 ±0.1 kpc, core radius Rcore=1.4 ± 0.1 pc, limitingradius Rlim=11.6 ± 0.7 pc and total massmtot≈(5.3±1.6) × 103 M_ȯ.Large-scale mass segregation in NGC 2477 is reflected in the significantvariation of the MF slopes in different spatial regions of the cluster,and in the large number-density of giant stars in the core with respectto the cluster as a whole. For NGC 2516 we derive an age of 160 ±10 Myr, d_ȯ=0.44 ± 0.02 kpc, Rcore=0.6 ±0.1 pc, Rlim=6.2 ± 0.2 pc andmtot≈(1.3±0.2) × 103 M_ȯ.Mass-segregation in NGC 2516 shows up in the MFs. Six of the 11 clusterspresent a slope break in the MF occurring at essentially the same massas that found for the field stars in Kroupa's universal IMF. The MFbreak is not associated to cluster mass, at least in the clusters inthis paper. In two clusters the low-mass end of the MF occurs above theMF break. However, in three clusters the MF break does not occur, atleast for the mass range m≥0.7 M_ȯ. One possibility isdynamical evolution affecting the MF slope distribution. We also searchfor relations of structural and evolutionary parameters with age andGalactocentric distance. The main results for the present sample are:(i) cluster size correlates both with age and Galactocentric distance;(ii) because of size and mass scaling, core and limiting radii, and coreand overall mass correlate; (iii) massive (m≥1000 M_ȯ) andless-massive clusters follow separate correlation paths on the planecore radius and overall mass; (iv) MF slopes of massive clusters arerestricted to a narrow range, while those of the less-massive onesdistribute over a wider range. Core and overall MF flattening is relatedto the ratio (τ) of age to relaxation time. For large values ofτ the effects of large-scale mass segregation and low-mass starsevaporation can be observed in the MFs. In this sense, τ appears tocharacterize the evolutionary state of the clusters. We conclude thatappreciable slope flattenings in the overall MFs of the less-massiveclusters take ~6 times longer to occur than in the core, while in themassive clusters they take a time ~13 times longer. We investigatecluster parameters equivalent to those determining the fundamental planeof ellipticals. These parameters are: overall mass, projected massdensity and core radius. We conclude that in the present sample there isevidence of a fundamental plane. Larger samples are necessary to pindown this issue.
| From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main-Sequence Stellar Evolution in Open Clusters
I explore the current ability of both white dwarf cooling theory andmain-sequence stellar evolution theory to accurately determine stellarpopulation ages by comparing ages derived using both techniques for openclusters ranging from 0.1 to 4 Gyr. I find good agreement between whitedwarf and main-sequence evolutionary ages over the entire age rangecurrently available for study. I also find that directly comparingmain-sequence turnoff ages to white dwarf ages is only weakly sensitiveto realistic levels of errors in cluster distance, metallicity, andreddening. Additional detailed comparisons between white dwarf andmain-sequence ages have tremendous potential to refine and calibrateboth of these important clocks, and I present new simulations ofpromising open cluster targets. The most demanding requirements forthese white dwarf studies are very deep (V>=25-28) clusterobservations made necessary by the faintness of the oldest white dwarfs.
| UVES observations of the Canis Major overdensity
We present the first detailed chemical abundances for three giant starswhich are candidate members of the Canis Major overdensity, obtained byusing FLAMES-UVES at VLT. The stars, in the background of the opencluster NGC 2477, have radial velocities compatiblewith a membership to this structure. However, due to Galactic disccontamination, radial velocity by itself is unable to firmly establishmembership. The metallicities span the range -0.5 [Fe/H] +0.1. Assuming that at least one of the three stars is indeed a memberof CMa implies that this structure has undergone a high level ofchemical processing, comparable to that of the Galactic disc. The mostmetal-rich star of the sample, EIS 6631, displaysseveral abundance ratios which are remarkably different from those ofGalactic stars: [α/Fe] -0.2, [Cu/Fe] +0.25, [La/Fe] +0.6, [Ce/Fe] +0.8 and [Nd/Fe] +0.6. These ratios makeit likely that this star was formed in an external galaxy.Based on observations obtained in the ESO Director's Discretionary Timeprogram 272-B.5017.
| Detection of the Canis Major galaxy at (l;b) = (244° -8°) and in the background of Galactic open clusters
We report on the detection of main-sequence stars belonging to therecently identified Canis Major (CMa) galaxy in a field located at~=4.°2 from the centre of the stellar system. With main-sequencefitting we obtain a distance modulus (m-M)0= 14.5 +/- 0.3 tothe dwarf, corresponding to a distance of Dsolar~= 8.0 +/-1.2 kpc, in full agreement with previous estimates based on thephotometric parallax of M-giants. From the comparison with theoreticalisochrones, we constrain the age of the main population of the CMasystem in the range ~4-10 Gyr. A blue plume of likely younger stars (age<1-2 Gyr) is also identified. The available colour-magnitude diagramsof open clusters that may be projected on to the main body of CMa arealso briefly analysed. The position, distance and stellar population ofthe old open clusters Arp-Madore 2 and Tombaugh 2 strongly suggest thatthey are physically associated with the CMa galaxy. Using our ownphotometry and data from the Two-Micron All-Sky Survey and the GuideStar Catalogue 2.2 we demonstrate that the claim by Momany et al. thatthe CMa overdensity is entirely due to the Galactic warp is notsupported by the existing observations, once all the available pieces ofinformation are taken into account. It is shown that the CMa overdensityclearly emerges at a heliocentric distance of ~8 kpc above anyoverdensity possibly produced by the Galactic warp.
| The astrophysics of cool white dwarfs
Electronic Article Available from Elsevier Science.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| Red giants in open clusters. XI. Membership, duplicity, and structure of NGC 2477
New, accurate radial velocities and photoelectric UBV photometry of 83red-giant candidates in the field of the rich, intermediate-age opencluster NGC 2477 ([Fe/H]= -0.05, age ≃1 Gyr)are presented and discussed. From 49 constant-velocity members we find amean cluster velocity of +7.32±0.13 km s-1 and confirmthe membership of 76 of the stars. Among the cluster members, weidentify 26 definite and 1 probable spectroscopic binaries and determineorbits for 13 of these systems, with periods ranging from 40 to 4578days. The binary frequency is thus rather high (27/76 = 36%). Theobserved internal radial velocity dispersion of the cluster, asdetermined from the single member stars, is 0.93 km s-1,corrected for the small average observational error of 0.22 kms-1. Fitting King-type models to the observed stellar densitydistribution and velocity dispersion, and assuming a distance of 1.25kpc, we find the core and tidal radii of NGC 2477 tobe 1.8 and 8.1 pc, respectively, and estimate that the mass of clusterstars down to V = 17, corresponding to 1 Mȯ, is atleast 5400 Mȯ. The substantial differential reddening ofNGC 2477 requires a more detailed study beforedefinitive isochrone fits can be made.Based on observations collected with the Danish 1.54-m and ESO 1-mtelescopes at the European Southern Observatory, La Silla, Chile, andwith the University of Toronto 0.6-m telescope at Las CampanasObservatory, Chile.Full Tables 1 and 2 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?J/A+A/423/189
| 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.
| Probing the Canis Major stellar over-density as due to the Galactic warp
Proper-motion, star counts and photometric catalog simulations are usedto explain the detected stellar over-density in the region of CanisMajor, claimed to be the core of a disrupted dwarf galaxy (CMa, Martinet al. \cite{martin04}, MNRAS, 348, 12; Bellazzini et al. \cite{luna04},[arXiv:astro-ph/0311119]), as due to the Galactic warp and flare in theexternal disk. We compare the kinematics of CMa M-giant selected samplewith surrounding Galactic disk stars in the UCAC2 catalog and find nopeculiar proper motion signature: CMa stars mimic thick disk kinematics.Moreover, when taking into account the Galactic warp and flare of thedisk, 2MASS star count profiles reproduce the CMa stellar over-density.This star count analysis is confirmed by direct comparison withsynthetic color-magnitude diagrams simulated with the Besançonmodels (Robin et al. \cite{robin03}, A&A, 409, 523) that include thewarp and flare of the disk. The presented evidence casts doubt on theidentification of the CMa over-density as the core of a disrupted MilkyWay satellite. This however does not make clear the origin ofover-densities responsible for the ring structure in the anticenterdirection of the Galactic halo (Newberg et al. \cite{newberg02}, ApJ,569, 245; Yanny et al. 2003, ApJ, 588, 824).
| 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.
| 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.
| 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.
| WIYN Open Cluster Study - XVI. Optical/infrared photometry and comparisons with theoretical isochrones
We present combined optical/near-infrared photometry (BVIK) for six openclusters - M35, M37, NGC 1817, NGC 2477, NGC 2420 and M67. The openclusters span an age range from 150 Myr to 4 Gyr and have metalabundances from [Fe/H]=-0.27 to +0.09 dex. We have utilized these datato test the robustness of theoretical main sequences constructed byseveral groups as denoted by the following designations - Padova,Baraffe, Y2, Geneva and Siess. The comparisons of the modelswith the observations have been performed in the [MV,(B-V)0], [MV, (V-I)0] and[MV, (V-K)0] colour-magnitude diagrams as well asthe distance-independent [(V-K)0, (B-V)0] and[(V-K)0, (V-I)0] two-colour diagrams. We concludethat none of the theoretical models reproduces the observational data ina consistent manner over the magnitude and colour range of the unevolvedmain sequence. In particular, there are significant zero-point and shapedifferences between the models and the observations. We speculate thatthe crux of the problem lies in the precise mismatch between theoreticaland observational colour-temperature relations. These results underscorethe importance of pursuing the study of stellar structure and stellarmodelling with even greater intensity.
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