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The Infrared Ca II Triplet as Metallicity Indicator
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
| Red giants in open clusters. XII. Six old open clusters NGC 2112, 2204, 2243, 2420, 2506, 2682
Aims.We studied the membership and binarity of 123 red giants in six oldopen clusters, NGC 2112, 2204, 2243, 2420, 2506 and 2682, to define moreprecisely the evolutionary path on the red-giant branch. Methods: Theanalysis is based on 185 radial-velocity observations with the Coravelspectrographs and available photometric data. Results: The membershipof 93 red giants was confirmed on the basis of the radial velocities.Seven definitive spectroscopic binaries were identified and 11additional stars are suspected of being binaries. The binary frequency(19%) is slightly lower than average. This is partly due to the smallnumber of observations secured for each star. Orbital elements have beendetermined for the first time for the BaII star NGC 2420-173 (D) andthose of the other BaII star NGC 2420-250 (X) have been improved. Thevalues of the cluster mean velocities have been significantly improved. Conclusions: With the new membership estimates and binary detections,the existing CCD data allow precise definition of the red-giant loci. Anumber of stars in NGC 2506, 2420 and 2204 appear to define anasymptotic branch, the position of which differs significantly from thatpredicted by the models.Based on observations collected with the Danish 1.54-m and ESO 1-mtelescopes at the European Southern Observatory, La Silla, Chile.
| 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
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| The Orbits of 48 Globular Clusters in a Milky Way-like Barred Galaxy
The effect of a barred potential (such as the one of the Milky Way) onthe Galactic orbits of 48 globular clusters for which absolute propermotions are known is studied. The orbital characteristics are comparedwith those obtained for the case of an axisymmetric Galactic potential.Tidal radii are computed and discussed for both the better knownaxisymmetric case and that including a bar. The destruction rates due tobulge and disk shocking are calculated and compared in both Galacticpotentials.
| Empirical color transformations between SDSS photometry and other photometric systems
Aims.We present empirical color transformations between the SloanDigital Sky Survey (SDSS) ugriz photometry and the Johnson-Cousins UBVRIsystem and Becker's RGU system, respectively. Owing to the magnitude ofdata that is becoming available in the SDSS photometric system it isparticularly important to be able to convert between this new system andtraditional photometric systems. Unlike earlier publishedtransformations we based our calculations on stars actually measured bythe SDSS with the SDSS 2.5-m telescope. The photometric database of theSDSS provides in a sense a single-epoch set of "tertiary standards"covering more than one quarter of the sky. Our transformations shouldfacilitate their use to easily and reliably derive the correspondingapproximate Johnson-Cousins or RGU magnitudes. Methods: .The SDSSsurvey covers a number of areas that were previously established asstandard fields in the Johnson-Cousins system, in particular, fieldsestablished by Landolt and by Stetson. We used these overlapping fieldsto create well-photometered star samples on which our calculatedtransformations are based. For the RGU photometry we used fieldsobserved in the framework of the new Basel high-latitude field starsurvey. Results: .We calculated empirical color transformationsbetween SDSS photometry and Johnson-Cousins UBVRI and Becker's RGUsystem. For all transformations we found linear relations to besufficient. Furthermore we showed that the transformations between theJohnson-Cousins and the SDSS system have a slight dependence onmetallicity.
| Wide-Field CCD Photometry around Nine Open Clusters
In this paper we study the evolution of the core and corona of nine openclusters using the projected radial density profiles derived fromhomogeneous CCD photometric data obtained with the 105 cm Kiso Schmidttelescope. The age and galactocentric distance of the target clustersvary from 16 to 2000 Myr and 9 to 10.8 kpc, respectively. Barring Be 62,which is a young open cluster, other clusters show a uniform reddeningacross the cluster region. The reddening in Be 62 varies fromE(B-V)min=0.70 mag to E(B-V)max=1.00 mag. Thecoronae of six of the clusters in the present sample are found to beelongated; however, on the basis of the present sample it is notpossible to establish any correlation between the age and shape of thecore. The elongated core in the case of the young cluster Be 62 mayreflect the initial conditions in the parental molecular cloud. Theother results of the present study are as follows: (1) Core radiusrc and corona size rcn/cluster radiusrcl are linearly correlated. (2) The rc,rcn, and rcl are linearly correlated with thenumber of stars in that region. (3) In the age range 10-1000 Myr, thecore and corona shrink with age. (4) We find that in the galactocentricdistance range 9-10 kpc, the core and corona/cluster extent of theclusters increase with the galactocentric distance.
| 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.
| Extended Strömgren Photoelectric Photometry in NGC 752
Photoelectric photometry on the extended Strömgren system (uvbyCa)is presented for 7 giants and 21 main-sequence stars in the old opencluster NGC 752. Analysis of the hk data for the turnoff stars yields anew determination of the cluster mean metallicity. From 10 single-starmembers, [Fe/H]=-0.06+/-0.03, where the error quoted is the standarderror of the mean and the Hyades abundance is set at [Fe/H]=+0.12. Thisresult is unchanged if all 20 stars within the limits of the hkmetallicity calibration are included. The derived [Fe/H] is in excellentagreement with past estimates, using properly zeroed m1 data,transformed moderate-dispersion spectroscopy, and recent high-dispersionspectroscopy.
| UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
| Galactic Orbits of Globular Clusters in a Barred Galaxy
We study the effect of a bar in the galactic orbits of forty-fiveglobular clusters whose absolute proper motions are known. The orbitalcharacteristics of the orbits are compared with those obtained for thecase of an axisymmetric galactic potential. Tidal radii are computed anddiscussed for both cases.
| vbyCaHβ CCD Photometry of Clusters. VI. The Metal-deficient Open Cluster NGC 2420
CCD photometry on the intermediate-band vbyCaHβ system is presentedfor the metal-deficient open cluster NGC 2420. Restricting the data toprobable single members of the cluster using the CMD and the photometricindices alone generates a sample of 106 stars at the cluster turnoff.The average E(b-y)=0.035+/-0.003 (s.e.m.) or E(B-V)=0.050+/-0.004(s.e.m.), where the errors refer to internal errors alone. With thisreddening, [Fe/H] is derived from both m1 and hk, using b-yand Hβ as the temperature index. The agreement among the fourapproaches is reasonable, leading to a final weighted average of[Fe/H]=-0.37+/-0.05 (s.e.m.) for the cluster, on a scale where theHyades has [Fe/H]=+0.12. When combined with the abundances from DDOphotometry and from recalibrated low-resolution spectroscopy, the meanmetallicity becomes [Fe/H]=-0.32+/-0.03. It is also demonstrated thatthe average cluster abundances based on either DDO data orlow-resolution spectroscopy are consistently reliable to 0.05 dex orbetter, contrary to published attempts to establish an open clustermetallicity scale using simplistic offset corrections among differentsurveys.
| Open clusters in the Third Galactic Quadrant. II. The intermediate age open clusters NGC 2425 and NGC 2635
We analyse CCD broad band (UBV(RI)_C) photometric data obtained in thefields of the poorly studied open clusters NGC 2425and NGC 2635. Both clusters are found to be ofintermediate age thus increasing the population of open clusters knownto be of the age of, or older than, the Hyades. More explicitly, we findthat NGC 2425 is a 2.2 Gyr old cluster, probably ofsolar metallicity, located at 3.5 kpc from the Sun. NGC2635 is a Hyades age (600 Myr) cluster located at a distanceof 4.0 kpc from the Sun. Its Colour Magnitude Diagram reveals that it isextremely metal poor for its age and position, thus making it a veryinteresting object in the context of Galactic Disk chemical evolutionmodels.
| Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters
New photometric data in the Washington system are presented for redgiant candidates in NGC 1817 and 2251, two open clusters located towardsthe Galactic anticentre direction. In the case of NGC 2251, theWashington data are supplemented with new UBV and David DunlapObservatory (DDO) photoelectric photometry. Published radial velocitiesare used to separate field stars from cluster giants. The photometricdata yield an effective temperature and metal abundance for each clustermember. Five independent Washington abundance indices yield meanmetallicities of [Fe/H]= 0.25 +/- 0.04 for NGC 1817 and 2251,respectively. From combined BV and DDO data, we also derive E(B-V) =0.21 +/- 0.03 and [Fe/H]DDO=-0.14 +/- 0.05 for NGC 2251. Bothobjects are then found to be on the metal-poor side of the distributionof open clusters, their metallicities being compatible with theexistence of a radial abundance gradient in the disc. Using the WEBDAOpen Cluster data base and the available literature, we re-examined theoverall properties of a sample of 30 clusters located towards theGalactic anticentre with the distances, ages and metallicitiesavailable. This cluster sample presents no evidence of an abundancegradient perpendicular to the Galactic plane, nor is an age-metallicityrelation found. However, a radial abundance gradient of -0.093 dexkpc-1 is derived over a Galactocentric distance of 14 kpc, agradient which is in keeping with most recent determinations. This valuepractically does not change when all clusters with basic parametersknown up to this date are considered.
| 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.
| Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic openclusters are studies in this work, in which 27 Galactic open clusters ofages >=1 Gyr are selected as the working sample. Based on thephotometric observations of these open clusters, a synthetic integratedspectrum has been made for the stellar population of each cluster. Theeffects of blue straggler (BS) stars on the conventional simple stellarpopulation (SSP) model are analyzed on an individual cluster basis. Itis shown that the BSs, whose positions in the color-magnitude diagramscannot be predicted by the current single-star evolution theory, requiresignificant modifications to the integrated properties of theoreticalSSP model. The synthesized integrated spectral energy distributions(ISEDs) of our sample clusters are dramatically different from those ofSSPs based on an isochrone only. The BS-corrected ISEDs of stellarpopulations show systematic enhancements toward shorter wavelengths inthe spectra. When measured with broadband colors in unresolvableconditions, the age of a stellar population can be seriouslyunderestimated by the conventional SSP model. Therefore, considering thecommon existence of BS components in real stellar populations, we shouldexpect considerable alterations of the conventional ISEDs when we applythe technique of evolutionary population synthesis to more complicatedstellar systems.
| Radial velocities and membership of stars in the old, distant open cluster Berkeley 29
Multi slit spectroscopy at the Telescopio Nazionale Galileo was employedto measure radial velocities for 20 stars in the direction of the oldopen cluster Berkeley 29, the farthest known in our Galaxy. Membershipinformation was derived for stars all along the red giant branch, inparticular near its tip, and on the red clump. The sample of bona fidecluster members was used to revise the cluster distance to ˜ 15kpc, on the basis of an empirical comparison with the red clump in openclusters with known distances. A metallicity [Fe/H] = -0.74 ±0.18 was also estimated using the colours of spectroscopically confirmedred giant stars.Based on observations made with the Italian Telescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Centro Galileo Galileiof the INAF (Istituto Nazionale di Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
| Seven-Color Photometry of the Open Cluster NGC 1647 Area
The area of the open cluster NGC 1647 in Taurus is investigated by CCDphotometry in the Vilnius seven-color system. Magnitudes and colorindices are determined for 433 stars down to V = 15.0 mag in the 45arcmin diameter area. For 252 of them photometric spectral andluminosity classes, interstellar reddenings, extinctions and distancesare obtained. According to the CDS WEBDA database, 89 of them have ahigh cluster membership probability. Their mean distance from the Sunis 555±74 pc, excluding four stars which seem to be field stars.The main sequence starts at spectral class B7 V which corresponds to acluster age of about 150 million years. Cluster members show adifferential interstellar extinction ranging from 0.8 to 1.8 mag. Themean extinction of the cluster stars is 1.12±0.25 mag.Interstellar extinction in the area is dominated by the Taurus darkcloud complex at 160 pc. Color excesses of individual stars correlatewell with the 100 μm dust thermal emission intensity. The clustershape is investigated by counting stars down to K=15.6 mag and is foundto be elongated in the direction roughly perpendicular to the Milky Way,with the flattening 0.4.
| Y2 Isochrones with an Improved Core Overshoot Treatment
Convective core overshoot affects stellar evolution rates and the datingof stellar populations. In this paper, we provide a patch to theY2 isochrones with an improved treatment of convective coreovershoot. The new tracks cover the transition mass range from noconvective core to a fully developed convective core. We compare theimproved isochrones to CMDs of a few well-observed open star clusters inthe Galaxy and the Large Magellanic Cloud. Finally, we discuss futureprospects for improving the treatment of core overshoot with the help ofasteroseismology.
| vbyCaHβ CCD Photometry of Open Clusters: NGC 2420 and NGC 6791
As part of an ongoing program to refine the fundamental characteristicsof open clusters for use in studying stellar and galactic evolution, CCDintermediate and narrow-band photometry has been obtained of thenorthern old open clusters NGC 2420 and NGC 6791 using the 0.9m WIYNtelescope at KPNO. For NGC 2420, the critical question is the clustermetallicity. Long considered to be moderately metal-poor relative to thesun ([Fe/H] ˜ -0.4), recent photometric observations have claimeda solar abundance for the object. By isolating likely cluster membersusing a combination of spatial distribution and photometric indices, asample of over 100 highly probable members with high precisionphotometry near the cluster turnoff has been generated. From Hβphotometric analysis, the cluster reddening is well defined at E(B-V) =0.054, with a standard error of the mean of 0.004 mag. Adopting thisreddening, one can derive metallicity from both the m1 and hkindices, leading to a weighted average of [Fe/H] = -0.35 with anuncertainty < 0.05 dex. These values are totally consistent withprevious cluster studies and exclude the possibility that NGC 2420 hassolar metallicity. Similar analysis is underway for NGC 6791 and will bereported on at the meeting.We acknowledge the generosity of the University of Kansas GeneralResearch Fund and the WIYN Open Cluster Survey consortium.
| Morphology of Galactic Open Clusters
We analyzed the shapes of Galactic open clusters by the star-countingtechnique with the Two Micron All Sky Survey star catalog database.Morphological parameters such as the ellipticity and size have beenderived via stellar density distribution, weighed by clusteringprobability. We find that most star clusters are elongated, even for theyoungest star clusters of a few million years old, which are locatednear the Galactic disk. The shapes of young star clusters must reflectthe conditions in the parental molecular clouds and during the clusterformation process. As an open cluster ages, stellar dynamics cause theinner part of the cluster to circularize, but the overall radius getslarger and the stellar density becomes sparser. We discuss how theinternal relaxation process competes with Galactic external perturbationduring cluster evolution.
| 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.
| A Large Stellar Evolution Database for Population Synthesis Studies. I. Scaled Solar Models and Isochrones
We present a large and updated stellar evolution database for low-,intermediate-, and high-mass stars in a wide metallicity range, suitablefor studying Galactic and extragalactic simple and composite stellarpopulations using population synthesis techniques. The stellar massrange is between ~0.5 and 10 Msolar with a fine mass spacing.The metallicity [Fe/H] comprises 10 values ranging from -2.27 to 0.40,with a scaled solar metal distribution. The initial He mass fractionranges from Y=0.245, for the more metal-poor composition, up to 0.303for the more metal-rich one, with ΔY/ΔZ~1.4. For eachadopted chemical composition, the evolutionary models have been computedwithout (canonical models) and with overshooting from the Schwarzschildboundary of the convective cores during the central H-burning phase.Semiconvection is included in the treatment of core convection duringthe He-burning phase. The whole set of evolutionary models can be usedto compute isochrones in a wide age range, from ~30 Myr to ~15 Gyr. Bothevolutionary models and isochrones are available in severalobservational planes, employing an updated set of bolometric correctionsand color-Teff relations computed for this project. Thenumber of points along the models and the resulting isochrones isselected in such a way that interpolation for intermediate metallicitiesnot contained in the grid is straightforward; a simple quadraticinterpolation produces results of sufficient accuracy for populationsynthesis applications.We compare our isochrones with results from aseries of widely used stellar evolution databases and perform someempirical tests for the reliability of our models. Since this work isdevoted to scaled solar chemical compositions, we focus our attention onthe Galactic disk stellar populations, employing multicolor photometryof unevolved field main-sequence stars with precise Hipparcosparallaxes, well-studied open clusters, and one eclipsing binary systemwith precise measurements of masses, radii, and [Fe/H] of bothcomponents. We find that the predicted metallicity dependence of thelocation of the lower, unevolved main sequence in the color magnitudediagram (CMD) appears in satisfactory agreement with empirical data.When comparing our models with CMDs of selected, well-studied, openclusters, once again we were able to properly match the whole observedevolutionary sequences by assuming cluster distance and reddeningestimates in satisfactory agreement with empirical evaluations of thesequantities. In general, models including overshooting during theH-burning phase provide a better match to the observations, at least forages below ~4 Gyr. At [Fe/H] around solar and higher ages (i.e., smallerconvective cores) before the onset of radiative cores, the selectedefficiency of core overshooting may be too high in our model, as well asin various other models in the literature. Since we also providecanonical models, the reader is strongly encouraged to always comparethe results from both sets in this critical age range.
| 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.
| 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.
| Abundances of Red Giants in the Old Open Cluster Collinder 261
We present an analysis of high-dispersion echelle spectra of four giantstars in the 8 Gyr open cluster Cr 261 obtained with the Cerro TololoInter-American Observatory's 4 m telescope. Abundances were determinedrelative to the bright, slightly metal-poor disk giant Arcturus. Cr 261is found to have a mean [Fe/H]=-0.22+/-0.05 (mean error). Oxygenabundances, determined from the forbidden [O I] lines, show roughlysolar or slightly depleted abundance ratios. The α-elements Mg,Ca, and Ti also show scaled solar abundance ratios, with suggestionsthat the Si abundance is slightly enhanced. The odd-Z elements Na and Alare significantly enhanced relative to scaled solar abundances. Theabundance results for Cr 261 are compared with those fromhigh-dispersion studies of other old open clusters in the literature,which, overall, show similar behavior in all these elements. None of theabundance ratios show significant trends with overall cluster [Fe/H] orwith cluster age.
| Using Optical and Near-Infrared Photometry to Test MACHO Lens Candidates
We obtained new VLT/ISAAC H-band observations for five MACHO LMC sourcestars and adjacent LMC field regions. After combining our near-IRphotometry with Hubble Space Telescope/PC BVRI optical photometry, wecompared the MACHO objects to the adjacent field stars in a variety ofcolor-magnitude and color-color diagrams. These diagnostic diagrams werechosen to be sensitive to our hypothesis that at least some of the MACHOlenses were foreground Galactic disk or thick-disk M dwarfs. For thefive lensed objects we studied, our hypothesis could be ruled out formain-sequence lens masses >~0.1 Msolar for distances outto 4 kpc. On the other hand, the fact that LMC MACHO 5, an object not inour study, has been recently found to have just such a foreground lenshighlights that the remainder of the LMC MACHO objects should besearched for the signature of their lenses using our photometrictechnique or via near-IR spectroscopy. We also constructed diagnosticcolor-color diagrams sensitive to determining reddening for theindividual MACHO source stars and found that these five objects did notshow evidence for significant additional reddening. At least these fiveMACHO objects are thus also inconsistent with the LMC self-lensinghypothesis.Based on data obtained at ESO (VLT/UT1) under project 66.B-0326.
| An empirical test of the theoretical population corrections to the red clump absolute magnitude
The mean absolute magnitude of the local red clump (RC),MRCλ, is a very well-determined quantityowing to the availability of accurate Hipparcos parallaxes for severalhundred RC stars, potentially allowing it to be used as an accurateextragalactic distance indicator. Theoretical models predict that the RCmean magnitude is dependent on both age and metallicity and,furthermore, that these dependences are non-linear. This suggests that apopulation correction, ΔMRCλ, basedon the star formation rate (SFR) and age-metallicity relation (AMR) ofthe system in question, should be applied to the local RC magnitudebefore it can be compared to the RC in any other system in order to makea meaningful distance determination.Using a sample of eight Galactic open clusters and the Galactic globularcluster 47 Tuc, we determine the cluster distances, and hence the RCabsolute magnitude in V, I and K, by applying our empiricalmain-sequence (MS) fitting method, which utilizes a large sample oflocal field dwarfs with accurate Hipparcos parallaxes. The nine clustershave metallicities in the range -0.7 <=[Fe/H]<=+0.02 and ages from1 to 11 Gyr, enabling us to make a quantitative assessment of the ageand metallicity dependences of ΔMRCλpredicted by the recent theoretical models of Girardi & Salaris andSalaris & Girardi. We find excellent agreement between the empiricaldata and the models in all three passbands, with no statisticallysignificant trends or offsets, thus fully confirming the applicabilityof the models to single-age, single-metallicity stellar populations.Since, from the models, ΔMRCλ is acomplicated function of both metallicity and age, if this method is usedto derive distances to composite populations, it is essential to have anaccurate assessment of the SFR and AMR of the system in question, iferrors of several tenths of a magnitude are to be avoided. Using recentdeterminations of the SFR and AMR for four systems - the Large and SmallMagellanic Clouds, Carina and the solar neighbourhood - we examine thequantity IRCobs-KRCobs,which is the difference between the mean magnitude of the RC in the Iband and the K band. Comparing the theoretical predictions with the mostrecent observational data, we find complete agreement between theobservations and the models, thus confirming even further theapplicability of the population corrections predicted from theory.
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