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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
| Canis Major the trusty Dog.
Not Available
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| Morphological analysis of open clusters' propertiesII. Relationships projected onto the galactic plane
A morphological analysis study of open clusters' properties has beenachieved for a sample of 160 UBVCCD open star clusters of approximately128,000 stars near the galactic plane. The data was obtained and reducedfrom using the same reduction procedures, which makes this catalogue thelargest homogeneous source of open clusters' parameters.
| Population effects on the red giant clump absolute magnitude: the K band
We present a detailed analysis of the behaviour of the red clump K-bandabsolute magnitude (MRCK) in simple and compositestellar populations, in light of its use as a standard candle fordistance determinations. The advantage of usingMRCK, following recent empirical calibrations ofits value for the solar neighbourhood, arises from its very lowsensitivity to extinction by interstellar dust. We show that, as in thecase of the V- and I-band results, MRCK is acomplicated function of the stellar metallicity Z and age t. In general,MRCK is more sensitive to t and Z thanMRCI, for high t and low Z. Moreover, for agesabove ~1.5 Gyr, MRCK decreases with increasing Z,i.e. the opposite behaviour with respect to MRCVand MRCI. We provide data and equations that allowthe determination of the K-band population correctionΔMRCK (the difference between the red clumpbrightness in the solar neighbourhood and in the population underscrutiny) for any generic stellar population. These data complement theresults presented by Girardi & Salaris for the V- and I-band. Weshow how data from galactic open clusters consistently support ourpredicted ΔMRCV ,ΔMRCI and ΔMRCKvalues. Multiband VIK population corrections for various galaxy systemsare provided. They can be used in conjunction with the method devised byAlves et al., in order to derive simultaneously reddening and distancefrom the use of VIK observations of red clump stars. We have positivelytested this technique on the Galactic globular cluster 47 Tuc, for whichboth an empirical parallax-based main-sequence-fitting distance andreddening estimates exist. We have also studied the case of using only Vand I photometry, recovering consistent results for both reddening anddistance. Application of this method to an OGLE-II field, and theresults by Alves et al., confirm a Large Magellanic Cloud distancemodulus of about 18.50, in agreement with the Hubble Space Telescopeextragalactic distance scale zero-point.
| 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.
| The intermediate-age open cluster NGC 2112
We report on BVI charge-coupled device photometry of a field centred onthe region of the intermediate-age open cluster NGC 2112 down to V= 21.Owing to the smaller field coverage, we are able to limit the effect offield star contamination that, in the past, hampered precisedeterminations of the cluster age and distance. In this way, we provideupdated estimates of the fundamental parameters for NGC 2112. Havingextended the photometry to the I passband, we are able to construct acolour-colour diagram, from which we infer a reddening E(B-V) = 0.63 +/-0.14 mag. The comparison of the colour-magnitude diagram withtheoretical isochrones leads to a distance of 850 +/- 100 pc and an ageof 2.0 +/- 0.3 Gyr. While the distance is in agreement with previousdeterminations, the age turns out to be much better constrained andsignificantly lower than previous estimates.
| Integrated photometric characteristics of galactic open star clusters
Integrated UBVRI photometric parameters of 140 galactic open clustershave been computed. Integrated I(V-R)0 and I(V-I)0colours as well as integrated parameters for 71 star clusters have beenobtained for the first time. These, in combination with published data,altogether 352 objects, are used to study the integrated photometriccharacteristics of the galactic open clusters. The I(MV)values range from -9.0 to -1.0 mag corresponding to a range in totalmass of the star clusters from ~ 25 to 4*E4 Msun.The integrated colours have a relatively narrow range, e.g., I(B-V){_0}varies from -0.4 to 1.2 mag. The scatter in integrated colours at agiven integrated magnitude can be understood in terms of differences infraction of red giants/supergiants in the clusters. The observedintegrated magnitudes and colours agree with the synthetic ones, exceptthe dependences of I(V-R)0 and I(V-I)0 colours forclusters younger than ~ 100 Myrs and also of the integrated magnitudesof oldest clusters. The large sample provides the most accurate agedependence of integrated magnitudes and colours determined so far. Theluminosity function of the I(MV) has a peak around -3.5 magand its slope indicates that only ~ 1% of the open clusters in thegalactic disc are brighter than I(MV)=-11 mag. No variationhas been found of integrated magnitude with galactocentric distance andmetallicity.
| The intermediate-age open cluster NGC 2158
We report on UBVRI CCD photometry of two overlapping fields in theregion of the intermediate-age open cluster NGC 2158 down to V =21. Byanalysing colour-colour (CC) and colour-magnitude diagrams (CMDs) weinfer a reddening E (B -V )=0.55+/-0.10, a distance of 3600+/-400pc, andan age of about 2Gyr. Synthetic CMDs constructed with these parameters(but fixing E (B -V )=0.60 and [Fe/H]=-0.60), and including binaries,field contamination and photometric errors, yield a good description ofthe observed CMD. The elongated shape of the clump of red giants in theCMD is interpreted as resulting from a differential reddening of aboutΔE (B -V )=0.06 across the cluster, in the direction perpendicularto the Galactic plane. NGC 2158 turns out to be an intermediate-age opencluster with an anomalously low metal content. The combination of theseparameters, together with the analysis of the cluster orbit, suggeststhat the cluster belongs to the old thin disc population.
| WIYN Open Cluster Study. X. The K-Band Magnitude of the Red Clump as a Distance Indicator
In an effort to improve the utility of the helium-burning red clumpluminosity as a distance indicator, we explore the sensitivity of theK-band red clump absolute magnitude [MK(RC)] to metallicityand age. We rely upon JK photometry for 14 open clusters and twoglobular clusters from the Second Incremental Data Release of the TwoMicron All Sky Survey (2MASS) Point Source Catalog. The distances,metallicities, and ages of the open clusters are all on an internallyconsistent system, while the K(RC) values are measured from the 2MASSdata. For clusters younger than ~2 Gyr, MK(RC) is insensitiveto metallicity but shows a dependence on age. In contrast, for clustersolder than ~2 Gyr, MK(RC) is influenced primarily by themetallicity of the population and shows little or no dependence on theage. Theoretical red clump models based on the formalism of Girardi etal. reinforce this finding. Over comparable metallicity and age ranges,our average MK(RC) value is in accord with that based onsolar neighborhood red clump stars with Hipparcos parallaxes. Lastly, wecompute the distance to the open cluster NGC 2158 using our red clumpcalibration. Adopting an age of 1.6+/-0.2 Gyr and [Fe/H]=-0.24+/-0.06,our calibration yields a distance of (m-M)V=14.38+/-0.09.
| 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.
| Mapping the interstellar dust with near-infrared observations: An optimized multi-band technique
We generalize the technique of Lada et al. (\cite{1994ApJ...429..694L})to map dust column density through a molecular cloud (NICE) to anoptimized multi-band technique (NICER) that can be applied to anymulti-band survey of molecular clouds. We present a first application toa ~ 625 deg2 subset of the Two Micron All Sky Survey (2MASS)data and show that when compared to NICE, the optimized NICER technique(i) achieves the same extinction peak values, (ii) improves the noisevariance of the map by a factor of 2 and (iii) is able to reach 3 sigmadust extinction measurements as low as AV =~ 0.5 magnitudes,better than or equivalent to classical optical star count techniques andbelow the threshold column density for the formation of CO, thebrightest H2 tracer in radio-spectroscopy techniques. Theapplication of the NICER techniques to near-infrared data obtained witha 8 meter-class telescope with a state-of-the-art NIR camera, such asthe VLT-ISAAC combination, will be able to achieve dynamic ranges frombelow 1021 protons cm-2 to over 1023protons cm-2 (AV in the range [0.3, 60]) andspatial resolutions <10'', making use of a single and straightforwardcolumn density tracer, extinction by interstellar dust.
| Morphological analysis of open clusters' propertiesI. Properties' estimations
A sample of 160 UBVCCD observations of open star clusters near thegalactic plane has been studied, and a catalogue of their propertiesobtained. The main photometrical properties have been re-estimated selfconsistently and the results have been compared with those of Lynga[Lynga, G., 1987. Catalog of Open Cluster Data, 5th Edition, StellarData Centers, Observatoire de Strasbourg, France].
| An optical and near IR study of the old open cluster NGC 2141
We report CCD optical (B and V passbands) and near IR (J and K bands)observations in the region of the old open cluster NGC 2141. Bycombining the two sets of photometry (500 stars in common) we derive newestimates of the cluster's fundamental parameters. We confirm that thecluster is 2.5 Gyrs old, but, with respect to previous investigations,we obtain a slightly larger reddening (E(B-V) = 0.40), and a slightlyshorter distance (3.8 kpc) from the Sun. Finally, we present theLuminosity Function (LF) in the V band, which is another age indicator.We provide a good fit for the age range inferred from isochrones byassuming the Kroupa et al. (\cite{r18}) IMF up to MV = 5.0.We interpret the disagreement at fainter magnitudes as evidence of masssegregation. Based on observations taken at ESO La Silla and TIRGO.Table~2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/372/879
| Exploring Halo Substructure with Giant Stars. I. Survey Description and Calibration of the Photometric Search Technique
We have begun a survey of the structure of the Milky Way halo, as wellas the halos of other Local Group galaxies, as traced by theirconstituent giant stars. These giant stars are identified vialarge-area, CCD photometric campaigns. Here we present the basis for ourphotometric search method, which relies on the gravity sensitivity ofthe Mg I triplet+MgH features near 5150 Å in F-K stars, and whichis sensed by the flux in the intermediate-band DDO51 filter. Ourtechnique is a simplified variant of the combined Washington/DDO51four-filter technique described by Geisler, which we modify for thespecific purpose of efficiently identifying distant giant stars forfollow-up spectroscopic study: We show here that for most stars theWashington T1-T2 color is correlated monotonicallywith the Washington M-T2 color with relatively low scatter;for the purposes of our survey, this correlation obviates the need toimage in the T1 filter, as originally proposed by Geisler. Tocalibrate our (M-T2, M-DDO51) diagram as a means todiscriminate field giant stars from nearby dwarfs, we utilize newphotometry of the main sequences of the open clusters NGC 3680 and NGC2477 and the red giant branches of the clusters NGC 3680, Melotte 66,and ω Centauri, supplemented with data on field stars, globularclusters and open clusters by Doug Geisler and collaborators. Bycombining the data on stars from different clusters, and by takingadvantage of the wide abundance spread within ω Centauri, weverify the primary dependence of the M-DDO51 color on luminosity anddemonstrate the secondary sensitivity to metallicity among giant stars.Our empirical results are found to be generally consistent with thosefrom analysis of synthetic spectra by Paltoglou & Bell. Finally, weprovide conversion formulae from the (M, M-T2) system to the(V, V-I) system, corresponding reddening laws, as well as empirical redgiant branch curves from ω Centauri stars for use in derivingphotometric parallaxes for giant stars of various metallicities (butequivalent ages) to those of ω Centauri giants.
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