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K-band magnitude of the red clump as a distance indicator
We have investigated how the K-band magnitude of the red clump [M_K(RC)]depends on age and metallicity, using 2MASS infrared data for a sampleof 24 open clusters with known distances. We show that a constant valueof M_K(RC)=-1.57 ± 0.05 is a reasonable assumption to use indistance determinations for clusters with metallicity between -0.5 and+0.4 dex and age between 108.5 and 109.9 years.Figures 8 and 9 are available in electronic form at http://www.aanda.org
| Caroline Herschel's catalogue of nebulae
Not Available
| Inverting Color-Magnitude Diagrams to Access Precise Star Cluster Parameters: A Bayesian Approach
We demonstrate a new Bayesian technique to invert color-magnitudediagrams of main-sequence and white dwarf stars to reveal the underlyingcluster properties of age, distance, metallicity, and line-of-sightabsorption, as well as individual stellar masses. The advantages ourtechnique has over traditional analyses of color-magnitude diagrams areobjectivity, precision, and explicit dependence on prior knowledge ofcluster parameters. Within the confines of a given set of often-usedmodels of stellar evolution, a single mapping of initial to finalmasses, and white dwarf cooling, and assuming photometric errors thatone could reasonably achieve with the Hubble Space Telescope, ourtechnique yields exceptional precision for even modest numbers ofcluster stars. For clusters with 50-400 members and one to a few dozenwhite dwarfs, we find typical internal errors ofσ([Fe/H])<=0.03 dex, σ(m-MV)<=0.02 mag, andσ(AV)<=0.01 mag. We derive cluster white dwarf ageswith internal errors of typically only 10% for clusters with only threewhite dwarfs and almost always <=5% with 10 white dwarfs. Theseexceptional precisions will allow us to test white dwarf cooling modelsand standard stellar evolution models through observations of whitedwarfs in open and globular clusters.WIYN Open Cluster Study XXIII. The WIYN Observatory is a joint facilityof the University of Wisconsin-Madison, Indiana University, YaleUniversity, and the National Optical Astronomy Observatory.
| 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.
| 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.
| Caroline Herschel as observer
Not Available
| 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.
| 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.
| Deepsky delights.
Not Available
| Astrophysical supplements to the ASCC-2.5. II. Membership probabilities in 520 Galactic open cluster sky areas
We present a catalogue (CSOCA ) of stars residing in 520 Galactic opencluster sky areas which is the result of the kinematic (proper motion)and photometric member selection of stars listed in the homogeneousAll-sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). We describethe structure and contents of the catalogue, the selection procedureapplied, and the proper motion and photometric membership constraintsadopted. In every cluster area the CSOCA contains the complete list ofthe ASCC-2.5 stars regardless of their membership probability. Forevery star the CSOCA includes accurate J2000 equatorial coordinates,proper motions in the Hipparcos system, BV photometric data in theJohnson system, proper motion and photometric membership probabilities,as well as angular distances from the cluster centers for about 166 000ASCC-2.5 stars. If available, trigonometric parallaxes, spectral types,multiplicity and variability flags from the ASCC-2.5, and radialvelocities with their errors from the Catalogue of Radial Velocities ofGalactic Stars with high precision Astrometric Data (CRVAD) are alsogiven.
| 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.
| 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.
| The Subsystem of Open Clusters in the Post-Hipparcos Era: Cluster Structural Parameters and Proper Motions
The wide neighborhoods of 401 open clusters are analyzed using themodern, high-precision, homogeneous ASCC-2.5 all-sky catalog. More than28000 possible cluster members (including about 12500 most probablemembers) are identified using kinematic and photometric criteria. Starcounts with the ASCC-2.5 and USNO-A2.0 catalogs are used to determinethe angular and linear radii of the cluster cores and coronas, whichexceed the previously published values by factors of two to three. Thesegregation (differing central concentration) of member stars bymagnitude is observed. The mean proper motions are determined directlyin the Hipparcos system for 401 clusters, for 183 of them for the firsttime. The heliocentric distances of 118 clusters are determined for thefirst time based on color-magnitude diagrams for their identifiedmembers.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Metallicities of Old Open Clusters
We present radial velocities and metallicities for a sample of 39 openclusters with ages greater than about 700 million years. For 24 clustersnew moderate-resolution spectroscopic data obtained with multiobjectspectrographs on the Kitt Peak National Observatory and the Cerro TololoInter-American Observatory 4 m telescopes are used to determine radialvelocities and mean cluster metallicities. These new results arecombined with data published previously by Friel & Janes to providea sample of 459 giants in 39 old open clusters, which are used toinvestigate radial abundance gradients in the Galactic disk. Based on anupdated abundance calibration of spectroscopic indices measuring Fe andFe-peak element blends, this larger sample yields an abundance gradientof -0.06+/-0.01 dex kpc-1 over a range in Galactocentricradius of 7 to 16 kpc. There is a slight suggestion of a steepening ofthe abundance gradient with increasing cluster age in this sample, butthe significance of the result is limited by the restricted distancerange for the youngest clusters. The clusters show no correlation ofmetallicity with age in the solar neighborhood. Consistent with theevidence for a steepening of the gradient with age, the clusters in theouter disk beyond 10 kpc show a suggestion at the 1.5 σ level of adependence of metallicity on age.
| Proper motions of open clusters based on the TYCHO2 Catalogue. II. Clusters farther than 1 kpc
We determined the mean absolute proper motion of 94 open clusterssituated farther than 1 kpc from the Sun. The results are derived fromthe stellar proper motion data given in the Tycho2 Catalogue. The meanproper motion of the clusters and membership probability of individualstars were obtained from the proper motion data by applying thestatistical method proposed by Sanders (\cite{Sanders1971}). Themeasurements made use of a large number of stars, usually several tens,for each cluster. The total number of stars investigated in the fieldsof the clusters is 4864 of which 2021 were considered members. For 55clusters, this is the first determination of the proper motion. Based onobservations of the ESA Hipparcos satellite. Tables 1 to 95 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/388/168
| 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.
| A study of spatial structure of galactic open star clusters
In order to study the relation between the core and corona in galacticstar clusters, the spatial structure of 38 rich open star clusters hasbeen studied using radial density profiles derived from the photometricdata of the Digital Sky Survey. The shape of the radial density profileindicates that the corona, most probably, is the outer region around thecluster. It can exist from the very beginning of the cluster formationand dynamical evolution is not the reason for its occurrence. The studydoes not find any relation between cluster size and age but indicatesthat the clusters with galacto-centric distances >9.5 kpc have largersizes. Further, we find that the average value of the core radius is1.3+/- 0.7 pc and that of annular width of the corona is 5.6+/- 1.9 pc,while average values of densities of cluster members in the core andcorona are 15.4+/- 9.9 star/pc2 and 1.6+/- 0.99star/pc2 respectively. Average field star contaminations inthe core and corona are ~ 35% and 80% respectively. In spite of smallerdensities in the coronal region, it contains ~ 75% of the clustermembers due to its larger area in comparison to the core region. Thisclearly demonstrates the importance of the coronal region in studiesdealing with the entire stellar contents of open star clusters as wellas their dynamical evolution. In contrast to the cluster cores, thestructure of coronal regions differs significantly from one cluster toother.
| 12C/13C in Metal-poor Field Halo Giants
We have estimated 12C/13C in 15 metal-poor(-2.4<=[Fe/H]<=-1.0) field halo giant stars from spectra of the13CO v=3-1 and v=2-0 band heads and surrounding12CO and 13CO R-branch lines. Our isotope ratiosare consistent with previous measurements for stars in our sample with12C/13C determined either from the infraredfirst-overtone bands of CO or from optical G-band spectra of CH and redsystem bands of CN. We have also compiled carbon isotope ratios from theliterature for a much larger sample of field and cluster red giantbranch (RGB) stars spanning a wide range of metallicities(-2.4<=[Fe/H]<=solar). Combining these data, we confirm thedecline of the isotope ratio as stars evolve up the RGB and we haveidentified a trend toward higher levels of mixing in more metal-poorstars. Standard RGB first dredge-up models do not predict the carbonisotope ratios that we observe in the more evolved (higher luminosity)metal-poor stars, but more recent models that account for other mixingmechanisms can explain these data; even for very metal-poor stars suchas those that we have observed in the Galactic halo.
| Statistics and supermetallicity: The metallicity of NGC 6791
For the old galactic cluster NGC 6791, Peterson & Green(\cite{pg98a}) and Chaboyer et al. (\cite{cgl99}) have found that [Fe/H]~ +0.4 dex. A second look at that conclusion is taken in this paper.Zero-point problems are reviewed for a high-dispersion analysis done byPeterson & Green, and it is found that accidental errors have notbeen determined rigorously for the results of that analysis. It is alsonoted that in a color-magnitude analysis performed by Chaboyer et al.,the important metallicity range between 0.0 and +0.3 dex is not exploredand hence is not ruled out. Moreover, that analysis does not yieldstatistically rigorous results, and it appears that such results may notbe produced in color-magnitude analysis of clusters in general. Resultsin the two cited papers and elsewhere are re-evaluated statistically,with an allowance being made for uncertainty in the cluster reddening.Apparently the best that can be said at present is that the clustermetallicity lies in the range from +0.16 to +0.44 dex. This conclusionis stressed by reviewing the immaturity of the underlying data base. Thepremature conclusion for a high metallicity turns out to be due largelyto neglect of accidental errors, though a tendency to ascribe too muchweight to high derived metallicities may also play a role.
| 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.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Chemical abundances in seven red giants of NGC 2360 and NGC 2447
Chemical abundances of about fifteen elements from oxygen to europiumare measured in seven red giants of the two open clusters NGC 2360 andNGC 2447. The effective temperatures of the giants are determinedspectroscopically by taking advantage of their known masses (~ 2Msolar in NGC 2360 and ~ 3 Msolar in NGC 2447) andbolometric magnitudes. The average iron abundances we obtain for the twoclusters are [Fe/H]=0.07 for NGC 2360 and [Fe/H]=0.03 for NGC 2447.Evolutionary stellar model calculations are performed in the mass range1 - 4 Msolar in order to analyze the surface Na and Oabundances predicted after the first dredge-up. The sodium abundanceshows a well defined correlation with stellar mass in the 2 - 3Msolar range. The agreement between our Na abundancedeterminations in NGC 2360 and our model predictions at 2Msolar is very good. In contrast, the overabundance in one ofthe three stars in NGC 2447 exceeds that predicted at 3Msolar by ~ 0.08 dex, which is significant compared to theobservational error bars. The effects of core overshooting, convectionprescription, metallicity and nuclear reaction rates on the Na surfacepredictions of our models are investigated. An oxygen deficiencyrelative to iron by 0.2 dex is measured in our stars, in disagreementwith our model predictions. Assuming that the Sun is 0.1-0.3 dexenriched in oxygen relative to neighbor stars could explain thediscrepancy. Based on observations collected at the European SouthernObservatory, La Silla, Chile (period 56) Table 3 is available only inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Foreground and background dust in star cluster directions
This paper compares reddening values E(B-V) derived from the stellarcontent of 103 old open clusters and 147 globular clusters of the MilkyWay with those derived from DIRBE/IRAS 100 mu m dust emission in thesame directions. Star clusters at |b|> 20deg showcomparable reddening values between the two methods, in agreement withthe fact that most of them are located beyond the disk dust layer. Forvery low galactic latitude lines of sight, differences occur in thesense that DIRBE/IRAS reddening values can be substantially larger,suggesting effects due to the depth distribution of the dust. Thedifferences appear to arise from dust in the background of the clustersconsistent with a dust layer where important extinction occurs up todistances from the Plane of ~ 300 pc. For 3 % of the sample asignificant background dust contribution might be explained by higherdust clouds. We find evidence that the Milky Way dust lane and higherdust clouds are similar to those of several edge-on spiral galaxiesrecently studied in detail by means of CCD imaging.
| The STACC Open Cluster Target List
Observations of variable stars offer a potential to test stellarstructure and evolution. The observations can be either of single,isolated stars, or of variable stars in clusters. The STACC group(Frandsen 1992) has for the last several years searched for openclusters with a population of delta Scuti stars. To make this searchmore efficient, we have produced a target list with a number ofpromising open clusters. The list includes parameters, finding charts,Colour-Magnitude diagrams (CM diagrams) and references for the clusters.This target list is presented here, and is thus made available toobservers interested in participating in the search for variable starsin open clusters. In this paper we describe the motivation, contents anduse of the STACC Open Cluster Target List. We also give some guidelineson how to make CCD observations of open clusters in order to search forvariable stars.
| How many low-mass stars do destroy (3) He?
We recall the current status of the long-standing (3) He problem, andits possible connection with chemical anomalies on the red giant branch.In this context, we collect in the literature all the availableobservations of the carbon isotopic ratio in field and cluster giantstars. Using the HIPPARCOS parallaxes, we get constraints on theevolutionary status of the field stars of the sample. This allows us toidentify the stars that have passed the luminosity function bump andpresent (12) C/(13) C ratios in disagreement with the standardpredictions of stellar evolutionary models. We determine statisticallywhat fraction of low mass stars experience an extra-mixing process onthe red giant branch, and are then expected to destroy their (3) He atthis evolutionary phase. The high number we get satisfies the galacticrequirements for the evolution of the (3) He abundance.
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