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Photometric study of distant open clusters in the second quadrant: NGC 7245, King 9, King 13 and IC 166
We present a UBV CCD photometric study of four open clusters, NGC 7245,King 9, IC 166 and King 13, located between l = 90° and 135°.All are embedded in a rich Galactic field. NGC 7245 and King 9 are closetogether in the sky and have similar reddenings. The distances and agesare: NGC 7245, 3.8 +/- 0.35 kpc and 400 Myr; King 9 (the most distantcluster in this quadrant), 7.9 +/- 1.1 kpc and 3.0 Gyr. King 13 is 3.1+/- 0.3 kpc distant and 300 Myr old. King 9 and IC 166 (4.8 +/- 0.5 kpcdistant and 1 Gyr old) may be metal-poor clusters (Z = 0.008), asestimated from isochrone fitting. The average value of the distance ofyoung clusters from the Galactic plane in the above longitude range andbeyond 2 kpc (-47 +/- 16 pc, for 64 clusters) indicates that the youngdisc bends towards the southern latitudes.
| Blue Straggler Stars in Galactic Open Clusters and the Simple Stellar Population Model
The presence of blue straggler stars (BSs) as secure members of Galacticopen clusters (OCs) poses a major challenge to the conventional pictureof simple stellar population (SSP) models. These are based on thestellar evolution theory of single stars, whereas the major formationmechanisms of BSs are all correlated with stellar interactions. We haveillustrated this in a previous study based on a small sample of old (age>=1 Gyr) Galactic OCs. However, for the purpose of demonstrating thecontributions of BSs to the conventional SSP models statistically andsystematically, a large database with sufficient coverage of age andmetallicity is definitely needed. The working sample now includes 100Galactic OCs with ages ranging from 0.1 to 10 Gyr. The contributions ofBSs to the integrated light of their host clusters are calculated on anindividual cluster basis. The general existence of BSs in our starcluster sample dramatically alters the predictions of conventional SSPmodels in terms of their integrated properties. Neglecting theconsequences of nonstandard evolutionary products, such as BSs, instellar populations, very large uncertainties can be made in analyzingtheir integrated spectral energy distributions at unresolvableconditions. The current work strongly suggests that when evolutionarypopulation synthesis technique is used to study the properties ofunresolved stellar populations in galaxies, the contributions of BSsshould be taken into account.
| 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.
| 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.
| 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.
| 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.
| 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
| 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.
| Near-infrared photometry of the intermediate age open clusters IC 166 and NGC 7789
We present and discuss new photometric data obtained with an IR camerain the J and K pass-bands for the intermediate age open clusters IC 166(936 stars in total) and NGC 7789 (1030 stars in total). IC 166 is apoorly studied open cluster for which no IR data was availablepreviously, while NGC 7789 is a well studied open cluster. We show thatIC 166 is of intermediate age (about 1.0 Gyr), with a reddening E(V-K) ~1.3 (corresponding to E(B-V) = 0.50) and a true distance modulus (m-M)_o= 13.25. These values are significantly different from previousdeterminations. NGC 7789 is found to be a 1.4 Gyr open cluster. Themetallicity derived with the method developed by Tiede et al (1997) isfound to be closer to the spectroscopic estimate than previousphotometric studies. Based on observations taken at TIRGO.
| Near-infrared photometry of the young open clusters NGC 1893 and Berkeley 86
We present photometry in the J and K near-infrared bands for two regionscentered on the young open clusters NGC 1893 and Berkeley 86. We study700 stars down to K = 17 in the field of NGC 1893, and about 2000 starsin the field of Berkeley 86 down to K ~ 16.5, for which near-infraredphotometry was insofar not available. Coupling J-K data with UBVphotometry taken from literature, we produce reddening correctedcolour-magnitude diagrams. We find that our data are consistent withprevious determinations: the clusters are roughly coeval with an agebetween 4 and 6 million years. The mean reddening (measured as E(J-K))values turn out to be 0.35 and 0.50 for NGC 1893 and Berkeley 86,respectively. Using colour-colour plots we discuss the presence ofcandidate pre-main sequence stars showing infrared excess. Candidatesare found in both cluster regions, confirming the young age of theseclusters. Based on observations taken at TIRGO
| On the Galactic disc age-metallicity relation
A comparison is made between the age-metallicity relations obtained fromfour different types of studies: F and G stars in the solarneighbourhood, analysis of open clusters, galactic structure studieswith the stellar population synthesis technique and chemical evolutionmodels. Metallicities of open clusters are corrected for the effects ofthe radial gradient, which we find to be -0.09 dex kpc^-1 and mostlikely constant in time. We do not correct for the vertical gradient,because its existence and value are not firmly established. Stars andclusters trace a similar age-metallicity relation, showing an excess ofrather metal-rich objects in the age range 5-9 Gyr. Galactic structurestudies tend to give a more metal-poor relation than chemical evolutionmodels. Neither relation explains the presence of old, relativelymetal-rich stars and clusters. This might be caused by uncertainties inthe ages of the local stars, or pre-enrichment of the disc with materialfrom the bulge, possibly as a result of a merger event in the earlyphases of the formation of our Galaxy.
| Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2556T&db_key=AST
| Catalogue of blue stragglers in open clusters.
An extensive survey of blue straggler candidates in galactic openclusters of both hemispheres is presented. The blue stragglers wereselected considering their positions in the cluster colour-magnitudediagrams.They were categorized according to the accuracy of thephotometric measurements and membership probabilities. An amount of 959blue straggler candidates in 390 open clusters of all ages wereidentified and classified. A set of basic data is given for everycluster and blue straggler. The information is arranged in the form of acatalogue. Blue stragglers are found in clusters of all ages. Thepercentage of clusters with blue stragglers generally grows with age andrichness of the clusters. The mean ratio of the number of bluestragglers to the number of cluster main sequence stars is approximatelyconstant up to a cluster age of about 10^8.6^ yr and rises for olderclusters. In general, the blue stragglers show a remarkable degree ofcentral concentration.
| The Old Open Clusters Of The Milky Way
The Galactic open clusters, in particular the oldest members, serve asexcellent probes of the structure and evolution of the Galactic disk.Individual clusters provide excellent tests of stellar and dynamicalevolution. Cluster spatial and age distributions provide insight intothe processes of cluster formation and destruction that have allowedsubstantial numbers of old open clusters to survive.Spectroscopic andphotometric data for the old clusters yield kinematic, abundance, andage information that clarifies the relationship between the old opencluster population and other Galactic populations. New samples of oldopen clusters, which span a large range in distance and age, are used todefine disk abundance gradients and the cluster age-metallicityrelationship, and they point to a complex history of chemical enrichmentand mixing in the disk.
| The galactic system of old star clusters: The development of the galactic disk
The vast majority of open clusters persist as clusters for no more thana few hundred million years, but the few which survive for much longerperiods constitute a unique sample for probing the evolution of thegalactic disk. In a charge coupled device (CCD) photometric survey forpossible old open clusters combined with previously publishedphotometry, we have developed a list of 72 clusters of the age of theHyades or older (Phelps (1994). Using our version of parameters based onthe luminosity difference between the main sequence turnoff and thehorizontal branch and on the color difference between the turnoff andthe giant branch, we have calculated a 'Morphological Age Index' (MAI)for the clusters in our list and for a sample of globular clusters. Wefind that the MAI is well correlated with the logarithm of cluster ages,as determined by fitting to theoretical isochrones. We conclude that theindex is a good measure of the relative ages of both globular and openclusters, although uncertainties in the models and residual metallicityeffects prevent the use of the MAI as a definitive calibration of actualcluster ages. The age distribution of the open clusters overlaps that ofthe globular clusters, indicating that the galactic disk began todevelop toward the end of the period of star formation in the galactichalo. The open cluster age distribution can be fit approximately with atwo-component exponential decay function; one component can beidentified as the tail of the dominant population of thin disk openclusters with an age scale factor of 200 Myr, and the other consists oflonger-lived clusters with an age scale of 4 Gyr. The young openclusters are distributed on the galactic plane almost symmetricallyabout the Sun with a scale height perpendicular to the galactic plane of55 pc. The old population consists of rich clusters found only in theouter disk, nore than RGC = 7.5 kpc from the galactic center;this population has a scale height of 375 pc. After accounting for thetwo exponential distributions of cluster ages, there are indications ofan excess of clusters in the age range of 5-7 Gyr; there may have beenlarge bursts of star formation in that period, or perhaps a largerproportion of the clusters forming at that time had advantageous orbitsfor survival. Either circumstance is consistent with the idea that thegalactic disk has been repeatedly disturbed, possibly in collisions orother interactions with external systems, resulting in the occasionalformation of clusters with relatively large velocities perpendicular tothe plane; these are the clusters that have survived until the present.Finally, the repeated accretion of low angular momentum material ontothe disk from the halo or beyond would also explain the observed radialcomposition gradient and the lack of a correlation between open clustermetallicity and age found by Friel & Janes (1993).
| The Galactic system of old open clusters: age calibration and age-metallicity relation
In this paper, we present a new homogeneous compilation of ages for thesystem of intermediate age and old open clusters of the Galaxy, and theaccompanying age-metallicity relation (AMR). This study stands on theanalysis by Carraro et al. (1991, 1993a,b), who have obtained goodestimates for the color excess, distance modulus, and age for a sampleof ten clusters, for which modern color-magnitude diagrams (CMD) of goodphotometric quality and spectroscopic data on the metallicity (Friel& Janes 1991, 1993) were available. Firstly, we revise the resultsby Carraro et al. (1991, 1993a,b) to take into account recentdevelopments in the libraries of stellar models (Alongi et al. 1993;Bressan et al. 1993; Fagotto et al. 1993), and secondly we presentuseful age calibrations based on the correlation between metallicity,age, and magnitude difference between the turn-off and red clumpluminosities. The age calibration does not depend on the color excess,distance modulus, but only weakly on the metallicity. With the aid ofthe new age calibration, we assign the age to a more numerous sample ofclusters. The resulting ages span the range from 0.5x10^9^yr for NGC5822 to 8.0x10^9^yr for NGC 6791. With such a compilation, and adoptingan homogeneous source for the metal content, we propose a new AMR forthe family of Galactic open clusters. The AMR is also corrected for theeffect of the gradient in metallicity across the Galactic Disk. Althoughat any given age the spread in metallicity is high, the AMR togetherwith the distribution of clusters with different age and metallicityacross the Galactic Plane, confirms previous suggestions that themetallicity of a cluster is more related to the position than the age.
| Development of the Galactic disk: A search for the oldest open clusters
In an extensive charge coupled devices (CCD) photometric survey ofpotential old open clusters, we have identified a number of systems thatare indeed old; some of them are among the oldest of the open clusters.Using our versions of two well-known morphological age indices, onebased on the luminosity difference between the main sequence turnoff andthe horizontal branch and the other on the color difference between theturnoff and the giant branch, we have ranked the open clusters inapproximate order of age. Our data together with previously publishedphotometry of other old open clusters, yields a catalogue of 72 clustersof the age of Hyades or older with 19 of the clusters as old or olderthan M67 (about 5 Gyr). Among the oldest open clusters are Be 17, Cr261, NGC 6791, Be 54, and AM 2. Be 17 and another old cluster, Lynga 7,are possibly as old as the youngest globulars. The data also suggestthat the formation rate of open clusters may have been higher early inthe history of the disk than at intermediate times since numerousclusters have survived from that time.
| Properties of the Galactic Old Open Clusters System
Not Available
| Metallicities and radial velocities of old open clusters
Medium resolution spectra of giant stars in 24 open clusters with agesgreater than about 1 Gyr have been obtained and analyzed to provideradial velocities accurate to 10-15 km/s and (Fe/H) metallicitiesaccurate to 0.15 dex. The radial velocities of the older cluster systemsare consistent with those expected from the reflex of solar motion andsimple Galactic rotation. The cluster kinematics suggest strongly thatthey are drawn from the old thin disk stellar population. An abundancegradient in the Galactic disk of -0.09 +/- 0.02 dex/kpc is found. Thisgradient is in good agreement with that found for samples of youngeropen clusters, suggesting that there has been little change in theGalactic radial abundance gradient over time. The cluster sample showsno correlation between age and metallicity, regardless of Galactocentricradius. The clusters show an unexpected correlation between the Mg linestrength and age of the cluster at a fixed metallicity. Possible reasonsfor this are discussed.
| An observational age-metallicity relation for the Galaxy
An observational Age-Metallicity Diagram for the Galaxy has beenconstructed on the base of star cluster data available in theliterature. This diagram exhibits two independent and simultaneousmetallicity enhancement processes identified with two dynamical galacticsubstructures: the thin and the thick disks. Within the observationaluncertainties both these processes can be satisfactory approximated byInfall-Inflow models of the chemical evolution with different high,although rapidly decaying with time, Star Formation and Infall Rates.The comparison of the galactic age-metallicity relations with that forthe Large Magellanic Cloud shows some similarities between processes ofthe chemical enhancement in the Large Magellanic Cloud and the Thin Disksubsystem in the Galaxy.
| Metallicities of open clusters
An examination is conducted of available metallicity data on openclusters, gathering the largest possible set of homogeneous estimationsof this parameter and determining whether open cluster metallicities arerelated to the spatial and age distributions of such objects. The'composite' C-M diagrams for the two metallicity groups discussedexhibit interesting morphological differences which may be characterizedas the presence of numbered blue stragglers at the two metal-richclusters and a lack of such members in two metal-poor ones.
| Blue stragglers as remnants of stellar mergers - The discovery of short-period eclipsing binaries in the globular cluster NGC 5466
Results are reported from a search for short-period variables among bluestragglers in the central region of NGC 5466, based on analysis of 248 Band V CCD images obtained with the U.S. Naval Observatory 1-m, PalomarObservatory 1.5-m, and Steward Observatory 2.3-m telescopes during1987-1989. The data are presented in extensive tables and graphs andcharacterized in detail. Nine variable blue stragglers are identified,of which three are eclipsing binaries with periods 0.298-0.511 d (twocontact binaries of W UMa type and one detached or semidetached binary)and six are pulsating SX Phe stars. Theoretical models indicate that allof the noneclipsing blue stragglers could be merged close binaries,although other formation mechanisms cannot be completely ruled out.
| Radial velocities of more old open clusters
Moderate-resolution CCD spectra in the Mg b region of red giants in 13old or metal-poor open clusters were obtained with the kitt Peak 2.1-mtelescope. Radial velocities for these giants, measured bycross-correlation techniques, are accurate to 10 km/s. For the sixclusters in the sample with previous measurements, the velocities are ingood agreement with published values. Results are presented for sevenclusters (NGC 1193, NGC 1817, NGC 6819, NGC 7142, Be 21, King 8, IC 166)with no previous radial-velocity measurements. The majority of clustersin the sample, supplemented with ten more clusters older than 1 Gyrwhich have published velocities, show radial velocities which areconsistent with those expected from the reflex of solar motion andsimple galactic rotation. A few of the clusters, including the newlymeasured NGC 1193 and NGC 1817, show surprisingly large radialvelocities, indicating significant deviation from the general rotationof the remaining old cluster system.
| Two populations of open clusters
The paper proposes a division of the local open clusters into twodifferent dislike subpopulations on the base of the metallicityenhancement history, spatial distribution in the Galaxy and morphologyof their C-M diagrams. The thin disk population objects exhibit highermetallicities and are more closely concentrated to the center and thedisk of the Galaxy than the thick disk clusters. The former contain alsofrequently blue stragglers while the later exhibit the presence of Clumpgiant members, typical for the thick disk globular clusters. Kinematicalproperties of the two considered open cluster subpopulations remainunknown due to the lack of observational data.
| Component Analysis of Open Clusters
Not Available
| Young stellar-gas complexes in the Galaxy
It is found that about 90 percent of OB-associations and o-b2 clusterssituated within 3 kpc of the sun can be united into complexes withdiameters of 150-700 pc. Almost all of these clusters contain giantmolecular clouds with a mass greater than about 100,000 solar masses. Anumber of complexes are associated with giant H I clouds; a few of thesmall complexes are situated in the HI caverns. The concentration ofOB-associations and young clusters in star complexes attests to theircommon origin in the supergiant gaseous clouds.
| Catalogue of UBV Photometry and MK Spectral Types in Open Clusters (Third Edition)
Not Available
| Red giants in old open clusters - A test for stellar evolution
A group of 38 among the oldest open clusters has been analyzed usingtheoretical isochrones derived by Ciardullo and Demarque (1977) andevolutionary tracks for central He-burning of Sweigart and Gross (1976).This analysis has been performed in two steps: - comparison of HRdiagrams with isochrones statistical check of the fit between observedluminosity distributions of red giant stars and the theoretical ones.The following conclusions have been reached: - clusters older than 2 or3 billion years (B-V) equal to or greater than 0.35 at MS turnoff pointgenerally agree with theoretical description and allow the determinationof reliable ages from the distribution of red giants, - for clustersyounger than 2 or 3 billion years, the theory fails in theinterpretation of their red giant stars and the statistical analysisrejects all the tested distributions. In fact, their behavior is similarto that of even younger clusters, in which the evolved stars, duringshell H-burning, have non-degenerate cores and therefore do not undergothe helium flash. It is suggested that such behavior could be attributedto overshooting from the convective core during central H-burning in themass range 1.3-2.2 solar masses.
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