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Elemental abundances of intermediate-age open cluster NGC 3680
We present a new abundance analysis of the intermediate-age Galacticopen cluster NGC 3680, based on high-resolution, high signal-to-noiseratio VLT/UVES spectroscopic data. Several element abundances arepresented for this cluster for the first time, but most notably wederive abundances for the light and heavy s-process elements Y, Ba, Laand Nd. The serendipitous measurement of the rare-earth r-processelement Gd is also reported. This cluster exhibits a significantenhancement of Na in giants as compared to dwarfs, which may be a proxyfor an O to Na anticorrelation as observed in Galactic globular clustersbut not open clusters. We also observe a step-like enhancement of heavys-process elements towards higher atomic number, contrary toexpectations from asymptotic giant branch nucleosynthesis models,suggesting that the r process played a significant role in thegeneration of both La and Nd in this cluster.
| Lithium in the Intermediate-Age Open Cluster, NGC 3680
High-dispersion spectra centered on the Li 6708 Å line have beenobtained for 70 potential members of the intermediate-age open clusterNGC 3680, with an emphasis on stars in the turnoff region of the clustercolor-magnitude diagram (CMD). A measurable Li abundance has beenderived for 53 stars, 39 of which have radial velocities and propermotions consistent with cluster membership. After being transferred tocommon temperature and abundance scales, previous Li estimates have beencombined to generate a sample of 49 members, 40 of which bracket thecluster Li-dip. Spectroscopic elemental analysis of eight giants andfive turnoff stars produces [Fe/H] = -0.17 ± 0.07 (sd) and -0.07± 0.02 (sd), respectively. We also report measurements of Ca, Si,and Ni which are consistent with scaled-solar ratios within the errors.Adopting [Fe/H] = -0.08 (Section 3.6), Y 2 isochronecomparisons lead to an age of 1.75 ± 0.1 Gyr and an apparentmodulus of (m - M) = 10.30 ± 0.15 for the cluster, placing thecenter of the Li-dip at 1.35 ± 0.03 M sun. Among thegiants, five of the nine cluster members are now known to havemeasurable Li with A(Li) near 1.0. A combined sample of dwarfs in theHyades and Praesepe is used to delineate the Li-dip profile at 0.7 Gyrand [Fe/H] = +0.15, establishing its center at 1.42 ± 0.02 Msun and noting the possible existence of a secondary dip onits red boundary. When evolved to the typical age of the clusters NGC752 (age = 1.45 Gyr, (m - M) = 8.4), IC 4651 (age = 1.5 Gyr, (m - M) =10.4), and NGC 3680, the Hyades/Praesepe Li-dip profile reproduces theobserved morphology of the combined Li-dip within the CMDs of theintermediate-age clusters while implying a metallicity dependence forthe central mass of the Li-dip given by M/M sun = 1.38± 0.04 + 0.4 ± 0.2 [Fe/H]. The implications of thesimilarity of the Li-dichotomy among giants in NGC 752 and IC 4651 andthe disagreement with the pattern among NGC 3680 giants are discussed.
| Red giants in open clusters. XIV. Mean radial velocities for 1309 stars and 166 open clusters
Context: Radial velocities have proved to be an efficient method formembership determination if there are at least 2 or 3 red giants in acluster. They are necessary for galactic studies, but are still missingfor many open clusters. Aims: We present the final catalogues of along-term observing programme performed with the two coravelspectrovelocimeters for red giants in open clusters. The main aims wereto detect spectroscopic binaries and determine their orbital parameters,determine the membership, and compute mean velocities for the stars andopen clusters. Methods: We computed weighted mean radial velocities for1309 stars from 10 517 individual observations, including the systemicradial velocities from spectroscopic orbits and for cepheids. Results:The final results are contained in three catalogues collecting 10 517individual radial velocities, mean radial velocities for 1309 redgiants, and mean radial velocities for 166 open clusters among whichthere are 57 new determinations. We identified 891 members and 418non-members. We discovered a total of 288 spectroscopic binaries, amongwhich 57 are classified as non-members. In addition 27 stars were judgedto be variable in radial velocities and they are all red supergiants. Conclusions: The present material, combined with recent absolute propermotions, will permit various investigation of the galactic distributionand space motions of a large sample of open clusters. However, thedistance estimates still remain the weakest part of the necessary data.This paper is the last one in this series devoted to the study of redgiants in open clusters based on radial velocities obtained with thecoravel instruments.Based on observations collected at the Haute-Provence Observatory(France) and on observations collected with the Danish 1.54-m telescopesat the European Southern Observatory, La Silla, Chile. Full Tables [seefull textsee full textsee full textsee full textsee full text] to [seefull textsee full textsee full textsee full textsee full text] are onlyavailable and Tables [see full textsee full textsee full textsee fulltextsee full text] and [see full textsee full textsee full textsee fulltextsee full text] are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/485/303
| Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic openclusters are studies in this work, in which 27 Galactic open clusters ofages >=1 Gyr are selected as the working sample. Based on thephotometric observations of these open clusters, a synthetic integratedspectrum has been made for the stellar population of each cluster. Theeffects of blue straggler (BS) stars on the conventional simple stellarpopulation (SSP) model are analyzed on an individual cluster basis. Itis shown that the BSs, whose positions in the color-magnitude diagramscannot be predicted by the current single-star evolution theory, requiresignificant modifications to the integrated properties of theoreticalSSP model. The synthesized integrated spectral energy distributions(ISEDs) of our sample clusters are dramatically different from those ofSSPs based on an isochrone only. The BS-corrected ISEDs of stellarpopulations show systematic enhancements toward shorter wavelengths inthe spectra. When measured with broadband colors in unresolvableconditions, the age of a stellar population can be seriouslyunderestimated by the conventional SSP model. Therefore, considering thecommon existence of BS components in real stellar populations, we shouldexpect considerable alterations of the conventional ISEDs when we applythe technique of evolutionary population synthesis to more complicatedstellar systems.
| uvbyCaHβ CCD Photometry of Clusters. IV. Solving the Riddle of NGC 3680
CCD photometry on the intermediate-band uvbyCaHβ system ispresented for the open cluster, NGC 3680. Restricting the data toprobable cluster members using the color-magnitude diagram (CMD) and thephotometric indices alone defines a sample of 34 stars at the clusterturnoff that imply E(b-y)=0.042+/-0.002 (s.e.m.) or E(B-V)=0.058+/-0.003(s.e.m.), where the errors refer to internal errors alone. With thisreddening, [Fe/H] is derived from both m1 and hk using bothb-y and Hβ as the temperature indices. The agreement among the fourapproaches is excellent, leading to final value of [Fe/H]=-0.14+/-0.03for the cluster and removing the apparent discrepancy between the pastuvby analyses and extensive results from the red giants. The primarysource of the photometric anomaly appears to be a zero-point offset inthe original m1 indices. Using the homogenized and combinedV, b-y data from a variety of studies transformed to B-V, the clusterCMD is compared with NGC 752, IC 4651, and the core-convective-overshootisochrones of Girardi et al. By interpolation to the proper metallicity,it is found that the E(B-V), m-M, and age for NGC 752, IC 4651, and NGC3680 are (0.03, 8.30, 1.55 Gyr), (0.10, 10.20, 1.7 Gyr), and (0.06,10.20, 1.85 Gyr), respectively. The revised age and metallicity sequenceand the color distribution of the giants provide evidence for thesuggestion that the giants defining the apparent clump in NGC 3680 arepredominantly first-ascent giants, as indicated by their Li abundance,while the clump stars in NGC 752, 0.1 mag bluer in B-V, areHe-core-burning stars. When combined with the color distribution in IC4651, it is suggested that over this modest age range where He-coreflash becomes important, the distribution of so-called clump starsswitches from being dominated by He-core-burning stars to first-ascentgiants in the bump phase.
| 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.
| 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
| Critical tests of stellar evolution in open clusters. II. Membership, duplicity, and stellar and dynamical evolution in NGC 3680.
Based on new, accurate photometry, radial velocities, and proper motionsfor the intermediate-age open cluster NGC 3680, we identify individualsingle and binary cluster members and field stars in thecolour-magnitude diagram (CMD). This basic step turns out to be crucialfor a proper understanding of the cluster CMD: ~60% of the stars arefound to be field stars, and over 50% of the cluster stars are binaries.No bona fide cluster star is found more than 1.5mag below the turnoff,and cluster stars below 1.4Msun_ are only found in binarysystems. The total present mass of NGC 3680 is ~100Msun_,excluding any as yet unseen stellar remnants, and its half-mass radiusis 3.3' (1.2pc). Comparison with plausible IMFs indicates that only ~3%of the original stars and <~10% of the mass now survive, ~30% of theinitial mass being in the form of massive stars that have now completedtheir evolution, and ~60% in low-mass stars which may now be located ina distant cluster halo or perhaps have been lost entirely. The singlemain-sequence cluster members form an extremely tight sequence in theCMD, with E_(b-y)_=0.034 and [Fe/H]=+0.11. A direct fit to the Hyadesmain sequence yields (m-M)_0_=10.5+/-0.2 for NGC 3680. Isochrones fromseveral stellar models have been fit to the cluster sequence. When basedon consistent uvby colour transformations and the above clusterparameters, these fits are very stable and show that standard models arenot acceptable for stars with the turnoff mass of NGC 3680. Overshootingmodels perform much better, but further refinement of the overshootingformalism seems to be needed. The age derived for NGC 3680 is1.45+/-0.3Gyr. The limiting factor in a precise comparison of theory andobservations is now the transformation from theoretical to observedparameters, particularly (broad-band) colours.
| The Age of NGC 3680 and a Test of Convective Overshoot
A new CCD BV color-magnitude diagram has been derived for theintermediate age open cluster NGC 3680. The quality of photometrycoupled with the best-to-date knowledge of cluster memberships allowsfor a detailed isochrone fit to the CMD. The theoretical isochrones havebeen constructed using the Yale Rotating Evolution Code (YREC) in itsnon-rotating mode and the OPAL opacities (circa 1991). Four sets ofisochrones have been calculated: one for the standard stellar modelcalibrated to the Sun and three others for models with different amountsof convective overshoot at the edge of the convective core, namely,0.15, 0.20 and 0.25Hp, where Hp is the pressurescale height at the core edge. All four sets of theoretical isochroneswere adjusted to an adopted distance modulus ofVdeg-MV=10.20 and reddening E(B-V)=0.075 whichleads to ages of 1.3+/-0.15, 1.5+/-0.15, 1.6+/-0.15 and 1.7+/-0.15 Gyr,respectively. The uncertainties in age mainly reflect a subjectivedecision in differentiating a good fit from a poorer one. The model witha convective overshoot of 0.20Hp seems to yield the best fitto the sharply curved upper main-sequence. Hence, the estimated age ofNGC 3680 is 1.6+/-0.15 Gyr. The adopted overshoot parameter ostensiblyis uncertain by +/-0.05H_p. For comparison, the same stellar models withovershoot of 0.25Hp were fit to the color-magnitude diagramof the cluster NGC 752, which is similar in age and composition to NGC3680. The age estimate for NGC 752 is 1.6+/-0.2 Gyr, assumingVdeg-MV=8.20 and E(B-V)=0.030 for the cluster.
| Critical tests of stellar evolution in open clusters. I. New photometry and radial velocities for NGC 3680.
We present new CCD photometry in the b and y colours of the Stroemgrenuvby system for 310 stars in a 13'x13' field centered on theintermediate-age open cluster NGC 3680. Careful cross-checks indicatethat previously published BV photometry of NGC 3680 is affected byrandom and/or systematic errors precluding its use in criticalcomparison with theoretical isochrone computations. Detailed notes onseveral individual stars are given. In addition, we present =~400 newphotoelectric radial-velocity observations of 109 stars obtained withthe CORAVEL scanner during the period 1988-1994. These data allowsubstantially complete identification of member and non-member stars inthe field, and of spectroscopic binaries in both groups. Rotationalvelocities have also been derived for the programme stars, and ourvelocity variability criteria for stars of all rotations are described.The further astrophysical discussion of the data, including thedefinition of radial-velocity membership criteria, theoretical isochronefitting, and the dynamical state of the cluster and the origin of its"bimodal turnoff", will appear in a separate paper (Nordstroem et al.1996).
| Red giants in open clusters. IV. NGC 3680 and IC 4651.
Radial-velocity observations of 35 red giants in the field of theintermediate-age open clusters NGC 3680 and IC 4651 are presented andanalysed for membership and duplicity. Five stars in NGC 3680 and one inIC 4651 were identified as non-members, and six and five spectroscopicbinaries, respectively, were discovered among the red giant members inthe two clusters. Orbits have been determined for seven of the binaries,four of which are still preliminary. The mean cluster velocities of NGC3680 and IC 4651 are 0.9+/-0.2km/s and -31.0+/-0.2km/s respectively. Themorphology of the red giant region in the colour magnitude diagramsshows a well-populated clump in both clusters. In both clusters, onecertain member star is clearly brighter and redder than the clump, whileIC 4651 shows, in addition, a probable concentration of stars on theascending giant branch.
| A proper-motion study of the open cluster NGC 3680
Relative proper motions and cluster membership probabilities have beendetermined for 2711 stars in a region containing the old open clusterNGC 3680. The Yale-Columbia 26 in. refractor plates used in this studycontain significant magnitude equation as well as color equation. Aprocedure is developed for transforming the measures from each of theplates into a common magnitude/color equation system, thus yieldingdifferential positions from plate to plate, (i.e., proper motions) whichare free of these systematic errors. For well measured stars the meanstandard error of the proper motions is +/- 0.35 mas/yr. Three differentmembership calculation techniques are described in detail. The best fitto the proper-motion distributions yields 74 cluster members. Almost allknown red giants are cluster member according to the proper motionmembership criterion. The luminosity function of NGC 3680 shows acharacteristic turnover at Mupsilon approximately equals +3mag and an essentially flat faint end. Monte-Carlo simulations are usedin order to explore the mutual dependence of the luminosity function andmembership probabilities. The bimodality of the upper main sequencefound in earlier photometric studies probably still exists even aftereliminating proper-motion nonmembers.
| Catalogue of proper motions in the region of NGC 3680.
Not Available
| Evolved GK stars near the sun. I - The old disk population
A sample of nearly two thousand GK giants with intermediate band, (R,I),DDO and Geneva photometry has been assembled. Astrometric data is alsoavailable for most of the stars. The some 800 members of the old diskpopulation in the sample yield accurate luminosities (from two sources),reddening values and chemical abundances from calibrations of thephotometric parameters. Less than one percent of the objects arepeculiar in the sense that the flux distribution is abnormal. Thepeculiarity is signaled by strong CH (and Ba II) and weak CH. The CH+stars are all spectroscopic binaries, probably with white dwarfcompanions, whereas the CH- stars are not. A broad absorption band,centered near 3500 A, is found in the CH+ stars whereas the CH- objectshave a broad emission feature in the same region. The intensity of theseabsorptions and emissions are independent of the intensity of abnormalspectral features. Ten percent of the old disk sample have a heavyelement abundance from one and a half to three times the solar value.The distribution of the heavy element abundances is nearly a normal onewith a peak near solar abundance and ranges three times to one sixthsolar. The distribution of the (U, V) velocities is independent of theheavy element abundance and does not appear to be random. Ten percent ofthe old disk stars show a CN anomaly, equally divided between CN strongand CN weak. Several stars of individual astrometric or astrophysicalimportance are isolated.
| A BV photographic and CCD analysis of the intermediate-age open cluster NGC 3680
Results are presented of a photographic, photoelectric, and CCD surveyon the BV system in the intermediate-age open cluster, NGC 3680. Directtests demonstrate significant magnitude-dependent differences betweenthe photometry of Eggen (1969) and the standard BV system. The clustercontains a well-defined red giant clump, but the turnoff region exhibitsscatter noticeably larger than expected from the derived errors. Theapparent luminosity function of the cluster shows a dramatic decline inprobable cluster members below 1 solar mass when compared to counts forthis region of the sky based upon Galactic models. Comparison with thetheoretical isochrones of VB85 produces an age of 1.9 x 10 to the 9th yrwith a large uncertainty depending upon the choice of luminosity orcolor as the primary age estimator.
| CCD ubvy photometry of the bimodal main-sequence cluster NGC 3680
CCD uvby photometry for the intermediate age, southern open cluster, NGC3680, is analyzed. For a reddening of E(b-y) = 0.034, a true clustermodulus of 9.74 + or - 0.20 and a cluster metallicity of Fe/H abundance= 0.10 + or - 0.09, based on 18 probable nonbinary members of thecluster brighter than V = 14. The color-magnitude diagram for thecluster suggests that, although the main sequence may be subject to thesame bimodal distibution as NGC 752, the likely source in both clustersis a combination of binaries and a sharply curved turnoff. Thecolor-magnitude diagram is compared to the theoretical isochrones ofBertelli et al. (1988), showing an age of (1.9 + or - 0.3) X 10 to the9th yr.
| Large and kinematically unbiased samples of G- and K-type stars. IV - Evolved stars of the old disk population
Modified Stromgren and (R,I) photometry, along with DDO and Genevaphotometry, are presented for a complete sample of evolved old-disk Gand K giants in the Bright Star Catalogue. Stars with ages of between1.5 x 10 to the 9th and 10 to the 10th yr are found to have anear-normal distribution of heavy element abundances, centered on anFe/H abundance ratio of -0.1 dex. The old disk clusters NGC 3680 and IC4651 contain red-straggler young-disk giants that are probablycontemporaries of the blue stragglers in the clusters.
| Yellow evolved stars in open clusters
This paper describes a program in which Galactic cluster post-AGBcandidates were first identified and then analyzed for clustermembership via radial velocities, monitored for possible photometricvariations, examined for evidence of mass loss, and classified ascompletely as possible in terms of their basic stellar parameters. Theintrinsically brightest supergiants are found in the youngest clusters.With increasing cluster age, the absolute luminosities attained by thesupergiants decline. It appears that the evolutionary tracks ofluminosity class II stars are more similar to those of class I than ofclass III. Only two superluminous giant star candidates are found inopen clusters.
| Washington photometry of open cluster giants - The metal-rich clusters
Abundances of more than one hundred giants in a large sample ofmetal-rich open clusters in the Galaxy are investigated with Washingtonphotometry. Most of these clusters have no high-dispersion spectroscopicabundance determinations. A revision of the empirical abundancecalibrations for Population I giants, based on a much improved data setover previous calibrations, is first presented. The M-T(1) (Feabundance) calibration changes slightly for stars with Fe/H of about-0.5. The C-M (Fe + CN abundance) calibration is substantially alteredfor abundances above solar. The net effect brings the two abundanceindices into close agreement and eliminates the apparent CN enhancementspreviously determined for several open clusters. An enhanced sensitivityto Fe abundance is found for both the M - T(1) and especially the C - Mindex over that determined in previous investigations. Indeed, Delta(C -M) has an Fe abundance sensitivity that exceeds any other photometricabundance index. In view of the relative rarity of CN strengthvariations in Pop I giants, this index is most useful as a metallicityindicator for these stars. Abundances with an uncertainty of about 0.1dex are derived for a total of 18 open clusters observed with theWashington system. The abundances are generally in good agreement withprior abundance analyses. No strong evidence is found for CN variationsor anomalies in any of the clusters. A comparison with Population Ifield giants shows that the open cluster giants have normal CNabundances for their FE abundances.
| Astrophysical properties of red giants in three open clusters older than the Hyades
The application to the program stars of two independent membershipcriteria based on BV and DDO photometry makes it possible to distinguisha few red field stars from the physical members of the clusters. Inparticular, two stars previously thought to be members of NGC 3680 byEggen (1969) and McClure (1972) are found not to be physically connectedwith this cluster. It is also found that the cluster giants in NGC 2482and IC 4651 have CN strengths nearly identical with the Hyades giants,whereas NGC 3680 is considerably poorer in CN than the above clusters.Within the limits of the DDO photometry, no firm evidence is seen forany intrinsic differences in CN strength among the giants in the threeclusters. This result is valid for the clump stars at the blue end ofthe giant branch in NGC 3680 and IC 4651. Both CMT(1)T(2) and DDO datalend support to the conclusion that IC 4651 is on the metal-rich side ofthe distribution of intermediate and old open clusters. The mass resultsindicate that the clumps stars in NGC 3680 and IC 4651 could haveundergone mass loss before reaching their core burning phase ofevolution.
| The color excesses and metallicities of the open clusters NGC 2360 and NGC 3680
Abstract image available at:http://adsabs.harvard.edu/abs/1972ApJ...172..615M
| The Old Galactic Cluster NGC 3680
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1969ApJ...155..439E&db_key=AST
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Observation and Astrometry data
| Constellation: | Centaure |
| Right ascension: | 11h25m26.23s |
| Declination: | -43°11'24.0" |
| Apparent magnitude: | 10.223 |
| Proper motion RA: | -4.6 |
| Proper motion Dec: | 1.1 |
| B-T magnitude: | 11.433 |
| V-T magnitude: | 10.323 |
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INHABITED SKY
