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|The Infrared Ca II Triplet as Metallicity Indicator|
From observations of almost 500 red giant branch stars in 29 Galacticopen and globular clusters, we have investigated the behavior of theinfrared Ca II triplet (8498, 8542, and 8662 Å) in the age range13 Gyr<=age<=0.25 Gyr and the metallicity range-2.2<=[Fe/H]<=+0.47. These are the widest ranges of ages andmetallicities in which the behavior of the Ca II triplet lines has beeninvestigated in a homogeneous way. We report the first empirical studyof the variation of the Ca II triplet lines' strength, for givenmetallicities, with respect to luminosity. We find that the sequencedefined by each cluster in the luminosity-ΣCa plane is not exactlylinear. However, when only stars in a small magnitude interval areobserved, the sequences can be considered as linear. We have studied theCa II triplet lines on three metallicity scales. While a linearcorrelation between the reduced equivalent width(W'V or W'I) and metallicityis found in the Carretta & Gratton and Kraft & Ivans scales, asecond-order term needs to be added when the Zinn & West scale isadopted. We investigate the role of age from the wide range of agescovered by our sample. We find that age has a weak influence on thefinal relationship. Finally, the relationship derived here is used toestimate the metallicities of three poorly studied open clusters:Berkeley 39, Trumpler 5, and Collinder 110. For the latter, themetallicity derived here is the first spectroscopic estimate available.
|Giants in the Local Region|
We present parameter and abundance data for a sample of 298 nearbygiants. The spectroscopic data for this work have a resolution ofR~60,000, S/N>150, and spectral coverage from 475 to 685 nm. Overalltrends in the Z>10 abundances are dominated by Galactic chemicalevolution, while the light-element abundances are influenced by stellarevolution, as well as Galactic evolution. We find several super-Li starsin our sample and confirm that Li abundances in the first giant branchare related to mixing depths. Once astration of lithium on the mainsequence along with the overall range of main-sequence lithiumabundances are taken into account, the lithium abundances of the giantsare not dramatically at odds with the predictions of standard stellarevolution. We find the giants to be carbon-diluted in accord withstandard stellar evolution and that the carbon and oxygen abundancesdetermined for the local giants are consistent with those found in localfield dwarfs. We find that there is evidence for systematic carbonvariations in the red giant clump in the sense that the blue side of theclump is carbon-poor (more diluted) than the red side.
|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.
|BATC 13 Band Photometry of the Open Cluster NGC 7789|
We present 13 band CCD intermediate-band spectrophotometry of a fieldcentered on the open cluster NGC 7789 from 400 to nearly 1000 nm, takenwith the Beijing-Arizona-Taiwan-Connecticut (BATC) Multi-Color Surveyphotometric system. By comparing observed spectral energy distributionsof NGC 7789 stars with theoretical ones, the fundamental parameters ofthis cluster are derived: an age of 1.4+/-0.1 Gyr, a distance modulus(m-M)0=11.27+/-0.04, a reddening E(B-V)=0.28+/-0.02, and ametallicity with the solar composition Z=0.019. When the surface densityprofile for member stars with limiting magnitudes of 19.0 in the BATC eband (λeff=4925 Å) is fitted by a King model, acore radius Rc=7.52' and a tidal radiusRt=28.84' are derived for NGC 7789. The observedmass function (MF) for main-sequence (MS) stars of NGC 7789 with massesfrom 0.95 to 1.85 Msolar is fitted with a power-law functionφ(m)~mα, and a slope α=-0.96 is derived.Strong mass segregation in NGC 7789 is reflected in the significantvariation of the concentration parametersC0=log(Rt/Rc) for member stars of NGC7789 within different mass ranges: C0=1.02 for the mostmassive stars and C0=0.37 for the lowest mass MS stars.Strong mass segregation in NGC 7789 is also indicated in the significantvariation of the slopes α in different spatial regions of thecluster: the MF for stars within the core region has α=-0.71, muchflatter than that for stars in external regions of the cluster(α=-1.20).
|Chemical Homogeneity in Collinder 261 and Implications for Chemical Tagging|
This paper presents abundances for 12 red giants of the old open clusterCollinder 261 based on spectra from the Very Large Telescope UVES.Abundances were derived for Na, Mg, Si, Ca, Mn, Fe, Ni, Zr, and Ba. Wefind that the cluster has a solar-level metallicity of [Fe/H]=-0.03 dex.However, most α- and s-process elements were found to be enhanced.The star-to-star scatter was consistent with the expected measurementuncertainty for all elements. The observed rms scatter is as follows:Na=0.07, Mg=0.05, Si=0.06, Ca=0.05, Mn=0.03, Fe=0.02, Ni=0.04, Zr=0.12,and Ba=0.03 dex. The intrinsic scatter was estimated to be less than0.05 dex. Such high levels of homogeneity indicate that chemicalinformation remains preserved in this old open cluster. We use thechemical homogeneity we have now established in Cr 261, the Hyades, andthe HR 1614 moving group to examine the uniqueness of the individualcluster abundance patterns, i.e., chemical signatures. We demonstratethat the three studied clusters have unique chemical signatures anddiscuss how other such signatures may be searched for in the future. Ourfindings support the prospect of chemically tagging disk stars to commonformation sites in order to unravel the dissipative history of theGalactic disk.Based on observations collected during ESO VLT-UT2 Program 73.D-0716A atthe European Southern Observatory, Paranal, Chile.
|The Benchmark Cluster Reddening Project. II. A Reddening Value for M67|
Since 1954, 70 reddening determinations have been published for M67.Those results are of very diverse quality, and they imply only that thevalue of E(B-V) for the cluster lies between -0.02 and +0.14 mag. Inaddition, this uncertainty is habitually concealed by long-standingcitation practices. In response, a reformed reddening protocol called``alternative scholarship'' is applied, with tenets including (1)rigorous published foundations for reddening values, (2) accountabilityfor all extant reddening results for M67, (3) explicit quality controlof reddening techniques, (4) use of techniques with limited metallicitysensitivity, (5) explicit zero-point control of input data (whenpossible), (6) a reddening zero point based ultimately on the dust-freesolar neighborhood, and (7) adherence to statistical norms. Results fromreddening maps are then set aside pending further examination of theirzero points and accidental errors. In addition, reddening values derivedfrom color-magnitude analysis and from averages of published reddeningresults are likewise set aside. Five techniques that satisfy the adoptedprotocol are then selected and applied. By averaging results fromcluster A and F stars and K giants, one finds that the mean value ofE(B-V) for M67 is 41+/-4 mmag. It is also found that extant results fromadditional techniques are consistent with this result or else can be setaside for plausible reasons. The analysis yields a collateral value of[Fe/H], and this is consistent with the derived reddening value. Usingpublished high-dispersion data, it is found that the mean cluster valueof [Fe/H] is -0.009+/-0.009 dex. For M67 and the Hyades combined, nomean metallicity difference between giants and stars near the mainsequence is found that is >=0.036 dex at 95% confidence.
|New catalogue of blue stragglers in open clusters|
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
|K-band magnitude of the red clump as a distance indicator|
We have investigated how the K-band magnitude of the red clump [M_K(RC)]depends on age and metallicity, using 2MASS infrared data for a sampleof 24 open clusters with known distances. We show that a constant valueof M_K(RC)=-1.57 ± 0.05 is a reasonable assumption to use indistance determinations for clusters with metallicity between -0.5 and+0.4 dex and age between 108.5 and 109.9 years.Figures 8 and 9 are available in electronic form at http://www.aanda.org
|Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters|
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
|A Photometric Search for Planets in the Open Cluster NGC 7086|
In an attempt to discover short-period, Jupiter-mass planets orbitingsolar-type stars in open clusters, we searched for planetary transits inthe populous and relatively unstudied open cluster NGC 7086. Acolor-magnitude diagram constructed from new B and V photometry ispresented, along with revised estimates of the cluster's color excess,distance modulus, and age. Several turnoff stars were observedspectroscopically in order to determine a color excess ofE(B-V)=0.83+/-0.02. Empirically fitting the main sequences of two youngopen clusters and the semiempirical zero-age main sequence of Vandenbergand Poll yielded a distance modulus of (V-MV)=13.4+/-0.3 mag.This corresponds to a true distance modulus of (m-M)0=10.8mag or a distance of 1.5 kpc to NGC 7086. These values were used withisochrones from the Padova group to obtain a cluster age of 100 Myr.Eleven nights of R-band photometry were used to search for planetarytransits. Differential magnitudes were constructed for each star in thecluster. Light curves for each star were produced on a night-to-nightbasis and inspected for variability. No planetary transits wereapparent; however, some interesting variable stars were discovered: apulsating variable that appears to be a member of the γ Dor classand four possible eclipsing binary stars, one of which actually may be amultiple system.
|On the current status of open-cluster parameters|
We aim to characterize the current status of knowledge on the accuracyof open-cluster parameters such as the age, reddening and distance.These astrophysical quantities are often used to study the globalcharacteristics of the Milky Way down to the very local stellarphenomena. In general, the errors of these quantities are neglected orset to some kind of heuristic standard value. We attempt to give somerealistic estimates for the accuracy of available cluster parameters byusing the independently derived values published in the literature. Intotal, 6437 individual estimates for 395 open clusters were used in ourstatistical analysis. We discuss the error sources depending ontheoretical as well as observational methods and compare our resultswith those parameters listed in the widely used catalogue by Dias et al.In addition, we establish a list of 72 open clusters with the mostaccurate known parameters which should serve as a standard table in thefuture for testing isochrones and stellar models.
|Survey for Transiting Extrasolar Planets in Stellar Systems. IV. Variables in the Field of NGC 1245|
The Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS)project is a search for planetary transits in open clusters. In thispaper we analyze the STEPSS observations of the open cluster NGC 1245 todetermine the variable-star content of the cluster. Out of 6787 starsobserved with V<22, of which ~870 are cluster members, we find 14stars with clear intrinsic variability that are potential clustermembers and 29 clear variables that are not cluster members. None ofthese variables have been previously identified. We present lightcurves, finder charts, and stellar/photometric data on these variableobjects. Several of the interacting binaries have estimated distancesconsistent with the cluster distance determined from isochrone fits tothe color-magnitude diagram. Four stars at the main-sequence turnoff ofthe cluster have light curves consistent with γ Doradusvariability. If these γ Doradus candidates are confirmed, theyrepresent the oldest and coolest members of this class of variablediscovered to date.
|Caroline Herschel's catalogue of nebulae|
|Astrophysics in 2005|
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
|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.
|Upper limits on the hot Jupiter fraction in the field of NGC 7789|
We describe a method of estimating the abundance of short-periodextra-solar planets based on the results of a photometric survey forplanetary transits. We apply the method to a 21-night survey with the2.5-m Isaac Newton Telescope of ~32000 stars in a ~0.5 ×0.5deg2 field including the open cluster NGC 7789. From thecolour-magnitude diagram, we estimate the mass and radius of each starby comparison with the cluster main sequence. We search for injectedsynthetic transits throughout the light curve of each star in order todetermine their recovery rate, and thus calculate the expected number oftransit detections and false alarms in the survey. We take properaccount of the photometric accuracy, time sampling of the observationsand criteria (signal-to-noise ratio and number of transits) adopted fortransit detection. Assuming that none of the transit candidates found inthe survey will be confirmed as real planets, we place conservativeupper limits on the abundance of planets as a function of planet radius,orbital period and spectral type.
|Extended Strömgren Photoelectric Photometry in NGC 752|
Photoelectric photometry on the extended Strömgren system (uvbyCa)is presented for 7 giants and 21 main-sequence stars in the old opencluster NGC 752. Analysis of the hk data for the turnoff stars yields anew determination of the cluster mean metallicity. From 10 single-starmembers, [Fe/H]=-0.06+/-0.03, where the error quoted is the standarderror of the mean and the Hyades abundance is set at [Fe/H]=+0.12. Thisresult is unchanged if all 20 stars within the limits of the hkmetallicity calibration are included. The derived [Fe/H] is in excellentagreement with past estimates, using properly zeroed m1 data,transformed moderate-dispersion spectroscopy, and recent high-dispersionspectroscopy.
|The Hipparcos mission and galactic open clusters and NGC 7538 star forming regions|
We present some results of a study of the galactic open cluster population. This study is based on the all-sky catalogue ASCC-2.5compiled from Tycho-2, Hipparcos and other catalogues. Screening theASCC-2.5 resulted in the identification of 520 known clusters and thedetection of 130 new ones. A uniform combined kinematic-photometriccluster membership was established for these objects and new uniformscales of cluster structure (angular sizes), kinematics (average propermotions and radial velocities), photometry (reddening and distance) andevolution (age) were established. Two parts of our more extended opencluster population study (some details on the spatial and agedistributions of open clusters) are presented here.
|Diameters of Open Star Clusters|
The present paper presents a tabulation of data on all 600 Galactic openclusters for which it is currently possible to calculate lineardiameters. As expected, the youngest ``clusters,'' with ages <15 Myr,contain a significant (>=20%) admixture of associations. Amongintermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, themedian cluster diameter is found to increase with age. Small, compactclusters are rare among objects with ages >1.5 Gyr. Open clusterswith ages >1 Gyr appear to form what might be termed a ``clusterthick disk,'' part of which consists of objects that were probablycaptured gravitationally by the main body of the Galaxy.
|The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting|
Seventy-two grids of stellar evolutionary tracks, along with the meansto generate isochrones and luminosity/color functions from them, arepresented in this investigation. Sixty of them extend (and encompass)the sets of models reported by VandenBerg et al. for 17 [Fe/H] valuesfrom -2.31 to -0.30 and α-element abundances corresponding to[α/Fe]=0.0, 0.3, and 0.6 (at each iron abundance) to the solarmetallicity and to sufficiently high masses (up to ~2.2Msolar) that isochrones may be computed for ages as low as 1Gyr. The remaining grids contain tracks for masses from 0.4 to 4.0Msolar and 12 [Fe/H] values between -0.60 and +0.49 (assumingsolar metal-to-hydrogen number abundance ratios): in this case,isochrones may be calculated down to ~0.2 Gyr. The extent of convectivecore overshooting has been modeled using a parameterized version of theRoxburgh criterion, in which the value of the free parameter at a givenmass and its dependence on mass have been determined from analyses ofbinary star data and the observed color-magnitude diagrams for severalopen clusters. Because the calculations reported herein satisfy manyempirical constraints, they should provide useful probes into theproperties of both simple and complex stellar populations.All of the model grids may be obtained from the Canadian Astronomy DataCenter(http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cvo/community/VictoriaReginaModels/).Included in this archive are (1) the interpolation software (FORTRAN 77)to produce isochrones, isochrone probability functions, luminosityfunctions, and color functions, along with instructions on how toimplement and use the software, (2) BVRI (VandenBerg & Clem 2003)and uvby (Clem et al. 2004) color-temperature relations, and (3)zero-age horizontal branch loci for all of the chemical compositionsconsidered.
|vbyCaHβ CCD Photometry of Clusters. VI. The Metal-deficient Open Cluster NGC 2420|
CCD photometry on the intermediate-band vbyCaHβ system is presentedfor the metal-deficient open cluster NGC 2420. Restricting the data toprobable single members of the cluster using the CMD and the photometricindices alone generates a sample of 106 stars at the cluster turnoff.The average E(b-y)=0.035+/-0.003 (s.e.m.) or E(B-V)=0.050+/-0.004(s.e.m.), where the errors refer to internal errors alone. With thisreddening, [Fe/H] is derived from both m1 and hk, using b-yand Hβ as the temperature index. The agreement among the fourapproaches is reasonable, leading to a final weighted average of[Fe/H]=-0.37+/-0.05 (s.e.m.) for the cluster, on a scale where theHyades has [Fe/H]=+0.12. When combined with the abundances from DDOphotometry and from recalibrated low-resolution spectroscopy, the meanmetallicity becomes [Fe/H]=-0.32+/-0.03. It is also demonstrated thatthe average cluster abundances based on either DDO data orlow-resolution spectroscopy are consistently reliable to 0.05 dex orbetter, contrary to published attempts to establish an open clustermetallicity scale using simplistic offset corrections among differentsurveys.
|Caroline Herschel as observer|
|Discovery of 13 New Variable Stars in the Field of the Open Cluster NGC 2168 (M35)|
A wide-field time-series CCD photometric survey of variable stars in thefield of the open cluster NGC 2168 was carried out using the BATCSchmidt telescope. In total 13 new variable stars are discovered withthree W UMa systems, one EA type and two EB type eclipsing binaries (oneof them could be a W UMa system), and seven pulsating stars includingthree candidates of δ Scuti stars.
|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.
|A dearth of planetary transits in the direction of NGC 6940|
We present results of our survey for planetary transits in the field ofNGC 6940. We think nearly all of our observed stars are field stars. Wehave obtained high precision (~3-10 mmag at the bright end) photometricobservations of ~50000 stars spanning 18 nights in an attempt toidentify low-amplitude and short-period transit events. We have used amatched filter analysis to identify 14 stars that show multiple eventsand four stars that show single transits. Of these 18 candidates, wehave identified two that should be further researched. However, none ofthe candidates is a convincing hot Jupiter.
|A survey for planetary transits in the field of NGC 7789|
We present results from 30 nights of observations of the open clusterNGC 7789 with the Wide Field Camera on the Isaac Newton Telescope, LaPalma. From ~900 epochs, we obtained light curves and Sloan r'-i'colours for ~33000 stars, with ~2400 stars having better than 1 per centprecision. We expected to detect ~2 transiting hot Jupiter planets if 1per cent of stars host such a companion and a typical hot Jupiter radiusis ~1.2RJ. We find 24 transit candidates, 14 of which we canassign a period. We rule out the transiting planet model for 21 of thesecandidates using various robust arguments. For two candidates, we areunable to decide on their nature, although it seems most likely thatthey are eclipsing binaries as well. We have one candidate exhibiting asingle eclipse, for which we derive a radius of1.81+0.09-0.00RJ. Three candidatesremain that require follow-up observations in order to determine theirnature.
|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.
|Chemical composition of evolved stars in the open cluster NGC 7789|
High-resolution spectra of six giants and three core-helium-burning“clump” stars in the open cluster NGC 7789 have beenobtained with the SOFIN spectrograph on the Nordic Optical Telescope toinvestigate abundances of up to 20 chemical elements. Abundances ofcarbon were studied using the C2 Swan (0, 1) band head at5635.5 Å. The wavelength interval 7980 8130 Å with strong CNfeatures was analysed in order to determine nitrogen abundances and12C/13C isotope ratios. The oxygen abundances weredetermined from the [O I] line at 6300 Å. The overall metallicityof evolved stars in the cluster was found to be close to solar([Fe/H]=-0.04±0.05). Compared with the Sun and other dwarf starsof the Galactic disk, mean abundances in the investigated giant starssuggest that carbon is depleted by about 0.2 dex, and nitrogen andoxygen are close to solar. In the clump stars investigated, carbon isdepleted by about 0.2 dex, the mean abundance of nitrogen is enhanced by0.26 dex and oxygen is lower by 0.14 dex. This has the effect oflowering the mean C/N ratios to the value of 1.9±0.5 in the giantstars and to the value of 1.3±0.2 in the clump stars. The mean12C/13C ratios are lowered to about the same valueof 9±1 in the giants and clump stars investigated. Concerningother chemical elements an overabundance of sodium is noticeable and ofsilicon and calcium one is suspected. Abundances of iron-group andheavier chemical elements in all nine stars were found to be close tosolar.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?A+A/431/933Based on observations made with the Nordic Optical Telescope, which isoperated on the island of La Palma jointly by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.
|Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC|
Open clusters potentially provide an ideal environment for the searchfor transiting extrasolar planets, since they feature a relatively largenumber of stars of the same known age and metallicity at the samedistance. With this motivation, over a dozen open clusters are now beingmonitored by four different groups. We review the motivations andchallenges for open cluster transit surveys for short-period giantplanets. Our photometric monitoring survey of Galactic southern openclusters, the Extrasolar Planet Occultation Research/Open Clusters(EXPLORE/OC) project, was designed with the goals of maximizing thechance of finding and characterizing planets and of providing astatistically valuable astrophysical result in the case of nodetections. We use the EXPLORE/OC data from two open clusters, NGC 2660and NGC 6208, to illustrate some of the largely unrecognized issuesfacing open cluster surveys, including severe contamination by Galacticfield stars (>80%) and the relatively low number of cluster membersfor which high-precision photometry can be obtained. We discuss how acareful selection of open cluster targets under a wide range of criteriasuch as cluster richness, observability, distance, and age can meet thechallenges, maximizing chances to detect planet transits. In addition,we present the EXPLORE/OC observing strategy to optimize planetdetection, which includes high-cadence observing and continuouslyobserving individual clusters rather than alternating between targets.
|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.
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