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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.
|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.
|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 (184.108.40.206) 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
|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|
|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.
|Toward the Detection of Transiting Hot Earths and Hot Neptunes in Open Clusters|
Radial velocity searches for extrasolar planets have recently detectedseveral very low mass (7-20 M_oplus) planets in close orbits withperiods less than 10 days. We consider the prospects for detecting theanalogs of these planets in Galactic open clusters via transits. Weoutline the requirements for constructing a transit survey that wouldallow one to probe such ``Hot Earths'' and ``Hot Neptunes.''Specifically, we present a simple criterion for detection that definesthe minimum aperture required to detect planets of a given radius in acluster at a given distance. Adopting photometric precisions that havebeen demonstrated in state-of-the-art variability surveys, we thenpredict the number of planets one could potentially detect withambitious transit surveys toward several open clusters. Dedicatedsurveys lasting more than 20 nights with Pan-STARRS toward the Hyadesand Praesepe could detect a handful of Hot Earths, if the majority ofstars host such planets. Similar surveys with larger aperture telescopes(eg CFHT, MMT), toward M67, M35, M50, and M37 could detect Hot Neptunes,provided that their frequency is greater than 1%. The majority ofplanets will be detected around M dwarfs; detecting Hot Neptunes aroundsuch primaries requires photometric precisions of approx 1%, whereas HotEarths require approx 0.1 %. We discuss potential hurdles in detectingand confirming small planets in ground-based surveys, includingcorrelated noise, false positives, and intrinsic stellar variability.
|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.
|UBVI CCD Photometry of the Old Open Cluster Berkeley 17|
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley17. Color-magnitude diagrams (CMDs) are constructed and reach well pastthe main-sequence turnoff for both clusters. Cluster ages are determinedby means of isochrone fitting to the cluster CMDs. These fits areconstrained to agree with spectroscopic metallicity and reddeningestimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188and 10.0+/-1.0 Gyr for Berkeley 17, where the errors refer touncertainties in the relative age determinations. These ages arecompared to the ages of relatively metal-rich inner halo/thick-diskglobular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyryounger than the thick-disk globular clusters. These results confirm thestatus of Berkeley 17 as one of the oldest known open clusters in theMilky Way, and its age provides a lower limit to the age of the Galacticdisk.
|The Bologna Open Cluster Chemical Evolution Project: Midterm Results from the Photometric Sample|
We describe a long-term project aimed at deriving information on thechemical evolution of the Galactic disk from a large sample of openclusters. The main property of this project is that all clusters areanalyzed in a homogeneous way to guarantee the robustness of the rankingin age, distance, and metallicity. Special emphasis is devoted to theevolution of the earliest phases of the Galactic disk evolution, forwhich clusters have superior reliability with respect to other types ofevolution indicators. The project is twofold: on one hand we derive theage, distance, and reddening (and indicative metallicity) byinterpreting deep and accurate photometric data with stellar evolutionmodels, and on the other hand, we derive the chemical abundances fromhigh-resolution spectroscopy. Here we describe our overall goals andapproaches and report on the midterm project status of the photometricpart, with 16 clusters already studied, covering an age interval from0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. Theimportance of quantifying the theoretical uncertainties by deriving thecluster parameters with various sets of stellar models is emphasized.Stellar evolution models assuming overshooting from convective regionsappear to better reproduce the photometric properties of the clusterstars. The examined clusters show a clear metallicity dependence on thegalactocentric distance and no dependence on age. The tight relationbetween cluster age and magnitude difference between the main-sequenceturnoff and the red clump is confirmed.
|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.
|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|
|Survey for Transiting Extrasolar Planets in Stellar Systems. II. Spectrophotometry and Metallicities of Open Clusters|
We present metallicity estimates for seven open clusters based onspectrophotometric indices from moderate-resolution spectroscopy.Observations of field giants of known metallicity provide a correlationbetween the spectroscopic indices and the metallicity of open clustergiants. We use χ2 analysis to fit the relation ofspectrophotometric indices to metallicity in field giants. The resultingfunction allows an estimate of the target-cluster giants' metallicitieswith an error in the method of +/-0.08 dex. We derive the followingmetallicities for the seven open clusters: NGC 1245, [M/H]=-0.14+/-0.04NGC 2099, [M/H]=+0.05+/-0.05 NGC 2324, [M/H]=-0.06+/-0.04 NGC 2539,[M/H]=-0.04+/-0.03 NGC 2682 (M67), [M/H]=-0.05+/-0.02 NGC 6705,[M/H]=+0.14+/-0.08 NGC 6819, [M/H]=-0.07+/-0.12. These metallicityestimates will be useful in planning future extrasolar planet transitsearches, since planets may form more readily in metal-richenvironments.
|High-resolution spectroscopy of the old open cluster Collinder 261: abundances of iron and other elements|
We present the analysis of high resolution spectra of six red giantstars in the old open cluster Collinder 261. Reddening values forindividual stars, derived from the relation between colours andtemperatures (deduced from our fully spectroscopic analysis) areconsistent with previous determinations based on photometry. For thiscluster we derive an iron abundance of [Fe/H] = - 0.03 ± 0.03. Wealso obtain the abundances of light metals (O, Na and Al),α-elements (Mg, Si, Ca, Ti), elements of the Fe-group (Sc, Cr, Mn,Co, Ni) and the neutron-capture element Ba. No intrinsic star-to-starscatter is present in any of these elements within our sample. Wecompare our findings with previous investigations on this cluster,discussing in detail differences in analysis methods and results.
|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.
|Variable stars in the field of open cluster NGC 6819 - II|
We report on the discovery of 141 further variable stars found in thefield of the open cluster NGC 6819. The stars were identified fromtime-series photometric data obtained on the Isaac Newton Telescope, LaPalma, during two observing runs covering the 19 nights between 1999June 22-30 and 1999 July 22-31. The variables found include 53 eclipsingbinaries, of which eight stars appear to be RS CVns, in addition to 70stars showing spot activity, 13 showing long-period variability and fivevariables of other types.
|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.
|A Robust Measure of Tidal Circularization in Coeval Binary Populations: The Solar-Type Spectroscopic Binary Population in the Open Cluster M35|
We present a new homogeneous sample of 32 spectroscopic binary orbits inthe young (~150 Myr) main-sequence open cluster M35. The distribution oforbital eccentricity versus orbital period (e-logP) displays a distincttransition from eccentric to circular orbits at an orbital period of ~10days. The transition is due to tidal circularization of the closestbinaries. The population of binary orbits in M35 provide a significantlyimproved constraint on the rate of tidal circularization at an age of150 Myr. We propose a new and more robust diagnostic of the degree oftidal circularization in a binary population based on a functional fitto the e-logP distribution. We call this new measure the ``tidalcircularization period.'' The tidal circularization period of a binarypopulation represents the orbital period at which a binary orbit withthe most frequent initial orbital eccentricity circularizes (defined ase=0.01) at the age of the population. We determine the tidalcircularization period for M35, as well as for seven additional binarypopulations spanning ages from the pre-main sequence (~3 Myr) to thelate main sequence (~10 Gyr), and use Monte Carlo error analysis todetermine the uncertainties on the derived circularization periods. Weconclude that current theories of tidal circularization cannot accountfor the distribution of tidal circularization periods with populationage.WIYN Open Cluster Study XXII.
|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.
|Infrared Photometry of NGC 6791|
We present deep JHK photometry of the old and metal-rich open clusterNGC 6791. The photometry reaches below the main-sequence turnoff toK~16.5 mag. We combine our photometry with that from Stetson et al. toprovide color-magnitude diagrams showing K versus J-K, K versus V-K, andV versus V-K. We study the slope of the red giant branch in the infraredbut find that it is not a useful metallicity indicator for the cluster,nor any metal-rich cluster that lacks a well-populated red giant branch,because it is not linear, as has often been assumed, in K versus J-K.The mean color of the red horizontal-branch/red clump stars provide anestimate of the cluster reddening, E(B-V)=0.14+/-0.04 mag for[Fe/H]=+0.4+/-0.1. The mean magnitudes of these stars also provide agood distance estimate, (m-M)0=13.07+/-0.04. Finally, we findthat the isochrones of Yi et al. provide optimal fits in V versus B-Vand V-K and K versus J-K and V-K for such values if [Fe/H] lies between+0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyr([Fe/H]=+0.3) and 7.5 Gyr ([Fe/H]=+0.5).Based on observations made with the Mayall 4 m Telescope of the NationalOptical Astronomy Observatory.
|The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters|
Spectra have been obtained of 21 white dwarfs (WDs) in the direction ofthe young, rich open star cluster NGC 2099. This represents anappreciable fraction (>30%) of the cluster's total WD population. Themean derived mass of the sample is 0.8 Msolar-about 0.2Msolar larger than the mean seen among field WDs. Asurprising result is that all of the NGC 2099 WDs have hydrogen-richatmospheres (DAs); none exhibit helium-rich ones (DBs) or any otherspectral class. The number ratio in the field at the temperatures of theNGC 2099 WDs is DA/DB ~ 3.5. While the probability of seeing no DB WDsin NGC 2099 solely by chance is ~2%, if we include WDs in other openclusters of similar age it then becomes highly unlikely that the dearthof DB WDs in young open clusters is just a statistical fluctuation. Weexplore possible reasons for the lack of DBs in these clusters andconclude that the most promising scenario for the DA/DB number ratiodiscrepancy in young clusters is that hot, high-mass WDs do not developlarge enough helium convection zones to allow helium to be brought tothe surface and turn a hydrogen-rich WD into a helium-rich one.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
|The Maria Mitchell Observatory Plate Collection as a Mirror of the Evolution of Astronomical Photographic Emulsions|
The recently digitized astronomical plate collection of the MariaMitchell Observatory (MMO) was used to study the evolution of limitingstellar magnitudes of the plates as various astronomical emulsions wereused over the years. The information on limiting magnitudes was obtainedby eye photometry of the weakest detectable stars on the digitizedimages of the open cluster NGC 6819. In total, 174 plate copiescontaining NGC 6819 and covering the period of time from the 1920s toabout 1990 were evaluated. A remarkable increase of the limitingmagnitude during these seven decades was revealed. The best plates withthe early astronomical emulsions, Speedway, Presto, and HiSpeed, showedlimiting magnitudes 14.4, 15.7, and 16.4, respectively, whereas thelimiting magnitude on the best plates with the post-1950 emulsions,103aO and IIaO (some hyper-sensitized with N2), are 17.4 and 17.2,respectively. The scatter of the limiting magnitudes from plate to plategradually decreased from about three magnitudes in the 1920s to aboutone magnitude in the 1980s. The scatter of the limiting magnitudes mustbe due not only to the scatter of the conditions of observations, butalso to the quality and homogeneity of the batches of plates, whichappears to steadily increase with time, together with the sensitivity ofthe plates.
|Stellar Photometry Using Old Photographic Plates|
The precision of various methods of stellar photometry on oldphotographic plates is investigated using the original plates or theirdigitized copies from the plate collection of the Maria MitchellObservatory (MMO), It is shown, in particular, that the simple and fastmethod of eye photometry is comparable in precision to the traditional"objective" methods using a microphotometer to measure the plates ofimage analysis software to measure digital copies of small parts of theplates obtained with a CCD camera. All these methods provide photometricaccuracy of +/-0.1-0.2 magnitude on the MMO plates. It is demonstratedthat the high-performance commercially available scanner AgfaScan T5000used for plate digitization at the MMO produces images that can bemeasured to a considerably higher precision of +/-0.05 magnitude, whichis sufficient for most purposes of photographic stellar photometry. Theresults of this investigation may be of interest to those who use oldplates for stellar photometry, as well as to those who look for anadequate, fast, and relatively inexpensive scanner to digitize theirplate archive.
|Photospheric Activity among Stars in Old Open Clusters: NGC 7789|
We present initial results of the search for photospheric stellaractivity in the open cluster NGC 7789 based on seven seasons ofbroadband V observations and one season of broadband BVR observations.We have achieved a precision as good as 0.9 mmag for the nightly averageV differential magnitudes of the best stars and a precision of 0.4 mmagfor the annual average V magnitudes. The brightness fluctuations of thephotometrically chosen main-sequence stars show a distinct positivecorrelation between the variations in V and those in B and R on thenight-to-night timescale; the amplitude of the fluctuations is alsolarger for the longer wavelengths. The V-band light curves of the annualmean differential magnitudes of the main-sequence stars are similar toboth the fluctuations observed in nearby bright stars and to spacecraftsolar irradiance data. The standard deviations of the variabilityincrease with B-V color for the main-sequence stars. From these results,we conclude that the variations are caused by phenomena associated withstellar activity.
|WIYN Open Cluster Study MOSAIC photometry of NGC 6819|
We have obtained deep CCD photometry in the VI passbands for a 1-degreefield around the open cluster NGC 6819. The color-magnitude diagram(CMD) shows a well-defined main sequence to the limit of the photometryat V 21. We will analyze the CMD and the luminosity function (LF)focusing in particular on radial variations in these diagrams. In thisway, we will investigate the effects of stellar dynamics on theappearance of the CMD and LF. This research is supported by NSF grantAST-0196212 to A. Sarajedini.
|On the Old Open Clusters M67 and NGC 188: Convective Core Overshooting, Color-Temperature Relations, Distances, and Ages|
The color-magnitude diagram of M67 is used to constrain the value of theparameter Fover in its ~1.3 Msolar turnoff stars:Fover is effectively the fraction of the maximum possibleextent of convective core overshooting predicted by the Roxburgh (1989,A&A, 211, 361) criterion. Isochrones that treat overshooting usingthe parameterized Roxburgh criterion are able to reproduce themorphology of the cluster turnoff, including the luminosity of the gap,if Fover~0.07 (assuming current best estimates for thereddening and metallicity). Previous studies have derived values ofFover near 0.5 in open clusters of similar metallicity buthaving turnoff masses >=1.55 Msolar, indicating that theovershooting parameter is a strong function of mass in the lowest massstars that retain convective cores throughout the main-sequence phase.NGC 188 appears to be too old for core overshooting to play any role inthe evolution of stars that are currently in the core H-burning phase,but the availability of well-calibrated BVRI photometry for this system,together with Landolt photometric standards, provides valuable tests ofthe color-Teff relations that apply to [Fe/H]~0.0 stars. Ouranalysis of color-color diagrams, in particular, suggests that theCastelli (1999, A&A, 346, 564) V-R and V-I transformations for lowergravity stars are more realistic than those published recently byVandenBerg & Clem (2003, AJ, 126, 778). The same color-colordiagrams also indicate that the differences in recent (V-I,V) diagramsfor M67 appear to be due, at least in part, to the difficulty ofdefining the standard VRI photometric system. The ages of M67 and NGC188 that have been derived in this study are 4.0 and 6.8 Gyr,respectively, which is consistent with other modern determinations.
|Metal Abundances in Extremely Distant Galactic Old Open Clusters. I. Berkeley 29 and Saurer 1|
We report on high-resolution spectroscopy of four giant stars in theGalactic old open clusters Berkeley 29 and Saurer 1 obtained with HIRESat the Keck Telescope. These two clusters possess the largestGalactocentric distances yet known for open star clusters and thereforeare crucial objects to probe the chemical pattern and evolution of theoutskirts of the Galactic disk. We find that [Fe/H]=-0.38+/-0.14 and-0.44+/-0.18 for Saurer 1 and Berkeley 29, respectively. On the basis ofthese data, we first revise the fundamental parameters of the clustersand then discuss them in the context of the Galactic disk radialabundance gradients. Both clusters seem to significantly deviate fromthe general trend, suggesting that the outer part of the Galactic diskunderwent a completely different evolution compared with the inner disk.In particular, Berkeley 29 is clearly associated with the Monocerosstream, whereas Saurer 1 exhibits very different properties. Theabundance ratios suggest that the chemical evolution of the outer diskwas dominated by the Galactic halo.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
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