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New Praesepe white dwarfs and the initial mass-final mass relation
We report the spectroscopic confirmation of four further white dwarfmembers of Praesepe. This brings the total number of confirmed whitedwarf members to 11, making this the second largest collection of theseobjects in an open cluster identified to date. This number is consistentwith the high-mass end of the initial mass function of Praesepe beingSalpeter in form. Furthermore, it suggests that the bulk of Praesepewhite dwarfs did not gain a substantial recoil kick velocity frompossible asymmetries in their loss of mass during the asymptotic giantbranch phase of evolution. By comparing our estimates of the effectivetemperatures and the surface gravities of WD0833+194, WD0840+190,WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks, wehave derived their masses to be in the range 0.72-0.76 Msolarand their cooling ages ~300 Myr. For an assumed cluster age of 625 +/-50 Myr, the inferred progenitor masses are between 3.3 and 3.5Msolar. Examining these new data in the context of theinitial mass-final mass relation, we find that it can be adequatelyrepresented by a linear function (a0 = 0.289 +/-0.051,a1 = 0.133 +/- 0.015) over the initial mass range 2.7-6Msolar. Assuming an extrapolation of this relation to largerinitial masses is valid and adopting a maximum white dwarf mass of 1.3Msolar, our results support a minimum mass for core-collapsesupernovae progenitors in the range ~6.8-8.6 Msolar.
| 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.
| 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.
| Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters
New photometric data in the Washington system are presented for redgiant candidates in NGC 1817 and 2251, two open clusters located towardsthe Galactic anticentre direction. In the case of NGC 2251, theWashington data are supplemented with new UBV and David DunlapObservatory (DDO) photoelectric photometry. Published radial velocitiesare used to separate field stars from cluster giants. The photometricdata yield an effective temperature and metal abundance for each clustermember. Five independent Washington abundance indices yield meanmetallicities of [Fe/H]= 0.25 +/- 0.04 for NGC 1817 and 2251,respectively. From combined BV and DDO data, we also derive E(B-V) =0.21 +/- 0.03 and [Fe/H]DDO=-0.14 +/- 0.05 for NGC 2251. Bothobjects are then found to be on the metal-poor side of the distributionof open clusters, their metallicities being compatible with theexistence of a radial abundance gradient in the disc. Using the WEBDAOpen Cluster data base and the available literature, we re-examined theoverall properties of a sample of 30 clusters located towards theGalactic anticentre with the distances, ages and metallicitiesavailable. This cluster sample presents no evidence of an abundancegradient perpendicular to the Galactic plane, nor is an age-metallicityrelation found. However, a radial abundance gradient of -0.093 dexkpc-1 is derived over a Galactocentric distance of 14 kpc, agradient which is in keeping with most recent determinations. This valuepractically does not change when all clusters with basic parametersknown up to this date are considered.
| 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 complete N-body model of the old open cluster M67
The old open cluster M67 is an ideal testbed for current clusterevolution models because of its dynamically evolved structure and richstellar populations that show clear signs of interaction betweenstellar, binary and cluster evolution. Here, we present the first trulydirect N-body model for M67, evolved from zero age to 4Gyr taking fullaccount of cluster dynamics as well as stellar and binary evolution. Ourpreferred model starts with 36000 stars (12000 single stars and 12000binaries) and a total mass of nearly 19000Msolar, placed in aGalactic tidal field at 8.0kpc from the Galactic Centre. Our choices forthe initial conditions and for the primordial binary population areexplained in detail. At 4Gyr, the age of M67, the total mass has reducedto 2000Msolar as a result of mass loss and stellar escapes.The mass and half-mass radius of luminous stars in the cluster are agood match to observations, although the model is more centrallyconcentrated than observations indicate. The stellar mass and luminosityfunctions (LFs) are significantly flattened by preferential escape oflow-mass stars. We find that M67 is dynamically old enough thatinformation about the initial mass function (IMF) is lost, both from thecurrent LF and from the current mass fraction in white dwarfs (WDs).The model contains 20 blue stragglers (BSs) at 4Gyr, which is slightlyless than the 28 observed in M67. Nine are in binaries. The bluestragglers were formed by a variety of means and we find formation pathsfor the whole variety observed in M67. Both the primordial binarypopulation and the dynamical cluster environment play an essential rolein shaping the population. A substantial population of short-periodprimordial binaries (with periods less than a few days) is needed toexplain the observed number of BSs in M67. The evolution and propertiesof two-thirds of the BSs, including all found in binaries, have beenaltered by cluster dynamics and nearly half would not have formed at alloutside the cluster environment. On the other hand, the clusterenvironment is also instrumental in destroying potential BSs from theprimordial binary population, so that the total number is in factslightly smaller than what would be expected from evolving the samebinary stars in isolation.
| The open-cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution
Recent studies of white dwarfs in open clusters have provided newconstraints on the initial-final mass relationship (IFMR) formain-sequence stars with masses in the range 2.5-6.5Msolar.We re-evaluate the ensemble of data that determines the IFMR and arguethat the IFMR can be characterized by a mean IFMR about which there isan intrinsic scatter. We investigate the consequences of the IFMR forthe observed mass distribution of field white dwarfs using populationsynthesis calculations. We show that while a linear IFMR predicts a massdistribution that is in reasonable agreement with the recent resultsfrom the Palomar-Green survey, the data are better fitted by an IFMRwith some curvature. Our calculations indicate that a significant (~28)percentage of white dwarfs originating from a single star evolution hasmasses in excess of ~0.8Msolar, obviating the necessity forpostulating the existence of a dominant population of high-mass whitedwarfs that arise from binary star mergers.
| The Age and Progenitor Mass of Sirius B
The Sirius AB binary system has masses that are well determined frommany decades of astrometric measurements. Because of the well-measuredradius and luminosity of Sirius A, we employed the TYCHO stellarevolution code to determine the age of the Sirius AB binary systemaccurately, at 225-250 Myr. Note that this fit requires the assumptionof solar abundance and the use of the new Asplund et al. primordialsolar metallicity. No fit to Sirius A's position is possible using theold Grevesse & Sauval scale. Because the Sirius B white dwarfparameters have also been determined accurately from space observations,the cooling age could be determined from recent calculations by Fontaineet al. or Wood to be 124+/-10 Myr. The difference in the two ages yieldsthe nuclear lifetime and mass of the original primary star,5.056+0.374-0.276 Msolar. This resultyields, in principle, the most accurate data point at relatively highmasses for the initial-to-final mass relation. However, the analysisrelies on the assumption that the primordial abundance of the Siriusstars was solar, based on membership in the Sirius supercluster. Arecent study suggests that its membership in the group is by no meanscertain.
| 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.
| 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 Initial-Final Mass Relationship: Spectroscopy of White Dwarfs in NGC 2099 (M37)
We present new observations of very faint white dwarfs (WDs) in the richopen star cluster NGC 2099 (M37). Following deep, wide-field imaging ofthe cluster using the Canada-France-Hawaii Telescope, we have nowobtained spectroscopic observations of candidate WDs using both theGemini Multi-Object Spectrograph on Gemini North and the Low-ResolutionImaging Spectrometer on Keck. Of our 24 WD candidates (all fainter thanV=22.4), 21 are spectroscopically confirmed to be bona fide WDs, four orfive of which are most likely field objects. Fitting 18 of the 21 WDspectra with model atmospheres, we find that most WDs in this clusterare quite massive (0.7-0.9 Msolar), as expected given thecluster's young age (650 Myr) and, hence, high turnoff mass (~2.4Msolar). We determine a new initial-final mass relationshipand almost double the number of existing data points from previousstudies. The results indicate that stars with initial masses between 2.8and 3.4 Msolar lose 70%-75% of their mass through stellarevolution. For the first time, we find some evidence of a metallicitydependence on the initial-final mass relationship.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.
| 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.
| 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.
| The astrophysics of cool white dwarfs
Electronic Article Available from Elsevier Science.
| A population model of the solar neighbourhood thin disc white dwarfs
We present a model of the solar neighbourhood (d < 100pc) white dwarf(WD) population and show the resulting effective temperature and massdistribution. Our model parameters, in particular the WD coolingtime-scale, are constrained by the models of Chabrier et al. The localinitial mass function and star formation rate (SFR, per unit volume inthe Galactic plane) of Schröder & Pagel are used for thecreation of the synthetic stars from which the WDs originate, as well asthe above authors' grid of evolutionary tracks. Furthermore, we considerin detail the significant depletion of the older (and cooler) WDs bydilution into the column, as caused by the dynamics of the `thin disc'.To verify our synthetic sample, especially the WD cooling time-scale, westudy the temperature distribution of a small but nearly complete,volume-limited sample of observed WDs, which we characterize by a singleand simple indicator R6300: the number ratio of WDs withTeff < 6300K over those with Teff > 6300K.After determination of the bias owing to a residual incompleteness withcool WDs, we find a corrected value of R6300 of 0.68(+/-0.24). This is in good agreement with our WD population model: itstemperature distribution yields R6300= 0.77.For a spherical volume around the Sun, with d < 100pc, our populationmodel suggests a total of about 13700 WDs (omiting part of the WDs witha binary system origin, an estimated 7 +/- 2 per cent of the totalcount). A subsample, limited to magnitude 19.0 (as expected for theEuropean Space Agency mission Gaia), would contain about 7750 WDs ofthis population model. With a less deep magnitude limit of B= 16.0, moretypical of current observed WD samples (SN Ia Progenitor surveY; SPY),the number of objects is reduced to only ~1350. We use this specificsynthetic subsample to test the completeness of the prospective SPY WDsample, which (with over 50 per cent of the candidates now observed) isalready the largest sample of WDs with high-resolution spectra. We findthat within d < 100pc and for B < 16.0 SPY will deliver a fairlycomplete (almost 80 per cent) sample.
| Interpreting the colour-magnitude diagrams of open star clusters through numerical simulations
We present detailed comparisons between high quality observationalcolour-magnitude diagrams (CMDs) of open star clusters and syntheticCMDs based on Monte Carlo numerical simulations. The comparisons accountfor all of the main parameters which determine the shape of the CMD fora stellar population. For the four clusters studied, NGC 6819, 2099(M37), 2168 (M35) and 2323 (M50), we derive reddening, distance, age,binary fraction, star formation rate and indicative metallicity bycomparing the locations and density of points in the observed CMDs tothe simulated CMDs. We estimate the uncertainties related to stellarevolution theories by adopting various sets of stellar models for all ofthe synthetic CMDs and discuss which stellar models provide thetheoretical CMDs that best reproduce the observations.
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| The Impact of Unresolved Binaries on Searches for White Dwarfs in Open Clusters
Many open clusters have a deficit of observed white dwarfs (WDs)compared with predictions of the number of stars that evolve into WDs.We evaluate the number of WDs produced in open clusters and the numberof those WDs that are detectable using photometric selection techniques.This calculation includes the effects of varying the initial massfunction (IMF), the maximum progenitor masses of WDs, and the binaryfraction. Differences between the calculated number of observable WDsand the actual number of WDs observed in a specific cluster thenindicate the true deficit of WDs that must be explained through effectssuch as dynamical evolution of the cluster or close binary evolution.Observations of WDs in three open clusters, the Hyades, Pleiades, andPraesepe, are compared to the calculated observable populations in thoseclusters. The results suggest that a large portion of the white dwarfdeficit may be explained by the presence of WDs in unresolved binarysystems. However, the calculated WD populations still overpredict thenumber of observable WDs in each cluster. While these calculationscannot determine the cause of this residual white dwarf deficit,potential explanations include a steep high-mass IMF, dynamicalevolution of the cluster, or an increased likelihood of equal-masscomponents in a binary system. Observations of complete WD samples inopen clusters covering a range of ages and masses can help todistinguish between these possibilities.
| WIYN Open Cluster Study. XIX. Main-Sequence-Fitting Distances to Open Clusters Using V-K Color-Magnitude Diagrams
We have combined existing optical magnitudes for stars in seven openclusters and 54 field stars with the corresponding JHKsphotometry from the Two Micron All Sky Survey (2MASS). Combining opticalwith near-IR photometry broadens the color baseline, minimizing theinfluence of photometric errors and allowing better discriminationbetween cluster stars and contaminating foreground and backgroundpopulations. The open clusters in this study include NGC 2516, M35, M34,NGC 3532, M37, M67, and NGC 188. The field stars we are using possesshigh-quality Hipparcos parallaxes and well-determined metal abundances,allowing us to empirically determine the dependence of V-K color onmetal abundance in the range -0.45<=[Fe/H]<=+0.35.Using this relation along with the parallaxes of the field stars, we areable to construct an unevolved main sequence in the [MV,(V-K)0] diagram for a specific abundance. These diagrams arethen used to fit to the cluster main sequences in the (V, V-K)color-magnitude diagram in order to estimate a distance for each opencluster. We find that the resultant distances are within the range ofdistances found in the literature via the main-sequence-fittingtechnique. It is hoped that this will spur an expansion of the current(limited) database of star clusters with high-quality V-K photometrydown to the unevolved main sequence.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| Red giants in open clusters. XI. Membership, duplicity, and structure of NGC 2477
New, accurate radial velocities and photoelectric UBV photometry of 83red-giant candidates in the field of the rich, intermediate-age opencluster NGC 2477 ([Fe/H]= -0.05, age ≃1 Gyr)are presented and discussed. From 49 constant-velocity members we find amean cluster velocity of +7.32±0.13 km s-1 and confirmthe membership of 76 of the stars. Among the cluster members, weidentify 26 definite and 1 probable spectroscopic binaries and determineorbits for 13 of these systems, with periods ranging from 40 to 4578days. The binary frequency is thus rather high (27/76 = 36%). Theobserved internal radial velocity dispersion of the cluster, asdetermined from the single member stars, is 0.93 km s-1,corrected for the small average observational error of 0.22 kms-1. Fitting King-type models to the observed stellar densitydistribution and velocity dispersion, and assuming a distance of 1.25kpc, we find the core and tidal radii of NGC 2477 tobe 1.8 and 8.1 pc, respectively, and estimate that the mass of clusterstars down to V = 17, corresponding to 1 Mȯ, is atleast 5400 Mȯ. The substantial differential reddening ofNGC 2477 requires a more detailed study beforedefinitive isochrone fits can be made.Based on observations collected with the Danish 1.54-m and ESO 1-mtelescopes at the European Southern Observatory, La Silla, Chile, andwith the University of Toronto 0.6-m telescope at Las CampanasObservatory, Chile.Full Tables 1 and 2 are only 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/423/189
| The relatively young, metal-poor and distant open cluster NGC 2324
We have obtained CCD photometry in the Johnson V, Kron-Cousins I andCT1 Washington systems for NGC 2324, a rich open clusterlocated 35° from the Galactic anticentre direction. We measuredV magnitudes and V-I colours for 2865 stars and T1 magnitudesand C-T1 colours for 1815 stars in an area of 13.6 arcmin× 13.6 arcmin. The comparison of the cluster colour-magnitudediagrams with isochrones of the Geneva group yield E(V-I) = 0.33± 0.07 and V-MV = 13.70 ± 0.15 for log t = 8.65(t = 440 Myr) and Z = 0.008 ([Fe/H] = -0.40), and E(C-T1) =0.40 ± 0.10 and T1-MT1 = 13.65 ±0.15 for the same age and metallicity level. The resulting E(V-I)reddening value implies E(B-V) = 0.25 ± 0.05 and a distance fromthe Sun of (3.8 ± 0.5) kpc. Star counts carried out within andoutside the cluster region allowed us to estimate the cluster angularradius as 5.3 arcmin ± 0.3 arcmin (5.9 pc). When using the E(B-V)reddening value here derived and the original Washington photometricdata of \citet{gcm91} for the stars confirmed as red cluster giants fromCoravel radial velocities, we found [Fe/H] = -0.31 ± 0.04, whichis in good agreement with the best fits of isochrones. Therefore, NGC2324 is found to be a relatively young, metal-poor and distant opencluster located beyond the Perseus spiral arm. A comparison of NGC 2324with 10 well-known open clusters of nearly the same age shows that thecluster metal abundance and its position in the Galaxy are consistentwith the existence of a radial abundance gradient of -0.07 dexkpc-1 in the Galactic disc.
| WIYN Open Cluster Study - XVI. Optical/infrared photometry and comparisons with theoretical isochrones
We present combined optical/near-infrared photometry (BVIK) for six openclusters - M35, M37, NGC 1817, NGC 2477, NGC 2420 and M67. The openclusters span an age range from 150 Myr to 4 Gyr and have metalabundances from [Fe/H]=-0.27 to +0.09 dex. We have utilized these datato test the robustness of theoretical main sequences constructed byseveral groups as denoted by the following designations - Padova,Baraffe, Y2, Geneva and Siess. The comparisons of the modelswith the observations have been performed in the [MV,(B-V)0], [MV, (V-I)0] and[MV, (V-K)0] colour-magnitude diagrams as well asthe distance-independent [(V-K)0, (B-V)0] and[(V-K)0, (V-I)0] two-colour diagrams. We concludethat none of the theoretical models reproduces the observational data ina consistent manner over the magnitude and colour range of the unevolvedmain sequence. In particular, there are significant zero-point and shapedifferences between the models and the observations. We speculate thatthe crux of the problem lies in the precise mismatch between theoreticaland observational colour-temperature relations. These results underscorethe importance of pursuing the study of stellar structure and stellarmodelling with even greater intensity.
| Setting up of the Strömvil System Standards and Standard Regions
CCD photometry of standard areas of the seven-color Strömvilphotometric system in some open and globular clusters is described. Theprimary standards are being measured with a two-channel photoelectricphotometer on the 1.5 meter telescope on Mt. Lemmon. CCD photometry ofstandard areas is in progress with the Vatican Advanced TechnologyTelescope on Mt. Graham and with the CASLEO 2.1 m telescope inArgentina. Two additional Strömvil projects are described.
| Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS): The Frequency of Planets in NGC 1245
We present first results from the Survey for Transiting ExtrasolarPlanets in Stellar Systems (STEPSS). Our goal is to assess the frequencyof close-in extrasolar planets around main-sequence stars in severalopen clusters. By concentrating on main-sequence stars in clusters ofknown (and varied) age, metallicity, and stellar density, we will gaininsight into how these various properties affect planet formation,migration, and survival. We show preliminary results from our 19 nightphotometric campaign of the old, solar metallicity cluster NGC 1245.Taking into account the photometric precision, observational windowfunction, transit probability, and total number of cluster membersmonitored, if no transits are detected, we can rule out > 5% ofmain-sequence stars in NGC 1245 have Jupiter-sized companions withseparations of a < 0.09 AU. If 1% of the stars in the cluster haveJupiter-sized companions evenly distributed in log(a) between 0.03 and0.3 AU, we expect to find about 2 transits. A preliminary search of ourlight curve data has revealed a transit with a depth of about 4%. Basedon its shape, it is likely to be a grazing binary eclipse rather than aplanetary transit, emphasizing the need for high temporal resolution intransit surveys.
| Proper Motions of Open Star Clusters and the Rotation Rate of the Galaxy
The mean proper motions of 167 Galactic open clusters withradial-velocity measurements are computed from the data of the Tycho-2catalog using kinematic and photometric cluster membership criteria. Theresulting catalog is compared to the results of other studies. The newproper motions are used to infer the Galactic rotation rate at the solarcircle, which is found to be ω0=+24.6±0.8 km s-1 kpc-1.Analysis of the dependence of the dispersion of ω0 estimates onheliocentric velocity showed that even the proper motions of clusterswith distances r>3 kpc contain enough useful information to be usedin kinematic studies demonstrating that the determination of propermotions is quite justified even for very distant clusters.
| Cool White Dwarfs
Old, cool white dwarfs convey valuable information about the earlyhistory of our Galaxy. They have been used to determine the age of thegalactic disk, several open clusters, and a globular cluster. We reviewthe current understanding of the physics of cool white dwarfs, includingtheir mass distribution, chemical evolution, magnetism, and cooling. Wealso examine the role of white dwarfs as tracers of various stellarpopulations, both in terms of observational searches and theoreticalmodels.
| The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)
We continue our study of rich Galactic clusters by presenting deep CCDobservations of both NGC 2168 (M35) and NGC 2323 (M50). Both clustersare found to be rich (NGC 2168 contains at least 1000 stars brighterthan V=22, and NGC 2323 contains ~2100 stars brighter than ourphotometric limit of V~23) and young (NGC 2168 age=180 Myr and for NGC2323 age=130 Myr). The color-magnitude diagrams for the clusters exhibitclear main sequences stretching over 14 mag in the (V, B-V)-plane.Comparing these long main sequences with those of earlier clusters inthe survey, as well as with the Hyades, has allowed for accuratedistances to be established for each cluster (for NGC 2168d=912+70-65 pc and for NGC 2323d=1000+81-75 pc). Analysis of the luminosity andmass functions suggests that, despite their young ages, both clustersare somewhat dynamically relaxed, exhibiting signs of mass segregation.This is especially interesting in the case of NGC 2323, which has an ageof only 1.3 times the dynamical relaxation time. The present photometryis also deep enough to detect all of the white dwarfs in both clusters.We discuss some interesting candidates that may be the remnants of quitemassive (M>=5Msolar) progenitor stars. The white dwarfcooling age of NGC 2168 is found to be in good agreement with themain-sequence turnoff age. These objects are potentially very importantfor setting constraints on the white dwarf initial-final massrelationship and the upper mass limit for white dwarf production.
| On the Galactic Disk Metallicity Distribution from Open Clusters. I. New Catalogs and Abundance Gradient
We have compiled two new open cluster catalogs. In the first one, thereare 119 objects with ages, distances, and metallicities available, whilein the second one, 144 objects have both absolute proper motion andradial velocity data, of which 45 clusters also have metallicity dataavailable. Taking advantage of the large number of objects included inour sample, we present an iron radial gradient of about -0.063+/-0.008dex kpc-1 from the first sample, which is quite consistentwith the most recent determination of the oxygen gradient from nebulaeand young stars, about -0.07 dex kpc-1. By dividing clustersinto age groups, we show that the iron gradient was steeper in the past,which is consistent with the recent result from Galactic planetarynebulae data, and also consistent with inside-out galactic diskformation scenarios. Based on the cluster sample, we also discuss themetallicity distribution, cluster kinematics, and space distribution. Adisk age-metallicity relation could be implied by those properties,although we cannot give conclusive result from the age- metallicitydiagram based on the current sample. More observations are needed formetal-poor clusters. From the second catalog, we have calculated thevelocity components in cylindrical coordinates with respect to theGalactic standard of rest for 144 open clusters. The velocitydispersions of the older clusters are larger than those of youngclusters, but they are all much smaller than that of the Galactic thickdisk stars.
| CCD photometric search for peculiar stars in open clusters. V. NGC 2099, NGC 3114, NGC 6204, NGC 6705 and NGC 6756
We have investigated 1008 objects in the area of five intermediate ageopen clusters (NGC 2099, NGC 3114, NGC 6204, NGC 6705 and NGC 6756) viathe narrow band Δ a-system. The detection limit for photometricpeculiarity is very low (always less than 0.009 mag) due to the highnumber of individual frames used (193 in total). We have detected sixpeculiar objects in NGC 6705 and NGC 6756 from which one in the latteris almost certainly an unreddened late type foreground star. Theremaining five stars are probably cluster members and bona fidechemically peculiar objects (two are łambda Bootis typecandidates). Furthermore, we have investigated NGC 3114, a cluster forwhich already photoelectric Δ a-measurements exist. A comparisonof the CCD and photoelectric values shows very good agreement. Again,the high capability of our CCD Δ a-photometric system to sort outtrue peculiar objects together with additional measurements from broador intermediate band photometry is demonstrated.Based on observations obtained at Complejo Astronómico elLeoncito (CASLEO), operated under the agreement between the ConsejoNacional de Investigaciones Científicas y Técnicas de laRepública Argentina and the National Universities of La Plata,Córdoba y San Juan; ESO-La Silla, UTSO-Las Campanas and L. FiglObservatory, Mt. Schöpfl (Austria).Figure 1 and Table 2 are only available in electronic form athttp://www.edpsciences.org
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