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Searching for Earth Analogs Around the Nearest Stars: The Disk Age-Metallicity Relation and the Age Distribution in the Solar Neighborhood
The chemical composition of Earth's atmosphere has undergone substantialevolution over the course of its history. It is possible, even likely,that terrestrial planets in other planetary systems have undergonesimilar changes; consequently, the age distribution of nearby stars isan important consideration in designing surveys for Earth analogs.Valenti & Fischer provide age and metallicity estimates for 1039 FGKdwarfs in the solar neighborhood. Using the Hipparcos catalog as areference to calibrate potential biases, we have extractedvolume-limited samples of nearby stars from the Valenti-Fischer dataset. Unlike other recent investigations, our analysis shows clearevidence for an age-metallicity relation in the local disk, albeit withsubstantial dispersion at any epoch. The mean metallicity increases from~-0.3 dex at a look-back time of ~10 Gyr to ~+0.15 dex at the presentday. Supplementing the Valenti-Fischer measurements with literaturemetallicity data to give a complete volume-limited sample, the agedistribution of nearby FGK dwarfs is broadly consistent with a uniformstar-formation rate over the history of the Galactic disk. In strikingcontrast, most stars known to have (gas giant) planetary companions areyounger than 5 Gyr; however, stars with planetary companions within 0.4AU have a significantly flatter age distribution, indicating that thosesystems are stable on timescales of many gigayears. Several of theolder, lower metallicity host stars have enhanced [α/Fe] ratios,implying membership of the thick disk. If the frequency of terrestrialplanets is also correlated with stellar metallicity, then the median ageof such planetary system is likely to be ~3 Gyr. We discuss theimplications of this hypothesis in designing searches for Earth analogsamong the nearby stars.
| 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.
| Element abundances in the metal-rich open cluster NGC 6253
Context: We have carried out a big FLAMES survey of 10 Galactic openclusters aiming at different goals. One of them is the determination ofchemical abundances, to put constraints on the radial metallicitygradient in the disk and its evolution. One of the sample clusters isthe very metal-rich NGC 6253. Aims: We have obtained UVES highresolution spectra of seven candidate cluster members (from the turn offup to the red clump) with the goal of determining the chemicalcomposition of NGC 6253 and of investigating its origin and role in theinterpretation of the radial metallicity gradient in the disk. Methods:Equivalent width analysis and spectral synthesis were performed usingMOOG and Kurucz model atmospheres. Results: We derived abundances ofFe, α-, and Fe-peak elements, the light element Na, and thes-process element Ba. Excluding two likely non-members and the clumpgiant, whose metallicity from equivalent widths is overestimated, wefind an average [Fe/H] = +0.36±0.07 (rms) for the cluster. Formost of the other elements we derive solar abundance ratios.Based on observations collected at ESO telescopes under program072.D-0550(A). Table 3 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/185
| 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 (130.79.128.5) 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
| A Hubble Space Telescope Survey of the Disk Cluster Population of M31. I. WFPC2 Pointings
As a follow-up to the automated cluster search carried out by Williams& Hodge, we have examined 39 Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) pointings to locate and study a comprehensivecollection of disk clusters. The Williams technique was effective infinding young clusters, but not intermediate-age or old clusters. Oursearches have shown that M31 has large numbers of these intermediate andolder open clusters, most of them undetected by both the Williams surveyand other ground-based searches. We present a catalog of 343 clustersdetected on the WFPC2 images. Extrapolation from our data indicates thatthe entire disk of M31 contains approximately 80,000 star clusters. Wehave carried out integrated multicolor photometry of these clusters toascertain their properties and to compare their properties with clustersystems of other galaxies. We show the cluster luminosity function, thecolor-magnitude diagram, the formation function, and the sizedistribution. Cluster densities and colors show trends with diskposition. An age distribution is derived and, although the ages are veryuncertain for the fainter clusters, there is evidence for clusterdynamical destruction at about the same rate as in our Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Investigating star formation in the young open cluster NGC 6383
Context: By studying young open clusters, the mechanisms important forstar formation over several Myr can be examined. For example, accretionrate as a function of rotational velocity can be investigated.Similarly, sequential star formation triggered by massive stars withhigh mass-loss rates can be studied in detail. Aims: We identifiedand characterized probable members of NGC 6383, as well as determinedcluster parameters. Methods: New Strömgren uvby CCD photometry,obtained by us, is presented. This new data, together with Johnson UBVand 2MASS data in the NIR, was used to investigate characteristics ofpre- as well as zero age main sequence cluster members. Results: Wepresent Strömgren uvby CCD photometry for 272 stars in the field ofNGC 6383 and derive its reddening, E(b-y) = 0.21(4) mag, as well asdistance, d = 1.7(3) kpc from the Sun. Several stars with NIR excess andobjects in the domain of the classical Herbig Ae/Be and T Tauri starswere detected. Two previously known variables were identified asrapidly-rotating PMS stars. The field population is clearly separatedfrom the probable members in the color-magnitude diagram. Conclusions:. NGC 6383 is a young open cluster, with an age of less than 4 Myr,undergoing continuous star formation. True pre-main sequence membersmight be found down to absolute magnitudes of +6 mag, with a variety ofrotational velocities and stellar activities.Based on observations at ESO-La Silla (Proposal 073.C-0144). Data tableis only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/157
| On the age of stars harboring transiting planets
Results of photometric surveys have brought to light the existence of apopulation of giant planets orbiting their host stars even closer thanthe hot Jupiters (HJ), with orbital periods below 3 days. The reason whyradial velocity surveys were not able to detect these very-hot Jupiters(VHJ) is under discussion. A possible explanation is that these close-inplanets are short-lived, being evaporated on short time-scales due to UVflux of their host stars. In this case, stars hosting transiting VHJplanets would be systematically younger than those in the radialvelocity sample. We have used the UVES spectrograph (VLT-UT2 telescope)to obtain high resolution spectra of 5 faint stars hosting transitingplanets, namely, OGLE-TR-10, 56, 111, 113 and TrES-1. Previouslyobtained CORALIE spectra of HD189733, and published data on the othertransiting planet-hosts were also used. The immediate objective is toestimate ages via Li abundances, using the Ca II activity-age relation,and from the analysis of the stellar rotational velocity. For the starsfor which we have spectra, Li abundances were computed as in Israelianet al. (2004, A&A, 414, 601) using the stellar parameters derived inSantos et al. (2006, A&A, 450, 825). The chromospheric activityindex S_US was built as the ratio of the flux within the core of the CaII H & K lines and the flux in two nearby continuum regions. Theindex S_US was calibrated to Mount Wilson index S_MW allowing thecomputation of the Ca II H & K corrected for the photosphericcontribution. These values were then used to derive the ages by means ofthe Henry et al. (1996) activity-age relation. Bearing in mind thelimitations of the ages derived by Li abundances, chromosphericactivity, and stellar rotational velocities, none of the stars studiedin this paper seem to be younger than 0.5 Gyr.
| 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.
| Multiband photometry towards the inner Galactic bulge: extinction, atmospheric parameters, and metallicities
Context: .We have obtained BVRI photometry for 21 stellar fields locatedin directions towards the central 3.5^circ of the Galaxy. Each field is10× 10 arcmin in size, and a total of 2 × 105stars were measured. Aims: .Previous analyses based on 2 MicronAll Sky Survey (2MASS) data have shown that these directions have arelatively low extinction (AK ≃ 0.25-0.30). Thecombined optical-infrared sample contains about 3.5 ×104 stars. This large dataset provides a more detailedextinction map within these surveyed areas than previously derived. Thelarge number of colour indexes available also yielded estimates ofeffective temperatures and metallicities for the inner bulge stars. Methods: .Extinction and effective temperature were estimated foreach star based on comparing the observed colours to model predictions.The extinction-corrected colour magnitude diagrams indicate the presenceof a broad red giant branch of inner bulge stars detached from thehelium-burning red clump of both bulge and disc stars. The red giantstars belonging to the bulge form a wide sequence mostly due to variablemetallicity. As previously done by other authors, we derived photometricmetallicities by comparing the position of each bulge red giant to a setof templates based on Galactic globular clusters. Results: .Weconfirm the existence of regions with AK < 0.30 in thesefields, as attested by analysis of the stellar colours. The resultingmetallicity distribution function is peaked at [Fe/H] ≃ -0.25 andextends to supersolar values up to [Fe/H] ≃ 0.3. This distributionis similar among the 4 regions, with relatively small field-to-fieldvariations within the central 1kpc. Our results are also in goodagreement with the metallicity distributions from other authors, bothphotometric and spectroscopic. Put together, this result strongly limitsthe existence of an abundance gradient in the inner bulge.
| SiO Masers in a Scutum Massive Star Cluster of Red Supergiants
We have detected five objects toward a Scutum massive star cluster ofred supergiants in the SiO J=1-0, v=1 or v=2 transitions. The radialvelocity data indicate that four of the detections are cluster membersand that the other, which is located close to the X-ray source AX1838-0655, is a foreground object. The high velocity resolution of themaser lines provides a more accurate determination of the radialvelocity (120 km s-1) and velocity dispersion (~2 kms-1) of the cluster and, hence, of the distance (6.5 kpc) andluminosities of the stars. We discuss the implications of thesemeasurements in constraining the relationships between SiO masers andmass-loss rate and modeling the cluster age and mass.
| uvbyCaHβ CCD Photometry of Clusters. VII. The Intermediate-Age Anticenter Cluster Melotte 71
CCD photometry on the intermediate-band uvbyCaHβ system ispresented for the anticenter, intermediate-age open cluster Melotte 71.Restricting the data to probable single members of the cluster using thecolor-magnitude diagram and the photometric indices alone generates asample of 48 F dwarfs on the unevolved main sequence. The averageE(b-y)=0.148+/-0.003 (standard error of the mean [s.e.m.]) orE(B-V)=0.202+/-0.004 (s.e.m.), where the errors refer to internal errorsalone. With this reddening, [Fe/H] is derived from both m1and hk, using Hβ and b-y as the temperature index, with excellentagreement among the four approaches and a final weighted average of[Fe/H]=-0.17+/-0.02 (s.e.m.) for the cluster, on a scale in which theHyades has [Fe/H]=+0.12. When adjusted for the higher reddeningestimate, the previous metallicity estimates from Washington photometryand from spectroscopy are now in agreement with the intermediate-bandresult. From comparisons to isochrones of appropriate metallicity, thecluster age and distance are determined as 0.9+/-0.1 Gyr and(m-M)=12.2+/-0.1 or (m-M)0=11.6+/-0.1. At this distance fromthe Sun, Mel 71 has a galactocentric distance of 10.0 kpc on a scale inwhich the Sun is 8.5 kpc from the Galactic center. Based on its age,distance, and elemental abundances, Mel 71 appears to be a less populousanalog to NGC 3960.
| Photometric and Coravel observations of red giant candidates in three open clusters: membership, binarity, reddening and metallicity
Aims.We present new CORAVEL radial-velocity observations andphotoelectric photometry in the UBV and DDO systems for a sample ofpotential members of the red-giant branches of NGC 6192, NGC 6208 andNGC 6268, three open clusters projected close to the Galactic centerdirection. We also examine the properties of a sample of 42 inner diskopen clusters projected towards almost the same direction as the threeclusters. Methods: .Cluster members and red field giants werediscriminated by using the CORAVEL radial-velocity data and by applyingtwo photometric criteria. Interstellar reddening and metal content ofthe clusters were derived from combined BV and DDO data. Results:.Cluster membership for five red giants in NGC 6192, three in NGC 6208and three in NGC 6268 has been confirmed by the analysis of thephotometric and kinematic data. Photometric membership probabilitiesshow very good agreement with those obtained from CORAVEL radialvelocities. Three new spectroscopic binaries were discovered among thered giants of NGC 6192 and NGC 6208. Mean radial velocities and E(B-V)colour excesses were derived. Conclusions: .The overallmetallicities were found to be nearly solar for NGC 6208 and above solarfor NGC 6192 and NGC 6268. Most of the clusters located closer than 2kpc from the Sun in the considered direction are slightly more reddenedthan the absorption resulting from the Baade's window absorption law.
| 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.
| Element abundances of unevolved stars in the open cluster M 67
Context: .The star-to-star scatter in lithium abundances observed amongotherwise similar stars in the solar-age open cluster M 67 is one of themost puzzling results in the context of the so called "lithium problem".Among other explanations, the hypothesis has been proposed that thedispersion in Li is due to star-to-star differences in Fe or otherelement abundances which are predicted to affect Li depletion. Aims: .The primary goal of this study is the determination of themetallicity ([Fe/H]), α- and Fe-peak abundances in a sample ofLi-poor and Li-rich stars belonging to M 67, in order to test thishypothesis. By comparison with previous studies, the presentinvestigation also allows us to check for intrinsic differences in theabundances of evolved and unevolved cluster stars and to draw moresecure conclusions on the abundance pattern of this cluster.Methods: .We have carried out an analysis of high resolution UVES/VLTspectra of eight unevolved and two slightly evolved cluster membersusing MOOG and measured equivalent widths. For all the stars we havedetermined [Fe/H] and element abundances for O, Na, Mg, Al, Si, Ca, Ti,Cr and Ni. Results: .We find an average metallicity [Fe/H] =0.03±0.01, in very good agreement with previous determinations.All the [ X/Fe] abundance ratios are very close to solar. Thestar-to-star scatter in [Fe/H] and [ X/Fe] ratios for all elements,including oxygen, is lower than 0.05 dex, implying that the largedispersion in lithium among cluster stars is not due to differences inthese element abundances. We also find that, when using a homogeneousscale, the abundance pattern of unevolved stars in our sample is verysimilar to that of evolved stars, suggesting that, at least in thiscluster, RGB and clump stars have not undergone any chemical processing.Finally, our results show that M 67 has a chemical composition that isrepresentative of the solar neighborhood.
| 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.
| 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.
| 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.
| Spectroscopic Abundance Analysis of Dwarfs in the Young Open Cluster IC 4665
We report a detailed spectroscopic abundance analysis for a sample of 18F-K dwarfs of the young open cluster IC 4665. Stellar parameters andelement abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe, and Ni have beenderived using the spectroscopic synthesis tool SME (Spectroscopy MadeEasy). Within the measurement uncertainties the iron abundance isuniform, with a standard deviation of 0.04 dex. No correlation is foundbetween the iron abundance and the mass of the stellar convective zoneor between the Li abundance and the Fe abundance. In other words, ourresults do not reveal any signature of accretion and therefore do notsupport the scenario that stars with planets (SWPs) acquire theiron-average higher metallicity compared to field stars via accretion ofmetal-rich planetary material. Instead, the higher metallicity of SWPsmay simply reflect the fact that planetary formation is more efficientin high-metallicity environs. However, since so many details of theplanetary system formation processes remain poorly understood, furtherstudies are needed for a final settlement of the problem of the highmetallicity of SWPs. The standard deviation of [Fe/H] deduced from ourobservations, taken as an upper limit on the metallicity dispersionamong the IC 4665 member stars, has been used to constrainprotoplanetary disk evolution, terrestrial and giant planets formation,and evolution processes. The total reservoir of heavy elements retainedby the nascent disks is limited, and high retention efficiency ofplanet-building material is supported. Under modest surface density, gasgiant planets are expected to form in locally enhanced regions or startefficient gas accretion when they only have a small core of a few Earthmasses. Our results do not support the possibility that the migration ofgas giants and the circularization of terrestrial planets' orbits areregulated by their interaction with a residual population ofplanetesimals and dust particles.
| On the ages of exoplanet host stars
We obtained spectra, covering the CaII H and K region, for 49 exoplanethost (EH) stars, observable from the southern hemisphere. We measuredthe chromospheric activity index, R'{_HK}. We compiled previouslypublished values of this index for the observed objects as well as theremaining EH stars in an effort to better smooth temporal variations andderive a more representative value of the average chromospheric activityfor each object. We used the average index to obtain ages for the groupof EH stars. In addition we applied other methods, such as: Isochrone,lithium abundance, metallicity and transverse velocity dispersions, tocompare with the chromospheric results. The kinematic method is a lessreliable age estimator because EH stars lie red-ward of Parenago'sdiscontinuity in the transverse velocity dispersion vs dereddened B-Vdiagram. The chromospheric and isochrone techniques give median ages of5.2 and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The medianage of F and G EH stars derived by the isochrone technique is 1-2 Gyrolder than that of identical spectral type nearby stars not known to beassociated with planets. However, the dispersion in both cases is large,about 2-4 Gyr. We searched for correlations between the chromosphericand isochrone ages and L_IR/L* (the excess over the stellarluminosity) and the metallicity of the EH stars. No clear tendency isfound in the first case, whereas the metallicy dispersion seems toslightly increase with age.
| Properties of five low-contrast open clusters in the third quadrant
We derive photometric, structural and dynamical evolution-relatedparameters of five as yet unstudied low-contrast open clusters locatedin the third quadrant using 2MASS data. The target clusters are Czernik31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply astatistical field-star decontamination procedure to infer on theintrinsic colour-magnitude diagram (CMD) morphology which is criticalfor such low-contrast objects. Consequently, it became possible toderive accurate reddening, age, distance from the Sun and Galactocentricdistance for the five clusters. In the structural andluminosity/mass-function analyses we apply a colour-magnitude filterwhich encompasses the cluster evolutionary CMD sequences and excludesstars with discrepant colours. Using this procedure we derive core andlimiting radii, mass function (MF) slope, total mass, mass density andrelaxation time. We derive ages in the range 140-1100 Myr,Galactocentric distances within 7.7-11.4 kpc, and total masses within360-2900 M_ȯ. Reflecting large-scale mass segregation, the MF slopein the core is significantly flatter than that in the halo of the fiveclusters. Although some of the present clusters are relatively youngerthan the Gyr-old clusters, they present evidence of advanced dynamicalevolution. This kind of study has become possible because of thephotometric uniformity and spatial coverage of 2MASS which allows aproper subtraction of the field-star contamination on the target CMDs.The present study indicates that low-contrast clusters can be studiedwith 2MASS, particularly after field-star subtraction, which isimportant since most of the unstudied open clusters belong to thisclass.
| Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS
We have performed a new and homogeneous analysis of all the Li dataavailable in the literature for main sequence stars (spectral-types fromlate F to K) in open clusters. In the present paper we focus on adetailed investigation of MS Li depletion and its time scales for starsin the 6350-5500 K effective temperature range. For the first time, wewere able to constrain the age at which non-standard mixing processes,driving MS Li depletion, appear. We have also shown that MS Li depletionis not a continuous process and cannot be simply described by at-α law. We confirm that depletion becomes ineffectivebeyond an age of 1-2 Gyr for the majority of the stars, leading to a Liplateau at old ages. We compared the empirical scenario of Li as afunction of age with the predictions of three non-standard models. Wefound that models including only gravity waves as main mixing processare not able to fit the Li vs. age pattern and thus this kind of mixingcan be excluded as the predominant mechanism responsible for Lidepletion. On the other hand, models including slow mixing induced byrotation and angular momentum loss, and in particular those includingalso diffusive processes not related to rotation, can explain to someextent the empirical evidence. However, none of the currently proposedmodels can fit the plateau at old ages.
| 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.
| Influence of Gravity Waves on the Internal Rotation and Li Abundance of Solar-Type Stars
The Sun's rotation profile and lithium content have been difficult tounderstand in the context of conventional models of stellar evolution.Classical hydrodynamic models predict that the solar interior mustrotate highly differentially, in disagreement with observations. It hasrecently been shown that internal waves produced by convection insolar-type stars produce an asymmetric, shear layer oscillation, similarto Earth's quasi-biennial oscillation, that leads to efficient angularmomentum redistribution from the core to the envelope. We presentresults of a model that successfully reproduces both the rotationprofile and the surface abundance of lithium in solar-type stars ofvarious ages.
| 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.
| Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane
We derive photometric, structural and dynamical evolution-relatedparameters of 11 nearby open clusters with ages in the range 70 Myr to 7Gyr and masses in the range ≈400 M_ȯ to ≈5300 M_ȯ. Theclusters are homogeneously analysed by means of J, H and KS2MASS photometry, which provides spatial coverage wide enough toproperly take into account the contamination of the cluster field byGalaxy stars. Structural parameters such as core and limiting radii arederived from the background-subtracted radial density profiles.Luminosity and mass functions (MFs) are built for stars later than theturnoff and brighter than the 2MASS PSC 99.9% completeness limit. Thetotal mass locked up in stars in the core and the whole cluster, as wellas the corresponding mass densities, are calculated by taking intoaccount the observed stars (evolved and main sequence) and extrapolatingthe MFs down to the H-burning mass limit, 0.08 M_ȯ. We illustratethe methods by analysing for the first time in the near-infrared thepopulous open clusters NGC 2477 and NGC 2516. For NGC 2477 we derive anage of 1.1 ± 0.1 Gyr, distance from the Sun d_ȯ=1.2 ±0.1 kpc, core radius Rcore=1.4 ± 0.1 pc, limitingradius Rlim=11.6 ± 0.7 pc and total massmtot≈(5.3±1.6) × 103 M_ȯ.Large-scale mass segregation in NGC 2477 is reflected in the significantvariation of the MF slopes in different spatial regions of the cluster,and in the large number-density of giant stars in the core with respectto the cluster as a whole. For NGC 2516 we derive an age of 160 ±10 Myr, d_ȯ=0.44 ± 0.02 kpc, Rcore=0.6 ±0.1 pc, Rlim=6.2 ± 0.2 pc andmtot≈(1.3±0.2) × 103 M_ȯ.Mass-segregation in NGC 2516 shows up in the MFs. Six of the 11 clusterspresent a slope break in the MF occurring at essentially the same massas that found for the field stars in Kroupa's universal IMF. The MFbreak is not associated to cluster mass, at least in the clusters inthis paper. In two clusters the low-mass end of the MF occurs above theMF break. However, in three clusters the MF break does not occur, atleast for the mass range m≥0.7 M_ȯ. One possibility isdynamical evolution affecting the MF slope distribution. We also searchfor relations of structural and evolutionary parameters with age andGalactocentric distance. The main results for the present sample are:(i) cluster size correlates both with age and Galactocentric distance;(ii) because of size and mass scaling, core and limiting radii, and coreand overall mass correlate; (iii) massive (m≥1000 M_ȯ) andless-massive clusters follow separate correlation paths on the planecore radius and overall mass; (iv) MF slopes of massive clusters arerestricted to a narrow range, while those of the less-massive onesdistribute over a wider range. Core and overall MF flattening is relatedto the ratio (τ) of age to relaxation time. For large values ofτ the effects of large-scale mass segregation and low-mass starsevaporation can be observed in the MFs. In this sense, τ appears tocharacterize the evolutionary state of the clusters. We conclude thatappreciable slope flattenings in the overall MFs of the less-massiveclusters take ~6 times longer to occur than in the core, while in themassive clusters they take a time ~13 times longer. We investigatecluster parameters equivalent to those determining the fundamental planeof ellipticals. These parameters are: overall mass, projected massdensity and core radius. We conclude that in the present sample there isevidence of a fundamental plane. Larger samples are necessary to pindown this issue.
| Abundances of Red Giants in Old Open Clusters. II. Berkeley 17
We present an analysis of high-dispersion echelle spectra of three giantstars in the 10 Gyr open cluster Berkeley 17 obtained with the KPNO 4 mTelescope. Abundances were determined relative to the bright, somewhatmetal-poor disk giant Arcturus. Be 17 is found to have a mean[Fe/H]=-0.10+/-0.09 (rms). Oxygen abundances, determined from theforbidden [O I] lines, and the α-elements Mg, Ca, and Ti showscaled solar abundance ratios, while the Si abundance is slightlyenhanced. The odd-Z elements Na and Al are significantly enhancedrelative to scaled solar abundances. These abundance patterns aresimilar to those seen in other old open clusters and suggest that theGalactic disk was enriched to solar abundance levels at very earlytimes.
| Determination of stellar ages from isochrones: Bayesian estimation versus isochrone fitting
We present a new method, using Bayesian estimation, to determine stellarages and their uncertainties from observational data and theoreticalisochrones. The result for an individual star is obtained as therelative posterior probability density as function of the age (“Gfunction”). From this can be derived the most probable age andconfidence intervals. The convoluted morphology of isochrones and strongnon-linearities make the age determination by any method difficult andsusceptible to statistical biases, and as a result age uncertaintieshavee often been underestimated in the literature. From simulations wefind that the G functions provide a general, robust and reliable way toquantify age information. Resulting age estimates are at least asaccurate as those obtained with conventional isochrone fitting methods,and in some cases much better, especially when the observationaluncertainties are large. We also find that undetected binaries, on thewhole, have a surprisingly small effect on the age determinations. For astellar sample, the individual G functions can be combined to derive thestar formation history of the population; this will be developed in aforthcoming paper. For a coeval population the combination simplifies tocomputing the product of the individual G functions, and we apply thatmethod to estimate the ages of the two open clusters IC 4651 and M 67,using Padova isochrones and photometric data from the literature. For IC4651 we find an estimated age of 1.56± 0.03 Gyr, assuming a truedistance modulus of 9.80. For M 67 we find 4.05± 0.05 Gyr fortrue distance modulus 9.48. The small formal errors of these ageestimates do not include the (much larger) uncertainties fromcalibration and model errors, but illustrate the statistical power ofcombining G functions. Our statistical approach to the age determinationproblem is well suited for the mass treatment of data resulting fromlarge-scale surveys such as the Gaia mission.
| Proper motion measurements as indicators of binarity in open clusters
We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyrusing UCAC2 proper motion data and 2MASS photometry, which allows us toreach stellar masses down to ≈0.7 M_ȯ. We employ in this work anapproach in which the background proper motion contribution isstatistically subtracted in order to obtain the cluster's intrinsicproper motion distribution. For each cluster we consider the projectedvelocity distributions in the core and off-core regions separately. Inthe projected velocity distribution of all sample clusters we find awell-defined low-velocity peak, as well as an excess in the number ofstars at larger velocities. The low-velocity peak is accounted for bythe random motion of the single stars, while the high-velocity excesscan be attributed to the large velocity changes produced by asignificant fraction of unresolved binaries in a cluster. We derivekinematic parameters of the single-star distribution, in particular theprojected velocity dispersion. The relatively large velocity dispersionsderived in this work may reflect the non-virialized state of theclusters. Based on the relative number of high-velocity (binary) andsingle stars, we inferred for the sample clusters unresolved binaryfractions in the range 15%≤f_bin≤54%, for both core and off-coreregions. Stars with a projected velocity exceeding the maximum reachedby the single-star distribution are identified in 2MASS J×(J-H)colour magnitude diagrams. The asymmetry observed in the distribution ofthese stars around the main sequence is consistent with models ofmain-sequence widening resulting from unresolved binaries combined with2MASS photometric uncertainties. The present results suggest that caremust be taken when applying proper-motion filters to sort out members,especially binaries in a star cluster. This paper shows that propermotions turn out to be a useful tool for identifying high-velocity starsas unresolved binary cluster members, and as a consequence, map andquantify the binary component in colour magnitude diagrams.
| 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.
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