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Ýlgili Makaleler
A Constraint on Zsolar from Fits of Isochrones to the Color-Magnitude Diagram of M67
The mass at which a transition is made between stars that have radiativeor convective cores throughout the core H burning phase is a fairlysensitive function of Z (particularly, the CNO abundances). As aconsequence, the ~4 Gyr, open cluster M67 provides a constraint onZsolar (and the solar heavy-element mixture) because (1)high-resolution spectroscopy indicates that this system has virtuallythe same metal abundances as the Sun, and (2) its turnoff stars havemasses just above the lower limit for sustained core convection on themain sequence. In this study, evolutionary tracks and isochrones usingthe latest MARCS model atmospheres as boundary conditions have beencomputed for 0.6-1.4 Msolar on the assumption of ametals mix (implying Zsolar~0.0125) based on the solarabundances derived by M. Asplund and collaborators using 3D modelatmospheres. These calculations do not predict a turnoff gap where oneis observed in M67. No such difficulty is found if the analysis usesisochrones for Zsolar=0.0165, assuming the Grevesse andSauval mix of heavy elements. Our findings, like the inferences fromhelioseismology, indicate a problem with the abundances of Asplund andcollaborators. However, it is possible that low-Z models with diffusiveprocesses taken into account will be less problematic.
| Galactic Orbits of Globular Clusters in a Barred Galaxy
We study the effect of a bar in the galactic orbits of forty-fiveglobular clusters whose absolute proper motions are known. The orbitalcharacteristics of the orbits are compared with those obtained for thecase of an axisymmetric galactic potential. Tidal radii are computed anddiscussed for both cases.
| 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.
| Photometric observations of Supernovae 2000E, 2001B, 2001V, and 2001X
CCD BVRI photometry is presented for two type Ia supernovae 2000E and2001V, for SN Ib 2001B and SN II-P 2001X. The parameters of light curvesand absolute magnitudes at maximum light are estimated. It is shown thatall four supernovae are typical for their classes considering the shapeof their light curves and maximum luminosity.
| Planetary nebulae as tracers of galaxy stellar populations
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
| Na, Al, and O Abundances of Open Clusters NGC 7142, NGC 6939, and IC 4756
We present an analysis of echelle spectra of stars in three openclusters obtained with the Hydra multiobject spectrograph on the WIYN3.5 m telescope. Abundances of Fe, O, Si, Ca, Na, Al, and Ni have beendetermined via equivalent width analysis and spectrum synthesis. Meanabundances for each cluster are compared to those of previous studiesand of other clusters in the literature, with emphasis on exploring theenhancements of Na and Al seen in many open clusters. All three clustersshow enhanced values of [Na/Fe] and [Al/Fe], while the abundances of Fe,O, Si, and Ca are consistent with their ages and locations in theGalactic disk.
| The old anticentre open cluster Berkeley 32: membership and fundamental parameters*
We have obtained medium- and low-resolution spectroscopy and BVI CCDimaging of Berkeley 32, an old open cluster which lies in the anticentredirection. From the radial velocities of 48 stars in the clusterdirection, we found that 31 of them, in crucial evolutionary phases, areprobable cluster members, with an average radial velocity of +106.7(σ= 8.5) km s-1.From isochrone fitting to the colour-magnitude diagrams of Berkeley 32,we have obtained an age of 6.3 Gyr, (m-M)0= 12.48 and E(B-V)= 0.10. The best fit is obtained with Z= 0.008. A consistent distance,(m-M)0~= 12.6 +/- 0.1, has been derived from the meanmagnitude of red clump stars with confirmed membership; we may assume(m-M)0~= 12.55 +/- 0.1.The colour-magnitude diagram of the nearby field observed to check forfield stars contamination looks intriguingly similar to that of theCanis Major overdensity.
| The Benchmark Cluster Reddening Project. III. A Comparison of Reddening Values Derived from F and K Stars in NGC 752
A protocol for reddening analysis developed in previous papers in thisseries is reviewed here and applied to NGC 752. The protocol includes(1) detailed scrutiny of the histories of reddening determinations forprogram clusters, (2) adherence to statistical norms, (3) qualitycontrol of all reddening techniques, (4) a primary focus on techniqueswith limited metallicity sensitivity, (5) zero-point control of sourcedata, (6) use of the solar neighborhood to establish a zero point forreddening values, (7) use of a standard permitting meaningful quantitiesto be as small as a few millimagnitudes, and (8) rejection of ad hocdata assessments. After certain published results are set aside, it isfound that a reddening value derived for F stars by using a techniquethat is insensitive to blanketing disagrees by about 0.04 mag with avalue derived for K giants by using DDO photometry. This disagreementdoes not appear to be an artifact of the use of questionable data, andit persists if additional techniques are considered. Partly because twocomparable reddening results for K giants are afterward found todisagree with each other, the problem is attributed to a blanketingeffect (with an unknown source) on the reddening values derived for theK giants. Pending a final resolution of that problem, the adopted valueof E(B-V) for the cluster (44+/-3.4 mmag) is based solely on the initialF-star analysis. A discussion of the potential usefulness of smallstandard errors like the one derived here for E(B-V) is given. In aparallel investigation, it is found that no definitive value of [Fe/H]can be assigned to NGC 752 at present because a statisticallysignificant disagreement between two published high-precisionmetallicities is detected.
| Multicolour CCD Photometry of Three RRab Stars
Multicolour CCD observations of three RRab stars are presented.
| Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data*
The MOST (Microvariability and Oscillations of Stars) satellite obtainsultraprecise photometry from space with high sampling rates and dutycycles. Astronomical photometry or imaging missions in low Earth orbits,like MOST, are especially sensitive to scattered light from Earthshine,and all these missions have a common need to extract target informationfrom voluminous data cubes. They consist of upwards of hundreds ofthousands of two-dimensional CCD frames (or subrasters) containing fromhundreds to millions of pixels each, where the target information,superposed on background and instrumental effects, is contained only ina subset of pixels (Fabry Images, defocused images, mini-spectra). Wedescribe a novel reduction technique for such data cubes: resolvinglinear correlations of target and background pixel intensities. Thisstep-wise multiple linear regression removes only those targetvariations which are also detected in the background. The advantage ofregression analysis versus background subtraction is the appropriatescaling, taking into account that the amount of contamination may differfrom pixel to pixel. The multivariate solution for all pairs oftarget/background pixels is minimally invasive of the raw photometrywhile being very effective in reducing contamination due to, e.g. straylight. The technique is tested and demonstrated with both simulatedoscillation signals and real MOST photometry.
| 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.
| New Eclipsing Variables in the Field of M67
We present four newly-discovered eclipsing systems (1 detached, 3 W UMasystems) in the field of the open cluster M67. Only two of the variables(the detached system and one W UMa) might be cluster members, but lowproper motion probabilities or unusual color-magnitude diagram positionscomplicate the situation.
| Cassini States with Dissipation: Why Obliquity Tides Cannot Inflate Hot Jupiters
Some short-period exoplanets (``hot Jupiters'') are observed by theirtransits to have anomalously large radii. It has been suggested thatthese planets are in a resonance involving persistent misalignment andsynchronous precession of their spin and orbital angular momenta (aCassini state) and that the attendant tidal heating inflates the planet.We argue against this. Using explicit tidal integrations, we show thatalthough an oblique Cassini state can dissipate many times the planet'srotational energy, the rate of dissipation must be much less thanhypothesized. Dissipation causes the planetary spin to lie at an angleto the plane containing the orbital and total angular momenta. Ifdissipation is too rapid, this angle becomes so large that Cassiniequilibrium is lost. A separate consideration limits the total energythat can be extracted from the orbit. The source of the torque on theorbit, either an oblique parent star or an inclined third body, alignswith the orbit as it absorbs the angular momentum shed by the planet.Alignment removes the orbital precession required by the Cassini state.In combination with observational bounds on the mass and semimajor axisof a possible second planet and with bounds on the stellar rotation andobliquity, these constraints make it very unlikely that obliquity tidescan be the explanation for inflated hot Jupiters, especially HD 209458b.
| 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.
| The Core Binary Fractions of Star Clusters from Realistic Simulations
We investigate the evolution of binary fractions in star clusters usingN-body models of up to 100,000 stars. Primordial binary frequencies inthese models range from 5% to 50%. Simulations are performed with theNBODY4 code and include a full mass spectrum of stars, stellarevolution, binary evolution, and the tidal field of the Galaxy. We findthat the overall binary fraction of a cluster almost always remainsclose to the primordial value, except at late times when a cluster isnear dissolution. A critical exception occurs in the central regions,where we observe a marked increase in binary fraction with time-asimulation starting with 100,000 stars and 5% binaries reached a corebinary frequency as high as 40% at the end of the core-collapse phase(occurring at 16 Gyr with ~20,000 stars remaining). Binaries aredestroyed in the core by a variety of processes as a cluster evolves,but the combination of mass segregation and creation of new binaries inexchange interactions produces the observed increase in relative number.We also find that binaries are cycled into and out of cluster cores in amanner that is analogous to convection in stars. For models of 100,000stars we show that the evolution of the core radius up to the end of theinitial phase of core collapse is not affected by the exact value of theprimordial binary frequency (for frequencies of 10% or less). We discussthe ramifications of our results for the likely primordial binarycontent of globular clusters.
| Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process
We use high-resolution spectra obtained with the ESO Very LargeTelescope to measure surface abundance patterns of 43 blue stragglerstars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs showsa significant depletion of carbon and oxygen with respect to thedominant population. This evidence would suggest the presence of CNOburning products on the BSS surface coming from a deeply peeled parentstar, as expected in the case of a mass transfer process. This is thefirst detection of a chemical signature clearly pointing to a specificBSS formation process in a globular cluster.Based on observations collected at the ESO-VLT (Cerro Paranal, Chile)under program 072.D-0337.
| Red giants in open clusters. XII. Six old open clusters NGC 2112, 2204, 2243, 2420, 2506, 2682
Aims.We studied the membership and binarity of 123 red giants in six oldopen clusters, NGC 2112, 2204, 2243, 2420, 2506 and 2682, to define moreprecisely the evolutionary path on the red-giant branch. Methods: Theanalysis is based on 185 radial-velocity observations with the Coravelspectrographs and available photometric data. Results: The membershipof 93 red giants was confirmed on the basis of the radial velocities.Seven definitive spectroscopic binaries were identified and 11additional stars are suspected of being binaries. The binary frequency(19%) is slightly lower than average. This is partly due to the smallnumber of observations secured for each star. Orbital elements have beendetermined for the first time for the BaII star NGC 2420-173 (D) andthose of the other BaII star NGC 2420-250 (X) have been improved. Thevalues of the cluster mean velocities have been significantly improved. Conclusions: With the new membership estimates and binary detections,the existing CCD data allow precise definition of the red-giant loci. Anumber of stars in NGC 2506, 2420 and 2204 appear to define anasymptotic branch, the position of which differs significantly from thatpredicted by the models.Based on observations collected with the Danish 1.54-m and ESO 1-mtelescopes at the European Southern Observatory, La Silla, Chile.
| Mergers of Close Primordial Binaries
We study the production of main-sequence mergers of tidally synchronizedprimordial short-period binaries. The principal ingredients of ourcalculation are the angular momentum loss rates inferred from thespin-down of open cluster stars and the distribution of binaryproperties in young open clusters. We compare our results with theexpected number of systems that experience mass transfer in thepost-main-sequence phases of evolution and compute the uncertainties inthe theoretical predictions. We estimate that main-sequence mergers canaccount for the observed number of single blue stragglers in M67.Applied to the blue straggler population, this implies that such mergersare responsible for about one-quarter of the population of halo bluemetal-poor stars and at least one-third of the blue stragglers in openclusters for systems older than 1 Gyr. The observed trends as a functionof age are consistent with a saturated angular momentum loss rate forrapidly rotating tidally synchronized systems. The predicted number ofblue stragglers from main-sequence mergers alone is comparable to thenumber observed in globular clusters, indicating that the net effect ofdynamical interactions in dense stellar environments is to reduce ratherthan increase the blue straggler population. A population of subturnoffmergers of order 3%-4% of the upper main sequence population is alsopredicted for stars older than 4 Gyr, which is roughly comparable to thesmall population of highly Li-depleted halo dwarfs. Other observationaltests are discussed.
| uvby - Hβ CCD photometry and membership segregation of the open cluster NGC 2682 (M 67)
Following deep astrometric and photometric study of the cluster NGC 2682(M 67), we are able to accurately determine its fundamental parameters.Being an old and rich cluster, M 67 is relevant for the analysis of theGalactic disk evolution. M 67 is well studied but lacks a wide and deepStrömgren photometric study. The brightest stars of the opencluster M 67 were used as uvby-Hβ standard stars in our studies ofNGC 1817 and NGC 2548, and the extension of the field covered, as wellas the number of observations, allowed us to obtain the best set ofStrömgren data ever published for this cluster. We discuss theresults of our CCD uvby-Hβ intermediate-band photometry, coveringan area of about 50 arcmin× 50 arcmin down to V ~ 19. Moreover, acomplete membership segregation based on astrometric and photometriccriteria is obtained. The photometric analysis of a selected sample ofstars yields a reddening value of E(b-y) = 0.03 ± 0.03, adistance modulus of V_0-MV = 9.7 ± 0.2 and [Fe/H] =0.01 ± 0.14. Through isochrone fitting we found an age of log t =9.6 ± 0.1 (4.2 ± 0.2 Gyr). A clump of approximately 60stars around V = 16, (b-y) = 0.4 could be interpreted as a population ofpre-cataclysmic variable stars (if members), or as a stream of fieldG-type stars placed at twice the distance of the cluster (ifnon-members).Tables 8-11 are only available in electronic form athttp://www.aanda.org Tables 3, 4 and 7 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/470/585
| A Hot DQ White Dwarf in the Open Star Cluster M35
We report the discovery of a hot DQ white dwarf, NGC 2168:LAWDS 28, thatis a likely member of the 150 Myr old cluster NGC 2168 (M35). Thespectrum of the white dwarf (WD) is dominated by C II features. Theeffective temperature is difficult to estimate but likely >~20,000 Kbased on the temperatures of hot DQ WDs with similar spectra. NGC2168:LAWDS 28 provides further evidence that hot DQ WDs may be the``missing'' high-mass helium-atmosphere white dwarfs. Based on publishedstudies, we find that the DBA WD LP 475-242 is likely a member of theHyades open cluster, as often assumed. These two white dwarfs are theentire sample of known He-atmosphere WDs in open clusters with turnoffmasses >=2 Msolar. Based on the number of known cluster DAwhite dwarfs and a redetermination of the H-atmosphere to He-atmosphereratio, commonly known as the DA:DB ratio, we reexamine the hypothesisthat the H- to He-atmosphere ratio in open clusters is the same as theratio in the field. Under this hypothesis, we calculate that fiveHe-atmosphere WDs are expected to have been discovered, with aprobability of finding fewer than three He-atmosphere white dwarfs of0.08, or at the ~2 σ level.
| Multisite campaign on the open cluster M67 - III. δ Scuti pulsations in the blue stragglers
We have made an asteroseismic analysis of the variable blue stragglersin the open cluster M67. The data set consists of photometrictime-series from eight sites using nine 0.6-2.1 m telescopes with atime-baseline of 43 d. In two stars, EW Cnc and EX Cnc, we detect thehighest number of frequencies (41 and 26) detected in δ Scutistars belonging to a stellar cluster, and EW Cnc has the second highestnumber of frequencies detected in any δ Scuti star. We havecomputed a grid of pulsation models that take the effects of rotationinto account. The distribution of observed and theoretical frequenciesshows that in a wide frequency range a significant fraction of theradial and non-radial low-degree modes are excited to detectableamplitudes. Despite the large number of observed frequencies we cannotconstrain the fundamental parameters of the stars. To make progress weneed to identify the degrees of some of the modes from eithermulticolour photometry or spectroscopy.
| The Metallicity of the Old Open Cluster NGC 6791
We have observed four red clump stars in the very old, metal-rich opencluster NGC 6791 to derive its metallicity using the high-resolutionspectrograph SARG mounted on the Galileo National Telescope (TNG). Usinga spectrum synthesis technique, we obtain an average value of[Fe/H]=+0.47 (+/-0.04, rms=0.08) dex. Our method was tested on μ Leo,a well-studied, metal-rich field giant. We also derive average oxygenand carbon abundances for NGC 6791 from synthesis of [O I] at 6300Å and C2 at 5086 Å, finding [O/Fe]~=-0.3 and[C/Fe]~=-0.2.Based on observations made with the Italian Telescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Fundación GalileoGalilei of the INAF (Istituto Nazionale di Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
| From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries
Stellar models that incorporate simple diffusion or shear-induced mixingare used to describe canonical extra mixing in low-mass red giants oflow and solar metallicity. These models are able to simultaneouslyexplain the observed Li and CN abundance changes along the upper redgiant branch (RGB) in field low-metallicity stars and match photometry,rotation, and 12C/13C ratios for stars in the oldopen cluster M67. The shear mixing model requires that main-sequence(MS) progenitors of upper RGB stars possessed rapidly rotating radiativecores and that specific angular momentum was conserved in each of theirmass shells during their evolution. We surmise that solar-type starswill not experience canonical extra mixing on the RGB because their moreefficient MS spin-down resulted in solid-body rotation, as revealed byhelioseismological data for the Sun. Thus, RGB stars in the old,high-metallicity cluster NGC 6791 should show no evidence for mixing intheir 12C/13C ratios. We develop the idea thatcanonical extra mixing in a giant component of a binary system may beswitched to its enhanced mode with much faster and somewhat deepermixing as a result of the giant's tidal spin-up. This scenario canexplain photometric and composition peculiarities of RS CVn binaries.The tidally enforced enhanced extra mixing might contribute to thestar-to-star abundance variations of O, Na, and Al in globular clusters.This idea may be tested with observations of12C/13C ratios and CN abundances in RS CVnbinaries.
| Population Synthesis in the Blue. IV. Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
We present a new set of model predictions for 16 Lick absorption lineindices from Hδ through Fe5335 and UBV colors for single stellarpopulations with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from-1.3 to +0.3, and variable abundance ratios. The models are based onaccurate stellar parameters for the Jones library stars and a new set offitting functions describing the behavior of line indices as a functionof effective temperature, surface gravity, and iron abundance. Theabundances of several key elements in the library stars have beenobtained from the literature in order to characterize the abundancepattern of the stellar library, thus allowing us to produce modelpredictions for any set of abundance ratios desired. We develop a methodto estimate mean ages and abundances of iron, carbon, nitrogen,magnesium, and calcium that explores the sensitivity of the variousindices modeled to those parameters. The models are compared to high-S/Ndata for Galactic clusters spanning the range of ages, metallicities,and abundance patterns of interest. Essentially all line indices arematched when the known cluster parameters are adopted as input.Comparing the models to high-quality data for galaxies in the nearbyuniverse, we reproduce previous results regarding the enhancement oflight elements and the spread in the mean luminosity-weighted ages ofearly-type galaxies. When the results from the analysis of blue and redindices are contrasted, we find good consistency in the [Fe/H] that isinferred from different Fe indices. Applying our method to estimate meanages and abundances from stacked SDSS spectra of early-type galaxiesbrighter than L*, we find mean luminosity-weighed ages of theorder of ~8 Gyr and iron abundances slightly below solar. Abundanceratios, [X/Fe], tend to be higher than solar and are positivelycorrelated with galaxy luminosity. Of all elements, nitrogen is the morestrongly correlated with galaxy luminosity, which seems to indicatesecondary nitrogen enrichment. If that interpretation is correct, thisresult may impose a lower limit of 50-200 Myr to the timescale of starformation in early-type galaxies. Unlike clusters, galaxies show asystematic effect whereby higher order, bluer, Balmer lines yieldyounger ages than Hβ. This age discrepancy is stronger for lowerluminosity galaxies. We examine four possible scenarios to explain thistrend. Contamination of the bluer indices by a metal-poor stellarpopulation with a blue horizontal branch cannot account for the data.Blue stragglers and abundance-ratio effects cannot be ruled out, as theycan potentially satisfy the data, even though this can only be achievedby resorting to extreme conditions, such as extremely high [O/Fe] orspecific blue-straggler frequencies. The most likely explanation is thepresence of small amounts of a young/intermediate-age stellar populationcomponent. We simulate this effect by producing two-component models andshow that they provide a reasonably good match to the data when the massfraction of the young component is typically a few percent. Ifconfirmed, this result implies star formation has been extended inearly-type galaxies, and more so in less massive galaxies, which seemsto lend support to the ``downsizing'' scenario. Moreover, it impliesthat stellar population synthesis models are capable of constraining notonly the mean ages of stellar populations in galaxies, but also theirage spread.
| Measuring Stellar Velocity Dispersions in Active Galaxies
We present stellar velocity dispersion (σ*)measurements for a significant sample of 40 broad-line (type 1) activegalaxies for use in testing the well-known relation black hole mass andstellar velocity dispersion. The objects are selected to contain Ca IItriplet, Mg I b triplet, and Ca H+K stellar absorption features in theiroptical spectra so that we may use them to perform extensive tests ofthe systematic biases introduced by both template mismatch andcontamination from the active galactic nucleus (AGN). We use the Ca IItriplet as a benchmark to evaluate the utility of the other spectralregions in the presence of AGN contamination. Broad Fe II emission,extending from ~5050 to 5520 Å, in combination with narrow coronalemission lines, can seriously bias σ* measurements fromthe Mg I b region, highlighting the need for extreme caution in its use.However, we argue that at luminosities constituting a moderate fractionof the Eddington limit, when the Fe II lines are both weak and smoothrelative to the stellar lines, it is possible to derive meaningfulmeasurements with careful selection of the fitting region. Inparticular, to avoid the contamination of coronal lines, we advocate theuse of the region 5250-5820 Å, which is rich in Fe absorptionfeatures. At higher AGN contaminations, the Ca H+K region may providethe only recourse for estimating σ*. These features arenotoriously unreliable, due to a strong dependence on spectral type, asteep local continuum, and large intrinsic broadening. Indeed, we find astrong systematic trend in comparisons of Ca H+K with other spectralregions. Luckily the offset is well described by a simple linear fit asa function of σ*, which enables us to remove the biasand thus extract unbiased σ* measurements from thisregion. We lay the groundwork for an extensive comparison between blackhole mass and bulge velocity dispersion in active galaxies, as describedin a companion paper by Greene & Ho.
| Hδ in the integrated light of galaxies: what are we actually measuring?
We present a cautionary study exploring the reliability of the H-deltaline in the integrated spectra of galaxies for determining galaxy ages.Our database consists of the observed integrated spectra of ~120early-type galaxies, of 7 metal-rich globular clusters in M31 and theGalactic globular cluster 47 Tuc, and of the open cluster M67. We havemeasured H-delta using index definitions designed to assesscontamination from the CN molecule in and around H-delta by choosingcombinations of bandpasses that both avoid and include a region of CNmolecular lines redward of H-delta. We find systematic differences inthe ages derived from H-delta measurements among the various definitionswhen extracting ages from H-delta in old stellar populations withenhanced CN bands due to non-solar abundance ratios. We propose thatneighboring CN lines have a strong effect on pseudocontinuum and centralbandpass levels. For stellar populations which have non-solar abundanceratios in C and/or N, population synthesis models that do not accountfor abundance ratio variations cannot reproduce accurately the CN 4216\AA band, which leads to a corresponding inaccuracy in reproducing thevarious H-delta indices. Hence, caution must be used when extractinggalaxy ages from the H-delta line in old stellar populations withsignificant non-solar abundance ratios.
| Deep, Low Mass Ratio Overcontact Binary Systems. VI. AH Cancri in the Old Open Cluster M67
CCD photometric light curves in the B and V bands obtained in 2001 andin the V band obtained in 2002 of AH Cnc in the old open cluster M67 arepresented. It is shown that AH Cnc is a total-eclipsing binary and itslight curves correspond to a typical A type according to Binnendijk'sclassification. The variations of the light curve around the primaryminimum and second maximum were found. Our nine epochs of light minimummonitored from 2001 to 2005, including others collected from theliterature, were used to create the first study of the period changes ofthe binary system. A cyclic oscillation with a period of 36.5 yr and anamplitude of 0.0237 days was discovered to be superposed on a continuousperiod increase (dP/dt=3.99×10-7 days yr-1).Weak evidence indicates that there exists another small-amplitude periodoscillation (A4=0.0035 days, P4=7.75 yr). Thesymmetric light curves in the B and V bands obtained in 2001 wereanalyzed with the 2003 version of the Wilson-Devinney code. It isconfirmed that AH Cnc is a deep overcontact binary system with a highdegree of overcontact f=58.5%+/-4.5% and a low mass ratio ofq=0.1682+/-0.0012. The existence of the third light and the cyclicperiod oscillation both may suggest that AH Cnc is a triple systemcontaining an unseen third body. The tertiary component may have playedan important role in the origin of the overcontact binary star byremoving angular momentum from the central system, which would cause itto have a short initial orbital period and thus evolve into anovercontact configuration by angular momentum loss. The long-term periodincrease can be interpreted as a mass transfer from the less massivecomponent to the more massive one. As the orbital period increases, thedecrease of the mass ratio will cause it finally to evolve into a singlerapid-rotating star when the system meets the more familiar criterionthat the orbital angular momentum be less than 3 times the total spinangular momentum. Therefore, AH Cnc may be a progenitor of the bluestraggler stars in M67.
| UBVRI Photometry of Stellar Structures throughout the Disk of the Barred Galaxy NGC 3367
We report new detailed surface U, B, V, R, and I photometry of 81stellar structures in the disk of the barred galaxy NGC 3367. The imagesshow many different structures, indicating that star formation is goingon in most parts of the disk. NGC 3367 is known to have a very highconcentration of molecular gas distribution in the central regions ofthe galaxy and bipolar synchrotron emission from the nucleus, with twolobes (at 6 kpc) forming a triple structure similar to a radio galaxy.We have determined the U, B, V, R, and I magnitudes and U-B, B-V, U-V,and V-I colors for the central region (nucleus), a region which includessupernova 2003 AA, and 79 star associations throughout NGC 3367. Theestimation of ages of star associations is very difficult due to severalfactors, among them the filling factor, metallicity, spatialdistribution of each structure, and the fact that we estimated themagnitudes with a circular aperture of 16 pixels in diameter, equivalentto 6.8''~1.4 kpc. However, even though the colors derived forNGC 3367 were similar to the colors expected of star clusters withtheoretical evolutionary star tracks developed for the LMC and had asimilar metallicity, NGC 3367 shows 56% of the observed structures withage type SWB I (a few tens of megayears), with seven sources outside thehigh surface brightness visible disk.
| Contact Binaries with Additional Components. I. The Extant Data
We have attempted to establish observational evidence for the presenceof distant companions that may have acquired and/or absorbed angularmomentum during the evolution of multiple systems, thus facilitating orenabling the formation of contact binaries. In this preliminaryinvestigation we use several techniques (some of themdistance-independent) and mostly disregard the detection biases ofindividual techniques in an attempt to establish a lower limit to thefrequency of triple systems. While the whole sample of 151 contactbinary stars brighter than Vmax=10 mag gives a firm lowerlimit of 42%+/-5%, the corresponding number for the much better observednorthern-sky subsample is 59%+/-8%. These estimates indicate that mostcontact binary stars exist in multiple systems.
| Tracing mixing in stars: new beryllium observations of the open clusters NGC 2516, Hyades, and M 67
Context: Determinations of beryllium abundance in stars, together withlithium, provide a key tool to investigate the so far poorly understoodextra-mixing processes at work in stellar interiors. Aims: We measuredBe in three open clusters, complementing existing Be surveys, and aimingat gathering a more complete empirical scenario of the evolution of Beas a function of stellar age and temperature. Methods: We analyzedVLT/UVES spectra of members of NGC 2516, theHyades, and M 67 to determinetheir Be and Li abundances. In the first two clusters we focused onstars cooler than 5400 K, while the M 67 sample includes stars warmerthan 6150 K, as well as two subgiants and two blue stragglers. We alsocomputed the evolution of Be for a 0.9 Mȯ star based onstandard evolutionary models. Results: We find different behavioursfor stars in different temperature bins and ages. Stars warmer than 6150K show Be depletion and follow a Be vs. Li correlation, while Be isundepleted in stars in the 6150{-}5600 K range. NGC 2516 members coolerthan 5400 K have not depleted any Be, while older Hyades of similartemperature show some depletion. Be is severely depleted in thesubgiants and blue stragglers. Conclusions: The results for warm starsare in agreement with those of previous studies, supporting thehypothesis that mixing in this temperature regime is driven by rotation.The same holds for the two subgiants that have evolved from the“Li gap”. This mechanism is instead not the dominant one forsolar-type stars. We show that Be depletion of cool Hyades cannot simplybe explained by the effect of increasing depth of the convective zone.Finally, the different Be content of the two blue stragglers suggeststhat they have formed by two different processes (i.e., collisions vs.binary merging).Based on observations collected at ESO-VLT, Paranal Observatory, Chile,Programme numbers 65.L-0427, 68.D-0491, 69.D-0454.
| Elemental Abundance Ratios in Stars of the Outer Galactic Disk. III. Cepheids
We present metallicities, [Fe/H], and elemental abundance ratios,[X/Fe], for a sample of 24 Cepheids in the outer Galactic disk based onhigh-resolution echelle spectra. The sample members have galactocentricdistances covering 12 kpc<=RGC<=17.2 kpc, making themthe most distant Galactic Cepheids upon which detailed abundanceanalyses have been performed. We find subsolar ratios of [Fe/H] andoverabundances of [α/Fe], [La/Fe], and [Eu/Fe] in the programstars. All abundance ratios exhibit a dispersion that exceeds themeasurement uncertainties. As seen in our previous studies of old openclusters and field giants, enhanced ratios of [α/Fe] and [Eu/Fe]reveal that recent star formation has taken place in the outer disk withType II supernovae preferentially contributing ejecta to theinterstellar medium and with Type Ia supernovae playing only a minorrole. The enhancements for La suggest that asymptotic giant branch starshave contributed to the chemical evolution of the outer Galactic disk.Some of the young Cepheids are more metal-poor than the older openclusters and field stars at comparable galactocentric distances. Thisdemonstrates that the outer disk is not the end result of the isolatedevolution of an ensemble of gas and stars. We showed previously that theolder open clusters and field stars reached a basement metallicity atabout 10-11 kpc. The younger Cepheids reach the same metallicity but atlarger galactocentric distances, roughly 14 kpc. This suggests that theGalactic disk has been growing with time, as predicted from numericalsimulations. The outer disk Cepheids appear to exhibit a bimodaldistribution for [Fe/H] and [α/Fe]. Most of the Cepheids continuethe trends with galactocentric distance exhibited by S. M. Andrievsky'slarger Cepheid sample, and we refer to these stars as the ``GalacticCepheids.'' A minority of the Cepheids show considerably lower [Fe/H]and higher [α/Fe], and we refer to these stars as the ``MergerCepheids.'' One signature of a merger event would be compositiondifferences between the Galactic and Merger Cepheids. The Cepheidssatisfy this requirement, and we speculate that the distinctcompositions suggest that the Merger Cepheids may have formed under theinfluence of significant merger or accretion events. The short lifetimesof the Cepheids reveal that the merger event may be ongoing, with theMonoceros Ring and Canis Major galaxy being possible merger candidates.This paper makes use of observations obtained at the National OpticalAstronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA), Inc., under contract fromthe National Science Foundation. We also employ data products from theTwo Micron All Sky Survey, which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center,California Institute of Technology, funded by the National Aeronauticsand Space Administration and the National Science Foundation.
| Towards absolute scales for the radii and masses of open clusters
Aims. In this paper we derive tidal radii and masses of open clusters inthe nearest kiloparsecs around the Sun. Methods: For each cluster, themass is estimated from tidal radii determined from a fitting ofthree-parameter King profiles to the observed integrated densitydistribution. Different samples of members are investigated. Results:For 236 open clusters, all contained in the catalogue ASCC-2.5, weobtain core and tidal radii, as well as tidal masses. The distributionsof the core and tidal radii peak at about 1.5 pc and 7-10 pc,respectively. A typical relative error of the core radius lies between15% and 50%, whereas, for the majority of clusters, the tidal radius wasdetermined with a relative accuracy better than 20%. Most of theclusters have tidal masses between 50 and 1000 m_ȯ, and for abouthalf of the clusters, the masses were obtained with a relative errorbetter than 50%.Full Table 2 is 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/468/151
| 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.
| Characterization of open cluster remnants
Context: Despite progress in the theoretical knowledge of open clusterremnants and the growing search for observational identifications inrecent years, open questions still remain. The methods used to analyzeopen cluster remnants and criteria to define them as physical systemsare not homogeneous. In this work we present a systematic method forstudying these objects that provides a view of their properties andallows their characterization. Aims: Eighteen remnant candidates areanalyzed by means of photometric and proper motion data. These dataprovide information on objects and their fields. We establish criteriafor characterizing open cluster remnants, taking observationaluncertainties into account. Methods: 2MASS J and H photometry isemployed (i) to study structural properties of the objects by means ofradial stellar density profiles, (ii) to test for any similarity betweenobjects and fields with a statistical comparison method applied to thedistributions of stars in the CMDs, and (iii) to obtain ages, reddeningvalues, and distances from the CMD, taking an index of isochrone fitinto account. The UCAC2 proper motions allowed an objective comparisonbetween objects and large solid angle offset fields. Results: Theobjective analysis based on the present methods indicates 13open-cluster remnants in the sample. Evidence of the presence of binarystars is found, as expected for dynamically evolved systems. Finally, weinfer possible evolutionary stages among remnants from the structure,proper motion, and CMD distributions. The low stellar statistics forindividual objects is overcome by means of the construction of compositeproper motion and CMD diagrams. The distributions of remnants in thecomposite diagrams resemble the single-star and unresolved binary stardistributions of open clusters.
| 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.
| Multisite campaign on the open cluster M67 - II. Evidence for solar-like oscillations in red giant stars
Measuring solar-like oscillations in an ensemble of stars in a cluster,holds promise for testing stellar structure and evolution morestringently than just fitting parameters to single field stars. Themost-ambitious attempt to pursue these prospects was by Gilliland et al.who targeted 11 turn-off stars in the open cluster M67 (NGC 2682), butthe oscillation amplitudes were too small (<20μmag) to obtainunambiguous detections. Like Gilliland et al. we also aim at detectingsolar-like oscillations in M67, but we target red giant stars withexpected amplitudes in the range 50-500μmag and periods of 1 to 8 h.We analyse our recently published photometry measurements, obtainedduring a six-week multisite campaign using nine telescopes around theworld. The observations are compared with simulations and with estimatedproperties of the stellar oscillations. Noise levels in the Fourierspectra as low as 27μmag are obtained for single sites, while thecombined data reach 19μmag, making this the best photometric timeseries of an ensemble of red giant stars. These data enable us to makethe first test of the scaling relations (used to estimate frequency andamplitude) with an homogeneous ensemble of stars. The detected excesspower is consistent with the expected signal from stellar oscillations,both in terms of its frequency range and amplitude. However, our resultsare limited by apparent high levels of non-white noise, which cannot beclearly separated from the stellar signal.
| 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.
| 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.
| BATC 13 Band Photometry of the Open Cluster NGC 7789
We present 13 band CCD intermediate-band spectrophotometry of a fieldcentered on the open cluster NGC 7789 from 400 to nearly 1000 nm, takenwith the Beijing-Arizona-Taiwan-Connecticut (BATC) Multi-Color Surveyphotometric system. By comparing observed spectral energy distributionsof NGC 7789 stars with theoretical ones, the fundamental parameters ofthis cluster are derived: an age of 1.4+/-0.1 Gyr, a distance modulus(m-M)0=11.27+/-0.04, a reddening E(B-V)=0.28+/-0.02, and ametallicity with the solar composition Z=0.019. When the surface densityprofile for member stars with limiting magnitudes of 19.0 in the BATC eband (λeff=4925 Å) is fitted by a King model, acore radius Rc=7.52' and a tidal radiusRt=28.84' are derived for NGC 7789. The observedmass function (MF) for main-sequence (MS) stars of NGC 7789 with massesfrom 0.95 to 1.85 Msolar is fitted with a power-law functionφ(m)~mα, and a slope α=-0.96 is derived.Strong mass segregation in NGC 7789 is reflected in the significantvariation of the concentration parametersC0=log(Rt/Rc) for member stars of NGC7789 within different mass ranges: C0=1.02 for the mostmassive stars and C0=0.37 for the lowest mass MS stars.Strong mass segregation in NGC 7789 is also indicated in the significantvariation of the slopes α in different spatial regions of thecluster: the MF for stars within the core region has α=-0.71, muchflatter than that for stars in external regions of the cluster(α=-1.20).
| Chemical Homogeneity in the Hyades
We present an abundance analysis of the heavy elements Zr, Ba, La, Ce,and Nd for Hyades F-K dwarfs based on high-resolution, highsignal-to-noise ratio spectra from Keck HIRES. The derived abundancesshow the stellar members to be highly uniform, although some elementsshow a small residual trend with temperature. The rms scatter for eachelement for the cluster members is as follows: Zr=0.055, Ba=0.049,Ce=0.025, La=0.025, and Nd=0.032 dex. This is consistent with themeasurement errors and implies that there is little or no intrinsicscatter among the Hyades members. Several stars thought to be nonmembersof the cluster based on their kinematics are found to deviate from thecluster mean abundances by about 2 σ. Establishing chemicalhomogeneity in open clusters is the primary requirement for theviability of chemically tagging Galactic disk stars to common formationsites in order to unravel the dissipative history of early diskformation.
| Multiaperture UBVRIzJHK Photometry of Galaxies in the Coma Cluster
We present a set of UBVRIzJHKs photometry for 745J+H-band-selected objects in a 22.5'×29.2'region centered on the core of the Coma Cluster. This includes 516galaxies and is at least 80% complete to H=16, with a spectroscopicallycomplete sample of 111 cluster members (nearly all with morphologicalclassification) for H<14.5. For each object we present total Kron(1980) magnitudes and aperture photometry. As an example, we use thesedata to derive color-magnitude relations for Coma early-type galaxies,measure the intrinsic scatter of these relations and its dependence ongalaxy mass, and address the issue of color gradients. We find that thecolor gradients are mild and that the intrinsic scatter about thecolor-magnitude relation is small (~0.05 mag in U-V and less than ~0.03in B-R, V-I, or J-K). There is no evidence that the intrinsic scattervaries with galaxy luminosity, suggesting that the cluster red sequenceis established at early epochs over a range of ~100 in stellar mass.
| Variable Stars in the Large Magellanic Cloud: Discovery of Extragalactic W UMa Binaries
We observed a field in the disk of the LMC on two consecutive nights insearch of rapid variable stars. We found two pulsating stars of typeRRab and δ Scuti and four binary stars; among the latter we foundone sdB or cataclysmic variable below the LMC blue main sequence andthree very close binary systems on the main sequence. At least one ofthe main-sequence binaries, and possibly all three, are the firstsolar-type (W UMa-type) contact binaries to be detected in anyextragalactic system and observed to obey the sameMV=MV(logP, B-V) calibration as the Galacticsystems. Given the selection effects due to small amplitudes at faintmagnitudes, the frequency of such binaries in the disk of the LMC withits large spread in population ages is not inconsistent with that in thedisk of our Galaxy and contrasts with the lack of binaries found inearlier observations of the much younger LMC cluster LW 55.Based on data obtained at Las Campanas Observatory, operated by theCarnegie Institution of Washington, during the University of Torontotime allocation.
| Whiting 1: the youngest globular cluster associated with the Sagittarius dwarf spheroidal galaxy
Context: Recently, Carraro (2005) drew attention to the remarkable starcluster Whiting 1 by showing that it lies about 40 kpc from the Sun andis therefore unquestionably a member of the Galactic halo (b=-60.6 deg).Its Color Magnitude Diagram (CMD) indicated that Whiting 1 is very young(~5 Gyr) for a globular cluster. It is very likely that Whiting 1originated in a dwarf galaxy that has since been disrupted by the MilkyWay. Aims: The main goals of this investigation were to constrainbetter the age, metallicity, and distance of Whiting 1 and to assesswhether it belongs to a stellar stream from the Sagittarius dwarfspheroidal galaxy (Sgr dSph). Methods: Deep CCD photometry in the BVIpass-bands obtained with the VLT is used to improve the quality of theCMD and to determine the cluster's luminosity function and surfacedensity profile. High-resolution spectrograms obtained with Magellan areused to measure the cluster's radial velocity and to place limits on itspossible metallicity. The measurements of distance and radial velocityare used to test the cluster's membership in the stellar streams fromthe Sgr dSph. Results: From our CMD of Whiting 1, we derive newestimates for the cluster's age (6.5+1.0-0.5 Gyr),metallicity (Z=0.004±0.001, [Fe/H] = -0.65), and distance(29.4+1.8-2.0 kpc). From echelle spectrograms ofthree stars, we obtain -130.6±1.8 km s-1 for thecluster's radial velocity and show from measurements of two infra-redCaII lines that the [Fe/H] of the cluster probably lies in the range-1.1 to -0.4. Both the luminosity function and the surface densityprofile suggest that the cluster has undergone tidal stripping by theMilky Way. We demonstrate that the position of Whiting 1 on the sky, itsdistance from the Sun, and its radial velocity are identical to withinthe errors of both the theoretical predictions of the trailing stream ofstars from the Sgr dSph galaxy and the previous observations of the Mgiant stars that delineate the streams. Conclusions: With the additionof Whiting 1, there is now strong evidence that 6 globular clustersformed within the Sgr dSph. Whiting 1 is particularly interestingbecause it is the youngest and among the most metal rich. The relativelyyoung age of Whiting 1 demonstrates that this dwarf galaxy was able toform star clusters for a period of at least 6 Gyr, and the age andmetallicity of Whiting 1 are consistent with the age-metallicityrelationship in the main body of the Sgr dSph. The presence now ofWhiting 1 in the Galactic halo provides additional support for the viewthat the young halo clusters originated in dwarf galaxies that have beenaccreted by the Milky Way.Based on observations with the ESO VLT at the Paranal Observatory, underthe program 76.D-0128. This paper includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile. Tablesof the individual photometric measurements are available at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/181
| 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.
| 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.
| Rotational mixing in low-mass stars. II. Self-consistent models of Pop II RGB stars
Aims.In this paper we study the effects of rotation in low-mass,low-metallicity RGB stars. Methods: .We present the firstevolutionary models taking into account self-consistently the latestprescriptions for the transport of angular momentum by meridionalcirculation and shear turbulence in stellar interiors as well as theassociated mixing processes for chemicals computed from the ZAMS to theupper RGB. We discuss the uncertainties associated with the physicaldescription of the rotational mixing in detail and carefully study theireffects on the rotation profile, diffusion coefficients, structuralevolution, lifetimes, and chemical signatures at the stellar surface. Wefocus in particular on the various assumptions concerning the rotationlaw in the convective envelope, the initial rotation velocitydistribution, the presence of μ-gradients, and the treatment of thehorizontal and vertical turbulence. Results: .This explorationleads to two main conclusions. (1) After completion of the firstdredge-up, the degree of differential rotation (and hence mixing) ismaximised in the case of a differentially rotating convective envelope(i.e., jCE(r) = const.), as anticipated in previous studies.(2) Even with this assumption, and contrary to some previous claims, thepresent treatment for the evolution of the rotation profile andassociated meridional circulation and shear turbulence does not lead toenough mixing of chemicals to explain the abundance anomalies inlow-metallicity field and globular cluster RGB stars observed around thebump luminosity. Conclusions: .This study raises questions thatneed to be addressed in the near future. These include, for example, theinteraction between rotation and convection and the trigger ofadditional hydrodynamical instabilities.
| Multicolour CCD measurements of visual double and multiple stars. III
Context: Recent CCD observations were performed in the period 1998-2004for a large sample of visual double and multiple stars selected from theHipparcos Catalogue and/or from the Gliese Catalogue of Nearby Stars. Aims: Accurate astrometric and photometric data allowing us tocharacterise the individual components are provided. These data arecompared to Hipparcos data or to data from an older epoch to assess thenature of the observed systems. Methods: We simultaneously apply aMoffat-Lorentz profile with a similar shape to all detected componentsand adjust the profile parameters from which we obtain the relativeastrometric position (epoch, position angle, angular separation) as wellas differential multi-colour photometry (filters (B)VRI). Results: Wethus acquired recent data for 71 visual systems of which 6 are orbitalbinaries, 27 are nearby, and 30 are multiple systems. In three of thesecases, the systems remained unresolved. 23 new components were detectedand measured. Two new visual double stars of intermediate separationwere also found. The estimated accuracies in relative position are0.04° and 0.01 arcsec respectively, while those in differentialphotometry are of the order of 0.01-0.02 mag in general. Conclusions:.The nature of the association of 55 systems is evaluated. New basicbinary properties are derived for 20 bound systems. Component coloursand masses are provided for two orbital binaries.Based on observations collected at the National AstronomicalObservatory, Rozhen, and the Astronomical Observatory, Belogradchik,both operated by the Institute of Astronomy, Bulgarian Academy ofSciences. Also based on data obtained by the Hipparcos astrometrysatellite. Appendix A is only available in electronic form athttp://www.aanda.org Tables 4-6 are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/641
| Detection of K_s-excess stars in the 14 Myr open cluster NGC 4755
Aims.We derive the structure, distribution of MS and PMS stars anddynamical state of the young open cluster NGC 4755. We explore thepossibility that, at the cluster age, some MS and PMS stars stillpresent infrared excesses related to dust envelopes and proto-planetarydiscs. Methods: .J, H and Ks 2MASS photometry is usedto build CMD and colour-colour diagrams, radial density profiles,luminosity and mass functions. Field-star decontamination is applied touncover the cluster's intrinsic CMD morphology and detect candidate PMSstars. Proper motions from UCAC2 are used to determine clustermembership. Results: .The radial density profile follows King'slaw with a core radius Rcore=0.7 ± 0.1 pc and alimiting radius Rlim=6.9 ± 0.1 pc. The cluster agederived from Padova isochrones is 14 ± 2 Myr. Field-stardecontamination reveals a low-MS limit at ≈1.4 M_ȯ. The core MF(χ=0.94 ± 0.16) is flatter than the halo's (χ=1.58± 0.11). NGC 4755 contains 285 candidate PMS stars of age 1{-}15 Myr, and a few evolved stars. The mass locked up in PMS, MS andevolved stars amounts to 1150 M_ȯ. Proper motions show thatK_s-excess MS and PMS stars are cluster members. K_s-excess fractions inPMS and MS stars are 5.4 ± 2.1% and 3.9 ± 1.5%respectively, consistent with the cluster age. The core is deficient inPMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halobut they are scarce in the core. Conclusions: .Compared to openclusters in different dynamical states studied with similar methods, NGC4755 fits relations involving structural and dynamical parameters in theexpected locus for its age and mass. On the other hand, the flatter coreMF probably originates from primordial processes related to parentmolecular cloud fragmentation and mass segregation over 14 Myr. Starformation in NGC 4755 began ≈14 Myr ago and proceeded for about thesame length of time. Detection of K_s-excess emission in member MS starssuggests that some circumstellar dust discs survived for 10^7 yr,occurring both in some MS and PMS stars for the age and spread observedin NGC 4755.
| S1242: a lithium-rich subgiant star in the open cluster M 67
Aims.We report the serendipitous discovery of a lithium-rich subgiantstar, S1242. This object is member of a large eccentricity spectroscopicbinary system in the solar-age open cluster M 67. Methods.UsingVLT/FLAMES-UVES observations and synthetic spectrum analysis, we derivedfor this star stellar parameters (temperature, gravity, metallicity androtational velocity) and a surface Li abundance (A_Li=2.7) larger thanthe higher abundance of M 67 stars near the turn-off. Results.Thehigh Li abundance we found in S1242 points to an excess of lithium atthe subgiant stage. We discuss two possible explanations for thisunusually high Li content in this binary system: a preservation of theLi at the turn-off stage due to tidal effects, or an efficient dredge-upof Li, hidden below the convective zone by atomic diffusion occuring inthe post turn-off stage.
| Galactic Interstellar Gas Cloud Mass Functions: A Simple Quantitative Approach
We present here a simple approach to understanding the gas cloud massdistribution function by simulating formation and destruction of gasclouds and gas clumps in the ISM. We include as relevant processescoagulation to form bigger clouds, as well as disruption by collisionsand the removal of gas by collapse to form stars. We evolve initial setsof preexisting gas clumps with a range of initial distribution functions(flat, Gaussian, fractal) for their physical parameters and withdifferent geometrical forms (spherical or elongated) for the individualclouds, and constrain them within an imaginary box representinggravitational bounding, applying the kinematic laws of nonelasticcollisions. The results agree well with observations of the massdistribution function of Galactic giant gas clouds if we choose aGaussian for the initial distribution function, and initial gas cloudswhich are quasi-spherical.
| Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
Aims.We report a deep X-ray survey of the young (~140 Myr), rich opencluster NGC 2516 obtained with the EPIC camera on board the XMM-Newtonsatellite. Methods: .By combining data from six observations, ahigh sensitivity, greater than a factor of 5 with respect to recentChandra observations, has been achieved. Kaplan-Meier estimators of thecumulative X-ray luminosity distribution are built, statisticallycorrected for non members contaminants and compared to those of thenearly coeval Pleiades. The EPIC spectra of the X-ray brightest starsare fitted using optically thin model plasma with one or two thermalcomponents. Results: .We detected 431 X-ray sources and 234 ofthem have as optical counterparts cluster stars spanning the entire NGC2516 Main Sequence. On the basis of X-ray emission and opticalphotometry, we indicate 20 new candidate members of the cluster; at thesame time we find 49 X-ray sources without known optical or infraredcounterpart. The X-ray luminosities of cluster stars span the range logLX (erg s-1) = 28.4-30.8. The representativetemperatures span the 0.3-0.6 keV (3.5-8 MK) range for the coolcomponent and 1.0-2.0 keV (12-23 MK) for the hot one; similar values arefound in other young open clusters like the Pleiades, IC 2391, andBlanco 1. While no significant differences are found in X-ray spectra,NGC 2516 solar type stars are definitely less luminous in X-rays thanthe nearly coeval Pleiades. The comparison with a previous ROSAT surveyreveals the lack of variability amplitudes larger than a factor of 2 insolar type stars in a 11 yr time scale of the cluster and thusactivity cycles like in the Sun are probably absent or have a differentperiod and amplitude in young stars.
| Low-Mass Dwarf Template Spectra from the Sloan Digital Sky Survey
We present template spectra of low-mass (M0-L0) dwarfs derived from over4000 Sloan Digital Sky Survey spectra. These composite spectra aresuitable for use as medium-resolution (R~1800) radial velocitystandards. We report mean spectral properties (molecular band-headstrengths, equivalent widths) and use the templates to investigate theeffects of magnetic activity and metallicity on the spectroscopic andphotometric properties of low-mass stars.
| 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 Benchmark Cluster Reddening Project. II. A Reddening Value for M67
Since 1954, 70 reddening determinations have been published for M67.Those results are of very diverse quality, and they imply only that thevalue of E(B-V) for the cluster lies between -0.02 and +0.14 mag. Inaddition, this uncertainty is habitually concealed by long-standingcitation practices. In response, a reformed reddening protocol called``alternative scholarship'' is applied, with tenets including (1)rigorous published foundations for reddening values, (2) accountabilityfor all extant reddening results for M67, (3) explicit quality controlof reddening techniques, (4) use of techniques with limited metallicitysensitivity, (5) explicit zero-point control of input data (whenpossible), (6) a reddening zero point based ultimately on the dust-freesolar neighborhood, and (7) adherence to statistical norms. Results fromreddening maps are then set aside pending further examination of theirzero points and accidental errors. In addition, reddening values derivedfrom color-magnitude analysis and from averages of published reddeningresults are likewise set aside. Five techniques that satisfy the adoptedprotocol are then selected and applied. By averaging results fromcluster A and F stars and K giants, one finds that the mean value ofE(B-V) for M67 is 41+/-4 mmag. It is also found that extant results fromadditional techniques are consistent with this result or else can be setaside for plausible reasons. The analysis yields a collateral value of[Fe/H], and this is consistent with the derived reddening value. Usingpublished high-dispersion data, it is found that the mean cluster valueof [Fe/H] is -0.009+/-0.009 dex. For M67 and the Hyades combined, nomean metallicity difference between giants and stars near the mainsequence is found that is >=0.036 dex at 95% confidence.
| New catalogue of blue stragglers in open clusters
We present a catalogue of blue-straggler candidates in galactic openclusters. It is based on the inspection of the colour-magnitude diagramsof the clusters, and it updates and supersedesthe first version(Ahumada & Lapasset 1995). A new bibliographical search was made foreach cluster, and the resulting information is organised into twotables. Some methodological aspects have been revised, in particularthose concerning the delimitation of the area in the diagrams where thestragglers are selected.A total of 1887 blue-straggler candidates have been found in 427 openclusters of all ages, doubling the original number. The catalogued starsare classified into two categories mainly according to membershipinformation.The whole catalogue (Tables 8, 9, notes, and references) is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/789
| The PM2000 Bordeaux proper motion catalogue (+11° ≤ δ ≤ +18°)
We present a proper motion catalogue of 2 670 974 stars, covering thedeclination zone +11° ≤ δ ≤ +18 °. Proper motionswere derived from the comparison of the positional M2000 catalogue(systematic observations of the Bordeaux Carte du Ciel Zone with themeridian circle, completed in 2000) with positions derived from thereduction of 512 Carte du Ciel plates of the Bordeaux zone (scanned atthe APM Cambridge), the AC2000.2 catalogue, the USNO-A2.0 catalogue andthe unpublished Yellow Sky (YS3) USNO catalogue. The catalogue has alimiting magnitude VM=16.2 (Bordeaux CCD meridian circlemagnitude) and is complete down to VM=15.4. Depending onmagnitude, the positional precision at mean epoch ranges from 50 to 70mas and the precision of proper motions varies from 1.5 mas/yr to 6mas/yr. Meridian VM magnitudes are provided for all objectstogether with additional photometry from the 2MASS catalogue whenavailable (99.5% of objects). Positions and proper motions are on theICRS (International Celestial Reference System). Systematic offsets in2MASS positions and in UCAC2 proper motions were revealed fromcomparisons with PM2000.
| Probing disk properties with open clusters
We use the open clusters (OCs) with known parameters available in theWEBDA database and in recently published papers to derive propertiesrelated to the disk structure such as the thin-disk scale height,displacement of the Sun above the Galactic plane, scale length and theOC age-distribution function. The sample totals 654 OCs, consistingbasically of Trumpler types I to III clusters whose spatial distributiontraces out the local geometry of the Galaxy. We find that the populationof OCs with ages younger than 200 Myr distributes in the disk followingan exponential-decay profile with a scale height ofzh=48±3 pc. For the clusters with ages in the range200 Myr to 1 Gyr we derive zh=150±27 pc. Clustersolder than 1 Gyr distribute nearly uniformly in height from the plane sothat no scale height can be derived from exponential fits. Consideringclusters of all ages we obtain an average scale height ofzh=57±3 pc. We confirm previous results thatzh increases with Galactocentric distance. The scale heightimplied by the OCs younger than 1 Gyr outside the Solar circle is afactor 1.4-2 larger than zh of those interior to theSolar circle. We derive the displacement of the Sun above the Galacticplane as zȯ=14.8±2.4 pc, which agrees withprevious determinations using stars. As a consequence of thecompleteness effects, the observed radial distribution of OCs withrespect to Galactocentric distance does not follow the expectedexponential profile, instead it falls off both for regions external tothe Solar circle and more sharply towards the Galactic center. Wesimulate the effects of completeness assuming that the observeddistribution of the number of OCs with a given number of stars above thebackground, measured in a restricted zone outside the Solar circle, isrepresentative of the intrinsic distribution of OCs throughout theGalaxy. Two simulation models are considered in which the intrinsicnumber of observable stars are distributed: (i) assuming the actualpositions of the OCs in the sample, and (ii) random selection of OCpositions. As a result we derive completeness-corrected radialdistributions which agree with exponential disks throughout the observedGalactocentric distance range 5-14 kpc, with scale lengths in the rangeRD=1.5-1.9 kpc, smaller than those inferred by means ofstars. In particular we retrieve the expected exponential-disk radialprofile for the highly depleted regions internal to the Solar circle.The smaller values of RD may reflect intrinsic differences inthe spatial distributions of OCs and stars. We derive a number-densityof Solar-neighbourhood (with distances from the Sundȯ≤1.3 kpc) OCs ofρȯ=795±70 kpc-3, which implies atotal number of (Trumpler types I to III) OCs of 730 of which47% would already have been observed. Extrapolation of thecompleteness-corrected radial distributions down to the Galactic centerindicates a total number of OCs in the range(1.8-3.7)×105. These estimates are upper-limits becausethey do not take into account depletion in the number of OCs bydynamical effects in the inner parts of the Galaxy. The observed andcompleteness-corrected age-distributions of the OCs can be fitted by acombination of two exponential-decay profiles which can be identifiedwith the young and old OC populations, characterized by age scales of100 Myr and 1.9 Gyr, respectively. This rules out evolutionaryscenarios based on constant star-formation and OC-disruption rates.Comparing the number of observed embedded clusters and candidates in theliterature with the expected fraction of very young OCs, derived fromthe observed age-distribution function, we estimate that 3.4-8% of theembedded clusters do actually emerge from the parent molecular clouds asOCs.
| 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
| Accretion of chemically fractionated material on a wide binary with a blue straggler
Context: .The components of the wide binary HIP 64030 = HD 113984 show alarge (about 0.25 dex) iron content difference (Desidera et al. 2006).The positions of the components on the color magnitude diagram suggestthat the primary is a blue straggler. Aims: .We studied theabundance difference of several elements besides iron, and we searchedfor stellar and substellar companions around the components to unveilthe origin of the observed iron difference. Methods: .Aline-by-line differential abundance analysis for several elements wasperformed for iron, while suitable spectral synthesis was performed forC, N, and Li. High precision radial velocities obtained with the iodinecell were combined with available literature data. Results: .Theanalysis of additional elements shows that the abundance difference forthe elements studied increases with increasing condensation temperature,suggesting that accretion of chemically fractionated material might haveoccurred in the system. Alteration of C and N likely due to CNOprocessing is also observed. We also show that the primary is aspectroscopic binary with a period of 445 days and moderateeccentricity. The minimum mass of the companion is0.17~Mȯ. Conclusions: .Two scenarios were exploredto explain the observed abundance pattern. In the first, all abundanceanomalies arise on the blue straggler. If this is the case, the dust-gasseparation may have been occurred in a circumbinary disk around the bluestraggler and its expected white dwarf companion, as observed in severalRV Tauri and post AGB binaries. In the second scenario, accretion ofdust-rich material occurred on the secondary. This would also explainthe anomalous carbon isotopic ratio of the secondary. Such a scenariorequires that a substantial amount of mass lost by the central binaryhas been accreted by the wide component. Further studies to compare thetwo scenarios are proposed.Based on observations collected at the EuropeanSouthern Observatory, Chile, using FEROS spectrograph(proposal ID: 70.D-0081), on observations made with the ItalianTelescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Fundacion GalileoGalilei of the INAF(Istituto Nazionale di Astrofisica) at the Spanish Observatorio delRoque de los Muchachosof the Instituto de Astrofisica de Canarias, and on observations made atMcDonald Observatory.
| Mass functions and structure of the young open cluster NGC 6611
We use J, H and KS 2MASS photometry to study colour-magnitude(CMDs) and colour-colour diagrams, structure and mass distribution inthe ionizing open cluster NGC 6611. Reddening variation throughout thecluster region is taken into account followed by field-stardecontamination of the CMDs. Decontamination is also applied to derivethe density profile and luminosity functions in the core, halo andoverall (whole cluster) regions. The field-star decontamination showedthat the lower limit of the main sequence (MS) occurs at ≈5 M_ȯ.Based on the fraction of KS excess stars in the colour-colourdiagram we estimate an age of 1.3±0.3 Myr which is consistentwith the presence of a large number of pre-main sequence (PMS) stars.The distance from the Sun was estimated from known O V stars in thecluster area and the turn-on stars connecting the PMS and MS, resultingin d_ȯ=1.8±0.5 kpc. The radial density distributionincluding MS and PMS stars is fitted by a King profile with a coreradius R_core=0.70±0.08 pc. The cluster density profile mergesinto the background at a limiting radius R_lim=6.5±0.5 pc. Fromthe field-star subtracted luminosity functions we derive the massfunctions (MFs) in the form φ(m)∝ m-(1+χ). Inthe halo and through the whole cluster the MFs have slopesχ=1.52±0.13 and χ=1.45±0.12, respectively, thusslightly steeper than Salpeter's IMF. In the core the MF is flat,χ=0.62±0.16, indicating some degree of mass segregation sincethe cluster age is a factor 2 larger than the relaxation time. Becauseof the very young age of NGC 6611, part of this effect appears to berelated to the molecular cloud-fragmentation process itself. We detect362±120 PMS stars. The total observed mass including detected MS(in the range 5{-}85 M_ȯ) and PMS stars amounts to 1600 M_ȯ,thus more massive than the Trapezium cluster. Compared to older openclusters of different masses, the overall NGC 6611 fits in the relationsinvolving structural and dynamical parameters. However, the core isatypical in the sense that it looks like an old/dynamically evolvedcore. Again, part of this effect must be linked to formation processes.
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