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The Monitor project: rotation of low-mass stars in the open cluster M34
We report on the results of a V- and i-band time-series photometricsurvey of M34 (NGC 1039) using the Wide Field Camera (WFC) on the IsaacNewton Telescope (INT), achieving better than 1 per cent precision perdata point for 13 <~ i <~ 17. Candidate cluster members wereselected from a V versus V - I colour-magnitude diagram over 14 < V< 24 (0.12 <~ M/Msolar <~ 1.0), finding 714candidates, of which we expect ~400 to be real cluster members (takinginto account contamination from the field). The mass function wascomputed, and found to be consistent with a lognormal distribution indN/d logM. Searching for periodic variable objects in the candidatemembers gave 105 detections over the mass range 0.25 ~ 5d), consistent with the work ofother authors at very low masses. Our results are interpreted in thecontext of previous work, finding that we reproduce the same generalfeatures in the rotational period distributions. A number of rapidrotators were found with velocities ~ a factor of 2 lower than in thePleiades, consistent with models of angular momentum evolution assumingsolid body rotation without needing to invoke core-envelope decoupling.
| Spectral synthesis analysis and radial velocity study of the northern F-, G- and K-type flare stars
In this paper, we present a study of the general physical and chemicalproperties and radial velocity monitoring of young active stars. Wederive temperatures, logg, [Fe/H], v sini and Rspec valuesfor eight stars. The detailed analysis reveals that the stars are nothomogeneous in their principal physical parameters or in the agedistribution. In 4/5, we found a periodic radial velocity signal whichoriginates in surface features; the fifth is surprisingly inactive andshows little variation.
| Oxygen from the λ7774 High-Excitation Triplet in Open Cluster Dwarfs: Hyades
Oxygen abundances have been derived from the near-IR, high-excitation OI λ7774 triplet in high-resolution, high signal-to-noise ratiospectra of 45 Hyades dwarfs using standard one-dimensional,plane-parallel LTE models. Effective temperatures of the stellar samplerange from 4319 to 6301 K, and the derived relative O abundances as afunction of Teff evince a trichotomous morphology. AtTeff>6100 K, there is evidence of an increase in the Oabundances with increasing Teff, consistent with non-LTE(NLTE) predictions. At intermediate Teff (5450K<=Teff<=6100 K), the O abundances are flat, andstar-to-star values are in good agreement, having a mean value of[O/H]=+0.25+/-0.02 however, systematic errors at the <~0.10 dex levelmight exist. The O abundances for stars with Teff<=5450 Kshow a striking increase with decreasing Teff, in starkcontrast to expectations and canonical NLTE calculations. The coolHyades triplet results are compared to those recently reported fordwarfs in the Pleiades cluster and the UMa moving group; qualitativedifferences between the trends observed in these stellar aggregatespoint to a possible age-related diminution of triplet abundance trendsin cool open cluster dwarfs. Correlations with age-related phenomena,i.e., chromospheric activity and photospheric spots, faculae, and/orplages, are investigated. No correlation with Ca II H+K chromosphericactivity indicators is observed. Multicomponent LTE ``toy'' models havebeen constructed in order to simulate photospheric temperatureinhomogeneities that could arise from the presence of starspots, and wedemonstrate that photospheric spots are a plausible source of thetriplet trends among the cool dwarfs.Based on observations obtained with the Mayall 4 m telescope at KittPeak National Observatory, a division of the National Optical AstronomyObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under cooperative agreement with theNational Science Foundation.This paper includes data taken with the Harlan J. Smith 2.7 m telescopeat the McDonald Observatory of the University of Texas at Austin.
| Monitor: transiting planets and brown dwarfs in star forming regions and young open clusters
The Monitor project is a large scale photometric monitoring survey often star forming regions and open clusters aged between 1 and 200 Myrusing wide-field optical cameras on 2-4 m telescopes worldwide. Theprimary goal of the project is to search for close-in planets and browndwarfs at young ages through the detection of transit events. Suchdetections would provide unprecedented constraints on planet formationand migration time-scales, as well as on evolutionary models of planetsand brown dwarfs in an age range where such constraints are very scarce.Additional science goals include rotation period measurements and theanalysis of flares and accretion-related variability.Workshop ``Ultralow-mass star formation and evolution'', see AN 326, No. 10www.ast.cam.ac.uk/suz/monitor/monitor.php
| Hyades Oxygen Abundances from the λ6300 [O I] Line: The Giant-Dwarf Oxygen Discrepancy Revisited1,
We present the results of our abundance analysis of Fe, Ni, and O inhigh signal-to-noise ratio, high-resolution Very Large Telescope UVESand McDonald 2dcoudé spectra of nine dwarfs and three giants inthe Hyades open cluster. The difference in Fe abundances derived from FeII and Fe I lines ([Fe II/H]-[Fe I/H]) and Ni I abundances derived frommoderately high-excitation (χ~4.20 eV) lines is found to increasewith decreasing Teff for the dwarfs. Both of these findingsare in concordance with previous results ofoverexcitation/overionization in cool young dwarfs. Oxygen abundancesare derived from the [O I] λ6300 line, with careful attentiongiven to the Ni I blend. The dwarf O abundances are in star-to-staragreement within uncertainties, but the abundances of the three coolestdwarfs (4573 K<=Teff<=4834 K) evince an increase withdecreasing Teff. Possible causes for the apparent trend areconsidered, including the effects of overdissociation of O-containingmolecules. O abundances are derived from the near-UV OH λ3167line in high-quality Keck HIRES spectra, and no such effects are found;indeed, the OH-based abundances show an increase with decreasingTeff, leaving the nature and reality of the cool dwarf [OI]-based O trend uncertain. The mean relative O abundance of the sixwarmest dwarfs (5075 K<=Teff<=5978 K) is[O/H]=+0.14+/-0.02, and we find a mean abundance of [O/H]=+0.08+/-0.02for the giants. Thus, our updated analysis of the [O I] λ6300line does not confirm the Hyades giant-dwarf oxygen discrepancyinitially reported by King & Hiltgen, suggesting that thediscrepancy was a consequence of analysis-related systematic errors. LTEoxygen abundances from the near-IR, high-excitation O I triplet are alsoderived for the giants, and the resulting abundances are approximately0.28 dex higher than those derived from the [O I] line, in agreementwith non-LTE predictions. Non-LTE corrections from the literature areapplied to the giant triplet abundances; the resulting mean abundance is[O/H]=+0.17+/-0.02, in decent concordance with the giant and dwarf [O I]abundances. Finally, Hyades giant and dwarf O abundances derived fromthe [O I] λ6300 line and high-excitation triplet, as well asdwarf O abundances derived from the near-UV OH λ3167 line, arecompared, and a mean cluster O abundance of [O/H]=+0.12+/-0.02 isachieved, which represents the best estimate of the Hyades O abundance.This paper includes data taken with the Harlan J. Smith 2.7 m and theOtto Struve 2.1 m telescopes at the McDonald Observatory of theUniversity of Texas at Austin.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California and theNational Aeronautics and Space Administration. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.
| 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.
| Revisiting the population of Galactic open clusters
We present results of a study of the galactic open cluster populationbased on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2,Hipparcos and other catalogues. The sample of optical clusters fromASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry planeof the clusters' distribution is determined to be at Z_0=-22±4pc, and the scale height of open clusters is only 56±3 pc. Thetotal surface density and volume density in the symmetry plane areΣ= 114 kpc-2 and D(Z_0)=1015 kpc-3,respectively. We find the total number of open clusters in the Galacticdisk to be of order of 105 at present. Fluctuations in thespatial and velocity distributions are attributed to the existence offour open cluster complexes (OCCs) of different ages containing up to afew tens of clusters. Members in an OCC show the same kinematicbehaviour, and a narrow age spread. We find, that the youngest clustercomplex, OCC 1 (log t<7.9), with 19 deg inclination to the Galacticplane, is apparently a signature of Gould's Belt. The most abundant OCC2 complex has moderate age (log t≈8.45). The clusters of thePerseus-Auriga group, having the same age as OCC 2, but differentkinematics are seen in breaks between Perseus-Auriga clouds. The oldest(log t≈8.85) and sparsest group was identified due to a large motionin the Galactic anticentre direction. Formation rate and lifetime ofopen clusters are found to be 0.23±0.03 kpc-2Myr-1 and 322±31 Myr, respectively. This implies atotal number of cluster generations in the history of the Galaxy between30 to 40. We estimate that less than about 10% of the total Galacticstellar disk population has ever passed an open cluster membership.
| A Search for Variable Stars and Planetary Occultations in NGC 2301. II. Variability
We obtained R-band time-series observations of the young, metal-richopen cluster NGC 2301 for 12 nights in 2004 February. B-band images werealso obtained, and color-magnitude diagrams with limits of R=19.5 andB=21.5 were produced. Only a small effort was made to determine clustermembership, as our magnitude limits are far deeper than previouslypublished values. Our photometric precision for the brightest 5 mag ofsources is 1-2 mmag. We determine that for the ~4000 stars that havetime-series data, 56% are variable, and of these, approximately 13% areobserved to exhibit periodic light curves ranging from tens of minutesto days. We present some examples of the light curves obtained, andproduce cuts in variability space based on parameters such as color andamplitude. The percentage of variability is approximately equal acrossall colors, with the majority of variables having amplitudes of 0.15 magor less. In general, redder stars show larger variability amplitudes. Wefind a smooth decline in the number of periodic variables toward longerperiod. This decline is probably due to a transition from intrinsic toextrinsic variability, and also due in part to our limited observingperiod of 12 nights. Essentially all the A and F main-sequence stars inour sample are variable (~2 mmag and larger), and most present complexlight curves containing multiple periods that are suggestive of theirinclusion in the δ Scuti and γ Doradus classes. A variablenon-cluster member giant and two variable white dwarf candidates arediscussed. Our equational description of variability is shown to be anexcellent predictive tool for determining the cumulative fraction ofvariables that will be observed in a photometric survey. Our entire dataset is available electronically.
| High-Resolution Spectroscopy of Ursa Major Moving Group Stars
We use new and extant literature spectroscopy to address abundances andmembership for UMa moving group stars. We first compare the UMa, Coma,and Hyades H-R diagrams via a homogeneous set of isochrones and findthat these three aggregates are essentially coeval; this (near)coevality can explain the indistinguishable distributions of UMa andHyades dwarfs in the chromospheric emission versus color plane. Ourspectroscopy of cool UMa dwarfs reveals striking abundanceanomalies-trends with Teff, ionization state, and excitationpotential-like those recently seen in young, cool M34, Pleaides, andHyades dwarfs. In particular, the trend of rising λ7774-based O Iabundance with declining Teff is markedly subdued in UMacompared to the Pleiades, suggesting a dependence on age or metallicity.Recent photometric metallicity estimates for several UMa dwarfs aremarkedly low compared to the group's canonical metallicity, and similardeviants are seen among cool Hyads as well. Our spectroscopy does notconfirm these curious photometric estimates, which seem to be calledinto question for cool dwarfs. Despite disparate sources of Li data, ourhomogeneous analysis indicates that UMa members evince remarkably smallscatter in the Li-Teff plane for Teff>=5200 K.Significant star-to-star scatter suggested by previous studies is seenfor cooler stars. Comparison with the consistently determined HyadesLi-Teff trend reveals differences that are qualitativelyconsistent with this cluster's larger [Fe/H] (and perhaps slightlygreater age). However, quantitative comparison with standard stellarmodels indicates the differences are smaller than expected, suggestingthe action of a fourth parameter beyond age, mass, and [Fe/H]controlling Li depletion. The UMa-Coma cool star Li abundances may showa slight 0.2 dex difference; however, this may be mass-independent andthus more consistent with a modest initial Li abundance difference.This paper includes data taken at the McDonald Observatory of theUniversity of Texas at Austin.Based on observations obtained at Kitt Peak National Observatory, adivision of the National Optical Astronomy Observatories, which isoperated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation.
| The Prospects for Finding Brown Dwarfs in Eclipsing Binary Systems and Measuring Brown Dwarf Properties
We present the results of a simulation to investigate the prospects ofmeasuring mass, age, radius, metallicity, and luminosity data for browndwarfs in fully eclipsing binary systems around dwarf spectral typesfrom late K to early M that could be identified by ultra-wide-fieldtransit surveys such as SuperWASP. These surveys will monitorapproximately a million K and M dwarfs with |b|>20deg(where blending of sources is not a significant problem) at a levelsufficient to detect transits of low-luminosity companions. We look atthe current observational evidence for such systems and suggest that ~1%of late K and early-to-mid M dwarfs could have a very close (~0.02 AU)brown dwarf companion. With this assumption, and using SuperWASP as anexample, our simulation predicts that ~400 brown dwarfs in fullyeclipsing binary systems could be discovered. All of these eclipsingbinaries could yield accurate brown dwarf mass and radius measurementsfrom radial velocity and follow-up light curve measurements. Byinferring the brown dwarf effective temperature distribution, assuming auniform age spread and an α=0.5 companion brown dwarf massfunction, the simulation estimates that brown dwarf brightness couldalso be measurable (at the 10% level) for ~60 of these binary systemsfrom near-infrared follow-up light curves of the secondary eclipse. Weconsider irradiation of these brown dwarfs by their primary stars andconclude that it would be below the 10% level for ~70% of them. Thismeans that in these cases, the measured brown dwarf brightnesses shouldessentially be the same as those of free-floating counterparts. Thepredicted age distribution of the primaries is dominated by youngsystems, and ~20 binaries could be younger than 1 Gyr. Irradiation willbe below the 10% level for ~80% of these. We suggest that many of theseyoung binary systems will be members of ``kinematic moving groups,''allowing their ages to be accurately constrained.
| Elemental abundances in the Blanco 1 open cluster
High-resolution spectroscopy is used to determine the detailed chemicalabundances of a group of eight F- and G-type stars in the young opencluster Blanco 1. An average [Fe/H] of +0.04 +/- 0.02 (internal error)+/- 0.04 (external error) is found, considerably lower than a previousspectroscopic estimate for this cluster. The difference is due mainly toour adoption of significantly cooler temperatures that are consistentwith both photometric and spectroscopic constraints. Blanco 1 exhibitssubsolar [Ni/Fe](-0.18 +/- 0.01 +/- 0.01), [Si/Fe](-0.09 +/- 0.02 +/-0.03), [Mg/Fe](-0.14 +/- 0.02 +/- 0.03) and [Ca/Fe](-0.09 +/- 0.03 +/-0.03); ratios that are not observed among nearby field stars. Thematerial from which Blanco 1 formed may not have been well mixed withinterstellar matter in the galactic disc, which tallies with its currentlocation about 240pc below the galactic plane. A simultaneous deficit ofNi and alpha elements with respect to Fe is hard to reconcile with mostpublished models of yields from supernovae of types Ia and II. Therevised abundances for Blanco 1 indicate that overall radiativeopacities in its stars, and hence convective zone properties at a givenmass, are similar to those in the Pleiades at approximately the sameage. This can explain a previous observation that the Li-depletionpatterns of G- and K-type stars in the two clusters areindistinguishable. The lower overall metallicity of Blanco 1 now makesit less attractive as a target for discovering transiting, short-periodexoplanets.
| Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
We study the kinematics of 129 W UMa binaries and we discuss itsimplications on the contact binary evolution. The sample is found to beheterogeneous in the velocity space. That is, kinematically younger andolder contact binaries exist in the sample. A kinematically young (0.5Gyr) subsample (moving group) is formed by selecting the systems thatsatisfy the kinematical criteria of moving groups. After removing thepossible moving group members and the systems that are known to bemembers of open clusters, the rest of the sample is called the fieldcontact binary (FCB) group. The FCB group is further divided into fourgroups according to the orbital period ranges. Then, a correlation isfound in the sense that shorter-period less-massive systems have largervelocity dispersions than the longer-period more-massive systems.Dispersions in the velocity space indicate a 5.47-Gyr kinematical agefor the FCB group. Compared with the field chromospherically activebinaries (CABs), presumably detached binary progenitors of the contactsystems, the FCB group appears to be 1.61 Gyr older. Assuming anequilibrium in the formation and destruction of CAB and W UMa systems inthe Galaxy, this age difference is treated as an empirically deducedlifetime of the contact stage. Because the kinematical ages (3.21, 3.51,7.14 and 8.89 Gyr) of the four subgroups of the FCB group are muchlonger than the 1.61-Gyr lifetime of the contact stage, the pre-contactstages of the FCB group must dominantly be producing the largedispersions. The kinematically young (0.5 Gyr) moving group covers thesame total mass, period and spectral ranges as the FCB group. However,the very young age of this group does not leave enough room forpre-contact stages, and thus it is most likely that these systems wereformed in the beginning of the main sequence or during thepre-main-sequence contraction phase, either by a fission process or mostprobably by fast spiralling in of two components in a common envelope.
| 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.
| A Robust Measure of Tidal Circularization in Coeval Binary Populations: The Solar-Type Spectroscopic Binary Population in the Open Cluster M35
We present a new homogeneous sample of 32 spectroscopic binary orbits inthe young (~150 Myr) main-sequence open cluster M35. The distribution oforbital eccentricity versus orbital period (e-logP) displays a distincttransition from eccentric to circular orbits at an orbital period of ~10days. The transition is due to tidal circularization of the closestbinaries. The population of binary orbits in M35 provide a significantlyimproved constraint on the rate of tidal circularization at an age of150 Myr. We propose a new and more robust diagnostic of the degree oftidal circularization in a binary population based on a functional fitto the e-logP distribution. We call this new measure the ``tidalcircularization period.'' The tidal circularization period of a binarypopulation represents the orbital period at which a binary orbit withthe most frequent initial orbital eccentricity circularizes (defined ase=0.01) at the age of the population. We determine the tidalcircularization period for M35, as well as for seven additional binarypopulations spanning ages from the pre-main sequence (~3 Myr) to thelate main sequence (~10 Gyr), and use Monte Carlo error analysis todetermine the uncertainties on the derived circularization periods. Weconclude that current theories of tidal circularization cannot accountfor the distribution of tidal circularization periods with populationage.WIYN Open Cluster Study XXII.
| The Dearth of Massive, Helium-rich White Dwarfs in Young Open Star Clusters
Spectra have been obtained of 21 white dwarfs (WDs) in the direction ofthe young, rich open star cluster NGC 2099. This represents anappreciable fraction (>30%) of the cluster's total WD population. Themean derived mass of the sample is 0.8 Msolar-about 0.2Msolar larger than the mean seen among field WDs. Asurprising result is that all of the NGC 2099 WDs have hydrogen-richatmospheres (DAs); none exhibit helium-rich ones (DBs) or any otherspectral class. The number ratio in the field at the temperatures of theNGC 2099 WDs is DA/DB ~ 3.5. While the probability of seeing no DB WDsin NGC 2099 solely by chance is ~2%, if we include WDs in other openclusters of similar age it then becomes highly unlikely that the dearthof DB WDs in young open clusters is just a statistical fluctuation. Weexplore possible reasons for the lack of DBs in these clusters andconclude that the most promising scenario for the DA/DB number ratiodiscrepancy in young clusters is that hot, high-mass WDs do not developlarge enough helium convection zones to allow helium to be brought tothe surface and turn a hydrogen-rich WD into a helium-rich one.Based on observations with Gemini (run ID GN-2002B-Q-11) and Keck.Gemini is an international partnership managed by the Association ofUniversities for Research in Astronomy, Inc., under a cooperativeagreement with the National Science Foundation. The W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA, was made possible by the generous financial support of the W. M.Keck Foundation.
| ULMSF conference summary
Ten years on from the discovery of the first brown dwarf and the firstexoplanet, how well have we progressed in our understanding of theselow-mass objects? In particular how well do we understand theirformation? The strong impression from this conference was that theformation of brown dwarfs was just a continuation of the star formationprocess, no special additional mechanism is indicated.
| The uBVI Photometric System. II. Standard Stars
Paper I of this series described the design of a CCD-based photometricsystem that is optimized for ground-based measurements of the size ofthe Balmer discontinuity in stellar spectra. This ``uBVI'' systemcombines the Thuan-Gunn u filter with the standard Johnson-Kron-CousinsBVI filters and can be used to discover luminous yellow supergiants inextragalactic systems and post-asymptotic giant branch stars in globularclusters and galactic halos. In the present paper we use uBVIobservations obtained on 54 nights with 0.9 m telescopes at Kitt Peakand Cerro Tololo to construct a catalog of standardized u magnitudes forstandard stars taken from the 1992 catalog of Landolt. We describe theselection of our 14 Landolt fields and give details of the photometricreductions, including red leak and extinction corrections,transformation of all the observations onto a common magnitude system,and the establishment of the photometric zero point. We present acatalog of u magnitudes of 103 stars suitable for use as standards. Weshow that data obtained with other telescopes can be transformed to ourstandard system with better than 1% accuracy.
| 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.
| 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.
| Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. VIII. The late-type stellar component
We present results of an investigation of the X-ray properties, agedistribution, and kinematical characteristics of a high-galacticlatitude sample of late-type field stars selected from the ROSAT All-SkySurvey (RASS). The sample comprises 254 RASS sources with opticalcounterparts of spectral types F to M distributed over six study areaslocated at |b| 20 °, and Dec ≥ -9 °. A detailed studywas carried out for the subsample of ~200 G, K, and M stars. Lithiumabundances were determined for 179 G-M stars. Radial velocities weremeasured for most of the 141 G and K type stars of the sample. Combinedwith proper motions these data were used to study the age distributionand the kinematical properties of the sample. Based on the lithiumabundances half of the G-K stars were found to be younger than theHyades (660 Myr). About 25% are comparable in age to the Pleiades (100Myr). A small subsample of 10 stars is younger than the Pleiades. Theyare therefore most likely pre-main sequence stars. Kinematically the PMSand Pleiades-type stars appear to form a group with space velocitiesclose to the Castor moving group but clearly distinct from the LocalAssociation.Based on observations collected at the German-Spanish AstronomicalCentre, Calar Alto, operated by the Max-Planck-Institut fürAstronomie, Heidelberg, jointly with the Spanish National Commission forAstronomy, and at the European Southern Observatory, La Silla, Chile.Tables A2-A4 are only available in electronic form athttp://www.edpsciences.org
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| Lithium abundances of the local thin disc stars
Lithium abundances are presented for a sample of 181 nearby F and Gdwarfs with accurate Hipparcos parallaxes. The stars are on circularorbits about the Galactic centre and, hence, are identified as belongingto the thin disc. This sample is combined with two published surveys toprovide a catalogue of lithium abundances, metallicities ([Fe/H]),masses, and ages for 451 F-G dwarfs, almost all belonging to the thindisc. The lithium abundances are compared and contrasted with publishedlithium abundances for F and G stars in local open clusters. The fieldstars span a larger range in [Fe/H] than the clusters for which [Fe/H]~=0.0 +/- 0.2. The initial (i.e. interstellar) lithium abundance of thesolar neighbourhood, as derived from stars for which astration oflithium is believed to be unimportant, is traced from logɛ(Li) =2.2 at [Fe/H]=-1 to logɛ(Li) = 3.2 at +0.1. This form for theevolution is dependent on the assumption that astration of lithium isnegligible for the stars defining the relation. An argument is advancedthat this latter assumption may not be entirely correct, and, theevolution of lithium with [Fe/H] may be flatter than previouslysupposed. A sharp Hyades-like Li dip is not seen among the field starsand appears to be replaced by a large spread among lithium abundances ofstars more massive than the lower mass limit of the dip. Astration oflithium by stars of masses too low to participate in the Li dip isdiscussed. These stars show little to no spread in lithium abundance ata given [Fe/H] and mass.
| XMM-Newton EPIC observations of stellar clusters and star forming regions
We report on observations of open clusters (OCs) and star formingregions (SFRs) obtained with the EPIC camera as part of the MissionScientist Guaranteed Time on XMM-Newton. These observations provide apowerful tool to investigate the evolution of coronal activity inlate-type convective stars and its dependence on magnetic fieldgeneration by dynamo processes. We discuss the motivations for thisprogram and present some results for the SFRs sigma Orionis (2-5Myr) and Taurus-Auriga (1-10 Myr) as well as for the OCs IC 2602(30 Myr), alpha Persei (50 Myr), Praesepe (600 Myr) andthe Hyades (600 Myr). We discuss imaging and spectral data providedby the EPIC MOS and PN detectors focussing on the determination of thecluster X-ray luminosity function and of the temperature structure,chemical abundances and time variability of cluster stars.Based on observations collected with the ESA mission XMM-Newton as partof the Mission Scientist (R. Pallavicini) Guaranteed Time
| Comparison of the Luminosity Functions of Open Clusters Based on USNO-A1 Data
The luminosity and mass functions of a group of Galactic open clustersare constructed by applying a statistical method to photometric datafrom the USNO-A1 catalog. Despite some limitations, this catalog can beused for statistical analyses in Galactic astronomy. Pairwisecomparisons of the derived cluster luminosity functions are performedfor five age intervals. The differences between the luminosity functionsof the open clusters are not statistically significant in most cases. Itis concluded that the luminosity functions are approximately universalthroughout a large volume in the solar neighborhood. Combined luminosityand mass functions are constructed for six age intervals. The slope ofthe mass spectrum may vary somewhat from cluster to cluster, and themean slope may be somewhat higher than the Salpetervalue.
| Magnesium Isotope Ratios in Hyades Stars
Using classical model atmospheres and an LTE analysis, Mg isotope ratios24Mg:25Mg:26Mg are measured in 32Hyades dwarfs covering effective temperatures4000K<=Teff<=5000K. We find no significant trend in anyisotope ratio versus Teff, and the mean isotope ratio is inexcellent agreement with the solar value. We determine stellarparameters and Fe abundances for 56 Hyades dwarfs covering4000K<=Teff<=6200K. For stars warmer than 4700 K, wederive a cluster mean value of [Fe/H]=0.16+/-0.02 (σ=0.1), in goodagreement with previous studies. For stars cooler than 4700 K, we findthat the abundance of Fe from ionized lines exceeds the abundance of Fefrom neutral lines. At 4700 K,[Fe/H]II-[Fe/H]I~=0.3dex, while at 4000 K[Fe/H]II-[Fe/H]I~=1.2dex. This discrepancy betweenthe Fe abundance from neutral and ionized lines likely reflectsinadequacies in the model atmospheres and the presence of non-LTE orother effects. Despite the inability of the models to reproduce theionization equilibrium for Fe, the Mg isotope ratios appear immune tothese problems and remain a powerful tool for studying Galactic chemicalevolution.Data presented here were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Oxygen in Open Cluster Dwarfs: Pleiades and M34
We analyze the high-excitation O I λ7774 triplet inhigh-resolution, moderate signal-to-noise ratio spectra of 15 Pleiadesand eight M34 open cluster dwarfs over the effective temperature rangeof 5048-6172 K. Relative O abundances have been derived using modelatmospheres interpolated from four different sets of ATLAS9 grids. Incontrast to existing non-LTE (NLTE) predictions, a dramatic increase inthe O I triplet abundance with decreasing temperature is seen for bothclusters, regardless of the atmospheric model. S I abundances of threePleiades stars derived from the high-excitation λ6053 featuremimic the O I abundance behavior. O abundances have also been derivedfrom the [O I] λ6300 feature in three Pleiades stars; theabundances exhibit a much lower mean value than do the cool dwarftriplet results. Inasmuch as the [O I] abundances are presumed to befree from NLTE effects, their mean values ([O/H]6300=+0.14)provide the current best estimate of the Pleiades O abundance. Spreadsin [O/H]Trip at a given temperature are also seen in bothclusters; no correlation is found between the spreads and thechromospheric emission measures.Based on observations obtained with the High Resolution Spectrograph onthe Hobby-Eberly Telescope, which is operated by McDonald Observatory onbehalf of the University of Texas at Austin, Pennsylvania StateUniversity, Stanford University, the Ludwig-Maximillians-Universitaet,Munich, and the George-August-Universitaet, Goettingen. Public Accesstime is available on the Hobby-Eberly Telescope through an agreementwith the National Science Foundation.
| Alkali-Activity Correlations in Open Clusters
We present a census of correlations between activity measures andneutral resonance lines of the alkali elements Li I and K I in openclusters and star-forming regions. The majority of very youngassociations and star formation regions show no evidence of Li-activitycorrelations, perhaps because their chromospheric activity indicatorshave a dominant origin in accretion processes with implied disk-clearingtimescales in the range of a few times 106 to~4×107 yr. Alkali-alkali and/or alkali-activitycorrelations are newly noted within IC 2391, M34, and perhaps Blanco 1and NGC 6475. Global X-ray luminosities are not as robust indicators astraditional optical indicators of alkali-activity correlations, nor areLi I-K I relations. Intracluster alkali-activity correlations are notglobal but are seen only within different intracluster subsamples,evincing rich behavior. Li- and K-activity correlations appear to gohand in hand, likely suggesting that at least some part of intraclusterLi variance is not due to real differential Li depletion. Although up to~90% of the star-to-star variance in Li I and K I within such asubsample can be related to that in optical chromospheric emission,significant Li dispersion above observational scatter may remain evenafter accounting for this. We suggest, for example, that at least threeindependent mechanisms (including a possible intracluster age spread)influence the distribution in the M34 Li-Teff plane. We arguethat Li-activity correlations are not illusory manifestations of aphysical Li-rotation connection. Although an unexpected correlationbetween Li, chromospheric emission, and the λ6455 Ca I feature incool M34 dwarfs indicates that the role of ``activity'' is played byspots/plages, we note that the alkali-activity correlations arequalitatively opposite in sign to other abundance anomalies beingrapidly delineated in active, young, cool stars.
| WIYN Open Cluster Study. XIX. Main-Sequence-Fitting Distances to Open Clusters Using V-K Color-Magnitude Diagrams
We have combined existing optical magnitudes for stars in seven openclusters and 54 field stars with the corresponding JHKsphotometry from the Two Micron All Sky Survey (2MASS). Combining opticalwith near-IR photometry broadens the color baseline, minimizing theinfluence of photometric errors and allowing better discriminationbetween cluster stars and contaminating foreground and backgroundpopulations. The open clusters in this study include NGC 2516, M35, M34,NGC 3532, M37, M67, and NGC 188. The field stars we are using possesshigh-quality Hipparcos parallaxes and well-determined metal abundances,allowing us to empirically determine the dependence of V-K color onmetal abundance in the range -0.45<=[Fe/H]<=+0.35.Using this relation along with the parallaxes of the field stars, we areable to construct an unevolved main sequence in the [MV,(V-K)0] diagram for a specific abundance. These diagrams arethen used to fit to the cluster main sequences in the (V, V-K)color-magnitude diagram in order to estimate a distance for each opencluster. We find that the resultant distances are within the range ofdistances found in the literature via the main-sequence-fittingtechnique. It is hoped that this will spur an expansion of the current(limited) database of star clusters with high-quality V-K photometrydown to the unevolved main sequence.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| WIYN Open Cluster Study: UBVRI CCD Photometry of M34
M34 is a thoroughly studied open cluster on its way to becoming afundamental WOCS cluster. We provide photometry of M34 in the U, B, V,R, and I filters in order to re-evaluate its fundamental properties.Observations were made on the WIYN 0.9m telescope on 17 January 2003using a field of 20' x 20' on the S2KB CCD. PSF photometry was appliedusing DAOPHOT II and IRAF. We discuss the reddening and metallicity,which were derived using all of the U-B, U-V, U-R, and U-I vs B-Vcolor-color diagrams. We also discuss the distance modulus and age,which were derived from application of Yale-Yonsei isochrones. This workhas been supported by the National Science Foundation under GrantsAST-0206202 (to CPD), AST-1039617 (REU site at Indiana U.) and by theNational Space Grant College and Fellowship Program as a Wisconsin SpaceGrant Consortium undergraduate scholar (to HG).
| Oxygen in Hyades G & K Dwarfs
Deriving accurate O abundances in various collections of Galacticstellar populations is critical to our understanding of Galacticchemical evolution. Unfortunately, measurable OI lines in the visiblespectral region are limited to two sets of features. The first is thestrong, high-excitation λ ; 7775 triplet, which is known to beaffected by NLTE conditions. The second is the pair of [OI] forbiddenlines at 6300 and 6363 ; {Å}, which are quite weak in solar-typestars ( 5 ; {and} ; 3 ; {mÅ}, respectively, in theSun) and are blended with other absorption features. Abundances derivedfrom the triplet are seen to be discrepantly high compared to thosederived from the [OI] λ 6300 forbidden line, which have beenshown to be immune from NLTE effects, in dwarfs with T {eff}greater than 6300 ; {K}. Indeed, current NLTE calculationspredict such behavior. More recently, our study of cool Pleiades and M34dwarfs (T {eff} < 5600 ; {K}) revealed a trend ofincreasing O abundances derived from the triplet with\textit{decreasing} T {eff}, contrary to current NLTEpredictions. We have continued our investigation of dwarf O abundanceswith the Hyades open cluster and present our preliminary results.This work was supported by the South Carolina Space Grant Consortiumthrough the Graduate Student Research Fellowship awarded to S.C.S., andby grants AST 00-86576 and AST 02-39518 to J.R.K. from the NationalScience Foundation.
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