תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion
We present NLTE Li abundances for 88 stars in the metallicity range -3.5< [Fe/H] < -1.0. The effective temperatures are based on theinfrared flux method with improved E(B-V) values obtained mostly frominterstellar Na I D lines. The Li abundances were derived through MARCSmodels and high-quality UVES+VLT, HIRES+Keck and FIES+NOT spectra, andcomplemented with reliable equivalent widths from the literature. Theless-depleted stars with [Fe/H] < -2.5 and [Fe/H] > -2.5 fall intotwo well-defined plateaus of ALi = 2.18 (? = 0.04) andALi = 2.27 (? = 0.05), respectively. We show that thetwo plateaus are flat, unlike previous claims for a steep monotonicdecrease in Li abundances with decreasing metallicities. At allmetallicities we uncover a fine-structure in the Li abundances of Spiteplateau stars, which we trace to Li depletion that depends on bothmetallicity and mass. Models including atomic diffusion and turbulentmixing seem to reproduce the observed Li depletion assuming a primordialLi abundance ALi = 2.64, which agrees well with currentpredictions (ALi = 2.72) from standard Big Bangnucleosynthesis. Adopting the Kurucz overshooting model atmospheresincreases the Li abundance by +0.08 dex to ALi = 2.72, whichperfectly agrees with BBN+WMAP.Based in part on observations obtained at the W. M. Keck Observatory,the Nordic Optical Telescope on La Palma, and on data from theHIRES/Keck archive and the European Southern Observatory ESO/ST-ECFScience Archive Facility.Table 1 is only available in electronic form athttp://www.aanda.org
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| NLTE analysis of CoI/CoII lines in spectra of cool stars with new laboratory hyperfine splitting constants
The analysis of stellar abundances for odd-Z Fe-peak elements requiresaccurate non-local thermodynamic equilibrium (NLTE) modelling ofspectral lines fully taking into account the hyperfine structure (HFS)splitting of lines. Here, we investigate the statistical equilibrium ofCo in the atmospheres of cool stars and the influence of NLTE and HFS onthe formation of Co lines and abundances. Significant departures fromLTE level populations are found for CoI number densities of excitedstates in CoII also differ from LTE at low metallicity. The NLTE levelpopulations are used to determine the abundance of Co in solarphotosphere, log? = 4.95 +/- 0.04dex, which is in agreement withthat in CI meteorites within the combined uncertainties. The spectrallines of CoI were calculated using the results of recent measurements ofhyperfine interaction constants by UV Fourier transform spectrometry.For CoII, the first laboratory measurements of HFS A and B factors wereperformed. These highly accurate A factor measurements (errors of theorder of 3-7 per cent) allow, for the first time, reliable modelling ofCoII lines in the solar and stellar spectra and, thus, a test of theCoI/CoII ionization equilibrium in stellar atmospheres. A differentialabundance analysis of Co is carried out for 18 stars in the metallicityrange -3.12 < [Fe/H] < 0. The abundances are derived by the methodof spectrum synthesis. At low [Fe/H], NLTE abundance corrections for CoIlines are as large as +0.6,..., + 0.8dex. Thus, LTE abundances of Co inmetal-poor stars are severely underestimated. The stellar NLTEabundances determined from the single UV line of CoII are lower by~0.5-0.6dex. The discrepancy might be attributed to possible blends thathave not been accounted for in the solar CoII line and its erroneousoscillator strength. The increasing [Co/Fe] trend in metal-poor stars,as calculated from the CoI lines under NLTE, can be explained if Co isoverproduced relative to Fe in massive stars. The models of Galacticchemical evolution are wholly inadequate to describe this trendsuggesting that the problem is in supernova yields.Based on observations collected at the European Southern Observatory,Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.E-mail: mbergema@mpa-garching.mpg.de (MB); j.pickering@imperial.ac.uk(JCP); gehren@usm.lmu.de (TG)
| The Velocity Distribution of Nearby Stars from Hipparcos Data. I. The Significance of the Moving Groups
We present a three-dimensional reconstruction of the velocitydistribution of nearby stars (lsim100 pc) using a maximum likelihooddensity estimation technique applied to the two-dimensional tangentialvelocities of stars. The underlying distribution is modeled as a mixtureof Gaussian components. The algorithm reconstructs the error-deconvolveddistribution function, even when the individual stars have unique errorand missing-data properties. We apply this technique to the tangentialvelocity measurements from a kinematically unbiased sample of 11,865main-sequence stars observed by the Hipparcos satellite. We explorevarious methods for validating the complexity of the resulting velocitydistribution function, including criteria based on Bayesian modelselection and how accurately our reconstruction predicts the radialvelocities of a sample of stars from the Geneva-Copenhagen Survey (GCS).Using this very conservative external validation test based on the GCS,we find that there is little evidence for structure in the distributionfunction beyond the moving groups established prior to the Hipparcosmission. This is in sharp contrast with internal tests performed hereand in previous analyses, which point consistently to maximal structurein the velocity distribution. We quantify the information content of theradial velocity measurements and find that the mean amount of newinformation gained from a radial velocity measurement of a single staris significant. This argues for complementary radial velocity surveys toupcoming astrometric surveys.
| Statistical equilibrium of silicon in the atmospheres of metal-poor stars
Aims: The statistical equilibrium of neutral and ionized silicon in theatmospheres of metal-poor stars is discussed. Non-local thermodynamicequilibrium effects (NLTE) are investigated and the silicon abundancesin metal-poor stars determined. Methods: We have used highresolution, high signal to noise ratio spectra from the UVESspectragraph at the ESO VLT telescope. Line formation calculations of Sii and Si ii in the atmospheres of metal-poor stars are presented foratomic models of silicon including 174 terms and 1132 line transitions.Recent improved calculations of Si i and Si ii photoionizationcross-sections are taken into account, and the influence of thefree-free quasi-molecular absorption in the Ly? wing isinvestigated by comparing theoretical and observed fluxes of metal-poorstars. All abundance results are derived from LTE and NLTE statisticalequilibrium calculations and spectrum synthesis methods. Results:It is found that the extreme ultraviolet radiation is very important formetal-poor stars, especially for the high temperature, very metal-poorstars. The radiative bound-free cross-sections also play a veryimportant role for these stars. Conclusions: NLTE effects for Siare found to be important for metal-poor stars, in particular for warmmetal-poor stars. It is found that these effects depend on thetemperature. For warm metal-poor stars, the NLTE abundance correctionreaches ~0.2 dex relative to standard LTE calculations. Our resultsindicate that Si is overabundant for metal-poor stars.Based on observations obtained in the frame of the ESO programme ID165.N-0276(A).
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 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/501/941
| Calibration of Strömgren uvby-H? photometry for late-type stars - a model atmosphere approach
Context: The use of model atmospheres for deriving stellar fundamentalparameters, such as T_eff, log g, and [Fe/H], will increase as we findand explore extreme stellar populations where empirical calibrations arenot yet available. Moreover, calibrations for upcoming large satellitemissions of new spectrophotometric indices, similar to the uvby-H?system, will be needed. Aims: We aim to test the power oftheoretical calibrations based on a new generation of MARCS models bycomparisons with observational photomteric data. Methods: Wecalculated synthetic uvby-H? colour indices from synthetic spectra.A sample of 367 field stars, as well as stars in globular clusters, isused for a direct comparison of the synthetic indices versus empiricaldata and for scrutinizing the possibilities of theoretical calibrationsfor temperature, metallicity, and gravity. Results: We show thatthe temperature sensitivity of the synthetic (b-y) colour is very closeto its empirical counterpart, whereas the temperature scale based uponH? shows a slight offset. The theoretical metallicity sensitivityof the m1 index (and for G-type stars its combination withc_1) is somewhat higher than the empirical one, based upon spectroscopicdeterminations. The gravity sensitivity of the synthetic c1index shows satisfactory behaviour when compared to obervations of Fstars. For stars cooler than the sun, a deviation is significant in thec1-(b-y) diagram. The theoretical calibrations of (b-y),(v-y), and c1 seem to work well for Pop II stars and lead toeffective temperatures for globular cluster stars supporting recentclaims that atomic diffusion occurs in stars near the turnoff point ofNGC 6397. Conclusions: Synthetic colours of stellar atmospherescan indeed be used, in many cases, to derive reliable fundamentalstellar parameters. The deviations seen when compared to observationaldata could be due to incomplete linelists but are possibly also due tothe effects of assuming plane-parallell or spherical geometry and LTE.Model colours are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/498/527
| Beryllium abundances in metal-poor stars
We have determined beryllium (Be) abundances for 25 metal-poor starsbased on the high-resolution and high signal-to-noise ratio spectra fromthe Very Large Telescope/Ultraviolet and Visual Echelle Spectrographdata base. Our results confirm that Be abundances increase with Fe,supporting the global enrichment of Be in the Galaxy. Oxygen abundancesbased on the [OI] forbidden line imply a linear relation with a slopeclose to one for the Be versus O trend, which indicates that Be isprobably produced in a primary process. Some strong evidence is foundfor the intrinsic dispersion of Be abundances at a given metallicity.The deviation of HD 132475 and HD 126681 from the general Be versus Feand Be versus O trends favours the predictions of the superbubble model.However, the possibility that such dispersion originates from theinhomogeneous enrichment in Fe and O of the protogalactic gas cannot beexcluded.Based on observations made with the European Southern Observatorytelescopes obtained from the ESO/ST-ECF Science Archive Facility.E-mail: tan@bao.ac.cn
| NLTE abundances of Mn in a sample of metal-poor stars
Aims: Following our solar work, we perform NLTE calculations of the Mnabundance for fourteen stars with [Fe/H] from 0 to -2.5, mainly to showhow NLTE affects Mn abundances in cool stars of different metallicities.Methods: The spectrum synthesis and Mn abundances are based onstatistical equilibrium calculations using various estimates for theinfluence of hydrogen collisions. Results: The NLTE abundances ofMn in all studied stars are systematically higher than the LTEabundances. At low metallicities, the NLTE abundance corrections may runup to 0.5-0.7 dex. Instead of a strong depletion of Mn relative to Fe inmetal-poor stars as found by the other authors, we only find slightlysubsolar values of [Mn/Fe] throughout the range of metallicitiesanalyzed here. Conclusions: The [Mn/Fe] trend in metal-poor starsis inconsistent with the predictions of galactic chemical evolutionmodels, where Mn is less produced than Fe.Based on observations collected at the European Southern Observatory,Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.
| Neutron-capture elements in halo, thick-disk, and thin-disk stars. Strontium, yttrium, zirconium, cerium
We derived Sr, Y, Zr, and Ce abundances for a sample of 74 cool dwarfsand subgiants with iron abundances, [Fe/H], between 0.25 and‑2.43. These estimates were obtained using synthetic spectra,assuming local thermodynamic equilibrium (LTE) for Y, Zr, and Ce,allowing for non-LTE conditions for Sr. We used high-resolution(λ/Δλ≅40 000 and 60 000) spectra withsignal-to-noise ratios between 50 and 200. We find that the Zr/Y, Sr/Y,and Sr/Zr ratios for the halo stars are the same in a wide metallicityrange (‑2.43 ≤ [Fe/H] ≤ ‑0.90), within the errors,indicating a common origin for these elements at the epoch of haloformation. The Zr/Y ratios for thick-disk stars quickly decrease withincreasing Ba abundance, indicating a lower rate of production of Zrcompared to Y during active thick-disk formation. The thick-disk andhalo stars display an increase in the [Zr/Ba] ratio with decreasing Baabundance and a correlation of the Zr and Eu overabundances relative toBa. The evolutionary behavior of the abundance ratios found for thethick-disk and halo stars does not agree with current models for theGalaxy’s chemical evolution. The abundance ratios of Y and Zr toFe and Ba for thin-disk stars, as well as the abundance ratios withineach group, are, on average, solar, though we note a slight decrease ofZr/Ba and Zr/Y with increasing Ba abundance. These results provideevidence for a dominance of asymptotic-giant-branch stars in theenrichment of the interstellar medium in heavy elements during thethin-disk epoch, in agreement with the predictions of thenucleosynthesis theory for the main s-process component.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| The lithium content of the Galactic Halo stars
Thanks to the accurate determination of the baryon density of theuniverse by the recent cosmic microwave background experiments, updatedpredictions of the standard model of Big Bang nucleosynthesis now yieldthe initial abundance of the primordial light elements withunprecedented precision. In the case of ^7Li, the CMB+SBBN value issignificantly higher than the generally reported abundances for Pop IIstars along the so-called Spite plateau. In view of the crucialimportance of this disagreement, which has cosmological, galactic andstellar implications, we decided to tackle the most critical issues ofthe problem by revisiting a large sample of literature Li data in halostars that we assembled following some strict selection criteria on thequality of the original analyses. In the first part of the paper wefocus on the systematic uncertainties affecting the determination of theLi abundances, one of our main goal being to look for the "highestobservational accuracy achievable" for one of the largest sets of Liabundances ever assembled. We explore in great detail the temperaturescale issue with a special emphasis on reddening. We derive four sets ofeffective temperatures by applying the same colour {T}_eff calibrationbut making four different assumptions about reddening and determine theLTE lithium values for each of them. We compute the NLTE corrections andapply them to the LTE lithium abundances. We then focus on our "best"(i.e. most consistent) set of temperatures in order to discuss theinferred mean Li value and dispersion in several {T}_eff and metallicityintervals. The resulting mean Li values along the plateau for [Fe/H]≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075when the lowest effective temperature considered is taken equal to 5700K and 6000 K respectively. This is a factor of 2.48 to 2.81 (dependingon the adopted SBBN model and on the effective temperature range chosento delimit the plateau) lower than the CMB+SBBN determination. We findno evidence of intrinsic dispersion. Assuming the correctness of theCMB+SBBN prediction, we are then left with the conclusion that the Liabundance along the plateau is not the pristine one, but that halo starshave undergone surface depletion during their evolution. In the secondpart of the paper we further dissect our sample in search of newconstraints on Li depletion in halo stars. By means of the Hipparcosparallaxes, we derive the evolutionary status of each of our samplestars, and re-discuss our derived Li abundances. A very surprisingresult emerges for the first time from this examination. Namely, themean Li value as well as the dispersion appear to be lower (althoughfully compatible within the errors) for the dwarfs than for the turnoffand subgiant stars. For our most homogeneous dwarfs-only sample with[Fe/H] ≤ 1.5, the mean Li abundances are A(L)_NLTE = 2.177±0.071 and 2.215±0.074 when the lowest effective temperatureconsidered is taken equal to 5700 K and 6000 K respectively. This is afactor of 2.52 to 3.06 (depending on the selected range in {T}_eff forthe plateau and on the SBBN predictions we compare to) lower than theCMB+SBBN primordial value. Instead, for the post-main sequence stars thecorresponding values are 2.260±0.1 and 2.235±0.077, whichcorrespond to a depletion factor of 2.28 to 2.52. These results,together with the finding that all the stars with Li abnormalities(strong deficiency or high content) lie on or originate from the hotside of the plateau, lead us to suggest that the most massive of thehalo stars have had a slightly different Li history than their lessmassive contemporaries. In turn, this puts strong new constraints on thepossible depletion mechanisms and reinforces Li as a stellartomographer.
| Chemical abundances in 43 metal-poor stars
We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe,Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood,most of them subgiants or turn-off-point stars with iron abundances[Fe/H] ranging from -0.4 to -3.0. About half of this sample has not beenspectroscopically analysed in detail before. Effective temperatures wereestimated from uvby photometry, and surface gravities primarily fromHipparcos parallaxes. The analysis is differential relative to the Sun,and was carried out with plane-parallel MARCS models. Various sources oferror are discussed and found to contribute a total error of about0.1-0.2 dex for most elements, while relative abundances, such as[Ca/Fe], are most probably more accurate. For the oxygen abundances,determined in an NLTE analysis of the 7774 Å triplet lines, theerrors may be somewhat larger. We made a detailed comparison withsimilar studies and traced the reasons for the, in most cases,relatively small differences. Among the results we find that [O/Fe]possibly increases beyond [Fe/H] = -1.0, though considerably less sothan in results obtained by others from abundances based on OH lines. Wedid not trace any tendency toward strong overionization of iron, andfind the excesses, relative to Fe and the Sun, of the α elementsMg, Si, and Ca to be smaller than those of O. We discuss someindications that also the abundances of different α elementsrelative to Fe vary and the possibility that some of the scatter aroundthe trends in abundances relative to iron may be real. This may supportthe idea that the formation of Halo stars occurred in smaller systemswith different star formation rates. We verify the finding by Gratton etal. (2003b, A&A, 406, 131) that stars that do not participate in therotation of the galactic disk show a lower mean and larger spread in [α/Fe] than stars participating in the general rotation. The latterstars also seem to show some correlation between [ α/Fe] androtation speed. We trace some stars with peculiar abundances, amongthese two Ba stars, HD 17072 and HD196944, the second already known to be rich in s elements.Finally we advocate that a spectroscopic study of a larger sample ofhalo stars with well-defined selection criteria is very important, inorder to add to the very considerable efforts that various groups havealready made.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Stars of Extragalactic Origin in the Solar Neighborhood
For 77 main-sequence F-G stars in the solar neighborhood with publishediron, magnesium, and europium abundances determined from high-dispersionspectra and with the ages estimated from theoretical isochrones, wecalculated the spatial velocities using Hipparcos data and the Galacticorbital elements. A comparison with the orbital elements of the globularclusters that are known to have been accreted by our Galaxy in the pastreveals stars of extragalactic origin. We show that the abundance ratiosof r- and alpha-elements in all the accreted stars differ sharply fromthose in the stars that are genetically associated with the Galaxy.According to current theoretical models, europium is produced mainly inlow-mass type-II supernovae (SNe II), while magnesium is synthesized inlarge quantities in high-mass SN II progenitors. Since all the oldaccreted stars of our sample exhibit a significant Eu overabundancerelative to Mg, we conclude that the maximum masses of the SN IIprogenitors outside the Galaxy were much lower than those inside it. Onthe other hand, only a small number of young accreted stars exhibit lownegative ratios [Eu/Mg] < 0. This can be explained by the delay ofprimordial star formation and the explosions of high-mass SNe II in arelatively small part of extragalactic space. We provide evidence thatthe interstellar medium was weakly mixed at the early evolutionarystages of the Galaxy formed from a single protogalactic cloud, and thatthe maximum mass of the SN II progenitors increased in it with timesimultaneously with the increase in mean metallicity.
| Neutron-Capture Elements in Halo, Thick-Disk, and Thin-Disk Stars: Neodymium
We have derived the LTE neodymium abundances in 60 cool stars withmetallicities [Fe/H] from 0.25 to -1.71 by applying a synthetic-spectrumanalysis to spectroscopic observations of NdII lines with a resolutionof λ/Δλ⋍60 000 and signal-to-noise ratios of100 200. We have improved the atomic parameters of NdII and blendinglines by analyzing the corresponding line pro files in the solarspectrum. Neodymium is overabundant with respect to iron in halo stars,[Nd/Fe]=0.33±0.09, with the [Nd/Fe] ratio decreasingsystematically with metallicity when [Fe/H]>-1. This reflects anonset of efficient iron production in type I supernovae during theformation of the thick disk. The [Nd/Ba] and [Nd/Eu] abundance ratiosbehave differently in halo, thick-disk, and thin-disk stars. Theobserved abundance ratios in halo stars, [Nd/Ba]=0.34±0.08 and[Nd/Eu]=-0.27±0.05, agree within the errors with the ratios ofthe elemental yields for the r-process. These results support theconclusion of other authors based on analyses of other elements that ther-process played the dominant role in the synthesis of heavy elementsduring the formation of the halo. The [Nd/Ba] and [Nd/Eu] ratios forthick-disk stars are almost independent of metallicity([Nd/Ba]=0.28(±0.03)-0.01(±0.04) [Fe/H] and[Nd/Eu]=-0.13(±0.03)+0.05(±0.04) [Fe/H]) but are smallerin absolute value than the corresponding ratios for halo stars,suggesting that the synthesis of s-process nuclei started during theformation of the thick disk. The s-process is estimated to havecontributed ⋍30% of the neodymium produced during this stage ofthe evolution of the Galaxy. The [Nd/Ba] ratio decreases abruptly by0.17 dex in the transition from the thick to the thin disk. Thesystematic decrease of [Nd/Ba] and increase of [Nd/Eu] with increasingmetallicity of thin-disk stars point toward a dominant role of thes-process in the synthesis of heavy elements during this epoch.
| Abundances of Na, Mg and Al in nearby metal-poor stars
To determine the population membership of nearby stars we exploreabundance results obtained for the light neutron-rich elements23Na and 27 Al in a small sample of moderatelymetal-poor stars. Spectroscopic observations are limited to the solarneighbourhood so that gravities can be determined from HIPPARCOSparallaxes, and the results are confronted with those for a separatesample of more metal-poor typical halo stars. Following earlierinvestigations, the abundances of Na, Mg and Al have been derived fromNLTE statistical equilibrium calculations used as input to line profilesynthesis. Compared with LTE the abundances require systematiccorrections, with typical values of +0.05 for [Mg/Fe], -0.1 for [Na/Fe]and +0.2 for [Al/Fe] in thick disk stars where [Fe/H] -0.6. Inmore metal-poor halo stars these values reach +0.1, -0.4, and +0.5,respectively, differences that can no longer be ignored.After careful selection of a clean subsample free from suspected orknown binaries and peculiar stars, we find that [Na/Mg] and [Al/Mg], incombination with [Mg/Fe], space velocities and stellar evolutionaryages, make possible an individual discrimination between thick disk andhalo stars. At present, this evidence is limited by the small number ofstars analyzed. We identify a gap at [Al/Mg] -0.15 and [Fe/H] -1.0 that isolates stars of the thick disk from those in the halo.A similar separation occurs at [Na/Mg] -0.4. We do not confirm theage gap between thin and thick disk found by Fuhrmann. Instead we findan age boundary between halo and thick disk stars, however, with anabsolute value of 14 Gyr that must be considered as preliminary. Whilethe stellar sample is by no means complete, the resulting abundancesindicate the necessity to revise current models of chemical evolutionand/or stellar nucleosynthesis to allow for an adequate production ofneutron-rich species in early stellar generations.Based on observations collected at the German-Spanish AstronomicalCenter, Calar Alto (CAHA H01-2.2-002) and at the European SouthernObservatory, Chile (ESO 67.D-0086).
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are only available in electronic form, at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Mg, Ba and Eu abundances in thick disk and halo stars
Our sample of cool dwarf stars from previous papers (Mashonkina &Gehren \cite{euba, eubasr}) is extended in this study including 15moderately metal-deficient stars. The samples of halo and thick diskstars have overlapping metallicities with [Fe/H] in the region from -0.9to -1.5, and we compare chemical properties of these two kinematicallydifferent stellar populations independent of their metallicity. Wepresent barium, europium and magnesium abundances for the new sample ofstars. The results are based on NLTE line formation obtained indifferential model atmosphere analyses of high resolution spectraobserved mainly using the UVES spectrograph at the VLT of the EuropeanSouthern Observatory. We confirm the overabundance of Eu relative to Mgin halo stars as reported in our previous papers. Eight halo stars show[Eu/Mg] values between 0.23 and 0.41, whereas stars in the thick andthin disk display a solar europium to magnesium ratio. The [Eu/Ba]values found in the thick disk stars to lie between 0.35 and 0.57suggest that during thick disk formation evolved low-mass stars startedto enrich the interstellar gas by s-nuclei of Ba, and the s-processcontribution to barium thus varies from 30% to 50%. Based on theseresults, and using the chemical evolution calculations by Travaglio etal. (\cite{eu99}), we estimate that the thick disk stellar populationformed on a timescale between 1.1 to 1.6 Gyr from the beginning of theprotogalactic collapse. In the halo stars the [Eu/Ba] values are foundmostly between 0.40 and 0.67, which suggests a duration of the haloformation of about 1.5 Gyr. For the whole sample of stars we present theeven-to-odd Ba isotope ratios as determined from hyperfine structureseen in the Ba Ii resonance line lambda 4554. As expected, the solarratio 82:18 (Cameron \cite{cam}) adjusts to observations of the Ba Iilines in the thin disk stars. In our halo stars the even-to-odd Baisotope ratios are close to the pure r-process ratio 54:46 (Arlandini etal. \cite{rs99}), and in the thick disk stars the isotope ratio isaround 65:35 (+/-10%). Based on these data we deduce for thick diskstars the ratio of the s/r-process contribution to barium as 30:70(+/-30%), in agreement with the results obtained from the [Eu/Ba]values. Based on observations collected at the European SouthernObservatory, Chile, 67.D-0086A, and the German Spanish AstronomicalCenter, Calar Alto, Spain.
| The eclipsing millisecond pulsar PSR J1740-5340 and its red straggler companion
We present a high-resolution echelle spectrum taken with the Very LargeTelescope and analyse archival Hubble Space Telescope photometry of therecently identified companion of the eclipsing millisecond radio pulsarPSR J1740-5340 in the globular cluster NGC6397. From the spectrum, we show that the companion is metalpoor, as expected for a member of NGC 6397. Usingsynthetic photometry, and assuming a true distance modulus of12.13+/-0.15 mag and a colour excess of EB-V=0.179, we derivea radius of 1.60+/-0.17 Rsun and an effective temperature of5410+/-50K, implying a luminosity of 2.0+/-0.4 Lsun (1sigmaerrors). These properties make it similar to the so-called``sub-subgiants'' in the old open cluster M 67 and``red stragglers'' in 47 Tuc, which have luminositiescomparable to those of turn-off stars, but cooler temperatures andlarger radii. The light curve of the companion is well described byellipsoidal variations, and despite the incomplete ( ~ 60%) phasecoverage, we are able to derive good constraints on a number of thesystem parameters. In particular, for the inclination we find a 2-sigmalower limit of i>48deg (i>46o at 3sigma ).Assuming a pulsar mass of 1.2
| O/Fe in metal-poor main sequence and subgiant stars
A study of the O/Fe ratio in metal-poor main sequence and subgiant starsis presented using the \ion{Oi},6300 Å line, the O I 7774 Åtriplet, and a selection of weak Fe Ii lines observed on high-resolutionspectra acquired with the VLT UVES spectrograph. The \ion{Oi line isdetected in the spectra of 18 stars with -2.4 < [Fe/H] < -0.5, andthe triplet is observed for 15 stars with Fe/H ranging from -1.0 to-2.7. The abundance analysis was made first using standard modelatmospheres taking into account non-LTE effects on the triplet: the\ion{Oi} line and the triplet give consistent results with [O/Fe]increasing quasi-linearly with decreasing [Fe/H] reaching [O/Fe] =~ +0.7at [Fe/H] = -2.5. This trend is in reasonable agreement with otherresults for [O/Fe] in metal-poor dwarfs obtained using standardatmospheres and both ultraviolet and infrared OH lines. There is alsobroad agreement with published results for [O/Fe] for giants obtainedusing standard model atmospheres and the \ion{Oi} line, and the OHinfrared lines, but the O I lines give higher [O/Fe] values which may,however, fall into place when non-LTE effects are considered. Whenhydrodynamical model atmospheres representing stellar granulation indwarf and subgiant stars replace standard models, the [O/Fe] from the\ion{Oi} and Fe Ii lines is decreased by an amount which increases withdecreasing [Fe/H]. These 3D effects on [O/Fe] is compounded by theopposite behaviour of the \ion{Oi} (continuous opacity effect) and Fe Iilines (excitation effect). The [O/Fe] vs. [Fe/H] relation remainsquasi-linear extending to [O/Fe] =~ +0.5 at [Fe/H] = -2.5, but with atendency of a plateau with [O/Fe] =~ +0.3 for -2.0 < [Fe/H] <-1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] =~ -1.0. Use ofthe hydrodynamical models disturbs the broad agreement between theoxygen abundances from the \ion{Oi} , O I, and OH lines, but 3D non-LTEeffects may serve to erase these differences. The [O/Fe] values from the\ion{Oi} line and the hydrodynamical model atmospheres for dwarfs andsubgiant stars are lower than the values for giants using standard modelatmospheres and the \ion{Oi}, and O I lines. Based on observationscollected at the European Southern Observatory, Chile (ESO Nos.65.L-0131, 65.L-0507, and 67.D-0439).
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
| Catalogue of [Fe/H] determinations for FGK stars: 2001 edition
The catalogue presented here is a compilation of published atmosphericparameters (Teff, log g, [Fe/H]) obtained from highresolution, high signal-to-noise spectroscopic observations. This newedition has changed compared to the five previous versions. It is nowrestricted to intermediate and low mass stars (F, G and K stars). Itcontains 6354 determinations of (Teff, log g, [Fe/H]) for3356 stars, including 909 stars in 79 stellar systems. The literature iscomplete between January 1980 and December 2000 and includes 378references. The catalogue is made up of two tables, one for field starsand one for stars in galactic associations, open and globular clustersand external galaxies. The catalogue is distributed through the CDSdatabase. Access to the catalogue with cross-identification to othersets of data is also possible with VizieR (Ochsenbein et al.\cite{och00}). The catalogue (Tables 1 and 2) is only available inelectronic form at the 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/373/159 and VizieRhttp://vizier.u-strasbg.fr/.
| Revised Magnesium Abundances in Galactic Halo and Disk Stars
A differential analysis of the magnesium abundances in 61 F-K dwarfs andsubgiants with metallicities -2.6<[Fe/H]<+0.2 is performed basedon published observational data. Fundamental parameters for 36 stars aredetermined: T eff from V-K and V-R; logg from HIPPARCOS parallaxes, and[Fe/H] and ξt from Fe II lines. The computations allow for non-LTEeffects in the formation of the Mg I lines. For most of the stars, thestandard errors in the Mg abundances do not exceed 0.07 dex. Themetallicity dependence of [Mg/Fe] is analyzed. Magnesium shows aconstant overabundance relative to Fe of 0.46±0.06 dex formetallicities -2.6<[Fe/H]<-0.7 Mg. The Mg overabundance decreasesabruptly to +0.27 dex at [Fe/H]⋍-0.7. At highermetallicities, the Mg abundance smoothly decreases to the solar value at[Fe/H]=0.0. Halo stars with metallicities [Fe/H]<-1.0 exhibit lowerMg overabundances ( ) compared to the [Mg/Fe] values for other starswith similar [Fe/H].
| Models for Old, Metal-Poor Stars with Enhanced α-Element Abundances. II. Their Implications for the Ages of the Galaxy's Globular Clusters and Field Halo Stars
The ages of globular clusters (GCs) and post-turnoff field stars in theGalactic halo are derived using new stellar evolutionary models thatexplicitly take into account the observed abundances of theα-elements. Whereas the distances of the field subgiantsconsidered in this study are based on Hipparcos parallax measurements,theoretical zero-age horizontal-branch (ZAHB) loci have been used to setthe GC distance scale. (As reported in Paper I, the latter imply RRLyrae luminosities that are within 0.1-0.15 mag, on the high side, ofthose found for field variables from Baade-Wesselink, trignometicparallax, and statistical parallax studies.) Both the field and clusterobservations indicate that the most metal-deficient stars are >~14Gyr old while those with [Fe/H] <~-1.3 are 2-3 Gyr younger. Unlessthe O/Fe (or, more generally, the α/Fe) number abundance ratiorises quite steeply with decreasing [Fe/H], it seems unlikely that asignificant age-metallicity relation can be avoided. Using what iseffectively the ΔVHBTO method of determiningrelative cluster ages, we find that the dispersion in age at any [Fe/H]less than ~-1.0 is small (<~10%-15%). Even Arp 2 and IC 4499, whichhad been previously categorized as ``young GCs,'' probably havenear-normal ages for their metallicities-though a more definitiveconclusion must await improved photometry for these systems. Ruprecht106 appears no more than 1-1.5 Gyr younger than M3, as opposed to the ~4Gyr age difference that others have found. However, Palomar 12 andTerzan 7 are undoubtedly young, and they provide strong evidence that,at [Fe/H] >~-1.0, the ages of GCs differ by as much as 4 Gyr (or~25%). (A much larger sample of metal-rich clusters must be studied toascertain whether the distribution of ages at higher metallicities isbroad or narrow: Pal 12 and Ter 7 could well be atypical and representthe tail of a distribution that is strongly peaked near 12 Gyr.)Importantly, for the best-observed systems, there is no obvious conflictbetween the relative age estimates based on theΔVHBTO method, on the one hand, and theΔ(B-V)TO,RGB technique, on the other. However, thereare some exceptions, like M68, M53, M13, and NGC 288 for which the twoapproaches give slightly different results, indicating that something inaddition to, or besides, age must be playing a role. From isochrone/ZAHBfits to the C-M diagrams (CMDs) of the outer-halo GCs Pal 3, Pal 4, andEridanus, we conclude that there is no more than a small dependence ofage on Galactocentric distance. Pal 4 seems to have very close to thesame age as NGC 362, NGC 1261, and NGC 1851, while Eridanus and Pal 3appear to be <~1 Gyr younger than most inner-halo systems of the samemetallicity. Unless our understanding of the HB phase of evolution isseriously in error, cluster-to-cluster age differences (at a givenmetallicity) are much too small for age to be the most important of thepossible second parameters in determining the morphology of thehorizontal branch (the first parameter being [Fe/H]). Finally, wesuggest that the GC distance scale as inferred from studies of nearby RRLyraes is not necessarily in conflict with that based on localsubdwarfs. A reconciliation of the two approaches is possible simply byadopting a particular metallicity scale for the globular clusters-onethat is much closer to the Zinn-West scale than to that recentlyproposed by Carretta & Gratton. Indeed, such a simple way ofachieving consistency between subdwarf-based distances and thoseinferred from RR Lyraes seems compelling. If this suggestion is correct,then the ``long'' distance scale and low estimates of GC ages are nottenable. In fact, the adoption of the ``short'' distance scale, implyingages >~15 Gyr for the most metal-poor GCs (and <~18.30 for thetrue distance modulus of the Large Magellanic Cloud), leads to muchimproved agreement between synthetic and observed CMDs in the vicinityof the turnoff. It does cause a mismatch between predicted and observedHB luminosities, in the sense that the models are too bright by~0.1-0.15 mag, but this may simply be an indication that currentconductive opacities or the assumed chemical abundance parameters (Y,[O/Fe], and/or [α/Fe]) are not quite right.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Abundances of light elements in metal-poor stars. III. Data analysis and results
We present the results of the analysis of an extensive set of new andliterature high quality data concerning Fe, C, N, O, Na, and Mg. Thisanalysis exploited the T_eff scale determined in Gratton et al. (1996a),and the non-LTE abundance corrections computed in Gratton et al.(1999a). Results obtained with various abundance indices are discussedand compared. Detailed comparison with models of galactic chemicalevolution will be presented in future papers of this series. Our non-LTEanalysis yields the same O abundances from both permitted and forbiddenlines for stars with T_eff >4600 K, in agreement with King (1993),but not with other studies using a lower T_eff -scale for subdwarfs.However, we obtain slightly smaller O abundances for the most luminousmetal-poor field stars than for fainter stars of similar metallicities,an effect attributed to inadequacies of the adopted model atmospheres(Kurucz 1992, with overshooting) for cool stars. We find a nearlyconstant O overundance in metal-poor stars ([Fe/H]<-0.8), at a meanvalue of 0.46+/- 0.02 dex (sigma =0.12, 32 stars), with only a gentleslope with [Fe/H] ( ~ -0.1); this result is different from the steeperslope recently obtained using OH band in the near UV. If only bonafideunmixed stars are considered, C abundances scale with Fe ones (i.e.[C/Fe]~ 0) down to [Fe/H] ~ -2.5. Due to our adoption of a differentT_eff scale, we do not confirm the slight C excess in the most metalpoor disk dwarfs (-0.8<[Fe/H]<-0.4) found in previousinvestigations. Na abundances scale as Fe ones in the high metallicityregime, while metal-poor stars present a Na underabundance. None of thefield stars analyzed belong to the group of O-poor and Na-rich starsobserved in globular clusters. Na is deficient with respect to Mg inhalo and thick disk stars; within these populations, Na deficiency maybe a slow function of [Mg/H]. Solar [Na/Mg] ratios are obtained for thindisk stars. Tables~ 2 to 9 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strabg.fr/Abstract.html
| Abundances of metal-weak thick-disc candidates
High-resolution spectra of five candidate metal-weak thick-disc starssuggested by Beers & Sommer-Larsen are analysed to determine theirchemical abundances. The low abundance of all the objects has beenconfirmed, with metallicity reaching [Fe/H]=-2.9. However, for threeobjects the astrometric data from the Hipparcos catalogue suggest theyare true halo members. The remaining two, for which proper-motion dataare not available, may have disc-like kinematics. It is therefore clearthat it is useful to address properties of putative metal-weakthick-disc stars only if they possess full kinematic data. For CS22894-19 an abundance pattern similar to those of typical halo stars isfound, suggesting that chemical composition is not a useful discriminantbetween thick-disc and halo stars. CS 29529-12 is found to be C-enhancedwith [C/Fe]=+1.0 other chemical peculiarities involve the s-processelements: [Sr/Fe]=-0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr],considerably larger than that found in more metal-rich carbon-richstars, but similar to those in LP 706-7 and LP 625-44, discussed byNorris et al. Hipparcos data have been used to calculate the spacevelocities of 25 candidate metal-weak thick-disc stars, thus allowing usto identify three bona fide members, which support the existence of ametal-poor tail of the thick disc, at variance with a claim to thecontrary by Ryan & Lambert.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קנטאור |
| התרוממות ימנית: | 14h01m02.12s |
| סירוב: | -38°03'03.2" |
| גודל גלוי: | 8.722 |
| מרחק: | 102.354 פארסק |
| תנועה נכונה: | -451.3 |
| תנועה נכונה: | -82.5 |
| B-T magnitude: | 9.175 |
| V-T magnitude: | 8.76 |
קטלוגים וכינוים:
|
