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Oxygen Abundances in Low- and High-α Field Halo Stars and the Discovery of Two Field Stars Born in Globular Clusters
Oxygen abundances of 67 dwarf stars in the metallicity range -1.6 <[Fe/H] < -0.4 are derived from a non-LTE analysis of the 777 nm O Itriplet lines. These stars have precise atmospheric parameters measuredby Nissen and Schuster, who find that they separate into three groupsbased on their kinematics and α-element (Mg, Si, Ca, Ti)abundances: thick disk, high-α halo, and low-α halo. We findthe oxygen abundance trends of thick-disk and high-α halo starsvery similar. The low-α stars show a larger star-to-star scatterin [O/Fe] at a given [Fe/H] and have systematically lower oxygenabundances compared to the other two groups. Thus, we find the behaviorof oxygen abundances in these groups of stars similar to that of theα elements. We use previously published oxygen abundance data ofdisk and very metal-poor halo stars to present an overall view (-2.3< [Fe/H] < +0.3) of oxygen abundance trends of stars in the solarneighborhood. Two field halo dwarf stars stand out in their O and Naabundances. Both G53-41 and G150-40 have very low oxygen and very highsodium abundances, which are key signatures of the abundance anomaliesobserved in globular cluster (GC) stars. Therefore, they are likelyfield halo stars born in GCs. If true, we estimate that at least 3%± 2% of the local field metal-poor star population was born inGCs.
| Silver and palladium help unveil the nature of a second r-process
Context. The rapid neutron-capture process, which created about half ofthe heaviest elements in the solar system, is believed to have beenunique. Many recent studies have shown that this uniqueness is not truefor the formation of lighter elements, in particular those in the atomicnumber range 38 < Z < 48. Among these, palladium (Pd) andespecially silver (Ag) are expected to be key indicators of a possiblesecond r-process, but until recently they have been studied only in afew stars. We therefore target Pd and Ag in a large sample of stars andcompare these abundances to those of Sr, Y, Zr, Ba, and Eu produced bythe slow (s-) and rapid (r-) neutron-capture processes. Hereby weinvestigate the nature of the formation process of Ag and Pd. Aims: We study the abundances of seven elements (Sr, Y, Zr, Pd, Ag, Ba,and Eu) to gain insight into the formation process of the elements andexplore in depth the nature of the second r-process. Methods: Byadopting a homogeneous one-dimensional local thermodynamic equilibrium(1D LTE) analysis of 71 stars, we derive stellar abundances using thespectral synthesis code MOOG, and the MARCS model atmospheres. Wecalculate abundance ratio trends and compare the derived abundances tosite-dependent yield predictions (low-mass O-Ne-Mg core-collapsesupernovae and parametrised high-entropy winds), to extractcharacteristics of the second r-process. Results: The sevenelements are tracers of different (neutron-capture) processes, which inturn allows us to constrain the formation process(es) of Pd and Ag. Theabundance ratios of the heavy elements are found to be correlated andanti-correlated. These trends lead to clear indications that asecond/weak r-process, is responsible for the formation of Pd and Ag. Onthe basis of the comparison to the model predictions, we find that theconditions under which this process takes place differ from those forthe main r-process in needing lower neutron number densities, lowerneutron-to-seed ratios, and lower entropies, and/or higher electronabundances. Conclusions: Our analysis confirms that Pd and Agform via a rapid neutron-capture process that differs from the mainr-process, the main and weak s-processes, and charged particlefreeze-outs. We find that this process is efficiently working down tothe lowest metallicity sampled by our analysis ([Fe/H] = - 3.3). Ourresults may indicate that a combination of these explosive sites isneeded to explain the variety in the observationally derived abundancepatterns.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (ID 65.L-0507(A), 67.D-0439(A), 68.B-0475(A),68.D-0094(A), 71.B-0529(A); P.I. F. Primas).Appendices are available inelectronic form at http://www.aanda.org
| Two distinct halo populations in the solar neighborhood. IV. Lithium abundances
Context. A previous study of F and G main-sequence stars in the solarneighborhood has revealed the existence of two distinct halo populationswith a clear separation in [?/Fe] for the metallicity range -1.4< [Fe/H] < -0.7. Taking into account the kinematics and ages ofthe stars, some Galactic formation models suggest that the "high-alpha"halo stars were formed in situ, whereas the "low-alpha" stars have beenaccreted from satellite galaxies. Aims: In this paper weinvestigate if there is a systematic difference in the lithiumabundances of stars belonging to the high- and low-alpha halopopulations. Methods: Equivalent widths of the Li i 6707.8Å resonance line are measured from high resolution VLT/UVES andNOT/FIES spectra and used to derive Li abundances on the basis of MARCSmodel atmospheres. Furthermore, masses of the stars are determined fromthe log Teff - log g diagram by interpolating betweenevolutionary tracks based on Yonsei-Yale models. Results: Thereis no significant systematic difference in the lithium abundances ofhigh- and low-alpha stars. For the large majority of stars with masses0.7 < M/M&sun; < 0.9 and heavy-element mass fractions0.001 ? Z < 0.006, the lithium abundance is well fitted by arelation A(Li) = a0 + a1 M + a2 Z +a3 M Z, where a0, a1, a2,and a3 are constants. Extrapolating this relation to Z = 0leads to a lithium abundance close to the primordial value predictedfrom standard Big Bang nucleosynthesis calculations and the WMAP baryondensity. The relation, however, does not apply to stars withmetallicities below [Fe/H] ? - 1.5. Conclusions: We suggestthat metal-rich halo stars were formed with a lithium abundance close tothe primordial value, and that lithium in their atmospheres has beendepleted in time with an approximately linear dependence on stellar massand Z. The lack of a systematic difference in the Li abundances of high-and low-alpha stars indicates that an environmental effect is notimportant for the destruction of lithium.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility (programs 65.L-0507, 67.D-0086, 67.D-0439,68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585,76.B-0133 and 77.B-0507).Table 1 is available in electronic form at http://www.aanda.org
| Two distinct halo populations in the solar neighborhood. III. Evidence from stellar ages and orbital parameters
Context. In Papers I and II of this series, we have found clearindications of the existence of two distinct populations of stars in thesolar neighborhood belonging to the metal-rich end of the halometallicity distribution function. Based on high-resolution, high S/Nspectra, it is possible to distinguish between "high-alpha" and"low-alpha" components using the [?/Fe] versus [Fe/H] diagram. Aims: Precise relative ages and orbital parameters are determined for67 halo and 16 thick-disk stars having metallicities in the range -1.4< [Fe/H] < -0.4 to better understand the context of the two halopopulations in the formation and evolution of the Galaxy. Methods: Ages are derived by comparing the positions of stars in the logTeff-log g diagram with isochrones from the Y2models interpolated to the exact [Fe/H] and [?/Fe] values of eachstar. The stellar parameters have been adopted from the precedingspectroscopic analyses, but possible systematic errors inTeff and log g are considered and corrected. With spacevelocities from Paper I as initial conditions, orbital integrations havebeen carried out using a detailed, observationally constrainedMilky Way model including a bar and spiral arms. Results: The "high-alpha" halo stars have ages 2-3 Gyr larger thanthe "low-alpha" ones, with some probability that the thick-disk starshave ages intermediate between these two halo components. The orbitalparameters show very distinct differences between the "high-alpha" and"low-alpha" halo stars. The "low-alpha" ones have rmax's to30-40 kpc, zmax's to ?18 kpc, and emax'sclumped at values greater than 0.85, while the "high-alpha" ones,rmax's to about 16 kpc, zmax's to 6-8 kpc, andemax values more or less uniformly distributed over 0.4-1.0. Conclusions: A dual in situ-plus-accretion formation scenariobest explains the existence and characteristics of these two metal-richhalo populations, but one remaining defect is that this model is notconsistent regarding the rmax's obtained for the in situ"high-alpha" component; the predicted values are too small. It appearsthat ? Cen may have contributed in asignificant way to the existence of the "low-alpha" component; recentmodels, including dynamical friction and tidal stripping, have producedresults consistent with the present mass and orbital characteristics of? Cen, while at the same time includingextremes in the orbital parameters as great as those of the "low-alpha"component.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility (programmes 65.L-0507, 67.D-0086, 67.D-0439,68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585,76.B-0133 and 77.B-0507).Tables 1 and 4 are available in electronic format http://www.aanda.org.
| Statistical equilibrium of silicon in the atmospheres of nearby metal-poor stars
Aims: We discuss the statistical equilibrium of neutral andionized silicon in the atmospheres of nearby metal-poor stars. Weinvestigated the effects of non-local thermodynamic equilibrium (NLTE)and determined the silicon abundances. Methods: We used highresolution, high signal-to-noise ratio spectra from the FOCESspectragraph at the DSAZ telescope. Line-formation calculations of Si iand Si ii in the atmospheres of nearby metal-poor stars are presented.All abundance results are derived from local thermodynamic equilibrium(LTE) and NLTE statistical equilibrium calculations and spectrumsynthesis methods. Results: We find that NLTE effects for Si iioptical lines are important for warm stars, and that they depend oneffective temperature. The Si abundances of thin and thick disc starsfollow distinct trends, as in the case of Mg. We find that [Si/Fe]gradually increases as [Fe/H] decreases in thin disc stars, while itremains around at ~+0.30 dex for halo and thick disc stars, the halostars showing larger scatter. Conclusions: The derived dependencebetween [Si/Fe] and [Fe/H] is inconsistent with the theoreticalpredictions of published model calculations for the chemical evolutionof the Galaxy. The nearly constant [Si/Mg] ratio with some scatter forhalo and thick disc stars suggests that the nucleosynthesis of siliconis closely coupled to that of Mg. In addition, our results do notsupport the suggestion that type Ia supernove produce significantamounts of silicon.Based on observations collected at the German-Spanish AstronomicalCenter, Calar Alto, Spain.
| Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba
Context. Current models of galaxy formation predict that the Galactichalo was assembled hierarchically. By measuring abundance ratios instars it may be possible to identify substructures in the halo resultingfrom this process. Aims: A previous study of 94 dwarf stars with-1.6 < [Fe/H] < -0.4 in the solar neighborhood has revealed theexistence of two distinct halo populations with a systematic differencein [?/Fe] at a given metallicity. In continuation of that work,abundances of Mn, Cu, Zn, Y, and Ba are determined for the same sampleof stars. Methods: Equivalent widths of atomic lines are measuredfrom high resolution VLT/UVES and NOT/FIES spectra and used to deriveabundance ratios from an LTE analysis based on MARCS model atmospheres.The analysis is made relative to two thick-disk stars, HD22879 and HD 76932, such that very precisedifferential values are obtained. Results: Systematic differencesbetween the "high-?" and "low-?" halo populations are foundfor [Cu/Fe], [Zn/Fe], and [Ba/Y], whereas there is no significantdifference in the case of [Mn/Fe]. At a given metallicity, [Cu/Fe] showsa large scatter that is closely correlated with a corresponding scatterin [Na/Fe] and [Ni/Fe]. Conclusions: The metallicity trends of[Cu/Fe], [Zn/Fe], and [Ba/Y] can be explained from existingnucleosynthesis calculations if the high-? stars formed in regionswith such a high star formation rate that only massive stars and type IIsupernovae contributed to the chemical enrichment. The low-?stars, on the other hand, most likely originate from systems with aslower chemical evolution, characterized by additional enrichment fromtype Ia supernovae and low-mass AGB stars.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility (programs 65.L-0507, 67.D-0086, 67.D-0439,68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585,76.B-0133 and 77.B-0507).Tables 1, 2, and excerpt of Table 3 areavailable in electronic form at http://www.aanda.orgTables 1, 2, andfull Table 3 are also available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A15
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| The origin of palladium and silver
Context. In recent years, progress has been made in understanding therapid neutron-capture process (r-process) and current studies suggestthe need for a second, so-called weak r-process. Observational proof ofthis possible second branch of the r-process may come from a detailedknowledge of the formation and evolution of silver (Ag) and to someextent palladium (Pd) abundances in halo stars. Aims: We studythe silver and palladium abundances of a large sample of stars (only afew measurements have been made so far) to gain insight into theformation process of lighter neutron-capture elements. Methods:We analysed a sample of stars (including both dwarfs and giants), forwhich we determined in a consistent manner stellar parameters andabundances. The abundances were derived via spectrum synthesis, makinguse of the MOOG synthetic spectrum code (1D LTE) and MARCS stellar modelatmospheres (1D LTE). Results: We derived the Ag and Pdabundances of 56 stars, the largest sample to date for which both Pd andAg have been studied. The stars span a metallicity range of 2.5 dex,from the metal-poor giants at [Fe/H] ~ -3.2 to the more metal-richdwarfs at [Fe/H] ~ -0.6. Both elements display rather flat trends withmetallicity, with some dispersion being present at the lowestmetallicities. Conclusions: The similarity detected in theevolutionary trends of both Ag and Pd is indicative of a common origin.Qualitative comparisons with some theoretical calculations seem toconfirm the need for a second r-process, responsible for theirformation. Further abundance studies (exploring more light n-captureelements) and comparisons with a wider variety of theoretical models areneeded to fully characterise the details and site of the formationmechanism(s).Appendix is only available in electronic form at http://www.aanda.org
| Stellar atmosphere parameters with MA?, a MAssive compression of ?2 for spectral fitting
MA? is a new tool to estimate parameters from stellar spectra. It isbased on the maximum likelihood method, with the likelihood compressedin a way that the information stored in the spectral fluxes isconserved. The compressed data are given by the size of the number ofparameters, rather than by the number of flux points. The optimumspeed-up reached by the compression is the ratio of the data set to thenumber of parameters. The method has been tested on a sample oflow-resolution spectra from the Sloan Extension for GalacticUnderstanding and Exploration (SEGUE) survey for the estimate ofmetallicity, effective temperature and surface gravity, with accuraciesof 0.24 dex, 130 K and 0.5 dex, respectively. Our stellar parameters andthose recovered by the SEGUE Stellar Parameter Pipeline agree reasonablywell. A small sample of high-resolution VLT-UVES spectra is also used totest the method and the results were compared to a more classicalapproach. The speed and multi-resolution capability of MA? combinedwith its performance compared with other methods indicates that it willbe a useful tool for the analysis of upcoming spectral surveys.
| 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
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| StarCAT: A Catalog of Space Telescope Imaging Spectrograph Ultraviolet Echelle Spectra of Stars
StarCAT is a catalog of high resolution ultraviolet spectra of objectsclassified as "stars," recorded by Space Telescope Imaging Spectrograph(STIS) during its initial seven years of operations (1997-2004). StarCATis based on 3184 echelle observations of 545 distinct targets, with atotal exposure duration of 5.2 Ms. For many of the objects, broadultraviolet coverage has been achieved by splicing echellegrams taken intwo or more FUV (1150-1700 Å) and/or NUV (1600-3100 Å)settings. In cases of multiple pointings on conspicuously variablesources, spectra were separated into independent epochs. Otherwise,different epochs were combined to enhance the signal-to-noise ratio(S/N). A post-facto correction to the {\sf calstis} pipeline data setscompensated for subtle wavelength distortions identified in a previousstudy of the STIS calibration lamps. An internal "fluxing" procedureyielded coherent spectral energy distributions (SEDs) for objects withbroadly overlapping wavelength coverage. The best StarCAT materialachieves 300 m s-1 internal velocity precision;absolute accuracy at the 1 km s-1 level; photometricaccuracy of order 4%; and relative flux precision several times better(limited mainly by knowledge of SEDs of UV standard stars). WhileStarCAT represents a milestone in the large-scale post-processing ofSTIS echellegrams, a number of potential improvements in the underlying"final" pipeline are identified.
| 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
| Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
Aims: Precise abundance ratios are determined for 94 dwarf starswith Teff K, -1.6 < [Fe/H] < -0.4, and distances D? 335 pc. Most of them have halo kinematics, but 16 thick-disk starsare included. Methods: Equivalent widths of atomic lines aremeasured from VLT/UVES and NOT/FIES spectra with resolutions R? 55000 and R ? 40 000, respectively. An LTE abundance analysis basedon MARCS models is applied to derive precise differential abundanceratios of Na, Mg, Si, Ca, Ti, Cr, and Ni with respect to Fe. Results: The halo stars fall into two populations, clearly separated in[?/Fe], where ? refers to the average abundance of Mg, Si,Ca, and Ti. Differences in [Na/Fe] and [Ni/Fe] are also present with aremarkably clear correlation between these two abundance ratios. Conclusions: The “high-?” stars may be ancient disk orbulge stars “heated” to halo kinematics by merging satellitegalaxies or they could have formed as the first stars during thecollapse of a proto-Galactic gas cloud. The kinematics of the“low-?” stars suggest that they have been accretedfrom dwarf galaxies, and that some of them may originate from the? Cen progenitor galaxy.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility.Tables 3 and 4 are also available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/L10Figures5-8 and Tables 1-4 are only available in electronic form at http://www.aanda.org
| 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
| 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
| Lithium Isotopic Abundances in Metal-poor Halo Stars
Very high quality spectra of 24 metal-poor halo dwarfs and subgiantshave been acquired with ESO's VLT/UVES for the purpose of determining Liisotopic abundances. The derived one-dimensional, non-LTE 7Li abundances from the Li I 670.8 nm line reveal a pronounceddependence on metallicity but with negligible scatter around this trend.Very good agreement is found between the abundances from the Li I 670.8nm line and the Li I 610.4 nm line. The estimated primordial 7Li abundance is7Li/H=(1.1-1.5)×10-10, which is a factor of3-4 lower than predicted from standard big bang nucleosynthesis with thebaryon density inferred from the cosmic microwave background.Interestingly, 6Li is detected in 9 of our 24 stars at the>=2 σ significance level. Our observations suggest theexistence of a 6Li plateau at the level oflogε6Li~0.8 however, taking into accountpredictions for 6Li destruction during the pre-main-sequenceevolution tilts the plateau such that the 6Li abundancesapparently increase with metallicity. Our most noteworthy result is thedetection of 6Li in the very metal-poor star LP 815-43. Sucha high 6Li abundance during these early Galactic epochs isvery difficult to achieve by Galactic cosmic-ray spallation andα-fusion reactions. It is concluded that both Li isotopes have apre-Galactic origin. Possible 6Li production channels includeprotogalactic shocks and late-decaying or annihilating supersymmetricparticles during the era of big bang nucleosynthesis. The presence of6Li limits the possible degree of stellar 7Lidepletion and thus sharpens the discrepancy with standard big bangnucleosynthesis.Based on observations collected at the European Southern Observatory,Paranal, Chile (observing programs 65.L-0131, 68.D-0091, and273.D-5043).
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| 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
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| Сазвежђа: | Једра |
| Ректацензија: | 10h17m14.89s |
| Deклинација: | -52°29'18.7" |
| Apparent магнитуда: | 10.042 |
| Proper motion RA: | 410.7 |
| Proper motion Dec: | 5.8 |
| B-T magnitude: | 10.545 |
| V-T magnitude: | 10.084 |
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INHABITED SKY
