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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.
| 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
| Lithium abundances of halo dwarfs based on excitation temperatures. II. Non-local thermodynamic equilibrium
Context. The plateau in the abundance of 7Li in metal-poorstars was initially interpreted as an observational indicator of theprimordial lithium abundance. However, this observational value is indisagreement with that deduced from calculations of Big Bangnucleosynthesis (BBN), when using the Wilkinson microwave anisotropyprobe (WMAP) baryon density measurements. One of the most importantfactors in determining the stellar lithium abundance is the effectivetemperature. In a previous study by the authors, new effectivetemperatures (Teff) for sixteen metal-poor halo dwarfs werederived using a local thermodynamic equilibrium (LTE) description of theformation of Fe lines. This new Teff scale reinforced thediscrepancy. Aims: For six of the stars from our previous studywe calculate revised temperatures using a non-local thermodynamicequilibrium (NLTE) approach. These are then used to derive a new meanprimordial lithium abundance in an attempt to solve the lithiumdiscrepancy. Methods: Using the code MULTI we calculate NLTEcorrections to the LTE abundances for the Fe i lines measured in the sixstars, and determine new Teff's. We keep other physicalparameters, i.e. log g, [Fe/H] and ?, constant at the valuescalculated in Paper I. With the revised Teff scale we derivenew Li abundances. We compare the NLTE values of Teff withthe photometric temperatures of Ryan et al. (1999, ApJ, 523, 654), theinfrared flux method (IRFM) temperatures of Meléndez &Ramírez (2004, ApJ, 615, L33), and the Balmer line wingtemperatures of Asplund et al. (2006, ApJ, 644, 229). Results: Wefind that our temperatures are hotter than both the Ryan et al. andAsplund et al. temperatures by typically ~110-160 K, but are stillcooler than the temperatures of Meléndez & Ramírez bytypically ~190 K. The temperatures imply a primordial Li abundance of2.19 dex or 2.21 dex, depending on the magnitude of collisions withhydrogen in the calculations, still well below the value of 2.72 dexinferred from WMAP + BBN. We discuss the effects of collisions on trendsof7Li abundances with [Fe/H] and Teff, as well asthe NLTE effects on the determination of log g through ionizationequilibrium, which imply a collisional scaling factor SH >1 for collisions between Fe and H atoms.
| Beryllium, Oxygen, and Iron Abundances in Extremely Metal-Deficient Stars
The abundance of beryllium in the oldest, most metal-poor stars acts asa probe of early star formation and Galactic chemical evolution. We haveanalyzed high-resolution, high signal-to-noise ratio Keck/HIRES spectraof 24 stars with [Fe/H] from -2.3 to -3.5 in order todetermine the history of Be abundance and explore the possibility of aBe plateau. We have determined stellar parameters of our samplespectroscopically, using equivalent widths of Fe I, Ti I, and Ti IIlines. We have determined O abundances from three OH features whichoccur in the same spectral region; this region is relatively uncrowdedand has a well determined continuum in these very/extremely metal-poorstars. We have supplemented this sample with reanalyzed spectra of 25stars from previous observations so that our total sample ranges in[Fe/H] from -0.5 to -3.5. Our results indicate that therelationship between Be and [Fe/H] continues to lower metallicities witha slope of 0.92 ± 0.04. Although there is no indication of aplateau with constant Be abundance, the four lowest metallicity stars(below [Fe/H] of -3.0) do show a Be enhancement relative to Fe atthe 1σ level. A single relationship between Be and [O/H] has aslope of 1.21 ± 0.08, but there is also a good fit with twoslopes: 1.59 above [O/H] = -1.6 and 0.74 for stars with [O/H]below -1.6. This change in slope could result from a change in thedominant production mechanism for Be. In the era of the formation of themore metal-poor stars, Be would be formed by acceleration of CNO atomsin the vicinity of SN II and in later times by high-energy cosmic-raysbombarding CNO in the ambient interstellar gas. We find an excellentcorrelation between [Fe/H] and [O/H] and show that [O/Fe] is near +1.0at [Fe/H] = -3.5 declining to 0 at [Fe/H] = 0.
| The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars
Aims: We present new measurements of the abundances of carbon and oxygenderived from high-excitation C i and O i absorption lines in metal-poorhalo stars, with the aim of clarifying the main sources of these twoelements in the early stages of the chemical enrichment of the Galaxy.Methods: We target 15 new stars compared to our previous study,with an emphasis on additional C/O determinations in the crucialmetallicity range -3 ⪉ [Fe/H]⪉ -2. The stellar effectivetemperatures were estimated from the profile of the Hβ line.Departures from local thermodynamic equilibrium were accounted for inthe line formation for both carbon and oxygen. The non-LTE effects arevery strong at the lowest metallicities but, contrary to what hassometimes been assumed in the past due to a simplified assessment, ofdifferent degrees for the two elements. In addition, for the 28 starswith [Fe/H] < -1 previously analysed, stellar parameters werere-derived and non-LTE corrections applied in the same fashion as forthe rest of our sample, giving consistent abundances for 43 halo starsin total. Results: The new observations and non-LTE calculationsstrengthen previous suggestions of an upturn in C/O towards lowermetallicity (particularly for [O/H] ⪉ -2). The C/O values derivedfor these very metal-poor stars are, however, sensitive to excitationvia the still poorly quantified inelastic H collisions. While these donot significantly affect the non-LTE results for C i, they greatlymodify the O i outcome. Adopting the H collisional cross-sectionsestimated from the classical Drawin formula leads to [C/O] ≈ 0 at[O/H] ≈ -3. To remove the upturn in C/O, near-LTE formation for O ilines would be required, which could only happen if the H collisionalefficiency with the Drawin recipe is underestimated by factors of up toseveral tens of times, a possibility which we consider unlikely. Conclusions: The high C/O values derived at the lowest metallicitiesmay be revealing the fingerprints of Population III stars or may signalrotationally-aided nucleosynthesis in more normal Population II stars.Based on data collected with the European Southern Observatory's VeryLarge Telescope (VLT) at the Paranal, Chile (programmes No. 67.D-0106and 73.D-0024) and with the Magellan Telescope at Las CampanasObservatory, Chile.
| Lithium abundances of halo dwarfs based on excitation temperature. I. Local thermodynamic equilibrium
Context: The discovery of the Spite plateau in the abundances of7Li for metal-poor stars led to the determination of anobservationally deduced primordial lithium abundance. However, after thesuccess of the Wilkinson Microwave Anisotropy Probe (WMAP) indetermining the baryon density, Ω_Bh2, there was adiscrepancy between observationally determined and theoreticallydetermined abundances in the case of 7Li. One of the mostimportant uncertain factors in the calculation of the stellar7Li abundance is the effective temperature, T_eff. Aims: We use sixteen metal-poor halo dwarfs to calculate new T_effvalues using the excitation energy method. With this temperature scalewe then calculate new Li abundances for this group of stars in anattempt to resolve the 7Li discrepancy. Methods: Usinghigh signal-to-noise (S/N ≈ 100) spectra of 16 metal-poor halodwarfs, obtained with the UCLES spectrograph on the AAT, measurements ofequivalent widths from a set of unblended Fe I lines are made. Theseequivalent widths are then used to calculate new T_eff values with theuse of the single line radiative transfer program WIDTH6, where we haveconstrained the gravity using either theoretical isochrones or theHipparcos parallax, rather than the ionization balance. The lithiumabundances of the stars are calculated with these temperatures. Results: The physical parameters are derived for the 16 programmestars, and two standards. These include T_eff, log g, [Fe/H],microturbulence and 7Li abundances. A comparison between thetemperature scale of this work and those adopted by others has beenundertaken. We find good consistency with the temperatures derived fromthe Hα line by Asplund et al. (2006, ApJ, 644, 229), but not withthe hotter scale of Meléndez & Ramírez (2004, ApJ,615, L33). We also present results of the investigation into whether anytrends between 7Li and metallicity or temperature are presentin these metal-poor stars.Appendix A is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/493/601
| Speckle interferometry of metal-poor stars in the solar neighborhood. II
The results of speckle interferometric observations of 115 metal-poorstars ([m/H] < ‑1) within 250 pc from the Sun and with propermotions µ ≳ 0.2″/yr, made with the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences,are reported. Close companions with separations ranging from0.034″ to 1″ were observed for 12 objects—G76-21,G59-1, G63-46, G135-16, G168-42, G141-47, G142-44, G190-10, G28-43,G217-8, G130-7, and G89-14—eight of them are astrometricallyresolved for the first time. The newly resolved systems include onetriple star—G190-10. If combined with spectroscopic and visualdata, our results imply a single:binary:triple:quadruple star ratio of147:64:9:1 for a sample of 221 primary components of halo and thick-diskstars.
| Sulphur and zinc abundances in Galactic halo stars revisited
Aims.Based on a new set of sulphur abundances in very metal-poor starsand an improved analysis of previous data, we aim at resolving currentdiscrepancies on the trend of S/Fe vs. Fe/H and thereby gain betterinsight into the nucleosynthesis of sulphur. The trends of Zn/Fe andS/Zn will also be studied. Methods: High resolution VLT/UVES spectra of40 main-sequence stars with -3.3 < [Fe/H] < -1.0 are used toderive S abundances from the weak λ 8694.6 S I line and thestronger λ λ 9212.9,9237.5 pair of S I lines. For onestar, the S abundance is also derived from the S I triplet at 1.046μm recently observed with the VLT infrared echelle spectrographCRIRES. Fe and Zn abundances are derived from lines in the blue part ofthe UVES spectra, and effective temperatures are obtained from theprofile of the Hβ line. Results: Comparison of sulphur abundancesfrom the weak and strong S I lines provides important constraints onnon-LTE effects. The high sulphur abundances reported by others for somemetal-poor stars are not confirmed; instead, when taking non-LTEcorrections into account, the Galactic halo stars distribute around aplateau at [S/Fe] ~ +0.2 dex with a scatter of 0.07 dex only. [Zn/Fe] isclose to zero for metallicities in the range -2.0 < [Fe/H] < -1.0but increases to a level of [Zn/Fe] ~ +0.1 to +0.2 dex in the range -2.7< [Fe/H] < -2.0. At still lower metallicities [Zn/Fe] risessteeply to a value of [Zn/Fe] ~ +0.5 dex at [Fe/H] = -3.2. Conclusions:The trend of S/Fe vs. Fe/H corresponds to the trends of Mg/Fe, Si/Fe,and Ca/Fe and indicates that sulphur in Galactic halo stars has beenmade by α-capture processes in massive SNe. The observed scatterin S/Fe is much smaller than predicted from current stochastic models ofthe chemical evolution of the early Galaxy, suggesting that either themodels or the calculated yields of massive SNe should be revised. Wealso examine the behaviour of S/Zn and find that departures from thesolar ratio are significantly reduced at all metallicities if non-LTEcorrections to the abundances of these two elements are adopted. Thiseffect, if confirmed, would reduce the usefulness of the S/Zn ratio as adiagnostic of past star-formation activity, but would bring closertogether the values measured in damped Lyman-alpha systems and inGalactic stars.Based on observations collected at the European Southern Observatory atParanal, Chile (programmes No. 67.D-0106, 73.D-0024 and CRIRES scienceverification program 60.A-9072). Table 1 and Appendices are onlyavailable in electronic form at http://www.aanda.org
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| uvby-β photometry of high-velocity and metal-poor stars. XI. Ages of halo and old disk stars
New uvby-β data are provided for 442 high-velocity and metal-poorstars; 90 of these stars have been observed previously by us, and 352are new. When combined with our previous two photometric catalogues, thedata base is now made up of 1533 high-velocity and metal-poor stars, allwith uvby-β photometry and complete kinematic data, such as propermotions and radial velocities taken from the literature. Hipparcos, plusa new photometric calibration for Mv also based on theHipparcos parallaxes, provide distances for nearly all of these stars;our previous photometric calibrations give values for E(b-y) and [Fe/H].The [Fe/H], V(rot) diagram allows us to separate these stars intodifferent Galactic stellar population groups, such as old-thin-disk,thick-disk, and halo. The X histogram, where X is our stellar-populationdiscriminator combining V(rot) and [Fe/H], and contour plots for the[Fe/H], V(rot) diagram both indicate two probable components to thethick disk. These population groups and Galactic components are studiedin the (b-y)0, Mv diagram, compared to theisochrones of Bergbusch & VandenBerg (2001, ApJ, 556, 322), toderive stellar ages. The two thick-disk groups have the meancharacteristics: ([Fe/H], V(rot), Age, σW') ≈ (-0.7dex, 120 km s-1, 12.5 Gyr, 62.0 km s-1), and≈(-0.4, 160, 10.0, 45.8). The seven most metal-poor halo groups,-2.31 ≤ [Fe/H] ≤ -1.31, show a mean age of 13.0 ± 0.2(mean error) Gyr, giving a mean difference from the WMAP results for theage of the Universe of 0.7 ± 0.3 Gyr. These results for the agesand components of the thick disk and for the age of the Galactic halofield stars are discussed in terms of various models and ideas for theformation of galaxies and their stellar populations.
| 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.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| Mg abundances in metal-poor halo stars as a tracer of early Galactic mixing
We present results of a detailed chemical analysis performed on 23main-sequence turnoff stars having -3.4 ≤ [Fe/H] ≤ -2.2, asample selected to be highly homogeneous in Teff and log(g).We investigate the efficiency of mixing in the early Galaxy by means ofthe [Mg/Fe] ratio, and find that all values lie within a total range of0.2 dex, with a standard deviation about the mean of 0.06 dex,consistent with measurement errors. This implies there is little or nointrinsic scatter in the early ISM, as suggested also by the most recentresults from high-quality VLT observations. These results are incontrast with inhomogeneous Galactic chemical evolution (iGCE) modelsadopting present supernova (SN) II yields, which predict a peak-to-peakscatter in [Mg/Fe] as high as 1 dex at very low metallicity, with acorresponding standard deviation of about 0.4 dex. We propose thatcooling and mixing timescales should be investigated in iGCE models toaccount for the apparent disagreement with present observations. Thecontrast between the constancy and small dispersion of [Mg/Fe] reportedhere and the quite different behaviour of [Ba/Fe] indicates, accordingto this interpretation, that Mg and Ba are predominantly synthesised indifferent progenitor mass ranges.Table \ref{gftable} is only available in electronic form athttp://www.edpsciences.org
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete 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/419/167
| 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.
| A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries
We report 25,563 radial velocity measurements for 1359 single-linedstars in the Carney-Latham sample of 1464 stars selected for high propermotion. For 171 of these, we present spectroscopic orbital solutions. Wefind no obvious difference between the binary characteristics in thehalo and the disk populations. The observed frequency is the same, andthe period distributions are consistent with the hypothesis that the twosets of binaries were drawn from the same parent population. Thissuggests that metallicity in general, and radiative opacities inparticular, have little influence over the fragmentation process thatleads to short-period binaries. All the binaries with periods shorterthan 10 days have nearly circular orbits, while the binaries withperiods longer than 20 days exhibit a wide range of eccentricities and amedian value of 0.37. For the metal-poor high-velocity halo binaries inour sample, the transition from circular to eccentric orbits appears tooccur at about 20 days, supporting the conclusion that tidalcircularization on the main sequence is important for the oldestbinaries in the Galaxy. Some of the results presented here usedobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
| The Spite Lithium Plateau: Ultrathin but Postprimordial
We have studied 23 very metal-poor field turnoff stars, specificallychosen to enable a precise measurement of the dispersion in the lithiumabundance of the Spite Li plateau. We concentrated on stars having anarrow range of effective temperature and very low metallicities([Fe/H]<~-2.5) to reduce the effects of systematic errors and havemade particular efforts to minimize random errors. A typical statisticalerror for our abundances is 0.033 dex (1 sigma), which represents afactor of 2 improvement on most previous studies. Our sample does notexhibit a trend with effective temperature, although the temperaturerange is limited. However, for -3.6<[Fe/H]<-2.3 we do recover adependence on metallicity at dA(Li)/d[Fe/H]=0.118+/-0.023 (1 sigma) dexper dex, almost the same level as discussed previously. Earlier claimsfor a lack of dependence of A(Li) on abundance are shown to have arisenprobably from noisier estimates of effective temperatures andmetallicities, which have erased the real trend. The dependence isconcordant with theoretical predictions of Galactic chemical evolution(GCE) of Li (even in such metal-poor stars) and with the published levelof ^6Li in two of the stars of our sample, which we use to infer the GCE^7Li contribution. One of the 23 stars, G186-26, was known already to bestrongly Li-depleted. Of the remaining 22 objects, 21 have abundancesconsistent with an observed spread about the metallicity trend of a mere0.031 dex (1 sigma). Because the formal errors are 0.033 dex, weconclude that the intrinsic spread is effectively zero at the verymetal-poor halo turnoff. This is established at much higher precisionthan previous studies (~0.06-0.08 dex). The essentially zero intrinsicspread leads to the conclusion that either these stars have all changedtheir surface Li abundances very uniformly, or else they exhibit closeto the primordial abundance sought for its cosmological significance. Wecannot rule out a uniform depletion mechanism, but economy of hypothesissupports the latter interpretation. The lack of spread in the A(Li)abundances limits permissible depletion by rotationally induced mixingmodels to less than 0.1 dex. Correcting for the GCE contribution to both^6Li and ^7Li, we infer a primordial abundance A(Li)_p~=2.00 dex, withthree systematic uncertainties of up to 0.1 dex each depending onuncertainties in the effective temperature scale, stellar atmospheremodels, and correction for GCE. (This value rests on aneffective-temperature zero-point set by Magain's and Bell & Oke'sb-y calibrations of metal-poor stars and the model atmospheres withoutconvective overshoot.) We predict that observations of Li in extremelylow-metallicity stars, having [Fe/H]<-3, will yield smaller A(Li)values than the bulk of stars in this sample, consistent with a lowprimordial abundance. The difference between our field star observationsand published M92 data suggests real field-to-cluster differences. Thismay indicate different angular momentum evolutionary histories, withinteractions between protostellar disks in the dense globular clusterenvironments possibly being responsible. Further study of Li in globularclusters and in very metal-poor field samples is required to clarify thesituation.
| Estimation of Stellar Metal Abundance. II. A Recalibration of the Ca II K Technique, and the Autocorrelation Function Method
We have recalibrated a method for the estimation of stellar metalabundance, parameterized as [Fe/H], based on medium-resolution (1-2Å) optical spectra (the majority of which cover the wavelengthrange 3700-4500 Å). The equivalent width of the Ca II K line (3933Å) as a function of [Fe/H] and broadband B-V color, as predictedfrom spectrum synthesis and model atmosphere calculations, is comparedwith observations of 551 stars with high-resolution abundances availablefrom the literature (a sevenfold increase in the number of calibrationstars that were previously available). A second method, based on theFourier autocorrelation function technique first described by Ratnatunga& Freeman, is used to provide an independent estimate of [Fe/H], ascalibrated by comparison with 405 standard-star abundances.Metallicities based on a combination of the two techniques for dwarfsand giants in the color range 0.30<=(B-V)_0<=1.2 exhibit anexternal 1 sigma scatter of approximately 0.10-0.20 dex over theabundance range -4.0<=[Fe/H]<=0.5. Particular attention has beengiven to the determination of abundance estimates at the metal-rich endof the calibration, where our previous attempt suffered from aconsiderable zero-point offset. Radial velocities, accurate toapproximately 10 km s^-1, are reported for all 551 calibration stars.
| Lithium Processing in Halo Dwarfs, and T eff, [Fe/H] Correlations on the Spite Plateau
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...458..543R&db_key=AST
| A survey of proper motion stars. 12: an expanded sample
We report new photometry and radial velocities for almost 500 stars fromthe Lowell Proper Motion Catalog. We combine these results with ourprior sample and rederive stellar temperatures based on the photometry,reddening, metallicities (using chi squared matching of our 22,500 lowSignal to Noise (S/N) high resolution echelle spectra with a grid ofsynthetic spectra), distances, space motions, and Galactic orbitalparameters for 1269 (kinematics) and 1261 (metallicity) of the 1464stars in the complete survey. The frequency of spectroscopic binariesfor the metal-poor ((m/H) less than or equal to -1.2) stars with periodsshorter than 3000 days is at least 15%. The spectroscopic binaryfrequency for metal-rich stars ((m/H) greater than -0.5) appears to belower, about 9%, but this may be a selection effect. We also discussspecial classes of stars, including treatment of the double-linedspectroscopic binaries, and identification of subgiants. Four possiblenew members of the class of field blue stragglers are noted. We pointout the detection of three possible new white dwarfs, six broad-lined(binary) systems, and discuss briefly the three already knownnitrogen-rich halo dwarfs. The primary result of this paper will beavailable on CD-ROM, in the form of a much larger table.
| The primordial lithium abundance from extreme subdwarfs: New observations
High-resolution (R approximately equals 28,000), high signal-to-noise(S/N approximately equals 100) spectra of the Li I lambda-6707 regionhave been obtained for 90 halo dwarfs and main-sequence turnoff starswith (Fe/H) approximately less than or equal to -2.2. The mean lithiumabundance at 6300 K is found to be N(Li) = 12 + log (Li/H) = 2.32 +/-0.20 (95% confidence interval), where the quoted uncertainty reflectsthe error in the absolute abundance zero point from all known sources,random and systematic. Contrary to the findings of Spite and Spite(1982), these data show a larger lithium abundance dispersion than canbe explained by observational errors alone. The standard deviation ofdata points about the mean trend, excluding all upper limits, is 0.13dex, while the typical relative abundance error due to uncertainties inthe temperatures and equivalent widths is 0.08-0.09 dex. A formaldispersion analysis in the temperature-equivalent width plane rejectsthe null hypothesis (i.e., no intrinsic dispersion) at a greater than 6sigma confidence level (100% - 10-8%). In order for theobserved scatter to be consistent with noise, the relative equivalentwidth and temperature errors must both be increased by approximately 55%from their typical values of 3 mA and 100 K (1 sigma), respectively. Atrend of declining N(Li) with decreasing stellar metallicity isidentified as evidence of lithium production by Galactic sources. Allexcess scatter about the N(Li)-(Fe/H) relation is attributed to thecombination of lithium production and a approximately 2 Gyr dispersion(1 sigma) in the halo metallicity-to-age relation. This additionalsource of lithium abundance variations from star to star also accountsfor the observed intrinsic dispersion about the Spite plateau. Thedetection of Li-6 in HD 84937 (Smith, Lambert, and Nissen 1993c)suggests that Galactic cosmic-ray alpha + alpha reactions are thedominant source of lithium production in the early interstellar medium.The rate of Li-6 production inferred from the N(Li)-(Fe/H) trend canaccount for the current abundance of Li-6 observed in the interstellarmedium toward zeta Oph, zeta Per (Meyer, Hawkins, & Wright 1993),and rho Oph (Lemoine et al. 1993). The primordial lithium fraction isestimated from the surface lithium abundances of the hottest, mostmetal-poor stars in this program: 2.22 +/- 0.20 dex.
| High-tangential-velocity stars
Two lists of high tangential velocity (greater than 100 km/s) starscontained in the Lowell Northern Hemisphere proper motion surveycatalogue have been compiled, the stellar distances being inferred fromeither trigonometric or spectroscopic parallaxes. The information givenincludes equatorial coordinates, corrected photographic magnitudes,proper motions, trigonometric or spectroscopic parallaxes, and spectraltypes.
| Instellar reddening in a high-latitude field around Omega Bootis
UBV observations have revealed an interstellar cloud with a visual totalextinction of 0.2 mag and a diameter of 1.5 pc at 60.5 deg galacticlatitude near Omega Bootis. The distance to the cloud is estimated atabout 25 pc. On the basis of the reddening values and by taking thebackground component into account, the intrinsic polarization of thecloud is calculated to be 0.0082 mag. The cloud's mass in neutralhydrogen is estimated at roughly 7 solar masses.
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