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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.
| 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.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| 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
| uvby-β photometry of high-velocity and metal-poor stars. X. Stars of very low metal abundance: Observations, reddenings, metallicities, classifications, distances, and relative ages
uvby(-β) photometry has been obtained for an additional 411 verymetal-poor stars selected from the HK survey, and used to derive basicparameters such as interstellar reddenings, metallicities, photometricclassifications, distances, and relative ages. Interstellar reddeningsadopted from the Schlegel et al. (\cite{schlegel}) maps agree well withthose from the intrinsic-color calibration of Schuster & Nissen(\cite{schuster89}). [Fe/H] values are obtained from the CaII K lineindex of the HK survey combined with the uvby and UBV photometry. Thec0,(b-y)0 diagram is seen to be very useful forclassifying these very metal-poor field stars into categories similar tothose derived from globular cluster color-magnitude diagrams; the HKsurvey has detected metal-poor candidates extending from the red-giantto the blue-horizontal branch, and from the horizontal branch tosubluminous stars. Distances derived from UBV photometry agreereasonably well with those from uvby, considering the paucity of goodcalibrating stars and the extrapolations required for the mostmetal-poor stars. These very metal-poor stars are compared to M 92 inthe c0,(b-y)0 diagram, and evidence is seen forfield stars 1-3 Gyrs younger than this globular cluster; uncertaintiesin the [Fe/H] scale for M 92 would only tend to increase this agedifference. Significant reddening uncertainties for M 92 are unlikelybut might decrease this difference. The significance of these youngervery metal-poor stars is discussed in the context of Galactic evolution,mentioning such possibilities as hierarchical star-formation/mass-infallof very metal-poor material and/or accretion events whereby thismaterial has been acquired from other (dwarf) galaxies with differentformation and chemical-enrichment histories.Based on observations collected at the H. L. Johnson 1.5 m telescope atthe Observatorio Astronómico Nacional at San Pedro Mártir,Baja California, México, and at the Danish 1.5 m telescope, LaSilla, Chile.Tables 1-9 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/422/527
| 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
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Rotation in Globular Cluster stars. Turn-off and subgiant stars in NGC 104, NGC 6397 and NGC 6752
We present a derivation of upper limits to the rotation of Main Sequencestars in three globular clusters using spectra obtained during timeallocated to the ESO Large Programs 165-L0263 and 167.D-0173, with UVESat VLT2 (Kueyen). The stars analyzed in this work do not show anyevidence of high values of rotational velocities as far as the outerlayers are concerned, in particular robust estimates for the upperlimits of the values of the mean projected rotational velocities areplaced, about /line{vrot sin i} = 3.5+/- 0.2 kms-1 and about /line{vrot sin i}=4.7+/-0.2 kms-1 for, respectively, the program turn-off and subgiantstars. On the basis of statistical considerations, assuming thatinclination of the rotational axis i is randomly oriented, and that allstars within the same group rotate at the same rate, we obtain estimatesfor the values of the true rotational velocities. These values are upperlimits to the true rotational velocities if there is some star-to-starscatter in rotational or macro-turbulent velocities. The mean values ofthese upper limits for vrot for the stars of the samespectral type averaged over the three clusters are found to berespectively, /line{v}rot<=3.5+/-0.4 km s-1 and/line{v}rot<=2.6+/-1.1 km s-1. Thus, theexplanation for the large rotational velocities found for the HorizontalBranch stars must be looked for either in the rotation of the core notdetectable in the outer layers (Sills & Pinsennault \cite{sill}) orin the acquisition of angular momentum during their evolution.Based on observations collected at the European Southern Observatory,Chile.
| 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.
| Stellar Mixing and the Primordial Lithium Abundance
We compare the properties of recent samples of the lithium abundances inhalo stars to one another and to the predictions of theoretical modelsincluding rotational mixing, and we examine the data for trends withmetal abundance. We apply two statistical tests to the data: aKolomorgorov-Smirnov (K-S) test sensitive to the behavior around thesample median, and Monte Carlo tests of the probability to draw theobserved number of outliers from the theoretical distributions. We findfrom a K-S test that in the absence of any correction for chemicalevolution, the Ryan, Norris, & Beers (RNB) sample is fullyconsistent with mild rotational mixing induced depletion and, therefore,with an initial lithium abundance higher than the observed value. Testsfor outliers depend sensitively on the threshold for defining theirpresence, but we find a 10%-45% probability that the RNB sample is drawnfrom the rotationally mixed models with a 0.2 dex median depletion withlower probabilities corresponding to higher depletion factors. Includingor excluding the one upper limit in the sample changes the absoluteprobabilities but does not affect the overall conclusions. When chemicalevolution trends (Li/H vs. Fe/H) are included in our analysis we findthat the dispersion in the RNB sample is not explained by chemicalevolution; the inferred bounds on lithium depletion from rotationalmixing are similar to those derived from models without chemicalevolution. Finally, we explore the differences between the RNB sampleand other halo star data sets. We find that differences in theequivalent width measurements are primarily responsible for differentobservational conclusions concerning the lithium dispersion in halostars. The different data sets are all consistent with mild stellardepletion, but the systematic errors arising from differentobservational data sets are a major component of the error budget andneed to be addressed. The implications for cosmology are discussed. Wefind that the standard big bang nucleosynthesis predicted lithiumabundance that corresponds to the deuterium abundance inferred fromobservations of high-redshift, low-metallicity QSO absorbers requireshalo star lithium depletion in an amount consistent with that from ourmodels of rotational mixing but inconsistent with no depletion.
| Oxygen in the Very Early Galaxy
Oxygen abundances in a sample of ultra-metal-poor subdwarfs have beenderived from measurements of the oxygen triplet at 7771-5 Å and OHlines in the near-UV performed in high-resolution and highsignal-to-noise ratio spectra obtained with WHT/UES, Keck I/HIRES, andVLT/UVES. Our Fe abundances were derived in LTE and then corrected fornon-LTE (NLTE) effects. The new oxygen abundances confirm previousfindings for a progressive linear rise in the oxygen-to-iron ratio witha slope -0.33+/-0.02 from solar metallicity to [Fe/H]~-3. A slightlyhigher slope would be obtained if the Fe NLTE corrections were notconsidered. Below [Fe/H]=-2.5 our stars show [O/Fe] ratios as high as~1.17 (G64-12), which can be interpreted as evidence for oxygenoverproduction in the very early epoch of the formation of the halo,possibly associated with supernova events with very massive progenitorstars. We show that the arguments against this linear trend given byFulbright & Kraft in 1999, based on the LTE Fe analysis of twometal-poor stars, cannot be sustained when an NLTE analysis isperformed. We discuss how the Fulbright & Kraft LTE ionizationbalance of Fe lines underestimates the gravity of the very metal-poorstar BD +23°3130 ([Fe/H]=-2.43) and how this leads to anunderestimation of the oxygen abundance derived from the forbidden line.Gravities from Hipparcos appear to be in good agreement with thosedetermined in NLTE, giving higher values than previously assumed, whichreduces the discrepancies between the oxygen abundances determined fromOH, triplet, and forbidden lines. Using one-dimensional models, ouranalysis of three oxygen indicators available for BD +23°3130 givesan average [O/Fe] ratio of 0.78+0.15-0.18. Thehigh oxygen abundances at very low metallicities do not pose a problemto theoretical modeling since there is a range of parameters in thecalculations of nucleosynthesis yields from massive stars at lowmetallicities that can accommodate our results. Based on data collectedat the Keck I, VLT, and William Herschel telescopes.
| 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.
| Stars in the Galactic Halo
Not Available
| A Search for Stars of Very Low Metal Abundance. III. UBV Photometry of Metal-weak Candidates
We present photoelectric UBV photometry for some 2600 metal-poor dwarfand giant candidates selected from the HKobjective-prism/interference-filter survey of Beers and colleagues.These data provide the basis for our ultraviolet-excess selection ofobjects having [Fe/H]<~-2.0 for a spectroscopic investigation inwhich we have obtained ~1-2 Å resolution spectra of some 1000objects. The present material may also prove useful for investigationsof the metal-poor dwarfs and giants more metal-rich than [Fe/H]=-2.0, aswell as the misclassified subdwarf O and B stars, white dwarfs, bluemetal-poor stars, and field horizontal-branch stars which appear in thecatalog.
| The evolution of ^6Li in standard cosmic-ray nucleosynthesis
We review the Galactic chemical evolution of ^6Li and compare theseresults with recent observational determinations of the lithium isotopicratio. In particular, we concentrate on so-called standard Galacticcosmic-ray nucleosynthesis in which Li, Be, and B are produced(predominantly) by the inelastic scattering of accelerated protons andalpha off of CNO nuclei in the ambient interstellar medium. If O/Fe isconstant at low metallicities, then the ^6Li versus Fe/H evolution - aswell as Be and B versus Fe/H - has difficulty in matching theobservations. However, recent determinations of Population II oxygenabundances, as measured via OH lines, indicate that O/Fe increases atlower metallicity; if this trend is confirmed, then the ^6Li evolutionin a standard model of cosmic-ray nucleosynthesis is consistent with thedata. We also show that another key indicator of ^6LiBeB origin is the^6Li/Be ratio which also fits the available data if O/Fe is not constantat low metallicity. Finally we note that ^6Li evolution in this scenariocan strongly constrain the degree to which ^6Li and ^7Li are depleted inhalo stars.
| 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.
| The primordial lithium abundance
Lithium abundances in a selected sample of halo stars have been revisedby using the new accurate infrared flux method (IRFM) effectivetemperatures by Alonso, Arribas & Martinez-Roger. From 41 plateaustars (T_eff>5700 and [Fe/H]<=-1.5) we found no evidence forintrinsic dispersion, a tiny trend with T_eff and no trend with [Fe/H].The trend with the T_eff is fully consistent with the standard Liisochrones of Deliyannis, Demarque & Kawaler, implying a primordialvalue for Li of A(Li)=2.238+/-0.012_1sigma+/-0.05_sys. The presentresults argue against any kind of depletion predicted by diffusion,rotational mixing or stellar winds. Therefore the Li observed inPopulation II stars provides a direct and reliable estimate of thebaryonic density which can rival other baryonic indicators such as thedeuterium in high-redshift systems. The present upwards revision ofprimordial Li in the framework of standard big bang nucleosynthesis(SBBN) gives, at 1sigma, two solutions for the baryonic density:Omega_Bh^2=0.0062^+0.0018_-0.0011 or Omega_Bh^2=0.0146^+0.0029_-0.0033.
| The empirical scale of temperatures of the low main sequence (F0V-K5V).
We have calibrated the effective temperatures of the low main sequencestars ranging spectral types from F0 to K5 versus [Fe/H] and colours(B-V), (R-I), (V-R), (V-I), (V-K), (J-H), (J-K) and ubvy-β, using alarge sample of dwarfs and subdwarfs. The effective temperatures, scaledto direct T_eff_ determinations via reliable angular diametermeasurements, were derived applying the InfraRed Flux Method with thenew grid of atmosphere models developed by Kurucz (1993). We have fittedpolynomial functions of the form θ_eff_=P(colour,[Fe/H]) usingthe least squares method. The precision of the fits ranges from 30K for(V-K) to 154K for (J-H). The new relations have been compared toprevious calibrations. We also provide the empirical intrinsic colours(U-B), (B-V), (R-I), (V-R), (V-I), (V-K), (J-H), (J-K) and β, inthe ranges: 4000K[Fe/H]>-2.5.
| Determination of effective temperatures for an extended sample of dwarfs and subdwarfs (F0-K5).
We have applied the InfraRed Flux Method (IRFM) to a sample of 475dwarfs and subdwarfs in order to derive their effective temperatureswith a mean accuracy of about 1.5%. We have used the new homogeneousgrid of theoretical model atmosphere flux distributions developed byKurucz (1991, 1993) for the application of the IRFM. The atmosphericparameters of the stars cover, roughly, the ranges:3500K<=T_eff_<=8000K -3.5<=[Fe/H]<=+0.53.5<=log(g)<=5. The monocromatic infrared fluxes at the continuum,and the bolometric fluxes are derived using recent results, whichsatisfy the accuracy requeriments of the work. Photometric calibrationshave been revised and applied to estimate metallicities, although directspectroscopic determinations were preferred when available. The adoptedinfrared absolute flux calibration, based on direct optical measurementsof angular stellar diameters, sets the effective temperatures determinedusing the IRFM on the same scale than those obtained by direct methods.We derive three temperatures, T_J_, T_H_ and T_K_, for each star usingthe monochromatic fluxes at different infrared wavelengths in thephotometric bands J, H, and K. They show good consistency over 4000 K,and no trend with wavelength may be appreciated. We provide a detaileddescription of the steps followed for the application of the IRFM, aswell as the sources of the errors associated to the different inputs ofthe method, and their transmission into the final temperatures. We alsoprovide comparison with previous works.
| 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
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
| Broad band JHK infrared photometry of an extended sample of late type dwarfs and subdwarfs.
The results of a long term programme of broad band JHK photometry, for asample of 360 late type stars, made at the Observatorio del Teide(Tenerife, Spain) are presented. Transformations between thesemagnitudes and those of several currently used systems (CIT (Elias etal. 1982 and Carney 1983), Johnson (Johnson 1966, and Lee 1970), and ESO(Bouchet et al. 1991)) are proposed. A comparison to the narrow-bandsystem of Selby et al. (1988) has been made, in order to check theaccuracy of the photometric system. A mean internal accuracy better than0.02mag in the three bands can be inferred from the comparison to thelarge number of stars in common with Carney (1983), and from thedispersion of the multiple measured stars. The list of standards, thefilter passbands and effective wavelengths, together with correlationsbetween the extinction coefficients, ultimately characterize thephotometric IR system of the Observatorio del Teide (TCS). Data ofcomparable quality previously published have been added in order tocomplete the sample. This way the final sample consists of 550 stars.From the analysis of optical and IR colour:colour diagrams, we maydeduce that the range F0-K0 is properly sampled for0.1>[Fe/H]>-3.0. In the range K0-M4, no reliable photometricestimates of metallicity can be assigned, and only a small number ofstars have spectroscopic determination of the metallicity. Nevertheless,after kinematical considerations, the stars in this spectral range arealso expected to sample the galactic populations of dwarfs. Themetallicity effects on the IR and optical colour:colour diagrams arebriefly discussed.
| On the calibration of the Strömgren photometric system for F and G stars.
Not Available
| 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.
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