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Публикации по объекту
Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion
We present NLTE Li abundances for 88 stars in the metallicity range -3.5< [Fe/H] < -1.0. The effective temperatures are based on theinfrared flux method with improved E(B-V) values obtained mostly frominterstellar Na I D lines. The Li abundances were derived through MARCSmodels and high-quality UVES+VLT, HIRES+Keck and FIES+NOT spectra, andcomplemented with reliable equivalent widths from the literature. Theless-depleted stars with [Fe/H] < -2.5 and [Fe/H] > -2.5 fall intotwo well-defined plateaus of ALi = 2.18 (? = 0.04) andALi = 2.27 (? = 0.05), respectively. We show that thetwo plateaus are flat, unlike previous claims for a steep monotonicdecrease in Li abundances with decreasing metallicities. At allmetallicities we uncover a fine-structure in the Li abundances of Spiteplateau stars, which we trace to Li depletion that depends on bothmetallicity and mass. Models including atomic diffusion and turbulentmixing seem to reproduce the observed Li depletion assuming a primordialLi abundance ALi = 2.64, which agrees well with currentpredictions (ALi = 2.72) from standard Big Bangnucleosynthesis. Adopting the Kurucz overshooting model atmospheresincreases the Li abundance by +0.08 dex to ALi = 2.72, whichperfectly agrees with BBN+WMAP.Based in part on observations obtained at the W. M. Keck Observatory,the Nordic Optical Telescope on La Palma, and on data from theHIRES/Keck archive and the European Southern Observatory ESO/ST-ECFScience Archive Facility.Table 1 is only available in electronic form athttp://www.aanda.org
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| 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.
| 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
| First stars VII - Lithium in extremely metal poor dwarfs
Context: .The primordial lithium abundance is a key prediction of modelsof big bang nucleosynthesis, and its abundance in metal-poor dwarfs (theSpite plateau) is an important, independent observational constraint onsuch models. Aims: .This study aims to determine the level andconstancy of the Spite plateau as definitively as possible fromhomogeneous high-quality VLT-UVES spectra of 19 of the most metal-poordwarf stars known. Methods: .Our high-resolution (R 43 000),high S/N spectra are analysed with OSMARCS 1D LTE model atmospheres andturbospectrum synthetic spectra to determine effective temperatures,surface gravities, and metallicities, as well as Li abundances for ourstars. Results: .Eliminating a cool subgiant and a spectroscopicbinary, we find 8 stars to have -3.5 < [Fe/H] < -3.0 and 9 starswith -3.0 < [Fe/H] < -2.5. Our best value for the mean level ofthe plateau is A(Li) =2.10± 0.09. The scatter around the mean isentirely explained by our estimate of the observational error and doesnot allow for any intrinsic scatter in the Li abundances. In addition,we conclude that a systematic error of the order of 200 K in any of thecurrent temperature scales remains possible. The iron excitationequilibria in our stars support our adopted temperature scale, which isbased on a fit to wings of the Hα line, and disfavour hotterscales, which would lead to a higher Li abundance, but fail to achieveexcitation equilibrium for iron. Conclusions: .We confirm thepreviously noted discrepancy between the Li abundance measured inextremely metal-poor turnoff stars and the primordial Li abundancepredicted by standard Big-Bang nucleosynthesis models adopting thebaryonic density inferred from WMAP. We discuss recent work explainingthe discrepancy in terms of diffusion and find that uncertaintemperature scales remain a major question.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Large Programme "First Stars", ID 165.N-0276(A);P.I. R. Cayrel). Tables 4-8 and Appendix A are only available inelectronic form at http://www.aanda.org
| Broadband UBVRCIC Photometry of Horizontal-Branch and Metal-poor Candidates from the HK and Hamburg/ESO Surveys. I.
We report broadband UBV and/or BVRCIC CCDphotometry for a total of 1857 stars in the thick-disk and halopopulations of the Galaxy. The majority of our targets were selected ascandidate field horizontal-branch or other A-type stars (FHB/A, N=576),or candidate low-metallicity stars (N=1221), from the HK and Hamburg/ESOobjective-prism surveys. Similar data for a small number of additionalstars from other samples are also reported. These data are being usedfor several purposes. In the case of the FHB/A candidates they are usedto accurately separate the lower gravity FHB stars from various highergravity A-type stars, a subsample that includes the so-called blue metalpoor stars, halo and thick-disk blue stragglers, main-sequence A-typedwarfs, and Am and Ap stars. These data are also being used to derivephotometric distance estimates to high-velocity hydrogen clouds in theGalaxy and for improved measurements of the mass of the Galaxy.Photometric data for the metal-poor candidates are being used to refineestimates of stellar metallicity for objects with availablemedium-resolution spectroscopy, to obtain distance estimates forkinematic analyses, and to establish initial estimates of effectivetemperature for analysis of high-resolution spectroscopy of the starsfor which this information now exists.
| Bright Metal-poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields
We present a sample of 1777 bright (91.0) metal-poor([Fe/H]<-2.0) giants of 9%+/-2%, which is lower than previouslyreported. However, the frequency rises to similar (>20%) and highervalues with increasing distance from the Galactic plane. Although thenumbers of stars at low metallicity are falling rapidly at the lowestmetallicities, there is evidence that the fraction of carbon-enhancedmetal-poor stars is increasing rapidly as a function of decliningmetallicity. For ~60 objects, high-resolution data have already beenobtained; one of these, HE 1327-2326, is the new record holder for themost iron-deficient star known.
| 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.
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| 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.
| The Effective Temperature Scale of FGK Stars. I. Determination of Temperatures and Angular Diameters with the Infrared Flux Method
The infrared flux method (IRFM) has been applied to a sample of 135dwarf and 36 giant stars covering the following regions of theatmospheric parameter space: (1) the metal-rich ([Fe/H]>~0) end(consisting mostly of planet-hosting stars), (2) the cool(Teff<~5000 K) metal-poor (-1<~[Fe/H]<~-3) dwarfregion, and (3) the very metal-poor ([Fe/H]<~-2.5) end. These starswere especially selected to cover gaps in previous works onTeff versus color relations, particularly the IRFMTeff scale of A. Alonso and collaborators. Our IRFMimplementation was largely based on the Alonso et al. study (absoluteinfrared flux calibration, bolometric flux calibration, etc.) with theaim of extending the ranges of applicability of their Teffversus color calibrations. In addition, in order to improve the internalaccuracy of the IRFM Teff scale, we recomputed thetemperatures of almost all stars from the Alonso et al. work usingupdated input data. The updated temperatures do not significantly differfrom the original ones, with few exceptions, leaving the Teffscale of Alonso et al. mostly unchanged. Including the stars withupdated temperatures, a large sample of 580 dwarf and 470 giant stars(in the field and in clusters), which cover the ranges3600K<~Teff<~8000K and -4.0<~[Fe/H]<~+0.5, haveTeff homogeneously determined with the IRFM. The meanuncertainty of the temperatures derived is 75 K for dwarfs and 60 K forgiants, which is about 1.3% at solar temperature and 4500 K,respectively. It is shown that the IRFM temperatures are reliable in anabsolute scale given the consistency of the angular diameters resultingfrom the IRFM with those measured by long baseline interferometry, lunaroccultation, and transit observations. Using the measured angulardiameters and bolometric fluxes, a comparison is made between IRFM anddirect temperatures, which shows excellent agreement, with the meandifference being less than 10 K for giants and about 20 K for dwarfstars (the IRFM temperatures being larger in both cases). This resultwas obtained for giants in the ranges 3800K
| 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
| Reappraising the Spite Lithium Plateau: Extremely Thin and Marginally Consistent with WMAP Data
The lithium abundance in 62 halo dwarfs is determined from accurateequivalent widths reported in the literature and an improved infraredflux method temperature scale. The Li abundance of 41 plateau stars(those with Teff>6000 K) is found to be independent oftemperature and metallicity, with a star-to-star scatter of only 0.06dex over a broad range of temperatures (6000K
| 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 New Procedure for the Photometric Parallax Estimation
We present a new procedure for photometric parallax estimation. The datafor 1236 stars provide calibrations between the absolute magnitudeoffset from the Hyades main-sequence and the ultraviolet-excess foreight different (B-V)0 colour-index intervals, (0.3 0.4),(0.4 0.5), (0.5 0.6), (0.6 0.7), (0.7 0.8), (0.8 0.9), (0.9 1.0) and(1.0 1.1). The mean difference between the original and estimatedabsolute magnitudes and the corresponding standard deviation are rathersmall, +0.0002 and +/-0.0613 mag. The procedure has been adapted to theSloan photometry by means of colour equations and applied to a set ofartificial stars with different metallicities. The comparison of theabsolute magnitudes estimated by the new procedure and the canonical oneindicates that a single colour-magnitude diagram does not supplyreliable absolute magnitudes for stars with large range of metallicity.
| 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.
| Catalogue of [Fe/H] determinations for FGK stars: 2001 edition
The catalogue presented here is a compilation of published atmosphericparameters (Teff, log g, [Fe/H]) obtained from highresolution, high signal-to-noise spectroscopic observations. This newedition has changed compared to the five previous versions. It is nowrestricted to intermediate and low mass stars (F, G and K stars). Itcontains 6354 determinations of (Teff, log g, [Fe/H]) for3356 stars, including 909 stars in 79 stellar systems. The literature iscomplete between January 1980 and December 2000 and includes 378references. The catalogue is made up of two tables, one for field starsand one for stars in galactic associations, open and globular clustersand external galaxies. The catalogue is distributed through the CDSdatabase. Access to the catalogue with cross-identification to othersets of data is also possible with VizieR (Ochsenbein et al.\cite{och00}). The catalogue (Tables 1 and 2) is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/373/159 and VizieRhttp://vizier.u-strasbg.fr/.
| High proper motion stars with declinations between -30o and -40o, and right ascensions between 00 h and 10 h 40 m
Proper motions, positions, finding charts and magnitudes are given for147 newly discovered stars with proper motions larger than 0.15arcsec/year. They were found in a search for high proper motion starscarried out in thirteen 5o x 5o areas, locatedbetween -30o and -40o in declination, and 00 h and10 h 40 m in right ascension. Their blue photographic magnitudes rangefrom approximately 9.0 to 18.5. Nine objects from the above sample haveproper motions larger than 0.4 (0.404 to 0.550) arcsec/year. The sametype of data is also presented for 149 Luyten Catalogue (LTT, Luyten\cite{Luyten57}) stars re-discovered during the above search. Anestimated precision level between 6 and 22 mas/year was achieved for theproper motions. Tables 2 and 3 are only available in electronic form atthe CDS, via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/725 Figures 4 and 5are only available in electronic form at http://www.edpsciences.org
| 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.
| Stellar Iron Abundances: Non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II inthe atmosphere of late-type stars. We used atomic models for Fe I and FeII having, respectively, 256 and 190 levels, as well as 2117 and 3443radiative transitions. Photoionization cross sections are from the IronProject. These atomic models were used to investigate non-LTE (NLTE)effects in iron abundances of late-type stars with different atmosphericparameters. We found that most Fe I lines in metal-poor stars are formedin conditions far from LTE. We derived metallicity corrections of about0.3 dex with respect to LTE values for the case of stars with[Fe/H]~-3.0. Fe II is found not to be affected by significant NLTEeffects. The main NLTE effect invoked in the case of Fe I isoverionization by ultraviolet radiation; thus classical ionizationequilibrium is far from being satisfied. An important consequence isthat surface gravities derived by LTE analysis are in error and shouldbe corrected before final abundance corrections. This apparently solvesthe observed discrepancy between spectroscopic surface gravities derivedby LTE analyses and those derived from Hipparcos parallaxes. A table ofNLTE [Fe/H] and log g values for a sample of metal-poor late-type starsis given.
| 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.
| Detection of the Li i lambda 6104 Transition in the Population II Star HD 140283
Lithium is one of the few primordially produced elements. The value ofthe primordial Li is taken to be that observed in metal-poor dwarfs,where it is not contaminated by stellar Li sources that act on longertimescales. The atmospheric abundance is currently derived from the Li Ilambda 6707 resonance transition, and the validity of the modelsemployed has been questioned (Kurucz). In this Letter, we report thefirst detection of the Li I lambda 6104 2 2P - 32D subordinate transition in the prototype Population II starHD 140283. The same Li abundance of ({Li}/{H})=1.4x10^{-10} is found tobe consistent with both the resonance and subordinate lines. The twolines form at different depths in the atmosphere, implying that theone-dimensional, homogeneous atmospheric models used in the abundancedetermination are essentially correct. When coupled with the standardbig bang yields, the Li in the halo dwarfs provides two solutions forthe baryon-to-photon ratio eta10=nb/n_?x1010 and for thepresent baryon density Omegabh270=0.0748 eta 10 : afirst solution at eta 10~1.8 , which is consistent with theeta 10 implied by the high deuterium values {D}/{H}~2x10^{-4}observed in some quasar absorption systems (Webb et al.); and a secondsolution at eta 10~4 , which is consistent, within theerrors, with the low deuterium {D}/{H}=3.4x10^{-5} measured in otherquasar absorption systems (Burles & Tytler).
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Spread of the lithium abundance in halo stars.
The observed scatter of the lithium abundance around a "plateau" isrevisited for three samples of stars for which the temperature has beendetermined from either the excitation temperature, or the dereddenedcolor (b-y)_o_ or the profile of the Halpha_wings. Systematicdifferences are noted between the three methods of temperaturedetermination. From sample to sample the rms observed scatter of thelithium abundance varies from 0.06dex to 0.08dex (to be compared to thevalue 0.13 previously found by Thorburn (1994), for another sample ofstars). We show that in all cases but one, this scatter is fullyexplained by the temperature and equivalent width errors. The intrinsicscatter, if real, is small.
| 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
| The first generations of stars
Up to a decade ago, searches for population III stars (i.e. withstrictly the chemical composition left by the Big Bang) had led to theresults that (1) no such star had been found, (2) stars withmetallicities significantly below [Fe/H] = -2.5 were exceedingly rare.Thanks to a major survey, undertaken by Beers, Preston and Shectman 18years ago, covering about 7500 square degrees in the sky, and down tomagnitude B =16.0, the situation has drastically changed. Theobservational limit towards the lowest metallicities is now about [Fe/H]= -4, i.e. 4 dex below the solar metallicity Zsolar = 0.02,(a level of pollution by supernova ejecta of only a few ppm), and over100 stars are known with metallicities [Fe/H] in the range -4 to -3. Thestudy of this sample, and of a few stars found more serendipitously, hasallowed a number of new conclusions: (i) The cosmological element7Li stays constant (prolongation of the Spite's plateau) downto the lowest metallicities, a great observational gift to the hot BigBang cosmology (ii) All heavier elements show a roughly linear increasewith the abundance of O (or even Fe if the metallicity is below [Fe/H] =-1), including the other light elements, Be and B. This last point hasled to a reappraisal of the current view that they were produced byspallation of interstellar nuclei by galactic cosmic rays, because therise of those elements with metallicity should then have been morequadratic than linear. An alternative new perspective is that theseelements are produced by spallation of the primary nuclei ejected by SNeii against protons of the interstellar medium. (iii) The ratio of thealpha elements (O, Si, Mg,...) to iron also stays constant down to thelowest metallicities, at about 3 times the solar value. (iv) Significantdeviations to a lockstep variation of the various elements within theiron-peak start to appear below [Fe/H] = -2.5. The strongest are adecrease of [Cr/Fe] and an increase of [Co/Fe] when [Fe/H] decreasesfrom -2.5 to -4.0. These trends are not explained by the current statusof explosive nucleosynthesis. (v) A great scatter of the abundances ofthe neutron capture elements relative to iron appears at very lowmetallicities. Similar scatter is seen for [Al/Fe]. A remarkable starwith [Fe/H] = -3.1, CS 22892-052, has been found, with a superb spectrumof the r-elements, involving over-abundances of those with respect toiron by factors ranging between 10 and 50. (vi) The kinematics of thevery metal-poor stars is similar to that of other halo stars, with acomplete lack of systemic rotation in an inertial frame, if not a smallamount of counter-rotation in the Galaxy. Evidence exists that thevelocity ellipsoid is radially elongated for stars within 10 kpc fromthe galactic center, whereas it is more spherical or even radiallycontracted at 20 kpc from the galactic center. (vii) The low metallicitystars were likely formed at an early cosmological epoch (z > 5 ifH0~ 65 km/s), before the Galaxy had developed a disk. The newviews concerning the sizes of the Ly? clouds open the possibilitythat the low-metallicity Ly? systems are large halos having theright metallicity for being protogalaxies, just forming early stellargenerations. (viii) One may wonder why, if more than 100 stars are knownwith metallicities between [Fe/H] = -4 to -3 no pop. III has been found,or even not one star near [Fe/H] = -5. Different kinds of explanationshave been proposed, with none conclusive at present. Either we havealready observed a pop. III star, but its pristine Big Bang compositionhas been corrupted by a small amount of interstellar matter accretedduring its 10 Gyr of orbiting in an already-enriched gas, or thecollective process of star formation has polluted the medium before ithas produced the low-mass stars we can still observe now, or, simpler,pop. III stars exist, but are sufficiently rare that we have not yetobserved a volume large enough to have found one.
| Lithium abundances in the most metal-deficient stars
Five halo dwarfs in the temperature range of the Population II Liplateau and all having (Fe/H) is less than -3.0 were found to havemildly depleted Li abundances, down on average by 0.15 dex, compared tohigher metallicity halo dwarfs of comparable temperature. Evolutionarymodels tracing Li abundances in halo stars in the presence of several Lidepletion mechanisms do not predict this difference. Four comparisonstars with -2.8 is less than (Fe/H) is less than -0.6 were found to bein excellent agreement with previous studies. Our sample also includesthe very metal poor dwarf, CS 22876-32 ((Fe/H) = -4.2), for which wereport a lithium abundance in the range log (n(Li)/n(H)) + 12.00 = 1.8 -2.0, based on our observation of a 15 mA Li line, contrary to anondetection reported by Molaro, but in agreement with recentmeasurements reported by Thorburn & Beers. We also trace thedependence of Li abundances on stellar effective temperatures, amountingto 0.03 dex per 100 K, in agreement with the predictions of depletionmodels. Observed Li abundances are normalized to an effectivetemperature near the hot end of the Spite plateau, 6200 K, and must beinterpreted with this renormalization in mind. Two general possibilitiesare considered for the lower Li abundances seen in stars with (Fe/H) isless than -3.0: (1) that a small amount of Li production has occurredduring halo formation, in which case the primordial Li abundance is nearlog (n(Li)/n(H)) + 12.00 = 2.05 or lower, or (2) that the most metalpoor halo stars have depleted their surface Li abundances in a fashionnot predicted by current models, and the primordial Li value liesinstead near 2.20. The difference in the implied primordial value altersthe constraints on various cosmological models, most notably the baryondensity resulting from the big bang. Until the difference between higherand lower metallicity halo stars is understood, it cannot be stated withcertainty whether observing more metal poor stars leads one closertoward or further away from the primordial Li abundance.
| 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.
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Наблюдательные данные и астрометрия
| Созвездие: | Печь |
| Прямое восхождение: | 03h19m35.31s |
| Склонение: | -32°50'43.2" |
| Видимая звёздная величина: | 10.837 |
| Собственное движение RA: | 27.7 |
| Собственное движение Dec: | -204.3 |
| B-T magnitude: | 11.474 |
| V-T magnitude: | 10.89 |
Каталоги и обозначения:
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