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The Most Metal-poor Stars. I. Discovery, Data, and Atmospheric Parameters
We report the discovery of 34 stars in the Hamburg/ESO Survey formetal-poor stars and the Sloan Digital Sky Survey that have [Fe/H] <~-3.0. Their median and minimum abundances are [Fe/H] = -3.1and -4.1, respectively, while 10 stars have [Fe/H] <-3.5. High-resolution, high signal-to-noise spectroscopicdata—equivalent widths and radial velocities—are presentedfor these stars, together with an additional four objects previouslyreported or currently being investigated elsewhere. We have determinedthe atmospheric parameters, effective temperature (T eff),and surface gravity (log g), which are critical in the determination ofthe chemical abundances and the evolutionary status of these stars.Three techniques were used to derive these parameters.Spectrophotometric fits to model atmosphere fluxes were used to derive Teff, log g, and an estimate of E(B - V); Hα,Hβ, and Hγ profile fitting to model atmosphere resultsprovided the second determination of T eff and log g; andfinally, we used an empirical T eff-calibrated Hδindex, for the third, independent T eff determination. Thethree values of T eff are in good agreement, although theprofile fitting may yield systematically cooler T eff values,by ~100 K. This collective data set will be analyzed in future papers inthe present series to utilize the most metal-poor stars as probes ofconditions in the early universe.This paper includes data obtained with the ANU 2.3 m Telescope atSiding Spring Observatory, Australia; the Magellan Clay Telescope at LasCampanas Observatory, Chile; the Keck I Telescope at the W. M. KeckObservatory, Hawaii, USA; and the VLT (Kueyen) of the European SouthernObservatory, Paranal, Chile (proposal 281.D-5015).
| Chemical abundances of distant extremely metal-poor unevolved stars
Context. The old Galactic halo stars hold the fossil record of theinterstellar medium chemical composition at the time of their formation.Most of the stars studied so far are relatively near to the Sun, thisprompts the study of more distant stars, both to increase the size ofthe sample and to search for possible variations of abundance patternsat greater distances. Aims: The purpose of our study is todetermine the chemical composition of a sample of 16 candidate extremelymetal-poor (EMP) dwarf stars, extracted from the Sloan Digital SkySurvey (SDSS). There are two main purposes: in the first place to verifythe reliability of the metallicity estimates derived from the SDSSspectra; in the second place to see if the abundance trends found forthe brighter nearer stars studied previously also hold for this sampleof fainter, more distant stars. Methods: We used the UVES at theVLT to obtain high-resolution spectra of the programme stars. Theabundances were determined by an automatic analysis with the MyGIsFOScode, with the exception of lithium, for which the abundances weredetermined from the measured equivalent widths of the Li i resonancedoublet. Results: All candidates are confirmed to be EMP stars,with [Fe/H] ≤ -3.0. The chemical composition of the sample of starsis similar to that of brighter and nearer samples. We measured thelithium abundance for 12 stars and provide stringent upper limits forthree other stars, for a fourth star the upper limit is not significant,owing to the low signal-to noise ratio of the spectrum. The "meltdown"of the Spite plateau is confirmed, but some of the lowest metallicitystars of the sample lie on the plateau. Conclusions: Theconcordance of the metallicities derived from high-resolution spectraand those estimated from the SDSS spectra suggests that the latter maybe used to study the metallicity distribution of the halo. The abundancepattern suggests that the halo was well mixed for all probedmetallicities and distances. The fact that at the lowest metallicitieswe find stars on the Spite plateau suggests that the meltdown depends onat least another parameter, besides metallicity.Based on spectra obtained with UVES at the 8.2 m Kueyen ESO telescope,programmes 078.D-0217 and 081.D.0373.Table 1 is available in electronicform at http://www.aanda.org
| An extremely primitive star in the Galactic halo
The early Universe had a chemical composition consisting of hydrogen,helium and traces of lithium; almost all other elements weresubsequently created in stars and supernovae. The mass fraction ofelements more massive than helium, Z, is known as `metallicity'. Anumber of very metal-poor stars has been found, some of which have a lowiron abundance but are rich in carbon, nitrogen and oxygen. Fortheoretical reasons and because of an observed absence of stars withZ<1.5×10-5, it has been suggested that low-massstars cannot form from the primitive interstellar medium until it hasbeen enriched above a critical value of Z, estimated to lie in the range1.5×10-8 to 1.5×10-6 (ref. 8),although competing theories claiming the contrary do exist. (We use`low-mass' here to mean a stellar mass of less than 0.8 solar masses,the stars that survive to the present day.) Here we report the chemicalcomposition of a star in the Galactic halo with a very low Z(<=6.9×10-7, which is 4.5×10-5times that of the Sun) and a chemical pattern typical of classicalextremely metal-poor stars--that is, without enrichment of carbon,nitrogen and oxygen. This shows that low-mass stars can be formed atvery low metallicity, that is, below the critical value of Z. Lithium isnot detected, suggesting a low-metallicity extension of the previouslyobserved trend in lithium depletion. Such lithium depletion implies thatthe stellar material must have experienced temperatures above twomillion kelvin in its history, given that this is necessary to destroylithium.
| The Stellar Abundances for Galactic Archaeology (SAGA) data base - II. Implications for mixing and nucleosynthesis in extremely metal-poor stars and chemical enrichment of the Galaxy
We discuss the characteristics of known extremely metal-poor (EMP) starsin the Galaxy using the Stellar Abundances for Galactic Archaeology(SAGA) data base. We find the transition of the initial mass function tobe at [Fe/H]˜-2 from the viewpoint of the distribution of carbonabundance and the frequency of carbon-enhanced stars. Analyses ofcarbon-enhanced stars in our sample suggest that nucleosynthesis inasymptotic giant branch (AGB) stars can contribute to carbon enrichmentin a different way depending on whether the metallicity is above orbelow [Fe/H]˜-2.5, which is consistent with the current models ofstellar evolution at low metallicity. For observed EMP stars, we confirmthat some, though not all, observed stars might have undergone at leasttwo types of extra mixing to change their surface abundances. One is thedepletion of lithium abundance during the early phase of the red giantbranch; the other is a decrease of the C/N ratio by one order ofmagnitude during the red giant branch phase. Observed small scatters ofabundances for ?-elements and iron-group elements suggest that thechemical enrichment of our Galaxy takes place in a well-mixedinterstellar medium. The abundance trends of ?-elements are highlycorrelated with each other including ?-enhanced and depletedstars, while the abundances of iron-group elements are subject todifferent slopes relative to the iron abundance. This implies that thesupernova yields of ?-elements are almost independent of mass andmetallicity, while those of iron-group elements have a metallicitydependence or mass dependence on the variable initial mass function. Theoccurrence of the hot-bottom burning for M? 5 M&sun; isconsistent with an initial mass function of the Galaxy peaked at˜10-12 M&sun;, compatible with the statistics ofcarbon-enhanced stars with and without s-process element enhancement andnitrogen-enhanced stars. For s-process elements, we find not only apositive correlation between carbon and s-process element abundances,but also an increasing slope of the abundance ratio between them withincreasing mass number of s-process elements. The dominant site of thes-process is still an open question because none of the known mechanismsfor the s-process is able to account for this observed correlation. Inspite of the evidence of AGB evolution in observed abundances of EMPstars, any evidence of binary mass transfer is elusive by pursuing theeffect of dilution in the convective envelope. We find the dependence ofsulphur and vanadium abundances on the effective temperatures, inaddition to the previously reported trends for silicon, scandium,titanium, chromium and cobalt.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| Multiplicity and Period Distribution of Population II Field Stars in Solar Vicinity
We examine a sample of 223 F, G, and early K metal-poor subdwarfs ([m/H]< -1) with high proper motions (?>0farcs2yr-1) at distances of up to 250 pc from the Sun. Bymeans of our own speckle interferometric observations conducted on the 6m Bolshoi Azimuthal Telescope of the Special Astrophysical Observatoryof the Russian Academy of Sciences, and the spectroscopic and visualdata taken from the literature, we determine the frequency of binary andmultiple systems in this sample. The ratio of single, binary, triple,and quadruple systems among 221 primary components of the sample is147:64:9:1. We show that the distribution of orbital periods of binaryand multiple subdwarfs is asymmetric in the range of up to P =1010 days, and has a maximum at P =102-103 days, which differs from the distribution,obtained for the thin disk G dwarfs. We estimated the number ofundetected companions in our sample. Comparing the frequency of binarysubdwarfs in the field and in the globular clusters, we show that theprocess of halo field star formation by the means of destruction ofglobular clusters is very unlikely in our Galaxy. We discuss themultiplicity of old metal-poor stars in nearby stellar streams.
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| 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.
| Searching for the metal-weak thick disc in the solar neighbourhood
An abundance analysis is presented of 60 metal-poor stars drawn fromcatalogues of nearby stars provided by Arifyanto et al. and Schuster etal. In an attempt to isolate a sample of metal-weak thick disc stars, weapplied the kinematic criteria Vrot >=100kms-1,|ULSR| <= 140kms-1 (LSR -local standard of rest) and |WLSR| <= 100kms-1.14 stars satisfying these criteria and having [Fe/H] <= -1.0 areincluded in the sample of 60 stars. Eight of the 14 have [Fe/H] >=-1.3 and may be simply thick disc stars of slightly lower than average[Fe/H]. The other six have [Fe/H] from -1.3 to -2.3 and are eithermetal-weak thick disc stars or halo stars with kinematics mimickingthose of the thick disc. The sample of 60 stars is completed by eightthick disc stars, 20 stars of a hybrid nature (halo or thick disc stars)and 18 stars with kinematics distinctive of the halo.
| 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.
| Beryllium in Ultra-Lithium-Deficient Halo Stars: The Blue Straggler Connection
There exists a small group of metal-deficient stars that have Liabundances well below the Li plateau that is defined by over 100unevolved stars with temperatures above 5800 K and values of[Fe/H]<-1.0. Abundances of Be have been determined for most of theseultra-Li-deficient stars in order to investigate the cause of the Lideficiencies. These Li-deficient stars have implications on the value ofprimordial Li. High-resolution and high signal-to-noise ratio spectrahave been obtained in the Be II spectral region near 3130 Å forsix ultra-Li-deficient stars with the Keck I telescope and its newUV-sensitive CCD on the upgraded HIRES. The spectrum synthesis techniquehas been used to determine Be abundances. All six stars are found tohave Be deficiencies also. Two have measurable but reduced Be, and fourhave only upper limits on Be. These results are consistent with the ideathat these Li- and Be-deficient stars are analogous to blue stragglers.The stars have undergone mass transfer events (or mergers) that destroyor dilute both Li and Be. The findings cannot be matched by the modelsthat predict that the deficiencies are due to extramixing in a subset ofhalo stars that were initially rapid rotators, with the possibleexception of one star, G139-8. Because the ultra-Li-deficient stars arealso Be-deficient, they appear to be genuine outliers in the populationof halo stars used to determine the Li plateau in that they no longerhave the Li in their atmospheres that was produced in the big bang.
| 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
| The non-LTE line formation of neutral carbon in late-type stars
Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE)line formation of neutral carbon in late-type stars in order to removesome of the potential systematic errors in stellar abundance analysesemploying C i features. Methods: .The statistical equilibrium codeMULTI was used on a grid of plane-parallel 1D MARCS atmosphericmodels. Results: .Within the parameter space explored, thehigh-excitation C i lines studied are stronger in non-LTE due to thecombined effect of line source function drop and increased line opacitydue to overpopulation of the lower level for the transitions considered;the relative importance of the two effects depends on the particularcombination of T{eff}, log g, [Fe/H] and [C/Fe] and on theanalysed C i line. As a consequence, the non-LTE abundance correctionsare negative and can be substantially so, for example -0.4 dex inhalo turn-off stars at [Fe/H] -3. The magnitude of the non-LTEcorrections is rather insensitive to whether inelastic H collisions areincluded or not. Conclusions: .Our results have implications onstudies of nucleosynthetic processes and on Galactic chemical evolutionmodels. When applying our calculated corrections to recent observationaldata, the upturn in [C/O] at low metallicity might still be present(thus apparently still necessitating contributions from massive Pop. IIIstars for the carbon production), but at a lower level and possibly witha rather shallow trend of -0.2 dex/dex below [O/H] -1.
| Permitted Oxygen Abundances and the Temperature Scale of Metal-poor Turnoff Stars
We use high-quality VLT/UVES published data of the permitted O I tripletand Fe II lines to determine oxygen and iron abundances in unevolved(dwarfs, turnoff, subgiants) metal-poor halo stars. The calculationshave been performed both in LTE and non-LTE (NLTE), employing effectivetemperatures obtained with the new infrared flux method (IRFM)temperature scale by Ramírez & Meléndez, and surfacegravities from Hipparcos parallaxes and theoretical isochrones. A newlist of accurate transition probabilities for Fe II lines, tied to theabsolute scale defined by laboratory measurements, has been used.Interstellar absorption has been carefully taken into account byemploying reddening maps, stellar energy distributions andStrömgren photometry. We find a plateau in the oxygen-to-iron ratioover more than 2 orders of magnitude in iron abundance(-3.2<[Fe/H]<-0.7), with a mean [O/Fe]=0.5 dex (σ=0.1 dex),independent of metallicity, temperature, and surface gravity. The flat[O/Fe] ratio is mainly due to the use of adequate NLTE corrections andthe new IRFM temperature scale, which, for metal-poor F/early G dwarfsis hotter than most Teff scales used in previous studies ofthe O I triplet. According to the new IRFM Teff scale, thetemperatures of turnoff halo stars strongly depend on metallicity, aresult that is in excellent qualitative and quantitative agreement withstellar evolution calculations, which predict that the Teffof the turnoff at [Fe/H]=-3 is about 600-700 K higher than that at[Fe/H]=-1. Recent determinations of Hα temperatures in turnoffstars are in excellent relative agreement with the new IRFMTeff scale in the metallicity range -2.7<[Fe/H]<-1,with a zero-point difference of only 61 K.
| Beryllium in Disk and Halo Stars: Evidence for a Beryllium Dispersion in Old Stars
The study of Be in stars of differing metal content can elucidate theformation mechanisms and the Galactic chemical evolution of the lightelement, Be. We have obtained high-resolution, high signal-to-noiseratio (S/N) spectra of the resonance lines of Be II in eight stars withthe High Dispersion Spectrograph (HDS) on the 8.2 m Subaru Telescope onMauna Kea. Abundances of Be have been determined through spectrumsynthesis. The stars with [Fe/H] values greater than -1.1 conform to thepublished general trend of Be versus Fe. We have confirmed the high Beabundance in HD 94028 and have found a similarly high Be abundance inanother star, HD 132475, at the same metallicity: [Fe/H]=-1.5. These twostars are 0.5-0.6 dex higher in Be than the Be-Fe trend. While thatgeneral trend contains the evidence for a Galaxy-wide enrichment in Beand Fe, the higher than predicted Be abundances in those two stars showsthat there are also local Be enrichments. Possible enrichment mechanismsinclude hypernovae and multiple supernova explosions contained in asuperbubble. One of our stars, G64-37, has a very low metallicity of[Fe/H]=-3.2 we have determined its Be abundance to look for evidence ofa Be plateau. Its Be abundance appears to extend the Be-Fe trend tolower Fe abundances without any evidence for a plateau, as had beenindicated by a high Be abundance in another very metal-poor star,G64-12. Although these two stars have similar Be abundances within theerrors, it could be that their different Be values indicate a Bedispersion even at the lowest metallicities.Based on observations obtained with the Subaru Telescope.
| Beryllium in the Ultra-Lithium-deficient, Metal-Poor Halo Dwarf G186-26
The vast majority of low-metal halo dwarfs show a similar amount of Li;this has been attributed to the Li that was produced in the big bang.However, there are nine known halo stars with T>5900 K and[Fe/H]<-1.0 that are ultra-Li-deficient. We have looked for Be in thevery low metallicity star G186-26, at [Fe/H]=-2.71, which is one of theultra-Li-deficient stars. This star is also ultra-Be-deficient. Relativeto Be in the Li-normal stars at [Fe/H]=-2.7, G186-26 is down in Be bymore than 0.8 dex. Of two potential causes for the Li deficiency-masstransfer in a pre-blue straggler or extra rotationally induced mixing ina star that was initially a very rapid rotator-the absence of Be favorsthe blue straggler hypothesis, but the rotation model cannot be ruledout completely.Based on data obtained with the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| 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.
| 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
| Sulphur and zinc abundances in Galactic stars and damped Lyα systems
High resolution spectra of 34 halo population dwarf and subgiant starshave been obtained with VLT/UVES and used to derive sulphur abundancesfrom the λ λ 8694.0, 8694.6 and λ λ 9212.9,9237.5 S I lines. In addition, iron abundances have been determined from19 Fe II lines and zinc abundances from the λ λ 4722.2,4810.5 lines. The abundances are based on a classical 1D, LTE modelatmosphere analysis, but effects of 3D hydrodynamical modelling on the[S/Fe], [Zn/Fe] and [S/Zn] ratios are shown to be small. We find thatmost halo stars with metallicities in the range -3.2 < [Fe/H] <-0.8 have a near-constant [S/Fe] ≃ +0.3; a least square fit to[S/Fe] vs. [Fe/H] shows a slope of only -0.04 ± 0.01. Among halostars with -1.2 < [Fe/H] < -0.8 the majority have [S/Fe] ≃+0.3, but two stars (previously shown to have low α/Fe ratios)have [S/Fe] ≃ 0.0. For disk stars with [Fe/H] > -1, [S/Fe]decreases with increasing [Fe/H] . Hence, sulphur behaves like othertypical α-capture elements, Mg, Si and Ca. Zinc, on the otherhand, traces iron over three orders of magnitude in [Fe/H], althoughthere is some evidence for a small systematic Zn overabundance ([Zn/Fe]≃ +0.1) among metal-poor disk stars and for halo stars with [Fe/H]< -2.0. Recent measurements of S and Zn in ten damped Lyαsystems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances inthe range -2.1 < [Zn/H] < -0.15 show an offset relative to the[S/Zn] - [Zn/H] relation in Galactic stars. Possible reasons for thisoffset are discussed, including low and intermittent star formationrates in DLAs.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).Table A1 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/415/993
| 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.
| The evolution of the C/O ratio in metal-poor halo stars
We report new measurements of carbon and oxygen abundances in 34 F and Gdwarf and subgiant stars belonging to the halo population and spanning arange of metallicity from [Fe/H] = -0.7 to -3.2 . The survey is based onobservations of four permitted lines of C I near 9100 Å and the OI,λ 7774 triplet, all recorded at high signal-to-noise ratioswith the UVES echelle spectrograph on the ESO VLT. The line equivalentwidths were analysed with the 1D, LTE, MARCS model atmosphere code todeduce C and O abundances; corrections due to non-LTE and 3D effects arediscussed. When combined with similar published data for disk stars, ourresults confirm the metallicity dependence of the C/O ratio known fromprevious stellar and interstellar studies: C/O drops by a factor of3-4 as O/H decreases from solar to 1/10 solar. Analysed withinthe context of standard models for the chemical evolution of the solarvicinity, this drop results from the metallicity dependence of the Cyields from massive stars with mass loss, augmented by the delayedrelease of C from stars of low and intermediate mass. The former is,however, always the dominant factor. Our survey has also uncoveredtentative evidence to suggest that, as the oxygen abundance decreasesbelow [O/H] = -1, [C/O] may not remain constant at [C/O] = -0.5, aspreviously thought, but increase again, possibly approaching near-solarvalues at the lowest metallicities ([O/H] ≲ -3). With the currentdataset this is no more than a 3σ effect and it may be due tometallicity-dependent non-LTE corrections to the [C/O] ratio which havenot been taken into account. However, its potential importance as awindow on the nucleosynthesis by Population III stars is a strongincentive for future work, both observational and theoretical, to verifyits reality.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).
| 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.
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | שועלון |
| התרוממות ימנית: | 20h24m45.42s |
| סירוב: | +25°03'07.1" |
| גודל גלוי: | 10.862 |
| תנועה נכונה: | 94.4 |
| תנועה נכונה: | -245.1 |
| B-T magnitude: | 11.221 |
| V-T magnitude: | 10.892 |
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