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Ýlgili Makaleler
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.
| 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.
| 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.
| Chemical abundances of very metal-poor stars
High-resolution and high signal-to-noise ratio spectra of 32 verymetal-poor stars were obtained with the Coudé echellespectrograph mounted on the 2.16-m telescope at the NationalAstronomical Observatories (Xinglong, China). Equivalent widths of FeI,FeII, OI, NaI, MgI, AlI, SiI, SiII, KI, CaI, ScII, TiI, VI, CrI, MnI,NiI, CuI and BaII lines were measured. Stellar effective temperatureswere determined by colour indices. Stellar surface gravities werecalculated from Hipparcos parallaxes and stellar evolutionary tracks.Photospheric abundances of 16 elements were derived by localthermodynamical equilibrium analysis. Stellar space motions (U, V, W)and Galactic orbital parameters were calculated. Based on kinematics,sample stars were separated into dissipative collapse and accretioncomponents of halo population. The global kinematics of the twocomponents were analysed. Element abundances were discussed as functionsof metallicities. The results of oxygen and α-elements abundanceconfirmed the previous works. The [K/Fe] shows a gradual systematicincrease toward a lower metallicity, such as in the case ofα-elements. The [Ba/Fe] trend suggests that the s-processdominated Ba production at least for the metal-poor stars with[Fe/H]> -2.0.
| 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.
| 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
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| 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.
| Heavy elements and chemical enrichment in globular clusters
High resolution (R 40 000) and high S/N spectra have been acquiredwith UVES on the VLT-Kueyen (Paranal Observatory, ESO Chile) for severalmain sequence turnoff stars (V 17 mag) and subgiants at the baseof the Red Giant Branch (V 16 mag) in three globular clusters (NGC6397, NGC 6752 and 47 Tuc/NGC 104) at different metallicities(respectively [Fe/H] ≃ -2.0; -1.5; -0.7). Spectra for a sample of25 field halo subdwarves have also been taken with equal resolution, buthigher S/N. These data have been used to determine the abundances ofseveral neutron-capture elements in these three clusters: strontium,yttrium, barium and europium. This is the first abundance determinationof these heavy elements for such unevolved stars in these three globularclusters. These values, together with the [Ba/Eu] and [Sr/Ba] abundanceratios, have been used to test the self-enrichment scenario. Acomparison is done with field halo stars and other well known Galacticglobular clusters in which heavy elements have already been measured inthe past, at least in bright giants (V 11-12 mag). Our resultsshow clearly that globular clusters have been uniformly enriched by r-and s-process syntheses, and that most of them seem to follow exactlythe same abundance patterns as field halo stars, which discards the``classical'' self-enrichment scenario for the origin of metallicitiesand heavy elements in globular clusters.Based on data collected at the European Southern Observatory with theVLT-UT2, Paranal, Chile (ESO-LP 165.L-0263).
| Chemical abundances of 10 metal-poor halo stars
We present observations of ten metal-poor halo stars with themetallicity range -2.3 < [Fe/H] < -1.4 and derive their stellarparameters, acquire some elemental abundances relative to iron anddiscuss the relation between the abundance ratio and the metallicity. Itwas found that oxygen abundances are nearly constant at a level of 0.6dex for our metal-poor halo stars when the non-LTE correction isconsidered. The α-elements (Mg, Si, Ca and Ti) are overabundantrelative to Fe and decrease with increasing metallicity. We alsoobtained a significant underabundant non-LTE [Na/Fe] ratio from Na I Dlines which have a large deviation from the LTE assumption. Scandium ismarginally overabundant with respect to iron and tends to decrease withincreasing metallicity like the α-elements. A nearly solar valueof [Cr/Fe] ratio and underabundant [Mn/Fe] ratio are obtained.
| 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
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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
| 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).
| New Metallicity Calibration Down to [Fe/H] = -2.75 dex
We have taken 88 dwarfs, covering the colour-index interval 0.37 <=(B-V)0 <= 1.07mag, with metallicities -2.70 <= [Fe/H]<= +0.26dex, from three different sources for new metallicitycalibration. The catalogue of Cayrel de Stroble et al. (2001), whichincludes 65% of the stars in our sample, supplies detailed informationon abundances for stars with determination based on high-resolutionspectroscopy. In constructing the new calibration we have used as`corner stones' 77 stars which supply at least one of the followingconditions: (i) the parallax is larger than 10mas (distance relative tothe Sun less than 100pc) and the galactic latitude is absolutely higherthan 30° (ii) the parallax is rather large, if the galactic latitudeis absolutely low and vice versa. Contrary to previous investigations, athird-degree polynomial is fitted for the new calibration: [Fe/H]=0.10 -2.76δ - 24.04δ2 + 30.00δ3. Thecoefficients were evaluated by the least-squares method, without regardto the metallicity of Hyades. However, the constant term is in the rangeof metallicity determined for this cluster, i.e.0.08<=[Fe/H]<=0.11dex. The mean deviation and the mean error inour work are equal to those of Carney (1979), for [Fe/H] >= -1.75dexwhere Carney's calibration is valid
| Lithium Abundance of Metal-poor Stars
High-resolution, high signal-to-noise ratio spectra have been obtainedfor 32 metal-poor stars. The equivalent widths of Li λ6708Åwere measured and the lithium abundances were derived. The averagelithium abundance of 21 stars on the lithium plateau is 2.33±0.02dex. The Lithium plateau exhibits a marginal trend along metallicity,dA(Li)/d[Fe/H] = 0.12±0.06, and no clear trend with the effectivetemperature. The trend indicates that the abundance of lithium plateaumay not be primordial and that a part of the lithium was produced inGalactic Chemical Evolution (GCE).
| 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.
| Abundances for metal-poor stars with accurate parallaxes. I. Basic data
We present element-to-element abundance ratios measured from highdispersion spectra for 150 field subdwarfs and early subgiants withaccurate Hipparcos parallaxes (errors <20%). For 50 stars new spectrawere obtained with the UVES on Kueyen (VLT UT2), the McDonald 2.7 mtelescope, and SARG at TNG. Additionally, literature equivalent widthswere taken from the works by Nissen & Schuster, Fulbright, andProchaska et al. to complement our data. The whole sample includes boththick disk and halo stars (and a few thin disk stars); most stars havemetallicities in the range -2<[Fe/H]<-0.6. We found our data, thatof Nissen & Schuster, and that of Prochaska to be of comparablequality; results from Fulbright scatter a bit more, but they are stillof very good quality and are extremely useful due to the large size ofhis sample. The results of the present analysis will be used inforthcoming papers to discuss the chemical properties of thedissipational collapse and accretion components of our Galaxy.Based in part on data collected at the European Southern Observatory,Chile, at the MacDonald Observatory, Texas, USA, and at the TelescopioNazionale Galileo, Canary Island, INAF,Italy-Spain.}\fnmsep\thanks{Table 1 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia\resizebox{8.8cm}{2.2mm}htpp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/187}
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are only available in electronic form, at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| Chemical Composition of Stars in the Galactic Halo
The chemical compositions of the atmospheres of six metal-poor stars areanalyzed. Spectra with signal-to-noise ratios of no less than 100 and aresolution of R≈17 000 were obtained using the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences.The abundances of Li, O, α-process elements (Mg, Si, Ca, Ti), Na,K, Sc, iron-peak elements (Cr, Mn, Fe, Ni, Cu, Zn), and s-processelements (Y, Ba) are derived. The star G251-54 ([Fe/H]=-1.55, T eff=5541K, logg=3.58) is deficient in some elements compared to both stars withsimilar metallicities and the Sun. The atmosphere of G251-54 has thefollowing elemental abundances relative to iron: [O/Fe]=+0.47,[α/Fe]≈-0.3, [Na/Fe]=-0.60, [Sc/Fe]=-0.57, [Cr, Ni, Fe]≈0,[Zn/Fe]=+0.16, [Cu/Fe]=-0.66, [Y/Fe]=-0.70, and [Ba/Fe]=-1.35. Theremaining five stars have metallicities in the range-1.6<[Fe/H]<-1.3 and normal abundances for this metallicityinterval and are used as reference stars for comparison with thechemical composition of G251-54. Possible explanations for deviations ofthe abundances of some elements from the mean relations established forhalo objects are discussed.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| 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.
| Abundances and Kinematics of Field Stars. II. Kinematics and Abundance Relationships
As an investigation of the origin of ``α-poor'' halo stars, weanalyze kinematic and abundance data for 73 intermediate-metallicitystars (-1>[Fe/H]>=-2) selected from Paper I of this series. We findevidence for a connection between the kinematics and the enhancement ofcertain element-to-iron ([X/Fe]) ratios in these stars. Statisticallysignificant correlations were found between [X/Fe] and galacticrest-frame velocities (vRF) for Na, Mg, Al, Si, Ca, and Ni,with marginally significant correlations existing for Ti and Y as well.We also find that the [X/Fe] ratios for these elements all correlatewith a similar level of significance with [Na/Fe]. Finally, we comparethe abundances of these halo stars against those of stars in nearbydwarf spheroidal (dSph) galaxies. We find significant differencesbetween the abundance ratios in the dSph stars and halo stars of similarmetallicity. From this result, it is unlikely that the halo stars in thesolar neighborhood, including even the ``α-poor'' stars, were oncemembers of disrupted dSph galaxies similar to those studied to date.
| Lithium 6104 Å in Population II stars
We have obtained échelle spectroscopy of 14 Population II objectsselected from those previously observed by Bonifacio & Molaro(1997). For one object, HD 140283, we obtained exquisite data with theHigh Dispersion Spectrograph on the Subaru Telescope, with S/N exceeding1000 per 0.018 Å pixel. Li abundances have been determined byspectral synthesis from both the 6708 Å resonance line and alsofrom 6104 Å subordinate feature. Firm detections of the weak linehave been made in seven objects, and upper limits are reported for theremainder. Our 6708 Å abundances agree with those reported byBonifacio & Molaro with a mean difference of only 0.019 +/- 0.009dex. Abundances from the 6104 Å line hint at a higher Li abundancethan that determined from the resonance feature, but this evidence ismixed; the weakness of the 6104 Å line and the large number ofupper limits make it difficult to draw firm conclusions.NLTE-corrections increase (rather than eliminate) the size of the(potential) discrepancy, and binarity appears unlikely to affect anyabundance difference. The effect of multi-dimensional atmospheres on theline abundances was also considered, although it appears that use of 3-D(LTE) models could again act to increase the discrepancy, if one isindeed present.
| The lithium content of the globular cluster NGC 6397
We make use of high resolution, high signal-to-noise ratio spectra of 12turn-off stars in the metal-poor globular cluster NGC 6397 to measureits lithium content. We conclude that they all have the same lithiumabundance A(Li) = 2.34 with a standard deviation of 0.056 dex. We usethis result, together with Monte Carlo simulations, to estimate that themaximum allowed intrinsic scatter is of the order of 0.035 dex. This isa new stringent constraint to be fulfilled by stellar models whichpredict Li depletion. We argue that although a mild depletion of 0.1-0.2dex, as predicted by recent models, cannot be ruled out, there is nocompelling reason for it. This fact, together with the good agreementwith the Li abundance observed in field stars, supports the primordialorigin of lithium in metal-poor stars. Taking the above value as theprimordial lithium abundance implies a cosmic baryonic density which iseither Omegab h2 = 0.016 +/- 0.004 orOmegab h2 = 0.005+0.0026-0.0006, from the predictions of standardbig bang nucleosynthesis. The high baryonic density solution is inagreement with recent results on the primordial abundance of deuteriumand 3He and on the estimates derived from the fluctuations ofthe cosmic microwave background. Based on observations made with the ESOVLT-Kueyen telescope at the Paranal Observatory, Chile, in the course ofthe ESO-Large program 165.L-0263.
| Detailed analysis of Balmer lines in cool dwarf stars
An analysis of Hα and Hβ spectra in a sample of 30 cool dwarfand subgiant stars is presented using MARCS model atmospheres based onthe most recent calculations of the line opacities. A detailedquantitative comparison of the solar flux spectra with model spectrashows that Balmer line profile shapes, and therefore the temperaturestructure in the line formation region, are best represented under themixing length theory by any combination of a low mixing-length parameteralpha and a low convective structure parameter y. A slightly lowereffective temperature is obtained for the sun than the accepted value,which we attribute to errors in models and line opacities. The programmestars span temperatures from 4800 to 7100 K and include a small numberof population II stars. Effective temperatures have been derived using aquantitative fitting method with a detailed error analysis. Ourtemperatures find good agreement with those from the Infrared FluxMethod (IRFM) near solar metallicity but show differences at lowmetallicity where the two available IRFM determinations themselves arein disagreement. Comparison with recent temperature determinations usingBalmer lines by Fuhrmann (\cite{fuhrmann98, fuhrmann00}), who employed adifferent description of the wing absorption due to self-broadening,does not show the large differences predicted by Barklem et al.(\cite{bpo:hyd}). In fact, perhaps fortuitously, reasonable agreement isfound near solar metallicity, while we find significantly coolertemperatures for low metallicity stars of around solar temperature.Based on observations collected at the Isaac Newton Telescope, La Palma,Spain, and McDonald Observatory, Texas, USA.
| A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue
We have identified 317 stars included in the Hipparcos astrometriccatalogue that have parallaxes measured to a precision of better than 15per cent, and the location of which in the(MV,(B-V)T) diagram implies a metallicitycomparable to or less than that of the intermediate-abundance globularcluster M5. We have undertaken an extensive literature search to locateStrömgren, Johnson/Cousins and Walraven photometry for over 120stars. In addition, we present new UBV(RI)C photometry of 201of these candidate halo stars, together with similar data for a further14 known metal-poor subdwarfs. These observations provide the firstextensive data set of RCIC photometry ofmetal-poor, main-sequence stars with well-determined trigonometricparallaxes. Finally, we have obtained intermediate-resolution opticalspectroscopy of 175 stars. 47 stars still lack sufficient supplementaryobservations for population classification; however, we are able toestimate abundances for 270 stars, or over 80 per cent of the sample.The overwhelming majority have near-solar abundance, with theirinclusion in the present sample stemming from errors in the colourslisted in the Hipparcos catalogue. Only 44 stars show consistentevidence of abundances below [Fe/H]=-1.0. Nine are additions to thesmall sample of metal-poor subdwarfs with accurate photometry. Weconsider briefly the implication of these results for clustermain-sequence fitting.
| 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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| Takýmyýldýz: | Basakci |
| Sað Açýklýk: | 12h25m34.96s |
| Yükselim: | +01°17'02.3" |
| Görünürdeki Parlaklýk: | 9.683 |
| özdevim Sað Açýklýk: | -31 |
| özdevim Yükselim: | -470.4 |
| B-T magnitude: | 10.17 |
| V-T magnitude: | 9.724 |
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