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Silver and palladium help unveil the nature of a second r-process
Context. The rapid neutron-capture process, which created about half ofthe heaviest elements in the solar system, is believed to have beenunique. Many recent studies have shown that this uniqueness is not truefor the formation of lighter elements, in particular those in the atomicnumber range 38 < Z < 48. Among these, palladium (Pd) andespecially silver (Ag) are expected to be key indicators of a possiblesecond r-process, but until recently they have been studied only in afew stars. We therefore target Pd and Ag in a large sample of stars andcompare these abundances to those of Sr, Y, Zr, Ba, and Eu produced bythe slow (s-) and rapid (r-) neutron-capture processes. Hereby weinvestigate the nature of the formation process of Ag and Pd. Aims: We study the abundances of seven elements (Sr, Y, Zr, Pd, Ag, Ba,and Eu) to gain insight into the formation process of the elements andexplore in depth the nature of the second r-process. Methods: Byadopting a homogeneous one-dimensional local thermodynamic equilibrium(1D LTE) analysis of 71 stars, we derive stellar abundances using thespectral synthesis code MOOG, and the MARCS model atmospheres. Wecalculate abundance ratio trends and compare the derived abundances tosite-dependent yield predictions (low-mass O-Ne-Mg core-collapsesupernovae and parametrised high-entropy winds), to extractcharacteristics of the second r-process. Results: The sevenelements are tracers of different (neutron-capture) processes, which inturn allows us to constrain the formation process(es) of Pd and Ag. Theabundance ratios of the heavy elements are found to be correlated andanti-correlated. These trends lead to clear indications that asecond/weak r-process, is responsible for the formation of Pd and Ag. Onthe basis of the comparison to the model predictions, we find that theconditions under which this process takes place differ from those forthe main r-process in needing lower neutron number densities, lowerneutron-to-seed ratios, and lower entropies, and/or higher electronabundances. Conclusions: Our analysis confirms that Pd and Agform via a rapid neutron-capture process that differs from the mainr-process, the main and weak s-processes, and charged particlefreeze-outs. We find that this process is efficiently working down tothe lowest metallicity sampled by our analysis ([Fe/H] = - 3.3). Ourresults may indicate that a combination of these explosive sites isneeded to explain the variety in the observationally derived abundancepatterns.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (ID 65.L-0507(A), 67.D-0439(A), 68.B-0475(A),68.D-0094(A), 71.B-0529(A); P.I. F. Primas).Appendices are available inelectronic form at http://www.aanda.org
| The barium isotopic fractions in five metal-poor stars
Context. Theory and observations of heavy element nucleosynthesis are inconflict with one-another. Theory states that in the most metal-poorstars, the rapid (r-) neutron-capture nucleosynthetic process would bedominant over the slow (s-) process. The most recent determinations ofr- and s-process yields do not support this. Aims: We providemeasurements of the Ba isotopic fractions for five metal-poor starsderived with a local thermodynamic equilibrium (LTE) analysis with 1Dmodel stellar atmospheres. This increases the comparisons with heavyelement nucleosynthesis theory. Methods: We use high resolution(R ≡ λ/Δλ = 90 000-95 000), very highsignal-to-noise (S/N > 500) spectra to determine the fraction of oddBa isotopes (fodd) by measuring subtle asymmetries in theprofile of the Ba ii line at 4554 Å. We also use two differentmacroturbulent broadening techniques, Gaussian and radial-tangential, tomodel the Fe lines of each star, and propagate each technique to modelmacroturbulent broadening in the Ba 4554 Å line. We conduct a 1Dnon-LTE (NLTE) treatment of the Fe lines in the red giant HD 122563 andthe subgiant HD 140283 in an attempt to improve the fitting. Wedetermine [Ba/Eu] ratios for the two giants in our study, HD 122563 andHD 88609, which can also be used to determine the relative contributionof the s- and r-processes to heavy-element nucleosynthesis, forcomparison with fodd. Results: We find mathematicalsolutions of fodd for HD 122563, HD 88609 and HD 84937 of-0.12 ± 0.07, -0.02 ± 0.09, and -0.05 ± 0.11respectively. BD + 26° 3578 yielded a value for fodd =0.08 ± 0.08. Only BD - 04° 3208 was found to have a physicalfodd ratio of 0.18 ± 0.08. This means that all starsexamined here show isotopic fractions more compatible with an s-processdominated composition. The [Ba/Eu] ratios in HD 122563 and HD 88609 arefound to be -0.20 ± 0.15 and -0.47 ± 0.15 respectively,which indicate instead an r-process signature. We report a betterstatistical fit to the majority of Fe profiles in each star whenemploying a radial-tangential broadening technique during our 1D LTEinvestigation. Conclusions: With the increase of the number ofstars for which the Ba isotope fraction fodd has beenmeasured, and the nature of their results, there is now a strongerargument to suggest that other synthesis codes that employ alternativeapproaches to radiative transfer (e.g. 3D hydrodynamics) have to beconsidered to tackle the high level of precision required for thedetermination of isotopic ratios. We have shown that, from a statisticalpoint of view, one must consider using a radial-tangential broadeningtechnique rather than a Gaussian one to model Fe line macroturbulenceswhen working in 1D. No improvement to Fe line fitting is seen whenemploying a NLTE treatment of the Fe lines.
| Beryllium and Alpha-element Abundances in a Large Sample of Metal-poor Stars
The light elements, Li, Be, and B, provide tracers for many aspects ofastronomy including stellar structure, Galactic evolution, andcosmology. We have made observations of Be in 117 metal-poor starsranging in metallicity from [Fe/H] = -0.5 to -3.5 with KeckI/HIRES. Our spectra are high resolution (~42,000) and high signal tonoise (the median is 106 per pixel). We have determined the stellarparameters spectroscopically from lines of Fe I, Fe II, Ti I, and Ti II.The abundances of Be and O were derived by spectrum synthesistechniques, while abundances of Fe, Ti, and Mg were found from manyspectral line measurements. There is a linear relationship between[Fe/H] and A(Be) with a slope of +0.88 ± 0.03 over three ordersof magnitude in [Fe/H]. We find that Be is enhanced relative to Fe;[Be/Fe] is +0.40 near [Fe/H] ~-3.3 and drops to 0.0 near [Fe/H]~-1.7. For the relationship between A(Be) and [O/H], we find agradual change in slope from 0.69 ± 0.13 for the Be-poor/O-poorstars to 1.13 ± 0.10 for the Be-rich/O-rich stars. Inasmuch asthe relationship between [Fe/H] and [O/H] seems robustly linear (slope =+0.75 ± 0.03), we conclude that the slope change in Be versus Ois due to the Be abundance. Much of the Be would have been formed in thevicinity of Type II supernova (SN II) in the early history of the Galaxyand by Galactic cosmic-ray (GCR) spallation in the later eras. AlthoughBe is a by-product of CNO, we have used Ti and Mg abundances asalpha-element surrogates for O in part because O abundances are rathersensitive to both stellar temperature and surface gravity. We find thatA(Be) tracks [Ti/H] very well with a slope of 1.00 ± 0.04. Italso tracks [Mg/H] very well with a slope of 0.88 ± 0.03. We havekinematic information on 114 stars in our sample and they divide equallyinto dissipative and accretive stars. Almost the full range of [Fe/H]and [O/H] is covered in each group. There are distinct differences inthe relationships of A(Be) and [Fe/H] and of A(Be) and [O/H] for thedissipative and the accretive stars. It is likely that the formation ofBe in the accretive stars was primarily in the vicinity of SN II, whilethe Be in the dissipative stars was primarily formed by GCR spallation.We find that Be is not as good a cosmochronometer as Fe. We have found aspread in A(Be) that is valid at the 4? level between [O/H] =-0.5 and -1.0, which corresponds to -0.9 and-1.6 in [Fe/H].
| New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey
We present a re-analysis of the Geneva-Copenhagen survey, which benefitsfrom the infrared flux method to improve the accuracy of the derivedstellar effective temperatures and uses the latter to build a consistentand improved metallicity scale. Metallicities are calibrated onhigh-resolution spectroscopy and checked against four open clusters anda moving group, showing excellent consistency. The new temperature andmetallicity scales provide a better match to theoretical isochrones,which are used for a Bayesian analysis of stellar ages. With respect toprevious analyses, our stars are on average 100 K hotter and 0.1 dexmore metal rich, which shift the peak of the metallicity distributionfunction around the solar value. From Strömgren photometry we areable to derive for the first time a proxy for [?/Fe] abundances,which enables us to perform a tentative dissection of the chemical thinand thick disc. We find evidence for the latter being composed of anold, mildly but systematically alpha-enhanced population that extends tosuper solar metallicities, in agreement with spectroscopic studies. Ourrevision offers the largest existing kinematically unbiased sample ofthe solar neighbourhood that contains full information on kinematics,metallicities, and ages and thus provides better constraints on thephysical processes relevant in the build-up of the Milky Way disc,enabling a better understanding of the Sun in a Galactic context.Catalogue (Table 2) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A138
| 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.
| 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.
| Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy
Aims: We investigate statistical equilibrium of Cr in theatmospheres of late-type stars to ascertain whether the systematicabundance discrepancy between Cr I and Cr II lines, as often found inprevious work, is due to deviations from local thermodynamic equilibrium(LTE). Furthermore, we attempt to interpret the Non-LTE (NLTE) trend of[Cr/Fe] with [Fe/H] using chemical evolution models for the solarneighborhood. Methods: NLTE calculations are performed for themodel of the Cr atom, comprising 340 levels and 6806 transitions intotal. We use the quantum-mechanical photoionization cross-sections ofNahar (2009) and investigate the sensitivity of the model to uncertaincross-sections for H I collisions. NLTE line formation is performed forthe MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with-3.2 < [Fe/H] < -0.5, and Cr abundances are derived by comparingthe synthetic and observed flux spectra. Results: We achieve goodionization equilibrium of Cr for models with different stellarparameters, if inelastic collisions with H I atoms are neglected. Thesolar NLTE abundance based on Cr I lines is 5.74 dex with ? = 0.05dex, which is ~0.1 dex higher than the LTE abundance. For the metal-poorstars, the NLTE abundance corrections to Cr I lines range from +0.3 to+0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range ofmetallicities analyzed here, which is consistent with current views onthe production of these iron peak elements in supernovae. Conclusions: The tendency of Cr to become deficient with respect to Fein metal-poor stars is an artifact caused by the neglect of NLTE effectsin the line formation of Cr i, and has no relation to any peculiarphysical conditions in the Galactic ISM or deficiencies ofnucleosynthesis theory.Table 4 is only available in electronic form at http://www.aanda.org
| Stellar atmosphere parameters with MA?, a MAssive compression of ?2 for spectral fitting
MA? is a new tool to estimate parameters from stellar spectra. It isbased on the maximum likelihood method, with the likelihood compressedin a way that the information stored in the spectral fluxes isconserved. The compressed data are given by the size of the number ofparameters, rather than by the number of flux points. The optimumspeed-up reached by the compression is the ratio of the data set to thenumber of parameters. The method has been tested on a sample oflow-resolution spectra from the Sloan Extension for GalacticUnderstanding and Exploration (SEGUE) survey for the estimate ofmetallicity, effective temperature and surface gravity, with accuraciesof 0.24 dex, 130 K and 0.5 dex, respectively. Our stellar parameters andthose recovered by the SEGUE Stellar Parameter Pipeline agree reasonablywell. A small sample of high-resolution VLT-UVES spectra is also used totest the method and the results were compared to a more classicalapproach. The speed and multi-resolution capability of MA? combinedwith its performance compared with other methods indicates that it willbe a useful tool for the analysis of upcoming spectral surveys.
| Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion
We present NLTE Li abundances for 88 stars in the metallicity range -3.5< [Fe/H] < -1.0. The effective temperatures are based on theinfrared flux method with improved E(B-V) values obtained mostly frominterstellar Na I D lines. The Li abundances were derived through MARCSmodels and high-quality UVES+VLT, HIRES+Keck and FIES+NOT spectra, andcomplemented with reliable equivalent widths from the literature. Theless-depleted stars with [Fe/H] < -2.5 and [Fe/H] > -2.5 fall intotwo well-defined plateaus of ALi = 2.18 (? = 0.04) andALi = 2.27 (? = 0.05), respectively. We show that thetwo plateaus are flat, unlike previous claims for a steep monotonicdecrease in Li abundances with decreasing metallicities. At allmetallicities we uncover a fine-structure in the Li abundances of Spiteplateau stars, which we trace to Li depletion that depends on bothmetallicity and mass. Models including atomic diffusion and turbulentmixing seem to reproduce the observed Li depletion assuming a primordialLi abundance ALi = 2.64, which agrees well with currentpredictions (ALi = 2.72) from standard Big Bangnucleosynthesis. Adopting the Kurucz overshooting model atmospheresincreases the Li abundance by +0.08 dex to ALi = 2.72, whichperfectly agrees with BBN+WMAP.Based in part on observations obtained at the W. M. Keck Observatory,the Nordic Optical Telescope on La Palma, and on data from theHIRES/Keck archive and the European Southern Observatory ESO/ST-ECFScience Archive Facility.Table 1 is only available in electronic form athttp://www.aanda.org
| 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
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| NLTE analysis of CoI/CoII lines in spectra of cool stars with new laboratory hyperfine splitting constants
The analysis of stellar abundances for odd-Z Fe-peak elements requiresaccurate non-local thermodynamic equilibrium (NLTE) modelling ofspectral lines fully taking into account the hyperfine structure (HFS)splitting of lines. Here, we investigate the statistical equilibrium ofCo in the atmospheres of cool stars and the influence of NLTE and HFS onthe formation of Co lines and abundances. Significant departures fromLTE level populations are found for CoI number densities of excitedstates in CoII also differ from LTE at low metallicity. The NLTE levelpopulations are used to determine the abundance of Co in solarphotosphere, log? = 4.95 +/- 0.04dex, which is in agreement withthat in CI meteorites within the combined uncertainties. The spectrallines of CoI were calculated using the results of recent measurements ofhyperfine interaction constants by UV Fourier transform spectrometry.For CoII, the first laboratory measurements of HFS A and B factors wereperformed. These highly accurate A factor measurements (errors of theorder of 3-7 per cent) allow, for the first time, reliable modelling ofCoII lines in the solar and stellar spectra and, thus, a test of theCoI/CoII ionization equilibrium in stellar atmospheres. A differentialabundance analysis of Co is carried out for 18 stars in the metallicityrange -3.12 < [Fe/H] < 0. The abundances are derived by the methodof spectrum synthesis. At low [Fe/H], NLTE abundance corrections for CoIlines are as large as +0.6,..., + 0.8dex. Thus, LTE abundances of Co inmetal-poor stars are severely underestimated. The stellar NLTEabundances determined from the single UV line of CoII are lower by~0.5-0.6dex. The discrepancy might be attributed to possible blends thathave not been accounted for in the solar CoII line and its erroneousoscillator strength. The increasing [Co/Fe] trend in metal-poor stars,as calculated from the CoI lines under NLTE, can be explained if Co isoverproduced relative to Fe in massive stars. The models of Galacticchemical evolution are wholly inadequate to describe this trendsuggesting that the problem is in supernova yields.Based on observations collected at the European Southern Observatory,Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.E-mail: mbergema@mpa-garching.mpg.de (MB); j.pickering@imperial.ac.uk(JCP); gehren@usm.lmu.de (TG)
| 6Li/7Li estimates for metal-poor stars
Context: The presence of the lithium-6 isotope in some metal-poor starsis a matter of surprise because of the high values observed.Non-standard models of Big Bang nucleosynthesis and pre-Galactic cosmicray fusion and spallation have been proposed to explain these values.However, the observations of this light isotope are challenging whichmay make some detections disputable. Aims: The goal was todetermine ^6Li/^7Li for a sample of metal-poor stars; three of them havebeen previously studied and the remaining two are new for this type ofstudy. The purpose was to increase, if possible, the number of lithium-6detections and to confirm previously published results. Methods:Spectra of the resonance doublet line of neutral lithium Li I 670.78 nmwere taken with the high dispersion spectrograph at the Subaru 8.2m-telescope for a sample of five metal-poor stars({-3.12≤[Fe/H]≤-2.19}). The contribution of lithium-6 to the totalobserved line profile was estimated from the 1D-LTE analysis of the lineasymmetry. Results: Observed asymmetries could be reproducedassuming isotopic abundance ratios ^6Li/^7Li of the order of: 0.004 forBD +26° 3578, 0.010 for BD +02°3375 and G 64-37, 0.025 for BD+20° 3603 and 0.047 for BD -04°3208. We found that these results were very sensitive toseveral of the assumptions made in the analysis, in particular, thetreatment of the residual structure in the analysed spectra. Our finalestimates for the errors are respectively Δ^6Li/^7Li = ±0.028, 0.029, 0.039, 0.025 and 0.039. Conclusions: The ^6Li/^7Liratios for the sample are comparable to or even lower than these errorvalues, so that detections of lithium-6 can not safely be claimeddespite of the high resolving power (R ˜ 95 000) and S/N(400-600).
| Beryllium, Oxygen, and Iron Abundances in Extremely Metal-Deficient Stars
The abundance of beryllium in the oldest, most metal-poor stars acts asa probe of early star formation and Galactic chemical evolution. We haveanalyzed high-resolution, high signal-to-noise ratio Keck/HIRES spectraof 24 stars with [Fe/H] from -2.3 to -3.5 in order todetermine the history of Be abundance and explore the possibility of aBe plateau. We have determined stellar parameters of our samplespectroscopically, using equivalent widths of Fe I, Ti I, and Ti IIlines. We have determined O abundances from three OH features whichoccur in the same spectral region; this region is relatively uncrowdedand has a well determined continuum in these very/extremely metal-poorstars. We have supplemented this sample with reanalyzed spectra of 25stars from previous observations so that our total sample ranges in[Fe/H] from -0.5 to -3.5. Our results indicate that therelationship between Be and [Fe/H] continues to lower metallicities witha slope of 0.92 ± 0.04. Although there is no indication of aplateau with constant Be abundance, the four lowest metallicity stars(below [Fe/H] of -3.0) do show a Be enhancement relative to Fe atthe 1σ level. A single relationship between Be and [O/H] has aslope of 1.21 ± 0.08, but there is also a good fit with twoslopes: 1.59 above [O/H] = -1.6 and 0.74 for stars with [O/H]below -1.6. This change in slope could result from a change in thedominant production mechanism for Be. In the era of the formation of themore metal-poor stars, Be would be formed by acceleration of CNO atomsin the vicinity of SN II and in later times by high-energy cosmic-raysbombarding CNO in the ambient interstellar gas. We find an excellentcorrelation between [Fe/H] and [O/H] and show that [O/Fe] is near +1.0at [Fe/H] = -3.5 declining to 0 at [Fe/H] = 0.
| Statistical equilibrium of silicon in the atmospheres of metal-poor stars
Aims: The statistical equilibrium of neutral and ionized silicon in theatmospheres of metal-poor stars is discussed. Non-local thermodynamicequilibrium effects (NLTE) are investigated and the silicon abundancesin metal-poor stars determined. Methods: We have used highresolution, high signal to noise ratio spectra from the UVESspectragraph at the ESO VLT telescope. Line formation calculations of Sii and Si ii in the atmospheres of metal-poor stars are presented foratomic models of silicon including 174 terms and 1132 line transitions.Recent improved calculations of Si i and Si ii photoionizationcross-sections are taken into account, and the influence of thefree-free quasi-molecular absorption in the Ly? wing isinvestigated by comparing theoretical and observed fluxes of metal-poorstars. All abundance results are derived from LTE and NLTE statisticalequilibrium calculations and spectrum synthesis methods. Results:It is found that the extreme ultraviolet radiation is very important formetal-poor stars, especially for the high temperature, very metal-poorstars. The radiative bound-free cross-sections also play a veryimportant role for these stars. Conclusions: NLTE effects for Siare found to be important for metal-poor stars, in particular for warmmetal-poor stars. It is found that these effects depend on thetemperature. For warm metal-poor stars, the NLTE abundance correctionreaches ~0.2 dex relative to standard LTE calculations. Our resultsindicate that Si is overabundant for metal-poor stars.Based on observations obtained in the frame of the ESO programme ID165.N-0276(A).
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full 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/501/941
| 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.
| Beryllium abundances in metal-poor stars
We have determined beryllium (Be) abundances for 25 metal-poor starsbased on the high-resolution and high signal-to-noise ratio spectra fromthe Very Large Telescope/Ultraviolet and Visual Echelle Spectrographdata base. Our results confirm that Be abundances increase with Fe,supporting the global enrichment of Be in the Galaxy. Oxygen abundancesbased on the [OI] forbidden line imply a linear relation with a slopeclose to one for the Be versus O trend, which indicates that Be isprobably produced in a primary process. Some strong evidence is foundfor the intrinsic dispersion of Be abundances at a given metallicity.The deviation of HD 132475 and HD 126681 from the general Be versus Feand Be versus O trends favours the predictions of the superbubble model.However, the possibility that such dispersion originates from theinhomogeneous enrichment in Fe and O of the protogalactic gas cannot beexcluded.Based on observations made with the European Southern Observatorytelescopes obtained from the ESO/ST-ECF Science Archive Facility.E-mail: tan@bao.ac.cn
| Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature withdetailed chemical abundance information (spanning –4.2lsim [Fe/H]lsim+0.3), and we compute their space velocities and Galactic orbitalparameters. We identify members of the inner and outer stellar halopopulations in our sample based only on their kinematic properties andthen compare the abundance ratios of these populations as a function of[Fe/H]. In the metallicity range where the two populations overlap(–2.5lsim [Fe/H] lsim–1.5), the mean [Mg/Fe] of the outerhalo is lower than the inner halo by –0.1 dex. For [Ni/Fe] and[Ba/Fe], the star-to-star abundance scatter of the inner halo isconsistently smaller than in the outer halo. The [Na/Fe], [Y/Fe],[Ca/Fe], and [Ti/Fe] ratios of both populations show similar means andlevels of scatter. Our inner halo population is chemically homogeneous,suggesting that a significant fraction of the Milky Way stellar halooriginated from a well-mixed interstellar medium. In contrast, our outerhalo population is chemically diverse, suggesting that anothersignificant fraction of the Milky Way stellar halo formed in remoteregions where chemical enrichment was dominated by local supernovaevents. We find no abundance trends with maximum radial distance fromthe Galactic center or maximum vertical distance from the Galactic disk.We also find no common kinematic signature for groups of metal-poorstars with peculiar abundance patters, such as the α-poor stars orstars showing unique neutron-capture enrichment patterns. Several starsand dwarf spheroidal systems with unique abundance patterns spend themajority of their time in the distant regions of the Milky Way stellarhalo, suggesting that the true outer halo of the Galaxy may have littleresemblance to the local stellar halo.
| NLTE abundances of Mn in a sample of metal-poor stars
Aims: Following our solar work, we perform NLTE calculations of the Mnabundance for fourteen stars with [Fe/H] from 0 to -2.5, mainly to showhow NLTE affects Mn abundances in cool stars of different metallicities.Methods: The spectrum synthesis and Mn abundances are based onstatistical equilibrium calculations using various estimates for theinfluence of hydrogen collisions. Results: The NLTE abundances ofMn in all studied stars are systematically higher than the LTEabundances. At low metallicities, the NLTE abundance corrections may runup to 0.5-0.7 dex. Instead of a strong depletion of Mn relative to Fe inmetal-poor stars as found by the other authors, we only find slightlysubsolar values of [Mn/Fe] throughout the range of metallicitiesanalyzed here. Conclusions: The [Mn/Fe] trend in metal-poor starsis inconsistent with the predictions of galactic chemical evolutionmodels, where Mn is less produced than Fe.Based on observations collected at the European Southern Observatory,Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.
| 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.
| Neutron-capture elements in halo, thick-disk, and thin-disk stars. Strontium, yttrium, zirconium, cerium
We derived Sr, Y, Zr, and Ce abundances for a sample of 74 cool dwarfsand subgiants with iron abundances, [Fe/H], between 0.25 and‑2.43. These estimates were obtained using synthetic spectra,assuming local thermodynamic equilibrium (LTE) for Y, Zr, and Ce,allowing for non-LTE conditions for Sr. We used high-resolution(λ/Δλ≅40 000 and 60 000) spectra withsignal-to-noise ratios between 50 and 200. We find that the Zr/Y, Sr/Y,and Sr/Zr ratios for the halo stars are the same in a wide metallicityrange (‑2.43 ≤ [Fe/H] ≤ ‑0.90), within the errors,indicating a common origin for these elements at the epoch of haloformation. The Zr/Y ratios for thick-disk stars quickly decrease withincreasing Ba abundance, indicating a lower rate of production of Zrcompared to Y during active thick-disk formation. The thick-disk andhalo stars display an increase in the [Zr/Ba] ratio with decreasing Baabundance and a correlation of the Zr and Eu overabundances relative toBa. The evolutionary behavior of the abundance ratios found for thethick-disk and halo stars does not agree with current models for theGalaxy’s chemical evolution. The abundance ratios of Y and Zr toFe and Ba for thin-disk stars, as well as the abundance ratios withineach group, are, on average, solar, though we note a slight decrease ofZr/Ba and Zr/Y with increasing Ba abundance. These results provideevidence for a dominance of asymptotic-giant-branch stars in theenrichment of the interstellar medium in heavy elements during thethin-disk epoch, in agreement with the predictions of thenucleosynthesis theory for the main s-process component.
| 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
| Lithium abundances in metal-poor stars
Aims.Lithium abundances for 19 metal-poor stars are determined usinghigh-resolution spectroscopy. The abundances of stars on the lithiumplateau are discussed. Methods: All abundance results are derived fromNLTE statistical equilibrium calculations and spectrum synthesismethods. Results: In agreement with previous analyses it is found thatexcitation and de-excitation due to hydrogen collisions are negligiblefor the lithium line formation process, while charge transfer reactionsare an important source of thermalization. However, the resulting NLTEeffects on the determination of lithium abundances for metal-poor starsare negligible (<0.06 dex). Conclusions: .The mean lithium abundancefor stars on the lithium plateau determined from NLTE analyses is A(Li)~ 2.26, while it is 2.21 dex when charge transfer reactions areincluded. The latter result enhances the discrepancy between theobserved lithium abundances and the primordial lithium abundance asinferred by the WMAP analysis of the cosmic microwave background. Thisdiscrepancy may be explained by metal diffusion.Based on observations collected at the Germany-Spanish AstronomicalCenter, Calar Alto, Spain.
| The usage of Strömgren photometry in studies of local group dwarf spheroidal galaxies. Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients
Aims.In this paper we demonstrate how Strömgren uvby photometry canbe efficiently used to: 1. identify red giant branch stars that aremembers in a dwarf spheroidal galaxy; 2. derive age-independentmetallicities for the same stars and quantify the associated errors. Methods: Strömgren uvby photometry in a 11 × 22 arcmin fieldcentered on the Draco dwarf spheroidal galaxy was obtained using theIsaac Newton Telescope on La Palma. Members of the Draco dSph galaxywere identified using the surface gravity sensitive c1 indexwhich discriminates between red giant and dwarf stars. Thus enabling usto distinguish the (red giant branch) members of the dwarf spheroidalgalaxy from the foreground dwarf stars in our galaxy. The method isevaluated through a comparison of our membership list with membershipclassifications in the literature based on radial velocities and propermotions. The metallicity sensitive m1 index was used toderive individual and age-independent metallicities for the members ofthe Draco dSph galaxy. The derived metallicities are compared to studiesbased on high resolution spectroscopy and the agreement is found to bevery good. Results: We present metallicities for 169 members of the redgiant branch in the Draco dwarf spheroidal galaxy (the largest sample todate). The metallicity distribution function for the Draco dSph galaxyshows a mean [Fe/H] = -1.74 dex with a spread of 0.24 dex. Thecorrelation between metallicity and colour for the stars on the redgiant branch is consistent with a dominant old, and coeval population.There is a possible spatial population gradient over the field with themost metal-rich stars being more centrally concentrated than themetal-poor stars.Based on observations made with the Isaac Newton Telescope, operated onthe Island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias. Guest User, Canadian Astronomy Data Centre, which isoperated by the Herzberg Institute of Astrophysics, National ResearchCouncil of Canada. Full Tables 2 and 6 are only available athttp://www.aanda.org
| A non-LTE study of neutral and singly-ionized calcium in late-type stars
Aims.Non-local thermodynamical equilibrium (NLTE) line formation forneutral and singly-ionized calcium is considered through a range ofspectral types when the Ca abundance varies from the solar value down to[Ca/H] = -5. We evaluate the influence of departures from LTE on Caabundance determinations and inspect the possibility of using Ca I / CaII line-strength ratios as indicators of surface gravity for extremelymetal-poor stars. Methods: .A comprehensive model atom for Ca Iand Ca II is presented. Accurate radiative and electron collisionalatomic data are incorporated. The role of inelastic collisions withhydrogen atoms in the statistical equilibrium of Ca I/II is estimatedempirically from inspection of their different influences on the Ca Iand Ca II lines in selected stars with well determined stellarparameters and high-quality observed spectra. Results: .Thedependence of NLTE effects on the atmospheric parameters is discussed.Departures from LTE significantly affect the profiles of Ca I lines overthe whole range of stellar parameters being considered. However, at[Ca/H] ≥ -2, NLTE abundance correction of individual lines have alow absolute value due to the different influence of NLTE effects online wings and the line core. At lower Ca abundances, NLTE leads tosystematically depleted total absorption in the line and positiveabundance corrections, exceeding +0.5 dex for Ca I λ 4226 at[Ca/H] = -4.9. In contrast, the NLTE effects strengthen the Ca II linesand lead to negative abundance corrections. NLTE corrections are small,≤0.02 dex, for the Ca II resonance lines, and they grow in absolutevalue with decreasing Ca abundance for the IR lines of multiplet 3d-4p,exceeding 0.4 dex in the metal-poor models with [Fe/H] ≤ -3. As atest and first application of the Ca I/II model atom, Ca abundances aredetermined on the basis of plane-parallel LTE model atmospheres for theSun, Procyon (F IV-V), and seven metal-poor stars, using high S/N andhigh-resolution spectra at visual and near-IR wavelengths. Lines of Ca Iand Ca II give consistent abundances for all objects (except Procyon)when collisions with hydrogen atoms are taken into account. The derivedabsolute solar Ca abundance (from Ca I and Ca II lines) is log\varepsilon_Ca,ȯ = 6.38 ± 0.06. For Procyon, the mean Caabundance from Ca I lines is markedly subsolar, [Ca/H] = -0.14 ±0.03. All metal-poor stars within our sample show an overabundance ofcalcium relative to iron with [Ca/Fe] abundance ratios of 0.26 to 0.46that are typical of the halo population. The W(Ca I 4226) / W(Ca II8498) equivalent width ratio is predicted to be sensitive to surfacegravity for extremely metal-poor stars, while this is not the case forthe ratio involving the Ca II resonance line(s).
| 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.
| Lithium Isotopic Abundances in Metal-poor Halo Stars
Very high quality spectra of 24 metal-poor halo dwarfs and subgiantshave been acquired with ESO's VLT/UVES for the purpose of determining Liisotopic abundances. The derived one-dimensional, non-LTE 7Li abundances from the Li I 670.8 nm line reveal a pronounceddependence on metallicity but with negligible scatter around this trend.Very good agreement is found between the abundances from the Li I 670.8nm line and the Li I 610.4 nm line. The estimated primordial 7Li abundance is7Li/H=(1.1-1.5)×10-10, which is a factor of3-4 lower than predicted from standard big bang nucleosynthesis with thebaryon density inferred from the cosmic microwave background.Interestingly, 6Li is detected in 9 of our 24 stars at the>=2 σ significance level. Our observations suggest theexistence of a 6Li plateau at the level oflogε6Li~0.8 however, taking into accountpredictions for 6Li destruction during the pre-main-sequenceevolution tilts the plateau such that the 6Li abundancesapparently increase with metallicity. Our most noteworthy result is thedetection of 6Li in the very metal-poor star LP 815-43. Sucha high 6Li abundance during these early Galactic epochs isvery difficult to achieve by Galactic cosmic-ray spallation andα-fusion reactions. It is concluded that both Li isotopes have apre-Galactic origin. Possible 6Li production channels includeprotogalactic shocks and late-decaying or annihilating supersymmetricparticles during the era of big bang nucleosynthesis. The presence of6Li limits the possible degree of stellar 7Lidepletion and thus sharpens the discrepancy with standard big bangnucleosynthesis.Based on observations collected at the European Southern Observatory,Paranal, Chile (observing programs 65.L-0131, 68.D-0091, and273.D-5043).
| 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.
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