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Публикации по объекту
An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| Calibration of Strömgren uvby-H? photometry for late-type stars - a model atmosphere approach
Context: The use of model atmospheres for deriving stellar fundamentalparameters, such as T_eff, log g, and [Fe/H], will increase as we findand explore extreme stellar populations where empirical calibrations arenot yet available. Moreover, calibrations for upcoming large satellitemissions of new spectrophotometric indices, similar to the uvby-H?system, will be needed. Aims: We aim to test the power oftheoretical calibrations based on a new generation of MARCS models bycomparisons with observational photomteric data. Methods: Wecalculated synthetic uvby-H? colour indices from synthetic spectra.A sample of 367 field stars, as well as stars in globular clusters, isused for a direct comparison of the synthetic indices versus empiricaldata and for scrutinizing the possibilities of theoretical calibrationsfor temperature, metallicity, and gravity. Results: We show thatthe temperature sensitivity of the synthetic (b-y) colour is very closeto its empirical counterpart, whereas the temperature scale based uponH? shows a slight offset. The theoretical metallicity sensitivityof the m1 index (and for G-type stars its combination withc_1) is somewhat higher than the empirical one, based upon spectroscopicdeterminations. The gravity sensitivity of the synthetic c1index shows satisfactory behaviour when compared to obervations of Fstars. For stars cooler than the sun, a deviation is significant in thec1-(b-y) diagram. The theoretical calibrations of (b-y),(v-y), and c1 seem to work well for Pop II stars and lead toeffective temperatures for globular cluster stars supporting recentclaims that atomic diffusion occurs in stars near the turnoff point ofNGC 6397. Conclusions: Synthetic colours of stellar atmospherescan indeed be used, in many cases, to derive reliable fundamentalstellar parameters. The deviations seen when compared to observationaldata could be due to incomplete linelists but are possibly also due tothe effects of assuming plane-parallell or spherical geometry and LTE.Model colours 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/498/527
| Halo Star Streams in the Solar Neighborhood
We have assembled a sample of halo stars in the solar neighborhood tolook for halo substructure in velocity and angular momentum space. Oursample (231 stars) includes red giants, RR Lyrae variable stars, and redhorizontal branch stars within 2.5 kpc of the Sun with [Fe/H] less than-1.0. It was chosen to include stars with accurate distances, spacevelocities, and metallicities, as well as well-quantified errors. Withour data set, we confirm the existence of the streams found by Helmi andcoworkers, which we refer to as the H99 streams. These streams have adouble-peaked velocity distribution in the z-direction (out of theGalactic plane). We use the results of modeling of the H99 streams byHelmi and collaborators to test how one might use vz velocityinformation and radial velocity information to detect kinematicsubstructure in the halo. We find that detecting the H99 streams withradial velocities alone would require a large sample (e.g.,approximately 150 stars within 2 kpc of the Sun and within 20° ofthe Galactic poles). In addition, we use the velocity distribution ofthe H99 streams to estimate their age. From our model of the progenitorof the H99 streams, we determine that it was accreted between 6 and 9Gyr ago. The H99 streams have [α/Fe] abundances similar to otherhalo stars in the solar neighborhood, suggesting that the gas thatformed these stars were enriched mostly by Type II supernovae. We havealso discovered in angular momentum space two other possiblesubstructures, which we refer to as the retrograde and progradeoutliers. The retrograde outliers are likely to be halo substructure,but the prograde outliers are most likely part of the smooth halo. Theretrograde outliers have significant structure in the vφdirection and show a range of [α/Fe], with two having low[α/Fe] for their [Fe/H]. The fraction of substructure stars in oursample is between 5% and 7%. The methods presented in this paper can beused to exploit the kinematic information present in future largedatabases like RAVE, SDSS-II/SEGUE, and Gaia.
| A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Oxygen abundances in metal-poor subgiants as determined from [O I], O I and OH lines
The debate on the oxygen abundances of metal-poor stars has its originin contradictory results obtained using different abundance indicators.To achieve a better understanding of the problem we have acquired highquality spectra with the Ultraviolet and Visual Echelle Spectrograph atVLT, with a signal-to-noise of the order of 100 in the near ultravioletand 500 in the optical and near infrared wavelength range. Threedifferent oxygen abundance indicators, OH ultraviolet lines around 310.0nm, the [O i] line at 630.03 nm and the O i lines at 777.1-5 nm wereobserved in the spectra of 13 metal-poor subgiants with-3.0≤[Fe/H]≤-1.5. Oxygen abundances were obtained from theanalysis of these indicators which was carried out assuming localthermodynamic equilibrium and plane-parallel model atmospheres.Abundances derived from O i were corrected for departures from localthermodynamic equilibrium. Stellar parameters were computed usingT_eff-vs.-color calibrations based on the infrared flux method andBalmer line profiles, Hipparcos parallaxes and Fe II lines. [O/Fe]values derived from the forbidden line at 630.03 nm are consistent withan oxygen/iron ratio that varies linearly with [Fe/H] as[O/Fe]=-0.09(±0.08)[Fe/H]+0.36(±0.15). Values based on theO i triplet are on average 0.19±0.22 dex(s.d.) higher than thevalues based on the forbidden line while the agreement between OHultraviolet lines and the forbidden line is much better with a meandifference of the order of -0.09±0.25 dex(s.d.). In general, ourresults follow the same trend as previously published results with theexception of the ones based on OH ultraviolet lines. In that case ourresults lie below the values which gave rise to the oxygen abundancedebate for metal-poor stars.
| Li and Be depletion in metal-poor subgiants
A sample of metal-poor subgiants has been observed with the UVESspectrograph at the Very Large Telescope and abundances of Li and Behave been determined. Typical signal-to-noise per spectral bin valuesfor the co-added spectra are of the order of 500 for the ion{Li}{i} line(670.78 nm) and 100 for the ion{Be}{ii} doublet lines (313.04 nm). Thespectral analysis of the observations was carried out using the Uppsalasuite of codes and marcs (1D-LTE) model atmospheres with stellarparameters from photometry, parallaxes, isochrones and Fe ii lines.Abundance estimates of the light elements were corrected for departuresfrom local thermodynamic equilibrium in the line formation. Effectivetemperatures and Li abundances seem to be correlated and Be abundancescorrelate with [O/H]. Standard models predict Li and Be abundancesapproximately one order of magnitude lower than main-sequence valueswhich is in general agreement with the observations. On average, ourobserved depletions seem to be 0.1 dex smaller and between 0.2 and 0.4dex larger (depending on which reference is taken) than those predictedfor Li and Be, respectively. This is not surprising since the initial Liabundance, as derived from main-sequence stars on the Spite plateau, maybe systematically in error by 0.1 dex or more, and uncertainties in thespectrum normalisation and continuum drawing may affect our Beabundances systematically.
| 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.
| Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman
We develop and test a method for the estimation of metallicities([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhancedmetal-poor (CEMP) stars based on the application of artificial neuralnetworks, regressions, and synthesis models to medium-resolution (1-2Å) spectra and J-K colors. We calibrate this method by comparisonwith metallicities and carbon abundance determinations for 118 starswith available high-resolution analyses reported in the recentliterature. The neural network and regression approaches make use of apreviously defined set of line-strength indices quantifying the strengthof the Ca II K line and the CH G band, in conjunction with J-K colorsfrom the Two Micron All Sky Survey Point Source Catalog. The use ofnear-IR colors, as opposed to broadband B-V colors, is required becauseof the potentially large affect of strong molecular carbon bands onbluer color indices. We also explore the practicality of obtainingestimates of carbon abundances for metal-poor stars from the spectralinformation alone, i.e., without the additional information provided byphotometry, as many future samples of CEMP stars may lack such data. Wefind that although photometric information is required for theestimation of [Fe/H], it provides little improvement in our derivedestimates of [C/Fe], and hence, estimates of carbon-to-iron ratios basedsolely on line indices appear sufficiently accurate for most purposes.Although we find that the spectral synthesis approach yields the mostaccurate estimates of [C/Fe], in particular for the stars with thestrongest molecular bands, it is only marginally better than is obtainedfrom the line index approaches. Using these methods we are able toreproduce the previously measured [Fe/H] and [C/Fe] determinations withan accuracy of ~0.25 dex for stars in the metallicity interval-5.5<=[Fe/H]<=-1.0 and with 0.2<=(J-K)0<=0.8. Athigher metallicity, the Ca II K line begins to saturate, especially forthe cool stars in our program, and hence, this approach is not useful insome cases. As a first application, we estimate the abundances of [Fe/H]and [C/Fe] for the 56 stars identified as possibly carbon-rich, relativeto stars of similar metal abundance, in the sample of ``strong G-band''stars discussed by Beers, Preston, and Shectman.
| 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.
| Chemical abundances in 43 metal-poor stars
We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe,Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood,most of them subgiants or turn-off-point stars with iron abundances[Fe/H] ranging from -0.4 to -3.0. About half of this sample has not beenspectroscopically analysed in detail before. Effective temperatures wereestimated from uvby photometry, and surface gravities primarily fromHipparcos parallaxes. The analysis is differential relative to the Sun,and was carried out with plane-parallel MARCS models. Various sources oferror are discussed and found to contribute a total error of about0.1-0.2 dex for most elements, while relative abundances, such as[Ca/Fe], are most probably more accurate. For the oxygen abundances,determined in an NLTE analysis of the 7774 Å triplet lines, theerrors may be somewhat larger. We made a detailed comparison withsimilar studies and traced the reasons for the, in most cases,relatively small differences. Among the results we find that [O/Fe]possibly increases beyond [Fe/H] = -1.0, though considerably less sothan in results obtained by others from abundances based on OH lines. Wedid not trace any tendency toward strong overionization of iron, andfind the excesses, relative to Fe and the Sun, of the α elementsMg, Si, and Ca to be smaller than those of O. We discuss someindications that also the abundances of different α elementsrelative to Fe vary and the possibility that some of the scatter aroundthe trends in abundances relative to iron may be real. This may supportthe idea that the formation of Halo stars occurred in smaller systemswith different star formation rates. We verify the finding by Gratton etal. (2003b, A&A, 406, 131) that stars that do not participate in therotation of the galactic disk show a lower mean and larger spread in [α/Fe] than stars participating in the general rotation. The latterstars also seem to show some correlation between [ α/Fe] androtation speed. We trace some stars with peculiar abundances, amongthese two Ba stars, HD 17072 and HD196944, the second already known to be rich in s elements.Finally we advocate that a spectroscopic study of a larger sample ofhalo stars with well-defined selection criteria is very important, inorder to add to the very considerable efforts that various groups havealready made.
| 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.
| Galactic Evolution of Sr, Y, And Zr: A Multiplicity of Nucleosynthetic Processes
In this paper we follow the Galactic enrichment of three easily observedlight n-capture elements: Sr, Y, and Zr. Input stellar yields have beenfirst separated into their respective main and weak s-process componentsand r-process component. The s-process yields from asymptotic giantbranch (AGB) stars of low to intermediate mass are computed, exploring awide range of efficiencies of the major neutron source, 13C,and covering both disk and halo metallicities. AGB stars have been shownto reproduce the main s-component in the solar system, i.e., thes-process isotopic distribution of all heavy isotopes with atomic massnumber A>90, with a minor contribution to the light s-processisotopes up to A~90. The concurrent weak s-process, which accounts forthe major fraction of the light s-process isotopes in the solar systemand occurs in massive stars by the operation of the 22Neneutron source, is discussed in detail. Neither the main s- nor the weaks-components are shown to contribute significantly to theneutron-capture element abundances observed in unevolved halo stars.Knowing the s-process distribution at the epoch of the solar systemformation, we first employed the r-process residuals method to infer theisotopic distribution of the r-process. We assumed a primary r-processproduction in the Galaxy from moderately massive Type II supernovae thatbest reproduces the observational Galactic trend of metallicity versusEu, an almost pure r-process element. We present a detailed analysis ofa large published database of spectroscopic observations of Sr, Y, Zr,Ba, and Eu for Galactic stars at various metallicities, showing that theobserved trends versus metallicity can be understood in light of amultiplicity of stellar neutron-capture components. Spectroscopicobservations of the Sr, Y, and Zr to Ba and Eu abundance ratios versusmetallicity provide useful diagnostics of the types of neutron-captureprocesses forming Sr, Y, and Zr. In particular, the observed [Sr, Y,Zr/Ba, Eu] ratio is clearly not flat at low metallicities, as we wouldexpect if Ba, Eu and Sr, Y, Zr all had the same r-processnucleosynthetic origin. We discuss our chemical evolution predictions,taking into account the interplay between different processes to produceSr-Y-Zr. Making use of the very r-process-rich and very metal-poor starslike CS 22892-052 and CS 31082-001, we find hints and discuss thepossibility of a primary process in low-metallicity massive stars,different from the ``classical s-process'' and from the ``classicalr-process'' that we tentatively define LEPP (lighter element primaryprocess). This allows us to revise the estimates of the r-processcontributions to the solar Sr, Y, and Zr abundances, as well as of thecontribution to the s-only isotopes 86Sr, 87Sr,and 96Mo.
| 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.
| 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.
| Catalogue of [Fe/H] determinations for FGK stars: 2001 edition
The catalogue presented here is a compilation of published atmosphericparameters (Teff, log g, [Fe/H]) obtained from highresolution, high signal-to-noise spectroscopic observations. This newedition has changed compared to the five previous versions. It is nowrestricted to intermediate and low mass stars (F, G and K stars). Itcontains 6354 determinations of (Teff, log g, [Fe/H]) for3356 stars, including 909 stars in 79 stellar systems. The literature iscomplete between January 1980 and December 2000 and includes 378references. The catalogue is made up of two tables, one for field starsand one for stars in galactic associations, open and globular clustersand external galaxies. The catalogue is distributed through the CDSdatabase. Access to the catalogue with cross-identification to othersets of data is also possible with VizieR (Ochsenbein et al.\cite{och00}). The catalogue (Tables 1 and 2) is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/373/159 and VizieRhttp://vizier.u-strasbg.fr/.
| Galactic [O/Fe] and [C/Fe] Ratios: The Influence of New Stellar Parameters
We consider the effects of recent NLTE gravities and Fe abundances onstellar [O/Fe] and [C/Fe] ratios. The NLTE parameters greatly reduce oreliminate the well-known discrepancy between CH- and C I-based Cabundances in metal-poor stars and previously seen trends ofatomic-based [C/Fe] and [O/Fe] with Teff. With the NLTEparameters, the metal-poor molecular-based [C/Fe] ratio maintains itsincrease with declining [Fe/H] this may also be demonstrated by therevised atomic-based ratios. [O/Fe] values derived from OH and O Ifeatures are considerably reduced and typically show improved agreementbut are 0.1-0.2 dex larger than those exhibited by the Lick-Texassyndicate's recent [O I]-based giant determinations. The revised [O/Fe]ratios still show an increase down to at least [Fe/H]~-2 we suggest thatrecent field giant data show an increase with similar slope. Evenadopting uniform NLTE parameters, study-to-study abundance differencescan be significant; moreover, different NLTE studies yield differinggravities and Fe abundances even after taking Teffdifferences into account. Comparison of metal-poor giant gravities andcluster abundances with isochrones, trigonometric gravities, andnear-turnoff cluster abundances yields conflicting indications aboutwhether the evolved gravities might be underestimated as suggested formetal-poor dwarfs. Regardless, we argue that even extreme gravityrevisions do not affect the [O/Fe]-[Fe/H] relation derived from theextant results. Combining what we believe the most reliable giant anddwarf data considered here, we find[O/Fe]=-0.184(+/-0.022)×[Fe/H]+0.019 with an rms scatter of only0.13 dex; there is no indication of a break or slope change atintermediate [Fe/H]. The gentle slope is in very reasonable agreementwith some chemical evolution models employing yields with small mass andmetallicity dependences. Finally, two notes are made concerning Naabundance-spatial position and element-to-element correlations in M13giants.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Estimation of Stellar Metal Abundance. II. A Recalibration of the Ca II K Technique, and the Autocorrelation Function Method
We have recalibrated a method for the estimation of stellar metalabundance, parameterized as [Fe/H], based on medium-resolution (1-2Å) optical spectra (the majority of which cover the wavelengthrange 3700-4500 Å). The equivalent width of the Ca II K line (3933Å) as a function of [Fe/H] and broadband B-V color, as predictedfrom spectrum synthesis and model atmosphere calculations, is comparedwith observations of 551 stars with high-resolution abundances availablefrom the literature (a sevenfold increase in the number of calibrationstars that were previously available). A second method, based on theFourier autocorrelation function technique first described by Ratnatunga& Freeman, is used to provide an independent estimate of [Fe/H], ascalibrated by comparison with 405 standard-star abundances.Metallicities based on a combination of the two techniques for dwarfsand giants in the color range 0.30<=(B-V)_0<=1.2 exhibit anexternal 1 sigma scatter of approximately 0.10-0.20 dex over theabundance range -4.0<=[Fe/H]<=0.5. Particular attention has beengiven to the determination of abundance estimates at the metal-rich endof the calibration, where our previous attempt suffered from aconsiderable zero-point offset. Radial velocities, accurate toapproximately 10 km s^-1, are reported for all 551 calibration stars.
| Ca II H and K Photometry on the UVBY System. III. The Metallicity Calibration for the Red Giants
New photometry on the uvby Ca system is presented for over 300 stars.When combined with previous data, the sample is used to calibrate themetallicity dependence of the hk index for cooler, evolved stars. Themetallicity scale is based upon the standardized merger of spectroscopicabundances from 38 studies since 1983, providing an overlap of 122evolved stars with the photometric catalog. The hk index producesreliable abundances for stars in the [Fe/H] range from -0.8 to -3.4,losing sensitivity among cooler stars due to saturation effects athigher [Fe/H], as expected.
| Barium Abundances in Extremely Metal-poor Stars
New, improved, barium abundances for 33 extremely metal-poor halo starsfrom the 1995 sample of McWilliam et al. have been computed. The mean[Ba/Eu] ratio for stars with [Fe/H] <= -2.4 is -0.69 +/- 0.06 dex,consistent with pure r-process nucleosynthesis within the measurementuncertainties. Although the [Sr/Fe] and [Ba/Fe] abundance ratios span arange of 2.6 dex, the mean values are approximately constant with[Fe/H]. This is consistent with a model of chemical evolution in whichthe parent clouds were enriched by small numbers of supernova events. Inthis model, the decreasing heavy-element dispersion with increasing[Fe/H] is simply due to the averaging of element yields from manysupernovae at higher [Fe/H]; however, it is necessary to increase thenumber of extremely metal-poor stars known in order to confirm thispicture. In addition to the random Sr component from the r-process, the[Sr/Ba] ratios indicate that there is a second, also random, source ofSr from an as yet unidentified nucleosynthesis site.
| Early evolution of the Galactic halo revealed from Hipparcos observations of metal-poor stars
The kinematics of 122 red giant and 124 RR Lyrae stars in the solarneighborhood are studied using accurate measurements of their propermotions obtained by the Hipparcos astrometry satellite, combined withtheir published photometric distances, metal abundances, and radialvelocities. A majority of these sample stars have metal abundances of(Fe/H) = -1 or less and thus represent the old stellar populations inthe Galaxy. The halo component, with (Fe/H) = -1.6 or less, ischaracterized by a lack of systemic rotation and a radially elongatedvelocity ellipsoid. About 16 percent of such metal-poor stars have loworbital eccentricities, and we see no evidence of a correlation between(Fe/H) and e. Based on the model for the e-distribution of orbits, weshow that this fraction of low-e stars for (Fe/H) = -1.6 or less isexplained by the halo component alone, without introducing the extradisk component claimed by recent workers. This is also supported by theabsence of a significant change in the e-distribution with height fromthe Galactic plane. In the intermediate-metallicity range, we find thatstars with disklike kinematics have only modest effects on thedistributions of rotational velocities and e for the sample at absolutevalue of z less than 1 kpc. This disk component appears to constituteonly 10 percent for (Fe/H) between -1.6 and -1 and 20 percent for (Fe/H)between -1.4 and -1.
| The Abundance of CN. Calcium and Heavy Elements in High Velocity Stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114..825E&db_key=AST
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| HD 195636: A Metal-Poor Rotator Near the HB/AGB Transition
The color temperature and Balmer jump inferred from UBV andStrömgren photometric indices, the low gravity and lowmetallicity derived from spectrum analysis, and weakness of all CHmolecular lines combine to suggest that HD 195636 is in anevolutionary state near the transition between the horizontal branchand asymptotic giant branch. The projected equatorial rotationalvelocity of HD 195636 is Ve sin i=25 km/s, a value at least2.5 times greater than that expected if known blue horizontal branchaxial rotators in globular clusters conserve envelope angular momentumduring horizontal branch evolution. Constancy of the radial velocityon time scales of 3, 10, and 90 days indicates that the axial rotationis not due to co-rotation in a short-period binary. Therefore, itseems most plausible that transfer of angular momentum from core toenvelope occurred during HB evolution. The Balmer line profiles arepeculiar. H? is abnormally shallow, as if the core were filledby emission, and higher members of the Balmer series are significantlybroader than those of HD 200564, a slightly cooler giant star ofsimilar metallicity. The space velocity of HD 195636 calculated for anassumed RHB/AGB luminosity, Mv=-0.5, is 470 km/sretrograde, a high but not extraordinary value.
| New beryllium observations in low-metallicity stars.
We present observations of the Be II 313.0nm resonance doublet in 14halo and old disk stars with metallicities ranging from [Fe/H]=-0.4 to=~-3.0 obtained with the CASPEC spectrograph of the ESO 3.6m telescopeat a FWHM=~8.6km/s resolution. Abundances are derived by means of thesynthetic spectra technique employing Kurucz (1993) atmospheric models,with enhanced α-elements and no overshooting. The derivedabundances together with those available in literature show that for-2.7<[Fe/H]<-0.8 Be correlates linearly with iron[Be]{prop.to}1.07(+/-0.08)[Fe/H], giving strength to previous results.However, a steeper correlation is still possible at metallicities lowerthan [Fe/H]<-1.4 with [Be]{prop.to} 1.6(+/-0.44)[Fe/H]. When iron isreplaced with oxygen, Be is found tracking closely oxygen up to solarvalues, without signs of breaking in correspondence of the onset of theGalactic disk. No evidence of intrinsic dispersion is found, ought tothe large errors involved in the Be abundance determinations, but forthree stars (HD 106516, HD 3795, HD 211998) a significant upper limit inthe Be abundance can be placed at =~1dex below the mean trend of theBe-Fe relation. For such stars non conventional mixing is required toexplain Be depletion. Be observations can be used to discriminatestrongly Li-depleted stars. These are the stars which show less Li thanthat expected by high energy cosmic rays production as deduced from Beobservations. The available Be observations imply that some of the starswhich contribute to the scatter in the Li-Fe diagram are Li-depletedstars. This result strongly supports the use of the upper envelope ofthe Li-Fe diagram to trace the Li galactic evolution, and argues for alow value for the primordial Li against models which predict substantialLi depletion in halo and old disk stars.
| Spread of the lithium abundance in halo stars.
The observed scatter of the lithium abundance around a "plateau" isrevisited for three samples of stars for which the temperature has beendetermined from either the excitation temperature, or the dereddenedcolor (b-y)_o_ or the profile of the Halpha_wings. Systematicdifferences are noted between the three methods of temperaturedetermination. From sample to sample the rms observed scatter of thelithium abundance varies from 0.06dex to 0.08dex (to be compared to thevalue 0.13 previously found by Thorburn (1994), for another sample ofstars). We show that in all cases but one, this scatter is fullyexplained by the temperature and equivalent width errors. The intrinsicscatter, if real, is small.
| The first generations of stars
Up to a decade ago, searches for population III stars (i.e. withstrictly the chemical composition left by the Big Bang) had led to theresults that (1) no such star had been found, (2) stars withmetallicities significantly below [Fe/H] = -2.5 were exceedingly rare.Thanks to a major survey, undertaken by Beers, Preston and Shectman 18years ago, covering about 7500 square degrees in the sky, and down tomagnitude B =16.0, the situation has drastically changed. Theobservational limit towards the lowest metallicities is now about [Fe/H]= -4, i.e. 4 dex below the solar metallicity Zsolar = 0.02,(a level of pollution by supernova ejecta of only a few ppm), and over100 stars are known with metallicities [Fe/H] in the range -4 to -3. Thestudy of this sample, and of a few stars found more serendipitously, hasallowed a number of new conclusions: (i) The cosmological element7Li stays constant (prolongation of the Spite's plateau) downto the lowest metallicities, a great observational gift to the hot BigBang cosmology (ii) All heavier elements show a roughly linear increasewith the abundance of O (or even Fe if the metallicity is below [Fe/H] =-1), including the other light elements, Be and B. This last point hasled to a reappraisal of the current view that they were produced byspallation of interstellar nuclei by galactic cosmic rays, because therise of those elements with metallicity should then have been morequadratic than linear. An alternative new perspective is that theseelements are produced by spallation of the primary nuclei ejected by SNeii against protons of the interstellar medium. (iii) The ratio of thealpha elements (O, Si, Mg,...) to iron also stays constant down to thelowest metallicities, at about 3 times the solar value. (iv) Significantdeviations to a lockstep variation of the various elements within theiron-peak start to appear below [Fe/H] = -2.5. The strongest are adecrease of [Cr/Fe] and an increase of [Co/Fe] when [Fe/H] decreasesfrom -2.5 to -4.0. These trends are not explained by the current statusof explosive nucleosynthesis. (v) A great scatter of the abundances ofthe neutron capture elements relative to iron appears at very lowmetallicities. Similar scatter is seen for [Al/Fe]. A remarkable starwith [Fe/H] = -3.1, CS 22892-052, has been found, with a superb spectrumof the r-elements, involving over-abundances of those with respect toiron by factors ranging between 10 and 50. (vi) The kinematics of thevery metal-poor stars is similar to that of other halo stars, with acomplete lack of systemic rotation in an inertial frame, if not a smallamount of counter-rotation in the Galaxy. Evidence exists that thevelocity ellipsoid is radially elongated for stars within 10 kpc fromthe galactic center, whereas it is more spherical or even radiallycontracted at 20 kpc from the galactic center. (vii) The low metallicitystars were likely formed at an early cosmological epoch (z > 5 ifH0~ 65 km/s), before the Galaxy had developed a disk. The newviews concerning the sizes of the Ly? clouds open the possibilitythat the low-metallicity Ly? systems are large halos having theright metallicity for being protogalaxies, just forming early stellargenerations. (viii) One may wonder why, if more than 100 stars are knownwith metallicities between [Fe/H] = -4 to -3 no pop. III has been found,or even not one star near [Fe/H] = -5. Different kinds of explanationshave been proposed, with none conclusive at present. Either we havealready observed a pop. III star, but its pristine Big Bang compositionhas been corrupted by a small amount of interstellar matter accretedduring its 10 Gyr of orbiting in an already-enriched gas, or thecollective process of star formation has polluted the medium before ithas produced the low-mass stars we can still observe now, or, simpler,pop. III stars exist, but are sufficiently rare that we have not yetobserved a volume large enough to have found one.
| Lithium in Short-Period Tidally Locked Binaries: A Test of Rotationally Induced Mixing
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...453..819R&db_key=AST
| Spectroscopic analyses of metal-poor stars. II. The evolutionary stage of subdwarfs.
Models of post-main sequence stellar evolution of VandenBerg & Bell(???) have been applied to determine spectroscopic masses and distancesfor metal-poor stars. Careful consideration of the most important errorsources published in more recent papers such as VandenBerg (???) for thefirst time allow us to draw firm statistical conclusions. It is shownthat the evolutionary calculations qualitatively fit to the observedstellar parameters whereas quantitatively they predict too high ages formetal-poor stars. As an important result we confirm that evolutionarysequences need to be calibrated with respect to their metal abundance inorder to use their absolute predictions of temperature and luminosity.It turns out that this can be achieved by a simple shift of theevolutionary tracks and isochrones in effective temperature with values{DELTA}log T_eff_<~0.03 which accounts for possible changes of themixing-length and the O/Fe ratio with metallicity. The stellarluminosities and surface gravities obtained from evolutionary models aremuch more reliable than their effective temperatures. Therefore weconclude that the accuracy of the corresponding spectroscopic stellargravities is systematically affected by deviations from LTE, inparticular along the subgiant sequence where systematic errors less than{DELTA}log g =~0.3 must be ascribed to the non-LTE ionizationequilibrium of Fe II/Fe I. In our spectroscopic analyses the strongdependence between surface gravity and abundances determined from Fe Ilines restricts the accuracy of Fe abundances in subgiants to 0.1 dex atbest. The most remarkable result of our evolutionary and kinematicinvestigations of halo stars refers to the large fraction of slightlyevolved subgiants among the so-called subdwarfs. Since conventionalphotometric approaches often assume that the great majority ofmetal-poor stars are dwarfs this results in distances that aresystematically too low for their samples. Consequently, significantdifferences are found when comparing evolutionary and kinematicparameters obtained from either photometric or spectroscopic data. Wedemonstrate this by comparing the space velocities of the stars. Itappears that stars with particularly high space velocities derived fromspectroscopic distances show very often much lower velocities based ontheir main sequence parallaxes. We find that results refering to mainsequence parallaxes are doubtful and can be used only with greatestcare. An advantageous side-effect of the application of spectroscopicdata to evolutionary calculations is the possibility to identify binarysystems that are either standing out from the Toomre diagram with theirunusually high space velocities, or from a log g - log T_eff_ diagramwith apparently contradictory luminosities.
| Beryllium and Boron Abundances of Metal-deficient Halo Stars and Accretion of Interstellar Matter
We discuss the correlation of light-element abundances with metallicityfor metal-deficient dwarfs in the Galactic halo. We show that such starscan experience some metal enrichment after their formation because ofaccreting interstellar material during repeated encounters withmolecular clouds in the Galactic plane. If light-element abundances areproduced by the secondary spallation of preexisting heavy elements bycosmic rays, then this accretion affects the light-element abundancesand metal abundances differently. We construct an analytic chemicalevolution model for the halo and disk which reproduces the observedabundance correlations and the halo metallicity distribution. We showthat the introduction of interstellar accretion leads to a lowmetallicity plateau in the correlation of light elements withmetallicity which would mimic the formation of such elements in the bigbang. We suggest here that the observation of a constant light-elementabundance at low metallicity may not be a signature of primordial originbut rather a measure of the average accretion rate from metal-enrichedgas in the Galactic plane.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Индеец |
| Прямое восхождение: | 21h06m34.75s |
| Склонение: | -49°57'50.3" |
| Видимая звёздная величина: | 9.097 |
| Собственное движение RA: | 192.4 |
| Собственное движение Dec: | -274.1 |
| B-T magnitude: | 9.797 |
| V-T magnitude: | 9.155 |
Каталоги и обозначения:
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