Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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
| Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
Aims: Precise abundance ratios are determined for 94 dwarf starswith Teff K, -1.6 < [Fe/H] < -0.4, and distances D? 335 pc. Most of them have halo kinematics, but 16 thick-disk starsare included. Methods: Equivalent widths of atomic lines aremeasured from VLT/UVES and NOT/FIES spectra with resolutions R? 55000 and R ? 40 000, respectively. An LTE abundance analysis basedon MARCS models is applied to derive precise differential abundanceratios of Na, Mg, Si, Ca, Ti, Cr, and Ni with respect to Fe. Results: The halo stars fall into two populations, clearly separated in[?/Fe], where ? refers to the average abundance of Mg, Si,Ca, and Ti. Differences in [Na/Fe] and [Ni/Fe] are also present with aremarkably clear correlation between these two abundance ratios. Conclusions: The “high-?” stars may be ancient disk orbulge stars “heated” to halo kinematics by merging satellitegalaxies or they could have formed as the first stars during thecollapse of a proto-Galactic gas cloud. The kinematics of the“low-?” stars suggest that they have been accretedfrom dwarf galaxies, and that some of them may originate from the? Cen progenitor galaxy.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility.Tables 3 and 4 are also available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/L10Figures5-8 and Tables 1-4 are only available in electronic form at http://www.aanda.org
| 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.
| 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 and star formation in the halo and in the thick disk
Context: Beryllium is a pure product of cosmic ray spallation. Thisimplies a relatively simple evolution in time of the beryllium abundanceand suggests its use as a time-like observable. Aims: Our goal is toderive abundances of Be in a sample of 90 stars, the largest sample ofhalo and thick disk stars analyzed to date. We study the evolution of Bein the early Galaxy and its dependence on kinematic and orbitalparameters, and investigate its use as a cosmochronometer. Abundances ofBe, Fe, and α-elements of 73 stars are employed to study theformation of the halo and the thick disk of the Galaxy. Methods:Beryllium abundances are determined from high-resolution, highsignal-to-noise UVES spectra with spectrum synthesis. Atmosphericparameters and abundances of α-elements are adopted from theliterature. Lithium abundances are used to eliminate mixed stars fromthe sample. The properties of halo and thick disk stars are investigatedin diagrams of log(Be/H) vs. [ α/H] , log(Be/H) vs. [Fe/H], and [α/Fe] vs. log(Be/H) and with orbital and kinematic parameters. Results: We present our observational results in various diagrams. (i)In a log(Be/H) vs. [Fe/H] diagram we find a marginal statisticaldetection of a real scatter, above what is expected from measurementerrors, with a larger scatter among halo stars. The detection of thescatter is further supported by the existence of pairs of stars withidentical atmospheric parameters and different Be abundances; (ii) in alog(Be/H) vs. [ α/Fe] diagram, the halo stars separate into twocomponents; one is consistent with predictions of evolutionary models,while the other has too high α and Be abundances and is chemicallyindistinguishable from thick disk stars. This suggests that the halo isnot a single uniform population where a clear age-metallicity relationcan be defined; (iii) In diagrams of R_min vs. [ α/Fe] andlog(Be/H), the thick disk stars show a possible decrease in [α/Fe] with R_min, whereas no dependence of Be with R_min is seen.This anticorrelation suggests that the star formation rate was lower inthe outer regions of the thick disk, pointing towards an inside-outformation. The lack of correlation for Be indicates that it isinsensitive to the local conditions of star formation.Based on observations made with ESO VLT, at Paranal Observatory, underprograms 076.B-0133 and 077.B-0507, and on data obtained from theESO/ST-ECF Science Archive Facility and the UVES Paranal ObservatoryProject 266.D-5655.Tables 1-3, 6 and Appendices A-C are only available in electronic format http://www.aanda.org
| 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
| Searching for the metal-weak thick disc in the solar neighbourhood
An abundance analysis is presented of 60 metal-poor stars drawn fromcatalogues of nearby stars provided by Arifyanto et al. and Schuster etal. In an attempt to isolate a sample of metal-weak thick disc stars, weapplied the kinematic criteria Vrot >=100kms-1,|ULSR| <= 140kms-1 (LSR -local standard of rest) and |WLSR| <= 100kms-1.14 stars satisfying these criteria and having [Fe/H] <= -1.0 areincluded in the sample of 60 stars. Eight of the 14 have [Fe/H] >=-1.3 and may be simply thick disc stars of slightly lower than average[Fe/H]. The other six have [Fe/H] from -1.3 to -2.3 and are eithermetal-weak thick disc stars or halo stars with kinematics mimickingthose of the thick disc. The sample of 60 stars is completed by eightthick disc stars, 20 stars of a hybrid nature (halo or thick disc stars)and 18 stars with kinematics distinctive of the halo.
| Mapping the Local Galactic Halo. I. Optical Photometry of Cool Subdwarf Candidates
Optical (BVRI) photometric measurements of a sample of 564 candidatecool subdwarfs in the nearby halo are presented. The stars generallyspan the color range 0.4<(B-V)<1.8 hence, the sample is composedof early F- through early M-type stars on the subdwarf sequence. Thesample is selected from the revised NLTT catalog of Gould and Salim andSalim and Gould via a reduced proper motion diagram. The photometry isprecise and accurate; in particular, for stars with 9
| 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
| Oxygen abundances in nearby stars. Clues to the formation and evolution of the Galactic disk
The abundances of iron and oxygen are homogeneously determined in asample of 523 nearby (d<150 pc) FGK disk and halo stars withmetallicities in the range -1.5<[Fe/H]<0.5. Iron abundances wereobtained from an LTE analysis of a large set of Fe I and Fe II lineswith reliable atomic data. Oxygen abundances were inferred from arestricted non-LTE analysis of the 777 nm O I triplet. We adopted theinfrared flux method temperature scale and surface gravities based onHipparcos trigonometric parallaxes. Within this framework, theionization balance of iron lines is not satisfied: the mean abundancesfrom the Fe I lines are systematically lower by 0.06 dex than those fromthe Fe II lines for dwarf stars of Teff>5500 K and[Fe/H]<0.0, and giant stars of all temperatures and metallicitiescovered by our sample. The discrepancy worsens for cooler and metal-richmain-sequence stars. We use the stellar kinematics to compute theprobabilities of our sample stars to be members of the thin disk, thickdisk, or halo of the Galaxy. We find that the majority of thekinematically-selected thick-disk stars show larger [O/Fe] ratioscompared to thin-disk stars while the rest show thin-disk abundances,which suggests that the latter are thin-disk members with unusual(hotter) kinematics. A close examination of this pattern for disk starswith ambiguous probabilities shows that an intermediate population withproperties between those of the thin and thick disks does not exist, atleast in the solar neighborhood. Excluding the stars with unusualkinematics, we find that thick-disk stars show slowly decreasing [O/Fe]ratios from about 0.5 to 0.4 in the -0.8<[Fe/H]<-0.3 range. Usinga simple model for the chemical evolution of the thick disk we show thatthis trend results directly from the metallicity dependence of the TypeII supernova yields. At [Fe/H]>-0.3, we find no obvious indication ofa sudden decrease (i.e., a "knee") in the [O/Fe] vs. [Fe/H] pattern ofthick-disk stars that would connect the thick and thin disk trends at ahigh metallicity. We conclude that Type Ia supernovae (SN Ia) did notcontribute significantly to the chemical enrichment of the thick disk.In the -0.8<[Fe/H]<+0.3 range, thin-disk stars show decreasing[O/Fe] ratios from about 0.4 to 0.0 that require a SN Ia contribution.The implications of these results for studies of the formation andevolution of the Galactic disk are discussed.Tables 4-6 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/465/271 Partially based onobservations obtained with the Hobby-Eberly Telescope, which is a jointproject of the University of Texas at Austin, the Pennsylvania StateUniversity, Stanford University, Ludwig-Maximilians-UniversitätMünchen, and Georg-August-Universität Göttingen; and datafrom the UVES Paranal Observatory Project (ESO DDT Program ID266.D-5655).
| 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.
| 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.
| Medium resolution spectroscopy in ω Centauri: abundances of 400 subgiant and turn-off region stars
Medium resolution spectra of more than 400 subgiant and turn-off regionstars in ω Centauri were analysed. The observations were performedat the VLT/Paranal with FORS2/MXU. In order to determine themetallicities of the sample stars, we defined a set of line indices(mostly iron) adjusted to the resolution of our spectra. The indices asdetermined for ω Cen were then compared to line indices from starsin the chemically homogeneous globular cluster M 55, in addition tostandard stars and synthetic spectra. The uncertainties in the derivedmetallicities are of the order of ±0.2 {dex}. Our study confirmsthe large variations in iron abundances found on the giant branch inearlier studies (-2.2 <[Fe/H]< -0.7 dex). In addition, we studiedthe α-element and CN/CH abundances. Stars of different metallicitygroups not only show distinct ages (Hilker et al. 2004, A&A, 422,L9), but also different behaviours in their relative abundances. Theα abundances increase smoothly with increasing metallicityresulting in a flat [ α/Fe] ratio over the whole observedmetallicity range. The combined CN+CH abundance increases smoothly withincreasing iron abundance. The most metal-rich stars are CN-enriched. Ina CN vs. CH plot, though, the individual abundances divide into CN- andCH-rich branches. The large abundance variations observed in our sampleof (unevolved) subgiant branch stars most probably have their origin inthe pre-enriched material rather than in internal mixing effects.Together with the age spread of the different sub-populations, ourfindings favour the formation of ω Centauri within a more massiveprogenitor.
| A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. I. Testing Giant Planet Formation and Migration Scenarios
We describe a high-precision Doppler search for giant planets orbiting awell-defined sample of metal-poor dwarfs in the field. This experimentconstitutes a fundamental test of theoretical predictions, which willhelp discriminate between proposed giant planet formation and migrationmodels. We present details of the survey, as well as an overallassessment of the quality of our measurements, making use of the resultsfor stars that show no significant velocity variation.
| 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.
| Elemental abundance survey of the Galactic thick disc
We have performed an abundance analysis for F- and G- dwarfs of theGalactic thick-disc component. A sample of 176 nearby (d<= 150pc)thick-disc candidate stars was chosen from the Hipparcos catalogue andsubjected to a high-resolution spectroscopic analysis. Using accurateradial velocities combined with the Hipparcos astrometry, kinematics (U,V and W) and Galactic orbital parameters were computed. We estimate theprobability for a star to belong to the thin disc, the thick disc or thehalo. With a probability P>= 70 per cent taken as certain membership,we assigned 95 stars to the thick disc, 13 to the thin disc, and 20 tothe halo. The remaining 48 stars in the sample cannot be assigned withreasonable certainty to one of the three components.Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,Cu, Zn, Y, Ba, Ce, Nd and Eu have been obtained. The abundances for thethick-disc stars are compared with those for the thin-disc members fromReddy et al. The ratios of α-elements (O, Mg, Si, Ca and Ti) toiron for thick-disc stars show a clear enhancement compared to thin-discmembers in the range -0.3 < [Fe/H] < -1.2. There are also otherelements - Al, Sc, V, Co, and possibly Zn - which show enhanced ratiosto iron in the thick disc relative to the thin disc. The abundances ofNa, Cr, Mn, Ni and Cu (relative to Fe) are very similar for thin- andthick-disc stars. The dispersion in abundance ratios [X/Fe] at given[Fe/H] for thick-disc stars is consistent with the expected scatter dueto measurement errors, suggesting a lack of `cosmic' scatter.A few stars classified as members of the thick disc by our kinematiccriteria show thin-disc abundances. These stars, which appear older thanmost thin-disc stars, are also, on average, younger than the thick-discpopulation. They may have originated early in the thin-disc history, andbeen subsequently scattered to hotter orbits by collisions. The thickdisc may not include stars with [Fe/H] > -0.3. The observedcompositions of the thin and thick discs seem to be consistent with themodels of galaxy formation by hierarchical clustering in a Lambda colddark matter (ΛCDM) universe.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| Analysis of medium resolution spectra by automated methods - Application to M 55 and ω Centauri
We have employed feedforward neural networks trained on syntheticspectra in the range 3800 to 5600 Å with resolutions of ~2-3Å to determine metallicities from spectra of about 1000main-sequence turn-off, subgiant and red giant stars in the globularclusters M 55 and ω Cen. The overall metallicity accuracies are ofthe order of 0.15 to 0.2 dex. In addition, we tested how well thestellar parameters log g and T_eff can be retrieved from such datawithout additional colour or photometric information. We find overalluncertainties of 0.3 to 0.4 dex for log g and 140 to 190 K for T_eff. Inorder to obtain some measure of uncertainty for the determined values of[Fe/H], log g and T_eff, we applied the bootstrap method for the firsttime to neural networks for this kind of parametrization problem. Thedistribution of metallicities for stars in ω Cen clearly shows alarge spread in agreement with the well known multiple stellarpopulations in this cluster.
| 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.
| Binary statistics among population II stars
Population II stars are old, metal-poor, Galactic halo stars with highproper motion. We have carried out a visual binary survey of 164 halostars in the solar neighborhood (median distance 100 pc), using infraredspeckle interferometry, adaptive optics, and wide field direct imaging.The sample is based on the lists of Population II stars of Carney et al.(1994) and Norris (1986), with reliable distances from HIPPARCOSmeasurements.At face value, we found 33 binaries, 6 triples, and 1 quadruple system.When we limit ourselves to K-band flux ratios larger than 0.1 (to avoidbackground contamination), the numbers drop to 9 binaries and 1 triple,corresponding to a binary frequency of 6 - 7 % above our angularresolution limit of about 0.1 arcsec. If we count all systems withK-band flux ratios greater than 0.01, we obtain 15 more binaries and 3more triples, corresponding to a binary frequency for projectedseparations in excess of 10 AU of around 20 %. This is to be comparedwith the frequency of spectroscopic binaries (up to a period of 3000days) of Population II stars of about 15 % (Latham et al. 2002). We alsodetermined a semi-major axis distribution for our visual Population IIbinary and triple systems, which appears to be remarkably different fromthat of Population I stars. Second epoch-observations must help confirmthe reality of our results.
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/419/167
| Sulphur and zinc abundances in Galactic stars and damped Lyα systems
High resolution spectra of 34 halo population dwarf and subgiant starshave been obtained with VLT/UVES and used to derive sulphur abundancesfrom the λ λ 8694.0, 8694.6 and λ λ 9212.9,9237.5 S I lines. In addition, iron abundances have been determined from19 Fe II lines and zinc abundances from the λ λ 4722.2,4810.5 lines. The abundances are based on a classical 1D, LTE modelatmosphere analysis, but effects of 3D hydrodynamical modelling on the[S/Fe], [Zn/Fe] and [S/Zn] ratios are shown to be small. We find thatmost halo stars with metallicities in the range -3.2 < [Fe/H] <-0.8 have a near-constant [S/Fe] ≃ +0.3; a least square fit to[S/Fe] vs. [Fe/H] shows a slope of only -0.04 ± 0.01. Among halostars with -1.2 < [Fe/H] < -0.8 the majority have [S/Fe] ≃+0.3, but two stars (previously shown to have low α/Fe ratios)have [S/Fe] ≃ 0.0. For disk stars with [Fe/H] > -1, [S/Fe]decreases with increasing [Fe/H] . Hence, sulphur behaves like othertypical α-capture elements, Mg, Si and Ca. Zinc, on the otherhand, traces iron over three orders of magnitude in [Fe/H], althoughthere is some evidence for a small systematic Zn overabundance ([Zn/Fe]≃ +0.1) among metal-poor disk stars and for halo stars with [Fe/H]< -2.0. Recent measurements of S and Zn in ten damped Lyαsystems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances inthe range -2.1 < [Zn/H] < -0.15 show an offset relative to the[S/Zn] - [Zn/H] relation in Galactic stars. Possible reasons for thisoffset are discussed, including low and intermittent star formationrates in DLAs.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).Table A1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/993
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| The evolution of the C/O ratio in metal-poor halo stars
We report new measurements of carbon and oxygen abundances in 34 F and Gdwarf and subgiant stars belonging to the halo population and spanning arange of metallicity from [Fe/H] = -0.7 to -3.2 . The survey is based onobservations of four permitted lines of C I near 9100 Å and the OI,λ 7774 triplet, all recorded at high signal-to-noise ratioswith the UVES echelle spectrograph on the ESO VLT. The line equivalentwidths were analysed with the 1D, LTE, MARCS model atmosphere code todeduce C and O abundances; corrections due to non-LTE and 3D effects arediscussed. When combined with similar published data for disk stars, ourresults confirm the metallicity dependence of the C/O ratio known fromprevious stellar and interstellar studies: C/O drops by a factor of3-4 as O/H decreases from solar to 1/10 solar. Analysed withinthe context of standard models for the chemical evolution of the solarvicinity, this drop results from the metallicity dependence of the Cyields from massive stars with mass loss, augmented by the delayedrelease of C from stars of low and intermediate mass. The former is,however, always the dominant factor. Our survey has also uncoveredtentative evidence to suggest that, as the oxygen abundance decreasesbelow [O/H] = -1, [C/O] may not remain constant at [C/O] = -0.5, aspreviously thought, but increase again, possibly approaching near-solarvalues at the lowest metallicities ([O/H] ≲ -3). With the currentdataset this is no more than a 3σ effect and it may be due tometallicity-dependent non-LTE corrections to the [C/O] ratio which havenot been taken into account. However, its potential importance as awindow on the nucleosynthesis by Population III stars is a strongincentive for future work, both observational and theoretical, to verifyits reality.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).
| High-Precision Near-Infrared Photometry of a Large Sample of Bright Stars Visible from the Northern Hemisphere
We present the results of 8 yr of infrared photometric monitoring of alarge sample of stars visible from Teide Observatory (Tenerife, CanaryIslands). The final archive is made up of 10,949 photometric measuresthrough a standard InSb single-channel photometer system, principally inJHK, although some stars have measures in L'. The core of this list ofstars is the standard-star list developed for the Carlos SánchezTelescope. A total of 298 stars have been observed on at least twooccasions on a system carefully linked to the zero point defined byVega. We present high-precision photometry for these stars. The medianuncertainty in magnitude for stars with a minimum of four observationsand thus reliable statistics ranges from 0.0038 mag in J to 0.0033 magin K. Many of these stars are faint enough to be observable with arraydetectors (42 are K>8) and thus to permit a linkage of the bright andfaint infrared photometric systems. We also present photometry of anadditional 25 stars for which the original measures are no longeravailable, plus photometry in L' and/or M of 36 stars from the mainlist. We calculate the mean infrared colors of main-sequence stars fromA0 V to K5 V and show that the locus of the H-K color is linearlycorrelated with J-H. The rms dispersion in the correlation between J-Hand H-K is 0.0073 mag. We use the relationship to interpolate colors forall subclasses from A0 V to K5 V. We find that K and M main-sequence andgiant stars can be separated on the color-color diagram withhigh-precision near-infrared photometry and thus that photometry canallow us to identify potential mistakes in luminosity classclassification.
| Abundances for metal-poor stars with accurate parallaxes. I. Basic data
We present element-to-element abundance ratios measured from highdispersion spectra for 150 field subdwarfs and early subgiants withaccurate Hipparcos parallaxes (errors <20%). For 50 stars new spectrawere obtained with the UVES on Kueyen (VLT UT2), the McDonald 2.7 mtelescope, and SARG at TNG. Additionally, literature equivalent widthswere taken from the works by Nissen & Schuster, Fulbright, andProchaska et al. to complement our data. The whole sample includes boththick disk and halo stars (and a few thin disk stars); most stars havemetallicities in the range -2<[Fe/H]<-0.6. We found our data, thatof Nissen & Schuster, and that of Prochaska to be of comparablequality; results from Fulbright scatter a bit more, but they are stillof very good quality and are extremely useful due to the large size ofhis sample. The results of the present analysis will be used inforthcoming papers to discuss the chemical properties of thedissipational collapse and accretion components of our Galaxy.Based in part on data collected at the European Southern Observatory,Chile, at the MacDonald Observatory, Texas, USA, and at the TelescopioNazionale Galileo, Canary Island, INAF,Italy-Spain.}\fnmsep\thanks{Table 1 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia\resizebox{8.8cm}{2.2mm}htpp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/187}
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries
We report 25,563 radial velocity measurements for 1359 single-linedstars in the Carney-Latham sample of 1464 stars selected for high propermotion. For 171 of these, we present spectroscopic orbital solutions. Wefind no obvious difference between the binary characteristics in thehalo and the disk populations. The observed frequency is the same, andthe period distributions are consistent with the hypothesis that the twosets of binaries were drawn from the same parent population. Thissuggests that metallicity in general, and radiative opacities inparticular, have little influence over the fragmentation process thatleads to short-period binaries. All the binaries with periods shorterthan 10 days have nearly circular orbits, while the binaries withperiods longer than 20 days exhibit a wide range of eccentricities and amedian value of 0.37. For the metal-poor high-velocity halo binaries inour sample, the transition from circular to eccentric orbits appears tooccur at about 20 days, supporting the conclusion that tidalcircularization on the main sequence is important for the oldestbinaries in the Galaxy. Some of the results presented here usedobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Орао |
| Ректацензија: | 19h13m20.72s |
| Deклинација: | -00°35'42.3" |
| Apparent магнитуда: | 9.253 |
| Proper motion RA: | -315.4 |
| Proper motion Dec: | -448.2 |
| B-T magnitude: | 9.796 |
| V-T magnitude: | 9.298 |
Каталог и designations:
|
