내용
사진
사진 업로드
DSS Images Other Images
관련 글
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.
| Speckle interferometry of metal-poor stars in the solar neighborhood. I
We report the results of speckle-interferometric observations of 109high proper-motion metalpoor stars made with the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences. Weresolve eight objects—G102-20, G191-55, BD+19° 1185A, G89-14,G87-45, G87-47, G111-38, and G114-25—into individual componentsand we are the first to astrometrically resolve seven of these stars.New resolved systems included two triple (G111-38, G87-47) and onequadruple (G89-14) star. The ratio ofsingle-to-binary-to-triple-to-quadruple systems among the stars of oursample is equal to 71:28:6:1.
| 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
| Measuring the Balmer Jump and the Effective Gravity in FGK Stars
It is difficult to accurately measure the effective gravity (logg) inlate-type stars using broadband (e.g., UBV or SDSS) or intermediate-band(uvby) photometric systems, especially when the stars can cover a rangeof metallicities and reddenings. However, simple spectroscopicobservational and data reduction techniques can yield accurate valuesfor logg through comparison of the Balmer jumps of low-resolutionspectra with recent grids of synthetic flux spectra.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The non-LTE line formation of neutral carbon in late-type stars
Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE)line formation of neutral carbon in late-type stars in order to removesome of the potential systematic errors in stellar abundance analysesemploying C i features. Methods: .The statistical equilibrium codeMULTI was used on a grid of plane-parallel 1D MARCS atmosphericmodels. Results: .Within the parameter space explored, thehigh-excitation C i lines studied are stronger in non-LTE due to thecombined effect of line source function drop and increased line opacitydue to overpopulation of the lower level for the transitions considered;the relative importance of the two effects depends on the particularcombination of T{eff}, log g, [Fe/H] and [C/Fe] and on theanalysed C i line. As a consequence, the non-LTE abundance correctionsare negative and can be substantially so, for example -0.4 dex inhalo turn-off stars at [Fe/H] -3. The magnitude of the non-LTEcorrections is rather insensitive to whether inelastic H collisions areincluded or not. Conclusions: .Our results have implications onstudies of nucleosynthetic processes and on Galactic chemical evolutionmodels. When applying our calculated corrections to recent observationaldata, the upturn in [C/O] at low metallicity might still be present(thus apparently still necessitating contributions from massive Pop. IIIstars for the carbon production), but at a lower level and possibly witha rather shallow trend of -0.2 dex/dex below [O/H] -1.
| Permitted Oxygen Abundances and the Temperature Scale of Metal-poor Turnoff Stars
We use high-quality VLT/UVES published data of the permitted O I tripletand Fe II lines to determine oxygen and iron abundances in unevolved(dwarfs, turnoff, subgiants) metal-poor halo stars. The calculationshave been performed both in LTE and non-LTE (NLTE), employing effectivetemperatures obtained with the new infrared flux method (IRFM)temperature scale by Ramírez & Meléndez, and surfacegravities from Hipparcos parallaxes and theoretical isochrones. A newlist of accurate transition probabilities for Fe II lines, tied to theabsolute scale defined by laboratory measurements, has been used.Interstellar absorption has been carefully taken into account byemploying reddening maps, stellar energy distributions andStrömgren photometry. We find a plateau in the oxygen-to-iron ratioover more than 2 orders of magnitude in iron abundance(-3.2<[Fe/H]<-0.7), with a mean [O/Fe]=0.5 dex (σ=0.1 dex),independent of metallicity, temperature, and surface gravity. The flat[O/Fe] ratio is mainly due to the use of adequate NLTE corrections andthe new IRFM temperature scale, which, for metal-poor F/early G dwarfsis hotter than most Teff scales used in previous studies ofthe O I triplet. According to the new IRFM Teff scale, thetemperatures of turnoff halo stars strongly depend on metallicity, aresult that is in excellent qualitative and quantitative agreement withstellar evolution calculations, which predict that the Teffof the turnoff at [Fe/H]=-3 is about 600-700 K higher than that at[Fe/H]=-1. Recent determinations of Hα temperatures in turnoffstars are in excellent relative agreement with the new IRFMTeff scale in the metallicity range -2.7<[Fe/H]<-1,with a zero-point difference of only 61 K.
| The lithium content of the Galactic Halo stars
Thanks to the accurate determination of the baryon density of theuniverse by the recent cosmic microwave background experiments, updatedpredictions of the standard model of Big Bang nucleosynthesis now yieldthe initial abundance of the primordial light elements withunprecedented precision. In the case of ^7Li, the CMB+SBBN value issignificantly higher than the generally reported abundances for Pop IIstars along the so-called Spite plateau. In view of the crucialimportance of this disagreement, which has cosmological, galactic andstellar implications, we decided to tackle the most critical issues ofthe problem by revisiting a large sample of literature Li data in halostars that we assembled following some strict selection criteria on thequality of the original analyses. In the first part of the paper wefocus on the systematic uncertainties affecting the determination of theLi abundances, one of our main goal being to look for the "highestobservational accuracy achievable" for one of the largest sets of Liabundances ever assembled. We explore in great detail the temperaturescale issue with a special emphasis on reddening. We derive four sets ofeffective temperatures by applying the same colour {T}_eff calibrationbut making four different assumptions about reddening and determine theLTE lithium values for each of them. We compute the NLTE corrections andapply them to the LTE lithium abundances. We then focus on our "best"(i.e. most consistent) set of temperatures in order to discuss theinferred mean Li value and dispersion in several {T}_eff and metallicityintervals. The resulting mean Li values along the plateau for [Fe/H]≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075when the lowest effective temperature considered is taken equal to 5700K and 6000 K respectively. This is a factor of 2.48 to 2.81 (dependingon the adopted SBBN model and on the effective temperature range chosento delimit the plateau) lower than the CMB+SBBN determination. We findno evidence of intrinsic dispersion. Assuming the correctness of theCMB+SBBN prediction, we are then left with the conclusion that the Liabundance along the plateau is not the pristine one, but that halo starshave undergone surface depletion during their evolution. In the secondpart of the paper we further dissect our sample in search of newconstraints on Li depletion in halo stars. By means of the Hipparcosparallaxes, we derive the evolutionary status of each of our samplestars, and re-discuss our derived Li abundances. A very surprisingresult emerges for the first time from this examination. Namely, themean Li value as well as the dispersion appear to be lower (althoughfully compatible within the errors) for the dwarfs than for the turnoffand subgiant stars. For our most homogeneous dwarfs-only sample with[Fe/H] ≤ 1.5, the mean Li abundances are A(L)_NLTE = 2.177±0.071 and 2.215±0.074 when the lowest effective temperatureconsidered is taken equal to 5700 K and 6000 K respectively. This is afactor of 2.52 to 3.06 (depending on the selected range in {T}_eff forthe plateau and on the SBBN predictions we compare to) lower than theCMB+SBBN primordial value. Instead, for the post-main sequence stars thecorresponding values are 2.260±0.1 and 2.235±0.077, whichcorrespond to a depletion factor of 2.28 to 2.52. These results,together with the finding that all the stars with Li abnormalities(strong deficiency or high content) lie on or originate from the hotside of the plateau, lead us to suggest that the most massive of thehalo stars have had a slightly different Li history than their lessmassive contemporaries. In turn, this puts strong new constraints on thepossible depletion mechanisms and reinforces Li as a stellartomographer.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/419/167
| Sulphur and zinc abundances in Galactic stars and damped Lyα systems
High resolution spectra of 34 halo population dwarf and subgiant starshave been obtained with VLT/UVES and used to derive sulphur abundancesfrom the λ λ 8694.0, 8694.6 and λ λ 9212.9,9237.5 S I lines. In addition, iron abundances have been determined from19 Fe II lines and zinc abundances from the λ λ 4722.2,4810.5 lines. The abundances are based on a classical 1D, LTE modelatmosphere analysis, but effects of 3D hydrodynamical modelling on the[S/Fe], [Zn/Fe] and [S/Zn] ratios are shown to be small. We find thatmost halo stars with metallicities in the range -3.2 < [Fe/H] <-0.8 have a near-constant [S/Fe] ≃ +0.3; a least square fit to[S/Fe] vs. [Fe/H] shows a slope of only -0.04 ± 0.01. Among halostars with -1.2 < [Fe/H] < -0.8 the majority have [S/Fe] ≃+0.3, but two stars (previously shown to have low α/Fe ratios)have [S/Fe] ≃ 0.0. For disk stars with [Fe/H] > -1, [S/Fe]decreases with increasing [Fe/H] . Hence, sulphur behaves like othertypical α-capture elements, Mg, Si and Ca. Zinc, on the otherhand, traces iron over three orders of magnitude in [Fe/H], althoughthere is some evidence for a small systematic Zn overabundance ([Zn/Fe]≃ +0.1) among metal-poor disk stars and for halo stars with [Fe/H]< -2.0. Recent measurements of S and Zn in ten damped Lyαsystems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances inthe range -2.1 < [Zn/H] < -0.15 show an offset relative to the[S/Zn] - [Zn/H] relation in Galactic stars. Possible reasons for thisoffset are discussed, including low and intermittent star formationrates in DLAs.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).Table A1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/993
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| The evolution of the C/O ratio in metal-poor halo stars
We report new measurements of carbon and oxygen abundances in 34 F and Gdwarf and subgiant stars belonging to the halo population and spanning arange of metallicity from [Fe/H] = -0.7 to -3.2 . The survey is based onobservations of four permitted lines of C I near 9100 Å and the OI,λ 7774 triplet, all recorded at high signal-to-noise ratioswith the UVES echelle spectrograph on the ESO VLT. The line equivalentwidths were analysed with the 1D, LTE, MARCS model atmosphere code todeduce C and O abundances; corrections due to non-LTE and 3D effects arediscussed. When combined with similar published data for disk stars, ourresults confirm the metallicity dependence of the C/O ratio known fromprevious stellar and interstellar studies: C/O drops by a factor of3-4 as O/H decreases from solar to 1/10 solar. Analysed withinthe context of standard models for the chemical evolution of the solarvicinity, this drop results from the metallicity dependence of the Cyields from massive stars with mass loss, augmented by the delayedrelease of C from stars of low and intermediate mass. The former is,however, always the dominant factor. Our survey has also uncoveredtentative evidence to suggest that, as the oxygen abundance decreasesbelow [O/H] = -1, [C/O] may not remain constant at [C/O] = -0.5, aspreviously thought, but increase again, possibly approaching near-solarvalues at the lowest metallicities ([O/H] ≲ -3). With the currentdataset this is no more than a 3σ effect and it may be due tometallicity-dependent non-LTE corrections to the [C/O] ratio which havenot been taken into account. However, its potential importance as awindow on the nucleosynthesis by Population III stars is a strongincentive for future work, both observational and theoretical, to verifyits reality.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries
We report 25,563 radial velocity measurements for 1359 single-linedstars in the Carney-Latham sample of 1464 stars selected for high propermotion. For 171 of these, we present spectroscopic orbital solutions. Wefind no obvious difference between the binary characteristics in thehalo and the disk populations. The observed frequency is the same, andthe period distributions are consistent with the hypothesis that the twosets of binaries were drawn from the same parent population. Thissuggests that metallicity in general, and radiative opacities inparticular, have little influence over the fragmentation process thatleads to short-period binaries. All the binaries with periods shorterthan 10 days have nearly circular orbits, while the binaries withperiods longer than 20 days exhibit a wide range of eccentricities and amedian value of 0.37. For the metal-poor high-velocity halo binaries inour sample, the transition from circular to eccentric orbits appears tooccur at about 20 days, supporting the conclusion that tidalcircularization on the main sequence is important for the oldestbinaries in the Galaxy. Some of the results presented here usedobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
| 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/.
| The galactic lithium evolution revisited
The evolution of the 7Li abundance in the Galaxy has beencomputed by means of the two-infall model of Galactic chemicalevolution. We took into account several stellar 7Li sources:novae, massive AGB stars, C-stars and Type II SNe. In particular, weadopted new theoretical yields for novae. We also took into account the7Li production from GCRs. In particular, the absolute yieldsof 7Li, as suggested by a recent reevaluation of thecontribution of GCR spallation to the 7Li abundance, havebeen adopted. We compared our theoretical predictions for the evolutionof 7Li abundance in the solar neighborhood with a newcompilation of data, where we identified the population membership ofthe stars on a kinematical basis. A critical analysis of extantobservations revealed a possible extension of the Li plateau towardshigher metallicities (up to [Fe/H] ~ -0.5 or even -0.3) with a steeprise afterwards. We conclude that 1) the 7Li contributionfrom novae is required in order to reproduce the shape of the growth ofA(Li) versus [Fe/H], 2) the contribution from Type II SNe should belowered by at least a factor of two, and 3) the 7Liproduction from GCRs is probably more important than previouslyestimated, in particular at high metallicities: by taking into accountGCR nucleosynthesis we noticeably improved the predictions on the7Li abundance in the presolar nebula and at the present timeas inferred from measures in meteorites and T Tauri stars, respectively.We also predicted a lower limit for the present time 7Liabundance expected in the bulge, a prediction which might be tested byfuture observations. Tables~3 and 4 are only 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/Abstract.html
| Lithium abundances in metal-poor stars. I. New observations
We present the lithium measurements of a continuing programme of lightelement abundances in metal-poor stars. New equivalent widths of the Lii lambda 670.8 nm resonance line in 67 metal-poor stars covering themetallicity range -3.5 <= [Fe/H] <= -0.4 are reported. For abouthalf of this sample, the observations presented here represent the firstmeasurement of the Li i line. The sample allowed a statisticalcomparison with previous measurements from other authors and a study ofthe consistency and reliability of the quoted error bars. This papershows that for most of the stars these error bars are good estimates ofthe true uncertainties associated with the determination of theequivalent widths of the Li i line. However, about 20% of the stars withtwo or more independent measurements show discrepancies in the Li iequivalent widths; in these cases, other sources of uncertainty notproperly taken into account (binarity effects, cosmic rays, imperfectflat-field correction, continuum determination, etc.) could also beimportant. Conclusions on the possible lithium abundance trends versuseffective temperature or metallicity and on any intrinsic scatter shouldbe treated cautiously until their robustness vis-a-vis these additionaluncertainties is proved. Based on observations made with the IsaacNewton and Nordic Optical Telescopes, which are operated on the islandof La Palma by the Isaac Newton Group and the NOT ScientificAssociation, respectively, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.
| 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.
| 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
| The primordial lithium abundance
Lithium abundances in a selected sample of halo stars have been revisedby using the new accurate infrared flux method (IRFM) effectivetemperatures by Alonso, Arribas & Martinez-Roger. From 41 plateaustars (T_eff>5700 and [Fe/H]<=-1.5) we found no evidence forintrinsic dispersion, a tiny trend with T_eff and no trend with [Fe/H].The trend with the T_eff is fully consistent with the standard Liisochrones of Deliyannis, Demarque & Kawaler, implying a primordialvalue for Li of A(Li)=2.238+/-0.012_1sigma+/-0.05_sys. The presentresults argue against any kind of depletion predicted by diffusion,rotational mixing or stellar winds. Therefore the Li observed inPopulation II stars provides a direct and reliable estimate of thebaryonic density which can rival other baryonic indicators such as thedeuterium in high-redshift systems. The present upwards revision ofprimordial Li in the framework of standard big bang nucleosynthesis(SBBN) gives, at 1sigma, two solutions for the baryonic density:Omega_Bh^2=0.0062^+0.0018_-0.0011 or Omega_Bh^2=0.0146^+0.0029_-0.0033.
| A Survey of Proper Motion Stars. XIII. The Halo Population
Based on our expanded sample of metallicities and kinematics for a largesample of stars selected from the Lowell Proper Motion Catalog, we studyseveral questions relating to the halo stellar population(s) in ourGalaxy. For [m/H]≤-1.4, there does not seem to be any variation with[m/H] in the mean values of the V velocity (i.e., angular momentumrelated to that in the disk) or the Galactic orbital eccentricities. Further, in spite of the strong kinematical biases in our sample, starswith very low metallicities are found that have small V velocities (highorbital angular momenta) and low orbital eccentricities. These resultscontradict the model that the metal-poor stars are a single populationthat is only the relic of the earliest stages of the Galaxy's collapse. There are signs that some of the metal-poor stars in the solarneighborhood are due to accretion events and, perhaps, also to theearliest stages of the formation of the Galactic disk. Regardingaccretion, we confirm Majewski's [ApJS, 78, 87 (1992)] finding of aretrograde rotation among stars that reach S kpc or more from the plane. These stars do not show any radial metallicity gradient, and may beyounger on average than dynamically hot, metal-poor stars closer to theplane. These latter stars show net prograde rotation and a radialmetallicity gradient, suggestive of a dissipative process in theearliest stages of disk formation. The correlation between metallicityand perigalacticon found by Ryan & Norris [AJ, 101, 1835 (1991a)]disappears when care is taken to exclude the stars that may have beenaccreted by our Galaxy. The field star results complement those forglobular clusters found by other workers, notably Zinn (1993), whoargued for two populations of metal-poor clusters, one apparently inretrograde rotation with no radial metallicity gradient and slightlyyounger ages, and the other with prograde rotation, a weak radialmetallicity gradient, and slightly older ages. The field stars andglobular clusters do differ slightly, however. Their metallicitydistributions differ, with the field stars showing a larger fraction ofthe most metal-poor stars. This could be caused by accretion of Dracodwarf galaxy-like objects, with very low metallicities and no globularclusters. We see in our data, particularly in the V vs>Rapo< plane, possible signs of large-scale kinematicsubstructure suggestive of specific accretion events. We also see signsfor the Preston et al. [AJ, 108, 538 (1994)] low-metallicity,intermediate kinematics, and younger age stellar population. However,the strength of the signal in our data suggests that a fairly largefraction of its stars may be old. On the other hand, the "away" versus"toward" mystery of Croswell et al. [Al, 93, 1445 (1987)] hasdisappeared: the numbers of stars approaching and receding from theplane agree with expectations. Finally, we point out that the model ofNorris [ApJ, 431, 645 (1994)] for a proto-disk population that is hotterdynamically than the accreted halo components does not agree with ourexpanded data sample. We suggest that the proto-disk component wasdynamically cooler when the mean metallicity was very low.
| Determination of effective temperatures for an extended sample of dwarfs and subdwarfs (F0-K5).
We have applied the InfraRed Flux Method (IRFM) to a sample of 475dwarfs and subdwarfs in order to derive their effective temperatureswith a mean accuracy of about 1.5%. We have used the new homogeneousgrid of theoretical model atmosphere flux distributions developed byKurucz (1991, 1993) for the application of the IRFM. The atmosphericparameters of the stars cover, roughly, the ranges:3500K<=T_eff_<=8000K -3.5<=[Fe/H]<=+0.53.5<=log(g)<=5. The monocromatic infrared fluxes at the continuum,and the bolometric fluxes are derived using recent results, whichsatisfy the accuracy requeriments of the work. Photometric calibrationshave been revised and applied to estimate metallicities, although directspectroscopic determinations were preferred when available. The adoptedinfrared absolute flux calibration, based on direct optical measurementsof angular stellar diameters, sets the effective temperatures determinedusing the IRFM on the same scale than those obtained by direct methods.We derive three temperatures, T_J_, T_H_ and T_K_, for each star usingthe monochromatic fluxes at different infrared wavelengths in thephotometric bands J, H, and K. They show good consistency over 4000 K,and no trend with wavelength may be appreciated. We provide a detaileddescription of the steps followed for the application of the IRFM, aswell as the sources of the errors associated to the different inputs ofthe method, and their transmission into the final temperatures. We alsoprovide comparison with previous works.
| Lithium Processing in Halo Dwarfs, and T eff, [Fe/H] Correlations on the Spite Plateau
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...458..543R&db_key=AST
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
| Broad band JHK infrared photometry of an extended sample of late type dwarfs and subdwarfs.
The results of a long term programme of broad band JHK photometry, for asample of 360 late type stars, made at the Observatorio del Teide(Tenerife, Spain) are presented. Transformations between thesemagnitudes and those of several currently used systems (CIT (Elias etal. 1982 and Carney 1983), Johnson (Johnson 1966, and Lee 1970), and ESO(Bouchet et al. 1991)) are proposed. A comparison to the narrow-bandsystem of Selby et al. (1988) has been made, in order to check theaccuracy of the photometric system. A mean internal accuracy better than0.02mag in the three bands can be inferred from the comparison to thelarge number of stars in common with Carney (1983), and from thedispersion of the multiple measured stars. The list of standards, thefilter passbands and effective wavelengths, together with correlationsbetween the extinction coefficients, ultimately characterize thephotometric IR system of the Observatorio del Teide (TCS). Data ofcomparable quality previously published have been added in order tocomplete the sample. This way the final sample consists of 550 stars.From the analysis of optical and IR colour:colour diagrams, we maydeduce that the range F0-K0 is properly sampled for0.1>[Fe/H]>-3.0. In the range K0-M4, no reliable photometricestimates of metallicity can be assigned, and only a small number ofstars have spectroscopic determination of the metallicity. Nevertheless,after kinematical considerations, the stars in this spectral range arealso expected to sample the galactic populations of dwarfs. Themetallicity effects on the IR and optical colour:colour diagrams arebriefly discussed.
| A survey of proper motion stars. 12: an expanded sample
We report new photometry and radial velocities for almost 500 stars fromthe Lowell Proper Motion Catalog. We combine these results with ourprior sample and rederive stellar temperatures based on the photometry,reddening, metallicities (using chi squared matching of our 22,500 lowSignal to Noise (S/N) high resolution echelle spectra with a grid ofsynthetic spectra), distances, space motions, and Galactic orbitalparameters for 1269 (kinematics) and 1261 (metallicity) of the 1464stars in the complete survey. The frequency of spectroscopic binariesfor the metal-poor ((m/H) less than or equal to -1.2) stars with periodsshorter than 3000 days is at least 15%. The spectroscopic binaryfrequency for metal-rich stars ((m/H) greater than -0.5) appears to belower, about 9%, but this may be a selection effect. We also discussspecial classes of stars, including treatment of the double-linedspectroscopic binaries, and identification of subgiants. Four possiblenew members of the class of field blue stragglers are noted. We pointout the detection of three possible new white dwarfs, six broad-lined(binary) systems, and discuss briefly the three already knownnitrogen-rich halo dwarfs. The primary result of this paper will beavailable on CD-ROM, in the form of a much larger table.
| The abundance of lithium in metal-poor subgiant stars
We have determined lithium abundances for a sample of 79 halo subgiants.The subgiant candidates were identified using uvby photometry fromseveral catalogs of metal-poor stars. The basic data werehigh-resolution, low-noise coude spectra in the 6700 A spectral region.Abundances of iron and calcium, derived from one Ca I and several Fe Ilines in our spectra, provided a metallicity discriminant for the starsin our sample. The subgiants with temperatures between 5500 and 4900 Kshow a steady decline of lithium abundance with advancing subgiantposition (and decreasing temperature). The observed trend is inqualitative agreement with recent theoretical models of lithium dilutionin metal-poor stars, especially if main-sequence diffusion is included.The initial lithium abundances in metal-poor stars may have beenslightly larger than that exhibited by stars near the main-sequenceturnoff. For stars with temperatures below 4900 K, the models predict nofurther dilution, but observed lithium abundances continue to declinewith decreasing temperature, indicating further lithium destruction onthe giant branch of metal-poor stars. In all postdilution subgiants, theobserved lithium abundances show more scatter than do stars at themain-sequence turnoff, suggesting variations in the main-sequencelithium destruction below the observable surface layers.
| Subdwarf Studies. III. The Halo Metallicity Distribution
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991AJ....101.1865R&db_key=AST
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
