Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
The origin and chemical evolution of carbon in the Galactic thin and thick discs*
In order to trace the origin and evolution of carbon in the Galacticdisc, we have determined carbon abundances in 51 nearby F and G dwarfstars. The sample is divided into two kinematically distinct subsampleswith 35 and 16 stars that are representative of the Galactic thin andthick discs, respectively. The analysis is based on spectral synthesisof the forbidden [CI] line at 872.7nm using spectra of very highresolution (R~ 220000) and high signal-to-noise ratio (S/N >~ 300)that were obtained with the Coudé Echelle Spectrograph (CES)spectrograph by the European Southern Observatory (ESO) 3.6-m telescopeat La Silla in Chile. We find that [C/Fe] versus [Fe/H] trends for thethin and thick discs are totally merged and flat for subsolarmetallicities. The thin disc that extends to higher metallicities thanthe thick disc shows a shallow decline in [C/Fe] from [Fe/H]~ 0 and upto [Fe/H]~+0.4. The [C/O] versus [O/H] trends are well separated betweenthe two discs (due to differences in the oxygen abundances) and bear agreat resemblance to the [Fe/O] versus [O/H] trends. Our interpretationof our abundance trends is that the sources that are responsible for thecarbon enrichment in the Galactic thin and thick discs have operated ona time-scale very similar to those that are responsible for the Fe and Yenrichment [i.e. SNIa and asymptotic giant branch (AGB) stars,respectively]. We further note that there exist other observational datain the literature that favour massive stars as the main sources forcarbon. In order to match our carbon trends, we believe that the carbonyields from massive stars then must be very dependent on metallicity forthe C, Fe and Y trends to be so finely tuned in the two discpopulations. Such metallicity-dependent yields are no longer supportedby the new stellar models in the recent literature. For the Galaxy, wehence conclude that the carbon enrichment at metallicities typical ofthe disc is mainly due to low- and intermediate-mass stars, whilemassive stars are still the main carbon contributor at low metallicities(halo and metal-poor thick disc).
| Analysis of 26 barium stars. II. Contributions of s-, r-, and p-processes in the production of heavy elements
Context: .Barium stars show enhanced abundances for the slow neutroncapture (s-process) heavy elements, so they are suitable objects forstudying s-process elements. Aims: .The aim of this work is toquantify the contributions of the s-, r-, and p-processes for the totalabundance of heavy elements from abundances derived for a sample of 26barium stars. The abundance ratios between these processes and neutronexposures were studied. Methods: .The abundances of the samplestars were compared to those of normal stars, thus identifying thefraction relative to the main component of the s-process. Results:.The fittings of the σ N curves (neutron-capture cross-sectiontimes abundance, plotted against atomic mass number) for the samplestars suggest that the material from the companion asymptotic giantbranch star had approximately the solar isotopic composition as concernsfractions of abundances relative to the s-process main component. Theabundance ratios of heavy elements, hs, ls, and s and the computedneutron exposure are similar to those of post-AGB stars. For some samplestars, an exponential neutron exposure fits the observed data well,whereas a single neutron exposure provides a better fit for others. Conclusions: .The comparison of barium and AGB stars supports thehypothesis of binarity for the barium star formation. Abundances ofr-elements that are part of the s-process path in barium stars areusually higher than those in normal stars, so barium stars also seemedto be enriched in r-elements, although to a lower degree thans-elements. No dependence on luminosity classes was found in theabundance-ratio behaviour among the dwarfs and giants of the sample ofbarium stars.
| Analysis of 26 barium stars. I. Abundances
Context: .We present a detailed analysis of 26 barium stars, includingdwarf barium stars, providing their atmospheric parameters (T_eff, logg, [Fe/H], v_t), and elemental abundances. Aims: .We aim atderiving gravities and luminosity classes of the sample stars, inparticular to confirm the existence of dwarf barium stars. Accurateabundances of chemical elements were derived. We present the abundanceratios between nucleosynthetic processes, by using Eu and Ba asrepresentatives of the r- and s-processes. Methods:.High-resolution spectra were obtained with the FEROS spectrograph atthe ESO-1.52 m Telescope, along with photometric data with Fotrap at theZeiss telescope at the LNA. The atmospheric parameters were derived inan iterative way, with temperatures obtained from colour-temperaturecalibrations. The abundances were derived using spectrum synthesis forLi, Na, Al, α-, iron-peak, s-, and r-element atomic lines, and forC and N molecular lines. Results: .Atmospheric parameters in therange 4300 < T_eff < 6500, -1.2 < [Fe/H] < 0.0, and 1.4≤ log g < 4.6 were derived, confirming that our sample containsgiants, subgiants, and dwarfs. The abundance results obtained for Li,Al, Na, α-, and iron-peak elements for the sample stars show thatthey are compatible with the values found in the literature for normaldisk stars in the same range of metallicities. Enhancements of C, N, andheavy elements relative to Fe, that characterise barium stars, werederived and showed that [X/Ba] vs. [Ba/H] and [X/Ba] vs. [Fe/H] presentdifferent behaviour as compared to [X/Eu] vs. [Eu/H] and [X/Eu] vs.[Fe/H], reflecting the different nucleosynthetic sites for the s- andr-processes.
| Boron Depletion in F and G Dwarf Stars and the Beryllium-Boron Correlation
Boron provides a special probe below the stellar surface since itsurvives to greater depths than do Li and Be. To search for B depletionswe have observed B in 13 F and G dwarfs with large Be depletions; forcomparison we have also obtained spectra of five stars that areundepleted in Li and Be. We have used HST with STIS to obtain spectra ofthe B I resonance line at 2497 Å. The spectral resolution is30,000 or 114,000, and the median signal-to-noise ratio is 70pixel-1. New Be and Li spectra have been obtained at Keck Iwith HIRES of four of the five standard stars at ~48,000 resolution.Abundances have been determined by the spectrum synthesis method withMOOG. A comparison between the standard stars and those with severe Bedepletions shows a distinct difference in the B abundances between thetwo groups of 0.22 dex. We have discovered a correlation between the Beand B abundances. The slope between A(Be) and A(B)NLTE is0.22+/-0.05 [where A(element)=logN(element)/N(H)+12.00], which, asexpected, is shallower than the slope between A(Li) and A(Be) of 0.38.We have normalized the light-element abundances to account for theobservation that the initial abundances are somewhat lower in lowermetallicity stars by employing recently published empirical relationsbetween Be and [Fe/H] and between B and [Fe/H]. The correlation betweenthe normalized A(Be) and A(B)NLTE has a slope of 0.18+/-0.06.The star with the largest Be depletion, HR 107, a main-sequence Ba star,also has the largest B depletion, with the B abundance lower by a factorof 3.5 relative to the standard stars.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Spectroscopic Verification of Barium Dwarf Candidates: The Analysis of HD 8270, HD 13551, and HD 22589
This work presents the abundance patterns of three barium dwarfcandidates, HD 8270, HD 13551, and HD 22589, based on high-resolutionoptical spectra. This work also reports the spectroscopic stellarparameters, temperature, and microturbulent velocity, as well as thestellar surface gravity from a solution of excitation and ionizationequilibria of Fe I and Fe II lines under the assumption of localthermodynamic equilibrium. The abundance analysis reveals that HD 8270,HD 13551, and HD 22589 have metallicities of [Fe/H]=-0.43, -0.28, and-0.12, respectively. It was found that the abundances of iron group andα-elements follow the abundance pattern of a disk population. Theheavy-element abundance patterns of the three stars show enhancements bya factor of 4-8 with respect to the Sun. The abundances of the s-processelements are discussed and compared with other barium giants and dwarfsthrough diagrams involving the indices [hs/ls] and [s/Fe]. Themetallicity distribution of barium giants and dwarfs is also discussed.Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under agreement withObservatório Nacional (Brazil).
| 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.
| α-, r-, and s-process element trends in the Galactic thin and thick disks
From a detailed elemental abundance analysis of 102 F and G dwarf starswe present abundance trends in the Galactic thin and thick disks for 14elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu).Stellar parameters and elemental abundances (except for Y, Ba and Eu)for 66 of the 102 stars were presented in our previous studies (Bensbyet al. [CITE], A&A, 410, 527, [CITE], A&A, 415, 155). The 36stars that are new in this study extend and confirm our previous resultsand allow us to draw further conclusions regarding abundance trends. Thes-process elements Y and Ba, and the r-element Eu have also beenconsidered here for the whole sample for the first time. With this newlarger sample we now have the following results: 1) smooth and distinctabundance trends that for the thin and thick disks are clearlyseparated; 2) the α-element trends for the thick disk show typicalsignatures from the enrichment of SN Ia; 3) the thick disk stellarsample is in the mean older than the thin disk stellar sample; 4) thethick disk abundance trends are invariant with galactocentric radii(R_m); 5) the thick disk abundance trends appear to be invariant withvertical distance (Z_max) from the Galactic plane. Adding furtherevidence from the literaure we argue that a merger/interacting scenariowith a companion galaxy to produce a kinematical heating of the stars(that make up today's thick disk) in a pre-existing old thin disk is themost likely formation scenario for the Galactic thick disk. The 102stars have -1 ≲ [Fe/H] ≲ +0.4 and are all in the solarneighbourhood. Based on their kinematics they have been divided into athin disk sample and a thick disk sample consisting of 60 and 38 stars,respectively. The remaining 4 stars have kinematics that make themkinematically intermediate to the two disks. Their chemical abundancesalso place them in between the two disks. Which of the two diskpopulations these 4 stars belong to, or if they form a distinctpopulation of their own, can at the moment not be settled. The 66 starsfrom our previous studies were observed with the FEROS spectrograph onthe ESO 1.5-m telescope and the CES spectrograph on the ESO 3.6-mtelescope. Of the 36 new stars presented here 30 were observed with theSOFIN spectrograph on the Nordic Optical Telescope on La Palma, 3 withthe UVES spectrograph on VLT/UT2, and 3 with the FEROS spectrograph onthe ESO 1.5-m telescope. All spectra have high signal-to-noise ratios(typically S/N≳ 250) and high resolution (R 80 000, 45 000,and 110 000 for the SOFIN, FEROS, and UVES spectra, respectively).Based on observations collected at the Nordic Optical Telescope on LaPalma, Spain, and at the European Southern Observatories on La Silla andParanal, Chile, Proposals # 65.L-0019(B), 67.B-0108(B), 69.B-0277. FullTables [see full text], [see full text] and [see full text] are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/185
| Lithium abundances of the local thin disc stars
Lithium abundances are presented for a sample of 181 nearby F and Gdwarfs with accurate Hipparcos parallaxes. The stars are on circularorbits about the Galactic centre and, hence, are identified as belongingto the thin disc. This sample is combined with two published surveys toprovide a catalogue of lithium abundances, metallicities ([Fe/H]),masses, and ages for 451 F-G dwarfs, almost all belonging to the thindisc. The lithium abundances are compared and contrasted with publishedlithium abundances for F and G stars in local open clusters. The fieldstars span a larger range in [Fe/H] than the clusters for which [Fe/H]~=0.0 +/- 0.2. The initial (i.e. interstellar) lithium abundance of thesolar neighbourhood, as derived from stars for which astration oflithium is believed to be unimportant, is traced from logɛ(Li) =2.2 at [Fe/H]=-1 to logɛ(Li) = 3.2 at +0.1. This form for theevolution is dependent on the assumption that astration of lithium isnegligible for the stars defining the relation. An argument is advancedthat this latter assumption may not be entirely correct, and, theevolution of lithium with [Fe/H] may be flatter than previouslysupposed. A sharp Hyades-like Li dip is not seen among the field starsand appears to be replaced by a large spread among lithium abundances ofstars more massive than the lower mass limit of the dip. Astration oflithium by stars of masses too low to participate in the Li dip isdiscussed. These stars show little to no spread in lithium abundance ata given [Fe/H] and mass.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| The Correlation of Lithium and Beryllium in F and G Field and Cluster Dwarf Stars
Although Li has been extensively observed in main-sequence field andcluster stars, there are relatively fewer observations of Be. We haveobtained Keck HIRES spectra of 36 late F and early G dwarfs in order tostudy the Li-Be correlation we found previously in the temperatureregime of 5900-6650 K. The sample size for this temperature range withdetectable and (usually) depleted Li and Be is now 88, including Li andBe abundances in both cluster and field stars. Therefore we can nowinvestigate the influence of other parameters such as age, temperature,and metallicity on the correlation. The Be spectra at 3130 Å weretaken over six nights from 1999 November to 2002 January and have aspectral resolution of ~48,000 and a median signal-to-noise ratio (S/N)of 108 pixel-1. We obtained Li spectra of 22 stars with theUniversity of Hawaii 88 inch (2.2 m) telescope and coudéspectrograph with a spectral resolution of ~70,000 and a median S/N of110 pixel-1. We have redetermined the effective temperaturesfor all the stars and adopted other parameters from published data orempirical relations. The abundances of both Li and Be in the stars weobserved were determined from spectrum synthesis with MOOG 2002. Thepreviously observed Li equivalent widths for some of our Be stars wereused with the new temperatures and MOOG 2002 in the ``blends'' mode. Forthe 46 field stars from this and earlier studies we find a linearrelation between A(Li) and A(Be) with a slope of 0.375+/-0.036. Over theTeff range 5900-6650 K, we find the modest scatter about theBe-Li relation to be significantly correlated with Teff andperhaps also [Fe/H]. Dividing the sample into two temperature regimes of6300-6650 K (corresponding to the cool side of the Li-Be dip) and5900-6300 K (corresponding to the Li ``plateau'') reveals possible smalldifferences in the slopes for the two groups, 0.404+/-0.034 and0.365+/-0.049, respectively. When we include the cluster stars (Hyades,Pleiades, Praesepe, UMa Group, and Coma), the slope for the fulltemperature range (88 stars) is essentially the same, at 0.382+/-0.030,as for the field stars alone. For the hotter temperature group of 35Li-Be dip stars in the field and in clusters the slope is higher, at0.433+/-0.036, while for the cooler star group (54 stars) the slope is0.337+/-0.031, different by more than 1 σ. This small differencein the slope is predicted by the theory of rotationally induced mixing.The four stars with [Fe/H] less than -0.4 are all below the best-fitrelation, i.e., there is more Be depletion at a given A(Li) or less Beab initio. The youngest stars, i.e., Pleiades, have less depletion ofboth Li and Be. This too is predicted by rotationally induced slowmixing. Combining the Be results from both field and cluster stars, wefind that there are stars with undepleted Be, i.e., near the meteoriticvalues of 1.42 dex, at all temperatures from 5500 to 6800 K. Depletionsof Be of up to and even exceeding 2 orders of magnitude are commonbetween 6000 and 6700 K.
| Galactic Evolution of Sr, Y, And Zr: A Multiplicity of Nucleosynthetic Processes
In this paper we follow the Galactic enrichment of three easily observedlight n-capture elements: Sr, Y, and Zr. Input stellar yields have beenfirst separated into their respective main and weak s-process componentsand r-process component. The s-process yields from asymptotic giantbranch (AGB) stars of low to intermediate mass are computed, exploring awide range of efficiencies of the major neutron source, 13C,and covering both disk and halo metallicities. AGB stars have been shownto reproduce the main s-component in the solar system, i.e., thes-process isotopic distribution of all heavy isotopes with atomic massnumber A>90, with a minor contribution to the light s-processisotopes up to A~90. The concurrent weak s-process, which accounts forthe major fraction of the light s-process isotopes in the solar systemand occurs in massive stars by the operation of the 22Neneutron source, is discussed in detail. Neither the main s- nor the weaks-components are shown to contribute significantly to theneutron-capture element abundances observed in unevolved halo stars.Knowing the s-process distribution at the epoch of the solar systemformation, we first employed the r-process residuals method to infer theisotopic distribution of the r-process. We assumed a primary r-processproduction in the Galaxy from moderately massive Type II supernovae thatbest reproduces the observational Galactic trend of metallicity versusEu, an almost pure r-process element. We present a detailed analysis ofa large published database of spectroscopic observations of Sr, Y, Zr,Ba, and Eu for Galactic stars at various metallicities, showing that theobserved trends versus metallicity can be understood in light of amultiplicity of stellar neutron-capture components. Spectroscopicobservations of the Sr, Y, and Zr to Ba and Eu abundance ratios versusmetallicity provide useful diagnostics of the types of neutron-captureprocesses forming Sr, Y, and Zr. In particular, the observed [Sr, Y,Zr/Ba, Eu] ratio is clearly not flat at low metallicities, as we wouldexpect if Ba, Eu and Sr, Y, Zr all had the same r-processnucleosynthetic origin. We discuss our chemical evolution predictions,taking into account the interplay between different processes to produceSr-Y-Zr. Making use of the very r-process-rich and very metal-poor starslike CS 22892-052 and CS 31082-001, we find hints and discuss thepossibility of a primary process in low-metallicity massive stars,different from the ``classical s-process'' and from the ``classicalr-process'' that we tentatively define LEPP (lighter element primaryprocess). This allows us to revise the estimates of the r-processcontributions to the solar Sr, Y, and Zr abundances, as well as of thecontribution to the s-only isotopes 86Sr, 87Sr,and 96Mo.
| 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.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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.
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog 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/398/721
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 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/ cgi-bin/qcat?J/ A+A/394/927
| Europium abundances in F and G disk dwarfs
Europium abundances for 74 F and G dwarf stars of the galactic disk havebeen determined from the 4129.7 Å Eu II line. The stars wereselected from the sample of Edvardsson et al. (1993) and [Eu/Fe] shows asmaller scatter and a slightly weaker trend with [Fe/H] than found byWoolf et al. (1995). The data of the two analyses are homogenized andmerged. We also discuss the adopted effective temperature scale. Basedon observations carried out at the European Southern Observatory, LaSilla, Chile. Tables 2 and 6 are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcal?J/A+A/381/500
| Beryllium in F and G Field Dwarfs from High-Resolution Canada-France-Hawaii Telescope Spectra
It is important to add observations of Be to the huge arsenal of Liobservations in order to identify the mechanisms operating in stellarinteriors that alter the surface composition of the light elements.Beryllium is more resistant to destruction than is Li, so information onthe abundances of both Li and Be reveals more information on theinternal processes than either element does alone. We have madeobservations of Be II at 3131 Å in 46 solar-type stars from theCanada-France-Hawaii Telescope with high spectral resolution and highsignal-to-noise ratios (S/N). Our Li I 6707 Å data for 39 of thesestars come from our high-resolution, high-S/N observations with theUniversity of Hawai`i 88 inch (2.2 m) telescope and coudéspectrograph and Keck I High Resolution Echelle Spectrometer and, forsix stars, from the literature. Most of the stars in our sample are Fand G dwarfs with Teff between 6100 and 6600 K and with[Fe/H] between -0.6 and +0.2. The abundances of Be have been determinedthrough spectrum synthesis, while Li has been analyzed as a blend tofind the Li abundance. We find a large range in both Li and Be in thesestars; for Be it is at least 2.5 dex and for Li at least 3 dex. However,there is an excellent correlation between Li and Be, as discovered byDeliyannis et al. from a smaller sample. We find that in the range ofTeff of 5850 K (near the Li ``peak'' in open clusters) to6680 K (at the bottom of the Li ``gap'' as defined by the Hyades), Liand Be appear to be depleted together. The slope of this remarkablelogarithmic relation is 0.36: as Li is reduced by a factor of 10, Be isreduced by only 2.2 times. There is some scant evidence for a change inthe slope between the cooler stars and the hotter stars such that thecooler stars deplete more Li relative to Be than the hotter stars. Theseresults are well matched by models that incorporate rotationally inducedslow mixing of the stellar surface material with the deeper layers ofthe star.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| Lithium abundances for 185 main-sequence stars: Galactic evolution and stellar depletion of lithium
We present a survey of lithium abundances in 185 main-sequence fieldstars with 5600 <~ Teff <~ 6600 K and -1.4 <~ [Fe/H]<~ +0.2 based on new measurements of the equivalent width of thelambda 6708 Li I line in high-resolution spectra of 130 stars and areanalysis of data for 55 stars from Lambert et al. (\cite{Lambert91}).The survey takes advantage of improved photometric and spectroscopicdeterminations of effective temperature and metallicity as well as massand age derived from Hipparcos absolute magnitudes, offering anopportunity to investigate the behaviour of Li as a function of theseparameters. An interesting result from this study is the presence of alarge gap in the log varepsilon (Li) - Teff plane, whichdistinguishes ``Li-dip'' stars like those first identified in the Hyadescluster by Boesgaard & Tripicco (\cite{Boesgaard86}) from otherstars with a much higher Li abundance. The Li-dip stars concentrate on acertain mass, which decreases with metallicity from about 1.4Msun at solar metallicity to 1.1 Msun at [Fe/H] =~-1.0. Excluding the Li-dip stars and a small group of lower mass starswith Teff < 5900 K and log varepsilon (Li) < 1.5, theremaining stars, when divided into four metallicity groups, may show acorrelation between Li abundance and stellar mass. The dispersion aroundthe log varepsilon (Li)-mass relation is about 0.2 dex below [Fe/H] =~-0.4 and 0.3 dex above this metallicity, which cannot be explained byobservational errors or differences in metallicity. Furthermore, thereis no correlation between the residuals of the log varepsilon (Li)-massrelations and stellar age, which ranges from 1.5 Gyr to about 15 Gyr.This suggests that Li depletion occurs early in stellar life and thatparameters other than stellar mass and metallicity affect the degree ofdepletion, e.g. initial rotation velocity and/or the rate of angularmomentum loss. It cannot be excluded, however, that a cosmic scatter ofthe Li abundance in the Galaxy at a given metallicity contributes to thedispersion in Li abundance. These problems make it difficult todetermine the Galactic evolution of Li from the data, but a comparisonof the upper envelope of the distribution of stars in the log varepsilon(Li) - [Fe/H] plane with recent Galactic evolutionary models by Romanoet al. (\cite{Romano99}) suggests that novae are a major source for theLi production in the Galactic disk; their occurrence seems to be theexplanation for the steep increase of Li abundance at [Fe/H] =~ -0.4.Based on observations carried out at Beijing Astronomical Observatory(Xinglong, PR China) and European Southern Observatory, La Silla, Chile.Table 1 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/371/943 and athttp://www.edpsciences.org
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| Binarity among Barium Dwarfs and CH Subgiants: Will They Become Barium Giants?
Not Available
| Abundances of light elements in metal-poor stars. III. Data analysis and results
We present the results of the analysis of an extensive set of new andliterature high quality data concerning Fe, C, N, O, Na, and Mg. Thisanalysis exploited the T_eff scale determined in Gratton et al. (1996a),and the non-LTE abundance corrections computed in Gratton et al.(1999a). Results obtained with various abundance indices are discussedand compared. Detailed comparison with models of galactic chemicalevolution will be presented in future papers of this series. Our non-LTEanalysis yields the same O abundances from both permitted and forbiddenlines for stars with T_eff >4600 K, in agreement with King (1993),but not with other studies using a lower T_eff -scale for subdwarfs.However, we obtain slightly smaller O abundances for the most luminousmetal-poor field stars than for fainter stars of similar metallicities,an effect attributed to inadequacies of the adopted model atmospheres(Kurucz 1992, with overshooting) for cool stars. We find a nearlyconstant O overundance in metal-poor stars ([Fe/H]<-0.8), at a meanvalue of 0.46+/- 0.02 dex (sigma =0.12, 32 stars), with only a gentleslope with [Fe/H] ( ~ -0.1); this result is different from the steeperslope recently obtained using OH band in the near UV. If only bonafideunmixed stars are considered, C abundances scale with Fe ones (i.e.[C/Fe]~ 0) down to [Fe/H] ~ -2.5. Due to our adoption of a differentT_eff scale, we do not confirm the slight C excess in the most metalpoor disk dwarfs (-0.8<[Fe/H]<-0.4) found in previousinvestigations. Na abundances scale as Fe ones in the high metallicityregime, while metal-poor stars present a Na underabundance. None of thefield stars analyzed belong to the group of O-poor and Na-rich starsobserved in globular clusters. Na is deficient with respect to Mg inhalo and thick disk stars; within these populations, Na deficiency maybe a slow function of [Mg/H]. Solar [Na/Mg] ratios are obtained for thindisk stars. Tables~ 2 to 9 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strabg.fr/Abstract.html
| A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out followingthe classical recipe: LTE line formation and homogeneous,plane-parallel, flux-constant, and LTE model atmospheres. We reviewdifferent results in the literature that have suggested significantinconsistencies in the spectroscopic analyses, pointing out thedifficulties in deriving independent estimates of the stellarfundamental parameters and hence, detecting systematic errors. Thetrigonometric parallaxes measured by the Hipparcos mission provideaccurate appraisals of the stellar surface gravity for nearby stars,which are used here to check the gravities obtained from thephotospheric iron ionization balance. We find an approximate agreementfor stars in the metallicity range -1.0<=[Fe/H]<=0, but thecomparison shows that the differences between the spectroscopic andtrigonometric gravities decrease toward lower metallicities for moremetal-deficient dwarfs (-2.5<=[Fe/H]<=-1.0), which casts a shadowupon the abundance analyses for extreme metal-poor stars that make useof the ionization equilibrium to constrain the gravity. The comparisonwith the strong-line gravities derived by Edvardsson and Fuhrmannconfirms that this method provide systematically larger gravities thanthe ionization balance. The strong-line gravities get closer to thephysical ones for the stars analyzed by Fuhrmann, but they are evenfurther away than the iron ionization gravities for the stars of lowergravities in Edvardsson's sample. The confrontation of the deviations ofthe iron ionization gravities in metal-poor stars, reported here withdepartures from the excitation balance found in the literature, showthat they are likely to be induced by the same physical mechanism.
| Stellar Iron Abundances: Non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II inthe atmosphere of late-type stars. We used atomic models for Fe I and FeII having, respectively, 256 and 190 levels, as well as 2117 and 3443radiative transitions. Photoionization cross sections are from the IronProject. These atomic models were used to investigate non-LTE (NLTE)effects in iron abundances of late-type stars with different atmosphericparameters. We found that most Fe I lines in metal-poor stars are formedin conditions far from LTE. We derived metallicity corrections of about0.3 dex with respect to LTE values for the case of stars with[Fe/H]~-3.0. Fe II is found not to be affected by significant NLTEeffects. The main NLTE effect invoked in the case of Fe I isoverionization by ultraviolet radiation; thus classical ionizationequilibrium is far from being satisfied. An important consequence isthat surface gravities derived by LTE analysis are in error and shouldbe corrected before final abundance corrections. This apparently solvesthe observed discrepancy between spectroscopic surface gravities derivedby LTE analyses and those derived from Hipparcos parallaxes. A table ofNLTE [Fe/H] and log g values for a sample of metal-poor late-type starsis given.
| 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
| The origin of carbon, investigated by spectral analysis of solar-type stars in the Galactic Disk
Abundance analysis of carbon has been performed in a sample of 80 late Fand early G type dwarf stars in the metallicity range{-1.06<=[Fe/H]<=0.26} using the forbidden [C i] line at 8727Angstroms. This line is presumably less sensitive to temperature,atmospheric structure and departures from LTE than alternative carboncriteria. We find that {[C/Fe]} decreases slowly with increasing{[Fe/H]} with an overall slope of -0.17+/-0.03. Our results areconsistent with carbon enrichment by superwinds of metal-rich massivestars but inconsistent with a main origin of carbon in low-mass stars.This follows in particular from a comparison between the relation of{[C/O]} with metallicity for the Galactic stars and the correspondingrelation observed for dwarf irregular galaxies. The significance ofintermediate-mass stars for the production of carbon in the Galaxy isstill somewhat unclear. Based on observations carried out at theEuropean Southern Observatory, La Silla, Chile.
| Photometric Abundance Calibration of delta Scuti Stars Using HK Photometry
The hk index has been used as a metallicity indicator for RR Lyraevariable stars. It is now being applied to the shorter period deltaScuti variables. Employing spectroscopic abundances of stars withpublished hk values and photometric indices calculated from stellaratmosphere models, a three-dimensional interpolation is used todetermine [Fe/H] from intrinsic b-y, c_1, and hk values. The resulting[Fe/H], log g, and T_eff values for 10 delta Scuti stars are presented.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Pisces |
| Ascensió Recta: | 00h28m20.10s |
| Declinació: | +10°11'23.0" |
| Magnitud Aparent: | 6.04 |
| Distancia: | 36.35 parsecs |
| Moviment propi RA: | 35.9 |
| Moviment propi Dec: | -204.4 |
| B-T magnitude: | 6.539 |
| V-T magnitude: | 6.083 |
Catàlegs i designacions:
|
