Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Lithium Abundances of F-, G-, and K-Type Stars: Profile-Fitting Analysis of the Li I 6708 Doublet
An extensive profile-fitting analysis was performed for the Li(+Fe)6707-6708Å feature of nearby 160 F-K dwarfs/subgiants (including27 planet-host stars) in the Galactic disk ( 7000 K ≳Teff ≳ 5000 K, -1 ≲ [Fe/H] ≲ +0.4), in orderto establish the photospheric lithium abundances of these stars. Thenon-LTE effect (though quantitatively insignificant) was taken intoaccount based on our statistical equilibrium calculations, which werecarried out on an adequate grid of models. Our results confirmed most ofthe interesting observational characteristics revealed by recentlypublished studies, such as the bimodal distribution of the Li abundancesfor stars at Teff ≳ 6000 K, the satisfactory agreementof the upper envelope of the A(Li) vs. [Fe/H] distribution with thetheoretical models, the existence of a positive correlation betweenA(Li) and the stellar mass, and the tendency of lower lithium abundancesof planet-host stars (as compared to stars without planets) at thenarrow ``transition'' region of 5900 K ≳ Teff ≳5800 K. The solar Li abundance derived from this analysis is 0.92 (H =12.00), which is by 0.24dex lower than the widely referenced standardvalue of 1.16.
| Spectroscopic Study on the Atmospheric Parameters of Nearby F--K Dwarfs and Subgiants
Based on a collection of high-dispersion spectra obtained at OkayamaAstrophysical Observatory, the atmospheric parameters (Teff,log g, vt, and [Fe/H]) of 160 mid-F through early-K starswere extensively determined by the spectroscopic method using theequivalent widths of Fe I and Fe II lines along with the numericaltechnique of Takeda et al. (2002, PASJ, 54, 451). The results arecomprehensively discussed and compared with the parameter values derivedby different approaches (e.g., photometric colors, theoreticalevolutionary tracks, Hipparcos parallaxes, etc.) as well as with thepublished values found in various literature. It has been confirmed thatour purely spectroscopic approach yields fairly reliable and consistentresults.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| 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.
| 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.
| Sodium abundances in nearby disk stars
We present sodium abundances for a sample of nearby stars. All resultshave been derived from NLTE statistical equilibrium calculations. Theinfluence of collisional interactions with electrons and hydrogen atomsis evaluated by comparison of the solar spectrum with very precise fitsto the Na I line cores. The NLTE effects are more pronounced inmetal-poor stars since the statistical equilibrium is dominated bycollisions of which at least the electronic component is substantiallyreduced. The resulting influence on the determination of sodiumabundances is in a direction opposite to that found previously for Mgand Al. The NLTE corrections are about -0.1 in thick-disk stars with[Fe/H] -0.6. Our [Na/Fe] abundance ratios are about solar forthick- and thin-disk stars. The increase in [Na/Fe] as a function of[Fe/H] for metal-rich stars found by Edvardsson et al. (\cite{EAG93}) isconfirmed. Our results suggest that sodium yields increase with themetallicity, and quite large amounts of sodium may be produced by AGBstars. We find that [Na/Fe] ratios, together with either [Mg/Fe] ratio,kinematic data or stellar evolutionary ages, make possible theindividual discrimination between thin- and thick-disk membership.Based on observations collected at the Germany-Spanish AstronomicalCenter, Calar Alto, Spain.Tables \ref{table2} and \ref{table3} are only available in electronicform at http://www.edpsciences.org
| Chemical enrichment and star formation in the Milky Way disk. III. Chemodynamical constraints
In this paper, we investigate some chemokinematical properties of theMilky Way disk, by using a sample composed by 424 late-type dwarfs. Weshow that the velocity dispersion of a stellar group correlates with theage of this group, according to a law proportional to t0.26,where t is the age of the stellar group. The temporal evolution of thevertex deviation is considered in detail. It is shown that the vertexdeviation does not seem to depend strongly on the age of the stellargroup. Previous studies in the literature seem to not have found it dueto the use of statistical ages for stellar groups, rather thanindividual ages. The possibility to use the orbital parameters of a starto derive information about its birthplace is investigated, and we showthat the mean galactocentric radius is likely to be the most reliablestellar birthplace indicator. However, this information cannot bepresently used to derive radial evolutionary constraints, due to anintrinsic bias present in all samples constructed from nearby stars. Anextensive discussion of the secular and stochastic heating mechanismscommonly invoked to explain the age-velocity dispersion relation ispresented. We suggest that the age-velocity dispersion relation couldreflect the gradual decrease in the turbulent velocity dispersion fromwhich disk stars form, a suggestion originally made by Tinsley &Larson (\cite{tinsley}, ApJ, 221, 554) and supported by several morerecent disk evolution calculations. A test to distinguish between thetwo types of models using high-redshift galaxies is proposed.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/517
| 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
| New Metallicity Calibration Down to [Fe/H] = -2.75 dex
We have taken 88 dwarfs, covering the colour-index interval 0.37 <=(B-V)0 <= 1.07mag, with metallicities -2.70 <= [Fe/H]<= +0.26dex, from three different sources for new metallicitycalibration. The catalogue of Cayrel de Stroble et al. (2001), whichincludes 65% of the stars in our sample, supplies detailed informationon abundances for stars with determination based on high-resolutionspectroscopy. In constructing the new calibration we have used as`corner stones' 77 stars which supply at least one of the followingconditions: (i) the parallax is larger than 10mas (distance relative tothe Sun less than 100pc) and the galactic latitude is absolutely higherthan 30° (ii) the parallax is rather large, if the galactic latitudeis absolutely low and vice versa. Contrary to previous investigations, athird-degree polynomial is fitted for the new calibration: [Fe/H]=0.10 -2.76δ - 24.04δ2 + 30.00δ3. Thecoefficients were evaluated by the least-squares method, without regardto the metallicity of Hyades. However, the constant term is in the rangeof metallicity determined for this cluster, i.e.0.08<=[Fe/H]<=0.11dex. The mean deviation and the mean error inour work are equal to those of Carney (1979), for [Fe/H] >= -1.75dexwhere Carney's calibration is valid
| 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
| Behavior of Sulfur Abundances in Metal-poor Giants and Dwarfs
LTE and non-LTE (NLTE) abundances of sulfur in six metal-poor giants and61 dwarfs (62 dwarfs including the Sun) were explored in the range of-3<~[Fe/H]<~+0.5 using high-resolution, high signal-to-noise ratiospectra of the S I 8693.9 and 8694.6 Å lines observed by us andmeasured by François and Clegg, Lambert, & Tomkin. NLTEeffects in S abundances are found to be small and practicallynegligible. The behavior of [S/Fe] versus [Fe/H] exhibits a linearincreasing trend without plateau with decreasing [Fe/H]. Combining ourresults with those available in the literature, we find that the slopeof the increasing trend is -0.25 in the NLTE behavior of [S/Fe], whichis comparable to that observed in [O/Fe]. The observed behavior of S mayrequire chemical evolution models of the Galaxy, in which scenarios ofhypernovae nucleosynthesis and/or time-delayed deposition intointerstellar medium are incorporated.
| 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
| Photometric Monitoring of Bright Be Stars. IV. 1996-1999
We report long-term UBV observations of 15 bright, active Be stars,namely: X Persei, EW Canis Majoris, θ Coronae Borealis, 4 (V839)Herculis, 88 (V744) Herculis, 66 (V2048) Ophiuchi, NW Serpentis, CXDraconis, 12 (V395) Vulpeculae, 28 (V1624) Cygni, QR Vulpeculae, 59(V832) Cygni, EW Lacertae, ο Andromedae, and KX Andromedae. Theobservations were made in 1996-1999 through the Automatic PhotometricTelescope Service in Arizona and through the American Association ofVariable Star Observers (AAVSO) photoelectric photometry program andhave been added to a database extending back 20 years. We describe thestars' recent behavior and also comment on the long-term behavior ofsome of them. They vary photometrically on timescales ranging from abouta day to many years.
| Revised Magnesium Abundances in Galactic Halo and Disk Stars
A differential analysis of the magnesium abundances in 61 F-K dwarfs andsubgiants with metallicities -2.6<[Fe/H]<+0.2 is performed basedon published observational data. Fundamental parameters for 36 stars aredetermined: T eff from V-K and V-R; logg from HIPPARCOS parallaxes, and[Fe/H] and ξt from Fe II lines. The computations allow for non-LTEeffects in the formation of the Mg I lines. For most of the stars, thestandard errors in the Mg abundances do not exceed 0.07 dex. Themetallicity dependence of [Mg/Fe] is analyzed. Magnesium shows aconstant overabundance relative to Fe of 0.46±0.06 dex formetallicities -2.6<[Fe/H]<-0.7 Mg. The Mg overabundance decreasesabruptly to +0.27 dex at [Fe/H]⋍-0.7. At highermetallicities, the Mg abundance smoothly decreases to the solar value at[Fe/H]=0.0. Halo stars with metallicities [Fe/H]<-1.0 exhibit lowerMg overabundances ( ) compared to the [Mg/Fe] values for other starswith similar [Fe/H].
| 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.
| 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
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| 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
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and 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,199 of the stars in Part I 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(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| 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.
| 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
| Correlated Depletion of Lithium and Beryllium in F Stars
It has been known for over a decade that Hyades F stars have severelydepleted their Li abundances (the "Li gap"), in sharp contrast to thepredictions of the standard stellar evolution theory. We began a Li andBe survey aimed at identifying the physical mechanism that creates theLi gap. We present here the first results of that survey, which includehigh-resolution (R = 48,000-120,000) and high signal-to-noise ratioobservations in 24 stars of the Li I lambda 6707.8 and/or the Be IIlambda 3131 doublets taken at the University of Hawaii 2.2 m,Canada-France-Hawaii 3.6 m, and Keck I 10 m telescopes. Our programstars with detections in both Li and Be define a clear trend thatsuggests (1) the surface Li and Be abundance depletions are correlatedand (2) surface Li diminishes more rapidly than surface Be. Our resultssuggest that correlated Li and Be depletion is a normal process that Fstars undergo. The Li-Be trend argues strongly against the mass-loss anddiffusion mechanisms and strongly supports slow mixing as the cause ofthe surface light-element deficiencies. Moreover, models withrotationally induced mixing are in better agreement with the data thanare models with wave-driven mixing.
| The ROSAT all-sky survey catalogue of optically bright main-sequence stars and subgiant stars
We present X-ray data for all main-sequence and subgiant stars ofspectral types A, F, G, and K and luminosity classes IV and V listed inthe Bright Star Catalogue that have been detected as X-ray sources inthe ROSAT all-sky survey; several stars without luminosity class arealso included. The catalogue contains 980 entries yielding an averagedetection rate of 32 percent. In addition to count rates, sourcedetection parameters, hardness ratios, and X-ray fluxes we also listX-ray luminosities derived from Hipparcos parallaxes. The catalogue isalso available in electronic form via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Nearby stars of the Galactic disk and halo
Model atmosphere analyses of echelle spectra of some fifty nearby F- andG-type stars are presented. The sample is confined to the main-sequence,turnoff and subgiant region, regardless of the metal abundance. On thebase of these data, spectroscopic parallaxes are calculated and comparedto the Hipparcos astrometry to explore the reliability of the derivedstellar parameters, notably the surface gravity and metal abundancescale. The spectroscopic distance scale is found in good agreement withthe Hipparcos parallaxes and is characterized by a 5% rms uncertainty.The results suggest a precision in log g of ~ 0.1 dex, and 0.05-0.10 dexfor the metallicity. There is also reasonable evidence for thespectroscopic effective temperature scale to be free of systematicerrors; typical uncertainties are assessed to ~ 80 K. The basicspectroscopic parameters are supplemented by data for themicroturbulence velocities, the projected rotational velocities, stellarradii and alpha -enhancement abundances, the latter represented by theelement magnesium. Stellar masses are also given, though many areprobably subject to small adjustments (typically 5%) in forthcominganalyses. The well-defined distance correlation is also demonstrated tobe an efficient means for an identification of spectroscopic binaries.The results are discussed in terms of a spectroscopically establisheddistance scale, the sites of the stellar populations in the[Mg/H]-[Fe/Mg] plane, a timescale for the Galactic thin disk, and thepotential of future Teff-log g Kiel diagrams for a precisedetermination of Galactic globular clusters ages. Based on observationsat the German Spanish Astronomical Center, Calar Alto, Spain.
| Rotation modulation or/and pulsation in O Andromedae. I. The photometric results of an international multisite multitechnique campaign
We present the photometry of a month-long international campaign on thevariable Be star o Andromedae. Excellent time coverage and photometricprecision permit a critical comparison for the first time between thepulsational and the rotational modulation hypotheses. A multiperiodicanalysis of data taken many years apart shows sets of close frequencies.The amplitude ratio between the ultraviolet and visible variations iswhat is expected for early-type star pulsation. But, the total amplitudeand the order of importance of the frequencies is very different betweenobservation campaigns. A simple double wave periodic curve accounts formost of the light variation: a rotation/modulation model is considered,with activity variations in or just above the photosphere. Any modelmust explain the observed changes in the amplitudes of the frequencecorresponding to the period and its first harmonic. A very simple modelwith two stable photospheric activity "features" is insufficient toexplain the small variations observed around the mean values of theperiod and its light amplitude. Thus we propose that the photosphere,which is very probably oblate and seen almost equator-on, is dividedinto zonal bands undergoing differential rotation.
| Revised ages for stars in the solar neighbourhood
New ages are computed for the stars from the Edvardsson et al. (1993)data set. The revised values are systematically larger toward older ages(t>4 Gyr), while they are slightly lower for t<4 Gyr. A similar,but considerably smaller trend is present when the ages are computedwith the distances based on Hipparcos parallaxes. The resultingage-metallicity relation has a small, but distinct slope of ~ em0.07dex/Gyr. Tables 3\to8 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or WWW at URLhttp://cdsweb.u-strasbg.fr/Abstract.html
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Андромеда |
| Ректацензија: | 23h07m45.40s |
| Deклинација: | +49°17'45.0" |
| Apparent магнитуда: | 5.7 |
| Даљина: | 34.095 parsecs |
| Proper motion RA: | 151.8 |
| Proper motion Dec: | 131.6 |
| B-T magnitude: | 6.179 |
| V-T magnitude: | 5.727 |
Каталог и designations:
|
