Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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).
| Chemical abundances of 32 mildly metal-poor stars
Context: .The formation scenario of the Galactic thick disk is anunresolved problem. Chemical abundances in long-lived dwarf stars of thethin and thick disks provide information of the Galactic diskformation.Aims.We present photospheric abundances of the O, Na, Mg, Al,Si, Ca, Sc, Ti, V, Cr, Mn, Ni, and Ba elements for 32 mildly metal-poorstars with [Fe/H] -0.7. According to their kinematics, age, and [α/Fe] , sample stars are identified to thin disk, thick disk, andhalo population memberships. Element abundances for sample stars arediscussed as a function of metallicity.Methods.High resolution and highsignal-to-noise ratio spectra were obtained with the CoudéEchelle Spectrograph mounted on the 2.16 m telescope at the NationalAstronomical Observatories (Xinglong, China). Effective temperatureswere estimated from colour indices, and surface gravities from Hipparcosparallaxes. Stellar abundances were determined from a differential LTEanalysis. The kinematics parameters were calculated from the parallax,proper motion, and radial velocity. Stellar ages were determined fromtheoretical stellar evolution tracks.Results.The average age of thethick disk stars is older than the thin disk stars. Our elementabundance results extend and confirm previous works. The oxygen andother α-elements (Mg, Si, Ca, and Ti) abundances of thin and thickdisk stars show distinct trends at [Fe/H]≤-0.60. The [Al/Fe]behaviour is exactly as an α-element, although the separation for[Na/Fe] of thin and thick disk stars is not clear. The elements V, Cr,and Ni follow Fe very closely, and there is no offset between thin andthick disk stars, but the Sc and Mn abundance trends of the thin andthick disk stars are different, and [Ba/Fe] of thin disk and thick diskstars shows different behaviour.
| 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.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| 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.
| 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
| Isochrone ages for field dwarfs: method and application to the age-metallicity relation
A new method is presented to compute age estimates from theoreticalisochrones using temperature, luminosity and metallicity data forindividual stars. Based on Bayesian probability theory, this methodavoids the systematic biases affecting simpler strategies and providesreliable estimates of the age probability distribution function forlate-type dwarfs. Basic assumptions concerning the a priori parameterdistribution suitable for the solar neighbourhood are combined with thelikelihood assigned to the observed data to yield the complete posteriorage probability. This method is especially relevant for G dwarfs in the3-15 Gyr range of ages, crucial to the study of the chemical anddynamical history of the Galaxy. In many cases, it yields markedlydifferent results from the traditional approach of reading the derivedage from the isochrone nearest to the data point. We show that thestrongest process affecting the traditional approach is that of stronglyfavouring computed ages near the end-of-main-sequence lifetime. TheBayesian method compensates for this potential bias and generallyassigns much higher probabilities to lower main-sequence ages, comparedwith short-lived evolved stages. This has a strong influence on anyapplication to galactic studies, especially given the presentuncertainties on the absolute temperature scale of the stellar evolutionmodels. In particular, the known mismatch between the model predictionsand the observations for moderately metal-poor dwarfs (-1 < [Fe/H]< -0.3) has a dramatic effect on the traditional age determination.We apply our method to the classic sample of Edvardsson et al., whoderived the age-metallicity relation (AMR) of 189 field dwarfs withprecisely determined abundances. We show how much of the observedscatter in the AMR is caused by the interplay between the systematicbiases affecting the traditional age determination, the colour mismatchwith the evolution models and the presence of undetected binaries. Usingnew parallax, temperature and metallicity data, our age determinationfor the same sample indicates that the intrinsic dispersion in the AMRis at most 0.15 dex and probably lower. In particular, we show that old,metal-rich objects ([Fe/H]~ 0.0 dex, age > 5 Gyr) and young,metal-poor objects ([Fe/H] < -0.5 dex, age < 6 Gyr) in manyobserved AMR plots are artefacts caused by too simple a treatment of theage determination. The incompatibility of those AMR plots with awell-mixed interstellar medium may therefore only be apparent.Incidentally, our results tend to restore confidence in the method ofage determination from the chromospheric activity for field dwarfs.
| Magnesium abundances in mildly metal-poor stars from different indicators
We present Mg abundances derived from high-resolution spectra usingseveral MgI and two high-excitation MgII lines for 19 metal-poor starswith [Fe/H] values between -1.1 and +0.2. The main goal is to search forsystematic differences in the derived abundances between the twoionization state lines. Our analysis shows that the one-dimensionallocal thermodynamic equilibrium (LTE) and non-LTE (N-LTE) study finds avery good agreement between these features. The [Mg/Fe] versus [Fe/H]relationship derived, despite the small sample of stars, is also inagreement with the classical figure of increasing [Mg/Fe] withdecreasing metallicity. We find a significant scatter however, in the[Mg/Fe] ratio at [Fe/H]~-0.6 which is currently explained as aconsequence of the overlap at this metallicity of thick- and thin-discstars, which were probably formed from material with differentnucleosynthesis histories. We speculate on the possible consequences ofthe agreement found between MgI and MgII lines on the very well-known Oproblem in metal-poor stars. We also study the [O/Mg] ratio in thesample stars using O abundances from the literature and find that thecurrent observations and nucleosynthetic predictions from Type IIsupernovae disagree. We briefly discuss some alternatives to solve thisdiscrepancy.
| 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.
| Stars of Extragalactic Origin in the Solar Neighborhood
For 77 main-sequence F-G stars in the solar neighborhood with publishediron, magnesium, and europium abundances determined from high-dispersionspectra and with the ages estimated from theoretical isochrones, wecalculated the spatial velocities using Hipparcos data and the Galacticorbital elements. A comparison with the orbital elements of the globularclusters that are known to have been accreted by our Galaxy in the pastreveals stars of extragalactic origin. We show that the abundance ratiosof r- and alpha-elements in all the accreted stars differ sharply fromthose in the stars that are genetically associated with the Galaxy.According to current theoretical models, europium is produced mainly inlow-mass type-II supernovae (SNe II), while magnesium is synthesized inlarge quantities in high-mass SN II progenitors. Since all the oldaccreted stars of our sample exhibit a significant Eu overabundancerelative to Mg, we conclude that the maximum masses of the SN IIprogenitors outside the Galaxy were much lower than those inside it. Onthe other hand, only a small number of young accreted stars exhibit lownegative ratios [Eu/Mg] < 0. This can be explained by the delay ofprimordial star formation and the explosions of high-mass SNe II in arelatively small part of extragalactic space. We provide evidence thatthe interstellar medium was weakly mixed at the early evolutionarystages of the Galaxy formed from a single protogalactic cloud, and thatthe maximum mass of the SN II progenitors increased in it with timesimultaneously with the increase in mean metallicity.
| 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 Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| Synthetic Lick Indices and Detection of α-enhanced Stars. II. F, G, and K Stars in the -1.0 < [Fe/H] < +0.50 Range
We present an analysis of 402 F, G, and K solar neighborhood stars, withaccurate estimates of [Fe/H] in the range -1.0 to +0.5 dex, aimed at thedetection of α-enhanced stars and at the investigation of theirkinematical properties. The analysis is based on the comparison of 571sets of spectral indices in the Lick/IDS system, coming from fourdifferent observational data sets, with synthetic indices computed withsolar-scaled abundances and with α-element enhancement. We useselected combinations of indices to single out α-enhanced starswithout requiring previous knowledge of their main atmosphericparameters. By applying this approach to the total data set, we obtain alist of 60 bona fide α-enhanced stars and of 146 stars withsolar-scaled abundances. The properties of the detected α-enhancedand solar-scaled abundance stars with respect to their [Fe/H] values andkinematics are presented. A clear kinematic distinction betweensolar-scaled and α-enhanced stars was found, although a one-to-onecorrespondence to ``thin disk'' and ``thick disk'' components cannot besupported with the present data.
| 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.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Subgiants as probes of galactic chemical evolution
Chemical abundances for 23 candidate subgiant stars have been derivedwith the aim at exploring their usefulness for studies of galacticchemical evolution. High-resolution spectra from ESO CAT-CES andNOT-SOFIN covered 16 different spectral regions in the visible part ofthe spectrum. Some 200 different atomic and molecular spectral lineshave been used for abundance analysis of 30 elemental species. Thewings of strong, pressure-broadened metal lines were used fordetermination of stellar surface gravities, which have been comparedwith gravities derived from HIPPARCOS parallaxes and isochronic masses.Stellar space velocities have been derived from HIPPARCOS and Simbaddata, and ages and masses were derived with recent isochrones. Only 12of the stars turned out to be subgiants, i.e. on the ``horizontal'' partof the evolutionary track between the dwarf- and the giant stages. Theabundances derived for the subgiants correspond closely to those ofdwarf stars. With the possible exceptions of lithium and carbon we findthat subgiant stars show no ``chemical'' traces of post-main-sequenceevolution and that they are therefore very useful targets for studies ofgalactic chemical evolution.Based on observations made at ESO, La Silla.Based on observations made at NOT, La Palma.Tables 1 and 2 are only available in electronic form athttp://www.edpsciences.org
| 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
| 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
| On the link between rotation, chromospheric activity and Li abundance in subgiant stars
The connection rotation-CaII emission flux-lithium abundance is analyzedfor a sample of bona fide subgiant stars, with evolutionary statusdetermined from HIPPARCOS trigonometric parallax measurements and fromthe Toulouse-Geneva code. The distribution of rotation and CaII emissionflux as a function of effective temperature shows a discontinuitylocated around the same spectral type, F8IV. Blueward of this spectraltype, subgiants have a large spread of values of rotation and CaII flux,whereas stars redward of F8IV show essentially low rotation and low CaIIflux. The strength of these declines depends on stellar mass. Theabundance of lithium also shows a sudden decrease. For subgiants withmass lower than about 1.2 Msun the decrease is located laterthan that in rotation and CaII flux, whereas for masses higher than 1.2Msun the decrease in lithium abundance is located around thespectral type F8IV. The discrepancy between the location of thediscontinuities of rotation and CaII emission flux and log n(Li) forstars with masses lower than 1.2 Msun seems to reflect thesensitivity of these phenomena to the mass of the convective envelope.The drop in rotation, which results mostly from a magnetic braking,requires an increase in the mass of the convective envelope less thanthat required for the decrease in log n(Li). The location of thediscontinuity in log n(Li) for stars with masses higher than 1.2Msun, in the same region of the discontinuities in rotationand CaII emission flux, may also be explained by the behavior of thedeepening of the convective envelope. The more massive the star is, theearlier is the increase of the convective envelope. In contrast to therelationship between rotation and CaII flux, which is fairly linear, therelationship between lithium abundance and rotation shows no cleartendency toward linear behavior. Similarly, no clear linear trend isobserved in the relationship between lithium abundance and CaII flux. Inspite of these facts, subgiants with high lithium content also have highrotation and high CaII emission flux.
| Abundances for metal-poor stars with accurate parallaxes. I. Basic data
We present element-to-element abundance ratios measured from highdispersion spectra for 150 field subdwarfs and early subgiants withaccurate Hipparcos parallaxes (errors <20%). For 50 stars new spectrawere obtained with the UVES on Kueyen (VLT UT2), the McDonald 2.7 mtelescope, and SARG at TNG. Additionally, literature equivalent widthswere taken from the works by Nissen & Schuster, Fulbright, andProchaska et al. to complement our data. The whole sample includes boththick disk and halo stars (and a few thin disk stars); most stars havemetallicities in the range -2<[Fe/H]<-0.6. We found our data, thatof Nissen & Schuster, and that of Prochaska to be of comparablequality; results from Fulbright scatter a bit more, but they are stillof very good quality and are extremely useful due to the large size ofhis sample. The results of the present analysis will be used inforthcoming papers to discuss the chemical properties of thedissipational collapse and accretion components of our Galaxy.Based in part on data collected at the European Southern Observatory,Chile, at the MacDonald Observatory, Texas, USA, and at the TelescopioNazionale Galileo, Canary Island, INAF,Italy-Spain.}\fnmsep\thanks{Table 1 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia\resizebox{8.8cm}{2.2mm}htpp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/187}
| 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.
| Abundances and Kinematics of Field Stars. II. Kinematics and Abundance Relationships
As an investigation of the origin of ``α-poor'' halo stars, weanalyze kinematic and abundance data for 73 intermediate-metallicitystars (-1>[Fe/H]>=-2) selected from Paper I of this series. We findevidence for a connection between the kinematics and the enhancement ofcertain element-to-iron ([X/Fe]) ratios in these stars. Statisticallysignificant correlations were found between [X/Fe] and galacticrest-frame velocities (vRF) for Na, Mg, Al, Si, Ca, and Ni,with marginally significant correlations existing for Ti and Y as well.We also find that the [X/Fe] ratios for these elements all correlatewith a similar level of significance with [Na/Fe]. Finally, we comparethe abundances of these halo stars against those of stars in nearbydwarf spheroidal (dSph) galaxies. We find significant differencesbetween the abundance ratios in the dSph stars and halo stars of similarmetallicity. From this result, it is unlikely that the halo stars in thesolar neighborhood, including even the ``α-poor'' stars, were oncemembers of disrupted dSph galaxies similar to those studied to date.
| 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
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| 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
| Levels of coronal and chromospheric activity in late-type stars and various types of dynamo waves
We analyze the X-ray emission and chromospheric activity of late-type F,G, and K stars studied in the framework of the HK project. More powerfulcoronas are possessed by stars displaying irregular variations of theirchromospheric emission, while stars with cyclic activity arecharacterized by comparatively modest X-ray luminosities and ratios ofthe X-ray to bolometric luminosity L X/L bol. This indicates that thenature of processes associated with magnetic-field amplification in theconvective envelope changes appreciably in the transition from small tolarge dynamo numbers, directly affecting the character of the(α-Ω) dynamo. Due to the strong dependence of both thedynamo number and the Rossby number on the speed of axial rotation,earlier correlations found between various activity parameters and theRossby number are consistent with our conclusions. Our analysis makes itpossible to draw the first firm conclusions about the place of solaractivity among analogous processes developing in active late-type stars.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Kanatli At |
| Sað Açýklýk: | 21h52m29.90s |
| Yükselim: | +28°47'36.0" |
| Görünürdeki Parlaklýk: | 5.53 |
| Uzaklýk: | 27.663 parsek |
| özdevim Sað Açýklýk: | -58.4 |
| özdevim Yükselim: | -61.8 |
| B-T magnitude: | 6.021 |
| V-T magnitude: | 5.572 |
Kataloglar ve belirtme:
|
INHABITED SKY
