Contents
Images
Upload your image
DSS Images Other Images
Related articles
Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| Potassium abundances in nearby metal-poor stars
Aims.The potassium abundances for 58 metal-poor stars are determinedusing high-resolution spectroscopy. The abundance trends in stars ofdifferent population are discussed. Methods: .All abundanceresults have been derived from NLTE statistical equilibrium calculationsand spectrum synthesis methods. Results: .The NLTE corrections aresignificant (-0.20 to -0.55 dex) and they depend on the effectivetemperatures and surface gravities. The potassium abundances of thindisk, thick disk and halo stars show distinct trends, such as in thecase of the α-elements. [K/Fe] gradually increases with a decreasein [Fe/H] for thin disk stars, [K/Fe] of thick disk stars is nearlyconstant at [K/Fe] ~ +0.30 dex; halo stars also have nearly constantvalues of [K/Fe] ~ +0.20 dex. Conclusions: .The deriveddependence between [K/Fe] and [Fe/H] is in agreement with thetheoretical prediction of published model calculations of the chemicalevolution of the Galaxy. The nearly constant [K/Mg] ratio with smallscatter suggests that the nucleosynthesis of potassium is closelycoupled to the α-elements.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Fine structure in the phase space distribution of nearby subdwarfs
We analysed the fine structure of the phase space distribution functionof nearby subdwarfs using data extracted from various catalogues.Applying a new search strategy based on Dekker's theory of galacticorbits, we found four overdensely populated regions in phase space.Three of them were correlated with previously known star streams: theHyades-Pleiades and Hercules streams in the thin disk of the Milky Wayand the Arcturus stream in the thick disk. In addition we find evidencefor another stream in the thick disk, which resembles closely theArcturus stream and probably has the same extragalactic origin.
| 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.
| 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.
| 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.
| 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.
| Heavy elements and chemical enrichment in globular clusters
High resolution (R 40 000) and high S/N spectra have been acquiredwith UVES on the VLT-Kueyen (Paranal Observatory, ESO Chile) for severalmain sequence turnoff stars (V 17 mag) and subgiants at the baseof the Red Giant Branch (V 16 mag) in three globular clusters (NGC6397, NGC 6752 and 47 Tuc/NGC 104) at different metallicities(respectively [Fe/H] ≃ -2.0; -1.5; -0.7). Spectra for a sample of25 field halo subdwarves have also been taken with equal resolution, buthigher S/N. These data have been used to determine the abundances ofseveral neutron-capture elements in these three clusters: strontium,yttrium, barium and europium. This is the first abundance determinationof these heavy elements for such unevolved stars in these three globularclusters. These values, together with the [Ba/Eu] and [Sr/Ba] abundanceratios, have been used to test the self-enrichment scenario. Acomparison is done with field halo stars and other well known Galacticglobular clusters in which heavy elements have already been measured inthe past, at least in bright giants (V 11-12 mag). Our resultsshow clearly that globular clusters have been uniformly enriched by r-and s-process syntheses, and that most of them seem to follow exactlythe same abundance patterns as field halo stars, which discards the``classical'' self-enrichment scenario for the origin of metallicitiesand heavy elements in globular clusters.Based on data collected at the European Southern Observatory with theVLT-UT2, Paranal, Chile (ESO-LP 165.L-0263).
| 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
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/419/167
| Abundances of Na, Mg and Al in nearby metal-poor stars
To determine the population membership of nearby stars we exploreabundance results obtained for the light neutron-rich elements23Na and 27 Al in a small sample of moderatelymetal-poor stars. Spectroscopic observations are limited to the solarneighbourhood so that gravities can be determined from HIPPARCOSparallaxes, and the results are confronted with those for a separatesample of more metal-poor typical halo stars. Following earlierinvestigations, the abundances of Na, Mg and Al have been derived fromNLTE statistical equilibrium calculations used as input to line profilesynthesis. Compared with LTE the abundances require systematiccorrections, with typical values of +0.05 for [Mg/Fe], -0.1 for [Na/Fe]and +0.2 for [Al/Fe] in thick disk stars where [Fe/H] -0.6. Inmore metal-poor halo stars these values reach +0.1, -0.4, and +0.5,respectively, differences that can no longer be ignored.After careful selection of a clean subsample free from suspected orknown binaries and peculiar stars, we find that [Na/Mg] and [Al/Mg], incombination with [Mg/Fe], space velocities and stellar evolutionaryages, make possible an individual discrimination between thick disk andhalo stars. At present, this evidence is limited by the small number ofstars analyzed. We identify a gap at [Al/Mg] -0.15 and [Fe/H] -1.0 that isolates stars of the thick disk from those in the halo.A similar separation occurs at [Na/Mg] -0.4. We do not confirm theage gap between thin and thick disk found by Fuhrmann. Instead we findan age boundary between halo and thick disk stars, however, with anabsolute value of 14 Gyr that must be considered as preliminary. Whilethe stellar sample is by no means complete, the resulting abundancesindicate the necessity to revise current models of chemical evolutionand/or stellar nucleosynthesis to allow for an adequate production ofneutron-rich species in early stellar generations.Based on observations collected at the German-Spanish AstronomicalCenter, Calar Alto (CAHA H01-2.2-002) and at the European SouthernObservatory, Chile (ESO 67.D-0086).
| Sodium Abundances in Stellar Atmospheres with Differing Metallicities
The non-LTE sodium abundances of 100 stars with metallicities-3<[Fe/H]<0.3 are determined using high-dispersion spectra withhigh signal-to-noise ratios. The sodium abundances [Na/Fe] obtained areclose to the solar abundance and display a smaller scatter than valuespublished previously. Giants (logg<3.8) with [Fe/H]<-1 do notdisplay overabundances of sodium, and their sodium abundances do notshow an anticorrelation with the oxygen abundance, in contrast toglobular-cluster giants. They likewise do not show sodium-abundancevariations with motion along the giant branch. No appreciable decreasein the sodium abundance was detected for dwarfs (logg>3.8) withmetallicities -2<[Fe/H]<-1. The observed relation between [Na/Fe]and [Fe/H] is in satisfactory agreement with the theoreticalcomputations of Samland, which take into account the metallicitydependence of the sodium yield and a number of other factors affectingthe distribution of elements in the Galaxy during the course of itsevolution.
| 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.
| 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
| α Element Abundances in Mildly Metal-Poor Stars
We present [O/Fe] and other α-elements/Fe ratios in a sample of 24mildly metal-poor stars. The sample stars are thought to be brighterthan 9.0 magnitude and have available uvby photometric data. Also, basedon the typical LTE abundance analysis, we find that [Si/Fe] and [Ca/Fe]are correlated with each other. Combining the kinematic data and themetallicity, we can classify the sample stars into three groups. Anabundance analysis shows some evidence that these groups are chemicallydiscrete from each other. Further, the general trend of a decreasingoverabundance of the α elements with increasing metallicity hasbeen confirmed.
| A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries
We report 25,563 radial velocity measurements for 1359 single-linedstars in the Carney-Latham sample of 1464 stars selected for high propermotion. For 171 of these, we present spectroscopic orbital solutions. Wefind no obvious difference between the binary characteristics in thehalo and the disk populations. The observed frequency is the same, andthe period distributions are consistent with the hypothesis that the twosets of binaries were drawn from the same parent population. Thissuggests that metallicity in general, and radiative opacities inparticular, have little influence over the fragmentation process thatleads to short-period binaries. All the binaries with periods shorterthan 10 days have nearly circular orbits, while the binaries withperiods longer than 20 days exhibit a wide range of eccentricities and amedian value of 0.37. For the metal-poor high-velocity halo binaries inour sample, the transition from circular to eccentric orbits appears tooccur at about 20 days, supporting the conclusion that tidalcircularization on the main sequence is important for the oldestbinaries in the Galaxy. Some of the results presented here usedobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
| 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.
| Abundances of Cu and Zn in metal-poor stars: Clues for Galaxy evolution
We present new observations of copper and zinc abundances in 90metal-poor stars, belonging to the metallicity range -3<[Fe/H]<-0.5. The present study is based on high resolutionspectroscopic measurements collected at the Haute Provence Observatoire(R= 42 000, S/N>100). The trend of Cu and Zn abundances as a functionof the metallicity [Fe/H] is discussed and compared to that of otherheavy elements beyond iron. We also estimate spatial velocities andgalactic orbital parameters for our target stars in order to disentanglethe population of disk stars from that of halo stars using kinematiccriteria. In the absence of a firm a priori knowledge of thenucleosynthesis mechanisms controlling Cu and Zn production, and of therelative stellar sites, we derive constraints on these last from thetrend of the observed ratios [Cu/Fe] and [Zn/Fe] throughout the historyof the Galaxy, as well as from a few well established properties ofbasic nucleosynthesis processes in stars. We thus confirm that theproduction of Cu and Zn requires a number of different sources (neutroncaptures in massive stars, s-processing in low and intermediate massstars, explosive nucleosynthesis in various supernova types). We alsoattempt a ranking of the relative roles played by different productionmechanisms, and verify these hints through a simple estimate of thegalactic enrichment in Cu and Zn. In agreement with suggestionspresented earlier, we find evidence that type Ia Supernovae must play arelevant role, especially for the production of Cu. Based on the spectracollected with the 1.93-m telescope of Haute Provence Observatory.
| 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
| Analysis of neutron capture elements in metal-poor stars
We derived model atmosphere parameters (Teff, log g, [Fe/H],Vt) for 90 metal-deficient stars (-0.5<[Fe/H]<-3),using echelle spectra from the ELODIE library (Soubiran et al.\cite{soubet98}). These parameters were analyzed and compared withcurrent determinations by other authors. The study of the followingelements was carried out: Mg, Si, Ca, Sr, Y, Ba, La, Ce, Nd, and Eu. Therelative contributions of s- and r-processes were evaluated andinterpreted through theoretical computations of the chemical evolutionof the Galaxy. The chemical evolution models (Pagel &Tautvaišienė \cite{pagta95}; Timmes et al. \cite{timet95})depict quite well the behaviour of [Si/Fe], [Ca/Fe] with [Fe/H]. Thetrend of [Mg/Fe] compares more favourably with the computations of Pagel& Tautvaišienė (\cite{pagta95}) than those of Timmes etal. (\cite{timet95}). The runs of n-capture elements vs. metallicity aredescribed well both by the model of Pagel & Tautvaišienė(\cite{pagta95}, \cite{pagta97}) and by the model of Travaglio et al.(\cite{travet99}) at [Fe/H]>-1.5, when the matter of the Galaxy issufficiently homogeneous. The analysis of n-capture element abundancesconfirms the jump in [Ba/Fe] at [Fe/H]=-2.5. Some stars from our sampleat [Fe/H]<-2.0 show a large scatter of Sr, Ba, Y, Ce. This scatter isnot caused by the errors in the measurements, and may reflect theinhomogeneous nature of the prestellar medium at early stages ofgalactic evolution. The matching of [Ba/Fe], [Eu/Fe] vs. [Fe/H] with theinhomogeneous model by Travaglio et al. (\cite{travet01a}) suggests thatat [Fe/H]<-2.5, the essential contribution to the n-rich elementabundances derives from the r-process. The main sources of theseprocesses may be low mass SN II. The larger dispersion of s-processelement abundances with respect to alpha -rich elements may arise bothfrom the birth of metal-poor stars in globular clusters with followingdifferent evolutionary paths and (or) from differences in s-elementenrichment in Galaxy populations. Based on spectra collected at theObservatoire de Haute-Provence (OHP), France
| On the stellar content of the open clusters Melotte 105, Hogg 15, Pismis 21 and Ruprecht 140
CCD observations in the B, V and I passbands have been used to generatecolour-magnitude diagrams reaching down to V ~ 19 mag for two slightlycharacterized (Melotte 105 and Hogg 15) and two almost unstudied (Pismis21 and Ruprecht 140) open clusters. The sample consists of about 1300stars observed in fields of about 4arcmin x4arcmin . Our analysis showsthat neither Pismis 21 nor Ruprecht 140 are genuine open clusters sinceno clear main sequences or other meaningful features can be seen intheir colour-magnitude diagrams. Melotte 105 and Hogg 15 are openclusters affected by E(B-V) = 0.42 +/- 0.03 and 0.95 +/- 0.05,respectively. Their distances to the Sun have been estimated as 2.2 +/-0.3 and 2.6 +/- 0.08 kpc, respectively, while the corresponding agesestimated from empirical isochrones fitted to the Main Sequence clustermembers are ~ 350 Myr and 300 Myr, respectively. The present data arenot consistent with the membership of the WN6 star HDE 311884 to Hogg15. Tables 2 to 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.793.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/370/931
| The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters
High dispersion spectra (R>~ 40 000) for a quite large number ofstars at the main sequence turn-off and at the base of the giant branchin NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLTUT2). The [Fe/H] values we found are -2.03+/- 0.02+/- 0.04\ and -1.42+/-0.02+/- 0.04\ for NGC 6397 and NGC 6752 respectively, where the firsterror bars refer to internal and the second ones to systematic errors(within the abundance scale defined by our analysis of 25 subdwarfs withgood Hipparcos parallaxes). In both clusters the [Fe/H]'s obtained forTO-stars agree perfectly (within a few percent) with that obtained forstars at the base of the RGB. The [O/Fe]=0.21+/- 0.05 value we obtainfor NGC 6397 is quite low, but it agrees with previous results obtainedfor giants in this cluster. Moreover, the star-to-star scatter in both Oand Fe is very small, indicating that this small mass cluster ischemically very homogenous. On the other hand, our results show clearlyand for the first time that the O-Na anticorrelation (up to now seenonly for stars on the red giant branches of globular clusters) ispresent among unevolved stars in the globular cluster NGC 6752, a moremassive cluster than NGC 6397. A similar anticorrelation is present alsofor Mg and Al, and C and N. It is very difficult to explain the observedNa-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixingscenario; we think it requires some non internal mechanism. Based ondata collected at the European Southern Observatory, Chile.
| Distances, Ages, and Epoch of Formation of Globular Clusters
We review the results on distances and absolute ages of Galacticglobular clusters (GCs) obtained after the release of the Hipparcoscatalog. Several methods aimed at the definition of the Population IIlocal distance scale are discussed, and their results compared,exploiting new results for RR Lyraes in the Large Magellanic Cloud(LMC). We find that the so-called short distance and long distancescales may be reconciled whether or not a consistent reddening scale isadopted for Cepheids and RR Lyrae variables in the LMC. Emphasis isgiven in the paper to the discussion of distances and ages of GCsderived using Hipparcos parallaxes of local subdwarfs. We find that theselection criteria adopted to choose the local subdwarfs, as well as thesize of the corrections applied to existing systematic biases, are themain culprit for the differences found among the various independentstudies that first used Hipparcos parallaxes and the subdwarf fittingtechnique. We also caution that the absolute age of M92 (usuallyconsidered one of the oldest clusters) still remains uncertain due tothe lack of subdwarfs of comparable metallicity with accurateparallaxes. Distances and ages for the nine clusters discussed in aprevious paper by Gratton et al. are rederived using an enlarged sampleof local subdwarfs, which includes about 90% of the metal-poor dwarfswith accurate parallaxes (Δπ/π<=0.12) in the wholeHipparcos catalog. On average, our revised distance moduli are decreasedby 0.04 mag with respect to Gratton et al. The corresponding age of theGCs is t=11.5+/-2.6 Gyr, where the error bars refer to the 95%confidence range. The relation between the zero-age horizontal branch(ZAHB) absolute magnitude and metallicity for the nine program clustersturns out to beMV(ZAHB)=(0.18+/-0.09)([Fe/H]+1.5)+(0.53+/-0.12) Thanks toHipparcos the major contribution to the total error budget associatedwith the subdwarf fitting technique has been moved from parallaxes tophotometric calibrations, reddening, and metallicity scale. This totaluncertainty still amounts to about +/-0.12 mag. We then compare thecorresponding (true) LMC distance modulusμLMC=18.64+/-0.12 mag with other existing determinations.We conclude that at present the best estimate for the distance of theLMC is μLMC=18.54+/-0.03+/-0.06, suggesting that distancesfrom the subdwarf fitting method are ~1 σ too long. Consequently,our best estimate for the age of the GCs is revised to Age=12.9+/-2.9Gyr (95% confidence range). The best relation between ZAHB absolutemagnitude and metallicity isMV(ZAHB)=(0.18+/-0.09)([Fe/H]+1.5)+(0.63+/-0.07). Finally, wecompare the ages of the GCs with the cosmic star formation rate recentlydetermined by studies of the Hubble Deep Field (HDF), exploiting thedeterminations of ΩM=0.3 andΩΛ=0.7 provided by Type Ia supernovae surveys.We find that the epoch of formation of the GCs (at z~3) matches well themaximum of the star formation rate for elliptical galaxies in the HDF asdetermined by Franceschini et al. Based on data from the Hipparcosastrometry satellite.
| Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,high signal-to-noise ratio survey of 168 stars, most of which aremetal-poor dwarfs. We follow a self-consistent LTE analysis technique todetermine the stellar parameters and abundances, and we estimate theeffects of random and systematic uncertainties on the resultingabundances. Element-to-iron ratios are derived for key α-, odd-Z,Fe-peak, and r- and s-process elements. Effects of non-LTE on theanalysis of Fe I lines are shown to be very small on average.Spectroscopically determined surface gravities are derived that arequite close to those obtained from Hipparcos parallaxes.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Πήγασος |
| Right ascension: | 21h26m42.91s |
| Declination: | +05°26'29.9" |
| Apparent magnitude: | 8.491 |
| Distance: | 76.805 parsecs |
| Proper motion RA: | 166.6 |
| Proper motion Dec: | -245.6 |
| B-T magnitude: | 9.171 |
| V-T magnitude: | 8.548 |
Catalogs and designations:
|
