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The New HiVIS Spectropolarimeter and Spectropolarimetric Calibration of the AEOS Telescope
We have designed, built, and calibrated a new spectropolarimeter for theHiVIS spectrograph (R ~ 12,000-49,000) on the Advanced Electro-OpticalSystem (AEOS) telescope. We have also conducted a polarizationcalibration of the telescope and instrument. In this paper, we introducethe design and use of the spectropolarimeter, in addition to a newdata-reduction package we have developed, and then discuss thepolarization calibration of the spectropolarimeter and the AEOStelescope. We used observations of unpolarized standard stars at manypointings to measure the telescope-induced polarization and compare itwith a Zemax model. The telescope induces polarization of 1%-6%, with astrong variation with wavelength and pointing, consistent with theexpected altitude and azimuth variation. We then used scattered sunlightas a linearly polarized source to measure the telescope'sspectropolarimetric response to linearly polarized light. Finally, wemade an all-sky map of the telescope's polarization response tocalibrate future spectropolarimetry.
| Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra
The LTE abundances of sulfur (S) of 21 metal-poor stars and one normalstar were explored in the metallicity range of -3 < [Fe/H] ≤ 0,based on the equivalent widths of the S I (1) 9212, 9237Å and S I(6) 8693, 9894Å lines measured on high-resolution spectra, whichwere observed by the OAO 1.88-m telescope equipped with HIDES. Our mainresults are: (1) The abundances derived from the S I (6) lines areconsistent with those from the S I (1) lines among our sample stars inthe range of [Fe/H] > -2 with an average difference of +0.03 ±0.05 dex, whereas a significant discrepancy is observed in the range of[Fe/H] ≤ -2. (2) The behavior of [S(6)/Fe], versus [Fe/H] of ourhalo sample stars exhibits a nearly flat trend with an average of +0.62± 0.09 dex in the range of -3 < [Fe/H] < -1.25, and shows adistribution around +0.29 dex in -1.25 ≤ [Fe/H] ≤ -0.7. Oursample stars with -1.25 ≤ [Fe/H] ≤ -0.5 follow an increasingtrend with decreasing [Fe/H]. The behavior of [S(1)/Fe] of our samplestars also shows essentially the same trend as [S(6)/Fe], though it isquantitatively different. (3) The S behavior in the range of -3 <[Fe/H] ≤ 0 inferred from the abundances of multiplets 6 and 1 arequalitatively consistent with each other, and may be represented by acombination of a nearly flat trend and a linearly increasing trend withdecreasing [Fe/H]. A transition of the trend is likely to occur at[Fe/H] -1.5 dex.
| 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.
| The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| 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.
| 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.
| A Spectral Atlas of F and G Stars
We present an atlas of a group of bright stars in the range of spectralclasses F--G and luminosity classes I--V. The spectra were obtainedwith spectral resolution R 15,000 within spectral region4500--6620 Å. Typical spectra of stars with different metallicity[Fe/H] are included. We also show the digital version of the spectraldata in FITS format.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| The Correlation of Lithium and Beryllium in F and G Field and Cluster Dwarf Stars
Although Li has been extensively observed in main-sequence field andcluster stars, there are relatively fewer observations of Be. We haveobtained Keck HIRES spectra of 36 late F and early G dwarfs in order tostudy the Li-Be correlation we found previously in the temperatureregime of 5900-6650 K. The sample size for this temperature range withdetectable and (usually) depleted Li and Be is now 88, including Li andBe abundances in both cluster and field stars. Therefore we can nowinvestigate the influence of other parameters such as age, temperature,and metallicity on the correlation. The Be spectra at 3130 Å weretaken over six nights from 1999 November to 2002 January and have aspectral resolution of ~48,000 and a median signal-to-noise ratio (S/N)of 108 pixel-1. We obtained Li spectra of 22 stars with theUniversity of Hawaii 88 inch (2.2 m) telescope and coudéspectrograph with a spectral resolution of ~70,000 and a median S/N of110 pixel-1. We have redetermined the effective temperaturesfor all the stars and adopted other parameters from published data orempirical relations. The abundances of both Li and Be in the stars weobserved were determined from spectrum synthesis with MOOG 2002. Thepreviously observed Li equivalent widths for some of our Be stars wereused with the new temperatures and MOOG 2002 in the ``blends'' mode. Forthe 46 field stars from this and earlier studies we find a linearrelation between A(Li) and A(Be) with a slope of 0.375+/-0.036. Over theTeff range 5900-6650 K, we find the modest scatter about theBe-Li relation to be significantly correlated with Teff andperhaps also [Fe/H]. Dividing the sample into two temperature regimes of6300-6650 K (corresponding to the cool side of the Li-Be dip) and5900-6300 K (corresponding to the Li ``plateau'') reveals possible smalldifferences in the slopes for the two groups, 0.404+/-0.034 and0.365+/-0.049, respectively. When we include the cluster stars (Hyades,Pleiades, Praesepe, UMa Group, and Coma), the slope for the fulltemperature range (88 stars) is essentially the same, at 0.382+/-0.030,as for the field stars alone. For the hotter temperature group of 35Li-Be dip stars in the field and in clusters the slope is higher, at0.433+/-0.036, while for the cooler star group (54 stars) the slope is0.337+/-0.031, different by more than 1 σ. This small differencein the slope is predicted by the theory of rotationally induced mixing.The four stars with [Fe/H] less than -0.4 are all below the best-fitrelation, i.e., there is more Be depletion at a given A(Li) or less Beab initio. The youngest stars, i.e., Pleiades, have less depletion ofboth Li and Be. This too is predicted by rotationally induced slowmixing. Combining the Be results from both field and cluster stars, wefind that there are stars with undepleted Be, i.e., near the meteoriticvalues of 1.42 dex, at all temperatures from 5500 to 6800 K. Depletionsof Be of up to and even exceeding 2 orders of magnitude are commonbetween 6000 and 6700 K.
| 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.
| Boron Benchmarks for the Galactic Disk
Sixteen Population I solar-type dwarfs have been selected to ascertainthe baseline B abundance in the Galactic disk for a range of a factor of4 in metallicity: from [Fe/H] of -0.5 to +0.1. All the stars selectedare undepleted in Be, which ensures that they have also retained theirfull initial abundance of B. Evaluation of the trend of B with Feprovides a means to study the evolution of B in the Galactic disk. Weobserved 16 bright stars around the B I 2497 Å line, using theSTIS echelle spectrograph on HST. New observations of Li and Be in somestars were made, and previous abundance studies of Li and Be in thesestars were reevaluated using revised parameters and a modified spectralsynthesis code for consistency with the B measurements. Abundances of Bwere calculated by spectrum synthesis with the revised MOOG code, whichaccounts for the increased opacity in the UV due to metals; the LTE Babundances were then corrected for non-LTE effects. Four additionalstars with undepleted Be have HST B observations, which increase oursample to 20. For these disk stars there is a shallow slope for B versusFe and Be versus Fe, such that as Fe increases by a factor of 4, B andBe increase by 1.7 times. The slope for BLTE versus Fe is0.31+/-0.09, for BNLTE versus Fe 0.40+/-0.12, and for Beversus Fe 0.38+/-0.14. We have estimated the effect of additional UVopacity from Mg and find that an increase of 0.3 dex in Mg results in ahigher B abundances by 0.1 dex for all the disk stars. Individual starsare not consistently above (or below) the mean in both B and Be,implying that the star-to-star differences are not due to variations inthe elemental content of the ``natal'' clouds. We find that the trend ofB abundance with [Fe/H] is consistent with the general trend observed inhalo stars. If we connect the halo and disk stars, then an increase inthe Fe abundance by 103 is accompanied by increases of 100times in B and 550 times in Be. However, fitting two separate relationsfor the disk and the halo stars results in a somewhat steeper slope forBe for the halo stars (1.08+/-0.07) relative to the disk stars(0.38+/-0.14). This is the case for B also in LTE, with Bhalo(0.90+/-0.07) versus Bdisk (0.32+/-0.12). However, the NLTE Babundance increases more slowly for halo stars than the Be abundancedoes; since this is not predicted by light-element synthesis ordepletion, we suggest that a full NLTE analysis would be preferable tomaking the (small) corrections to the LTE abundances. Some of the lowestmetallicity stars are thought to have only upper limits on the Babundance; if that is the case, the NLTE B slope is steeper, nearing1.0. The abundance of B in the disk stars is observed to be a factor of~15+7-5 more than the abundance of Be in thesestars, a result consistent with the predictions of Galactic cosmic-ray(GCR) spallation, B/Be=15+/-5. The upper envelope for Li versus Feyields Li/B and Li/Be ratios that, when coupled with models andpredictions, indicate that 20%-45% of Li might be produced by GCRs.While there is no evidence to support the production of B by neutrinospallation, we cannot rule it out.Based on observations obtained with the NASA/ESA Hubble Space Telescope(HST) through the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS5-26555.
| The Structure of the Local Interstellar Medium. II. Observations of D I, C II, N I, O I, Al II, and Si II toward Stars within 100 Parsecs
Moderate- and high-resolution measurements(λ/Δλ>~40,000) of interstellar resonance lines ofD I, C II, N I, O I, Al II, and Si II (hereafter called light ions) arepresented for all available observed targets located within 100 pc thatalso have high-resolution observations of interstellar Fe II or Mg II(heavy ions) lines. All spectra were obtained with the Goddard HighResolution Spectrograph or the Space Telescope Imaging Spectrographinstrument aboard the Hubble Space Telescope. Currently, there are 41sight lines to targets within 100 pc with observations that include aheavy ion at high resolution and at least one light ion at moderate orhigh resolution. We present new measurements of light ions along 33 ofthese sight lines and collect from the literature results for theremaining sight lines that have already been analyzed. For all of thenew observations we provide measurements of the central velocity,Doppler width parameter, and column density for each absorptioncomponent. We greatly increase the number of sight lines with usefullocal interstellar medium (LISM) absorption-line measurements of lightions by using knowledge of the kinematic structure along a line of sightobtained from high-resolution observations of intrinsically narrowabsorption lines, such as Fe II and Mg II. We successfully fit theabsorption lines with this technique, even with moderate-resolutionspectra. Because high-resolution observations of heavy ions are criticalfor understanding the kinematic structure of local absorbers along theline of sight, we include 18 new measurements of Fe II and Mg II in anAppendix. We present a statistical analysis of the LISM absorptionmeasurements, which provides an overview of some physicalcharacteristics of warm clouds in the LISM, including temperature andturbulent velocity. This complete collection and reduction of allavailable LISM absorption measurements provides an important databasefor studying the structure of nearby warm clouds, including ionization,abundances, and depletions. Subsequent papers will present models forthe morphology and physical properties of individual structures (clouds)in the LISM.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS AR-09525.01A. Theseobservations are associated with program 9525.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| 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.
| Speckle Observations of Binary Stars with the WIYN Telescope. IV. Differential Photometry
Five hundred seventy-six magnitude difference measures are presented for260 binary stars. These measures are derived from CCD-based speckleobservations taken at the WIYN 3.5 m telescope at Kitt Peak NationalObservatory during the period 1997-2000. Separations of the systemsrange from over 1" down to near the diffraction limit of the telescope.A study of multiple measures of the same targets indicates that themeasures have a typical uncertainty of better than 0.13 mag per 2 minuteobservation, and that multiple observations can be averaged to arrive atsmaller uncertainties. Results presented here are also compared, insofaras it is possible, with measures in the Hipparcos Catalogue and toprevious studies using adaptive optics. No major systematic errors wereidentified.The WIYN Observatory is a joint facility of the University ofWisconsin-Madison, Indiana University, Yale University, and the NationalOptical Astronomy Observatory.
| Cu and Zn in the early Galaxy
We present Cu and Zn abundances for 38 FGK stars, mostly dwarfs,spanning a metallicity range between solar and [Fe/H] = -3. Theabundances were obtained using Kurucz's local thermal equilibrium (LTE)model atmospheres and the near-UV lines of Cu I 3273.95 Å and Zn I3302.58 Å observed at high spectral resolution. The trend of[Cu/Fe] versus [Fe/H] is almost solar for [Fe/H] > -1 and thendecreases to a plateau <[Cu/Fe]> = -0.98 at [Fe/H] < -2.5,whereas the [Zn/Fe] trend is essentially solar for [Fe/H] > -2 andthen slightly increases at lower metallicities to an average value of<[Zn/Fe]> = +0.18. We compare our results with previous work onthese elements, and briefly discuss them in terms of nucleosynthesisprocesses. Predictions of halo chemical evolution fairly reproduce thetrends, especially the [Cu/Fe] plateau at very low metallicities, but toa lesser extent the higher [Zn/Fe] ratios at low metallicities,indicating possibly missing yields.
| 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
| Galactic evolution of nitrogen
We present detailed spectroscopic analysis of nitrogen abundances in 31unevolved metal-poor stars analysed by spectral synthesis of the near-UVNH band at 3360 Å observed at high resolution with varioustelescopes. We found that [N/Fe] scales with that of iron in themetallicity range -3.1 <[Fe/H]<0 with the slope 0.01±0.02.Furthermore, we derive uniform and accurate (N/O) ratios using oxygenabundances from near-UV OH lines obtained in our previous studies. Wefind that a primary component of nitrogen is required to explain theobservations. The NH lines are discovered in the VLT/UVES spectra of thevery metal-poor subdwarfs G64-12 and LP815-43 indicating that thesestars are N rich. The results are compared with theoretical models andobservations of extragalactic H II regions and Damped Lyα systems.This is the first direct comparison of the (N/O) ratios in these objectswith those in Galactic stars.
| 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 correlation of elemental abundances with kinematics among galactic disk stars
We have performed the detailed analysis of 174 high-resolution spectraof FGK dwarfs obtained with the ELODIE echelle spectrograph at theObservatoire de Haute-Provence. Abundances of Fe, Si and Ni have beendetermined from equivalent widths under LTE approximation, whereasabundances of Mg have been determined under NLTE approximation usingequivalent widths of 4 lines and profiles of 5 lines. Spatial velocitieswith an accuracy better than 1 km s-1, as well as orbits,have been computed for all stars. They have been used to define 2subsamples kinematically representative of the thin disk and the thickdisk in order to highlight their respective properties. A transitionoccurs at [Fe/H] =-0.3. Stars more metal-rich than this value have aflat distribution with Zmax;<1 kpc and σW<20 km s-1, and a narrow distribution of [α/Fe].There exist stars in this metallicity regime which cannot belong to thethin disk because of their excentric orbits, neither to the thick diskbecause of their low scale height. Several thin disk stars areidentified down to [Fe/H] =-0.80. Their Mg enrichment is lower thanthick disk stars with the same metallicity. We confirm from a largersample the results of Feltzing et al. (\cite{felt03}) and Bensby et al.(\cite{ben03}) showing a decrease of [α/Fe] with [Fe/H] in thethick disk interpreted as the signature of the SNIa which haveprogressively enriched the ISM with iron. However our data suggest thatthe star formation in the thick disk stopped when the enrichment was[Fe/H] =-0.30, [Mg/Fe] =+0.20, [Si/Fe] =+0.17. A vertical gradient in[α/Fe] may exist in the thick disk but should be confirmed with alarger sample. Finally we have identified 2 new candidates of the HR1614moving group.Based on spectra collected with the ELODIE spectrograph at the 1.93-mtelescope of the Observatoire de Haute Provence (France).Tables 3 and 8 are 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/418/551
| New Metallicity Calibration Down to [Fe/H] = -2.75 dex
We have taken 88 dwarfs, covering the colour-index interval 0.37 <=(B-V)0 <= 1.07mag, with metallicities -2.70 <= [Fe/H]<= +0.26dex, from three different sources for new metallicitycalibration. The catalogue of Cayrel de Stroble et al. (2001), whichincludes 65% of the stars in our sample, supplies detailed informationon abundances for stars with determination based on high-resolutionspectroscopy. In constructing the new calibration we have used as`corner stones' 77 stars which supply at least one of the followingconditions: (i) the parallax is larger than 10mas (distance relative tothe Sun less than 100pc) and the galactic latitude is absolutely higherthan 30° (ii) the parallax is rather large, if the galactic latitudeis absolutely low and vice versa. Contrary to previous investigations, athird-degree polynomial is fitted for the new calibration: [Fe/H]=0.10 -2.76δ - 24.04δ2 + 30.00δ3. Thecoefficients were evaluated by the least-squares method, without regardto the metallicity of Hyades. However, the constant term is in the rangeof metallicity determined for this cluster, i.e.0.08<=[Fe/H]<=0.11dex. The mean deviation and the mean error inour work are equal to those of Carney (1979), for [Fe/H] >= -1.75dexwhere Carney's calibration is valid
| Intermediate-metallicity, high-velocity stars and Galactic chemical evolution
High signal-to-noise ratio spectra were obtained of 10high-proper-motion stars having -1 <~[Fe/H] < 0, and a comparablenumber of disc stars. All but two of the high-proper-motion stars wereconfirmed to have [Fe/H] > -1.0, some approaching solar metallicity,but, even so, earlier measurements overestimated the metallicities andvelocities of some of these stars. Models of stellar populations wereused to assign membership probabilities to the Galactic components towhich the high-velocity stars might belong. Many were found to be moreprobably thick-disc than halo objects, despite their large spacemotions, and two might be associated with the inner Galaxy. It may benecessary to reassess contamination of previous halo samples, such asthose used to define the metallicity distribution, to account forcontamination by high-velocity thick-disc stars, and to considerpossible subcomponents of the halo.The change in [α/Fe] ratios at [Fe/H]~=-1.0 is often used toconstrain the degree and timing of Type Ia supernova nucleosynthesis inGalactic chemical-evolution models. [Ti/Fe] values were measured foreight of the high-velocity stars. Both high- and low-[Ti/Fe] halo starsexist; likewise high- and low-[Ti/Fe] thick-disc stars exist. Weconclude that the [Ti/Fe]`break' is not well defined for a givenpopulation; nor is there a simple, continuous evolutionary sequencethrough the break. Implications for the interpretation of the[α/Fe] break in terms of SN Ia time-scales and progenitors arediscussed. The range of [Ti/Fe] found for high-velocity (low rotation)thick-disc stars contrasts with that for the low-velocity (highrotation) thick-disc sample studied by Prochaska et al.
| Non-LTE Analysis of the Sodium Abundance of Metal-Poor Stars in the Galactic Disk and Halo
We performed an extensive non-LTE analysis of the neutral sodium linesof Na I 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halostars of types F-K covering a wide metallicity range (-4 <≈[Fe/H] <≈ +0.4), using our own data as well as data collectedfrom the literature. For comparatively metal-rich disk stars (-1<≈ [Fe/H] <≈ +0.4) where the weaker 6154/6161 linesare the best abundance indicators, we confirmed [Na/Fe] 0 with an"upturn" (i.e., a shallow/broad dip around -0.5 <≈ [Fe/H]<≈ 0) as already reported in previous studies. For themetal-deficient halo stars, where the much stronger 5890/5896 or8183/8195 lines subject to considerable (negative) non-LTE correctionsamounting to 0.5 dex have to be used, our analysis suggests mildly"subsolar" [Na/Fe] values down to -0.4 (with a somewhat largescatter of ± 0.2 dex) on the average at the typical halometallicity of [Fe/H] -2, followed by a rise again to a near-solarratio of [Na/Fe] 0 at the very metal-poor regime [Fe/H] -3to -4. These results are discussed in comparison with the previousobservational studies along with the theoretical predictions from theavailable chemical evolution models.
| Classification of Metal-Deficient Dwarfs in the Vilnius Photometric System
Methods used for the quantitative classification of metal-deficientstars in the Vilnius photometric system are reviewed. We present a newcalibration of absolute magnitudes for dwarfs and subdwarfs, based onHipparcos parallaxes. The new classification scheme is applied to asample of Population II visual binaries.
| 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.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
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Osservazione e dati astrometrici
| Costellazione: | Ercole |
| Ascensione retta: | 18h07m01.50s |
| Declinazione: | +30°33'43.0" |
| Magnitudine apparente: | 5.04 |
| Distanza: | 15.654 parsec |
| Moto proprio RA: | 0 |
| Moto proprio Dec: | 0 |
| B-T magnitude: | 5.672 |
| V-T magnitude: | 5.125 |
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