Contents
Images
Upload your image
DSS Images Other Images
Related articles
Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion
We present NLTE Li abundances for 88 stars in the metallicity range -3.5< [Fe/H] < -1.0. The effective temperatures are based on theinfrared flux method with improved E(B-V) values obtained mostly frominterstellar Na I D lines. The Li abundances were derived through MARCSmodels and high-quality UVES+VLT, HIRES+Keck and FIES+NOT spectra, andcomplemented with reliable equivalent widths from the literature. Theless-depleted stars with [Fe/H] < -2.5 and [Fe/H] > -2.5 fall intotwo well-defined plateaus of ALi = 2.18 (? = 0.04) andALi = 2.27 (? = 0.05), respectively. We show that thetwo plateaus are flat, unlike previous claims for a steep monotonicdecrease in Li abundances with decreasing metallicities. At allmetallicities we uncover a fine-structure in the Li abundances of Spiteplateau stars, which we trace to Li depletion that depends on bothmetallicity and mass. Models including atomic diffusion and turbulentmixing seem to reproduce the observed Li depletion assuming a primordialLi abundance ALi = 2.64, which agrees well with currentpredictions (ALi = 2.72) from standard Big Bangnucleosynthesis. Adopting the Kurucz overshooting model atmospheresincreases the Li abundance by +0.08 dex to ALi = 2.72, whichperfectly agrees with BBN+WMAP.Based in part on observations obtained at the W. M. Keck Observatory,the Nordic Optical Telescope on La Palma, and on data from theHIRES/Keck archive and the European Southern Observatory ESO/ST-ECFScience Archive Facility.Table 1 is only available in electronic form athttp://www.aanda.org
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| NLTE analysis of CoI/CoII lines in spectra of cool stars with new laboratory hyperfine splitting constants
The analysis of stellar abundances for odd-Z Fe-peak elements requiresaccurate non-local thermodynamic equilibrium (NLTE) modelling ofspectral lines fully taking into account the hyperfine structure (HFS)splitting of lines. Here, we investigate the statistical equilibrium ofCo in the atmospheres of cool stars and the influence of NLTE and HFS onthe formation of Co lines and abundances. Significant departures fromLTE level populations are found for CoI number densities of excitedstates in CoII also differ from LTE at low metallicity. The NLTE levelpopulations are used to determine the abundance of Co in solarphotosphere, log? = 4.95 +/- 0.04dex, which is in agreement withthat in CI meteorites within the combined uncertainties. The spectrallines of CoI were calculated using the results of recent measurements ofhyperfine interaction constants by UV Fourier transform spectrometry.For CoII, the first laboratory measurements of HFS A and B factors wereperformed. These highly accurate A factor measurements (errors of theorder of 3-7 per cent) allow, for the first time, reliable modelling ofCoII lines in the solar and stellar spectra and, thus, a test of theCoI/CoII ionization equilibrium in stellar atmospheres. A differentialabundance analysis of Co is carried out for 18 stars in the metallicityrange -3.12 < [Fe/H] < 0. The abundances are derived by the methodof spectrum synthesis. At low [Fe/H], NLTE abundance corrections for CoIlines are as large as +0.6,..., + 0.8dex. Thus, LTE abundances of Co inmetal-poor stars are severely underestimated. The stellar NLTEabundances determined from the single UV line of CoII are lower by~0.5-0.6dex. The discrepancy might be attributed to possible blends thathave not been accounted for in the solar CoII line and its erroneousoscillator strength. The increasing [Co/Fe] trend in metal-poor stars,as calculated from the CoI lines under NLTE, can be explained if Co isoverproduced relative to Fe in massive stars. The models of Galacticchemical evolution are wholly inadequate to describe this trendsuggesting that the problem is in supernova yields.Based on observations collected at the European Southern Observatory,Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.E-mail: mbergema@mpa-garching.mpg.de (MB); j.pickering@imperial.ac.uk(JCP); gehren@usm.lmu.de (TG)
| An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| Statistical equilibrium of silicon in the atmospheres of metal-poor stars
Aims: The statistical equilibrium of neutral and ionized silicon in theatmospheres of metal-poor stars is discussed. Non-local thermodynamicequilibrium effects (NLTE) are investigated and the silicon abundancesin metal-poor stars determined. Methods: We have used highresolution, high signal to noise ratio spectra from the UVESspectragraph at the ESO VLT telescope. Line formation calculations of Sii and Si ii in the atmospheres of metal-poor stars are presented foratomic models of silicon including 174 terms and 1132 line transitions.Recent improved calculations of Si i and Si ii photoionizationcross-sections are taken into account, and the influence of thefree-free quasi-molecular absorption in the Ly? wing isinvestigated by comparing theoretical and observed fluxes of metal-poorstars. All abundance results are derived from LTE and NLTE statisticalequilibrium calculations and spectrum synthesis methods. Results:It is found that the extreme ultraviolet radiation is very important formetal-poor stars, especially for the high temperature, very metal-poorstars. The radiative bound-free cross-sections also play a veryimportant role for these stars. Conclusions: NLTE effects for Siare found to be important for metal-poor stars, in particular for warmmetal-poor stars. It is found that these effects depend on thetemperature. For warm metal-poor stars, the NLTE abundance correctionreaches ~0.2 dex relative to standard LTE calculations. Our resultsindicate that Si is overabundant for metal-poor stars.Based on observations obtained in the frame of the ESO programme ID165.N-0276(A).
| Beryllium abundances and star formation in the halo and in the thick disk
Context: Beryllium is a pure product of cosmic ray spallation. Thisimplies a relatively simple evolution in time of the beryllium abundanceand suggests its use as a time-like observable. Aims: Our goal is toderive abundances of Be in a sample of 90 stars, the largest sample ofhalo and thick disk stars analyzed to date. We study the evolution of Bein the early Galaxy and its dependence on kinematic and orbitalparameters, and investigate its use as a cosmochronometer. Abundances ofBe, Fe, and α-elements of 73 stars are employed to study theformation of the halo and the thick disk of the Galaxy. Methods:Beryllium abundances are determined from high-resolution, highsignal-to-noise UVES spectra with spectrum synthesis. Atmosphericparameters and abundances of α-elements are adopted from theliterature. Lithium abundances are used to eliminate mixed stars fromthe sample. The properties of halo and thick disk stars are investigatedin diagrams of log(Be/H) vs. [ α/H] , log(Be/H) vs. [Fe/H], and [α/Fe] vs. log(Be/H) and with orbital and kinematic parameters. Results: We present our observational results in various diagrams. (i)In a log(Be/H) vs. [Fe/H] diagram we find a marginal statisticaldetection of a real scatter, above what is expected from measurementerrors, with a larger scatter among halo stars. The detection of thescatter is further supported by the existence of pairs of stars withidentical atmospheric parameters and different Be abundances; (ii) in alog(Be/H) vs. [ α/Fe] diagram, the halo stars separate into twocomponents; one is consistent with predictions of evolutionary models,while the other has too high α and Be abundances and is chemicallyindistinguishable from thick disk stars. This suggests that the halo isnot a single uniform population where a clear age-metallicity relationcan be defined; (iii) In diagrams of R_min vs. [ α/Fe] andlog(Be/H), the thick disk stars show a possible decrease in [α/Fe] with R_min, whereas no dependence of Be with R_min is seen.This anticorrelation suggests that the star formation rate was lower inthe outer regions of the thick disk, pointing towards an inside-outformation. The lack of correlation for Be indicates that it isinsensitive to the local conditions of star formation.Based on observations made with ESO VLT, at Paranal Observatory, underprograms 076.B-0133 and 077.B-0507, and on data obtained from theESO/ST-ECF Science Archive Facility and the UVES Paranal ObservatoryProject 266.D-5655.Tables 1-3, 6 and Appendices A-C are only available in electronic format http://www.aanda.org
| Calibration of Strömgren uvby-H? photometry for late-type stars - a model atmosphere approach
Context: The use of model atmospheres for deriving stellar fundamentalparameters, such as T_eff, log g, and [Fe/H], will increase as we findand explore extreme stellar populations where empirical calibrations arenot yet available. Moreover, calibrations for upcoming large satellitemissions of new spectrophotometric indices, similar to the uvby-H?system, will be needed. Aims: We aim to test the power oftheoretical calibrations based on a new generation of MARCS models bycomparisons with observational photomteric data. Methods: Wecalculated synthetic uvby-H? colour indices from synthetic spectra.A sample of 367 field stars, as well as stars in globular clusters, isused for a direct comparison of the synthetic indices versus empiricaldata and for scrutinizing the possibilities of theoretical calibrationsfor temperature, metallicity, and gravity. Results: We show thatthe temperature sensitivity of the synthetic (b-y) colour is very closeto its empirical counterpart, whereas the temperature scale based uponH? shows a slight offset. The theoretical metallicity sensitivityof the m1 index (and for G-type stars its combination withc_1) is somewhat higher than the empirical one, based upon spectroscopicdeterminations. The gravity sensitivity of the synthetic c1index shows satisfactory behaviour when compared to obervations of Fstars. For stars cooler than the sun, a deviation is significant in thec1-(b-y) diagram. The theoretical calibrations of (b-y),(v-y), and c1 seem to work well for Pop II stars and lead toeffective temperatures for globular cluster stars supporting recentclaims that atomic diffusion occurs in stars near the turnoff point ofNGC 6397. Conclusions: Synthetic colours of stellar atmospherescan indeed be used, in many cases, to derive reliable fundamentalstellar parameters. The deviations seen when compared to observationaldata could be due to incomplete linelists but are possibly also due tothe effects of assuming plane-parallell or spherical geometry and LTE.Model colours 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/498/527
| NLTE abundances of Mn in a sample of metal-poor stars
Aims: Following our solar work, we perform NLTE calculations of the Mnabundance for fourteen stars with [Fe/H] from 0 to -2.5, mainly to showhow NLTE affects Mn abundances in cool stars of different metallicities.Methods: The spectrum synthesis and Mn abundances are based onstatistical equilibrium calculations using various estimates for theinfluence of hydrogen collisions. Results: The NLTE abundances ofMn in all studied stars are systematically higher than the LTEabundances. At low metallicities, the NLTE abundance corrections may runup to 0.5-0.7 dex. Instead of a strong depletion of Mn relative to Fe inmetal-poor stars as found by the other authors, we only find slightlysubsolar values of [Mn/Fe] throughout the range of metallicitiesanalyzed here. Conclusions: The [Mn/Fe] trend in metal-poor starsis inconsistent with the predictions of galactic chemical evolutionmodels, where Mn is less produced than Fe.Based on observations collected at the European Southern Observatory,Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.
| Accurate fundamental parameters for lower main-sequence stars
We derive an empirical effective temperature and bolometric luminositycalibration for G and K dwarfs, by applying our own implementation ofthe Infrared Flux Method to multiband photometry. Our study is based on104 stars for which we have excellent BV(RI)C JHKSphotometry, excellent parallaxes and good metallicities.Colours computed from the most recent synthetic libraries (ATLAS9 andMARCS) are found to be in good agreement with the empirical colours inthe optical bands, but some discrepancies still remain in the infrared.Synthetic and empirical bolometric corrections also show fair agreement.A careful comparison to temperatures, luminosities and angular diametersobtained with other methods in the literature shows that systematiceffects still exist in the calibrations at the level of a few per cent.Our Infrared Flux Method temperature scale is 100-K hotter than recentanalogous determinations in the literature, but is in agreement withspectroscopically calibrated temperature scales and fits well thecolours of the Sun. Our angular diameters are typically 3 per centsmaller when compared to other (indirect) determinations of angulardiameter for such stars, but are consistent with the limb-darkeningcorrected predictions of the latest 3D model atmospheres and also withthe results of asteroseismology.Very tight empirical relations are derived for bolometric luminosity,effective temperature and angular diameter from photometric indices.We find that much of the discrepancy with other temperature scales andthe uncertainties in the infrared synthetic colours arise from theuncertainties in the use of Vega as the flux calibrator. Angulardiameter measurements for a well-chosen set of G and K dwarfs would go along way to addressing this problem.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Metallicity and absolute magnitude calibrations for UBV photometry
Calibrations are presented here for metallicity ([Fe/H]) in terms of theultraviolet excess, [δ(U - B) at B - V = 0.6, hereafterδ0.6], and also for the absolute visual magnitude(MV) and its difference with respect to the Hyades(ΔMHV) in terms of δ0.6 and(B - V), making use of high-resolution spectroscopic abundances from theliterature and Hipparcos parallaxes. The relation[Fe/H]-δ0.6 has been derived for dwarf plus turn-offstars, and also for dwarf, turn-off, plus subgiant stars classifiedusing the MV-(B - V)0 plane of Fig. 11, which iscalibrated with isochrones from Bergbusch & VandenBerg (and alsoVandenBerg & Clem). The [Fe/H]-δ0.6 relations inour equations (5) and (6) agree well with those of Carney, as can beseen from Fig. 5(a). Within the uncertainties, the zero-points,+0.13(+/-0.05) of equation (5) and +0.13(+/-0.04) of equation (6), arein good agreement with the photometric ones of Cameron and of Carney,and close to the spectroscopic ones of Cayrel et al. and of Boesgaard& Friel for the Hyades open cluster. Good quantitative agreementbetween our estimated [Fe/H] abundances with those from uvby-βphotometry and spectroscopic [Fe/H]spec values demonstratesthat our equation (6) can be used in deriving quality photometric metalabundances for field stars and clusters using UBV data from variousphotometric surveys.For dwarf and turn-off stars, a new hybrid MV calibration ispresented, based on Hipparcos parallaxes withσπ/π <= 0.1 and with a dispersion of +/-0.24in MV. This hybrid MV calibration containsδ0.6 and (B - V) terms, plus higher order cross-termsof these, and is valid for the ranges of +0.37 <= (B - V)0<= +0.88,- 0.10 <= δ0.6 <= +0.29 and 3.44<= MV <= 7.23. For dwarf and turn-off stars, therelation for ΔMHV is revised and updated interms of (B - V) and δ0.6, for the ranges of -0.10<= δ0.6 <= +0.29, and +0.49 <= (B -V)0 <= +0.89, again making use of Hipparcos parallaxeswith σπ/π <= 0.1. These parallaxes formetal-poor dwarf and turn-off stars in our sample reveal that thedifference of ΔMHV(B - V) relative to Hyadesat (B - V) = +0.70 should be 1.37mag, instead of the 1.58mag given byLaird et al. In general, Hipparcos parallaxes are larger thanground-based ones, causing a divergence of ourΔMHV(B - V,δ0.6) relation(the solid line in Fig. 15b), from the one of Laird et al. (the dashedline) for the range +0.10 <= δ0.6 <= +0.29 ourabsolute magnitudes are fainter, as has been confirmed for localsubdwarfs by Reid. Our final calibrations forΔMHV(B - V, δ0.6),equations (16) and (17), are third-order polynomials inδ0.6, pass through the origin, and provide photometricdistances in reasonable agreement with those obtained directly fromHipparcos parallaxes (Fig. 18).
| 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.
| 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.
| The lithium content of the Galactic Halo stars
Thanks to the accurate determination of the baryon density of theuniverse by the recent cosmic microwave background experiments, updatedpredictions of the standard model of Big Bang nucleosynthesis now yieldthe initial abundance of the primordial light elements withunprecedented precision. In the case of ^7Li, the CMB+SBBN value issignificantly higher than the generally reported abundances for Pop IIstars along the so-called Spite plateau. In view of the crucialimportance of this disagreement, which has cosmological, galactic andstellar implications, we decided to tackle the most critical issues ofthe problem by revisiting a large sample of literature Li data in halostars that we assembled following some strict selection criteria on thequality of the original analyses. In the first part of the paper wefocus on the systematic uncertainties affecting the determination of theLi abundances, one of our main goal being to look for the "highestobservational accuracy achievable" for one of the largest sets of Liabundances ever assembled. We explore in great detail the temperaturescale issue with a special emphasis on reddening. We derive four sets ofeffective temperatures by applying the same colour {T}_eff calibrationbut making four different assumptions about reddening and determine theLTE lithium values for each of them. We compute the NLTE corrections andapply them to the LTE lithium abundances. We then focus on our "best"(i.e. most consistent) set of temperatures in order to discuss theinferred mean Li value and dispersion in several {T}_eff and metallicityintervals. The resulting mean Li values along the plateau for [Fe/H]≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075when the lowest effective temperature considered is taken equal to 5700K and 6000 K respectively. This is a factor of 2.48 to 2.81 (dependingon the adopted SBBN model and on the effective temperature range chosento delimit the plateau) lower than the CMB+SBBN determination. We findno evidence of intrinsic dispersion. Assuming the correctness of theCMB+SBBN prediction, we are then left with the conclusion that the Liabundance along the plateau is not the pristine one, but that halo starshave undergone surface depletion during their evolution. In the secondpart of the paper we further dissect our sample in search of newconstraints on Li depletion in halo stars. By means of the Hipparcosparallaxes, we derive the evolutionary status of each of our samplestars, and re-discuss our derived Li abundances. A very surprisingresult emerges for the first time from this examination. Namely, themean Li value as well as the dispersion appear to be lower (althoughfully compatible within the errors) for the dwarfs than for the turnoffand subgiant stars. For our most homogeneous dwarfs-only sample with[Fe/H] ≤ 1.5, the mean Li abundances are A(L)_NLTE = 2.177±0.071 and 2.215±0.074 when the lowest effective temperatureconsidered is taken equal to 5700 K and 6000 K respectively. This is afactor of 2.52 to 3.06 (depending on the selected range in {T}_eff forthe plateau and on the SBBN predictions we compare to) lower than theCMB+SBBN primordial value. Instead, for the post-main sequence stars thecorresponding values are 2.260±0.1 and 2.235±0.077, whichcorrespond to a depletion factor of 2.28 to 2.52. These results,together with the finding that all the stars with Li abnormalities(strong deficiency or high content) lie on or originate from the hotside of the plateau, lead us to suggest that the most massive of thehalo stars have had a slightly different Li history than their lessmassive contemporaries. In turn, this puts strong new constraints on thepossible depletion mechanisms and reinforces Li as a stellartomographer.
| 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.
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| The Solar Neighborhood. VIII. Discovery of New High Proper Motion Nearby Stars Using the SuperCOSMOS Sky Survey
Five new objects with proper motions between 1.0" and 2.6"yr-1 have been discovered via a new Research Consortium onNearby Stars (RECONS) search for high proper motion stars using theSuperCOSMOS Sky Survey. The first portion of the search, discussed here,is centered on the south celestial pole and covers declinations -90°to -57.5d. Photographic photometry from SuperCOSMOS and JHKsnear-infrared photometry from the Two Micron All Sky Survey for starsnearer than 10 pc are combined to provide a suite of newMKs-color relations useful for estimatingdistances to main-sequence stars. These relations are then used toderive distances to the new proper motion objects, as well as previouslyknown stars with μ>=1.0" yr-1 (many of which have notrigonometric parallaxes) recovered during this phase of the survey.Four of the five new stars have red dwarf colors, while one is a nearbywhite dwarf. Two of the red dwarfs are likely to be within the RECONS 10pc sample, and the white dwarf probably lies between 15 and 25 pc. Amongthe 23 known stars recovered during the search, there are threeadditional candidates for the RECONS sample that have no trigonometricparallaxes.
| The distance to NGC 5904 (M 5) via the subdwarf main sequence fitting method
We present a determination of the distance modulus of the globularcluster NGC 5904 (M 5), obtained by means of the subdwarf main-sequencefitting on the (V,V-I) color-magnitude diagram. The subdwarf sample hasbeen selected from the HIPPARCOS catalog in a metallicity rangehomogeneous with the cluster ([Fe/H] ≃ -1.1). Both the cluster andthe subdwarfs have been observed with the sametelescope+instrument+filters setup (namely, ESO-NTT equipped with theSUSI2 camera), in order to preserve homogeneity and reduce systematicuncertainties. A set of archival HST data has then been used to obtain adeep and precise ridge line. These have been accurately calibrated inthe ground photometric system by using the NTT data and used to fit thecluster distance modulus. By adopting the most commonly accepted valuesfor the reddening, E(B-V) = 0.035 and 0.03, we obtain respectivelyμ0 = 14.44 ± 0.09 ± 0.07 andμ0 = 14.41 ± 0.09 ± 0.07, in agreement withrecent determinations.Based on data collected at ESO-La Silla, Chile, (GTO 63.L-0717) and fromHST archival data (GO 8310).
| The Luminosity Function and Color-Magnitude Diagram of the Globular Cluster M12
In this paper we present the V and I luminosity functions andcolor-magnitude diagrams derived from wide-field(23'×23') BVI photometry of theintermediate-metallicity ([Fe/H]~-1.3) Galactic globular cluster M12.Using observed values (and ranges of values) for the clustermetallicity, reddening, distance modulus, and age, we compare these datawith recent α-enhanced stellar evolution models for low-massmetal-poor stars. We describe several methods of making comparisonsbetween theoretical and observed luminosity functions to isolate theevolutionary timescale information that the luminosity functionscontain. We find no significant evidence of excesses of stars on the redgiant branch, although the morphology of the subgiant branch in theobserved luminosity function does not match theoretical predictions in asatisfactory way. Current uncertainties in Teff-colortransformations (and possibly also in other physics inputs to themodels) make more detailed conclusions about the subgiant branchmorphology impossible. Given the recent constraints on cluster ages fromthe WMAP experiment, we find that good-fitting models that do notinclude He diffusion (both color-magnitude diagrams and luminosityfunctions) are too old (by ~1-2 Gyr) to adequately represent the clusterluminosity function. The inclusion of helium diffusion in the modelsprovides an age reduction (compared with nondiffusive models) that isconsistent with the age of the universe being 13.7+/-0.2 Gyr.
| 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.
| 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).
| 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}
| 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.
| Mg, Ba and Eu abundances in thick disk and halo stars
Our sample of cool dwarf stars from previous papers (Mashonkina &Gehren \cite{euba, eubasr}) is extended in this study including 15moderately metal-deficient stars. The samples of halo and thick diskstars have overlapping metallicities with [Fe/H] in the region from -0.9to -1.5, and we compare chemical properties of these two kinematicallydifferent stellar populations independent of their metallicity. Wepresent barium, europium and magnesium abundances for the new sample ofstars. The results are based on NLTE line formation obtained indifferential model atmosphere analyses of high resolution spectraobserved mainly using the UVES spectrograph at the VLT of the EuropeanSouthern Observatory. We confirm the overabundance of Eu relative to Mgin halo stars as reported in our previous papers. Eight halo stars show[Eu/Mg] values between 0.23 and 0.41, whereas stars in the thick andthin disk display a solar europium to magnesium ratio. The [Eu/Ba]values found in the thick disk stars to lie between 0.35 and 0.57suggest that during thick disk formation evolved low-mass stars startedto enrich the interstellar gas by s-nuclei of Ba, and the s-processcontribution to barium thus varies from 30% to 50%. Based on theseresults, and using the chemical evolution calculations by Travaglio etal. (\cite{eu99}), we estimate that the thick disk stellar populationformed on a timescale between 1.1 to 1.6 Gyr from the beginning of theprotogalactic collapse. In the halo stars the [Eu/Ba] values are foundmostly between 0.40 and 0.67, which suggests a duration of the haloformation of about 1.5 Gyr. For the whole sample of stars we present theeven-to-odd Ba isotope ratios as determined from hyperfine structureseen in the Ba Ii resonance line lambda 4554. As expected, the solarratio 82:18 (Cameron \cite{cam}) adjusts to observations of the Ba Iilines in the thin disk stars. In our halo stars the even-to-odd Baisotope ratios are close to the pure r-process ratio 54:46 (Arlandini etal. \cite{rs99}), and in the thick disk stars the isotope ratio isaround 65:35 (+/-10%). Based on these data we deduce for thick diskstars the ratio of the s/r-process contribution to barium as 30:70(+/-30%), in agreement with the results obtained from the [Eu/Ba]values. Based on observations collected at the European SouthernObservatory, Chile, 67.D-0086A, and the German Spanish AstronomicalCenter, Calar Alto, Spain.
| On the lithium content of the globular cluster M 92
I use literature data and a new temperature calibration to determine theLi abundances in the globular cluster M 92. Based on the same data,Boesgaard et al. have claimed that there is a dispersion in Liabundances in excess of observational errors. This result has beenbrought as evidence for Li depletion in metal-poor dwarfs. In thepresent note I argue that there is no strong evidence for intrinsicdispersion in Li abundances, although a dispersion as large as 0.18 dexis possible. The mean Li abundance, A(Li) = 2.36, is in good agreementwith recent results for field stars and TO stars in the metal-poorglobular cluster NGC 6397. The simplest interpretation is that thisconstant value represents the primordial Li abundance.
| 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
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| Models of Metal-poor Stars with Gravitational Settling and Radiative Accelerations. II. The Age of the Oldest Stars
Isochrones for ages between 12 and 18 Gyr have been derived from theevolutionary tracks presented in Paper I (Richard et al.) for massesfrom 0.5 to 1.0 Msolar and initial chemical abundancescorresponding to (1) Y=0.2352, Z=1.69×10-4([Fe/H]=-2.31,[α/Fe]=0.3) and (2) Y=0.2370,Z=1.69×10-3 ([Fe/H=-1.31,[α/Fe]=0.3). These arethe first models for Population II stars in which both gravitationalsettling and radiative accelerations have been taken into account.Allowance for these diffusive processes leads to a 10%-12% reduction inage at a given turnoff luminosity. However, in order for the diffusivemodels to satisfy the constraints from Li and Fe abundance data (seePaper I) and to reproduce the observed morphologies of globular cluster(GC) color-magnitude diagrams (CMDs) in a straightforward way, extramixing just below the boundary of the convective envelope seems to benecessary. Indeed, when additional turbulent mixing is invoked, theresultant models are able to satisfy all of these constraints, as wellas those provided by the CMDs of local subdwarfs, rather well. Moreover,they imply an age near 13.5 Gyr for M92, which is one of the mostmetal-deficient (and presumably one of the oldest) of the Galaxy's GCs,if the field subgiant HD 140283 is used to derive the cluster distance.Comparisons of field subdwarfs and subgiants with a recently publishedfiducial for M5 suggests that the cluster has [Fe/H]<~-1.4, inconflict with some estimates based on high-resolution spectroscopy, ifthe metallicities of the field stars are to be trusted. In addition, anage of ~11.5 Gyr is found for M5, irrespective of whether diffusive ornondiffusive isochrones are employed in the analysis. The implicationsof our results for the extragalactic distance scale and for the Hubbleconstant are briefly discussed in the context of the presently favoredΩM~0.35, ΩΛ~0.65 cosmologicalmodel.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Dorade |
| Right ascension: | 05h13m05.28s |
| Declination: | -59°38'44.3" |
| Apparent magnitude: | 9.472 |
| Proper motion RA: | 938.2 |
| Proper motion Dec: | 511.7 |
| B-T magnitude: | 9.996 |
| V-T magnitude: | 9.516 |
Catalogs and designations:
|
INHABITED SKY
