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Evidence of Tidal Debris from ? Cen in the Kapteyn Group
This paper presents a detailed kinematic and chemical analysis of 16members of the Kapteyn moving group. The group does not appear to bechemically homogenous. However, the kinematics and the chemicalabundance patterns seen in 14 of the stars in this group are similar tothose observed in the well-studied cluster, ? Centauri (?Cen). Some members of this moving group may be remnants of the tidaldebris of ? Cen, left in the Galactic disk during the merger eventthat deposited ? Cen into the Milky Way.
| A holistic approach to carbon-enhanced metal-poor stars
Context. Carbon-enhanced metal-poor (CEMP) stars are known to haveproperties that reflect the nucleosynthesis of the first low- andintermediate-mass stars, because most have been polluted by anow-extinct AGB star. Aims: By considering abundances in thevarious CEMP subclasses separately, we try to derive parameters (such asmetallicity, mass, temperature, and neutron source) characterising AGBnucleosynthesis from the specific signatures imprinted on theabundances, and separate them from the impact of thermohaline mixing,first dredge-up, and dilution associated with the mass transfer from thecompanion. Methods: To place CEMP stars in a broader context, wecollect abundances for about 180 stars of various metallicities (fromsolar to [Fe/H] =-4), luminosity classes (dwarfs and giants), andabundance patterns (e.g. C-rich and poor, Ba-rich and poor), from bothour own sample and the literature. Results: We first show thatthere are CEMP stars that share the properties of CEMP-s stars andCEMP-no stars (which we refer to as CEMP-low-s stars). We also show thatthere is a strong correlation between Ba and C abundances in the s-onlyCEMP stars. This represents a strong detection of the operation of the13C neutron source in low-mass AGB stars. For the CEMP-rsstars (seemingly enriched with elements from both the s- andr-processes), the correlation of the N abundances with abundances ofheavy elements from the 2nd and 3rd s-process peaks bears instead thesignature of the 22Ne neutron source. Since CEMP-rs starsalso exhibit O and Mg enhancements, we conclude that extremely hotconditions prevailed during the thermal pulses of the contaminating AGBstars. We also note that abundances are not affected by the evolution ofthe CEMP-rs star itself (especially by the first dredge-up). Thisimplies that mixing must have occurred while the star was on the mainsequence, and that a large amount of matter must have been accreted soas to trigger thermohaline mixing. Finally, we argue that most CEMP-nostars (with neutron-capture element abundances comparable to non-CEMPstars) are likely the extremely metal-poor counterparts of CEMPneutron-capture-rich stars. We also show that the C enhancement inCEMP-no stars declines with metallicity at extremely low metallicity([Fe/H] < -3.2). This trend is not predicted by any of the currentAGB models.Tables 1-4 are only available in electronic form at http://www.aanda.org
| Non-LTE abundances of Mg and K in extremely metal-poor stars and the evolution of [O/Mg], [Na/Mg], [Al/Mg], and [K/Mg] in the Milky Way
Aims: LTE abundances of light elements in extremely metal-poor(EMP) stars have been previously derived from high quality spectra. Newderivations, free from the NLTE effects, will better constrain themodels of the Galactic chemical evolution and the yields of the veryfirst supernovae. Methods: The NLTE profiles of the magnesium andpotassium lines have been computed in a sample of 53 extremelymetal-poor stars with a modified version of the program MULTI andadjusted to the observed lines in order to derive the abundances ofthese elements. Results: The NLTE corrections for magnesium andpotassium are in good agreement with the works found in the literature.The abundances are slightly changed, reaching a better precision: thescatter around the mean of the abundance ratios has decreased. Magnesiummay be used with confidence as reference element. Together withpreviously determined NLTE abundances of sodium and aluminum, the newratios are displayed, for comparison, along the theoretical trendsproposed by some models of the chemical evolution of the Galaxy, usingdifferent models of supernovae.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory (Large Programme “First Stars”, ID165.N-0276; P.I.: R. Cayrel.
| Accuracy of spectroscopy-based radioactive dating of stars
Context. Combined spectroscopic abundance analyses of stable andradioactive elements can be applied for deriving stellar ages. Theachievable precision depends on factors related to spectroscopy,nucleosynthesis, and chemical evolution. Aims: We quantify theuncertainties arising from the spectroscopic analysis, and compare theseto the other error sources. Methods: We derive formulae for theage uncertainties arising from the spectroscopic abundance analysis, andapply them to spectroscopic and nucleosynthetic data compiled from theliterature for the Sun and metal-poor stars. Results: We obtainedready-to-use analytic formulae of the age uncertainty for the cases ofstable+unstable and unstable+unstable chronometer pairs, and discuss theoptimal relation between to-be-measured age and mean lifetime of aradioactive species. Application to the literature data indicates that,for a single star, the achievable spectroscopic accuracy is limited toabout ±20% for the foreseeable future. At present, theoreticaluncertainties in nucleosynthesis and chemical evolution models form theprecision bottleneck. For stellar clusters, isochrone fitting provides ahigher accuracy than radioactive dating, but radioactive dating becomescompetitive when applied to many cluster members simultaneously,reducing the statistical errors by a factor ?{N}. Conclusions: Spectroscopy-based radioactive stellar dating would benefitfrom improvements in the theoretical understanding of nucleosynthesisand chemical evolution. Its application to clusters can provide strongconstraints for nucleosynthetic models.
| 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.
| First stars XII. Abundances in extremely metal-poor turnoff stars, and comparison with the giants
Context: The detailed chemical abundances of extremely metal-poor (EMP)stars are key guides to understanding the early chemical evolution ofthe Galaxy. Most existing data, however, treat giant stars that may haveexperienced internal mixing later. Aims: We aim to compare theresults for giants with new, accurate abundances for all observableelements in 18 EMP turnoff stars. Methods: VLT/UVES spectra at R~ 45 000 and S/N ~ 130 per pixel (?? 330-1000 nm) areanalysed with OSMARCS model atmospheres and the TURBOSPECTRUM code toderive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, andBa. Results: For Ca, Ni, Sr, and Ba, we find excellentconsistency with our earlier sample of EMP giants, at all metallicities.However, our abundances of C, Sc, Ti, Cr, Mn and Co are ~0.2 dex largerthan in giants of similar metallicity. Mg and Si abundances are ~0.2 dexlower (the giant [Mg/Fe] values are slightly revised), while Zn is again~0.4 dex higher than in giants of similar [Fe/H] (6 stars only). Conclusions: For C, the dwarf/giant discrepancy could possibly have anastrophysical cause, but for the other elements it must arise fromshortcomings in the analysis. Approximate computations of granulation(3D) effects yield smaller corrections for giants than for dwarfs, butsuggest that this is an unlikely explanation, except perhaps for C, Cr,and Mn. NLTE computations for Na and Al provide consistent abundancesbetween dwarfs and giants, unlike the LTE results, and would be highlydesirable for the other discrepant elements as well. Meanwhile, werecommend using the giant abundances as reference data for Galacticchemical evolution models.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory, Chile (Large Programme “First Stars”,ID 165.N-0276; P.I.: R. Cayrel, and Programme 078.B-0238; P.I.: M.Spite). Appendices A-C are only available in electronic form athttp://www.aanda.org Table 7 is only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/519
| Potassium Abundances in Red Giants of Mildly to Very Metal-Poor Globular Clusters
A non-LTE analysis of K I resonance lines at 7664.91 and 7698.97 A wascarried out for 15 red giants belonging to three globular clusters ofdifferent metallicity (M 4, M 13, and M 15) along with two referenceearly-K giants (rho Boo and alpha Boo), in order to check whether the Kabundances are uniform within a cluster and to investigate the behaviorof [K/Fe] ratio at the relevant metallicity range of -2.5 <[Fe/H]< -1. We confirmed that [K/H] (as well as [Fe/H]) is almosthomogeneous within each cluster to a precision of < ~0.1 dex, thoughdubiously large deviations are exceptionally seen for two peculiar starsshowing signs of considerably increased turbulence in the upperatmosphere. The resulting [K/Fe] ratios are mildly supersolar by a fewtenths of dex for three clusters, tending to gradually increase from~+0.1-0.2 at [Fe/H] ~-1 to ~+0.3 at [Fe/H] ~ -2.5. This result connectsreasonably well with the [K/Fe] trend of disk stars (-1 < [Fe/H]) andthat of extremely metal-poor stars (-4 <[Fe/H] < -2.5). That is,[K/Fe] appears to continue a gradual increase from [Fe/H]~0 toward alower metallicity regime down to [Fe/H]~-3, where a broad maximum of[K/Fe]~+0.3-0.4 is attained, possibly followed by a slight downturn at[Fe/H]<~-3.
| The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy
We examine the Pb and Th abundances in 27 metal-poor stars(-3.1< [Fe/H] <-1.4) whose very heavy metal (Z >56) enrichment was produced only by the rapid (r-) nucleosynthesisprocess. New abundances are derived from Hubble Space Telescope/SpaceTelescope Imaging Spectrograph, Keck/High Resolution EchelleSpectrograph, and Very Large Telescope/UV-Visual Echelle Spectrographspectra and combined with other measurements from the literature to forma more complete picture of nucleosynthesis of the heaviest elementsproduced in the r-process. In all cases, the abundance ratios among therare earth elements and the third r-process peak elements considered(La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solarsystem r-process abundance distribution. We compare the stellarobservations with r-process calculations within the classical"waiting-point" approximation. In these computations a superposition of15 weighted neutron-density components in the range 23 <=lognn <= 30 is fit to the r-process abundance peaks tosuccessfully reproduce both the stable solar system isotopicdistribution and the stable heavy element abundance pattern between Baand U in low-metallicity stars. Under these astrophysical conditions,which are typical of the "main" r-process, we find very good agreementbetween the stellar Pb r-process abundances and those predicted by ourmodel. For stars with anomalously high Th/Eu ratios (the so-calledactinide boost), our observations demonstrate that any nucleosyntheticdeviations from the main r-process affect—at most—only theelements beyond the third r-process peak, namely Pb, Th, and U. Ourtheoretical calculations also indicate that possible r-process abundance"losses" by nuclear fission are negligible for isotopes along ther-process path between Pb and the long-lived radioactive isotopes of Thand U.
| The evolution of field early-type galaxies in the FDF and WHDF
We explore the properties of 24 field early-type galaxies in theredshift range 0.20 < z < 0.75 down to MB <= -19.30in a sample extracted from the FORS Deep Field and the William HerschelDeep Field. Target galaxies were selected on the basis of a combinationof luminosity, spectrophotometric type, morphology and photometricredshift or broad-band colours. High signal-to-noise ratiointermediate-resolution spectroscopy has been acquired at the Very LargeTelescope, complemented by deep high-resolution imaging with theAdvanced Camera for Surveys onboard the Hubble Space Telescope (HST) andadditional ground-based multiband photometry. All galaxy spectra wereobserved under subarcsecond conditions and allow us to derive accuratekinematics and stellar population properties of the galaxies. To clarifythe low level of star formation detected in some galaxies, we identifythe amount of active galactic nuclei (AGN) activity in our sample usingarchive data of Chandra and XMM-Newton X-ray surveys. None of thegalaxies in our sample was identified as secure AGN source based ontheir X-ray emission. The rest-frame B- and K-band scaling relations ofthe Faber-Jackson relation and the Fundamental Plane display a moderateevolution for the field early-type galaxies. Lenticular (S0) galaxiesfeature on average a stronger luminosity evolution and bluer rest-framecolours which can be explained that they comprise more diverse stellarpopulations compared to elliptical galaxies. The evolution of the FP canbe interpreted as an average change in the dynamical (effective)mass-to-light ratio of our galaxies as = -0.74 +/- 0.08. The M/Levolution of these field galaxies suggests a continuous mass assembly offield early-type galaxies during the last 5 Gyr, which gets supported byrecent studies of field galaxies up to z ~ 1. Independent evidence forrecent star formation activity is provided by spectroscopic ([OII]emission, H?) and photometric (rest-frame broad-band colours)diagnostics. Based on the H? absorption feature we detect a weakresidual star formation for galaxies that accounts for 5-10 per cent inthe total stellar mass of these galaxies. The co-evolution in theluminosity and mass of our galaxies favours a downsizing formationprocess. We find some evidence that our galaxies experienced a period ofstar formation quenching, possible triggered by AGN activity that is ingood agreement with recent results on both observational and theoreticalside.Based on observations collected at the European Southern Observatory,Cerro Paranal, Chile using the Very Large Telescope under PIDs65.O-0049, 66.A-0547, 68.A-0013, 69.B-0278B and 70.B-0251A, and onobservations made with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. These observations are associated with programmes9502 and 9908.E-mail: afritz@gemini.edu
| Evolution of the barium abundance in the early Galaxy from a NLTE analysis of the Ba lines in a homogeneous sample of EMP stars
Context: Barium is a key element in constraining the evolution of the(not well understood) r-process in the first galactic stars andcurrently the Ba abundances in these very metal-poor stars were mostlymeasured under the Local Thermodynamical Equilibrium (LTE) assumption,which may lead in general to an underestimation of Ba. Aims: We presenthere determinations of the barium abundance taking into account thenon-LTE (NLTE) effects in a sample of extremely metal-poor stars (EMPstars): 6 turnoff stars and 35 giants. Methods: The NLTE profiles ofthe three unblended Ba II lines (4554 Å, 5853 Å, 6496Å) have been computed. The computations were made with a modifiedversion of the MULTI code, applied to an atomic model of the Ba atomwith 31 levels of Ba I, 101 levels of Ba II, and compared to theobservations. Results: The ratios of the NLTE abundances of bariumrelative to Fe are slightly shifted towards the solar ratio. In the plotof [Ba/Fe] versus [Fe/H], the slope of the regression line is slightlyreduced as is the scatter. In the interval -3.3 <[Fe/H] < -2.6,[Ba/Fe] decreases with a slope of about 1.4 and a scatter close to 0.44.For [Fe/H] <-3.3 the number of stars is not sufficient to decidewhether [Ba/Fe] keeps decreasing (and then CD-38:245 should beconsidered as a peculiar “barium-rich star”) or if a plateauis reached as soon as [Ba/Fe] ≈ -1. In both cases the scatter remainsquite large, larger than what can be accounted for by the measurementand determination errors, suggesting the influence of a complex processof Ba production, and/or inefficient mixing in the early Galaxy.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory (Large Programme “First Stars”, ID165.N-0276; P.I.: R. Cayrel.
| Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature withdetailed chemical abundance information (spanning –4.2lsim [Fe/H]lsim+0.3), and we compute their space velocities and Galactic orbitalparameters. We identify members of the inner and outer stellar halopopulations in our sample based only on their kinematic properties andthen compare the abundance ratios of these populations as a function of[Fe/H]. In the metallicity range where the two populations overlap(–2.5lsim [Fe/H] lsim–1.5), the mean [Mg/Fe] of the outerhalo is lower than the inner halo by –0.1 dex. For [Ni/Fe] and[Ba/Fe], the star-to-star abundance scatter of the inner halo isconsistently smaller than in the outer halo. The [Na/Fe], [Y/Fe],[Ca/Fe], and [Ti/Fe] ratios of both populations show similar means andlevels of scatter. Our inner halo population is chemically homogeneous,suggesting that a significant fraction of the Milky Way stellar halooriginated from a well-mixed interstellar medium. In contrast, our outerhalo population is chemically diverse, suggesting that anothersignificant fraction of the Milky Way stellar halo formed in remoteregions where chemical enrichment was dominated by local supernovaevents. We find no abundance trends with maximum radial distance fromthe Galactic center or maximum vertical distance from the Galactic disk.We also find no common kinematic signature for groups of metal-poorstars with peculiar abundance patters, such as the α-poor stars orstars showing unique neutron-capture enrichment patterns. Several starsand dwarf spheroidal systems with unique abundance patterns spend themajority of their time in the distant regions of the Milky Way stellarhalo, suggesting that the true outer halo of the Galaxy may have littleresemblance to the local stellar halo.
| NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
Aims. Aluminium is a key element to constrain the models of the chemicalenrichment and the yields of the first supernovae. But obtaining preciseAl abundances in extremely metal-poor (EMP) stars requires that thenon-LTE effects be carefully taken into account. Methods: The NLTEprofiles of the blue resonance aluminium lines have been computed in asample of 53 extremely metal-poor stars with a modified version of theprogram MULTI applied to an atomic model of the Al atom with 78 levelsof Al I and 13 levels of Al II, and compared to the observations. Results: With these new determinations, all the stars of the sample showa ratio Al/Fe close to the solar value: [Al/Fe] = -0.06±0.10 witha very small scatter. These results are compared to the models of thechemical evolution of the halo using different models of SN II and arecompatible with recent computations. The sodium-rich giants are notfound to be also aluminium-rich and thus, as expected, the convection inthese giants only brings to the surface the products of the Ne-Na cycle.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory, Chile (Large Programme “First Stars”,ID 165.N-0276(A); P.I.: R. Cayrel).
| First stars. VIII. Enrichment of the neutron-capture elements in the early Galaxy
Context: Extremely metal-poor (EMP) stars in the halo of the Galaxy aresensitive probes of the production of the first heavy elements and theefficiency of mixing in the early interstellar medium. The heaviestmeasurable elements in such stars are our main guides to understandingthe nature and astrophysical site(s) of early neutron-capturenucleosynthesis. Aims: Our aim is to measure accurate, homogeneousneutron-capture element abundances for the sample of 32 EMP giant starsstudied earlier in this series, including 22 stars with [Fe/H]< -3.0. Methods: Based on high-resolution, high S/N spectra from the ESOVLT/UVES, 1D, LTE model atmospheres, and synthetic spectrum fits, wedetermine abundances or upper limits for the 16 elements Sr, Y, Zr, Ba,La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb in all stars. Results: As found earlier, [Sr/Fe], [Y/Fe], [Zr/Fe] and [Ba/Fe] arebelow Solar in the EMP stars, with very large scatter. However, we finda tight anti-correlation of [Sr/Ba], [Y/Ba], and [Zr/Ba] with [Ba/H] for-4.5 <[Ba/H] < -2.5, also when subtracting the contribution of themain r-process as measured by [Ba/H]. Spectra of even higher S/N ratioare needed to confirm and extend these results below [Fe/H] ≃ -3.5.The huge, well-characterised scatter of the [n-capture/Fe] ratios in ourEMP stars is in stark contrast to the negligible dispersion in the [α/Fe] and [Fe-peak/Fe] ratios for the same stars found in Paper V. Conclusions: These results demonstrate that a second(“weak” or LEPP) r-process dominates the production of thelighter neutron-capture elements for [Ba/H] < -2.5. The combinationof very consistent [ α/Fe] and erratic [n-capture/Fe] ratiosindicates that inhomogeneous models for the early evolution of the haloare needed. Our accurate data provide strong constraints on futuremodels of the production and mixing of the heavy elements in the earlyGalaxy.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (program ID 165.N-0276(A); P.I: R. Cayrel).
| Halo Star Streams in the Solar Neighborhood
We have assembled a sample of halo stars in the solar neighborhood tolook for halo substructure in velocity and angular momentum space. Oursample (231 stars) includes red giants, RR Lyrae variable stars, and redhorizontal branch stars within 2.5 kpc of the Sun with [Fe/H] less than-1.0. It was chosen to include stars with accurate distances, spacevelocities, and metallicities, as well as well-quantified errors. Withour data set, we confirm the existence of the streams found by Helmi andcoworkers, which we refer to as the H99 streams. These streams have adouble-peaked velocity distribution in the z-direction (out of theGalactic plane). We use the results of modeling of the H99 streams byHelmi and collaborators to test how one might use vz velocityinformation and radial velocity information to detect kinematicsubstructure in the halo. We find that detecting the H99 streams withradial velocities alone would require a large sample (e.g.,approximately 150 stars within 2 kpc of the Sun and within 20° ofthe Galactic poles). In addition, we use the velocity distribution ofthe H99 streams to estimate their age. From our model of the progenitorof the H99 streams, we determine that it was accreted between 6 and 9Gyr ago. The H99 streams have [α/Fe] abundances similar to otherhalo stars in the solar neighborhood, suggesting that the gas thatformed these stars were enriched mostly by Type II supernovae. We havealso discovered in angular momentum space two other possiblesubstructures, which we refer to as the retrograde and progradeoutliers. The retrograde outliers are likely to be halo substructure,but the prograde outliers are most likely part of the smooth halo. Theretrograde outliers have significant structure in the vφdirection and show a range of [α/Fe], with two having low[α/Fe] for their [Fe/H]. The fraction of substructure stars in oursample is between 5% and 7%. The methods presented in this paper can beused to exploit the kinematic information present in future largedatabases like RAVE, SDSS-II/SEGUE, and Gaia.
| Hubble Space Telescope Observations of Chromospheres in Metal-Deficient Field Giants
Hubble Space Telescope high-resolution spectra of metal-deficient fieldgiants more than double the stars in previous studies, span ~3 mag onthe red giant branch, and sample an abundance range [Fe/H] = -1 to -3.These stars, in spite of their age and low metallicity, possesschromospheric fluxes of Mg II (λ2800) that are within a factor of4 of Population I stars, and they give signs of a dependence on themetal abundance at the lowest metallicities. The Mg II k line widthsdepend on luminosity and correlate with metallicity. Line profileasymmetries reveal outflows that occur at lower luminosities(MV = -0.8) than detected in Ca K and Hα lines inmetal-poor giants, suggesting mass outflow occurs over a larger span ofthe red giant branch than previously thought and confirming that the MgII lines are good wind diagnostics. These results do not support amagnetically dominated chromosphere, but they appear more consistentwith some sort of hydrodynamic or acoustic heating of the outeratmospheres.
| Supernova Nucleosynthesis in Population III 13-50 Msolar Stars and Abundance Patterns of Extremely Metal-poor Stars
We perform hydrodynamic and nucleosynthesis calculations ofcore-collapse supernovae (SNe) and hypernovae (HNe) of Population (Pop)III stars. We provide new yields for the main-sequence mass ofMMS=13-50 Msolar and the explosion energy ofE=(1-40)×1051 ergs to apply to chemical evolutionstudies. Our HN yields based on the mixing-fallback model of explosionsreproduce the observed abundance patterns of extremely metal-poor (EMP)stars (-4<[Fe/H]<-3), while those of very metal-poor (VMP) stars(-3<[Fe/H]<-2) are reproduced by the normal SN yields integratedover the Salpeter initial mass function. Moreover, the observed trendsof abundance ratios [X/Fe] against [Fe/H] with small dispersions for theEMP stars can be reproduced as a sequence resulting from the variouscombination of MMS and E. This is because we adopt theempirical relation that a larger amount of Fe is ejected by more massiveHNe. Our results imply that the observed trends with small dispersionsdo not necessarily mean the rapid homogeneous mixing in the earlyGalactic halo at [Fe/H]<-3 but can be reproduced by the``inhomogeneous'' chemical evolution model. In addition, we examine howthe modifications of the distributions of the electron mole fractionYe and the density in the presupernova models improve theagreement with observations. In this connection, we discuss possiblecontributions of nucleosynthesis in the neutrino-driven wind and theaccretion disk.
| A Search for Nitrogen-enhanced Metal-poor Stars
Theoretical models of very metal-poor intermediate-mass asymptotic giantbranch (AGB) stars predict a large overabundance of primary nitrogen.The very metal-poor, carbon-enhanced, s-process-rich stars, which arethought to be the polluted companions of now extinct AGB stars, providedirect tests of the predictions of these models. Recent studies of thecarbon and nitrogen abundances in metal-poor stars have focused on themost carbon-rich stars, leading to a potential selection bias againststars that have been polluted by AGB stars that produced large amountsof nitrogen and hence have small [C/N] ratios. We call these starsnitrogen-enhanced metal-poor (NEMP) stars and define them as having[N/Fe]>+0.5 and [C/N]<-0.5. In this paper we report on the [C/N]abundances of a sample of 21 carbon-enhanced stars, all but three ofwhich have [C/Fe]<+2.0. If NEMP stars were made as easily ascarbon-enhanced metal-poor (CEMP) stars, then we expected to findbetween two and seven NEMP stars. Instead, we found no NEMP stars in oursample. Therefore, this observational bias is not an importantcontributor to the apparent dearth of N-rich stars. Our [C/N] values arein the same range as values reported previously in the literature (-0.5to +2.0), and all stars are in disagreement with the predicted [C/N]ratios for both low- and high-mass AGB stars. We suggest that thedecrease in [C/N] from the low-mass AGB models is due to enhancedextramixing, while the lack of NEMP stars may be caused by unfavorablemass ratios in binaries or the difficulty of mass transfer in binarysystems with large mass ratios.Based on observations obtained at Cerro Tololo Inter-AmericanObservatory and Kitt Peak National Observatory, a division of theNational Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation.
| Improved Laboratory Transition Probabilities for Hf II and Hafnium Abundances in the Sun and 10 Metal-poor Stars
Radiative lifetimes from laser-induced fluorescence measurements,accurate to ~+/-5%, are reported for 41 odd-parity levels of Hf II. Thelifetimes are combined with branching fractions measured using Fouriertransform spectrometry to determine transition probabilities for 150lines of Hf II. Approximately half of these new transition probabilitiesoverlap with recent independent measurements using a similar approach.The two sets of measurements are found to be in good agreement for linesin common. Our new laboratory data are applied to refine the hafniumphotospheric solar abundance and to determine hafnium abundances in 10metal-poor giant stars with enhanced r-process abundances. For the Sunwe derive logɛ(Hf)=0.88+/-0.08 from four lines; the uncertaintyis dominated by the weakness of the lines and their blending by otherspectral features. Within the uncertainties of our analysis, ther-process-rich stars possess constant Hf/La and Hf/Eu abundance ratios,logɛ(Hf/La)=-0.13+/-0.02(σ=0.06) andlogɛ(Hf/Eu)=+0.04+/-0.02 (σ=0.06). The observed averagestellar abundance ratio of Hf/Eu and La/Eu is larger than previousestimates of the solar system r-process-only value, suggesting asomewhat larger contribution from the r-process to the production of Hfand La. The newly determined Hf values could be employed as part of thechronometer pair, Th/Hf, to determine radioactive stellar ages.
| NLTE determination of the sodium abundance in a homogeneous sample of extremely metal-poor stars
Context: Abundance ratios in extremely metal-poor (EMP) stars are a goodindication of the chemical composition of the gas in the earliest phasesof the Galaxy evolution. It had been found from an LTE analysis that atlow metallicity, and in contrast with most of the other elements, thescatter of [Na/Fe] versus [Fe/H] was surprisingly large and that, ingiants, [Na/Fe] decreased with metallicity. Aims: Since it iswell-known that the formation of sodium lines is very sensitive tonon-LTE effects, to firmly establish the behaviour of the sodiumabundance in the early Galaxy, we have used high quality observations ofa sample of EMP stars obtained with UVES at the VLT, and we have takeninto account the non-LTE line formation of sodium. Methods: Theprofiles of the two resonant sodium D lines (only these sodium lines aredetectable in the spectra of EMP stars) have been computed in a sampleof 54 EMP giants and turn-off stars (33 of them with [Fe/H]<-3.0)with a modified version of the code MULTI, and compared to the observedspectra. Results: With these new determinations in the range {-4<[Fe/H]< -2.5}, both [Na/Fe] and [Na/Mg] are almost constant witha low scatter. In the turn-off stars and "unmixed" giants (located inthe low RGB): [Na/Fe] = -0.21 ± 0.13 or [Na/Mg] = -0.45 ±0.16. These values are in good agreement with the recent determinationsof [Na/Fe] and [Na/Mg] in nearby metal-poor stars. Moreover we confirmthat all the sodium-rich stars are "mixed" stars (i.e., giants locatedafter the bump, which have undergone an extra mixing). None of theturn-off stars is sodium-rich. As a consequence it is probable that thesodium enhancement observed in some mixed giants is the result of a deepmixing.
| Broadband UBVRCIC Photometry of Horizontal-Branch and Metal-poor Candidates from the HK and Hamburg/ESO Surveys. I.
We report broadband UBV and/or BVRCIC CCDphotometry for a total of 1857 stars in the thick-disk and halopopulations of the Galaxy. The majority of our targets were selected ascandidate field horizontal-branch or other A-type stars (FHB/A, N=576),or candidate low-metallicity stars (N=1221), from the HK and Hamburg/ESOobjective-prism surveys. Similar data for a small number of additionalstars from other samples are also reported. These data are being usedfor several purposes. In the case of the FHB/A candidates they are usedto accurately separate the lower gravity FHB stars from various highergravity A-type stars, a subsample that includes the so-called blue metalpoor stars, halo and thick-disk blue stragglers, main-sequence A-typedwarfs, and Am and Ap stars. These data are also being used to derivephotometric distance estimates to high-velocity hydrogen clouds in theGalaxy and for improved measurements of the mass of the Galaxy.Photometric data for the metal-poor candidates are being used to refineestimates of stellar metallicity for objects with availablemedium-resolution spectroscopy, to obtain distance estimates forkinematic analyses, and to establish initial estimates of effectivetemperature for analysis of high-resolution spectroscopy of the starsfor which this information now exists.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| First stars IX - Mixing in extremely metal-poor giants. Variation of the 12C/13C, [Na/Mg] and [Al/Mg] ratios
Context: .Extremely metal-poor (EMP) stars preserve a fossil record ofthe composition of the ISM when the Galaxy formed. It is crucial,however, to verify whether internal mixing has modified their surfacecomposition, especially in the giants where most elements can bestudied. Aims: .We aim to understand the CNO abundance variationsfound in some, but not all EMP field giants analysed earlier. Mixingbeyond the first dredge-up of standard models is required, and itsorigin needs clarification. Methods: .The 12C/^{13C}ratio is the most robust diagnostic of deep mixing, because it isinsensitive to the adopted stellar parameters and should be uniformlyhigh in near-primordial gas. We have measured 12C and ^{13C} abundances in 35 EMP giants (including 22 with {[Fe/H] <-3.0}) from high-quality VLT/UVES spectra analysed with LTE modelatmospheres. Correlations with other abundance data are used to studythe depth of mixing. Results: .The 12C/^{13C} ratio isfound to correlate with [C/Fe] (and Li/H), and clearly anti-correlatewith [N/Fe], as expected if the surface abundances are modified by CNOprocessed material from the interior. Evidence for such deep mixing isobserved in giants above {log L/Lȯ = 2.6}, brighter thanin less metal-poor stars, but matching the bump in the luminosityfunction in both cases. Three of the mixed stars are also Na- andAl-rich, another signature of deep mixing, but signatures of the ONcycle are not clearly seen in these stars. Conclusions: .Extramixing processes clearly occur in luminous RGB stars. They cannot beexplained by standard convection, nor in a simple way by rotatingmodels. The Na- and Al-rich giants could be AGB stars themselves, but aninhomogeneous early ISM or pollution from a binary companion remainpossible alternatives.
| Galaxy merging and number vs. apparent magnitude relation for the universe with a time-decaying cosmological term
Aims. An attempt is made to constrain the values of the cosmologicalparameters together with the galaxy merging factor η on the basis ofa comparison between the observed galaxy number counts vs. theirapparent magnitudes relation (N-m relation) with those theoreticallyconstructed for the universe with a time-decaying cosmological termΛ. Methods. We assume that the galaxy number density evolutioncan be represented sufficiently well by a function of the redshift z ofthe form φ^*(z)∝ (1+z)^η. Three variations of thecosmological term with time τ are considered, (1) Λ∝τ-l, (2) Λ ∝ a-m with a being thescale factor, and (3) Λ ∝ Hn with H the Hubbleparameter. The optimum ranges for the decaying parameters (l, m, and n),the density parameters ΩΛ,0 andΩm,0, as well as T_mg (the timescale for the merger ofa pair of galaxies) and the redshift z_mg for the first onset of galaxymerger are sought based on statistical analysis using likelihoodfunctions given by χ2 evaluations. Results. In the caseof the type I models, for instance, we find thatl=0.75+0.55-0.75, T_mg = 0.3 Gyr, z_mg=3,ΩΛ,0=0.71+0.17-0.09 andΩm,0=0.29+0.09-0.17, from which acosmic age of 17.2+1.3-1.2 Gyr results. Thesemodel parameters lead to η=1.73+0.14-0.07. Asa consistency check, we have also carried out computations of the cosmicmicrowave background radiation (CMBR) spectrum, and have madecomparisons with WMAP measurements. We found that it is necessary tosomewhat modify the parameter values obtained above on account of thehigh sensitivity of η to the value of T_mg. The final model that wasfound to account for both the observed N-m relation and the WMAPmeasurements of the CMBR spectrum is as follows: z_mg=3.0, T_mg=0.2 Gyr,l=0.04, {ΩΛ,0}=0.77, η =2.2269. The age ofthis model universe is 14.6 Gyr, which is still sufficiently high tocope with the "new" cosmic age problem.
| Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman
We develop and test a method for the estimation of metallicities([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhancedmetal-poor (CEMP) stars based on the application of artificial neuralnetworks, regressions, and synthesis models to medium-resolution (1-2Å) spectra and J-K colors. We calibrate this method by comparisonwith metallicities and carbon abundance determinations for 118 starswith available high-resolution analyses reported in the recentliterature. The neural network and regression approaches make use of apreviously defined set of line-strength indices quantifying the strengthof the Ca II K line and the CH G band, in conjunction with J-K colorsfrom the Two Micron All Sky Survey Point Source Catalog. The use ofnear-IR colors, as opposed to broadband B-V colors, is required becauseof the potentially large affect of strong molecular carbon bands onbluer color indices. We also explore the practicality of obtainingestimates of carbon abundances for metal-poor stars from the spectralinformation alone, i.e., without the additional information provided byphotometry, as many future samples of CEMP stars may lack such data. Wefind that although photometric information is required for theestimation of [Fe/H], it provides little improvement in our derivedestimates of [C/Fe], and hence, estimates of carbon-to-iron ratios basedsolely on line indices appear sufficiently accurate for most purposes.Although we find that the spectral synthesis approach yields the mostaccurate estimates of [C/Fe], in particular for the stars with thestrongest molecular bands, it is only marginally better than is obtainedfrom the line index approaches. Using these methods we are able toreproduce the previously measured [Fe/H] and [C/Fe] determinations withan accuracy of ~0.25 dex for stars in the metallicity interval-5.5<=[Fe/H]<=-1.0 and with 0.2<=(J-K)0<=0.8. Athigher metallicity, the Ca II K line begins to saturate, especially forthe cool stars in our program, and hence, this approach is not useful insome cases. As a first application, we estimate the abundances of [Fe/H]and [C/Fe] for the 56 stars identified as possibly carbon-rich, relativeto stars of similar metal abundance, in the sample of ``strong G-band''stars discussed by Beers, Preston, and Shectman.
| Supernova Neutrino Effects on r-Process Nucleosynthesis in Black Hole Formation
Very massive stars with mass >=8 Msolar culminate theirevolution by supernova explosions, which are presumed to be the mostviable candidates for the astrophysical sites of r-processnucleosynthesis. If the models for the supernova r-process are correct,then the results of nucleosynthesis could also put a significantconstraint on the remnants of supernova explosions, i.e., a neutron staror black hole. In the case of very massive core collapse for aprogenitor mass 20-40 Msolar, a remnant stellar black hole isthought to be formed. Intense neutrino flux from the neutronized coreand the neutrinosphere might suddenly cease during the Kelvin-Helmholtzcooling phase because of the black hole formation. It is important andinteresting to explore the observable consequences of such a neutrinoflux truncation. It has recently been argued in the literature that eventhe neutrino mass can be determined from the time delay of the deformedneutrino energy spectrum after the cessation of neutrino ejection(neutrino cutoff effect). Here we study the expected theoreticalresponse of the r-process nucleosynthesis to the neutrino cutoff effectin order to look for another independent signature of this phenomenon.We found a sensitive response of the r-process yield if the neutrinocutoff occurs after the critical time when the expanding materials inthe neutrino-driven wind drop out of nuclear statistical equilibrium(NSE). The r-process nucleosynthesis yields drastically change if thecutoff occurs during the r-process, having maximal effect on the changein abundance of 232Th and 235,238U. There is alarge probability of finding this effect in elemental abundances ofr-process-enhanced metal-deficient halo stars whose chemical compositionis presumed to be affected by Population III supernovae in the earlyGalaxy. Using this result, connected with future detection of the timevariation of the SN neutrino spectrum, we are able to identify when theblack hole formation occurs in the course of SN collapse.
| Hubble Space Telescope Observations of Heavy Elements in Metal-Poor Galactic Halo Stars
We present new abundance determinations of neutron-capture elements Ge,Zr, Os, Ir, and Pt in a sample of 11 metal-poor(-3.1<=[Fe/H]<=-1.6) Galactic halo giant stars, based on HubbleSpace Telescope UV and Keck I optical high-resolution spectroscopy. Thestellar sample is dominated by r-process-rich stars such as thewell-studied CS 22892-052 and BD +17°3248 but also includes ther-process-poor, bright giant HD 122563. Our results demonstrate thatabundances of the third r-process peak elements Os, Ir, and Pt in thesemetal-poor halo stars are very well correlated among themselves and withthe abundances of the canonical r-process element Eu (determined inother studies), thus arguing for a common origin or site for r-processnucleosynthesis of heavier (Z>56) elements. However, the large (andcorrelated) scatters of [Eu, Os, Ir, Pt/Fe] suggest that the heaviestneutron-capture r-process elements are not formed in all supernovae. Incontrast, the Ge abundances of all program stars track their Feabundances, very well. An explosive process on iron peak nuclei (e.g.,the α-rich freezeout in supernovae), rather than neutron capture,appears to have been the dominant synthesis mechanism for this elementat low metallicities: Ge abundances seem completely uncorrelated withEu. The correlation (with very small scatter) of Ge and Fe abundancessuggests that Ge must have been produced rather commonly in stars, evenat early times in the Galaxy, over a wide range of metallicity. The Zrabundances show much the same behavior as Ge with (perhaps) somewhatmore scatter, suggesting some variations in abundance with respect toFe. The Zr abundances also do not vary cleanly with Eu abundances,indicating a synthesis origin different than that of heavierneutron-capture elements. Detailed abundance distributions for CS22892-052 and BD +17°3248, combining the new elementaldeterminations for Os-Pt and recently published Nd and Ho measurements,show excellent agreement with the solar system r-process curve from theelements Ba to Pb. The lighter n-capture elements, including Ge, ingeneral fall below the same solar system r-process curve that matchesthe heavier elements.
| Kinematic and chemical evolution of early-type galaxies
We investigate in detail 13 early-type field galaxies with0.2
| First stars VI - Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements
We have investigated the poorly-understood origin of nitrogen in theearly Galaxy by determining N abundances from the NH band at 336 nm in35 extremely metal-poor halo giants, with carbon and oxygen abundancesfrom Cayrel et al. (\cite{CDS04}, A&A, 416, 1117), usinghigh-quality ESO VLT/UVES spectra (30 of our 35 stars are in the range-4.1 < [Fe/H] < -2.7 and 22 stars have [Fe/H] < -3.0). Nabundances derived both from the NH band and from the CN band at 389 nmfor 10 stars correlate well, but show a systematic difference of 0.4dex, which we attribute to uncertainties in the physical parameters ofthe NH band (line positions, gf values, dissociation energy, etc.).Because any dredge-up of CNO processed material to the surface maycomplicate the interpretation of CNO abundances in giants, we have alsomeasured the surface abundance of lithium in our stars as a diagnosticof such mixing. Our sample shows a clear dichotomy between two groups ofstars. The first group shows evidence of C to N conversion through CNcycling and strong Li dilution, a signature of mixing; these stars aregenerally more evolved and located on the upper Red Giant Branch (RGB)or Horizontal Branch (HB). The second group has [N/Fe] < 0.5, showsno evidence for C to N conversion, and Li is only moderately diluted;these stars belong to the lower RGB and we conclude that their C and Nabundances are very close to those of the gas from which they formed inthe early Galaxy, they are called ``unmixed stars''. The [O/Fe] and[(C+N)/Fe] ratios are the same in the two groups, confirming that thedifferences between them are caused by dredge-up of CN-processedmaterial in the first group, with negligible contributions from the O-Ncycle. The ``unmixed'' stars reflect the abundances in the early Galaxy:the [C/Fe] ratio is constant (about +0.2 dex) and the [C/Mg] ratio isclose to solar at low metallicity, favouring a high C production bymassive zero-metal supernovae. The [N/Fe] and [N/Mg] ratios scatterwidely. Their mean values in each metallicity bin decrease withincreasing metallicity, but this trend could be a statistical effect.The larger values of these ratios define a flat upper plateau ([N/Mg] =0.0, [N/Fe] = +0.1), which could reflect higher values within a widerange of yields of zero-metal SNe II. Alternatively, by analogy with theDLAs, the lower abundances ([N/Mg] = -1.1, [N/Fe] = -0.7) could reflectgenerally low yields from the first SNe II, the other stars being Nenhanced by winds of massive Asymptotic Giant Branch (AGB) stars. Sinceall the stars show clear [α/Fe] enhancements, they were formedbefore any significant enrichment of the Galactic gas by SNe Ia, andtheir composition should reflect the yields of the first SNe II.However, if massive AGB stars or AGB supernovae evolved more rapidlythan SNe Ia and contaminated the ISM, our stars would also reflect theyields of these AGB stars. At present it cannot be decided whetherprimary N is produced primarily in SNe II or in massive AGB stars, or inboth. The stellar N abundances and [N/O] ratios are compatible withthose found in Damped Lyman-α (DLA) systems. They extend thewell-known DLA ``plateau'' at [N/O] ≈ -0.8 to lower metallicities,albeit with more scatter; no star is found below the putative ``low[N/α] plateau'' at [N/O] ≈ -1.55 in DLAs.Based on observations obtained with the ESO VLT under ESO programme ID165.N-0276(A). This work has made use of the SIMBAD database.
| The Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| 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.
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